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Engineering Education Studies 1

DATESTAMPHERE

Insert

this

way

Donald

E D U C 6 5 0 4

HEATHER

Intro to University

23/4/2012

1499

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COUPLINGS

Couplings are devices that are ulilised to connect two shafts together for the purpose of transmitting power.

The purpose of couplings is to join two rotating pieces of equipment together allowing for a small amount of

misalignment or end movement in each piece of equipment.

Substantial savings can be made by selecting, installing and maintaining couplings correctly which will result in

reduced maintenance costs and downtime.

Shaft couplings have several purposes when used with machinery with the most common reasons being:

To introduce mechanical flexibility

To allow for misalignment of shafts

To reduce shock transmission loads from one shaft to another

To introduce protection against overloading of machinery parts

To change the vibration characteristics of rotating units

Types of Couplings:

Rigid:

A rigid coupling can be used to connect two separate systems, such as a motor and a pump, or can be

utilised to repair a connection within a single system. A rigid coupling could also be used to reduce

shock and wear at a point where two shafts meet.

Rigid couplings are used when it is necessary to have precise alignment and the ability to hold the two

pieces of machinery securely in place. By aligning the two shafts and aligning them securely helps to

maximise performance and increase the expected life of the machine. Rigid coupling are available in

two types:

i) Sleeve style – they are a single tube of material with an inside diameter that is the same size of

the shaft. The sleeve slips over the shafts with screws that can be tightened to the top of each

shaft.

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ii) Clamped rigid coupling – they are made with two parts that fit together around the shaft and form

a sleeve. They are secured by screws which pass all the way through the coupling and into the

second half to hold it rigidly in place. They are used to connect shafts that are fixed in place.

Constant-velocity couplings:

Constant velocity couplings allow a shaft to transmit power at varying angles and at a constant speed.

The constant velocity coupling was used in drive trains in cars with front wheel drive.

The Thompson Coupling consists of two cardan joints assembled within each other eliminating the use

of a shaft, along with a control yoke that geometrically constrains their alignment. The control yoke

has minimal inertia and generates virtually no vibration.

Shaft alignment and coupling setup need to be disconnected and connected easily. There should be

some allowance for misalignment between the two adjacent shaft rotation axes. Coupling

maintenance should be relatively easy by performing visual inspections, cleaning couplings regularly

and checking for signs of wear. Detection of potential coupling failure would be abnormal noise,

vibration and an indication of lubricant leakage.

Coupling

The coupling will fit over the shaft and be bolted together with M12 bolts. The reason is it needs to

withstand a lot of vibration. The coupling will be used to transverse power from an electric motor to

centrifugal pump to pump water for use in an agriculture farm. The precise alignment is necessary to

minimise vibration and maintenance.

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Incorrectly aligned shafts.

The alteration to radius on the coupling body is to allow for more stress and strain to be absorbed. The

radius will assist the coupling by reinforcing a weak point. When a coupling is manufactured it needs

to be designed to maximise its ability to transverse power without failure. The radius is designed to

allow the component to have more strength at a point that would be weak.

There are several methods which could be used to manufacture the coupling.

Machining:

The coupling could be machined out of a billet. The process would need to be accurate and fine

tolerances would need to be maintained. The emergence of Computer Numerical Control is how it

would need to be produced for it to be a viable proposition cost wise. The machine would cut the

profile and drill and bore the hole in one process. The advent of technology would also allow the

machine to drill the four 12mm diameter holes while the coupling is in the machine. The chuck has

the ability to rotate at a given angle anywhere within the 360° rotation utilising a universal head to

drill holes. The keyway could be broached out to size.

Drop Forging:

The coupling could be drop forged to shape. If a material is worked by the application of localised

compressive forces, it is said to have been forged.

“Drop Forging is “it may be squeezed or hammered in between special dies so that the metal flows

and takes up the shape of the die cavity, Shlenkler 1986.

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The machines utilise 2 dies that take up the shape of the coupling and have a capacity of up to 10 MN.

Drop forging is a mass production process using one die or a series of dies. Dimensional accuracy is

not high with drop forging however the couplings outside surfaces do not need machining. The bore

will be machined, holes drilled and keyway cut. When a components finished size is known the

forging could allow for balance by allowing the mass taken out by the keyway being applied exactly

opposite. This will result in the coupling when completed being balanced which will assist in less

vibration when turning at high speed.

Material:

The material I would use mild steel. Mild steel ‘contains 0.15 – 0.3%C. Wrought forms are used as

RSJ and other structural members, shafting, levers and various forgings”. Higgins 1977.

Mild steel is readily available and reasonably cheap compared to other materials. Mild steel is easily

formed, machined and does not harden much if cooled quickly, and is quite ductile. Copeland 2000

It should be noted that mild steel, like all steels (except stainless steels), readily corrode, and that the

product of corrosion (commonly known as rust) is porous, thus promoting further corrosion.

The radius is being applied to the coupling to add strength at the intersection of the two diameters.

When two diameters meet which have been machined the flow lines of the material in this case “fibre”

is cut. This reduces the strength of the coupling and is a weak point at which the material may fail due

to stresses. The coupling could develop cracks at this point running along the flow lines.

“High stress concentration is likely to cause failure along exposed fibres and so weaken the

component” Higgins 1977

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The material being drop forged will have a radius at this point. The material has been upset from bar

stock with the result being flow lines following the contour of the coupling. This result is the radius at

the intersection of the two diameters being strengthened.

Billet or bar stock

Flow lines following outside profile.

Section displaying flow lines.

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Conclusion:

By adding a radius at the point where diameters meet strengthens components so they will be able to

be utilised for longer periods. This results in less maintenance of the component, ability of the

component to withstand greater forces for longer periods and less down time replacing the component.

The time it would take to connect the coupling correctly and fixing both components securely in

position would offset costs utilising different couplings or methods to transmit power. The final

process could be trying to minimise corrosion by painting the coupling, and installing a cover to

protect the coupling from direct weather conditions. This could also be a safety measure so no

rotating parts of the machinery are exposed.

Bibliography

Copeland, P. L. (2000). Engineering Studies, The Definitive Guide. Australia; Anno Domini Pty Lty.

Higgins, R. A. (1977). Properties of Engineering Materials. Great Britain; The Chaucer Press.

Schlenker. B. R. (1986). Introduction to Material Science. [SI. ed]. Milton; The Jacaranda Press.

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Shaft Washer

Nut Key

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Coupling

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Cross section 3D Drawing

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Drawing displaying radii.

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