Keys, Cotter and
Knuckle JointsUNIT 5 KEY, COTTER AND KNUCKLE
5.3 Types of Key
5.4 Gib Head Key
5.5 Cotter and Cotter Joint
5.6 Sleeve Cotter Joint
5.7 Socket and Spigot Cotter Joint
5.8 Joining of Rods
5.9 Knuckle Joint
5.11 Answers to SAQs
There are many situations where two parts of machines are required to be restrained. For
example two rods may be joined coaxially and when they are pulled apart they should not
separate i.e. should not have relative motion and continue to transmit force. Similarly if a
cylindrical part is fitted on another cylinder (the internal surface of one contacting the
external surface of the other) then there should be no slip along the circle of contact.
Such situations of no slip or no displacements are achieved through placing a third part or
two parts at the jointing regions. Such parts create positive interference with the jointing
parts and thus prevent any relative motion and thus help transmit the force. You will
remember that the rivets in a riveted joint had exactly the same role as they prevented the
slipping of one plate over the other (in lap joint) and moving away of one plate from
there (in butt joint). The rivets provided positive interference against the relative motion
of the plate.
Knuckle joint is yet another to join rods to carry axial force. It is named so because of its
freedom to move or rotate around the pin which joins two rods, a motion which naturally
exists at finger joints or knee. A knuckle joint is understood to be a hinged joint in which
projection in one part enters the recess is the other part and two are held together by
passing a pin through coaxial holes in two parts. This joint can not sustain compressive
force because of possible rotation about the pin. In this unit we will study other
interfering parts for geometrically different jointing parts.
After studying this unit, you should be able to understand
• what is a key,
• what are the types of key,
• how to draw a key,
• the parts that are joined by key,
• how are the keys made,
• what is a cotter,
Machine Drawing • what are the types of cotter,
• how to draw cotter joint,
• how to make a pin joint,
• how is a knuckle joint constructed, and
• how is a knuckle joint drawn.
A shaft rotates in its bearings and transmits torque. A shaft always carry upon at some
other part like gear or pulley. That part of the gear or pulley which sits on the shaft by
surrounding the shaft on all its circumference is called the hub. The hub and the shaft are
provided with a positive interfering part which is called a key.
The key is a prismatic bar inserted between the shaft and the hub so that it passes through
both or one of them. It may be tapered or of uniform cross section. When placed in
position the shaft and mating part rotate as a single unit without any slipping. The torque
then can pass from shaft to mating part and vice versa. Apparently if the key is to pass
through one or both the mating parts a proper groove, called keyway must be made.
5.3 TYPES OF KEY
Several of the keys used in practice are shown in Figure 5.1. In these figures 1 is shaft
and 2 is surrounding hub of the mating part and 3 is the key. The length of the key is
perpendicular to the plane of the paper and often is equal to the length of the hub. Shaft is
Figure 5.1 : Types of Key
Round key is a cylinder and requires a hole to pass. Half of the hole is in the shaft and
other half in the hub. It is used when load is low and shaft diameter is small. Making of
hole is not easy and costly if made separately in two halves in two parts. Since the
cylindrical holes do not have sharp corners they still represent a better choice. Taper
round keys produce tighter joint. The taper may be as gentle as 1 : 100.
Saddle key is shown in Figure 5.1(b). It sits on the curved surface of shaft and fits in the
rectangular slot of hub. No keyway in the staff is required and frictional force between
the seat of key and surface of the shaft is responsible for transmission of the torque.
Either for transmission of light torque or holding the mating part in position during
assembly such saddle key is used.
Key on Flat is similar to saddle key on three sides except at the bottom where it is flat.
It will of course require a flat narrow surface machined on the shaft, while it fits into the
keyway made in the hub. Such flat region machined on the surface of the shaft does not
affect the strength because much material is not removed no corners are created as will
happen if keyway is machined.
Keys, Cotter and
Knuckle JointsFlat key or rectangular key Figure 5.1(d) and square key Figure 5.1(f) are essentially
same and used universally between shaft and any mating part like gear and pulley.
Very large torque or power can be transmitted by both but square key is often preferred
for equal strength in shear and crushing.
Splines Figure 5.1(e) can be regarded as keys integral with the shaft. The shafts are
weakened by creating keyways whose depth could be as large as 1/4 of diameter of the
shaft. Hence, splines are created on the shaft surface fit into the grooves made in the
mating part. Splines are routinely used when mating parts are required to slide on the
shaft. Examples are change gear boxed in automobile. The cross section of the splines
may be rectangular, triangular or involute. A spline normally has larger width (w) and
smaller height (h) as shown in Figure 5.1(e). There may be four, six or 10 splines and
both w and h reduce with increasing number of splines. w and h for permanent splined
connections are respectively 0.28d and 0.09d for four splines, 0.278d and 0.056d for six
splines and 0.17d and 0.05d for 10 splines. For sliding the dimensions increase.
