Military Institute of Science & Technology
MACHINE DESIGN SESSIONAL
AEAS-320
Exp No: 2
Chapter: Key
Objectives:
1. To familiarize with different keys.
2. To solve key related problems.
3. To calculate the effect of keyway.
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 considerable
crushing and shearing stresses.
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.
Properties of various kinds of Sunk Keys
Rectangular sunk 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.
Square sunk key
w=t=d/4
Parallel sunk key
The parallel sunk keys may be of rectangular or square section uniform in width and thickness
throughout.
Gib-head key
Width, w = d / 4 ; and thickness at large end, t = 2w / 3 = d / 6
Forces acting on a Sunk Key
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.
Forces (F) due to the torque transmitted by the shaft. These forces produce shearing and
compressive (or crushing) stresses in the key.
Necessary Formulas
Considering shearing of the key,
The tangential shearing force acting at the circumference of the key:
F = Area resisting shearing × Shear stress
=l×w×τ
Torque transmitted by the shaft:
T= F x d/2 = l × w × τ x d/2
Considering crushing of the key,
The tangential shearing force acting at the circumference of the key:
F = Area resisting crushing × Crushing stress
Torque transmitted by the shaft:
T= F x d/2
Where, 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, t = Thickness of key, and τ
and σc = Shear and crushing stresses for the material of key.
Note: The permissible crushing stress for the usual key material is at least twice the permissible
shearing stress.
Effect of Keyways
A keyway is a slot or recess in a shaft and hub of the pulley to accommodate a key. Due to
keyway, the torsional strength of the shaft is reduced. The following relation for the weakening
effect of the keyway is based on the experimental results by H.F. Moore.
