Gear Nomenclature

Design Parameters

Gear Terminology

What is a gear ?Gear is a toothed wheel used to - Change direction in which a force is applied. - Increase / reduce the force or distance over which

force is applied.- Transmit motion from one to another shaft when

meshed with another toothed wheel.

Basic types of gears:

Spur - external & internal

Helical - external & internal

Bevel - straight & spiral

Worm & Rack

Splines - Straight & involute

Gear Terminology

Spur Gear

Internal Spur

Gear Terminology

Spur gears : Spur gears connect parallel shafts, have

involute teeth that are parallel to the shafts, and can have either internal or external teeth.

Notes:

•Spur gears are inexpensive to manufacture.

•They cause no axial thrust between gears.

•They give lower performance, but may be satisfactory in low speed or simple applications

Gear Terminology

Helical Gear

Gear Terminology

Helical gears :

Helical gears also connect parallel shafts, but the involute teeth are cut at an angle (called the helix angle) to the axis of rotation. Note that two mating helical gears must have equal helix angle but opposite hand. These are found in automotive transmissions, and any application requiring high speed rotation and good performance. Notes:

•Helical gears run smoother and more quietly than spurs (due to continuous tooth mating).

•They have a higher load capacity (teeth have a greater cross section).

•They are more expensive to manufacture.

•Helical gears create axial thrust.

Gear Terminology

Bevel Gears Straight & SpiralGear Terminology

Bevel gears :

Bevel gears connect intersecting axes, and come in several types. For bevel gears, the pitch surface is a cone, (it was a cylinder in spur and helical gears) and

Mating spiral gears can be modeled as two cones in rolling contact.

Types of bevel gears:

Straight bevel: These are like spur gears, the teeth have no helix angle. Spiral bevel gears: Teeth have a spiral angle which gives performance improvements much like helical gears

Gear Terminology

Spiral bevel GearGear Terminology

Hypoid Gear Gear Terminology

Hypoid gears :

Similar to spiral bevel gears, but connect non-parallel shafts that do not intersect. The pitch surface of a hypoid gear is a hyperboloid of revolution (rather than a cone, the pitch surface in bevel gears), hence the name. Hypoid pinions (the smaller driving gear) are stronger than spiral bevel pinions because the helix angle of the pinion is larger than that of the gear. Hypoid gears are stronger, operate more quietly, and can be used for higher reduction ratios than spiral bevel gears. They also have sliding action along the teeth, potentially reducing efficiency. Hypoid gears are found in auto differentials.

Hypoid tooth profile has unequal pressure angles & unequal profile cirvatures on the two sides of the tooth.

Below center offset-pinion has LH spiral.

Above center offset-pinion has RH spiral.

Gear Terminology

Worm & Worm Gear Gear Terminology

Worm Gears :

The driving gear is called a worm, and typically has 1, 2, or four teeth. The low number of teeth on the worm can result in a very large velocity ratio. Can carry high loads. Because of sliding action, efficiency is low.

Gear Terminology

Rack and Pinion :

These transmit rotary motion (from the pinion) to translational motion (of the rack). The rack is a gear with infinite radius; its teeth, although flat sided, are involute. The rack and pinion is commonly used in steering units and jacks.

Gear Terminology

Evaluation Of Gear TypesType Q. Rating Feature Application Comments

Spur Excellent Parallel shaft,high efficiency

All types,wide range ofV. Ratio

Simplest,low cost.Precise

Helical Good Parallel shaft,Very highspeed & loads

High speedtypes,

Complicatedthan Spurbut superiorin load,precise

Bevel Good-Fair IntersectingShaft, HighSpeed & loads

Right-anglemeshes,

Complex,limitation inachievingprecision

Worm Good-Fair Right-angleskew shaft,high V. ratio

High V. ratio& loads,Angularmeshes

Best choicefor highratio & rightangle drive

Gear Terminology

ModuleA unit of metric measurement indicating the size of the pitch of a gear.It relates to the ratio of the pitch diameter to the number of teeth, thus the module of a gear = pitch diameter (in millimetres) divided by the number of teeth in the gear.Module pitch is an actual dimension,

whereas Diametral Pitch is only a ratio. Normal Module: This is the value of the

module in the Normal Plane,

usually measured on the pitch line.

Transverse Module: This is the value of the

module in the Transverse Plane, usually

measured on the pitch line.

Gear Terminology

Gear Terminology

Addendum Modification Coefficient (x) = The amount by which the reference line is offset from the reference diameter when divided by the module mn. i.e., x = addendum modification/mn

Long addendum - a gear with a positive addendum modification coefficient (x) is said to have a 'long addendum'. Its value is defined as (1+x)mn

Short addendum - a gear with a negative addendum modification coefficient (x) is said to have a 'short addendum'. Its value is defined as (1-x)mn

Gear Terminology

Gear Terminology

Gear Terminology

Gear NomenclatureGear Terminology

Backlash

Gear Terminology

Gear Terminology

Gear Manufacturing

Design ParametersNo. of Teeth

Module / DP

Pressure Angle

Helix angle

Add. Modifications (If any)

Outer Diameter

Root Diameter

Pitch Circle Diameter

Base Circle Diameter

TIF Diameter

Semi-topping & Angle

Root Radius

DOP Or Span Over teeth

Mating Part Details (No. Of Teeth / Backlash/ C. D.)

Protuberance Details

Gear Manufacturing Processes

Quality Parameters

Quality Standard & Class

Profile Error

Lead Error

Lead Crowning

Tooth To Tooth Composite Error

Total Composite Error

PCD Runout

Surface Finish on Ground Areas

Type Of Fit & Class (For Splines)

K-Graph (Modified Profile i.e. tip relief, etc..)

K-Graph (Modified Lead i.e. crowned, tapered)

Gear Manufacturing

Control Parameters on a gear:

DOP or DIP: Measurement of circular tooth thickness or space width at pitch circle with help of measuring pins.

PCD run-out: The difference between maximum & minimum radial distance of pitch diameter from the gear axis.

Helix: Axial tooth trace of a gear flank along the face width.Profile: Locus of a point on straight line when rotated

around the base circle. Tooth to tooth Composite Error: Value of radial composite

deviation corresponding to one pitch (360° / Z), during one cycle of rotation in mesh with mating gear.

Total Composite Error: The difference between the maximum & minimum values of working center distance, during one rotation of work gear in tight mesh with mating gear.

Gear Manufacturing

Parameters Functional Importance

DOP or DIP: Tooth size

PCD run-out: Concentricity errors

Helix & Profile: Tooth bearing / Contact

Composite Errors: Pitch / CD variation

AGMA

Quality No.

Apprx. DIN

No.

Quality

Classification

Typical Application

4 125 116 1078 9-109 8-910 7-811 6-712 5-613 4-514 3-415 2-316 217 1

Ultra-Precision

Gear Quality Classification

Hand tools, Pumps, Clocks, Farm

machines, hoists,

Military navigation, Computer

equipments

Commercial

Precision

Turbines, Automotive, High-speed machines, Aircraft Engines

END