Machinery and Components

Adam AdgarSchool of Computing and Technology

Machine/Component Failures

►Major source of production downtime in most industrial plants

► Cost of production downtime is often higher than the expense involved with the repair or replacement

►Many machine/component failures can be averted, or at least the useful life can be extended

► An important part of this process is knowing the cause of the failure.

Machines

► Pumps► Compressors► Fans► Conveyor belts► Crushers► Mills► Gearboxes► Rollers

► Motors

Components

► Shafts► Bearings► Gears► Couplings► Pulleys + Belts► Sprockets + Chains► Impeller/Vanes

► Electrical (Rotor and Stator)

Shafts

► Rotating bars which transmit force, power, and motion

► Usually circular cross section► Torque is twisting force resisted by shaft

► Stresses on shafts Torque Tension Compression Bending Combinations

TENSION

TORSION BENDING

COMPRESSION

Bearings

► Hydrodynamic /journal bearings

Sliding action coefficient of friction

high 0.002 < f < 0.010

► Rolling-element /antifriction

Rolling action coefficient of friction

low 0.001 < f < 0.002

► A device that supports, guides, and reduces the friction of motion between fixed and moving machine parts.

► Two major types:

Journal Bearings► Shaft rotates inside a

bearing bore slightly larger than the shaft diameter

► Lubricant is supplied to the annular gap

► Portion of the shaft within the bearing is called the journal.

► At rest, there is metal-to-metal contact between the journal and the bearing, along the line of contact

► Once rotation begins, a lubricant film develops between the journal and the bearing

► As speed increases, a wedge of lubricant forms, supporting the shaft away from the bearing, and preventing wear.

Rolling Element Bearings► Make use of spherical or

cylindrical rolling elements captured between inner and outer rings

► The rolling elements support the load, and transmit rotation by rolling, rather than sliding

► Friction is fairly uniform with speed hence power loss is more predictable

► Experience much less wear at slower speeds than do journal bearings

OuterRace

InnerRace

Ball

Cage orSeparator

Bore

Rolling Element Bearings

► There are two types of rolling element bearings

Ball bearings Point contact High speed Low load

Roller bearings Line contact Low speed High load

Gears► Gears

Wheel, disk or bar with teeth on periphery

Teeth mesh with teeth on other gears

Simple gears transmit power directly from one shaft to another

Internal or external teeth

As two engaged gears rotate together, the same number of teeth on each gear pass the line between the centres

► Types of gears Gear train Bevel gears Internal and external

gears Worm gear drive

Gears► Used in many mechanical

devices► Typically required for one

(or a combination) of several different reasons:

to increase or decrease the speed of rotatione.g. increase torque

to reverse the direction of rotation

to move rotational motion to a different axis

to keep the rotation of two axes synchronized

► To create large ratios, gears are often connected together in gear trains

► Gear teeth provide advantages:

Slippage between the gears is prevented. Therefore, axles connected by gears are always synchronised exactly with one another.

It is possible to determine exact gear ratios by counting number of teeth in the two gears and divide one by the other, e.g. if one gear has 60 teeth and another has 20, the gear ratio when these two gears are connected together is 3:1.

Slight imperfections in actual diameter and circumference of two gears don't matter. The gear ratio is controlled by the number of teeth even if the diameters are a bit off.

Couplings

Couplings► Couplings are used to make co-axial connections

between two shafts► They serve two main purposes:

To allows first shaft (driver) to drive the second shaft (driven) at the same speedi.e. transmission of power

To compensate for minor amounts of misalignment and random movement between the two shafts.

► Couplings are either rigid or flexible Rigid couplings require very close alignment of the shafts

Sleeve coupling Flange coupling

Flexible couplings generally ‘sandwich’ something flexible in between, or connected to, rigid flanges attached to each shaft

Toroidal coupling

Pulleys and Belts ► V belts

create greater friction by wedging into the groove on the pulley

hence greater torque capacity

Between 70-96% efficiency

► Flat and round belts work very well require higher tension

than V belts to transmit the same torque

require more rigid shafts, larger bearings, etc.

Up to 98% efficiency

► Toothed belts don’t slip (synchronous) hence transmit torque at

a constant ratio good for applications

requiring precise timing very efficient more costly than other

types

Sprockets and Chains► Compared to belts, chains can

transmit more power for a given size, and can maintain more precise speed ratios.

► Like belts, chains may suffer from a shorter life than a gear drive.

► Flexibility is limited by the link-length, which can cause a non-uniform output at high speeds.

► Can be very efficient – around 98%.► User controls the length (with master links) is a

plus.► Sprockets wear out much more frequently than

pulleys

Motors

Motor Faults► Neglect

Dirt Surface dirt causes

overheating Internal dirt degrades

insulation and bearings

Bad Lubrication Too much or too

frequent Too infrequent Mixing incompatible

lubricants Poor Power Quality

Over/under voltage(especially under)

Unbalanced voltage Single-phasing

► Misapplication Under-sizing

Service factor is not for normal use

ASD Stresses Low speed + high torque

= overheating Induced bearing currents Standing wave

phenomena Poor Ventilation

Causes overheating Coupling & Belts

Misaligned couplings or sheaves = Bearing Failures

“Soft Foot”, I.e. bad shimming

Motor Faults► Severe Environment

Hot Ambient Requires de-rating

High Altitude Causes overheating -

requires de-rating Humidity

Motors in storage (zaps insulation or bearings)

Motors with significant off time (zaps insulation)

Requires internal or ambient heating

Airborne Contaminants Damages insulation or

bearings Kills by abrasion or

chemical deterioration

► Normal Wear Without unusual

stresses, motors sometimes last for tens of thousands of operating hours, but will eventually succumb to...

Bearing wear Insulation failure

Faults

► Mechanical Unbalance Misalignment Bearing Looseness Lubrication Cavitation

► Electrical Rotor Stator

UnbalanceBalanced Unbalanced

► Unbalance is the force exerted on a rotor due to the difference between the centre of rotation and the centre of mass

Centrelineof mass

Centrelineof rotation

Misalignment

AngularMisalignment

OffsetMisalignment

Bearing Faults► Revolution around outer

race ► Non-uniform radial tension

of bearing► Misalignment of outer race ► Wear of the outer race ► Cavities on the outer race► Wear of the inner race ► Cavities on the inner race ► Wear of balls, rollers or

cage ► Cavities, spallings of

balls/rollers► Complex defect► Slip of race► Defects of lubrication

► Causes Handling/Transportation Defects of

installation/maintenance High dynamic loads from

shaft imbalance, coupling

misalignment, and self-excited rotor oscillations

Bearing distortion Lubrication

Lack of Excess Impurities Degradation

Bearing Faults

►Defects can lead to seizure of the bearing breakage of cage Rapid or slow wear of bearing Pitting Cavities Cracks Increase in the friction coefficient

Monitoring/Diagnostic Techniques

►Many predictive maintenance technologies

► Examples include Vibration analysis Oil analysis Ultrasound Thermography Motor Current Signature Analysis

Typical Installation

Motor Driven Pump

VV V V

PR

P T

A

T TT

Self Study

► Read B&K Application Note on Detecting Faulty Rolling Element Bearings (pp 1-2)