**Engineering Machanics Terms and Definitions :-**

ABSOLUTE MOTION – Motion of a body in relation to some other body which is at rest.

ACCELERATION – Rate of change of velocity with respect to time, of a particle which is in motion. It is a vector quantity.

ADDENDUM – The radial distance from the pitch circle to the top of the tooth.

ANGULAR ACCELERATION – The time rate of change of angular velocity.

ANGULAR VELOCITY – The time rate of change of angular displacement of a point rotating about a fixed axis (expressed in radians per unit time) Angular velocity of a machine part is often expressed in revolutions per minute (RPM) and is denoted by n.

ANGULAR VELOCITY OF PRECISION – The rate of change in the direction of the plane of rotation of a rotating disc.

ARC OF CONTACT – The arc traced out along the pitch circle while one pair of teeth of gear wheels is in contact (divided into arc of approach and arc of recess).

BEVEL GEARING – Gearing arrangement in which the axes of the shafts connected by gears intersect.

CAM – A reciprocating, oscillating or rotating body which imparts reciprocating or oscillating motion to a second body, called the FOLLOWER with which it is in contact.

CAM PROFILE – The surface profile of the cam that decides the desired motion of the follower.

CENTRIFUGAL FORCE – Radial outward force acting on a body moving along a circular path with uniform velocity.

CENTRIFUGAL GOVERNOR – The effort of the governor is obtained from the change in centrifugal force on (usually) two rotating masses, known as balls, when an increase or decrease in the governor speed occurs.

CENTRIPETAL FORCE – The force that must act radially inward in order to constrain a particle to follow a curved path at uniform velocity.

CIRCULAR PITCH – Length of arc round the pitch circle between the corresponding points on adjacent teeth of a gear.

COMPLEX MECHANISMS – Mechanisms which have two or more floating links.

COMPOSITION OF VECTORS – Composition refers to the adding together of any number of vectors. The sum is called their resultant and the vectors are called the components of the resultant.

COMPOUND CHAIN – A kinematic chain in which there are more than four pairs.

COMPOUND GEAR TRAIN – A gear train containing compound gears i.e., gears, two or more in number integral with one another being used on the same shaft.

COMPOUND PENDULUM – A rigid body suspended vertically so as to oscillate with small amplitude under the action of gravity.

CONSERVATION OF ENERGY – The total energy possessed by a system of moving bodies is at every instant constant, provided no energy is rejected to or received from a source external to the system.

CONSERVATION OF MOMENTUM – For a system of moving bodies which is not acted upon by any external forces, the sum of the moments remain constant.

CONTROLLING FORCE OF A GOVERNOR – The inward radial force exerted on each ball of a centrifugal governor by the arms, springs etc., which are attached to it.

CURVILINEAR MOTION – A translation in which points in the body move along curved path (motion of a wheel).

CYCLE OF MOTION – Motion of a mechanism when it moves through all its possible configurations and returns to its starting position. The time required for one cycle is called PERIOD.

CYCLOIDAL TEETH – Profile of the teeth formed by the locus of a point on a circle rolling on the inside (for the flank) and on the outside (for the face) of the pitch circle.

CYLINDRICAL CAM – Type of cam in which the motion of the follower is controlled by a path traced out on the surface of a cylinder which is rotating about its axis.

DEAD WEIGHT GOVERNOR – The governor in which the radius of the ball path is controlled by levers and weights, the latter being usually attached to the control sleeve.

DEDENDUM – The radial distance from the pitch circle to the bottom of the tooth space.

DIAMETRAL PITCH – Number of teeth per inch diameter.

DISC CAM – An irregular disc rotating about a fixed axis and imparting reciprocating or oscillating motion to a follower in a plane at right angles to the cam axis.

DYNAMICS OF MACHINES – Treatment with the forces acting on the parts of a machine and the motions resulting from these forces.

DYNAMOMETER – A device for measuring the forces or couples which tend to change the state of rest or of uniform motion of a body.

