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# Glossary of Common Mechanical Terms

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Some of the basic terminologies that every mechanical engineering student need to know are covered in this article.

Acceleration: Acceleration is the time rate of change of velocity of a body. It is always produced by a force acting on a body. Acceleration is measured in meters per second (m/s2).

Component forces: The individual forces that are the equivalent of the resultant.

Concurrent forces: Forces whose lines of action or directions pass through a common point or meet at a common point.

Crank: A sideline that revolves relative to the frame.

Crank-rocker mechanism: A four-bar linkage characterized by the ability of the shorter side link to resolve through 360° while the opposing link rocks or oscillates.

Couple: Couple is a two equal and opposite parallel forces that act at diametrically opposite points on a body to cause it to rotate around a point or an axis through its center.

Displacement: Distance measured from a fixed reference point in a specified direction; it is  a vector quantity; units are measured in inches, feet, miles, centimeters, meters, and kilometers.

Double-crank mechanism: A four-bar linkage characterized by the ability of both of its side links to oscillate while the shortest link (opposite the foundation link) can resolve through 360°.

Dynamics: The study of forces that act on bodies, not in equilibrium, both balanced and unbalanced; it accounts for the masses and acceleration of the parts as well as the external forces acting on the mechanisms. Dynamics is a combination of kinetics and kinematics.

Efficiency of machines: Efficiency of machines is the ratio of machine’s  output divided by its input is typically expressed as a percent. There are energy or power losses in all moving machinery caused primarily by friction. This causes inefficiency, so a machine’s output is always less than its input; both output and input must be expressed in the same units of power or energy. This ratio is always a fraction, is multiplied by 100 to obtain a percent. It can also be determined by dividing the machine’s mechanical advantage by its velocity ratio and multiplying that ratio by 100 to get a percent.

Energy: A physical quantity present in three-dimensional  space in which forces can act on a body or particle to bring about physical change; it is capacity for doing work. Energy can take many forms, including mechanical, electrical, electromagnetic, chemical, thermal, solar and nuclear. Energy and work are related and measured in the same units: foo-pounds, ergs or joules; it cannot be destroyed, but it can be wasted.

• Kinetic energy is the kind of energy a body has when it is in motion. Examples are rolling soccer, a speeding automobile, or a flying airplane.
• Potential energy is the kind of energy that a body has because of its position or state. Examples are a concrete block poised at the edge of a building, a shipping container suspended above the ground by a crane or a roadside bomb.

Equilibrium: In mechanics, a condition of balance or static equilibrium between opposing forces. An example is when there are equal forces at both ends of a seesaw resting on a fulcrum.

Force: Strength or energy acting on a body to push or pull it; it is required to produce acceleration. Except for gravitation, one body cannot exert a force on another body unless the two are in contact. The earth exerts a force of attraction on bodies, whether they are in contact or not. Force is measured in units newtons (kg-m/s2).

Fulcrum: A pivot point or edge about which objects are free to rotate.

Kinematic chain: A combination of links and pairs without a fixed link.

Kinematics: The study of the motions of bodies without considering how the variables of force and mass influence the motion. It is desired as the geometry of motion.

Kinetics: The study of the effects of external forces including gravity upon the motions of physical bodies.

Lever: A simple machine that uses opposing torque around a fulcrum to perform work.

Linear motion: Motion in a straight line. An example is when a car is driving on a straight road.

Link: A rigid body with pins or fasteners at its ends to connect it to other. rigid bodies s it can transmit a force or motion. All machines contain at least one link, either in a fixed position relative to the earth or capable of moving the machine and the link during the motion; this link is the frame or fixed link of the machine.

Linkages: Mechanical assemblies consisting of two or more levers connected to produce the desired motion. They can also be mechanisms consisting of rigid bodies and lower pairs.

Machine: An assembly of mechanisms or parts or mechanisms capable of transmitting force, motion, and energy from a power source; the objective of a machine is to overcome some form of resistance to accomplish the desired result. Machines are used for carrying out two functions. They are

1. The transmission of relative motion and
2. The transmission of force; both require that the machine is strong and rigid. While both machines and mechanisms are combinations of rigid bodies capable of definite relative motions, machines transform energy, but mechanisms do not. A simple machine is an elementary mechanism. Examples are the lever, wheel, and axle, pulley, inclined plane, wedge, and screw.

