ME-3011D – Machine Design-I
Dr.R PRABHU SEKAR,
Assistant Professor,
Mechanical Engineering Department,
National Institute of Technology Calicut.
Keys
A key can be defined as a machine element which is used to
connect the transmission shaft to rotating machine elements like
pulleys, gears, sprockets or flywheels.
A keyed joint consisting of shaft, hub and key
Components of Keyed Joint
Function of Key
The primary function of the key is to transmit the torque from the
shaft to the hub of the mating element and vice versa.
The second function of the key is to prevent relative rotational
motion between the shaft and the joined machine element like gear
or pulley.
In most of the cases, the key also prevents axial motion between
two elements.
Key way
A slot machined either on the shaft or in the hub to accommodate
the key is called keyway.
The keyway is usually cut by a vertical or horizontal milling cutter.
The keyway results in stress concentration in the shaft and the part
becomes weak. This is the main drawback of a keyed joint.
Keys are made of plain carbon steels in order to withstand shear
and compressive stresses resulting from transmission of torque.
Types of Keys
(i) Saddle key and sunk key
(ii) Square key and flat key
(iii) Taper key and parallel key
(iv) Key with and without Gib-head
(v) Woodruff Key
(vi) Feather Key
The selection of the type of key for a given application depends
upon the following factors:
(i) power to be transmitted;
(ii) tightness of fit;
(iii) cost.
Saddle Keys
A saddle key is a key which fits in the keyway of the hub only.
Saddle key requires keyway only on the hub.
(a)Hollow Saddle Key
A hollow saddle key has a concave
surface at the bottom to match the
circular surface of the shaft.
(b) Flat saddle key
A flat saddle key has a flat surface at
the bottom and it sits on the flat
surface machined on the shaft.
saddle keys are suitable for light duty or low power transmission
as compared with sunk keys.
Sunk Keys
A sunk key is a key in which half the thickness of the key fits into
the keyway on the shaft and the remaining half in the keyway on the
hub.
Therefore, keyways are required both on the shaft as well as the hub
of the mating element.
Sunk key is suitable for heavy duty application, since there is no
possibility of the key to slip around the shaft.
It is a positive drive. This is the main advantage of the sunk key
over the saddle key.
Sunk Keys
For square Key
For flat Key
Flat Key with rectangular
Square Key
cross section
b = width of key (mm), h = height or thickness of key (mm),
l = length of key (mm), d = diameter of shaft (mm)
Sunk Keys
Square keys are used in general industrial machinery.
Flat keys are more suitable for machine tool applications,
Sunk keys with square or rectangular cross sections are classified
into two groups, namely, parallel and taper keys.
Sunk Keys
A parallel key is a sunk key which is uniform in width as well as
height throughout the length of the key.
A taper key is uniform in width but tapered in height.
The standard taper is 1 in 100.
The bottom surface of the key is straight and the top surface is
given a taper.
The reason for providing taper is
1. To provide tightness of the joint and prevents loosing of the parts
2. Due to taper, it is easy to remove the key and dismantle the joint.
Gib-head Taper Key
Tapered keys are often provided with Gib-head to facilitate
removal.
The taper surface facilitates easy removal of the key, particularly
with Gib-head
Feather Key
A feather key is a parallel key which is fixed either to the shaft or to
the hub and which permits relative axial movement between them.
Feather Key
The feather key is a particular type of sunk key with uniform width
and height.
A feather key, which is fixed to the shaft by means of
two cap screws, having countersunk-heads.
There is a clearance fi t between the key and the keyway in the hub.
Therefore, the hub is free to slide over the key.
At the same time, there is no relative rotational movement between
the shaft and the hub.
Therefore, the feather key transmits the torque and at the same time
permits some axial movement of the hub.
Feather keys are used where the parts mounted on the shaft are
required to slide along the shaft such as clutches or gear shifting
devices.
Woodruff Key
A Woodruff key is a sunk key in the form of an almost semicircular
disk of uniform thickness.
The keyway in the shaft is in the form of a semicircular recess with
the same curvature as that of the key.
The bottom portion of the Woodruff key fits into the circular
keyway in the shaft.
The keyway in the hub is made in the usual manner.
Woodruff keys are used on tapered shafts in machine tools and
automobiles.
Design of Square and Flat Keys
A square key is a particular type of flat key, in which the height is
equal to the width of the cross-section.
The moment couple due to load P
acting on the key is equal to torque Mt.
The resisting torque is equal to the
moment couple due to load P’ which
prevents the to roll in the keyway.
It is assumed that the force P is
tangential to the shaft diameter. Forces Acting on Key
Design of Square and Flat Keys
The design of square or flat key is based on two criteria,
(i) Failure due to shear stress and
(ii) Failure due to compressive stress.
Design of Square and Flat Keys
The shear stress in the plane AB is given by,
Substitute P value in shear stress Eqn., shear stress is given by
Design of Square and Flat Keys
The failure due to compressive stress will occur on surfaces AC or
DB.
It is assumed that
h = height of key (mm)
The compressive stress c in the key is given by,
Design of Square and Flat Keys
For square key, h = b
Therefore, the compressive stress induced in a square key due to
the transmitted torque is twice the shear stress.
Problem-1
It is required to design a square key for fixing a gear on a shaft of 25
mm diameter. The shaft is transmitting 15 kW power at 720 rpm to the
gear. The key is made of steel (Syt = 460 N/mm2) and the factor of
safety is 3. For key material, the yield strength in compression can be
assumed to be equal to the yield strength in tension. Determine the
dimensions of the key.
