Knuckle Joint: Definition, Assembly, and Application

A knuckle joint is a mechanical joint that connects two rods or bars at an angle, allowing limited angular movement.

It consists of a pin or rod inserted into a series of alternating holes in the two components, enabling rotation and axial movement. Knuckle joints are commonly used in applications where flexibility and articulation are required, such as in linkages and suspension systems.

What is a Knuckle joint?

A knuckle joint is a mechanical joint used to connect two rods that are under a tensile load. When there is a requirement of a small amount of flexibility or angular movement is necessary. There is always an axial or linear line of action of load.

A knuckle joint is used to join two rods that are subjected to the action of tensile loads. However, the rods can support compressive loads if the joint is directed. A knuckle joint can be easily disconnected for adjustment or repair. The knuckle joint is used to transmit axial tensile force.

Typically, these types of joints are used in bicycle chain links, tie rod joints for roof trusses, valve rod joints with centric rods, pump rod joints, tension links in bridges, and also found in a variety of lever and rod connections.

The shaft is formed into an eye at one end and a fork at the other, with an eye in each fork. The knuckle pin slightly passes through both the eye and fork hole and it is achieved with a collar and taper pin.

The knuckle pin can be stopped from rotating into the fork by using a tiny stop, pin, peg, or snug. The fork and eye edges are machined, the openings are precisely drilled, and the pins are turned to produce a joint of higher quality. Steel or iron can be used to create the joint.

Knuckle Joint Parts or Assembly

The knuckle joint assembly consists of the following major components:

knuckle joint diagram
  • Single eye
  • Double-eye or fork
  • Knuckle pin.

At one end of the rod, a single eye is formed and a double eye is formed at the other end of the rod. Both, single and double eyes are connected by a knuckle pin inserted through the eye. The knuckle pin has a head at one end and another end there is a taper pin or split pin.

For gripping purposes, all three holes are concentric and a pin is used to secure the joint. Now, when the two eyes are pulled apart, the pin holds them together. The solid rod portion of the joint in this case is much stronger than the portion through which the pin passes.

Materials used in Knuckle joint parts

Knuckle joints can be made from various materials like aluminum, stainless steel, structural steel, magnesium, and gray cast iron have been studied and it was found that knuckle joints made of aluminum have the highest factor of safety for 50 KN loading conditions. Hence knuckle joints made of aluminum are best suited for 50 KN loading conditions.

The knuckle joint used in some industries is generally made from a combination of cast iron and stainless steel. The rapid growth of technology in recent decades led to a reduction in the cost and weight of the materials as a result accidents have been reduced and safety has increased. Knuckle joints may be cast or fabricated or forged.

Teflon can also replace with cast iron. Recent research has proved that if we use composite materials instead of cast iron, there will be many advantages such as the making of knuckle joints being easy, having maximum safety, and being eco-friendly.

application of the Knuckle Joint

Following are the knuckle joint applications:

  • The joint between the tie rod joint of a roof truss.
  • The joint Tension link in the bridge structure.
  • Tie the rod joint of the jib crane.
  • Link of roller chain, bicycle chain, and Chain straps of watches.
  • Connecting rods between locomotive wheels.
  • The valve mechanism of a reciprocating engine.
  • The fulcrum for the lever.
  • The knuckle joint is also used in Tractor wheel alignment parts.
  • In an automobile, a knuckle is supporting the skeleton for wheel assembly.
  • It uses downhole string on some occasions like the flush eccentric work barrel steps, the lateral drilling, Re-entrance into the bottom of the well, fishing, and so on.
  • knuckle joints are mainly used to join coaches of the train.
  • It is used in the windshield wipers of the vehicle.
  • It also uses in Robotic arms or Earth Movers like Crain or tanks etc.

Advantages of knuckle joint

The advantages of knuckle joints are as follows:

  • Knuckle joints can withstand large tensile loads.
  • It has good mechanical rigidity.
  • Also, it is simple to manufacture and set up.
  • It can be easily assembled and dismantled.
  • Design is simple and easy.
  • Fewer parts are less costly and more reliable.
  • It can permit angular movement between rods.
  • High repeating accuracy even with varying material thickness and tensile strengths.
  • Reduced impact shock and high system rigidity.
  • Long tool life.

The disadvantage of knuckle joints

The disadvantages of knuckle joints are as follows:

  • This joint cannot withstand large compressive loads.
  • It permits angular movement in only one plane.
  • It is not as flexible as a universal joint.

Failure of Knuckle Joints

Failure of knuckle joints may cause accidents so it is necessary to design knuckle joints to withstand tension without failure.

The modes of failure are:

  • Shear failure of Knuckle pin(single shear).
  • Crushing of pin against the rod.
  • Tensile failure of flat end bar.
  • Tensile failure of solid rod
  • Bending of knuckle pin,
  • Tensile failure across knuckle pin single eye or rod end hole
  • Shear failure of single eye end beyond knuckle pinhole (double Shear).
  • Bending of a pin in the knuckle pinhole of a single-eye end.
  • Tensile failure of double eye end at knuckle pinhole.
  • Shear failure of double eye end beyond knuckle pinhole
  • Bearing failure of the pin in the knuckle pinhole of the double eye end,

Design of Knuckle joints

  • Selection of Material: As the joint is subjected to reversible stresses, the stresses induced will be repeated and the joint may fail due to fatigue. Hence the selection of material is carried out by considering low carbon steel which will withstand stresses developed in the above case and the cost of material will be within range.
  • Properties of material: Hence select C-30 as material for all parts [Hint: we can select between C-15 to C-45]
  • Selection of FOS: As the joint is subjected to reversible load and to account for fatigue failure, let us select a higher FOS.
  • Permissible stresses: Maximum principal stress, shear stress, Imperial relation
  • Design of rod: Consider the rod is subjected to direct tensile stress ς = P / π d2
  • A design of knuckle pin
  • Design of single-eye end
  • Design of double-eye end
  • Miscellaneous Dimensions: Pinhead diameter and Thickness of pinhead