Machinery: A term generally meaning various combinations of machines and mechanisms.

Mass: Mass is the quantity of matter in a body indicating its inertia. Mass also initiates gravitational attraction. Mass is measured in ounces, pounds, kilograms, grams and tons.

Mechanical advantage: The ratio of the load (or force F) exerted by an operator. If friction is considered in determining mechanical advantage, or it has been determined by the actual testing, the ratio W/F is the mechanical advantage. However, if the machine is assumed to operate without friction, the ratio W/F is the theoretical mechanical advantage.

Mechanics: A branch of physics concerned with the motions of objects and their response to forces. Descriptions of mechanics begin with definitions of such quantities as acceleration, displacement, force, mass, time and velocity.

Mechanism: In mechanisms, mechanism refers to two or more rigid or resistant bodies connected together by joints so they exhibit definite relative motions with respect to one another. Mechanisms are divided into two classes:

1. Planar: Two-dimensional mechanisms whose relative motions are in one plane or parallel planes.
2. Spatial: Three-dimensional mechanisms whose relative motions are not all in the same or parallel planes.

Moment of force or torque: The product of the force acting to produce a turning effect and the perpendicular distance of its line of action from the point or axis of rotation. the perpendicular distance is called the moment arm or the lever arm torque. It is measured in pound-inches (lb-in), pound-feet (lb-ft) or newton-meters (N-m).

Moment of inertia: A physical quantity giving a measure of the rotational inertia of a body about a specified axis of rotation; it depends on the mass, size and shape of the body.

Non-concurrent forces: Forces whose lines of action do not meet at a common point.

Non-coplanar forces: Forces that do not act in the same place.

Oscillating motion: Repetitive forward and backward circular motion such as that of a clock pendulum.

Pair: A joint between the surfaces of two rigid bodies that keeps them in contact and relatively movable. It might be as simple as a pin, bolt or hinge between two links or as complex as a universal joint between two links. There are two kinds of pairs in mechanisms classified by the type of contact between the two bodies of the pair: lower pairs and higher pairs.

• Lower pairs are surface-contact pairs classed either as revolute or prismatic. Examples: a hinged door is a revolute pair and a sash window is a prismatic pair.
• Higher pairs include point, line or curve pairs. Examples: paired rollers, cams and followers and meshing gear tooth.

Power: Power is defined as the time rate of doing work. It is measured in foot-pounds per second (ft-lb/s), foot-pounds per minute (ft-lb/min), horsepower, watts, kilowatts, newton-meters/s, ergs/s and joules/s.

Reciprocating motion: Repetitive back and forth linear motion as that of a piston in an internal combustion engine.

Resultant: In a system of forces, it is single force equivalent to the entire system. When the resultant of a system of forces is zero, the system is in equilibrium.

Rotary motion: Circular motion as in the turning of a bicycle wheel.

Skeleton outline: A simplified geometrical line drawing showing the fundamentals of a simple machine devoid of the actual details of its construction. It gives all of the geometrical information needed for determining the relative motions of the main links. The relative motions of these links might be completed circles, semicircles or arcs or even straight lines.

Statics: The study of bodies in equilibrium, either at rest or in uniform motion.

Torque: An alternative name for the moment of inertia.

Velocity: Velocity is the time rate of change with respect to distance. It is measured in feet per second (ft/s), feet per minute (ft/min), meters per second (m/s) or kilometers per hour (km/hr).

Velocity ratio: Velocity ratio is the distance movement of the effort divided by the distance of movement of the load per second for a machine. Velocity ratio has no units.

Weight: The force on a body due to the gravitational attraction of the Earth; weight W = mass (m) * acceleration (g) due to the Earth’s gravity; the mass of a body is constant but g, and therefore ‘W’ vary slightly over the Earth’s surface.

Work: Work is the product of force and distance: the distance an object moves in the direction of force. Work is not done if the force exerted on a body fails to move that body. Work, like energy, is measured in units of ergs, joules or foot-pounds.