# Keys and Coupling

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2090

### Keys and Coupling

1. Introduction :Â A key is a piece of mild steel inserted between theÂ shaft and hub or boss of the pulley to connect these togetherÂ in order to prevent relative motion between them. It isÂ always inserted parallel to the axis of the shaft. Keys areÂ used as temporary fastenings and are subjected to considerableÂ crushing and shearing stresses. A key way is a slot orÂ recess in a shaft and hub of the pulley to accommodate aÂ key.

2. Types of Keys :Â The following types of keys are important from theÂ subject point of view :
1. Sunk keys , 2. Saddle keys , 3. Tangent keys,Â 4. Round keys, and 5. Splines.
We shall now discuss the above types of keys, inÂ detail, in the following pages.

9. #### Strength of a Sunk Key.

10. Effect of Keyways :Â A little consideration will show that the keyway cut into the shaft reduces the load carryingÂ capacity of the shaft. This is due to the stress concentration near the corners of the keyway andÂ reduction in the cross-sectional area of the shaft. It other words, the torsional strength of the shaft isÂ reduced. The following relation for the weakening effect of the keyway is based on the experimentalÂ results by H.F. Moore. Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â e = 1 – 0.2 (w/d) – 1.1 (h/d) Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  where e = Shaft strength factor. It is the ratio of the strength of the shaft with
keyway to the strength of the same shaft without keyway,
w = Width of keyway,
d = Diameter of shaft, and
h = Depth of keyway =Â Thickness of key (t)/2 Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â It is usually assumed that the strength of the keyed shaft is 75% of the solid shaft, which isÂ somewhat higher than the value obtained by the above relation.
In case the keyway is too long and the key is of sliding type, then the angle of twist is increasedÂ in the ratio kÎ¸ as given by the following relation : Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â kÎ¸ = 1 + 0.4 (w/d) + 0.7 (h/d) Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â where kÎ¸ = Reduction factor for angular twist.

11. #### Shaft Couplings.

12. Requirements of a GoodÂ Shaft Coupling :Â A good shaft coupling should have the following requirements :
1. It should be easy to connect or disconnect.
2. It should transmit the full power from one shaft to the other shaft without losses.
3. It should hold the shafts in perfect alignment.
4. It should reduce the transmission of shock loads fromÂ one shaft to another shaft.
5. It should have no projecting parts.

21. #### Universal Coupling.

Reference A Textbook of Machine Design by R.S. Khurmi and J.K. Gupta

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