Keys, Cotters and Pin Joints
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Keys
Keys are machine elements used to prevent
relative rotational movement between a shaft
and the parts mounted on it, such as pulleys,
gears, wheels, couplings, etc.
For making the joint, grooves or keyways are
cut on the surface of the shaft and in the hub
of the part to be mounted.
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Keys
Keys are classified into three types :
Saddle keys,
Sunk keys
Round keys
These are taper keys, with uniform width but tapering in thickness on the upper side. These
are made in two forms: hollow and flat.
Hollow saddle key: it has a concave shaped bottom to suit the curved surface of the shaft,
on which it is used.
Flat saddle key: It is similar to the hollow saddle key, except that the
bottom surface of it is flat.
The two types of saddle keys discussed above are suitable for light
duty only. However, the flat one is slightly superior compared to the
hollow type.
Saddle keys are liable to slip around the shaft when used under heavy
loads.
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Sunk keys
These are the standard forms of keys used in practice :
It may be either square or rectangular in cross section.
The end may be squared or rounded.
These keys are used for heavy duty, as the fit between the key and the shaft
is positive.
Sunk keys may be classified as:
1. taper keys,
2. parallel or feather keys
3. splines
4. woodruff keys.
5. Round keys
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Taper Sunk keys
These keys are square or rectangular in cross-section,
uniform in width but tapered in thickness.
The bottom surface of the key is straight and the top surface
is tapered
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Parallel or Feather keys-Sunk keys
These keys are uniform in width and thickness as well.
These keys are used when the parts (gears, clutches,
etc.) mounted are required to slide along the shaft;
permitting relative axial movement.
The feather key may be fitted into the keyway provided
on the shaft by two or more screws (Fig. 6.5) or into
the hub of the mounting (Fig. 6.6)
These keys are of three types:
1. peg feather key: the key and the mounting move
axially as one unit.
2. single headed feather key
3. double headed feather key: These heads prevent
the axial movement of the key in the hub
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Splines keys-Sunk keys
Splines are keys made integral with the shaft, by cutting equi-
spaced grooves of uniform cross-section.
The shaft with splines is called a splined shaft.
The splines on the shaft, fit into the corresponding recesses in the
hub of the mounting, with a sliding fit, providing a positive drive
and at the same time permitting the latter to move axially along the
shaft.
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Woodruff keys-Sunk keys
It is a sunk key, in the form of a segment of a circular disc of uniform
thickness (Fig. 6.8 a).
The keyway in the shaft is in the form of a circular recess to the same
curvature as the key.
Woodruff key is mainly used on tapered shafts of machine tools and
automobiles.
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Round keys-Sunk keys
Round keys are of circular cross-section, usually tapered along the
length.
A round key fits in the hole drilled partly in the shaft and partly in the hub.
The mean diameter of the pin may be taken as 0.25 D, where D is shaft
diameter.
Round keys are generally used for light duty, where the loads are not
considerable.
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Cotter Joints
A cotter is a flat wedge shaped piece, made of steel.
It is uniform in thickness but tapering in width, generally on one
side; the usual taper being.
The lateral (bearing) edges of the cotter and the bearing slots
are generally made semi-circular instead of straight.(Fig.6.10)
This increases the bearing area and permits drilling while
making the slots.
The cotter is locked in position by means of a screw. as shown
in Fig. 6.11.
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Cotter Joints
Cotter joints are used to connect two rods, subjected to
tensile or compressive forces along their axes.
These joints are not suitable where the members are under
rotation.
The following are some of the commonly used cotter joints:
1. Cotter joint with sleeve
2. Cotter joint with socket and spigot ends
3. Cotter joint with a gib
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Cotter joint with sleeve
This is the simplest of all cotter joints, used for fastening
two circular rods.
The slots in the rods and sleeve are made slightly wider
than the width of cotter.
The relative positions of the slots are such, that when a
cotter is driven into its position, it permits wedging action
and pulls the rod into the sleeve.
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Cotter joint with socket and spigot ends
This joint is also used to fasten two circular
rods.
In this, the rod ends are modified instead of
using a sleeve. One end of the rod is formed
into a socket and the other into a spigot (Fig.
6.13) and slots are cut.
After aligning the socket and spigot ends, a
cotter is driven-in through the slots, forming the
joint.
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Cotter joint with a gib
This joint is generally used to connect two rods of square or
rectangular cross-section.
To make the joint, one end of the rod is formed into a U-
fork, into which, the end of the other rod fits in.
When a cotter is driven-in, the friction between the cotter
and straps of the U-fork, causes the straps to open. This is
prevented by the use of a gib.
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Pin Joint
In a pin joint, a pin is used to fasten two rods that are
under the action of a tensile force; although the rods
may support a compressive force if the joint is guided.
Some pin joints such as universal joints, use two pins and
are used to transmit power from one rotating shaft to
another.
A pin joint permits a small amount of flexibility or one rod
may be positioned at an angle (in the plane containing the
rods) with respect to the other rod, after providing suitable
guides.
Unlike in cotter joints, the pin in a pin joint is not driven in
with a force fit, but is inserted in the holes with a
clearance fit.
The pin is held in position, by means of a taper pin or a
split pin provided at its end.
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