Design of keys

 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 aroused as temporary fastenings and are
subjected to consider-able crushing and
shearing stresses

 1. Sunk keys,
 2. Saddle keys,

 3. Tangent keys,

 4.Round keys, and

 5. Spines.

 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 type
 1 Rectangular sunk key
 2. Square sunk key
 3. Parallel sunk key
 4.Gib-head key
 5. Feather key
 6. Woodruff key

 A rectangular sunk key is shown in above Fig.
The usual proportionsof this key are :

 width of key, w=d /4;
 thickness of key, t = 2 w / 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

 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

 The parallel sunk keys may be of rectangular or
square section uniform in width and thickness
throughout. It may be noted that a parallel key
is a taper less
 and is used where the pulley, gear or other
mating piece is required to slide along the shaft

 It is a rectangular sunk key with a head at one
end known as gib head .
 It is usually provided to facilitate the removal
of key.

 The usual proportions of the gib head key are :
 Width, w = d / 4 ;
 thickness at large end, t = 2 w / 3 = d /6

 A key attached to one member of a pair and
which permits relative axial movement is
known as feather key .
 It is a special type of parallel key which
transmits a turning moment and also permits
axial movement. It is fastened either to the
shaft or hub, the key being a sliding fit in the
key way of the moving piece

 The woodruff key is an easily adjustable key. It
is a piece from a cylindrical disc having
segmental cross-section in front view as shown
in Fig.
 A woodruff key is capable of tilting in a recess
milled out in the shaft by a cutter having the
same curvature as the disc from which the key
is made. This key is largely used in machine
tool and automobile construction

 The main advantages of a woodruff key
1.It accommodates itself to any taper in the hub
or boss of the mating piece.
2. It is useful on tapering shaft ends. Its extra
depth in the shaft prevents any tendency to
turnover in its keyway.
 The disadvantages are :
1.The depth of the keyway weakens the shaft.
2.It can not be used as a feather

 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 as shown
in Fig.
 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

The tangent keys are fitted in pair at right angles as shown in Fig. Each
key is to with standtorsion in one direction only. These are used in
large heavy duty shafts.

 The round keys, as shown in Fig (a), are
circular in section and fit into holes drilled
partly in the shaft and partly in the hub.
 They have the advantage that their keyways
may be drilled and reamed after the mating
parts have been assembled. Round keys are
usually considered to be most appropriate for
low power drive
 Sometimes the tapered pin, as shown in Fig.
(b), is held in place by the friction between the
pin and the reamed tapered hole

 Sometimes, keys are made integral with the shaft
which fits in the keyways broached in the hub.
Such shafts are known as splined shafts as shown
in Fig.
 These shafts usually have four, six, ten or
sixteensplines. The splined shafts are relatively
stronger than shafts having a single keyway.
 The splined shafts are used when the force to be
transmitted is large in proportion to the size of the
shaft as in automobile transmission and sliding
gear transmissions.
 By using splined shafts, we obtain axial movement
as well as positive drive is obtained

 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.

 The distribution of the forces along the length of the key is not uniform
because the forces are concentrated near the torque-input end.
 The non-uniformity of distribution is caused by the twisting of the shaft
within the hub.
 In designing a key, forces due to fit of the key are neglected and it is
assumed that the distribution of forces along the length of key is uniform.

The forces acting on a key for a clockwise torque being
transmitted from a shaft to a hub are shown in Fig

 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.
 T =Thickness of key, and
 τ =Shear stresses for the material of key
 σc=crushing stresses for the material of key

 A little consideration will show that due to the
power transmitted by the shaft, the key may
fail due to shearing or crushing.
 Considering shearing of the key, the tangential
shearing force acting at the circumference of the
shaft,
 F =Area resisting shearing × Shear stress
 F= L ×w×τ
 ∴Torque transmitted by the shaft
 T = F × d/2 = l ×w×τ × d/2……………(i)

 Considering crushing of the key, the tangential
crushing force acting at the circumference of
the shaft,
 F =Area resisting crushing × Crushing stress
 F=l × t/2× σc
 ∴Torque transmitted by the shaft,
 T =F × d/2 = l × t/2 × σc× d/2……………(ii)

 The key is equally strong in shearing and
crushing
 EQUATION (i) = (ii)
 L ×w×τ × d/2= l × t/2 × σc× d/2
W/t = σc/2τ…………………………(iii)
 The permissible crushing stress for the usual
key material is at least twice the permissible
shearing stress.
 Therefore from above equation we have w = t
 In other words, a square key is equally strong
in shearing and crushing

 In order to find the length of the key to
transmit full power of the shaft, the shearing
strength of the key is equal to the torsional
shear strength of the shaft.
 We know that the shearing strength of key,
T =l × w × τ × d/2…………………(iv)
 torsional shear strength of the shaft
T = (π/16) × d³× τ1………..(v)
Taking τ1= Shear stress for the shaft material

 From equations ( iv ) and (v), we have

w × l× τ × d/2= (π/16) ×d³ × τ1

for sunk key width= (w)= d/4
 When the key material is same as that of the
shaft, then τ = τ1

l=1.571d

Design of keys

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Design of keys