Why 77 Teeth?

[nanaimo73]

I've read that EMD freight gearing was 65:12, and passenger gearing was 58:19, and earlier passenger gearing was 52:25. Was there a mechanical reason that that caused EMD to always use 77 teeth in total?

[erikem]

 I would guess that EMD would have a standard size for the gear tooth, so the number of teeth would be proportional to the combined circumference of both gears. Since the spacing between the motor shaft and axle is constant, then the total circumference would be constant, hence the number of teeth would be constant.

It's a good idea to choose the number of teeth for each gear so that the greatest common denominator is 1 (i.e. the numbers are relatively prime).

- Erik

[edbenton]

Also so the Pinion and Ring gear Never hit the same gear the Same time around and devolped a wear patteren.  You will see the same in Rear End gears. 

[beaulieu]

nanaimo73

I've read that EMD freight gearing was 65:12, and passenger gearing was 58:19, and earlier passenger gearing was 52:25. Was there a mechanical reason that that caused EMD to always use 77 teeth in total?

 

There were more combinations used than that. The most common gear ratio for freight was 62:15. This was good for 65 mph. in the postwar years. With the development of the stronger D77 series motors they could safely rotate faster allowing 70 mph with the same gear ratio. Many railroads chose the 61:16 gearing for dual-service locomotives such as some GP7 and GP9s  that worked secondary passenger services or local freights, this gearing was good for 70 mph early or 75 mph later. Rock Island, and GN (and later BN) bought most of their modern freight power with 60:17 gearing giving a top speed of 79 mph. 59:18, 58:19, and 57:20 were not rare either (57:20 was used on Santa Fe SD45, F45, and SD45-2 locomotives in the Super C pool, while 58:19 and 59:18 were used by the UP). The rise of unit coal trains and pooled power caused the demise of the variety of gear ratios with virtually everyone settling on 62:15 until it was replaced with a "Fine Mesh" design to handle the greater motor torque of more powerful motors like the D87 and D90.

The reason that the 65:12 gear ratio quickly fell from popularity is due to the fact that the 12 gear pinion needed to be so small that a press fit gear could not be used and the pinion teeth had to be cut into the motor shaft itself. The reason it was initially popular was the relatively high minimum continuous speeds of early EMD motors. Early motors with the 62:15 gearing had MCS in the 14 to 17 mph range which many railroads felt was too high for work on heavy grades, the same motors when used with 65:12 gearing had MCS in the 9 to 11 mph range. The 65:12 gearing was common on freight EMDs through the Korean War era, but quickly became rare for locomotives built after that.

[Phoebe Vet]

I don't know about railroads, but in helicopters ratios are intentionally designed to not divide evenly in order to reduce harmonic vibrations.

[oltmannd]

edbenton

Also so the Pinion and Ring gear Never hit the same gear the Same time around and devolped a wear patteren.  You will see the same in Rear End gears. 

Bingo! (the gear on the axle is normally referred to as a bull gear.)

[oltmannd]

beaulieu

...with virtually everyone settling on 62:15 until it was replaced with a "Fine Mesh" design to handle the greater motor torque of more powerful motors like the D87 and D90.

Bingo, again!

[Paul_D_North_Jr]

erikem

 I would guess that EMD would have a standard size for the gear tooth, so the number of teeth would be proportional to the combined circumference of both gears. Since the spacing between the motor shaft and axle is constant, then the total circumference would be constant, hence the number of teeth would be constant.

It's a good idea to choose the number of teeth for each gear so that the greatest common denominator is 1 (i.e. the numbers are relatively prime).

- Erik

That makes a lot of sense. For whatever further insight this might add: If we call the constant spacing from the centerlines of the nose-hung motor shaft to the axle "S", that becomes the constant total of the radii of the 2 gears - let's call them "Rm" and "Ra", respectively. The combined circumference would be 2 x Pi x S = 2 x Pi x (Rm + Ra) = (2 x Pi x Rm) + (2 x Pi x Ra) for each gear individually.

And if you notice, as Dale originally surmised, all of the ratios listed by beaulieu DO total to 77 teeth ! So of course the "pitch" of the EMD standard tooth would be 1 / 77th of the combined circumference as calculated above. But I've always wondered how much of a pain that oddball dimension would be to design, detail-dimension, layout, and machine/ fabricate - what's 1/77th of of inch, anyways ? A little smaller than 1/64th, 0.01299" . . .

- Paul North.