L. Pradhapan, TPL Pune

Backlash measurement in Gears/ Gear Trains

What is Backlash?

The amount of clearance between mated gear teeth. It can be seen when the direction of

movement is reversed and the slack or lost motion is taken up before the reversal of motion is

complete.

Why back lash is required?

Minimum amount of backlash is maintained to prevent jamming. Reasons for the presence of

backlash include allowing for lubrication, manufacturing errors, deflection under load, and thermal

expansion.

What are the actors affecting backlash?

Factors affecting the amount backlash required in a gear train include errors in profile, pitch,

tooth thickness, helix angle and center distance, and run-out. The greater the accuracy the smaller

the backlash needed. Backlash is most commonly created by cutting the teeth deeper into the gears

than the ideal depth. Another way of introducing backlash is by increasing the center distances

between the gears.

L. Pradhapan, TPL Pune

Backlash pertaining to gear train:

In a gear train, backlash is cumulative. When a gear-train is reversed the driving gear is turned a

short distance, equal to the total of all the backlashes, before the final driven gear begins to rotate.

How backlash is measured?

Backlash is measured normal (perpendicular) to the tooth surface on pitch circle with the help of

a dial indicator as shown in below. Backlash is measured at pinion while holding the gear rigid.

Case Study: Measurement of backlash of pinion of gearbox used in Sugar Industry.

Initial information received from Client: Pinion was checked by client at their premises and backlash

was found 0.19mm against requirement of 0.4 to 0.66mm. Since the backlash was less, pinion was

kept unused for want of clarification.

Investigation by TPL on 08.07.14 at client premises and the observations are as below

The method followed by client for the measurement of backlash was wrong. They checked as

follows.

Pinion with shaft assembly Backlash measured on pinion shaft end

L. Pradhapan, TPL Pune

Backlash was measured at pinion shaft end (as shown in above figure) instead of at pinion gear

tooth, which resulted in backlash of 0.19mm and the pinion was kept unused.

Subsequently, the backlash was measured at pinion normal to gear tooth and observed backlash

was 0.59mm which was within acceptable limit of 0.4 to 0.66mm. Method of measurement is shown

below.

Backlash measurement at pinion gear tooth

Conclusion: Even though angle of deflection is same, backlash varied due to variation in pinion pitch

circle diameter and shaft diameter. Since pinion shaft diameter is less in this case compared to pitch

circle diameter of pinion (shown in below figure), backlash measured at shaft end is less compared

to backlash measured at pinion tooth. Hence, backlash measurement of pinion at correct location is

very important and measurement at wrong location resulted in almost rejection of acceptable

pinion.

Backlash measured at pinion tooth

(cord length is more)

Backlash measured at pinion shaft

end (cord length is less)

Diameter of pinion shaft

Pitch circle diameter of pinion