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How to Reverse Engineer Spur Gears

November 1, 2023

How to Reverse Engineer Spur Gears

A question we get quite often is whether Grob can reverse engineer spur gears for a customer.  Many times a customer may not know the exact parameters of the gear they need, but they do have a sample part in front of them.

So, to help the customers try and figure out what they have, we wanted to come up with a short guide on how to help speed up the process.

Tools you’ll need:

  • Micrometer (recommended), Calipers, or Pi Tape
  • Calculator

What we’re after:

  • Tooth Count
  • Diametral Pitch
  • Pressure Angle

Step 1:

Count the number of teeth on the gear.

Step 2:

Measure the outside diameter with the micrometer or calipers as indicated by the picture.

If the gear has an odd number of teeth, you can approximate it by trying to rock the measuring device back and forth to the adjacent teeth.

Step 3:

Now use the following formula to estimate the Diametral Pitch of the gear.

Diametral Pitch Formula for Spur Gears
Diametral Pitch = (Tooth Count +2)/(Outside Diameter)



Tooth Count – 16
Outside Diameter – 1.005 inches

Diametral Pitch = (16+2)/1.005
Diametral Pitch = 17.91044

Chances are you’ll get a number with a long decimal after it.  If the number is close to a whole number, that is probably the diametral pitch.  It is very difficult to get an exact number using this method because of the manufacturing tolerances on the gears outside diameter.

So in our example, we got 17.91044, which is very close to 18. So it very well could have a diametral pitch of 18. There is also a chance that it is a combination pitch gear where the gear tooth geometry won’t correlate to the outside diameter.

This formula should give you a good idea where to start but it needs to be verified with a CMM or matched with a known gear to be sure.

Step 4:

The last thing we need to figure out is the pressure angle.  Note, this can be challenging unless you have experience doing it.  The following illustration shows the comparison between a gear with 14.5° and 20° pressure angles.  Notice how the 20° is thinner on top and wider at the base. Gears with a 14.5° pressure angle tend to go almost straight up at the base of the tooth.

Gear Pressure Angle Comparison - 14.5° & 20° - Grob Inc.

Need a Spur Gear?

Grob will cold formed spur gears with a diametral pitch between 6 and 48, in either 14.5° or 20° pressure angles.  You can use this guide to help estimate what gear you may have, and while it should still be inspected prior to a purchase, it still helps us figure out if we have the capabilities to manufacture the gear you require.

Click Here to Learn More About Grob’s Stock & Custom Spur Gears

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