Defintion - Gear

ADDENDUM (a) :

It is the height by which a tooth projects beyond the pitch circle or pitch line.

AXIAL MOVEMENT :

Endwise movement of input or output shafts, sometimes called endplay, is usually expressed in thousands of an inch.

BASE DIAMETER (Db) :

It is the diameter of the base cylinder from which the involute portion of a tooth profile is generated.

BACKLASH (B) :

It is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles. As actually indicated by measuring devices, backlash may be determined variously in the trans-verse, normal, or axial-planes, and either in the direction of the pitch circles or on the line of action. Such measure-ments should be corrected to corresponding values on transverse pitch circles for general comparisons.

BACKLASH :

Rotational movement of the output shaft when holding the input shaft stationary and rotating the output shaft alternately clockwise and counter clockwise. Backlash may be expressed in thousands of an inch measured at a specific radius at the output shaft.

BORE LENGTH :

It is the total length through a gear, sprocket, or coupling bore.

CENTER DISTANCE :

On a single reduction reducer, this is the distance between the center lines of the input and output shafts. Shaft center lines may be parallel or at right angles to one another. The center distance of multiple stage reducers usually refers to the lowest speed stage (last reduction).

CIRCULAR PITCH (p) :

It is the distance along the pitch circle or pitch line between corresponding profiles of adjacent teeth.

CIRCULAR THICKNESS (t) :

It is the length of arc between the two sides of a gear tooth on the pitch circle, unless other-wise specified.

CLEARANCE-OPERATING (c) :

It is the amount by which the dedendum in a given gear exceeds the addendum of its mating gear.

CONTACT RATIO (mc) :

In general, the number of angular pitches through which a tooth surface rotates from the beginning to the end of contact.

DEDENDUM (b) :

It is the depth of a tooth space below the pitch line. It is normally greater than the addendum of the mating gear to provide clearance.

DIAMETRAL PITCH (P) :

It is the ratio of the number of teeth to the pitch diameter.

EFFICIENCY :

The amount of output power of the reducer as compared to the amount of input power. It is usually stated as a percentage.

FACE WIDTH (F) :

It is the length of the teeth in an axial plane.

FILLET RADIUS (rf) :

It is the radius of the fillet curve at the base of the gear tooth.

FULL DEPTH TEETH :

Are those in which the working depth equals 2.000 divided by the normal diametral pitch.

GEAR :

It is a machine part with gear teeth. When two gears run together, the one with the larger number of teeth is called the gear.

HUB DIAMETER :

It is outside diameter of a gear, sprocket or coupling hub.

HUB PROJECTION :

It is the distance the hub extends beyond the gear face.

INPUT HORSEPOWER :

The amount of power applied to the input shaft of a reducer by the prime mover is its input horsepower. It is often used as a selection basis for power transmission components, and it appears in the rating tables of drive manufacturer’s published data. Remember that input horse-power ratings represent the maximum amount of power that the reducer can safely handle

INVOLUTE TEETH :

Of spur gears, helical gears and worms are those in which the active portion of the profile in the transverse plane is the involute of a circle.

KEYWAY :

It is the machined groove running the length of the bore. A similar groove is machined in the shaft and a key fits into this opening.

LONG AND SHORT-ADDENDUM TEETH :

Are those of engaging gears (on a standard designed center distance) one of which has a long addendum and the other has a short addendum.

MECHANICAL RATING :

The maximum power or torque that a speed reducer can transmit, based on the strength and durability of its components, is it’s mechanical rating. Obviously, the reducer may be rated no higher than the strength or durability of its weakest component. Reducers typically have a safety margin of two to three on their mechanical ratings. Thus, a reducer can withstand momentary overloads of 200-300% of its mechanical rating during a startup or other brief overload situations.

MOUNTING POSITION :

The relationship of the input and output shafts relative to the floor line.

NORMAL DIAMETRAL PITCH (Pn) :

It is the value of the diametral pitch as calculated in the normal plane of a helical gear or worm.

