Key:
A key is a piece of mild steel inserted between the shaft and hub or boss of the pulley to connect these together in order to prevent relative motion between them. It is always inserted parallel to the axis of the shaft. Keys are used as temporary fastenings and are subjected to consider- able crushing and shearing stresses. A keyway is a slot or recess in a shaft and hub of the pulley to accommodate a key.
Types of Keys:
The following types of keys are important from the subject point of view :
1. Sunk keys, 2. Saddle keys, 3. Tangent keys, 4. Round keys, and 5. Splines.
Sunk Keys:
The sunk keys are provided half in the keyway of the shaft and half in the keyway of the hub or boss of the pulley. The sunk keys are of the following types :
1. Rectangular Sunk Key:
A rectangular sunk key is shown in Fig. 13.1. The usual proportions of this key are :
Width of key, w = d / 4 ; and thickness of key, t = 2w / 3 = d / 6
where; d = Diameter of the shaft or diameter of the hole in the hub.
The key has taper 1 in 100 on the top side only.
2. Square Sunk Key:
The only difference between a rectangular sunk key and a square sunk key is that its width and thickness are equal, i.e. w = t = d / 4.
Saddle Keys:
The saddle keys are of the following two types :
1. Flat saddle key, and 2. Hollow saddle key.
A flat saddle key is a taper key which fits in a keyway in the hub and is flat on the shaft. It is likely to slip round the shaft under load. Therefore it is used for comparatively light loads.
A hollow saddle key is a taper key which fits in a keyway in the hub and the bottom of the key is shaped to fit the curved surface of the shaft. Since hollow saddle keys hold on by friction, therefore these are suitable for light loads. It is usually used as a temporary fastening in fixing and setting eccentrics, cams etc.
Proportions of standard parallel, tapered and gib head keys
Forces acting on a Sunk Key:
When a key is used in transmitting torque from a shaft to a rotor or hub, the following two types of forces act on the key :
1. Forces (F1) due to fit of the key in its keyway, as in a tight fitting straight key or in a tapered key driven in place. These forces produce compressive stresses in the key which are difficult to determine in magnitude.
2. Forces (F) due to the torque transmitted by the shaft. These forces produce shearing and compressive (or crushing) stresses in the key.
Strength of a Sunk Key or Design Procedure of a Sunk Key
Let; T = Torque transmitted by the shaft,
F = Tangential force acting at the circumference of the shaft,
d = Diameter of shaft,
l = Length of key,
w = Width of key.s
t = Thickness of key, and
τ and σc = Shear and crushing stresses for the material of key.
WATCH THIS VIDEO FOR DESIGN OF SUNK KEY
Example 1. Design the rectangular key for a shaft of 50 mm diameter. The shearing and crushing stresses for the key material are 42 MPa and 70 MPa.
Solution:
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