Design of material handeling equipment

DESIGN OF MATERIAL HANDELING SYSTEMPREPARED BY:SAGAR CHOKSHIASST. PROFESSORMECHANICAL DEPT.CSPIT-CHANGA.

INTRODUCTION TO MATERIAL HANDLING

In any industrial process, the product being manufactured passes through various phases and it needs to be transported from place to place. This could involve processes such as transporting of raw material to the machines and then shifting the machines from one station to the other and finally to the store or warehouse. This involves the use of material handling equipment. The simplest form of material handling is manual, where the material is shifted from one machine to the other by a worker. However, in large production setups, where the production rates are high and the product to be handled is such that manual transportation is not possible, sophisticated material handling systems would be required. Material handling system does not contribute directly to the product value, but it adds to the cost of the product and is therefore sometimes is referred to as a necessary evil. In fact, the least handling would be the best handling.Thus, while designing the material handling system, care should be taken that the selected equipment will give optimum productivity. Unnecessary sophistication should be avoided as it would lead to increase of product cost.

**Prof.Sagar Chokshi (MEDept)

Prof.Sagar Chokshi (MEDept)

SYSTEM CONCEPT FOR MATERIAL HANDLING

A material handling system design should have certain parameters which should be satisfied by the system. These are listed as below:Material handling systems suggested should not obstruct or create problems to the product cycle. They should work hand in hand, without requiring any major modifications in the factory layouts.The systems designed should be easy to upgrade and should not become obsolete with small changes in the product cycle.Any future expansions or changes should not be at prohibitive costs.The system should be so selected that it will achieve optimum productivity. Unnecessary sophistication should be avoided, and the simplest system which would work should be used.The system should have a low initial and operational cost. A high initial cost, would put financial constraints on its implementation, as it does not directly contribute to product costs.The rate of return on investment should justify the Solution adopted.



The basic objectives that a material handling system should fulfill are:Quick and Precise Pick-up of LoadsQuick and Efficient Transfer of Load with a Planned Time IntervalTransport of Loads in Planned QuantitySafe Transport of the Load without any DamagesAccuracy in Delivering at the DestinationAutomation with Minimum Human ElementLow Initial and Operational CostsSimple and Easy to MaintainSafe OperationBASIC OBJECTIVES OF MATERIAL HANDLING SYSTEM **Prof.Sagar Chokshi (MEDept)


The types of load handled by a material handling system can be of two basic types:

Bulk LoadUnit Load and Containerization. Bulk Load:A bulk load is in the form of particles, powders or lumps of homogeneous materials, which cannot be counted by numbers. Examples of bulk loads are : iron ore, coal, sand, cement, mineral, stone, clay etc. A bulk material is classified by its bulk density, lump-size, flowability, abrasiveness and other miscellaneous characteristics. This property based classification plays an important role in the design of material handling systems.

TYPES OF LOAD**Prof.Sagar Chokshi (MEDept)


*

Bulk Material Character Classification


PropertyMaterial CharacteristicsClassification

SizeVery fine - 150 micron sieveFine - Less than 3 mmGranular - Less than or equal to 12 mmLumpy - Lumps over 12 mmIrregular - InterlockingABCDE

FlowabilityVery free flowingFree flowingAverage flowingSluggish1234AbrasivenessNon-abrasiveAbrasiveVery AbrasiveSharp5678MiscellaneousCharacteristicsBuilds up and hardensGenerates static electricityFlammabilityVery dustyContains explosive dustHighly corrosiveHygroscopicPacks under pressureVery light and fluffy - may be windsweptElevated TemperatureFGJLNSUXY.Z


Unit Loads:Unit loads are those loads which can be counted by numbers or units Examples of unit loads would be a machine component, a complete machine, a building block, or containers in the form of boxes. Generally unit loads are specified by its weight. The important properties to be considered in the design of material handling equipment for unit loads would be :

The weight of each unit.The support or bearing surface where the unit would be placed.The brittleness and hardness of the material.The surface finish.Temperature of the unit.*Prof.Sagar Chokshi (MEDept)*


*

Containerisation:In a high productivity cycle, handling small sized packets or units is inconvenient and uneconomical. In such cases, containers of different sizes and shapes are used for handling such materials. Many a times, certain quantities of bulk material can be placed in a container which can then be handled as a unit load. This process of converting a bulk load into a unit load is referred to as containerisation.The advantage of this process is that even bulk material can be easily expressed in units : for example, 10 containers of 500 kg each. In some cases, various containers can be further stacked over one another and each stack can then be treated as a unit. This allows for a better check on the produced material. Besides that, hoisting equipment are generally designed for handling loads, by their weights. With containerisation, bulk materials can be treated as unit loads and can be specified by its weight.

*Prof.Sagar Chokshi (MEDept)*


Consideration in the design of container:Limit the variety of containers by way of sizes and shapes.Each container should be designed based on the requirements of the material it has to handle.Containers should not be very large as it would increase the loading on the handling systems. At the same time, a small container would increase the number of containers and hamper productivity. Containers should be stackable. This reduces storage costs, as the vertical space in a store can be utilised. Stackable containers should be designed in such a manner, that they do not collapse when stacked.The containers should at all times be properly coated or painted. Colour coding and numbering of containers should also be done so that they can be easily recognised. Wooden or plastic containers are rust free, but have limitations on their life, cleanliness and can also be hazardous in case of fires.Steel containers are sturdy and have a long life. Steel containers can be of a solid design or made of wire mesh. Wire mesh containers are suitable as they are lighter in weight and are self cleaning. They also produce less noise in handling, as compared to a solid container. Wire mesh containers allows the components to be washed, without removing them from the containers. Besides this, the components and the containers dry and cool faster.**Prof.Sagar Chokshi (MEDept)


Stackable container



CLASSIFICATION OF MATERIAL HANDLING EQUIPMENT*Prof.Sagar Chokshi (MEDept)*


HOISTING EQUIPMENTThis comprises of a group of equipment which are used primarily for lifting or lowering of unit load or components in batches. These equipment can move the loads in vertical direction and can then transfer them from one point to the other in the horizontal direction, within the operating zone of the equipment. Hoists can further be classified into three categories.