ELLIPTIC TRAMMEL – An instrument used for drawing ellipses.

ENERGY – Capacity for doing work.

EPICYCLIC GEAR TRAINS – Gear trains in which the axis of one or more gears moves relative to the frame. The gear at the centre is called the SUN, and gears whose axes move are called PLANETS. Also called PLANETARY GEARS.

EPICYCLOID – The locus of a point on the circumference of a circle which rolls outside a circular arc, without slipping.

FLOATING LINK – A link in a mechanism which does not have a fixed center of rotation (e.g., coupler in a four bar linkage).

FORCE – The entity which when acts on a body can cause a change in its velocity or direction or both.

FRAME – That part of a machine which is stationary and which supports the moving parts.

FRICTION DRIVE – Drive in which the rotation of one body causes another body in contact with it to rotate due to sufficient friction between the bodies.

GEAR CLEARANCE – The radial distance from the top of the tooth to the bottom of the tooth space in a mating gear unit.

GEAR TRAIN – Unit composed of two or more gears in mesh for the purpose of transmitting motion from one shaft to another.

GOVERNOR – Device that controls the mean speed of an engine over a period of time, as distinct from the flywheel, which limits the fluctuation of speed during one cycle but is not able to prevent a change in mean speed from cycle to cycle.

GOVERNOR EFFORT – Mean force exerted at a sleeve due to a 1% change in speed of governor.

GOVERNOR POWER – Work done at the sleeve for a 1% change in speed, equal to the governor effort times the sleeve displacement.

GYROSCOPIC ACCELERATION – The rate of change of angular velocity of precision of a rotating disc.

HELICAL MOTION – Motion of a body in which each point in the body describes a helix. Helix is the locus of a point which rotates about an axis at a fixed distance and at the same time moves parallel to the axis.

HELICAL GEARING – A type of spur gearing in which although the axes of the shafts are parallel, the teeth are cut on helices instead of straight across the wheels parallel to the axis.

HIGHER PAIRS – Types of kinematic pairs, namely, two elements generally have line or point contact and the pair must be force closed in order to provide completely constrained motion.

HUNTING OF GOVERNOR – The governor is said to hunt if the engine speed is caused to fluctuate continually above and below the mean speed.

HYPOCYCLOID – The locus of a point on the circumference of a circle which rolls inside a circular arc without slipping.

IMPULSE – Time integral of the impulsive force acting on a body.

IMPULSIVE FORCE – Force that acts on a body for an extremely short interval of time and makes the body to move. Occur in collisions, in explosions, in the striking of a nail by a hammer or of a pile by a tup or monkey.

INERTIA GOVERNOR – Governor in which the position of the flyballs are affected by the rate of change of speed of the governor shaft.

INSTANT CENTRE – (1) A point in one body about which another body is rotating either permanently or at the instant (2) A point common to two bodies having the same linear velocity in both magnitude and direction in each.

INTERMITTENT MOTION MECHANISM – A linkage which converts continuous motion into intermittent motion (e.g., indexing mechanism).

INVOLUTE – The locus of a point on a straight line which rolls, without slipping, on the circumference of a circle, or alternatively the locus of a point on the chord which is held taught and unwound from a cylinder.

INVOLUTE TEETH – The outline of a tooth traced out by a point on a chord unwrapped from a circle (known as base circle).

ISOCHRONISM OF GOVERNOR – A governor is said to be isochronous, if , neglecting friction, the equilibrium speed is the same for all radii of the flyballs.

KENNEDY’S THEOREM – Any three bodies having plane motion relative to one another have three instant centers, and they lie in a straight line.

KINEMATIC CHAIN – A group of links either joined together or arranged in a manner that permits them to move relative to one another.

KINEMATIC DIAGRAM – A scale drawing representing the machine so that only the dimensions which affect its motions are recorded.

KINEMATIC PAIR – Two bodies in contact, between which there is relative motion and this motion is completely constrained e.g., turning pair, sliding pair, screw pair.

