Gear Design Improvements Lead to Higher Load Carrying Capacity

As demand grows for improved power transmission, gear designers must explore new design concepts to increase load-carrying capacity. One of the techniques, addendum modification (also known as profile shift), decreases gear tooth stress through tooth redesign. The gear addendum is defined as the distance between the pitch circle and the tooth tip circle (see figure below), which determines the overall gear height.

An increase or decrease of the addendum is known as addendum modification. Gear designers must have a critical understanding of addendum modification to improve gear performance.

This post discusses the improvements gained through profile shift designs.

Profile Shift Designs Explored

Profile shift aims to avoid interference as the gear teeth mesh. Interference creates stress at the root of the tooth, which can reduce load-carrying capacity.

Other design techniques used in avoiding interference include undercutting at the root of the tooth, stubbing the mating gear tooth, or minimizing the number of teeth based on pressure angle.

These techniques may lead to reduced tooth strength and compromise the reliability of the gear system. Profile shift design eliminates these introduced ramifications and significantly improves the load-carrying capacity of the gears. In miniature gear applications with less tooth space, the profile shift method helps reduce the number of teeth to eliminate interference issues.

Summary of Simulation Runs

Different pressure angles, modules and number of teeth were investigated to determine the effects of the profile shift on root stress of an involute spur gear tooth to help evaluate individual performance improvements.

The KISSsoft gear design tool calculated root stress and tooth thickness for different values of the profile shift coefficient when pressure angles, gear modules, and the number of teeth were changed.

The profile shift coefficient for each series of simulations were measured at -0.5 to +0.5 in 0.1 increments.

Three specific simulations were run:

•	Change in Pressure Angle Simulation. Root stress and tooth thickness are plotted against different profile shift coefficients for various pressure angles. See these results in Addendums 1-2 in the appendix.
•	Change in Number of Teeth Simulation. Root stress and tooth thickness are plotted against different Profile Shift Coefficient for a various number of teeth. See these results in Addendums 3-4 in the appendix.
•	Change in Gear Module Simulation. Root stress and tooth thickness are plotted against different profile shift coefficients for various gear modules from 0.5 to 2.5mm. See these results in Addendums 5-6 in the appendix below.

Results of Simulation Runs

1	Root stress decreased drastically with an increase in profile shift coefficient. Stress also decreased with an increase in pressure angle, number of teeth, and gear module.
2	Tooth thickness at the root (critical section) increases with a positive profile shift coefficient and with an increase in pressure angle, number of teeth, and gear module.
3	Root stress further decreases when both the drive and the driven gear are modified in concert. Root stress is at its lowest value when the drive gear is at the maximum positive profile shift correction and the driven gear is at the maximum negative correction.

Conclusion

When the proper profile shift coefficient is selected, the root stress on gear teeth decreases significantly, while a greater thickness at the root will increase the strength and load-carrying capacity of the gear system. These design changes result in a more robust and reliable gearing system versus non-profile shift designs. In addition to all these, a smaller number of teeth on gear can be employed in miniature products and the interference can be avoided.

Are you a design engineer wanting to partner on a new gear design? Reach out to Portescap here to start collaborating!

Appendix

Addendum 1. Root stress vs. profile shift coefficient for various pressure angles.

Addendum 2. Tooth thickness vs. profile shift coefficient for various pressure angles.

Addendum 3. Root stress vs. profile shift coefficient for various number of teeth.

Addendum 4. Tooth thickness vs. profile shift coefficient for various number of teeth.

Addendum 5. Root stress vs. profile shift coefficient for various gear modules.

Addendum 6. Tooth thickness vs. profile shift coefficient for various gear modules.

References

•	‘Handbook of Gear Design’ by Gitin M Maitra, Tata McGraw-Hill, 1994.
•	Theory of machines” by R. S. Khurmi and J. K. Gupta, Eurasia Publishing House (Pvt.) Ltd.
•	‘The Effects of Addendum Modification Coefficient on Tooth stresses of Spur Gear’ by Durmus Gunay, Halil Ozer and Alpay Aydemir, Sakarya University.