Post on 17-Oct-2014
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