Pure Hoisting Equipment CranesElevators*Prof.Sagar Chokshi (MEDept)*


Pure Hoisting Equipment The first category includes the pure hoisting equipment such as Jacks, Winches, Hand hoists and pulley blocks. These generally are used for vertical lifting with very limited horizontal movement. A typical arrangement of a hoist is as shown in Fig.



Cranes:One of the most widely used type of power driven cranes are the EOT cranes as shown in Fig.. (Electric Overhead Travelling Crane). It can serve a large area of floor space and does not occupy floor space, as it is mounted overhead. These cranes have three motions, i.e. hoisting motion, the long travel motion and the cross motion. With these three motions, any space on the shop floor within its working range can be covered.



Cranes:



Portal Jib Crane

Fig. Portal Jib Crane **Prof.Sagar Chokshi (MEDept)


Cont..Besides the EOT cranes, other kind of cranes are the jib cranes, cantilever cranes, mobile cranes, wall cranes. Fig. shows a portal jib crane which has an inclined member called as a jib. This is supported by a wire rope or a structure attached to the frame. The crane has a revolving base which can be rotated for transfer of load from one place to the other. Cranes could also be of the mobile type. In this case, the jib cranes are mounted on trucks, crawlers 'or locomotives. In general, cranes have four possible motions :Hoisting Motion : This is used to lift or lower the load.Derrieking or Luffing Motion : This motion which is imparted to the jib so as to move in a vertical plane. It allows the load line to be brought, near or farther away from the centre of the .crane.Slewing Motion : This is the revolution of the whole structure of the crane including the jib about the revolving base.Long travel motion : This motion is applicable when the crane has to be bodily moved on rail track or along a road.



ElevatorsThese are the devices which are used to lift bulk materials in a vertical or inclined plane. Bucket type elevator shown in Fig. is an example of a vertical elevator.Elevators of this design predominate in the bulk handling of free-flowing, fine and loose materials with small to medium size lumps. Buckets, mounted at spaced intervals, are loaded by scooping up material from the boot or by feeding the material into them. Material is discharged by centrifugal action as the buckets pass over the head wheel connected to the driving pulley.Other types of elevators include cage or cars being guided and raised using ropeways and counter weights.



CONVEYING EQUIPMENT

Conveing equipment consists of a number of equipment used for generally handling bulk load. However, they are also occasionally used for unit loads. These are continuous flow kind of systems and do not have a separate lifting or lowering gear. The major merits of using conveyors are

They can be used for horizontal, vertical or inclined transfer of material.They can be installed at floor level, work station level or at a higher level. In cases, where the installation is done at a higher level, valuable floor space can be saved.Transfer rates in conveyors are extremely high as compared to others.Conveyors are based on a continuous flow cycle and a continuous flow of material is aensured.Transfer rates can be easily controlled by raising or lowering the speed of the conveyor. In case of improvements in the product cycle, the rate at which the material reaches a station can thus be controlled easily.Conveyors can be started and stopped easily with the press of a button and they come to a halt within a short time. This allows easy control of the arriving and departing material at each workstation.Most of the conveyors are generally laid down in straight lines. This reduces the route taken by the material substantially.**Prof.Sagar Chokshi (MEDept)


Cont..Conveyors can be used extensively in assembly lines. In this case, the conveyor serves a dual role in conveying the material and also acting as a processing line, where operations are done. These kind of conveyors are often used in the automobile industry. The speed at which the conveyor moves, helps in maintaining the productivity.They can conveniently handle all kinds of loads and in all kinds of conditions. They can be used to load furnaces, paint units or high pressure cleaning units or even handle explosive and hazardous substances.Conveyors do not necessarily require an operator for handling the material. They are extremely mechanised and are suitable for companies involved in automated production.Conveyors can be designed for working in clean and controlled environments. They can be used in food and pharmaceutical industries, where the products can be handled and then packed mechanically, without any human contact.When designed properly, conveyors are reliable and require minimum maintenance. **Prof.Sagar Chokshi (MEDept)


Cont..(Limitations)Any change in the product cycle, which changes the destination of the material or the sequence would mean reworking the complete layout of the conveyor. This would involve high costs.Though the conveyors can handle high transfer rates, they are not suitable in handling very heavy unit loads.Conveyors operate only on fixed routes and no deviations are possible in that.Conveyors are rapid transfer systems but are inflexible. They can only be suitable in plants, where the work content does not change at all.



*

Cont..

Conveyors find application in transfer of minerals and ores from the mines to the store. They can also used to load ships or trucks. Another application is in the automobile industry where it also serves as a process line. Conveyors can be used in the food and dairy industry, hottling plants and pharmaceutical companies. Some of the commonly used conveying systems are mentioned below



Belt Conveyors: **Prof.Sagar Chokshi (MEDept)


Cont..A belt conveyor consists of two end pulleys, with a continuous loop of material that rotates about them. The pulleys are powered, moving the belt and the material on the belt forward. The belt span is supported by idlers spaced along the length of the conveyor. Conveyor belts are extensively used to transport industrial and agricultural materials, such as grain, coal, ores, etc. Conveyor belts are also used in self-unloading bulk freighters. The longest conveyor belt in the world is in Western Sahara. It is 100 km long, from the phosphate mines of Bu Craa to the coast south of El-Aaiun. The longest single belt conveyor runs from Meghalaya in India to Sylhet in Bangladesh. It is 17 kms long and Conveys limestone and shale. Belt conveyors allow large volumes of material to move rapidly. A sketch of the belt conveyor is as shown in Fig. A detailed discussion on belt conveyors is explained in the later articles.



Chain ConveyorsThese conveyors are similar in working principles to the belt conveyors. In this case, an endless chain is made to run over the head and tail sprockets. Just as in belt conveyors, idlers are provided to support the chains in the load carrying and return tracks. Chain conveyors have a better ability to absorb impact loads and have also less stretchability under load. They can transport large loads and can be used in applications where the conveyed material is hot. However, these conveyors are more expensive and are also noisy as compared to belt conveyors. They require proper lubrication and the chains wear significantly faster, if the conveying speeds are increased. Chain conveyors are generally suitable for unit loads.



