Keys Design
Key is a machine member employed at the interface of pair of mating male and female circular
cross-sectioned members to prevent relative angular motion between these mating. Keys are
used as temporary fastenings and are subjected to considerable crushing and shearing stresses.
Keyway is a slot or recess in a shaft and hub of the pulley to accommodate a key.
F
Classifications of Keys
1. Constant cross-section
2. Variable cross-section
3.
Types of Constant Cross-sectioned Keys
1. Square Key 3. Round Key
2. Flat Key
4. Barth Key
Square key – has a square cross-section with half of its depth sunk in the shaft and half in the
hub.
Flat key – has a regular cross section with a smaller dimension placed in the radial direction
with half sunk in the shaft and half in the hub and is used where the weakening of the shaft by
the keyway is serious.
Round Key – has circular cross-section.
Barth Key – is a square key with bottom two corners beveled.
h
h
w
D
w
D
Square Key
𝑤 = ℎ = 𝐷/4
Flat Key
𝑤 = 𝐷/4
d
D
Round Key
Barth Key
Types Variable Cross-sectioned Keys
1. Woodruff key - consist of one-half of a circular disk fitting into a rectangular keyway
in the female member and a semi-circular keyway in the male member.
2. Saddle key - is a flat key used without a keyway in the shaft.
3. Gib-head key - is a flat or rectangular key with a special gib-head to facilitate easy
driving and removal of the key.
Woodruff key
Other Types of Key
1. Feather key
2. Kennedy key
3. Pin Key
1. Feather key – is one which has a tight fit into one member and a loose sliding fit in the
mating member thus allowing the hub to move along the shaft but prevents rotation on
the shaft.
2. Kennedy key – are tapered square keys with the diagonal dimension in a circumferential
direction.
3. Pin key
Considering power transmitted by the shaft, the key may fail due to shearing or crushing.
Power of Key
𝑃 = 2𝜋 𝑇𝑁
Force Transmitted, F
𝑇
𝑇
𝐹= 𝑟=𝐷
F
r
T
2
Where: F = tangential force acting at the circumference of the shaft
T = torque transmitted by the shaft
F
Shearing Stress on the key
𝐹
h
2𝑇
𝜏 = 𝑤𝐿 = 𝑤𝐿𝐷 --------------1
D
Compressive Stress on the Key
2𝐹
4𝑇
𝜎𝑐 = ℎ𝐿 = ℎ𝐿𝐷 ---------------2
Where: D = diameter of shaft
L = length of key
w = width of key
h = thickness of key and
τ = shearing stress of key
𝜎𝑐 = compressive stress
F = tangential force acting at the circumference of the shaft
T = torque transmitted by the shaft
The square key is equally strong in shearing and crushing since w = h.
w
Dimension of Keys made of same Material
When the key is made of the same material as the shaft the length of the key required
to transmit the full power capacity of the shaft is determined by equating the shear strength of
the key to the torsional shear strength of the shaft.
2𝑇
16𝑇
1
= 𝜋𝐷3 𝑥 0.75
and w = D/4
𝐷𝐿𝑤
L = 1.18 D
Where:
L = length of the key
w = width of the key
D = shaft diameter