A mechanical joint is a section of a

machine A machine is a physical system using Power (physics), power to apply Force, forces and control Motion, movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to na ...

which is used to connect one or more 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 Kinematics is a subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause them to move. Kinematics, as a fiel ...

of these mechanical parts of the machine in one

degree of freedom Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

, and restrict movement in one or more others.

A pin joint, also called a revolute joint, is a one- degree-of-freedom

kinematic pair In classical mechanics, a kinematic pair is a connection between two physical objects that imposes constraints on their relative movement (kinematics). German engineer Franz Reuleaux introduced the kinematic pair as a new approach to the study ...

. It constrains the motion of two bodies to pure rotation along a common axis. The joint doesn't allow translation, or sliding linear motion. This is usually done through a rotary bearing. It enforces a cylindrical contact area, which makes it a lower

, also called a full joint.

Prismatic

A prismatic joint provides a linear sliding movement between two bodies, and is often called a slider, as in the

slider-crank linkage A slider-crank linkage is a four-link mechanism with three revolute joints and one prismatic, or sliding, joint. The rotation of the crank drives the linear movement the slider, or the expansion of gases against a sliding piston in a cylinder ...

. A prismatic pair is also called as sliding pair. A prismatic joint can be formed with a polygonal cross-section to resist rotation. The relative position of two bodies connected by a prismatic joint is defined by the amount of linear slide of one relative to the other one. This one parameter movement identifies this joint as a one

. Prismatic joints provide single-axis sliding often found in

hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...

and

pneumatic cylinder Pneumatic cylinders (sometimes known as air cylinders) are mechanical devices which use the power of compressed gas to produce a force in a reciprocating linear motion. Like hydraulic cylinders, something forces a piston to move in the desir ...

Ball

In an automobile, ball joints are spherical bearings that connect the

control arm In automotive suspension, a control arm, also known as an A-arm, is a hinged suspension link between the chassis and the suspension upright or hub that carries the wheel. In simple terms, it governs a wheel's vertical travel, allowing it to move ...

s to the

steering knuckle 300px, Double Wishbone Suspension In automotive suspension, a steering knuckle or upright is that part which contains the wheel hub or spindle, and attaches to the suspension and steering components. The terms ''spindle'' and ''hub'' are someti ...

s. They are used on virtually every automobile made and work similarly to the ball-and-socket design of the human

hip joint In vertebrate anatomy, hip (or "coxa"Latin ''coxa'' was used by Celsus in the sense "hip", but by Pliny the Elder in the sense "hip bone" (Diab, p 77) in medical terminology) refers to either an anatomical region or a joint. The hip region is ...

. A ball joint consists of a bearing stud and socket enclosed in a casing; all these parts are made of

steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...

. The bearing stud is tapered and threaded, and fits into a tapered hole in the steering knuckle. A protective encasing prevents dirt from getting into the joint assembly. Usually, this is a rubber-like boot that allows movement and expansion of

lubricant A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...

. Motion-control ball joints tend to be retained with an internal spring, which helps to prevent

vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, su ...

problems in the linkage. The "offset" ball joint provides means of movement in systems where

thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...

and contraction, shock, seismic motion, and

torsional In the field of solid mechanics, torsion is the twisting of an object due to an applied torque. Torsion is expressed in either the pascal (Pa), an SI unit for newtons per square metre, or in pounds per square inch (psi) while torque is expressed ...

motions, and forces are present.

Knuckle

A knuckle joint is used to connect the two rods which are under the

tensile load Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...

when there is requirement of a small amount of flexibility or when angular movement is necessary. There is always axial or linear line of action of load. The knuckle joint assembly consists of the following major components: # Single eye. # Double eye or fork # Knuckle pin. At one end of the rod the a single eye is formed and a double eye is formed at the other end of the rod. Both, single and double eye are connected by a pin inserted through the 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: # Shear failure of pin (single shear). # Crushing of pin against rod. # Tensile failure of flat end bar. Application: # Tie rod joint of roof truss. # Tension link in bridge structure. # Link of roller chain. # Tie rod joint of jib crane. # The knuckle joint is also used in tractor.

Turnbuckle

The buckle or a coupler is a mechanical joint used to connect two members which are subjected to tensile loading which require slight adjustment of length or tension under loaded conditions. It consists of central hexagonal nut called coupler and tie rod having right-hand and left-hand threads. A coupler of hexagonal shape is to facilitate the turning of it with a spanner or sometime a hole is provided in the nut so that a tommy bar can be inserted for rotating it. As the coupler rotates, the tie rods are either pulled together or pushed apart depending upon the direction of the rotation coupler. Normally the tie rods are made of steel, while the coupler is made of steel or C.I. Application : # To tighten the members of the roof truss. # Used to connect link in a mechanism to transfer motion # Used between the two railway wagon or bogies. # To tighten the cable or stay ropes of electric distribution poles.

Cotterpin

This is mainly used to connect rigidly two rods which transmit motion in the axial direction, without rotation. These joints may be subjected to tensile or compressive forces along the axes of the rods. The very famous example is the joining of piston rod's extension with the connecting rod in the cross head assembly. Advantages: *Quick assembly and disassembly is possible *It can take tensile as well as compressive force. Application: *Joint between piston rod and cross head of a steam engine *Joint between valve rod and its steam *A steam engine connecting rod strap end *Foundation bolt

Bolted

A Bolted Joint is a Mechanical Joint which is the most popular choice for connecting two members together. It is easy to design and easy to procure parts for, making it a very popular design choice for many applications. Advantage: * Joints are easily assembled/ disassembled by using a torque wrench or other Fastener tooling. * Clamped members can be axially tensioned at variable preloads. Disadvantage: * Threaded components can fail from Fatigue Failure. * Joints can come loose, requiring re-torqueing. Application: *Pipe flanges *Automotive engines *Wind Power Generator Foundation Bolts

Screw

Universal

References

{{reflist, 30em Kinematics Rigid bodies Mechanical engineering Hardware (mechanical) Mechanical fasteners

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