Screw Conveyors :

Fig. shows a screw conveyor, which comprises of rotating screw placed in a tube. Depending on the material to be handled, different shapes are given to the screw. As the screw: rotates, the material is pushed towards the discharge end.A screw conveyor can handle cold as well as hot materials. They are also effective in transferring materials such as coal, slurries, fine powders and agrigultural products. Screw conveyors can be arranged inclined or sometimes even vertical.



Cont..**Prof.Sagar Chokshi (MEDept)


Apron Conveyors :

Apron conveyors are used to handle various bulk and unit loads in the horizontal and inclined directions. These type of conveyor are constructed of a series of parallel endless roller chains running on tracks supported by the frame. Overlapping metal plates are attached to the chain to form a moving bed. These types of conveyors are suitable for withdrawing loads from hoppers bins, etc. and can usually handle heavy, large lumped, abrasive and hot material. A apron conveyor chain is as shown in Fig.



SURFACE AND OVER HEAD EQUIPMENTSurface or overhead equipment is the group of equipment which are used for handling unit or bulk loads in hatches on a horizontal surface. This group of equipment can be in the form of fork lifts, trucks, lorries, railway cars and wagons, overhead monorails, scrapers and skidders.



Cont..A fork lift is a powered industrial truck used to lift and transport materials, often by means of steel forks inserted under the load. Fork lifts are most commonly used to move loads stored on pallets. Fig. shows the sketch of a typical fork lift. Fork lift trucks are available in many variations and load capacities. In a typical warehouse setting most fork lifts used have load capacities of around one to five tons. In addition to a control to raise and lower the forks, the operator can tilt the mast to compensate for a load's tendency to angle the blades toward the ground and risk slipping off the forks. Tilt also provides a limited ability to operate on non- level ground.Scrapers or skidders are a special type of heavy vehicles used in for pulling carts along a pathway.



BASIC PRINCIPLES IN SELECTION OF MATERIAL HANDLING EQUIPMENT

In the previous article, we saw that there are different types of material handling equipment, with their own characteristics. In many cases, different types of equipment can be used for thesame kind of an operation. Certain basic principles and factors are used in identifying the type of equipment that should be used for a given situation. These factors are discussed below:



1.Direction of Load TravelIn a material handling system, the payload could be required to be transported in to vertical, horizontal or inclined directions. The direction of this travel is important in selecting the kind of material handling system that should be chosen for that application. For example, if the direction of load travel is vertical or close to vertical, then hoists, cranes or elevators are used. If the load travel is horizontal, then the different types of conveyors are preferred



2.Length of Load Travel :In case of machine shops, where the load travel happens in relatively smaller areas, hoists, EOT or fork lifts are the ideal means of material handling. On the other hand, in industries such as mining, where the material handling occurs over a large length, conveyors are preferred.



3.Properties and Characteristics of the material being handled The material being handled can be either of bulk type or can be a unit load. Each of this material would also have other inherent properties. For example, materials like iron ore are lumpy, with average flowability and are abrasive. On the other hand materials such as aluminium nitrate are granular and also contain explosive dust and are highly corrosive. Besides these, materials could also possess other properties such as their hardness, surface finish, temperatures, and tendency to form cake. Depending on these characteristics and properties of the material being handled, a proper system should be selected



4.The rate of flow of the material:

The rate of flow of material directly affects the productivity of manufacturing. In applications, where a large flow rate of material is required, conveyors and bucket elevators are preferred. These equipment would give a continuous flow of material. On the-other hand, if the rate at which the material flow happens is not that large, equipment such as cranes, EOT or fork lifts can be used.



5.Kind of the Production Process :

The kind of production process done to the material is an important parameter in selecting the equipment for handling it, For example, if the production process is that of an assembly line, then the material has to flow at a particular rate, within which certain set operations must be done by the operator at each station. The rate at which this flow occurs would completely define the productivity of the whole cycle. This kind of production process is typically seen in the automobile industry. In such cases, conveyors are preferred



6.Methods of Loading and Unloading

The method of loading the material at entry and discharge also plays a role in selection of a material handling system. If bulk materials are used, then gravity is the chosen form of feeding and discharging the material. Bucket elevators use the buckets to scoop the material and then it is discharged at another level using centrifugal forces and gravity. In case of unit loads, the units or containers are generally stacked one on top of each other. In such cases, cranes, EoT or fork lifts are suitable.



7.Existing Layout and Conditions of the Workspace:Many a times, the material handling system may be added on to an existing shop floor. In such a case, the existing layout of the workspace should be carefully studied to facilitate an easy installation of a particular material handling system.Besides, this, conditions such as type and design of building, climatic conditions and the type of energy available should be studied before firolising the kind of equipment to be installed.



8.Initial and Operational Costs:

Costs are a very important factor in the selection of a material handling system. It has been stressed earlier that a handling system does not increase the value of the product, but it increases its cost. Thus, this increase in cost should be justified in terms of savings achieved by way of increased productivity and reduction in costs due to mechanisation. A thought should be given on these aspects before selecting a material handling system.Besides, the high initial costs, the operational costs of the system should also be considered while selecting the system.



BELT CONVEYORSA belt conveyor consists of an endless belt of a resilient material Connected between two pulleys and moved by rotating one of the pulleys through a drive unit gear box, which is connected to an electric motor. The driving pulley end is called as the head end, and the pulley is called as the head pulley. Conversely, the other pulley is at the tail end and is referred to as the tail pulley as shown in Fig. Material is conveyed by placing it on the belt, through a feeder. As the belt rotates, the material is carried with it to the other end, where it is then dropped in the discharge chute. It should however be noted that the discharge can be arranged at any point along the run by means of special discharge devices.As the belt rotates, due to the weight of the belt and the conveyed material, the belt will sag. To support this sag, rollers called as idler pulleys are placed on both sides i.e. the carrying side and the return side. Closely spaced idlers are placed at the loading point, as there is some impact due to the falling material and overcrowding of the material in this region.