KINEMATICS OF MACHINES – A study of the relative motion of machine parts e.g., displacement, velocity and acceleration.

KINETICS – Study which deals with the inertia force arising from the combined effect of the mass and the motion of the parts.

LINEAR ACCELERATION – The time rate of change of liner velocity.

LINEAR VELOCITY – The time rate of change of linear displacement of a point or body.

LINK – Name given to anybody which has relative motion to another. Also called ELEMENT. A RIGID LINK is one whose deformations are so small that they can be neglected in determining the motions of various other links in a machine. A belt or chain is a FLEXIBLE LINK.

LOWER PAIRS – Types of kinematic pairs, namely two elements have surface contact and when relative motion takes place, the surface of one element slides over the surface of the other element.

MACHINE – A combination of resisting bodies, with successfully constrained relative motions, which is used for transmitting or transporting available energy so as to do some particular kind of work e.g., electrical motor, internal combustion engine.

MASS OF A BODY – The property of a body which determines its resistance to change its velocity.

MECHANISM – A constrained kinematic chain which means the motion of anyone link will give a definite, predictable motion to each of the others.

MODULE – Reciprocal of diametral pitch.

MOMENTUM – The product of the mass and velocity of a body.

NORMAL ACCELERATION – The time rate of change of velocity of a point in a direction normal to its path. This results from a change in the direction of its linear velocity.

OLDHAM COUPLING – A mechanism for connecting two shafts having parallel misalignment. The coupling transmits a constant velocity ratio.

PAIR – Two bodies in contact constitute a pair. LOWER PAIRING exists when two surfaces are in contact. HIGHER PAIRING refers to the contact which exists at a point or along a line.

PANTOGRAPH – Mechanism used to reproduce to an enlarged or reduced scale and as exactly as possible the path described by a given point.

PARALLEL MECHANISMS – Linkages which give parallel motion (e.g., pantograph which is used for reducing or enlarging drawings and maps, also used for grinding cutting tools or cutting torches to duplicate complicated shapes).

PATH OF CONTACT – The path traced out by the point of contact between a pair of teeth (may be divided into approach and recess).

PINION – The small of the two mating gear wheels.

PITCH CIRCLES – Equivalent rolling circles for a pair of mating gears.

PITCH CIRCLE DIAMETER – The diameter of a circle which by a pure rolling action would transmit the same motion as the actual gearwheel.

PITCH LINE – The point of contact of two circles.

PITCH SURFACES – The cylindrical surfaces of the equivalent rolling circles for a pair of mating gears.

PLANE MOTION – A body has plane motion if all the points move in planes which are parallel to some reference plane (called plane of motion).

POSITIVE DRIVE – The drive that exists in a direct contact mechanism if motion of the driving link compels the follower to move (e.g., cam and follower).

POWER – Rate of doing work or work done in unit time.

PRECESSIONAL MOTION – The change in the direction of the plane of rotation of a rotating disc.

PRESSURE ANGLE – Angle between the common normal and the tangent at the pitch point in a gear drive. Also called ANGLE OF OBLIQUITY.

QUICK RETURN MECHANISM – The mechanism used in machine tools such as shapers and power driven saws for the purpose of giving the reciprocating cutting tool a slow cutting stroke and a quick return stroke with a constant velocity of the driving crank.

RACK – A portion of a gear wheel which has an infinitely large number of teeth.

RACHETS – Mechanisms used to transform motion of rotation or translation into intermittent rotation or translation.

RECTILINEAR MOTION – A motion wherein all points of the body move in straight line paths (e.g., piston motion).

RELATIVE INSTANTANEOUS CENTER – In the case of two bodies, it is the point about which either of them appears to turn (at that instant) if the other is considered fixed (e.g., if two links in a mechanism are pinned together, the pin becomes the relative instantaneous center, if the two bodies are in pure rolling contact, the point of contact is the relative instantaneous center).

RELATIVE MOTION – A body has motion relative to another body only if there is a difference in their absolute motions.

RESOLUTION OF VECTORS – Resolution refers to the breaking down of a vector into any number of component vectors.

