Gear Accuracy

5. Gear Accuracy / High accuracy in a gear denotes a gear that will produce less errors.

A gear must work in transmitting rotation/power from one gear axis to another, efficiently and quietly. To improve gear accuracy is to improve the performance of a gear.

Gear accuracy can be loosely classified into 3 types
The datum of gear accuracy is the centerline (gear axis) of a gear
Higher accuracy gears have less errors

Precision in involute tooth profile -> Profile Deviation
Precision in tooth face / tooth trace -> Helix Deviation
Precision in positioning of teeth / tooth-spaces
- Precision in tooth positioning -> Single Pitch Deviation
- Precision in pitch -> Total Cumulative Pitch Deviation
- Variation of the position of a ball inserted in each tooth space, around the gear -> Runout Error of Gear Teeth

Fig.5.1 Gear Accuracy
Fig. 5.1 Gear Accuracy

(Important Gear Terminology and Gear Nomenclature in Fig 5.1)

Tooth trace
Tooth profile
Pitch

Profile Deviation (Fα)
Fig.5.2 total Profile Deviation Fα
Fig. 5.2　Total Profile Deviation Fα

(Important Gear Terminology and Gear Nomenclature in Fig 5.2)

Addendum
Dedendum
Evaluation range
Active length
Usable length
Designed profile
Actual profile

Helix Deviation (Fβ)
Fig.5.3 Total helix deviation Fβ
Fig. 5.3 Total helix deviation Fβ

(Important Gear Terminology and Gear Nomenclature in Fig 5.3)

Evaluation range
Face width
Design helix
Actual helix

Pitch Deviation

The pitch value is measured on a measurement-circle where the center is the gear axis.

(a) Single Pitch Deviation (fpt)
The deviation between actual measurement pitch value and theoretical circular pitch.
(b) Total Cumulative Pitch Deviation (Fp)
Evaluated by measuring the accumulative pitch deviation of the total amount of gear teeth, where the overall amplitude of accumulative pitch error curve is the total cumulative pitch deviation.

Fig.5.4 pitch Deviation
Fig. 5.4　Pitch Deviation

Runout Error of Gear Teeth (Fr)

Runout error is measured by indicating the position of a pin or ball inserted in each tooth space around the gear and taking the largest difference. The values of runout include eccentricity.

Fig.5.5 Runout error of a 16-tooth gear
Fig. 5.5　Runout error of a 16-tooth gear

Total Radial Composite Deviation (Fi)

Tooth profile / Pitch / Tooth space are factors to evaluate gear accuracy by measuring a single gear. There is also another method to evaluate gear accuracy, which is the double flank meshing test method; a measurement of a gear meshed with the master gear. This method measures the variation in the center distance when the gear is rotated one revolution, in a tight mesh with a master gear.
Figure 5.6 is the test result for the 30-tooth gear. It shows 30 small mountain-like waves, representing the tooth-to-tooth radial composite deviation. The value of total radial composite deviation would be similar to the sum of runout error and tooth-to-tooth radial composite deviation.

Fig5.6 Test result of Total Radial Composite Deviation
Fig 5.6 Test result of Total Radial Composite Deviation

(Important Gear Terminology and Gear Nomenclature in Fig 5.6)

Total radial composite deviation
Tooth-to-tooth radial composite deviation