Design of Machine Members-I

Cottered Joints:

A cotter is a flat wedge shaped piece

(either on one side or both sides) from

varies from 1 in 48 to 1 in 24 and

provided. The locking device may be

cotter. The cotter is usually made of

fastening and is used to connect rigidly two

tensile or compressive forces. It is u

a reciprocating steam engine, a pisto

connecting rod etc.

Types of Cotter Joints

Following are the three commonly used cotter j

1. Socket and spigot cotter joint,

Socket and Spigot Cotter Joint

In a socket and spigot cotter joint, one

of end as shown in Fig., and the other

The end of the rod which goes into

in the socket and spigot. A cotter is

temporary connection between the tw

its direction and hence the cot

compressive loads. The compressive loa

Fig. Sock

Design of Socket and Spigot Cotter Joint

The socket and spigot cotter joint is s

Let P = Load carried by the rods,

Lecture Notes - 49

wedge shaped piece of rectangular cross-section and its width

both sides) from one end to another for an easy adjustment.

to 1 in 24 and it may be increased up to 1 in 8, if a locking

device may be a taper pin or a set screw used on the lower

usually made of mild steel or wrought iron. A cotter joint is a

to connect rigidly two co-axial rods or bars which are subjecte

It is usually used in connecting a piston rod to the crosshead

am engine, a piston rod and its extension as a tail or pump rod,

hree commonly used cotter joints to connect two rods by a cotter:

Socket and spigot cotter joint, 2. Sleeve and cotter joint, and 3. Gib and cotter joint.

r Joint

cotter joint, one end of the rods (say A) is provided with a socket

and the other end of the other rod (say B) is inserted into

which goes into a socket is also called spigot. A rectangular hole

cotter is then driven tightly through a hole in order to

between the two rods. The load is usually acting axially, but it

cotter joint must be designed to carry both the tensile

sive loads. The compressive load is taken up by the collar on the spigot.

Fig. Socket and spigot cotter joint

ket and Spigot Cotter Joint

t and spigot cotter joint is shown in Fig.

= Load carried by the rods,

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and its width is tapered

y adjustment. The taper

8, if a locking device is

lower end of the

cotter joint is a temporary

which are subjected to axial

iston rod to the crosshead of

tail or pump rod, strap end of

connect two rods by a cotter:

Gib and cotter joint.

provided with a socket type

is inserted into a socket.

rectangular hole is made

hole in order to make the

cting axially, but it changes

carry both the tensile and

ollar on the spigot.

Design of Machine Members-I

d = Diameter of the rods,

d1 = Outside diameter of socket,

d2 = Diameter of spigot or inside diameter of socket,

d3 = Outside diameter of spigot co

t1= Thickness of spigot collar,

d4 = Diameter of socket collar,

c = Thickness of socket collar,

b = Mean width of cotter,

t = Thickness of cotter,

l = Length of cotter,

a = Distance from the end of the slot to the end of rod,

σt = Permissible tensile stress for the rods m

τ = Permissible shear stress for the cot

σc = Permissible crushing stress f

The dimensions for a socket and

various modes of failure as discussed

1. Failure of the rods in tension

From this equation, diameter of the

2. Failure of spigot in tension across the weakest section (or slot)

From this equation, the diameter

determined. In actual practice, the thickness of cotter is usually take

3. Failure of the rod or cotter in crushing

Lecture Notes - 49

= Diameter of the rods,

= Outside diameter of socket,

= Diameter of spigot or inside diameter of socket,

= Outside diameter of spigot collar,

= Thickness of spigot collar,

= Diameter of socket collar,

= Thickness of socket collar,

= Mean width of cotter,

= Distance from the end of the slot to the end of rod,

= Permissible tensile stress for the rods material,

Permissible shear stress for the cotter material, and

= Permissible crushing stress for the cotter material.

a socket and spigot cotter joint may be obtained by consid

as discussed below:

f the rods in tension

quation, diameter of the rods (d) may be determined.

Failure of spigot in tension across the weakest section (or slot)

the diameter of spigot or inside diameter of socket (

In actual practice, the thickness of cotter is usually taken as d2 / 4.

