Technical Documents  Documentos Técnicos: Gear Terminology
addendum: The radial distance between the top land and the pitch circle.
addendum circle: The circle defining the outer diameter of the gear.
circular pitch: The distance along the pitch circle from a point on one
tooth to a corresponding point on an adjacent tooth. It is also the sum of
the tooth thickness and the space width, measured in inches or millimeters.
clearance: The radial distance between the bottom land and the clearance
circle.
contact ratio: The ratio of the number of teeth in contact to the number
of those not in contact.
dedendum circle: The theoretical circle through the bottom lands of a
gear.
dedendum: The radial distance between the pitch circle and the dedendum
circle.
depth: A number standardized in terms of pitch. Fulldepth teeth have a
working depth of 2/P. If the teeth have equal addenda (as in standard
interchangeable gears), the addendum is 1/P. Fulldepth gear teeth have a
larger contact ratio than stub teeth, and their working depth is about 20%
more than that of stub gear teeth. Gears with a small number of teeth
might require undercutting to prevent one interfering with another during
engagement.
diametral pitch (P): The ratio of the number of teeth to the pitch diameter.
A measure of the coarseness of a gear, it is the index of tooth size
when U.S. units are used, expressed as teeth per inch.
pitch: A standard pitch is typically a whole number when measured as a
diametral pitch (P). Coarsepitch gears have teeth larger than a diametral
pitch of 20 (typically 0.5 to 19.99). Finepitch gears usually have
teeth of diametral pitch greater than 20. The usual maximum fineness is
120 diametral pitch, but involutetooth gears can be made with diametral
pitches as fine as 200, and cycloidal tooth gears can be made with diametral
pitches to 350.
pitch circle: A theoretical circle upon which all calculations are based.
pitch diameter: The diameter of the pitch circle, the imaginary circle
that rolls without slipping with the pitch circle of the mating gear, measured
in inches or millimeters.
pressure angle: The angle between the tooth profile and a line perpendicular
to the pitch circle, usually at the point where the pitch circle and
the tooth profile intersect. Standard angles are 20 and 25º. The pressure
angle affects the force that tends to separate mating gears. A high pressure
angle decreases the contact ratio, but it permits the teeth to have
higher capacity and it allows gears to have fewer teeth without undercutting.
Gear Dynamics Terminology
backlash: The amount by which the width of a tooth space exceeds the
thickness of the engaging tooth measured on the pitch circle. It is the
shortest distance between the noncontacting surfaces of adjacent teeth.
gear efficiency: The ratio of output power to input power, taking into
consideration power losses in the gears and bearings and from windage
and churning of lubricant.
gear power: A gear’s load and speed capacity, determined by gear
dimensions and type. Helical and helicaltype gears have capacities to
approximately 30,000 hp, spiral bevel gears to about 5000 hp, and worm
gears to about 750 hp.
gear ratio: The number of teeth in the gear (larger of a pair) divided by
the number of teeth in the pinion (smaller of a pair). Also, the ratio of the
speed of the pinion to the speed of the gear. In reduction gears, the ratio
of input to output speeds.
gear speed: A value determined by a specific pitchline velocity. It can be
increased by improving the accuracy of the gear teeth and the balance of
rotating parts.
undercutting: Recessing in the bases of gear tooth flanks to improve
clearance.
Gear Classification
External gears have teeth on the outside surface of a disk or wheel. Internal gears have teeth on the inside surface of a cylinder.
Spur gears are cylindrical gears with teeth that are straight and parallel to
the axis of rotation. They are used to transmit motion between parallel
shafts.
Rack gears have teeth on a flat rather than a curved surface that provide
straightline rather than rotary motion.
Helical gears have a cylindrical shape, but their teeth are set at an angle
to the axis. They are capable of smoother and quieter action than spur
gears. When their axes are parallel, they are called parallel helical gears,
and when they are at right angles they are called helical gears.
Herringbone and worm gears are based on helical gear geometry.
Herringbone gears are double helical gears with both righthand and
lefthand helix angles side by side across the face of the gear. This geometry
neutralizes axial thrust from helical teeth.
Worm gears are crossedaxis helical gears in which the helix angle of
one of the gears (the worm) has a high helix angle, resembling a screw.
Pinions are the smaller of two mating gears; the larger one is called the
gear or wheel.
Bevel gears have teeth on a conical surface that mate on axes that intersect,
typically at right angles. They are used in applications where there are
right angles between input and output shafts. This class of gears includes
the most common straight and spiral bevel as well as the miter and hypoid.
Straight bevel gears are the simplest bevel gears. Their straight teeth
produce instantaneous line contact when they mate. These gears provide
moderate torque transmission, but they are not as smooth running
or quiet as spiral bevel gears because the straight teeth engage with
fullline contact. They permit medium load capacity.
Spiral bevel gears have curved oblique teeth. The spiral angle of curvature
with respect to the gear axis permits substantial tooth overlap.
Consequently, teeth engage gradually and at least two teeth are in contact
at the same time. These gears have lower tooth loading than
straight bevel gears, and they can turn up to eight times faster. They
permit high load capacity.
Miter gears are mating bevel gears with equal numbers of teeth and
with their axes at right angles.
Hypoid gears are spiral bevel gears with offset intersecting axes.
Face gears have straight tooth surfaces, but their axes lie in planes perpendicular
to shaft axes. They are designed to mate with instantaneous
point contact. These gears are used in rightangle drives, but they have
low load capacities.
Designing a properly sized gearbox is not a simple task and tables or
manufacturer’s recommendations are usually the best place to look for
help. The amount of power a gearbox can transmit is affected by gear
size, tooth size, rpm of the faster shaft, lubrication method, available
cooling method (everything from nothing at all to forced air), gear materials,
bearing types, etc. All these variables must be taken into account to
come up with an effectively sized gearbox. Don’t be daunted by this. In
most cases the gearbox is not designed at all, but easily selected from a
large assortment of offtheshelf gearboxes made by one of many manufacturers.
Let’s now turn our attention to more complicated gearboxes
that do more than just exchange speed for torque.
Worm Gears
Worm gear drives get their name from the unusual input gear which
looks vaguely like a worm wrapped around a shaft. They are used primarily
for high reduction ratios, from 5:1 to 100s:1. Their main disadvantage
is inefficiency caused by the worm gear’s sliding contact with the
worm wheel. In larger reduction ratios, they can be self locking, meaning
when the input power is turned off, the output cannot be rotated. The following
section discusses an unusual double enveloping, internallylubricated
worm gear layout that is an attempt to increase efficiency and the
life of the gearbox.
