GEAR TERMINOLOGY

CONTENTS

TERMINOLOGY

MANUFACTURING

ERRORS

GEAR AND PINION

Gears are the most common means of transmitting power in mechanical engineering.

Gear or wheel. The larger of two interacting gears.

Pinion. The smaller gear in a pair.

Base circle : It is a circle from which involute form is generated. Only the base circle on a gear is fixed and unalterable.

Base pitch (pb): It is the distance from one face of a tooth to the corresponding face of an adjacent tooth on the same gear, measured along the base circle. Sometimes called the 'normal pitch'.

BASE CIRCLE

Pitch is the distance between a point on one tooth and the corresponding point on an adjacent tooth.

Pitch circle. A circle, centered on and perpendicular to the axis, and passing through the pitch point. Sometimes also called the 'pitch line', although it is a circle.

Pitch diameter (D). Diameter of a pitch circle. The nominal gear size is usually the pitch diameter.

PITCH CIRCLE

Circular pitch (p). The distance from one face of a tooth to the corresponding face of an adjacent tooth on the same gear, measured along the pitch circle.

Diametral pitch (Pd). The ratio of the number of teeth to the pitch diameter.

Module (m). The module of a gear is equal to the pitch diameter divided by the number of teeth.

ADDENDUM AND DEDENDUM

Addendum : The radial distance from the pitch surface to the outermost point of the tooth.  

Dedendum : The radial distance from the depth of the tooth to the pitch surface.

A point of contact is any point at which two tooth profiles touch each other.

The pitch point is the point of tangency of two pitch circles and is on the line of centers.

POINT OF CONTACT

The line of action is

also called path of action.

It is the straight line passing through the pitch point and tangent to both base circles.

LINE OF ACTION

LANDS

Bottom land is the surface at the bottom of a gear tooth space adjoining the fillet.

Top land is the (sometimes flat) surface of the top of a gear tooth.

ANGLES ASSOCIATED WITH GEARS

Helix angle (ψ). The angle between a tangent to the helix and the gear axis. It is zero in case of a spur gear.

Lead angle (λ). The angle between a tangent to the helix and a plane perpendicular to the axis. It is the complement of the helix angle which is usually given for helical gears.

Pressure angle (ø). The complement of the angle between the direction, that the teeth exert force on each other, and the line joining the centers of the two gears. The pressure angle is a constant for a given gear.

MANUFACTURING OF GEARS

The gears (gear teeth) are generally made by one of the

following 2 methods.

1. Reproducing method, in which the cutting tool is a formed involve cutter, which forms the gear teeth profiles by reproducing the shape of the cutter itself. In this method, each tooth space is cut independently of the other tooth spaces.

2. Generating method, in which the cutting tool (hob) forms the profiles of several teeth simultaneously during constant relative motion of the tool and blank.

SOURCES OF ERRORS

The various sources of errors in the gear made by reproducing method be due to

1. incorrect profile on the cutting tool,

2. incorrect positioning of the tool in relation to the work and,

3. incorrect indexing of the blank.

SOURCES OF ERRORS

The sources of error in the gear made by generating method are:

1. Errors in the manufacture of the cutting tool

2. Errors in positioning the tool in relation to the work and

3. Errors in the relative motion of the tool and blank during the generating operation.

GEAR ERRORS

Pitch error It is the difference between actual and design pitch.

  Cyclic error

Error occurs in each revolution of gear.

Undulation It is the periodical departure of the actual tooth surface from the design surface.

GEAR ERRORS

Run outTotal range of a fixed indicator with the contact points applied to a surface rotated, without axial movement, about a fixed axis.

  Periodic error

  Error occurs at regular intervals.

Profile errorThe maximum distance of any point on the tooth profile form to the design profile.

GEAR ERRORS

Radial run out 

Run out is measured along a perpendicular to the axis of rotation.

   Axial run out

  Run out is measured parallel to the axis of rotation.

Eccentricity

It is the half radial run out.

BACKLASH

Backlash is the error in motion that occurs when gears change direction.

It exists because there is always some gap between the trailing face of the driving tooth and the leading face of the tooth on the driven gear, and that gap must be closed before force can be transferred in the new direction.

REFERENCE

Engineering MetrologyR.K. jain

www.wikipedia.org

www.gearshub.com