A mechanical joint is a section of a

machine A machine is a physical system that uses power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromol ...

which is used to connect one or more mechanical parts 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 In physics, kinematics studies the geometrical aspects of motion of physical objects independent of forces that set them in motion. Constrained motion such as linked machine parts are also described as kinematics. Kinematics is concerned with s ...

of these mechanical parts of the machine in one

degree of freedom In many scientific fields, the degrees of freedom of a system is the number of parameters of the system that may vary independently. For example, a point in the plane has two degrees of freedom for translation: its two coordinates; a non-infinites ...

, 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 stud ...

. 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 (also commonly referred to as a crank-slider linkage) is a four-link mechanism with three revolute joint, revolute joints and one prismatic joint, prismatic (sliding) joint. The naming convention of slider-crank and cra ...

. A prismatic pair is also called as sliding pair. A prismatic joint can be formed with a polygonal

cross-section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture and engineering 3D * Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) ...

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 () 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 counterpart of pneumatics, which concer ...

and

pneumatic cylinder Pneumatic cylinder, also known as air cylinder, is a mechanical device which uses the power of compressed gas to produce a force in a reciprocating linear motion. Like in a hydraulic cylinder, something forces a piston to move in the desired ...

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 mo ...

s to the steering knuckles. 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, the hip, or coxaLatin ''coxa'' was used by Celsus in the sense "hip", but by Pliny the Elder in the sense "hip bone" (Diab, p 77) (: ''coxae'') in medical terminology, refers to either an anatomical region or a joint o ...

. 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 of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

. 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, ...

. 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. Vibration may be deterministic if the oscillations can be characterised precisely (e.g. the periodic motion of a pendulum), or random if the os ...

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 increase in length, area, or volume, changing its size and density, in response to an increase in temperature (usually excluding phase transitions). Substances usually contract with decreasing temp ...

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 could be defined as strain or angular deformation, and is measured by the angle a chosen section is rotated from its equilibrium position. Th ...

motions, and forces are present.

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 *Foundation bolts

Screw

Universal

References

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