Mechanical joint

A mechanical joint is a section of a machine which is used to connect one mechanical part to another. Mechanical joints may be temporary or permanent, most types are designed to be disassembled. Most mechanical joints are designed to allow relative movement of these mechanical parts of the machine in one degree of freedom (insert LINK), and restrict movement one one or more others. Mechanical joints are much cheaper and are usually bought ready assembled. (please add other joints like: U-joint, ball joint, prismatic joint)

A knuckle joint is used to connect the two rods which are under the tensile load, when there is requirement of small amount of flexibility or angular moment is necessary. There is always axial or linear line of action of load.

The knuckle joint assembly consist of following major components:

1.Single eye.

2.Double eye or fork

3.Knuckle pin.

At one end of the rod the single eye is formed and double eye is formed at the other end of the rod.Both, single and double eye are connected by a pin inserted through eye.The pin has a head at one end and at other end there is a taper pin or split pin. For gripping purpose the ends of the rod are of octagonal forms.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.

The modes of failure are:

1.Shear failure of pin (single shear).

2.Crushing of pin against rod.

3.Tensile failure of flat end bar.

Application:

1.Tie rod joint of roof truss.

2.Tension link in bridge structure.

3.Link of roller chain.

4.Tie rod joint of jib crane.

5.The knuckle joint is also used in tractor.

The assembly diagram of knuckle joint is as shown in fig.

The dimension of knuckle joints are

Diameter of rod = d

Diameter of knuckle pin = d_p

Outside diameter of single eye = d_oe

Outside diameter of double eye = d_od

Thickness of single eye = t

Thickness of fork = t₁

Axial tensile force on rod = P

(1) Diameter of rod

Consider the rod is subjected to a direct tensile stress

ς = P /πd²

From above equation, diameter of rod 'd' is obtained.

(2) Design of pin (d_p)

(a) Consider the failure of pin under double shear due to tensile force.

Therefore, direct shear stress induced in knuckle pin is given by Equation

ς= P/ 2A = (P/2) / (π/4)d_p² = 2P/ πd_p²

(b) Failure of knuckle pin in bending

Assume there is no clearance or slack but in actual, knuckle pin is loose in forks to permit angular moment of one with respect to other, so it is subjected to bending moment in addition to shear, consider uniformly distributed load along the portion of pin.

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