Keys and Coupling
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Introduction :Ā A key is a piece of mild steel inserted between theĀ shaft and hub or boss of the pulley to connect these togetherĀ in order to prevent relative motion between them. It isĀ always inserted parallel to the axis of the shaft. Keys areĀ used as temporary fastenings and are subjected to considerableĀ crushing and shearing stresses. A key way is a slot orĀ recess in a shaft and hub of the pulley to accommodate aĀ key.
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Types of Keys :Ā The following types of keys are important from theĀ subject point of view :
1. Sunk keys , 2. Saddle keys , 3. Tangent keys,Ā 4. Round keys, and 5. Splines.
We shall now discuss the above types of keys, inĀ detail, in the following pages. -
Sunk Keys.
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Saddle Keys.
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Tangent Keys.
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Round Keys.
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Splines.
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Forces acting on a SunkĀ Key.
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Strength of a Sunk Key.
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Effect of Keyways :Ā A little consideration will show that the keyway cut into the shaft reduces the load carryingĀ capacity of the shaft. This is due to the stress concentration near the corners of the keyway andĀ reduction in the cross-sectional area of the shaft. It other words, the torsional strength of the shaft isĀ reduced. The following relation for the weakening effect of the keyway is based on the experimentalĀ results by H.F. Moore. Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā e = 1 – 0.2 (w/d) – 1.1 (h/d) Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā 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 =Ā Thickness of key (t)/2 Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā It is usually assumed that the strength of the keyed shaft is 75% of the solid shaft, which isĀ 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 increasedĀ in the ratio kĪø as given by the following relation : Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā kĪø = 1 + 0.4 (w/d) + 0.7 (h/d) Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā where kĪø = Reduction factor for angular twist. -
Shaft Couplings.
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Requirements of a GoodĀ Shaft Coupling :Ā 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 fromĀ one shaft to another shaft.
5. It should have no projecting parts. -
Types of Shaft Couplings.
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Sleeve or Muff Coupling.
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Clamp or CompressionĀ Coupling.
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Flange Coupling.
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Design of Flange Coupling.
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Flexible Coupling.
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Bushed Pin FlexibleĀ Coupling.
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Oldham Coupling.
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Universal Coupling.
Reference A Textbook of Machine Design by R.S. Khurmi and J.K. Gupta
