Achieving Gear Design Success

When evaluating a new or existing spur gear design, there are important areas that should be evaluated.


After a gear design is completed, enough of the gear, stress, and life data is established so that the gear set can be completely described and all of the relevant gear data specified in order to manufacture and rate the gears involved. From this gear data, the following items should be evaluated:

Contact Ratio: This usually varies between 1.1 on the low end to over 2.0 for high contact ratio gears. Normal values are typically 1.2 to 1.8. Contact ratios over 2.0 are a special case and need to be evaluated using different specialized tools.

Backlash: There are tables and data within AGMA standards and textbooks that have recommendations for the proper amount of backlash. The backlash should be enough to enable adequate lubrication film between the gear teeth at the appropriate operating speeds in order to always prevent metal-to-metal contact of the gear teeth, even in the worst-case conditions.

Gear and Pinion Top Land Thicknesses: The teeth should obviously not be pointed and should have adequate top land thicknesses even after anticipated wear. There are also metallurgical factors to be considered such as tip breakage. If the gears are carburized, the top lands should typically be greater than the case depth by a sufficient amount.

Pressure Angle, Operating and Standard: These values will normally range from 14.5 degrees to 30 degrees or more. Typical values for pressure angle are 20 to 25 degrees. The operating pressure angle can be much different than standard, and the pressure angle can also affect the separating forces.

Amount and Type of Gear Tooth Geometry Modification:

  • Long and short addendum
  • Non-standard outside diameters
  • Modified circular tooth thicknesses
  • Non-standard whole depths
  • Any or all of these resulting changes can cause a change in the operating center distance and other gear data.

Gear Tooth Tip to Root Clearances: Standard values range from 0.157/DP to 0.35/DP and should be sufficient to provide adequate clearance for the oil film and, in high-speed gear meshes, for the pockets of high velocity air.

Slip Ratios and Sliding Velocities: High values for these items could put the gears at risk of scoring, scuffing, or other surface distress-related damage, which is also related to lubrication.

Face Width and Face Width to Pitch Diameter Ratio: Too much face width or too high face width/diameter ratio could result in uneven contact across the face width of the gears or insufficient backing of the material under the gear or pinion teeth, the effective shaft under the gear teeth.

Amount of Approach and Recess Action: Too much approach action increases friction and can adversely affect lubrication of the gear teeth and potentially contribute to or cause surface distress.

SAP Angle: This is a manufacturing consideration; typically, SAP angles less than 5 degrees can make the gear difficult to manufacture.

Gear Tooth Bending and Surface Contact Stresses: Comparing the bending stresses of the pinion versus the gear and its equivalent life provides an indication of which of these members is stronger or has more life. Ask the question: Am I comfortable with the pinion or gear being stronger than the other member?

Power ratings of the bending and surface contact stresses should also be compared to see if the gear set is stronger in bending or surface contact fatigue.

Taking a comprehensive look at the stresses this way allows you to compare modes of failure and which gear is predicted to fail first and in what manner. For example, sometimes it is preferred for a gear or pinion to fail first in surface contact fatigue (pitting) relative to bending (tooth breakage by bending fatigue) because pitting is visible on the gear teeth and progressive, while bending fatigue is neither. Sometimes it is desirable for a design to have stresses that are more equal, and other times gear lives based on these stresses are preferred to be more equal. Other times, some other relationship between stresses and life is called for and is best.

The intent of evaluating and checking these values is to determine both if they are what was intended in the gear design phase and within acceptable limits and in general, if anything looks out of order, is poor gear design practice, or outside of acceptable limits. There is considerable judgment involved in all of this that comes from experience, knowledge, intuition, and gut feelings. So, this advice is only intended for the experienced, qualified gear design engineer.

For example, a strong gear set might have small numbers of gear and pinion teeth, coarse DP, higher pressure angle, lower contact ratio, and more gear tooth geometry modifications. A smooth, quiet, and higher speed set may have more teeth and finer DP, lower pressure angle, and a higher contact ratio. Changing even one of these parameters listed above can change one or many of the other gear design elements.

In this way, all gear designs are a compromise and an exercise in optimization and juggling all of the various input and output gear data in order to achieve the desired results. Therefore, a good gear designer knows and understands the various interactions and relationships of the different gear design elements and the patterns and behaviors between them. This is not magic, but it does take a considerable amount of experience and completing this process successfully many times.

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Rick Miller
is president/sole member of Innovative Drive Solutions LLC, which provides gear design engineering services for optimizing gears, gearboxes, and other geared devices. Miller has created over 300 original gearbox system designs. He is a member of AGMA, SAE, and ASME, and he holds three patents. He is vice chair of the AGMA Vehicle Gearing Committee. For more information, visit