Cont..The belt is subjected to tension and it being from a resilient material is prone to elongation. This reduces the tension in the belts. Reduction in tension causes slackness of the belt on the pulleys resulting in slippage and loss in power. To compensate for this, a tensioning device called as take-up arrangement is used. Generally, belt conveyors carry payload only on the upper side. This is the carrying side, whereas the other side carries no payload and is called as the return side.



Flat Belt Arrangement

Belt conveyors can be flat or troughed. A flat belt conveyor is that in which the belt runs flat on the carrying and return side over straight rollers or idlers as shown in Fig. Flat belt conveyors are suitable for low loads, low capacity and relatively lesser speeds. They are cheaper and require less maintenance and are suitable for handling unit loads or bulk loads of small lump size.



Cont.. Five Idler ArrangementTroughed Belt Arrangement **Prof.Sagar Chokshi (MEDept)


Cont..A troughed belt conveyor is the one in which the belt forms a trough on the carrying side. A trough is formed by a set of idlers or rollers arranged as shown in Fig. A transition distance is required to convert a flat returning belt into a troughed arrangement.Two, three or five roll idlers can be used for troughing. The troughing angles are defined by standards and are selected from a range of 15, 20, 25, 30, 35, 40, 45. Troughed belt conveyors are used for higher capacities, higher speeds and for handling material of large lump size. It is also suitable for incline or decline type of conveyors



ADVANTAGES OF A BELT CONVEYOR OVER OTHER CONVEYORS Belt conveyors are an important component of the material handling system. They can be operated over long distances over any kind of terrain, as compared to other conveyers.They have a high load carrying capacity and can carry all kinds of loads in an economical manner.Belt conveyers are noiseless as compared to chain conveyers. They neither pollute nor deafen the ears of people working in the premises.They are much more simpler to maintain and do not require any major lubrication system unlike what would be required in case of chain conveyors.The reliability of belt conveyors has been proved over a long period by its use in the industry.Belt conveyors are environmentally more acceptable than others.They have a relatively low labour and low energy requirements as compared to other conveying equipment.Unlike screw conveyors, belt conveyors can be easily used for performing processing functions in a production line.



TYPES OF BELT CONVEYOR LAYOUTS**Prof.Sagar Chokshi (MEDept)


METHODS OF LOADING AND DISCHARGING OF CONVEYORSThe successful working of a belt conveyor requires that the conveyor be loaded and discharged properly. Ideal loading exists when the material is placed centrally on the belt at a velocity equal to that of the belt. Ideal discharge would happen if all the conveyed material is discharged at the desired point and in the desired direction. In practice ideal conditions are seldom achieved resulting in excessive belt wear, spillage and pollution which lead to higher running costs. Loading the material onto the belt involves many considerations. The two most important of these are

The material should be placed centrally on the belt.The material should be fed in the direction of belt travel and at a speed as near as possible to that of the belt. Delivering the material at a different velocity to that of the belt creates turbulence in the material at the loading point. This may result in a material build-up which could disturb the conditions of ideal loading.**Prof.Sagar Chokshi (MEDept)


Cont..Loading methods:

a). Hopper Based Loading :



Cont.The simplest method of loading a material is by a hopper as shown in Fig. 8.13.1. The material is fed to the conveyor through a hopper and a feeder as shown in Fig. The feeder imparts the necessary velocity to the material, so that it roughly matches the belt speed.A variation of this arrangement is when the hopper is fixed to a travelling machine as shown in Fig. 8.13.1 (b). In this case the hopper travels along the path of the conveyor, distributing the material over the belt region, thereby reducing volume build-up at one place.



Processing Unit Based Loading :

In certain cases, the feeder maybe connected to a processing unit such as a crusher or a mixer. Fig. shows a schematic of this arrangement. In such cases, the material may enter the feeder at a very high velocity. The feeder must be designed to limit this speed. Another form of limiting high lump velocities is to hang baffle bars or chains in the path of the lumps.



Loading from a Preceding Conveyor:In some cases, the feed to the conveyor can be given through another conveyor. This arrangement is as shown in Fig. This kind of feeding or loading is generally seen, when the layout involves paths of different inclinations.Materials carried by a belt can be discharged in many different ways. A few of the more commonly encountered ways are discussed briefly.



Cont..



Methods of Discharging:(a) Head End Discharge*Prof.Sagar Chokshi (MEDept)*


Cont..

Simplest dischargeWay of discharge occurs by gravity.Widely used.**Prof.Sagar Chokshi (MEDept)


b). Both End Discharge :**Prof.Sagar Chokshi (MEDept)


Cont..In some applications, materials are required to be discharged at both. ends. In this case the material can be unloaded at the head end when the conveyor runs in the forward direction and over the tail end, when it reverses. This reversal can be done by having a reversible motor. This arrangement is shown in Fig.



c). Plow Discharge



Cont..Plows are used as to divert some of the excess material on the belt. A discharge plow consists of an inclined blade as shown in Fig.. Only fine and free flowing materials can be discharged by means of plows. Plow discharge is effective only in the case where the belt is flat. It is however possible to use plows to skim materials from the top of a troughed belt. The discharge plows should be set at an angle less than 35 to the belt centre line. To prevent leakage under the plow it should be fitted with a rubber strip on the underside. If the portion of the belt under the plow is supported by a metal slide plate the discharge efficiency is further increased. The major disadvantage with this arrangement is that the belt is loaded due to the plow and wears much faster.



(d) Tripper Discharge

Trippers are used to discharge material before the head pulley in applications where the use of plows is not suitable. Atypical fixed tripper pulley arrangement is shown in Fig. The upper pulley acts as a tripper pulley to discharge the material into a chute which directs the material to one or both sides of the belt.



Cont..



Cont..In the case of moving trippers the tripper arrangement reciprocates along the length of the belt as shown in Fig. This arrangement is supported on wheels which are made to run in tracks installed on the sides of the conveyor belt. The tripper arrangement can then distribute the material over a larger area, thereby preventing any volume build-up at a point.