NORMAL PLANE :

It is the plane normal to the tooth surface at a pitch point and perpendicular to the pitch plane. For a helical gear this plane can be normal to one tooth at a point laying in the plane surface. At such point, the normal plane contains the line normal to the tooth surface and this is normal to the pitch circle.

NORMAL PRESSURE ANGLE (øn) :

In a normal plane of helical tooth.

OUTPUT HORSEPOWER :

The amount of power avail-able at the output shaft of a reducer is its output horsepower. Due to losses caused by inefficiency, output horsepower is always less than input horse-power.

OUTSIDE DIAMETER (Do) :

It is the diameter of the addendum (outside) circle.

OVERHUNG LOAD :

The input or the output shaft of a speed reducer can be subject to an overhung load; that is, to a force applied at right angles to the shaft, beyond its outermost bearing. Such a force is a shaft bending load resulting from a gear, pulley, sprocket or other external drive member. Besides the tendency to bend the shaft, the overhung load (that is, the radial force on the shaft) is reacted to by the shaft in it’s bearings. Therefore, the overhung load creates loads that the bearings must be able to support without damage.

PITCH CIRCLE :

It is the circle derived from a number of teeth and a specified diametral or circular pitch. Circle on which spacing or tooth profiles is established and from which the tooth proportions are constructed.

PITCH CYLINDER :

It is the cylinder of diameter equal to the pitch circle.

PINION :

It is a machine part with gear teeth. When two gears run together, the one with the smaller number of teeth is called the pinion.

PITCH DIAMETER (D) :

It is the diameter of the pitch circle. In parallel shaft gears, the pitch diameters can be determined directly from the center distance and the number of teeth.

PRESSURE ANGLE (ø) :

It is the angle at a pitch point between the line of pressure which is normal to the tooth surface, and the plane tangent to the pitch surface. In involutes teeth, pressure angle is often described also as the angle between the line of action and the line tangent to the pitch circle. Standard pressure angles are established in connection with standard gear-tooth proportions.

PRESSURE ANGLE—OPERATING (ør) :

It is determined by the centre distance at which the gears operate. It is the pres-sure angle at the operating pitch diameter.

PRIME MOVER :

The machine that provides power to a drive is its prime mover. The most frequently encountered prime movers include electric motors, internal combustion engines, hydraulic motors and air motors. The type of prime mover used can affect the speed reducer during operation. For example, an electric motor runs relatively smoothly in comparison to an internal combustion engine.

ROOT DIAMETER (Dr) :

It is the diameter at the base of the tooth space.

SERVICE FACTORS :

Numbers which modify the loads which must be considered in selecting a speed reducer are called service factors. They vary with the type of service in which the reducer is to be used, the kind of prime mover involved and the duty cycle. The service factor can be a multiplier applied to the known load, which redefines the load in accordance with the conditions at which the drive will be used, or it can be a divisor applied to catalogue reducer ratings, thus redefining the rating in accordance with drive conditions. The service factor is usually applied to the speed reducer, but can also be applied to the name plate rating of the prime mover.

THERMAL RATING :

The maximum power or torque that a speed reducer can transmit continuously, based on its ability to dissipate heat generated by friction, is called its thermal rating.

TIP RELIEF :

It is an arbitrary modification of a tooth profile whereby a small amount of material is removed near the tip of the gear tooth.

THRUST LOAD :

Forces imposed on a shaft parallel to the shaft axis. Such a force is called a thrust load. It is often encountered on shafts driving mixers, fans, blowers and similar machines. When a thrust load acts on a speed reducer, you must be sure that the thrust load rating of the reducer is high enough that it’s shafts and bearings can absorb the load.

UNDERCUT :

It is a condition in generated gear teeth when any part of the fillet curve lies inside a line drawn tangent to the working profile at its point of juncture with the fillet.

WHOLE DEPTH (ht) :

It is the total depth of a tooth space, equal to addendum plus addendum, equal to the working depth plus variance.

WORKING DEPTH (hk) :

It is the depth of engagement of two gears; that is, the sum of their addendums.