Machine Design 2 Terms

Gear

A rotating machine part having cut teeth or cogs that mesh with another toothed part to transmit torque.

Transmission

Two or more gears working together in tandem to produce mechanical advantage through a gear ratio.

Mechanical advantage

The advantage produced when gears with unequal numbers of teeth are combined, affecting rotational speed and torque.

Gear ratio

The number of teeth in the gear divided by the number of teeth in the pinion.

Spur gear

The most common type of gear, having straight teeth and usually mounted on parallel shafts.

External spur gear

A gear with teeth formed on the outer surface of a cylinder or cone.

Internal spur gear

A gear with teeth formed on the inner surface of a cylinder or cone.

Annular gear

The larger gear in an internal spur gear arrangement.

Helical gear

A cylindrical gear with helicoidally shaped teeth; also called a twisted spur gear.

Twisted spur gear

Another name for a helical gear.

Double helical gear

A gear having two sets of helical teeth arranged in a V shape to cancel axial thrust.

Herringbone gear

Another name for a double helical gear.

Rack and pinion

A gear arrangement used to transform rotary motion into linear motion and vice versa.

Rack

A straight toothed member that meshes with a round pinion to produce linear motion.

Pinion

The smaller gear when two gears are in mesh.

Pin gearing

A type of gearing in which the distance from the center of each gear to the point of contact changes as the gears rotate, causing the gear ratio and output speed to vary.

Bevel gear

A gear used to connect intersecting shafts and change the direction of shaft rotation, usually with shafts approximately 90 degrees apart.

Plain bevel gear

A bevel gear having straight teeth parallel to the generators of the cone.

Straight bevel gear

The simplest form of bevel gear, having straight teeth and resembling a conical spur gear.

Crown gear

A gear whose teeth project at right angles to the plane of the wheel and resemble the points of a crown.

Spiral bevel gear

A bevel gear having spiral teeth with a helical angle that engage progressively across the tooth.

Hypoid gear

A gear similar to a spiral bevel gear but with shaft centerlines that do not intersect; commonly found in automobile differentials.

Hyperboloidal gear

A gear system using two hyperboloids of revolution to provide transmission between two skew axes.

Worm

A gear having one tooth in the form of a screw thread.

Worm wheel

A helical gear with inclined teeth designed to engage with a thread-like worm.

Worm gearing

A gear arrangement in which a worm drives a worm wheel.

Worm and wheel helical gears

A gearing arrangement where the worm always drives the worm wheel and the mechanism operates very quietly.

Face gear

A circular disc with a ring of teeth cut on one side, with teeth tapered toward the center.

Double enveloping worm gear

A worm gear having a radially changing pitch diameter that increases the number and amount of tooth shear area.

Non-circular gear

A gear designed for special purposes such as ratio variations and axle displacement oscillations.

Epicyclic gear

A planetary gearing system consisting of one or more planet gears revolving about a central sun gear.

Planetary gearing

Another term for epicyclic gearing.

Planet gear

An outer gear in an epicyclic system that revolves around the central sun gear.

Sun gear

The central gear around which the planet gears revolve in an epicyclic gear system.

Carrier

A movable arm on which the planet gears of an epicyclic system are mounted.

Annulus

The outer ring gear of an epicyclic gearing system that meshes with the planet gears.

Sun and planet gear

A gearing mechanism associated with converting reciprocal motion into rotary motion in steam engines.

Harmonic drive

A specialized proprietary gearing mechanism.

Harmonic drive gear

A specialized gearing mechanism that uses components such as a circular spline, flexspline, and wave generator.

Cage gear

A gear having cylindrical rods as teeth arranged in a circle around the axle.

Lantern gear

Another name for a cage gear.

Lantern pinion

Another name for a cage gear having cylindrical rods for teeth.

Idler gear

The intermediate gear in a gear train arrangement.

Polyethylene

An inexpensive thermoplastic widely used for bottles and other containers.

Nylon

An expensive thermoplastic with load-carrying capacity up to 250°F, low coefficient of friction, and good wear resistance; used for bearings and may be machined or molded.

Zytel

A trade name for nylon resins.

Teflon

Tetrafluoroethylene; a relatively expensive material processed more like powder metallurgy and capable of making a tough bearing with a low dry coefficient of friction.

Tolerance

The permissible variation in the size of a dimension.