Cont.**Prof.Sagar Chokshi (MEDept)


REQUIREMENTS OF THE BELT USED IN A CONVEYOR SYSTEMThe belt is one of the most important and expensive components of a typical conveyor system and is subjected to tough working conditions. A belt should have the following requirementsHigh Strength : The belt is subjected to tensile loads. It is also subjected to other loads due to scrapers, plows. The material when fed also creates an impact load on the belt. All these conditions require the belt to have high strength.Low Self Weight : The belt is continuously driven over the pulleys. The power requirement to drive this belt is depended on its weight. A low self weight of the belt reduces the power requirement and increases the material conveying capacity.High Wear Resistance : The belts are subjected to rough working conditions over a long period of time. Besides this, scrapers, plows and other cleaners further create wear as they rub over the belt surface. The belt should thus have a high wear resistance to survive these tough working conditions.Low elastic and permanent elongation : Any elongation in the belt reduces the tension created in the belt. This would reduce the power transmitting capacity of the belt. A good belt should thus have a low elastic and permanent elongation.



Cont..Flexibility : The belt should have the necessary flexibility in the longitudinal and lateral planes. In many cases, belts are made to run by forming a trough over the pulleys. The belt material should have the necessary flexibility to mould over the idlers.High Resistance to Ply Separation : Belts are made from plies, which are bonded with a rubber element. The bonding of the plies should be such that it does not separate out due to the repeated bending of the belt over the pulleys.Low water absorption capability (hygroscopicity) Water if it gets absorbed by the belt increases the weight of the belt. This would result in increased power consumption and reduced conveying capability. Besides this, water if absorbed reduces the dimensional stability of the belt.Suitable for working in tough environmental conditions : The belt is subjected to tough environmental conditions such as humidity, extreme heat or cold. The belt material should be good enough to ensure that it works with optimum results under such working environment and has a long service life.



CONSTRUCTIONAL DETAILS OF A CONVEYOR BELT:Conveyor belts generally are composed of three main components.

CarcassSkimsCovers (carry cover and pulley cover )**Prof.Sagar Chokshi (MEDept)


1. CarcassThe reinforcement usually found on the inside of a conveyor belt is referred to as the carcass. The carcass provides the strength to the belt and absorbs the forces acting on the belt. The functions of a carcass include the following :

Provide the tensile strength necessary to move the loaded belt.Absorb the impact of the impinging material being loaded onto the conveyor belt.Provide the bulk and lateral stiffness required for the load support.Belts are connected at the ends by splicing them with belt fasteners. The carcass should provide the necessary strength to hold the fasteners.

The carcass is normally rated by the manufacturer in terms of maximum permissible operating tension. The carcass can of two major types

Fabric Ply Type.Steel Cord Type.**Prof.Sagar Chokshi (MEDept)


a). Fabric Ply TypeThe most common carcass design is made up of layers or plies of woven fabrics bonded together. The fabric is made from a variety of materials. Some of these are listed in Table with their characteristics.



Cont..A plain weave or twill weave carcass is as shown in Fig. It is usually bound together with rubber and consists of warp yarns (yarns used lengthwise) and the fill yarns (yarns used crosswise) passing over and under each other. This means that both these yarns are crimped and they assume a sine-wave-like configuration. This fact, combined with the basic characteristics of the fibre used give the belt its stretch characteristics. A ply belt uses different weaving methods impregnated with a rubber bond, depending on the requirement of the belt. The number of such plies would determine the strength of the belt. If a large number of plies are used, it may affect the flexibility of the belt. In such cases, the belts cannot be troughed. Generally, the belt with the lesser plies, but stronger fabric are preferred. Conventional plied belting constructions, using all synthetic carcasses and elastomer covers are recommended for :Mud Rock Mining - Aggregate, sand and oreGeneral purpose applicationsForest productsSoft Minerals - Coal, Potash, Phosphates, GrainUnit handling - Parcels, Baggage, Mail.



Plain Weave Design of Carcass



b). Steel Cord TypeA steel cord type belt uses a layer of uniformly tensioned steel cords as the strength members. This cord is encased in rubber as shown in Fig.The top cover of the rubber is about two times the bottom cover. The steel cord runs parallel along the length of the belt and are normally supported by transverse textile reinforcement. Steel cord belts are generally used in high tension applications or where the low stretch requirement is a necessity. Steel cord belts are designated with the suffix ST. A typical steel cord belt would be ST-1800 which would have a minimum tensile strength of 1800 kN/m.



Cont..



Cont..The major advantage of a steel cord type belt are:

A much higher breaking strength as compared to fabric type conveyor belt.Very low stretch.The flexibility of a steel cord type belt is more than that of a fabric type. This allows the conveyor to be run on smaller sized pulleys.These belts can be easily troughed, as the steel cords are placed parallel to the belt length and do not cause any bending resistance.The fabric belt is of a layered construction, thereby reducing the homogeneity between the various rubber layers. In a steel cord belt, there is one rubber mass, without any discontinuity. This results in a stronger construction.**Prof.Sagar Chokshi (MEDept)


2.Skims :The rubber, PVC or urethane between the plies is called as a skim. Skims are important contributors to internal belt adhesions, impact resistance, and play a significant role in determining the belt load support and troughability.Improper skims can also adversely affect belt performance and can lead to ply separation failure.



3. Covers (carry cover and pulley cover)Covers are used in conveyor belt constructions to protect the conveyor belt carcass and also to extend its service life. In addition to these, covers aiso provide the finished belt with a wide variety of desirable properties such asTextures.Cleanability.A specific coefficient of friction.A specific colour.Cut resistance.Enhanced impact resistance.Hardness.Fire, oil and chemical resistance



Cont..Covers are generally made from rubber or rubber like compounds. The primary requirement in selecting a cover is the kind of material to be conveyed and the environment in which the belt functions. In addition to selecting proper compounds for cover material, it is also necessary to determine the proper cover thickness. The thickness of a cover is influenced by the amount of abuse and wear the belt will receive.Textures are applied to the belt cover surface to provide for appropriate gripping action. Many textures and cleat patterns are available which function very well in handling free flowing materials such as wood chips, grain, sand, aggregate and fertilizer. This enables the conveyor to handle inclines and also to increase the friction.