Failure of the rod or cotter in crushing

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considering the

of socket (d2) may be

Design of Machine Members-I

From this equation, the induced crushing

4. Failure of the socket in tension

From this equation, outside diameter of socket (

5. Failure of cotter in shear

From this equation, width of cotter (

6. Failure of the socket collar in crushing

From this equation, the diameter of socke

7. Failure of socket end in shearing

From this equation, the thickness of

Lecture Notes - 49

quation, the induced crushing stress may be checked.

ocket in tension across the slot

this equation, outside diameter of socket (d1) may be determined.

quation, width of cotter (b) is determined.

ocket collar in crushing

quation, the diameter of socket collar (d4) may be obtained.

Failure of socket end in shearing

quation, the thickness of socket collar (c) may be obtained.

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Design of Machine Members-I

8. Failure of rod end in shear

From this equation, the distance from

obtained.

9. Failure of spigot collar in crushing

From this equation, the diameter of t

10. Failure of the spigot collar in shearing

From this equation, the thickness of

11. Failure of cotter in bending

The maximum bending moment occurs at the centre of

Lecture Notes - 49

the distance from the end of the slot to the end of the rod (

Failure of spigot collar in crushing

quation, the diameter of the spigot collar (d3) may be obtained.

pigot collar in shearing

quation, the thickness of spigot collar (t1) may be obtained.

Failure of cotter in bending

m bending moment occurs at the centre of the cotter and is given by

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end of the rod (a) may be

he centre of the cotter and is given by

Design of Machine Members-I

We know that section modulus of the cotte

Bending stress induced in the cotter,

This bending stress induced in the

the cotter.

12. The length of cotter (l) in taken as 4

13. The taper in cotter should not excee

locking device must be provided.

14. The draw of cotter is generally taken as 2 to 3 mm.

Notes: 1. when all the parts of the joint

of diameter of the rod (d) are generally adopte

d1 = 1.75 d , d2 = 1.21 d , d3 = 1.5

,t1 = 0.45 d , e = 1.2 d.

Taper of cotter = 1 in 25, and draw of cotter =

2. If the rod and cotter are made of steel or wrought iron, then τ = 0.8 σ

taken.

References:

1. Machine Design - V.Bandari .

2. Machine Design – R.S. Khurmi

3. Design Data hand Book - S MD Jalaludin.

Lecture Notes - 49

hat section modulus of the cotter,

tress induced in the cotter,

induced in the cotter should be less than the allowable bending

) in taken as 4 d.

cotter should not exceed 1 in 24. In case the greater taper is required,

device must be provided.

The draw of cotter is generally taken as 2 to 3 mm.

the parts of the joint are made of steel, the following proportions

) are generally adopted:

= 1.5 d , d4 = 2.4 d , a = c = 0.75 d , b = 1.3 d, l = 4

= 1 in 25, and draw of cotter = 2 to 3 mm.

If the rod and cotter are made of steel or wrought iron, then τ = 0.8 σt and σc = 2 σ

V.Bandari .

R.S. Khurmi

S MD Jalaludin.

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allowable bending stress of

taper is required, then a

ng proportions in terms

= 4 d , t = 0.31 d

= 2 σt may be

Design of Machine Members-I

Problem:

Design and draw a cotter joint to support a

in tension. The material used is carbon

be used. The load is applied statically.

stress = 35 MPa and crushing stress

Lecture Notes - 50

otter joint to support a load varying from 30 kN in compression

material used is carbon steel for which the following allowable stresses

applied statically. Tensile stress = compressive stress = 50 MPa

35 MPa and crushing stress = 90 MPa.

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compression to 30 kN

allowable stresses may

50 MPa ; shear

Design of Machine Members-I

Lecture Notes - 50

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Design of Machine Members-I

References:

1. Machine Design - V.Bandari .

2. Machine Design – R.S. Khurmi

3. Design Data hand Book - S MD Jalaludin.

Lecture Notes - 50

V.Bandari .

R.S. Khurmi

S MD Jalaludin.

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