Gear with Pinion

Warm and Warm wheel

Spur Gear Terms

& Calculations

• Three spur gear tooth forms are generally used with pressure angles of 14½, 20, and 25 degrees. The 14½ degree tooth form is being replaced and made obsolete by the 20 and 25 degree forms. Figure 1. illustrates these three pressure angles as applied to a gear rack with all teeth being the same depth. The larger pressure angle makes teeth with a much larger base, which makes these teeth much stronger and also allows the production of gears with fewer teeth. Any two gears in mesh with each other must be of the same pressure angle. When gear tooth measurements are to be made with gear tooth calipers, the chordal tooth thickness and the chordal addendum must be calculated.

• Addendum.The radial distance from the pitch circle to the outside diameter.• Dedendum.The radial distance between the pitch circle and the root

diameter.• Circular thickness.The distance of the arc along the pitch circle from one

side of a gear tooth to the other.• Circular pitch.The length of the arc of the pitch circle from one point on a

tooth to the same point on the adjacent tooth.• Pitch diameter.The diameter of the pitch circle.• Outside diameter.The major diameter of the gear.• Root diameter.The diameter of the root circle measured from the bottom of

the tooth spaces.• Chordal addendum.The distance from the top of the tooth to the chord

connecting the circular thickness arc.• Chordal thickness.The thickness of a tooth on a straight line or chord on

the pitch circle.• Whole depth.The total depth of a tooth space equal to the sum of the

addendum and dedendum.• Working depth.The depth of engagement of two mating gears.• Clearance.The amount by which the tooth space is cut deeper than the

working depth• .Backlash.The amount by which the width of a tooth space exceeds the

thickness of the engaging tooth on the pitch circles.• Diametral pitch.The number of gear teeth to each inch of pitch diameter.• Pressure angle.The angle between a tooth profile and a radial line at the

pitch circle.• Center distance.The distance between the centers of the pitch circles.

Coupling

• A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.

• The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end movement or both. By careful selection, installation and maintenance of couplings, substantial savings can be made in reduced maintenance costs and downtime.

Uses

• Shaft couplings are used in machinery for several purposes, the most common of which are the following.[1]

• To provide for the connection of shafts of units that are manufactured separately such as a motor and generator and to provide for disconnection for repairs or alternations.

• To provide for misalignment of the shafts or to introduce mechanical flexibility.

• To reduce the transmission of shock loads from one shaft to another.

• To introduce protection against overloads. • To alter the vibration characteristics of rotating units.