PrePared by
AjAy kumar
AssociAte professor
Mechanical engineering departMent
Kiet, ghaziabad
Keys and Couplings
Types of keys, splines, Selection of square & flat keys,
Strength of sunk key, Couplings, Design of rigid and flexible
couplings.
Keys
A key is the piece inserted in an axial direction between a shaft
and hub of the mounted machine element such as pulley or gear
etc.,
to prevent relative rotation….
may allow sliding movement along the shaft if required.
Keys are temporary fastening and are always made of mild
steel because they are subjected to shearing and compressive
stresses caused by the torque they transmit.
a keyway is the groove cut in the shaft or hub to accommodate a
key. Key ways can be milled horizontally or vertically .
Classification of keys
• Saddle keys
» Hollow saddle key
» Flat saddle key
• Sunk keys
» Taper sunk keys
» Parallel sunk keys
» Feather keys
» Woodruff key (adjustable key)
• Round keys
» Parallel pin
» Taper pin
–.
Hollow saddle key
This key has curved underside so that it can be placed on the curved
surface of the shaft. The keyway is cut in the mating piece (hub) only
Saddle key are taper keys and are sunk into the hub only
Taper prevents axial movement along the shaft
Saddle key is suitable for light duty, since they rely on a friction drive
alone and are liable to slip on the shaft under load
Flat saddle key
• holding force is comparative large than the hollow saddle key
Flat surface on
the shaft
Let D = diameter of the shaft Width of the key,
W = D/4Nominal thickness at large end ,
T = W/3 = D/12
Length of the key , L = D to 1.5D
Taper on the top surface = 1:100
Sunk keys
Sunk keys are sunk in the shaft and the hub. These keys
are suitable for heavy duty since they rely on positive
drive.
Taper sunk keys:
• This is the standard form of the key and may be either of
rectangular or square cross-section. The key is sunk in the
shaft to a depth of half its nominal thickness when measured
at the side.
» Rectangular cross-section
let D = diameter of the shaft
width of the key W = D/4
nominal thickness T = (2/3)W = (1/6)D
» Square cross-section:
T=W
Sunk taper key
Parallel sunk key
It is uniform in width and
thickness throughout.
It is thus taper less and is
used where the pulley or
other mating piece is
required to slide along the
shaft.
It may be rectangular or
square cross-section and
their ends may be squared or
rounded.
Feather keys
It is a key attached to one member of a pair
It is a particular kind of parallel key that permits axial moment
A feather key is secured either to the shaft or to the hub, the key
being a sliding fit in the keyway of the machine element on
which it moves.
Double Headed Peg Key
Key
Woodruff keys
It is an adjustable sunk key in the form of a semi-circular disc of
uniform thickness.
The key fits into a semi-circular keyway in the shaft and the top of
the key fits into a plain rectangular key way in the hub of the
wheel.
Since the key and the key seat bear the same radius , it has the
advantage of adjusting itself to any taper of the slot of the hub or
boss of wheel.
Used in gear box of lathe, other machine tools and in automobiles.
Round key or Pin Key:
Spline shaft and hub
A spline shaft is used when the hub is
required to slide along the shaft. These
shafts are used mostly for sliding gear
application as in automotive gear box
and propeller shaft of aircraft.
A spline shaft in which are cut
equiangular longitudinal groove, the
metal between these groove forming
splines or feathers of uniform depth.
By this means the power transmitted is
equally divided amongst the number of
keys giving great strength and security
against total failure than by using a
single key.
Strength of a Sunk Key
T = Torque transmitted by 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 of key.
Torque transmitted by the shaft,(on the basis of shearing strength)
Torque transmitted by the shaft,(on the basis of crushing strength)
The key is equally strong in shearing and crushing, if
The permissible crushing stress for the usual key material is at least twice the
permissible shearing stress. Therefore w = t. In other words, a square key is equally
strong in shearing and crushing.
Capacity of Torque transmitted by shaft
Considering shearing strength criteria for designing
(Taking w = d/4) then
Example 13.1. 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.
Example 13.2. A 45 mm diameter shaft is made of steel with a yield strength of 400
Mpa. A parallel key of size 14 mm wide and 9 mm thick made of steel with a yield
strength of 340 MPa is to be used. Find the required length of key, if the shaft is loaded
to transmit the maximum permissible torque. Use maximum shear stress theory and
assume a factor of safety of 2.
Example 13.3. A 15 kW, 960 r.p.m. motor has a mild steel shaft of 40 mm diameter and
the extension being 75 mm. The permissible shear and crushing stresses for the mild
steel key are 56 MPa and 112 MPa. Design the keyway in the motor shaft extension.
Check the shear strength of the key against the normal strength of the shaft.