Nominal size

The closer the dimension must be to precisely 2.5 in.

Bilateral

The size of a part is permitted to be either larger or smaller than the given dimension.

Unilateral

The size of a part may be larger only, or smaller only, than the given dimension.

Allowance

The minimum specified difference between the dimensions of the pin or male part and the hole or female part.

Interference of metal (negative allowance)

The pin is larger than the hole.

Pitch circle

The basis of measurement of gears.

Pitch diameter

The size of a gear is its pitch-circle diameter in inches.

Pitch cylinder (pitch surface)

The pitch circle is the trace of the pitch cylinder as it intersects a plane normal to the axis.

Pitch point

The point of tangency of the pitch circles of meshing gears; for an individual gear, it is where the tooth profile cuts the standard pitch circle.

Addendum circle (outside circle)

The circle that bounds the outer ends of the teeth.

Addendum cylinder

Encloses a gear.

Addendum

The radial distance between the pitch circle and the addendum circle.

Dedendum circle (root circle)

The circle that bounds the bottoms of the teeth.

Dedendum

The radial distance from the pitch circle to the root circle, or to the bottom of the tooth space.

Whole depth

Equal to the addendum plus the dedendum.

Working depth

The radial distance from the addendum circle to the working-depth circle; it is the sum of the addendums of mating gears.

Clearance

The radial distance between the working-depth circle and the root circle; it is the dedendum minus the mating addendum.

Circular thickness (tooth thickness)

The width of the tooth measured along the pitch circle.

Chordal thickness

Tooth width measured along the chord at the pitch circle.

Width of space (tooth space)

The space between teeth measured along the pitch circle.

Backlash

The tooth space minus the circular thickness.

Face width

The length of teeth in an axial direction.

Face of the tooth

The surface of the tooth between the pitch cylinder and the addendum cylinder.

Flank

The surface of the tooth between the pitch and root cylinders.

Top land

The surface of the top of the tooth.

Bottom land

The surface of the bottom of the tooth space.

Angle of action

The angle through which the gear turns from the time a particular pair of teeth comes into contact until they go out of contact.

Arc of action

Subtends the angle of action.

Angle of approach

The angle through which the gear turns from the time a particular pair of teeth comes into contact until they are in contact at the pitch point.

Angle of recess

The angle through which the gear turns from the time a given pair of teeth are in contact at the pitch point until they pass out of mesh.

Velocity ratio

The angular velocity of the driver divided by the angular velocity of the driven gear.

Base circle

The circle from which the involute is generated.

Degree of involute

In gearing, this expression is used to define the base circle for a particular pitch circle.

Pressure angle

The angle between the direction of the force acting normal to the tooth profiles and the tangent to the pitch circles.

Pressure of obliquity

Another term for the pressure angle.

Line of action

All points of contact between mating teeth properly lie somewhere on this line, which is also the generating line.

Spacing, size

The pitch of a gear is a measure of the spacing, and usually also of the size, of the tooth.

Circular pitch

The distance in inches measured along the pitch circle from a point on one tooth to the corresponding point on an adjacent tooth.

Diametral pitch

The ratio of the number of teeth per inch of pitch diameter.

Base pitch

The distance in inches measured along the base circle from a point on one tooth to the corresponding point on an adjacent tooth.

Pitch angle

The angle subtended by an arc on the pitch circle equal in length to the circular pitch.

Length of action

The length of that part of the line of action between the initial and final points of contact.

Contact ratio

The ratio of the angle of action to the pitch angle, length of action to base pitch, or arc of action to circular pitch.

Law of gearing

For a pair of engaging teeth to transmit a constant velocity ratio, the common normal to the profiles at the point of contact must always pass through the pitch point.

Conjugate curves

Curves that satisfy the law of gearing.

Interference

Nonconjugate action.

Carefully cut gears

Represents the results that should be obtained with accurately ground hobs or shaping cutters.

First-class commercial gears

Represents the results that might be expected of gears cut by form cutters.

Precision gears

Represents the degree of accuracy expected in carefully ground or shaved teeth or teeth cut with extreme care using accurately ground tools.

Pitting

Surface fatigue failure caused by high contact stresses.

Abrasion

Wear caused by foreign matter such as grit or metal particles, or by failure of the oil film at low speed.

Scoring

Occurs when the oil film fails and high load and speed cause the surface metal to melt and smear down the profile.