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transmission capacity thumb rule

transmission capacity thumb rule

transmission capacity thumb rule

On another group someone mentioned "As a rule of thumb a transmission with the shaft spacing of the Corvair (77mm) is not good for more than 300 lb*ft."

Is there a more complete general rule for trans capacity?


Dan T

RE: transmission capacity thumb rule

Is there a more complete general rule for trans capacity?

Yes, the original manufacturer's rated torque capacity.

There is much more to a transmission than a input shaft.

RE: transmission capacity thumb rule

I've never heard anyone put a figure on that "rule of thumb" before. To GMIracing's point you have to consider bearings, gear width, gear teeth, materials, duty cycle & life expectancy, syncro design/materials, lubrication, speed that the max torque occurs at, etc.


RE: transmission capacity thumb rule

Although not a "transmission" per se, the normal shaft spacing in typical quick-change boxes and rearends is 3.5". These boxes handle upwards of 1,000 ft.lb. of torque. I can't see where a rule-of-thumb for transmission torque capacity would refer merely to shaft spacing; unless the spacing were so small as to limit bearing selection.

RE: transmission capacity thumb rule

I suppose if you limit the discussion to '60's mass-produced vehicle transmissions, the materials and heat treatment of the gears are all within a pretty confined range, such that you could develop such a ROT.  I'm not sure how useful it would be, tho.

RE: transmission capacity thumb rule

I have heard of a general formula used by the European industry to determine the maximum torque that can be safely transmitted for a given shaft center distance in a cascade automotive gearbox.  It was mentioned to me, about 20 years ago, by an Iveco engineer that had been in the business of designing truck gearboxes for about 20 years.
He referred to it as the "Chrysler formula" but I have found no reference other that that conversation to such formula.
It stands to reason that in a given center distance you are limited by the torque you can transmit.   The center distance will dictate the size of shafts, bearings and gears.  While it is true that you can increase the face width of the gear to carry more load or increase its durability, doing so will change the bearing center distance (on the same shaft) and thus will increase the shaft deflection and consequently limit the benefit of the increase in face width.
I know the fellow has retired but I will try to get in contact with him to get the "Chrysler formula".

RE: transmission capacity thumb rule

I'd like that, but don't go to much trouble, please.

Dan Timberlake

RE: transmission capacity thumb rule

The design rule of thumb I mentioned before is as follows:

Shafts Center distance [mm] = 30 x (torque [kgm])^(1/3)

This is from the older fellow I met years ago that designed most of Iveco truck transmissions.


RE: transmission capacity thumb rule

From a failing memory of racing one of these buggers in the early 60's---Because of the interesting design of the input shaft (first motion), using the Corvair layout to compare tq capacity with other generally "normal" trans is not valid, IMO.  The problems in the Corvair are fairly unique, however, failures in the trans were not all that common but "spyder gear" failures were.


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