Cont..The cover is usually the lowest cost component of the belt. The wear characteristics of the cover would depend on the kind of the material and on the size, weight, shape and discharge rate of the material conveyed. Sharp edges, especially on large pieces, can cut a cover badly in a very short time. On the other hand, if loading conditions are ideal, with the material being loaded in the direction of travel of the belt, and with only a slight impact onto the belt, even very sharp material may not seriously cut or wear the belt surface. Cover wear is also influenced by the loading area being on the horizontal compared to loading a conveyor belt on an incline which will result in the product bouncing around more before the load settles down. This increased product movement will adversely affect cover wear.Some of the standard cover materials, properties and their designations are listed in,q; Table.



Cont..Table:-8.15.4



Cont..Briefly, when recommending a belt, for a new system or as a replacement on an existing system, an objective study of the system and all the relevant operating conditions is of utmost importance. Belt selection must take into account six key criteria : system tension, load support, troughability, impact rating, pulley diameters, and covers (compound and thickness).



8.16 conveyor pulleyA conveyor arrangement uses the following types of pulleysDriving Pulleys (Head and Tail Pulleys).Snub PulleyIdlers



Cont..a). Driving Pulleys (Head and Tail Pulleys) :



Cont..The most commonly used conveyor pulleys are the standard steel pulleys as shown in Fig. 8.16.1. They are manufactured in a wide range of sizes and consist of a continuous rim and two end discs fitted in the hubs. To support large loads, the rim is supported within by stiffening discs welded to it. Pulleys may be straight faced or crowned. The crown helps in keeping the belt centred. The height of the crown is usually 0.5 % of the pulley width, but not less than 4 mm. The pulleys are lagged with grooves to improve the coefficient of friction. This also prevents the material from sticking to the pulley surface.The drive pulley are sometimes lagged by a rubber coating whenever necessary to increase the friction coefficient. This lagging should be in between 6 to 12 mm thick and its hardness should be lesser than the belt cover. The pulleys are generally manufactured from cast iron of grade FG200 or mild steel tubes.



Cont..Another type of a drive pulley is the self cleaning winged pulley as shown in Fig. 8.16.2. These are generally used as the tail or take-up pulley where the material tends to stick to the surface of the pulley. The self cleaning winged pulleys are lagged with a high tensile strength rubber or urethane material, encased in a steel jacket. This lagging compresses upon belt contact.



When the belt contact is broken, the rubber snaps back to its original height automatically cleaning itself. The pulley has a two cone arrangement to give strength and support to the



Cont..The size requirements of the conveyor are influenced by the diameter of the head and tail pulleys. These pulleys are subjected to maximum tension, and their diameter would thus be more than that of the other pulleys used. The smallest pulley diameter the belt can use in the conveyor system is an important factor in selecting the proper conveyor belt. The belt has to wrap around the head and tail pulleys under tension and the fatigue stress developed in the belt as it wraps and unwraps is influenced by the pulley diameter. Smaller the pulley diameter, more is the tendency of over stressing the bond between the individual plies and the skim. This can then result in ply separation and premature belt failure.



The pulley dimensions can be decided on the basis of the number of plies in the belt. The minimum pulley diameter Dmin can be roughly approximated by the formulaDalink x i (mm)(8.16.1)where,Number of plies**Prof.Sagar Chokshi (MEDept)


Multiplying factor which depends on the number of plies.125 (for 2 to 6 plies)150 (for 8 to 12 plies)If material of the ply and other drive details are available, the value of k can also be found by :k = k1 x k2(8.16.2) where,= Material factor for plies which depends on the material of the ply.k2 = Belt tension and arc of contact factor, which depends on the belt tension and the arc of contact made by the belt on the pulley.The values of kl and k2 are specified by the belt manufacturer.The pulley width can be taken as, w:B+2 xs. .(8.16.3) where,B = Belt width, mm= Side margin (taken as about 60 to 75 mm), mm



The calculated value should then be rounded off to the nearest standard size. Some of the standard sized pulleys are listed in Table 8.16.3. Most of the preferred sizes are based on the R10 series of prefeired numbers.Table 8.16.3 Standard Sized Pulleys200 mm250 mm315 mm400 mm500 mm630 turn800 mm1000 mm 1250 mm 1400 mm 1600 mm



b). Snub Pulley :Snub pulleys are those pulleys which help in increasing the angle of contact of the belt from 180 to as high as 2300 on the drive pulleys. This allows an increased power transmitting capacity to the belt. Snub pulleys can also be used to change the path of the belt for facilitating loading and unloading opmn ion:.



cont.. with Snub Pulleys Snub Pulley ArrangementsFig. 8.16.4**Prof.Sagar Chokshi (MEDept)


Cont..the ply.belt tensionIn most of the drives conveying abrasive materials, 2100 snub pulleys with a hard rubber lagging are used. Different types of snub pulley arrangements are shown in Fig. 8.16.4. A special kind of an arrangement is the tandem drive with snub pulley. In this case, two drive motors are used in tandem to connect to two drive pulleys as shown in Fig. 8.16.4 (b). This increases the angle of contact substantially and is suitable where the conveyor has to handle high loads..(8.16.3)c).IdlersConveyor belts are. supported on idlers. Idlers can be of two types :I.Carrying IdlerSome of the ased on the2.Return Idler



Cont..A carrying idler is used on the load carrying side of a conveyor and must support both the belt and the load carried by the belt while on the return side, the return idlers `must only support the empty return belt. The spacing or pitch of idlers has a direct bearing on the sag of the belt between the idler sets. The idlers on the carrying side should have a lower spacing as compared to that on the return side. As a thumb rule, the number of return idlers are half as that of the carrying idlers. Excessive sag in the belt between idlers results in a higher absorbed power for the conveyor and therefore the pitch or spacing of the idlers should be properly designed. Idlers serve the following functionsThey support the belt and the conveyed material on the upper run and the belt in the lower run with minimum frictional resistance. This support reduces the sagging of the belt.of the belt -ansmittingThe spacing of the idlers is reduced near the loading point, so as to support the belt due to the impact of the material in that region. This would prevent the belt from wearing quickly.



Cont..Idlers help in centering the belt and guiding it to the drive and snub pulleys.In a troughed belt conveyor, the shape of the belt is achieved by the idlers.In a troughed belt conveyor, lower nm is flat, whereas the upper run is troughed. Transition idlers of increasing angles as shown earlier in Fig. 8.10.3 (a) are used to achieve the required troughing.The different types of idlers and their characteristics are shown below.