The keys are normally prismatic with either rounded or flat ends. The flat key with
rounded ends is shown in Figure 5.2(a). No doubt it can also have flat ends as shown in
Figure 5.2(b). The keyways for flat or square keys are made with end mill, which will
end in semicircular ends. The keyways can also be made with discutters which can not be
used with rounded end keys. The rounded end keyways are shown in Figure 5.2(c).
(a) (b) (c)
The jib headed key as shown in Figure 5.1(g) is in fact a rectangular cross section
prismatic bar with taper (1 : 100) along the length and having a jib head at largest cross
section. It is inserted in the keyslot and head helps both in insertion and extraction of the
key. The jibhead, being a projection on the shaft, presents a hazard of collecting loose
garments or cottonwaste, hence should be protected. It may be pointed out here that a
taper key is not preferred in precise machines because it causes varying information of
the moting hub.
Woodruff key as shown in Figure 5.1(h) is a segment of a disc whose rounded part enters
the corresponding shape cut in the shaft. The key provides the advantage of easy
assembly and disassembly but weakens the shaft due to deep groove. The key is cut from
a disc of radius R = 0.4 D with w = 0.2 D. Its total depth is 95% of radius and radius is
0.4 D. Three fourths of depth is in shaft.
On a shaft of diameter 200 mm a flanged-hub is to be placed. The diameter of the
hub is 300 mm while its length is 200 mm. The flange is 500 mm diameter with a
width of 50 mm. The shaft and flanged hub are shown in Figures 5.3(a) and (b).
Draw the necessary views connecting the shaft with different keys.
5.4 GIB HEAD KEY
For convenience of insertion and extraction one end of a sunk rectangular flat key is
sometimes provided with a jib head. Such keys normally have taper in height but have
uniform thickness. The taper is generally 1 : 100, only in the upper surface. The keys are
provided with heavy rotating mass for which accuracy of outer surface does not matter
much, like heavy pulleys.
The depth of the key at the end is taken as D/6 and width as D/4. The height and the
length of the gib head are respectively 0.3D and 0.25D. D in this case is the diameter of
the shaft. Figure 5.4(a) shows a gib key and Figure 5.8(b) shows it fitted with the shaft.
On a 35 mm diameter shaft carries a pulley of 900 mm diameter whose hub tapers
from 75 mm at the arm to 70 mm at the edge and is 80mm long. Four arms, elliptic
in section taper from a1 = 26 mm to a = 20 mm and b1 = 12 mm to b = 30 mm.
Show the assembly of pulley with gib headed key and with part of the shaft whose
diameter increases to 45 mm from 35 mm suddenly with a radius of 5 mm at the
corner. The width of the pulley is 100 mm with a crown of 3 mm. Rim thickness at
edges, 8 mm.
Keys, Cotter and
Knuckle Joints5.5 COTTER AND COTTER JOINT
A cotter is a metallic strip of uniform thickness but tapers in width. The taper may be
very small like 1 : 100 but may be as large as 1 : 30. The cotter passes through slots made
in two coaxial parts and thus prevent the relative motion between them. The cotter can
pass through two specially made ends of two coaxial bars which may be circular in
section or rectangular or it may pass through sleeve put on the plain ends of rod
(two cotters will be needed). We shall now see both types of joints. The cotter joints are
used only to transmit axial pull between two rods and they are not made to rotate.
5.6 SLEEVE COTTER JOINT
Two plain cylindrical ends are made to butt each other and a single sleeve covers both.
Two slots are made in the sleeve, each coinciding with the slot in the rod end. The rod
end may be enlarged to compensate for the slot.
Figure 5.5 shows a cotter, a rod with enlarged end and a sleeve. Two cotters are need to
join two rods. The internal diameter of the sleeve match with the external diameter of the
rod and the slot matches with the cotter. Figure 5.6 shows two rod ends pushed in a
sleeve with a slight clearance at butting ends to accommodate cotters. The two views of
sleeve cotter joint are drawn in Figure 5.7.
Figure 5.7 : Two Views of Sleeve Cotter Joint
5.7 SOCKET AND SPIGOT COTTER JOINT
One end of a rod carries a socket while other end of another rod carries a spigot. The
socket is a hollow and spigot a solid cylinder with a collar. The socket also has a collar.
The spigot the socket and the cotter are shown in Figure 5.8.
Figure 5.8 : Cotter, Socket and Spigot
Figure 5.9 shows the spigot inserted into socket with their slots for receiving the
Figure 5.9 : Socket and Spigot Assembled
Draw the elevation and side view of cotter joint from three parts shown in
A rectangular fork ahead of a square section bar carries slot for a cotter and a gib
as shown in Figure 5.10. A square bar carries a slot at its end similar to that in the
fork and also shown in the above Figure. Assemble the four parts and draw
elevation, plan and side view of the assembly.
