EME 22342 : Unit 4 4.5 Gear Measurement

 Gear Measurement Introduction:

1. Introduction
·        Gears are mechanical elements generally circular in shape with number of evenly spaced teeth on its periphery.
·         Sometimes they may be in a straight form. They are mostly used in power or motion transmission when the distance between two shafts is less.
·         They are used in automobiles, machine tools and equipments, wrist watches, dial indicator, etc.
The gears are available in various types:
·         Spur gear  b. Helical gear  c. Bevel gear  d. Worm and worm wheel
·         The most commonly used form of gear teeth are: ·         Involute   ·         Cycloidal.
·         These have been already studied in the engineering drawing in the first year.
 ·         It is necessary to measure the various parameters of gears which are used in precision machines and devices.

Terminology of Spur Gear

·         Involute: It is the curve obtained by the locus of a point on straight line which rolls around a circle (fixed) without slipping. OR      It is the locus of a point on a piece of string which is unwanted from a stationary cylinder.
  Cycloid: It is the curve obtained by the locus of point on circle which rolls on a fixed straight line without slipping.

 Base circle: It is the circle from which involute form is generated. It is fixed and unalterable.

·         Pitch circle: It is the imaginary circle useful for design calculation.

·         Pitch circle diameter (P.C.D.): It is the diameter of circle which by pure rolling action would produce the same motion as the toothed gear wheel.

·         Module: It is the ratio of P.C.D. of gear to the number of teeth (N) module =P.C.D./N (unit mm)

·         Circular pitch: It is the arc distance measured around the pitch circle from the flank of one tooth to the flank on the next tooth.

·         Tooth thickness: It is the arc distance measured along the pitch circle from the intercept with one flank to its intercept with the other flank of the same tooth.

 

Backlash: It is the amount by which tooth space exceeds the thickness of an engaging tooth.

       Important parameters of gears for measurement

i.     i.Checking of involute profile of gear tooth.
ii. Measurement of tooth thickness.
iii. Measurement of circular pitch.
iii. Measurement of backlash, lead, Runout

       Analytical and functional testing of gears

·         In analytical method of gear testing, the various geometrical features are checked as well as

·         other parameters like hardness, alignment, etc. are also checked.

·         It involves checking of

·         i. Concentricity of gear tooth.

·         ii. Alignment of each tooth.

·         iii. Shape of gear tooth (profile).

·         iv. Diameters of gear.

·         This method is time consuming and it is not generally preferred in industries. It is useful for research

·         and development work in industries or laboratories.

   2 Functional testing (Rolling test)

It is the testing of the gear for actual use. It involves gear checking when it meshes with another gear

during power or motion transmission.

In this testing, the master or standard gear to check the errors present in the gear.

The testing gives idea about backlash, composite errors, pitch errors, etc.

This test is also called as “Rolling test” of gears.

     Rolling Test of Gears

• In this test, the gear to be tested is compared with a hardened and ground master gear.
• This test is carried out on a Parkinson gear tester machine. 
• It requires less time and gives accurate results.
• As the name indicates, the gear under test is rolled with master gear to find out the errors in tooth form, pitch errors, etc 

Questions

                1)  Define  a. Module  b. Tooth thickness

                2)  List parameters of gear for measurement

                3) State analytical testing of gears

                4) Explain rolling test of gears