Back to Pitch Surfaces on Gears…

This article continues the discussion of the question ‘How do I get gears from Design Accelerator to run in Dynamic Simulation?”

There are occasions where Simulation will reject your gears (Joints), stating that something is wrong or mismatched.  Fear not.  Just go take a break.

I found these occasions when cheating death with the heavy tweaks in the Gear Generator, and then trying to get the results into Simulation, or when I have made adjustments to gears after placing them into the Simulation Environment. When I go back to Simulation, Inventor says ‘no stinkin way’.

The problem stems from creating a gear assembly, and then adding the gears through Joints in Dynamic Simulation. When adjustments are made through the Gear Generator, 1 or 2 things are going to occur.

  • The parts will change relationship one-to-another, requiring their position in Simulation to need alteration. (remember Simulation Joints are unrelated to Assembly Constraints)
  • Often, excessive and obtuse Gear Generator alterations can cause mismatched parameters at minute levels

Inventor measures the distances between Gear Axis, and then relates that to the Gear Pitch Diameters. One simple addition or subtraction later, and Inventor can tell if things are on the up and up. Spur Gears for example must be placed apart exactly the sum of their radiuses (half the Pitch Diameter).

Gear 1 radius (1.00”) + Gear 2 radius ( 2.00”) = 3.00” of separation.

Not 3.000014”, but 3.000000”. Run your precision out to 6 places. As far as inventor is concerned it either is or it aint.


Open up the part and check these.  If the value does not match, your simulation Joints will have to be revised. It can be frustrating.


  • Get a notepad, draw out the problem schematic, and write down every measurement. You will find the mismatch in short order.
  • Use a good skeleton for complex gear assemblies. Make sure that the main dimensional constraint for spur gear placement is a direct tie between the 2 gear halves’ assembly axis.  In this manner getting it right is as fast as adding the sum of the Gear Pitch Radiuses in the single separation dimension in the master skeleton. (this would make a great article I bet)
  • Stay on the straight and narrow.  The Gear Generator is a great tool.  If you are wanting to get into and out of Simulation quickly, take the recommended gear design and go. If you try to micro massage the gears in the Gear Generator, you will end up with problems.

Additional discussion about fine tuning a Design Accelerator gear at sub-critical dimensions would be better in another article.

Next article, Selection Order ->