Spur Gears Again

Posted by Matt Reiland at 05/04/2001 3:59 PM EST

Engineer on team #226 from Troy, Troy Athens and General Motors CRW.

To all who are considering making their own transmissions next year:

Small parts has told me they will carry some mating 0.7 MOD gears next year if the rules don’t change to allow teams to buy them from any place they want. My question is to all of you what tooth counts are preferable for them to stock if they can only carry 2 or 3 different sizes? I asked them to do this since many teams don’t have access or funds to a shop that can do a very small batch of custom gears. I happen to be lucky enough to live here in MI where there are about 100 of them but it is different in other areas of the country.

Thanks for your reply’s


Posted by Joe Johnson at 05/04/2001 5:33 PM EST

Engineer on team #47, Chief Delphi, from Pontiac Central High School and Delphi Automotive Systems.

In Reply to: Spur Gears Again
Posted by Matt Reiland on 05/04/2001 3:59 PM EST:

I have screwed this up for two year in a row, but I pass on this advice free of charge.

Basically, the smallest gear you should try to mate with that motor should allow the bearing that goes around the shaft for the gear to clear the motor can.

I am pretty sure the motor is approx 45mm in diameter.

A good size shaft is 1/4 inch in dia. which means the bushing OD would be 3/8ths or about 9mm.

So… the total center distance should be at least 54mm/2 or 27mm.

The center distance is (Ngear+Npinion)*module/2
Solving for Ngear:

Ngear = 2*Centerdistance/module - Npinion

Ngear = 2*27/.7 - 12 = 65.XXX

I would round up to 66 teeth (this would be a gear with about the same diameter as the motor – not too bad to deal with).

It would also give you a pretty good ratio in the first stage of the transmission (66/12 = 5.5).

That is my recommendation.

Joe J.

Posted by Raul at 05/06/2001 7:01 PM EST

Engineer on team #111, Wildstang, from Rolling Meadows & Wheeling HS and Motorola.

In Reply to: 66 is the smallest I would go.
Posted by Joe Johnson on 05/04/2001 5:33 PM EST:

I would ask for 72,84 and 96 teeth.

However, it is also important that they use the right materials and gear thickness. Plastic gears will NOT do; we need steel or maybe brass. The thickness of the gear should be 6mm minimum to carry the dynamic loads.

Also, we need a useful hub design that can be mated to idler shafts. A max hole diameter of 6mm for the shaft will allow us the flexibility to drill/ream the hole to the shaft size we need. If the gear is solid, then fine; if it has spokes then the outside hub diameter needs to be at least 25mm.


Posted by James Jones at 05/07/2001 11:25 AM EST

Engineer on team #267, The Demolition Squad, from North Broward & St Andrews and Motorola.

In Reply to: Number of teeth is not the only issue
Posted by Raul on 05/06/2001 7:01 PM EST:

SPI generaly just orders from current suppliers and resells at a markup. PIC is one of the few places I know of that list .7 Module gears as a standard item. If they are going to get them from PIC the material choices in the catalog I have are SS or Aluminum. I would be happy with either one. We ran aluminum last year in the 84-88 tooth count range with no problems. I never did run a tooth stress on it but I know it survived our pretty brutal (10 ft/sec) drivetrain. I have the impression that the standard tooth stress calculations are overly conservative for our short life applications.