Notes on Gear Measurement

The Base Tangent Method Or David Brown tangent comparator Or Flange Micrometer

• The base tangent length is the distance between two parallel planes which are tangential to the opposite tooth flanks.
•  The number of teeth over which the measurement is to be made for a particular gear is based on its total number of teeth and pressure angle 
•  The purpose of tangent tester is to determine the mean value and variation in base tangent length.

It is measured between two parallel measuring surfaces tangential to unlike gear teeth flank.

• In this method, the span of a convenient number of teeth is measured with the help of the tangent comparator.
• Procedure
• In this method, the span of a convenient No. of teeth is measured by David brown tangent comparator. 
• The anvils of the comparator are first set of the base tangent length with the help of slip gauges . 
• Then, slip gauges are replaced by the gear which is to be measured. Reading are taken by the comparator.

Parkinson’s Gear Tester

Principle: It works on the principle of rolling test of gears with master gear.

Construction

The figures show Parkinson’s gear tester used for testing of spur gears. (They are also designed for bevel, helical and worm gears.

It mainly consists of

i. Master gear: A standard year known as master gear is mounted on a fixed vertical spindle. The spindle is mounted on a adjustable carriage whose position can be adjusted.

ii. Gear under test: The gear under test is mounted on a floating spring loaded carriage so that the master gear can mesh with the gear under best.

iii. Spring: The spring is provided between the floating carriage and the vertical carriage of master gear to maintain constant contact of both the gears.

iv. Dial indicator: The dial indicator is provided with electronic recorder to measure the error present in the gear.The distance of two spindles can be adjusted to accommodate

the various sizes of gears under test.

Working

• Initially the gear under test is mounted on vertical spindle and meshed with master gear.
• The gear under test is rotated slowly in mesh with the master gear. 
• During rotation, if there is any error present in the tooth form pitch error or runout, then it will cause variation in the canbe distance between master gear and gear under test spindles.
•  Thus, movements of carriage will be transmitted to the comparator and recorder and it will give (dial indicator) Output in the form of profile generated on graph paper as shown in figure.

• It gives combined error including profile, pitch, etc. but do not give individual errors.

Limitations

i. It can test maximum 300 mm diameter gear.

ii. Measurements are directly dependent upon the master gear/standard gear.

iii. It gives composite errors.

iv. There should be low friction and high sensitivity required for reliable performance.

Errors in Gear

•Following are various gear errors that can be checked while analytical inspection is carried out.

1)Profile 2) Spacing 3) Pitch 4) Run out 5) Gear Tooth Thickness 6) Lead 7) Backlash.

The Errors are –

1)Cumulative pitch error 2)The tooth thickness error 3)Cyclic error 4)Run out 5)Radial Run

Checking of Backlash

• Backlash in the gears is the play or clearance between the mating tooth surfaces.
• Backlash in the gear teeth results due to error in profile, pitch thickness of teeth, etc.
• If backlash is very less, it will increase the friction, noise and overheating of gears, and if it is more the power transmission will be poor.
• It is checked with the help of setup shown in the figure. In this set up, one gear is kept fixed and meshing gear is allowed to rotate.
• The dial indicator along with knife edge is provided for observation.

• During backlash measurement, the movable gear is rotated forward and backward to mesh with fixed gear.
• The movement of immovable gear causes the knife edge to move about pivot and other end of knife edge is attached to the dial gauge to show the amount of backlash present it should be 0.02 to 0.03 mm for precision gears.

Composite Error

• It means combination of profile error, pitch error, tooth thickness error, tooth alignment error, axial and radial runout, etc which are responsible for correct functioning of gears.
• The composite test of a gear is a method of inspection in which the work gear is rolled in tight double flank contact with a master gear.
• As the work gear is spring-loaded against the reference gear on the inspection machine. The composite action test is made on an inspection instrument that will allow variation in the center distance during rolling. 
• This variation in center distance will yield a "tooth-to-tooth" and a "total composite" indication that can be read on a simple dial indicator or recorded graphically.
• Composite inspection is useful to determine the general quality of a gear including size, runout, tooth to- tooth rolling action, etc. 
• It is not an appropriate method to determine individual tooth flank errors