Cont..Straight Roller Idlers with Rolling Contact Bearings :



Cont..These idlers use rolling contact bearings to rotate freely with the belt. They are suitable as carrying and return idlers. In light duty applications deep groove ball bearings can be used, whereas for heavy duty applications taper roller bearings can be used. The idler arrangement can be achieved by different methods of mounting the bearings. Seals are provided to prevent the conveying material from entering the bearings. The end caps for the rollers can be machined or press worked and are press fitted in the roller tube. The axle shaft in this case has flattened ends to prevent it from rotation.In many applications where the material has a tendency to stick to the idlers, especially in the return run, rubber linings or sleeves are coated on the roller tubes. This reduces the wear on the idlers. The rubber lining when worn out can be easily replaced.



Troughed IdlersThese idlers are used primarily, to support bulk load. They help in giving the trough shape to the belt. Troughed idlers are used on the carrying run and are formed by a set of idlers or rollers arranged as shown in Fig. 8.16.6. Two, three or five roll idlers can be used for troughing. The troughing angles are defined by standards and are selected from a range of 150, 20, 25, 30, 35, 400, 45. Troughed belt conveyors are used for higher capacities, higher speeds and for handling material of large lump size. It is also suitable for incline or decline type of conveyors.



Cont..c). Impact or Cushion Idlers



and the weight of the falling material creates impact on the belt. Impact idlers are made by fitting a thick sleeve of an elastomeric material such as rubber on plain rollers.(1). Rubber Cushion Return Idlers (fig 8.16.8)



Cont..

These are typically used as return idlers with a cushioning effect provided by rubber rings arranged as shown in Fig. 8.16.8. The rings also allow a quick and efficient cleaning of the idler from the adhering material on the belt.



Garland or Disc Type IdlersIn these type of idlers, trough ing is achieVed by a number of discs arranged as shown in the form of a garland. These idlers are suitable for self cleaning in applications where the material has the tendency to stick to the idlers. Refer Fig. 8.16.9.



f). Spring IdlersThese idlers use a spring to support the belt. The flexibility of the spring in the lateral plane creates a smooth trough for the belt. These idlers are generally used in cases where steelbands are used to convey the material and can support a much larger load. Refer Fig. 8.16.10.



8.17 BELT TAKE-UP DEVICESBelts are elastic in nature and they elongate with use. This results in reduced load carrying capacity of the belt. To maintain the tension in the belt under all operating conditions, take-up assemblies are required on all conveyors. The major functions of a take-up assembly are :To provide adequate tension in the belt as it leaves the drive pulley so as to prevent any slippage.To ensure that the belt has sufficient tension at the loading points.To compensate for belt elongation over the life of the belt.To provide extra length of belt if required for belt splicing.The location of the take-up, orientation of the take-up and design of the take-up depends on factors such as the length of the conveyor, the available headroom for the take-up and the mass of the counterweight. For horizontal take-ups the standard take-up travel is taken as about 1 % of the belt length or 400 mm, whichever is greater. For an inclined installation, this is taken as about 1.5 % of the belt length. Take-ups can be classified into two major categories



Cont..I.Fixed Take-up devices which are periodically adjusted by the operator.2. Automatic take-up devices which aim to provide a constant tension on the belt.The limitation of a fixed or manual take-up arrangement is that it requires manual checking from the operator. This would prevent perfect tensioning at all times. The conveyor would not work at its optimum levels at any instance. Besides this, there is a possibility of wrong tightening- due to oversight of the operator.In an automatic take-up, the tension in the belt is sensed by sensors and corrective action is taken under dynamic conditions. This results in perfect tensioning at all times, without any help from the operator. A large take-up travel or adjustment can be accommodated by this



arrangement. Some examples of take-up assemblies are discussed below.Screw Take-up Arrangement



Cont..A screw type arrangement is a fixed take-up device. In this arrangement, the take-up pulley (sometimes the drive pulley itself acts as the take-up pulley) rotates in two bearing blocks which may slide on stationary guideways with the help of two screws. The tension is created by tightening the screw periodically by the operator. Trapezoidal thread screws are preferred here to reduce the effort required in tightening the belt. This arrangement is compact and is suitable for small conveyors (upto 60 m length) operating under steady and light duty conditions. Fig. 8.17.1 shows a sketch of the screw take-up arrangement.



Vertical Gravity Take-up :**Prof.Sagar Chokshi (MEDept)


The vertical take up arrangement comprises of two bend pulleys over which the take-up pulley is arranged as shown in Fig. 8.17.2. The tension in the belt is altered by hanging weights. on the take-up pulley. The arrangement is very reliable and maintenance free. However, this would still be a fixed distance take-up arrangement and would require an operator to alter the weights. The limitations of this were already discussed earlier. Besides this, the arrangement requires a lot of head room under the pulleys making the system bulky.



Horizontal Gravity Take-up :Fig 8.17.3

This arrangement as shown in Fig. 8.17.3 comprises of a drive pulley mounted on a pulley trolley. The pulley trolley has four wheels which allow it to be guided along the length of the belt. A steel wire rope is held to the pulley trolley and using a bend 'and take-up pulley as shown, weights are suspended. The pulley trolley moves in a horizontal plane and hence the arrangement is called as the horizontal gravity take-up. This arrangement is suitable in places where sufficient head room is not available to install the vertical gravity take-up. The horizontal gravity take-up device is suitable for fabric belts but has all the limitations of a fixed take-up.



Motorised Winch Operated Take-up (Automatic Take-up) :The motorised winch operated take-up is an automatic arrangement which maintains a constant belt tension at all instances. This is achieved by having the arrangement as shown in Fig. 8.17.4. The drive pulley is placed on a sliding pulley trolley which is then connected to a winch with a wire rope.



Motorised Winch Operated Take-up (Automatic Take-up)Fig. 8.17.4**Prof.Sagar Chokshi (MEDept)


Cont..A sensor is placed to determine the tension in the belt. If the tension drops below a set value, it activates the winch motor. This would then pull the trolley thereby increasing the belt tension. Once the satisfactory tension is achieved, the sensor would switch off the winch motor. This results in an automatic adjustment. This kind of arrangement is preferred in large length conveyors working under heavy duty cycles.