Keys, Cotter and
Knuckle Joints5.8 JOINING OF RODS
If a problem is put before us to create a joint between two round bars to carry axial load
and use a pin to join them then a number of solution may come up. Some are shown in
Figure 5.11 shows how two rods can be joined with the help of a pin which passes
through holes. The ends are finished flat through half the diameter to match to form a
perfect cylinder when flats are placed in contact. Draw this joint in two views by taking
diameter of rod as 25 mm and diameter of pin as 10 mm.
Figure 5.11 : A Pin-Joint between Two Circular Section Rods
Figure 5.12 : A Pin Joint between Two Plates
Figure 5.13 : Another Pin-Joint between Two Plates
Figure 5.14 : A Knuckle Joint Joining Two Rods
Machine Drawing Figures 5.12 and 5.13. show the joints between the plates. Note how the changes are
introduced from Figures 5.12 to 5.13. It is also suggested that the plate ends can be cut
along broken lines.
Draw the joint shown in Figure 5.13 for plate 10 mm thick and two parts 1 (a) and
(b) each is 10 mm thick in plate1. The width of the plate is 25 mm and length can
be any thing. Redraw the two views of above drawing by cutting along broken
lines producing plates 15 mm wide. The pin diameter is 10 mm.
Figure 5.15 : Two View of the Joint of Figure 5.13
Figure 5.16 : Two Views of Modified Joint of Figure 5.13
5.9 KNUCLE JOINT
In earlier figures we developed a knuckle joint. That is a joint which connects two rods.
The parts that create the joint are made integral with the rods, i.e. they become the rod
ends. One is called fork which provides the recess and other is called eye which fits into
the recess. The ends are shaped properly to avoid sharp corners or sharp changes in the
radii. You must have noticed that in Figures 5.15 and 5.16 there is nothing to stop the pin
from sliding. Some restrictions like head in the pin and a stopper at the other end must be
provided. These can be seen in Figure 5.14 in which pin is marked 3. A collar with a hole
through which a taper pin or a split pin is pressed is used as a stopper. The collar is
marked 4 and taper pin as 5. Altogether there are five parts in a knuckle joint. These five
parts are shown separately in Figure 5.17.
Keys, Cotter and
Figure 5.17 : Five Parts of a Knuckle Joint
It is interesting to note that all dimension in a knuckle joint are related to diameter ‘d’ of
the rod. These rotations are shown in Figure 5.18 in which the five parts are assembled to
form the join.
You must draw the top view and side view. Parts of Figure 5.17 are assembled to make a
knuckle joint in Figure 5.18.
Figure 5.18 : A Knuckle Joint
All Dimensions in Terms of Rod Diameter d
For two rods of diameter 25 mm draw elevation and plan of a knuckle joint. Show
partial section of elevation. For inside and outside surfaces of fork take respective
radii of 14 and 32 mm.
Figure 5.19 : Elevation and Plan of a Knuckle Joint
Various types of keys are used in practice out of which the square key is most common
for gears and pulleys. By drawings it has been shown how do the keys fit in the
assembly. The cotter is another element that produces temporary joints. Different types of
cotter joints and their elements have been shown in drawing. The reader should
reproduce each drawing.
5.11 ANSWERS TO SAQs
Square key – w = 50 mm, h = 50 mm
Saddle key – w = 60 mm, h = 25 mm
Key on flat – w = 60 mm, h = 25 mm
Splines – 4 rectangular, w = 56 mm, h = 18 mm
Woodruff – w = 40 mm, Radius of key = 80 mm, Keyway depth = 57 mm,
Depth of the key = 76 mm
Round key – diameter = 50 mm
All the keys except woodruff will be equal to hub in length, i.e. 250 mm.
Figure 5.20 : Three Views of Shaft and Hub Assembly with a Square Key
Figure 5.21 : Shaft and Hub Assembly with a Woodruff Key
Keys, Cotter and
Figure 5.22 : Shaft and Hub Assembly with Round Key
Figure 5.23 : Splined Shaft and Hub Assembly
The shaft and hub assembly has been drawn for following four cases :
Figure 5.20 : Shaft and hub assembly with square key.
Figure 5.21 : Shaft and hub assembly with woodruff key.
Figure 5.22 : Shaft and hub assembly with round key.
Figure 5.23 : Shaft and hub assembly for splines.
You are advised to draw similar assemblies for saddle key and key on flat.
Gib key dimensions
6 mm, 0.6 0.6 35 21 mm
h H D= = = = = × =
9 mm, 12 mm
4 4 3 3
w B= = = = = =
(a) A Rectangular Key with Gib Head
(b) Rectangular Key with Gib Head Fitted between a Shaft and a Pulley
(Third Angle Projection)
Machine Drawing SAQ 3
The two views are drawn in Figure 5.25.
Figure 5.25 : Spigot Socket Cotter Joint Assembly Square
Figure 5.26 shows three views of assembled gib and cotter joint.
Figure 5.26 : Gib and Cotter Joint with Fork End