, diploma mechanical engineering , mechanical engineering , machine design , design of machine elements , knuckle joint , failures of knuckle joint under different streses , fork end , single eye end , knuckle pin

1. Design of Machine Elements
Design of Knuckle Joint.
Gaurav Mistry
Assistant Professor
Diwaliba Polytechnic, UTU.

2. ❑ Knuckle Joint
▪ A Knuckle joint is a temporary fastening and is used to connect two rods or bars which are
subjected to axial tensile forces.
▪ The joint permits slight angular movement between the rods and little offset between the
axis of the rods. As shown in the figure, the knuckle joint mainly consist of five parts: (1)
Single eye end, (2) Double eye end (fork), (3) Knuckle pin, (4) Collar and (5) Taper pin / Split
pin
Design of Machine Elements
2Gaurav MistryClick on Image for Video

3. ▪ Both the ends i.e. single eye and double
eye ends are connected by inserting
knuckle pin through the hole in both ends.
▪ The knuckle pin has a head at the top end
and a hole in the lower most end to
receive taper pin or split pin.
▪ In order to prevent the knuckle pin,
coming out from joint, the collar with the
hole is assembled on the knuckle pin and
the small pin (taper pin or split pin)is fixed
through the hole in the collar and knuckle
pin. Knuckle pin act like a fulcrum in the
joint.
Design of Machine Elements
3Gaurav Mistry
❑ Knuckle Joint

4. ➢ Advantages of the Knuckle joint:
1. Permits slight angular movement between the rods.
2. Easy to assemble and disassemble.
3. Permits little offset between the axes of the rods.
❑ Knuckle Joint
➢ Material for the Knuckle joint:
▪ Generally, all the parts of the joint are made of mild steel (M.S.)
▪ Wrought iron may also be used as it is easy to forge.
▪ When the joints are subjected to heavy loads and to restrict the size of the joint,
alloy steels may also be used.
Design of Machine Elements
4Gaurav Mistry
➢ Applications of the Knuckle joint:
1. To connect valve rod and eccentric rod of the steam engine.
2. To connect structural members under tension
3. To connect links in some material handling equipment.
4. In locomotive engine for connecting links for the air brake assembly.
➢ Disadvantages of the Knuckle joint:
1. The joint cannot withstand large compressive loads
2. It permits angular motion in only one plane
3. It is not as flexible as universal joint

5. ❑ Knuckle Joint
➢ Comparison between knuckle joint and cotter joint:
Design of Machine Elements
5Gaurav Mistry
Knuckle Joint Cotter Joint
Joint is obtained by using knuckle pin Joint is obtained by using wedge shape
element, cotter
Capable to carry tensile load Capable to carry both tensile and
compressive load.
Permits angular movement between rods As the joint is rigid, it can not permit
angular movement between rods
Mainly used to connect tension links Used to connect rods rigidly and can
permit axial movement of the joint
Less expensive compared to cotter joint More expensive compared to knuckle joint
Not preferred to connect rods having
rotational motions
Not preferred to connect rods having
rotational motions

6. ❑ Knuckle Joint Design
Design of Machine Elements
6Gaurav Mistry
➢ Design Procedure:
▪ The dimensions of various parts of the knuckle joint are fixed by empirical relations as given
below.
▪ If diameter of rod = d,
▪ Diameter of pin, d1 = d,
▪ Outer diameter of eye, d2 = 2d,
▪ Diameter of knuckle pin head
or collar, d3 = 1.5d,
▪ Thickness of single eye end, t = 1.25d,
▪ Thickness of fork, t1 = 0.75d,
▪ Thickness of pin head, t2 = 0.5d,
▪ Other dimensions are
shown in the figure.
▪ σt = Permissible tensile stress
for the rods material,
▪ τ = Permissible shear stress
for the cotter material, and
▪ σc = Permissible crushing stress
for the cotter material.

7. ❑ Knuckle Joint Design
Design of Machine Elements
7Gaurav Mistry
1. Failure of the solid rod in tension:
In determining the strength of the joint for the various methods of failure, it is assumed that
1. There is no stress concentration, and
2. The load is uniformly distributed over each part of the joint.
Due to these assumptions, the strengths are approximate, however they serve to indicate a
well proportioned joint.
Following are the various methods of failure of the joint :
The area resisting the tension in the rods is
Tensile strength of the rod is:
Equating this to load (P), we have:
From this equation, diameter of the rods (d)
may be determined.

8. ❑ Knuckle Joint Design
Design of Machine Elements
8Gaurav Mistry
2. Failure of the knuckle pin in shear: (Assuming that there is no
slack and clearance between the pin and the fork and hence there is no bending of the pin.)
Generally the diameter of knuckle pin is taken equal to diameter of rod
i.e. d1 = d
Since the pin is in double shear, therefore
cross-sectional area of the pin under shearing
and the shear strength of the pin
Equating this to the load (P) acting on the rod,
we have
From this equation the shear stress induced in
the knuckle pin is obtained . If it is less than
permissible shear stress, then only the pin is
safe for shear. Otherwise increase the
diameter.