REVERTED GEAR TRAIN – The compound gear train in which the first and the last gears are coaxial (e.g., units used in automobile transmission, lathe back gears, industrial speed reducers and in clocks).

ROLLING CONTACT – In a direct contact mechanism, rolling contact exists only if there is no sliding and hence the tangential components of velocities of the contact point on the two bodies are equal in magnitude and direction.

ROTATION – In rotation all points in a body remain at fixed distances from a line which is perpendicular to the plane of motion. This line is the AXIS OF ROTATION.

SCALAR QUANTITIES – Those quantities which have magnitude only (and no direction) e.g., distance, area, volume and time.

SENSITIVITY OF GOVERNOR – Ratio of the mean speed to the speed range of the governor over its limits of operation.

SIMPLE AND COMPOUND MECHANISM – A simple mechanism consists of three or four links. All other mechanisms or those consisting of more than four links are compound mechanisms. Compound mechanisms are usually made up of combinations of simple mechanisms.

SIMPLE GEAR TRAIN – A gear train in which there is only one gear on each shaft.

SIMPLE HARMONIC MOTION – A particle having rectilinear motion has simple harmonic motion if its acceleration is proportional to the displacement of the particle from a fixed point and is of opposite sign.

SKEW GEARING – Gearing arrangement in which the axes of the shafts connected by gears are non parallel and non-intersecting.

SLIDING CONTACT – Sliding exists in a direct contact mechanism whenever the bodies have relative motion along the tangent through their point of contact.

SPEED – The rate of change of magnitude of displacement with respect to time.

SPHERICAL MOTION – A point has spherical motion if it moves in three dimensional space and remains at a fixed distance from some fixed point. A body has spherical motion if each point in the body has spherical motion.

SPIRAL GEARING – A type of skew gearing, but differs in one respect i.e., the contact between pitch surfaces is point contact instead of line contact.

SPRING LOADED GOVERNOR – The governor in which the control of the flyballs is by springs operated directly on the balls or on the sleeve.

SPUR GEARING – Gearing arrangement in which the axes of the shafts connected by gears are parallel and the teeth are cut parallel to the axes.

STABILITY OF GOVERNOR – The governor is said to be stable if there is one equilibrium speed for each radius of rotation of the flyballs and this speed increases with the radius.

STATICS – Study which deals with forces which act on the various parts, when these parts are assumed to be without mass.

STRAIGHT LINE MECHANISMS – Linkages having a point that moves along a straight line or nearly along a straight line, without being guided by a plane surface (e.g., Watts’s mechanism, Scott Russell mechanism).

TANGENTIAL ACCELLERATION – The time rate of change of velocity of a point in a direction tangent to its path. This results from a change in its linear velocity.

TRANSLATION – A body has translation if it moves so that all straight lines in the body move in parallel positions.

UNCONSTRAINED KINEMATIC CHAIN – An arrangement of links wherein for a given motion of one of the links, each of the others is not constrained to move in a definite predictable manner.

UNIVERSAL JOINT – Joint used to connect intersecting shafts (e.g., Hooke or cardan joint).

VECTOR QUANTITIES – Those entities which have magnitude and direction. (e.g., displacement, velocity, acceleration and force). Vector quantities are represented by a straight line with an arrow head (magnitude is represented by its length and direction by the arrow head).

VELOCITY – When a particle is in motion, the rate of change of its displacement with respect to time. It is a vector quantity.

VELOCITY RATIO OF GEAR TRAIN – Ratio of the angular velocity of the first gear in the train to the angular velocity of the last gear.

WORM GEARING – A form of special gearing in which the axes of the driving and driven shafts are usually at right angles and the velocity ratio is high, the driving gear being of smaller diameter.

WORK – Product of the force acting on a body and the displacement of the body caused by that force.

WORKING SURFACE – With respect to a gear, the working surface above the pitch surface is called the FACE OF THE TOOTH and that below the pitch surface is called the FLANK OF THE TOOTH.