Knuckle Joint

A knuckle joint is used to connect two rods which are under the action of tensile loads. However, if the joint is guided, the rods may support a compressive load.

A knuckle joint may be readily disconnected for adjustments or repairs.

Its use may be found in the link of a cycle chain, tie rod joint for roof truss, valve rod joint with the eccentric rod, pump rod joint, tension link in bridge structure, and lever and rod connections of various types.

Construction of Knuckle Joint :  

• In the knuckle joint (the two views of which are shown in Fig.), one end of one of the rods is made into an eye and the end of the other rod is formed into a fork with an eye in each of the fork legs.
• The knuckle pin passes through both the eye hole and the fork holes and may be secured by means of a collar and taper pin or spilt pin. The knuckle pin may be prevented from rotating in the fork by means of a small stop, pin, peg, or snug.
• In order to get a better quality of joint, the sides of the fork and eye are machined, the hole is accurately drilled and the pin turned. The material used for the joint may be steel or wrought iron.

Knuckle Joint Parts : 

1. Eye,
2. Fork and
3. Pin as shown in Figure 

The eye is formed on one of the rods and the fork is formed on the other. The eye fits inside the fork and the pin is passed through both the fork and the eye. This pin is secured in its place by means of a split pin. The ends of the rods are made octagonal to some distance for better grip and are made a square for some portion before it is forged to make the eye and fork shapes.

Empirical relations are required to design a Knuckle joint.

The dimensions of various parts of the knuckle joint are fixed by empirical relations as given below. It may be noted that all the parts should be made of the same material i.e. mild steel or wrought iron.

When d= Diameter of rod
1. Diameter of knuckle pin d1=d
2. Outer diameter of eye d2=2d
3. Diameter of knuckle pinhead or collar d3=1.5d
4. Thickness of single eye rod t=1.25d
5. Thickness of double eye rod t1=1.25d
6. Thickness of knuckle pinhead or collar t2=1.25d

Design of Knuckle joint

Failure of rod in tension

Rod may fail in tension due to tensile load
Tensile strength of rod, P = /4 x² x

From this equation diameter of the rod may obtain

Diameter of knuckle pin in shearing

Since the pin is in double shear, the Shearing strength of pin P =/4 x 1² x

Value of d1 can be found here 1=d

Fix the dimensions using empirical relations;

Dia. Of pin = 1 =d
Outer dia. Of single or double eye = 2 =2d
Dia. Of knuckle pinhead and collar =3 = 1.5d
Thickness of single eye = t = 1.25d
Thickness of fork =1 = 0.75d
Thickness of collar pin =t2 = 0.5d

Checking the failure of a single eye in tension
= p/(d2-d1) x t

Checking the failure of a single eye in crushing
= p/d1 x t

Checking the failure of a single eye in shear
= p/(d2-d1) x t

Checking the failure of the double eye in tension
= p/2(d2-d1) x t1

Checking the failure of the double eye in crushing
= p/2d1 x t1 Checking the failure of the double eye in shear
= p/2(d2-d1) x t1

Advantages of Knuckle Joint are :

•  Simple to design and manufacture.
• Fewer parts – less cost more reliability.
• Simple to assemble and dismantle
• Knuckle joint can withstand large tensile loads
• Contains good mechanical rigidity
• Easy to manufacture and set up.
• Design is simple and easy.

Knuckle Joint Disadvantage:

• It is not suitable for large compressive loads.
• Permit angular movement in only one plane.

Applications of Knuckle joint:

1. Link of cycle chain
2. Tie rod joints for roof truss
3. Valve rod joint for eccentric rod pump rod joint
4. Tension link in bridge structure
5. Lever and rod connection of various types.
6. swing arm of two-wheeler
7. Connection of link rod of leaf springs in multi-axle vehicles
8. Piston, Piston Pin, Connecting Rod
9. Connections of leaf spring with chassis

Difference Between Knuckle Joint and cotter Joint

Question and Answers : 

1. Explain with a neat diagram about “ failure of knuckle pin in bending”

Ans: 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 the 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. Thus in the forks, a load P/2 acts through a distance of t1 / 3 from the inner edge and the bending moment will be maximum at the center of the pin. The value of the maximum bending moment is given by

2. What are some real-life applications of the Knuckle Joint?

1. Link of the suspension bridge
2. Valve mechanism of reciprocating engine
3. Bicycle chain
4. Fulcrum of lever
5. Tractor wheel alignment parts
6. Robotic arms

3. Which material is used for manufacturing of knuckle joint?

knuckle joint, all parts like Fork end, Knuckle pin, Taper pin, Collar and Eye end used material is Steel and Wrought Iron.

4. What is a knuckle joint?

Knuckle joint is a type of mechanical joint used in structures, to connect two intersecting cylindrical rods, whose axes lie on the same plane. It permits some angular movement between the cylindrical rods (in their plane). It is specially designed to withstand tensile loads.

Parts of a Knuckle Joint:

A typical knuckle joint has the following parts:

Fork end

Eye end

Knuckle pin

Collar

Taper pin

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