cotter and Knuckle design

1. Design of Cotter Joint

2. Review
Simple or Direct stresses
- Tensile, Compressive, Shear
- Stresses due to Bending and
Twisting Moments
(Their nature, distribution, Formulae
to compute their values)
Ductile and Brittle Materials, Their
Strength, Factor of Safety

3. Socket
Cotter Spigot
P P
Cotter Joint
Used to Connect Two Aligned Shafts
Transmitting a Pull or a Push
(Reciprocating Motion)

4. Failure of shaft: The shaft is under tension
Resisting area,
If ft is the safe tensile stress, strength equation is
2
4
A d


2
4
t
d f P


This gives the safe value of d - the diameter of the
shaft
,
y t
t
S
f
N

d
d

5. Failure of Cotter
The Cotter is in double shear
Area under shear = 2 x b x t
Force = P
Shear stresses induced
2
P P
A bt
  
For cotter to be safe in shear
 must be less than allowable
stresses i.e. Sy,s/N
From this obtain t , Usually b is taken equal to d,
i.e. diameter of the shaft
b
t

6. Failure of Spigot:
Different sections of the spigot are under different
types of loading (Different types of stress)
Tensile
Failure
Shear
Failure
d1
a
Crushing
at this face

7. d1
t
Tensile Failure :
2
1 1
4
t
d d t f P

 
 
 
 
Tensile
Failure
Shear
Failure
d1
a
Crushing
at this face
Shear failure: 1
2 s
ad f P

Crushing failure: 1 c
d tf P

These equations provide safe values of d1 and a
Value of t is calculated earlier

8. Socket: Different sections of the socket
are under different
types of loading (Different types of
stress)

9. ØD
Ød1
c
ØD1
Collar

10. Strength Equations
Tensile failure of the socket
Crushing failure of collar
Shear failure of collar end
   
2 2
1 1
4
t
D d D d t f P

 
   
 
 
 
1 1 C
D d tf P
 
1 1
2 ( ) S
c D d f P
 
t
ØD1
Ød1

11. Thickness of Collar on Spigot
Ød2
b
 
2 2
2 1
4
C
d d f P

 
Ød1
1 S
d bf P
 

12. Cotter Joint – Standard Proportions
d = Diameter of rod (calculated from tensile strength)
b = Width of cotter = 1.30d
t = Thickness of cotter = 0.31d
d1 = Diameter of spigot = 1.21d
a = Length of socket after the slot for cottar = 0.75d
b = Thickness of collar on spigot = d
D1 = Outer diameter of socket = 1.75d
D = Dia of Collar on socket = 2.4d
c = Thickness of collar on socket = d

13. Design of Knuckle Joint
Knuckle pin
Fork
Eye

14. The Fork

15. The shaft (Rod) is in tension
2
4
t
d f P


Fix up safe value of the diameter of the shaft

16. Deciding diameter of the Pin:
2
2
4
S
d f P

 
The pin is in double shear
Criterion of bearing pressure gives
OR
2
b
b
dap P
dbp P


Ød
a
b
b

17. The Eye
Tensile failure of the eye end
Bearing stress criterion
  t
D d af P
 
D
d
Thickness a
b
dap P


18. The Fork:
The strength equations are similar
to the Eye, thickness of each arm
is to be considered, Load is to be
taken as half – Two arms of the
fork are resisting the load

19.  
2 t
D d bf P
 
2 b
dbp P

Tensile failure
Criterion of bearing pressure

20. Conclusion
• Components of Cotter and Knuckle Joints were subjected to
simple stresses (Direct Stresses)
• Stresses induced were calculated mostly by the formulae
and they were compared with safe values of corresponding
strength (Critical stress/Factor of safety)
• Dimensions were decided OR Empirically assumed
dimensions were checked
P P
A A
 
 

21. THANK YOU