8.18 BELT CLEANING DEVICESAn important property a of rubber covered conveyor belts is the high coefficient of friction of rubber. This reduces the tendency for the material to slip on inclines. However, it also increases the difficulty of cleaning the belt.As the material is transported on a conveyor, a certain small percentage of product is not discharged as the belt passes around the head pulley. In instances where the material conveyed is wet and sticky, more material would carry over onto the return side of the conveyor. The problem with this carry over is that this residual material on the return belt passes over the return idlers and it dries and accumulates on it. Small heaps of product can be Seen forming beneath each return idler and these piles of material can grow rapidly and have a high cleanup cost over the life of the plant. To prevent this, belt scrapers are used at the head end of the conveyor. In extreme cases belt turnovers are used. These devices physically turn the return belt upside down along the return run thereby preventing the dirty side of the belt coming into contact with the idlers.Some of the devices for belt cleaning are discussed below



a). Belt Scraper :The belt scraper is the most common system used for belt cleaning. Belt scraper blades can be made from a variety of materials such as rubber, steel, porcelain and polyurethane. The arrangement comprises of a continuous transverse blade in contact with the dirty side as shown



Simple Belt Scraper

Thick Solid Rubber Blade**Prof.Sagar Chokshi (MEDept)


However, a simple belt scraper arrangement has certain limitationsThe flexibility of the blade allows some of the material to pass through.Vibration and chatter due to the rubbing action creates excessive blade wear and decreases the efficiency of the arrangement.The wear of the blade results in a loss of contact which would further reduce the cleaning efficiency.The blade wears unevenly, and cannot be adjusted in a simple belt scraper.Volume build-up on the blade and the holder reduces the cleaning efficiency.To counter these problems, a variety of new designs have been developed. Almost all scraper blades now have some means of automatic adjustment ensuring that the blade remains in contact with the belt. Some blades consist of a thick moulded section which is supported on the underside. This design, as shown in Fig. 8.18.1(b) will not bend easily in the direction of belt travel and adjustment of the blade relative to the holder is not required.



b). Rotating Belt Cleaners :These consist of power driven shafts or tubes to which brushes, rubber fingers or rubber profiles are attached. Rotation is in a direction opposite to the belt travel to get maximum cleaning action. Rotary belt cleaners can be driven by chain from the adjacent head pulley shaft, or by a separate drive. Two possible arrangements are shown in Fig. 8.18.2.



c). Water Spray and Wipers :Another method of cleaning the belt is with the use of high pressure water spray. This method can be effective in certain difficult applications. The spray is directed against the dirty surface of the conveyor belt by means of nozzles. A rubber scraper blade is then positioned behind the water spray, so as to remove excess water from the belt surface after it has been washed.



8.19 DRIVE UNIT FOR BELT CONVEYORSThe belt conveyors can be connected to the drive unit in different ways. Typically, a drive unit would comprise of an electric motor, coupling, V-belts and spur or worm gear boxes. Inclined conveyors may have a tendency to move backwards in the event of power failure or stoppage. In such cases, brakes are also a part of the drive unit. Brakes can also be used for horizontal conveyors for emergency stopping. The different ways of connecting the motor to the drive are shown in Fig. 8.19.1.Fig. 8.19.1 (a) shows gear motor directly connected to the drive shaft by a flexible coupling. This results in a simple, reliable and economical arrangement. In Fig. 8.19.1 (b), a parallel shaft external gear box is used in between the drive and the motor. Two couplings would be required in this case. Another arrangement can be achieved by having a primary reduction by a V-belt and a secondary reduction by a gear box arrangement as shown in Fig. 8.19.1 (c). In applications, where the drive torque is high and operating speeds are low, a



spiral bevel or worm gear box as shown in Fig. 8.19.1 (d) is used.



Coupling Motor(d) Drive through Spiral Bevelor Worm Gear Box(c) Drive through Primary reduction by V belt and secondary by gear boxFig. 8.19.1The approximate mechanical efficiency of speed reducers are listed in Table 8.19.2...Table 8.19.2 : Mechanical Efficiency of Speed Reduction Mechanisms



Cont..



TypeMechanical Efficiency, i%Roller Chain - Sprocket93 %V-Belt and Sheave94 %Generated Spur Gear - Single Stage94%Single Stage Helical / Herringbone Gear box95 %Worm Gear Drive ratio 20: 1 to 60: 170%Worm Gear Drive ratio 60: Ito 100: 150%lley RollerTypically, a gear boxes.failure or be used for he motor toy a flexible .19.1 (b), aD couplings ; a primary s shown in s are low, a



8.20 IMPORTANT TERMS FOR CONVEYOR DESIGN a). Angle of Repose and Angle of SurchargeConveyed BulkMaterial= Angle of Repose= Angle of SurchargeAngle of Repose and Angle of SurchargeFig. 8.20.1



Cont..If the material is to move or vibrate as in case of a conveyor belt, the pile tends to flatten out. Depending on the characteristics of the material and the type and amount of movement and vibration, this angle will decrease, anywhere from 5 to as much as 200. The resultant angle is known as the angle of surcharge, 4). It is important to note that the angle of surcharge will vary from conveyor to conveyor. The longer the distance the material is conveyed, the more the surcharge angle tends to flatten.At conveyor angles which are less than the angle of surcharge an appropriate textured surface would be required to lock the material on the belt surface, enabling the conveyor system to operate at that incline. For angles of incline greater than the angle of surcharge special means must be provided to handle the material such asI.CleatsPocketsBucketsCover belts, etc.b). FlowabilityThe flowability of the material determines the cross-section of the material load on the belt. It is measured by the angle of repose and angle of surcharge together with size and shape of the material, particles, roughness or smoothness of the surface of the particles, proportion of





*

*

*

Date post:	13-Apr-2017
Category:	Engineering
Upload:	sagar-chokshi
View:	1,251 times
Download:	3 times

Design of material handeling equipment

Engineering