9. ❑ Knuckle Joint Design
Design of Machine Elements
9Gaurav Mistry
3. Failure of the single eye end in tension:
The single eye or rod end may tear off due to the
tensile load. We know that area resisting
tearing
Tensile strength of the single eye end is:
Equating this to load (P), we have:
Generally the outer diameter of eye is taken twice the diameter of rod i.e. d2 = 2d and the
thickness of single eye end is taken as t = 1.25d
From this equation the tensile stress induced
in the eye is obtained . If it is more than
permissible tensile stress, then increase the
outer diameter of eye.

10. ❑ Knuckle Joint Design
Design of Machine Elements
10Gaurav Mistry
4. Failure of the single eye end in shear at area parallel to tensile load:
From this equation the shear stress induced in
the eye is obtained and checked with the
permissible shear stress.
The single eye or rod end may fail in shearing due to
tensile load. We know that area resisting double shearing
= 2
𝑑2
2
−
𝑑1
2
𝑡
and the shear strength of the eye
Equating this to the load (P), we have

11. ❑ Knuckle Joint Design
Design of Machine Elements
11Gaurav Mistry
5. Failure of the single eye end or knuckle pin in crushing under tensile load:
From this equation the crushing stress induced
in the eye is obtained . If it is more than
permissible crushing stress, then increase the
thickness of single eye.
Equating this to the load (P), we have
The single eye or pin may fail in crushing due
to the tensile load. We know that area resisting
crushing
Crushing strength of single eye or rod end
Generally the outer diameter of eye is taken twice the diameter of rod i.e. d2 = 2d and the
thickness of single eye end is taken as t = 1.25d

12. ❑ Knuckle Joint Design
Design of Machine Elements
12Gaurav Mistry
6. Failure of the forked end in tension:
From this equation the tensile stress induced
in the fork is obtained and checked with the
permissible tensile stress
Equating this to the load (P), we have
The forked end or double eye may fail in tension due to
the tensile load. We know that area resisting tearing
Generally the thickness of fork eye end is taken as t1 = 0.75d
Tearing strength of the forked end
(as there are two eye ends, the
area is doubled)

13. ❑ Knuckle Joint Design
Design of Machine Elements
13Gaurav Mistry
7. Failure of the forked end in shear at the area parallel to the tensile load:
Equating this to the load (P), we have
(as there are two eye ends, the
area is doubled)= 2
𝑑2
2
−
𝑑1
2
𝑥 2𝑡1
The forked end may fail in shearing due to the tensile load.
We know that area resisting Double shearing
Shearing strength of the forked end
From this equation the shear stress induced in
the fork eye is obtained and checked with the
permissible shear stress.

14. ❑ Knuckle Joint Design
Design of Machine Elements
14Gaurav Mistry
8. Failure of the forked end in crushing under tensile load:
Equating this to the load (P), we have
(as there are two eye ends, the
area is doubled)
The forked end or pin may fail in crushing due
to the tensile load. We know that area resisting
crushing
∴ Crushing strength of the forked end
From this equation the crushing stress induced in the fork
eye is obtained and checked with the permissible crushing
stress
Note: From the above failures of the joint, we see that the thickness of fork (t1) should
be equal to half the thickness of single eye (t / 2). But, in actual practice t1 > t / 2 in
order to prevent deflection or spreading of the forks which would introduce excessive
bending of pin.

15. ❑ Knuckle Joint Design
Design of Machine Elements
15Gaurav Mistry
9. Design of Knuckle Pin Head:
▪ Diameter of knuckle pin head or collar, d3 = 1.5d,
▪ Thickness of pin head, t2 = 0.5d,
▪ Other dimensions are determined using the empirical relations shown in the figure.
Design a knuckle joint to transmit 150 kN. The design stresses may be taken as 75 MPa in
tension, 60 MPa in shear and 150 MPa in compression.
❖ Knuckle Joint Numerical:

16. ❑ Knuckle Joint Design
Design of Machine Elements
16Gaurav Mistry
❖ Knuckle Joint Numerical:

17. ❑ Knuckle Joint Design
Design of Machine Elements
17Gaurav Mistry
❖ Knuckle Joint Numerical:

18. ❑ Knuckle Joint Design
Design of Machine Elements
18Gaurav Mistry
❖ Knuckle Joint Numerical:

19. ❑ Knuckle Joint Design
Design of Machine Elements
19Gaurav Mistry
In actual practice, the knuckle pin is loose in forks in order to permit angular
movement of one with respect to the other, therefore the pin is subjected to bending
in addition to shearing. By making the diameter of knuckle pin equal to the diameter
of the rod (i.e., d1 = d), a margin of strength is provided to allow for the bending of
the pin.
In case, the stress due to bending is taken into account, it is assumed that the load on
the pin is uniformly distributed along the middle portion (i.e. the eye end) and varies
uniformly over the forks as shown in Fig.

20. ❑ Knuckle Joint Design
Design of Machine Elements
20Gaurav Mistry
Or knuckle pin
Double
Double

21. ❑ Knuckle Joint Design
Design of Machine Elements
21Gaurav Mistry
REFERENCES:
1. A textbook of Machine design, R. S. Khurmi, S. Chand.
2. Design of Machine Elements, S. B. Soni, Atul prakashan.
3. www.google.com