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Transmission power rating

Transmission power rating

Transmission power rating

I'd like to know more about gearbox power rating... How much power can a gearbox handle? What are the causes of failure?

If a gearbox can take 100hp, then does that mean that:
a) it can take half the torque at twice the RPM?
b) it can take twice the torque at half the RPM?

in other words, is RPM or torque the critical factor? ANd where does failure first occur? At the gears themselves or shafts, dogs, etc?

RE: Transmission power rating

One of the ratings may be "thermal horsepower" and is strongly rpm related.

Tooth bending strength (geometry, metallurgy) sets an upper limit on torque, but the fatigue limit of the material in tooth contact is one of the main power limits too. The lubrication characteristics can pose limits from a few directions, like from speed/thermal direction, and contact surface tribology.

RE: Transmission power rating

thanks for the answers

Greg, i mean manual.

RE: Transmission power rating

Torque limits most applications. Power is probably (I'm no expert) all about dissipation of heat, not structual design or material properties that might cause mechaical failure.

- Steve

RE: Transmission power rating


The are many variables that can impose operational limits on transmissions. As many noted, with a conventional AT that has a rather low system efficiency at certain operating conditions and thus a high heat rejection rate, there would probably be an input power limit. For other cases, where fatigue life of gears, shafts, bearings, etc. is of concern, both the level of torque (load) and the rate of accumulation of load cycles is what matters. All other things being equal, fatigue cycles will accumulate faster at higher speeds. But the fatigue life of gears, etc. can vary greatly depending upon the combination of load/speeds it is subject to. For example, a gear may have an L10 life of 2000 hours at a mean load of X, but at a mean load of 2X that same gear may have an L10 life of only 100 hours (assuming the same speed).

To provide an answer to your question, you would need to provide a very detailed description of your analysis case and transmission design.


RE: Transmission power rating

Thanks Steve and Terry. i don't have a specific design, i was looking into the possibility of making some modifications to increase power in a motorcycle transmission, that's why i wanted to know where's the limit.

Also, anyone knows what steel grade is used for the gears?

RE: Transmission power rating


what type of heat treat?
case harden or through harden


RE: Transmission power rating


case harden

RE: Transmission power rating


for starters
For the gears (carburized steel)
aerospace stuff AMS 6265 (AISI 9310 steel)
it obtains case hardness of 60 HRc min , core 33-43 Hrc
have a it heat treated to case harden to .020-.025 effective case depth after grinding.

for gears & shafts (nitrided)
again aerospace stuff AMS 6471 (nitralloy 135)
Nitrided steel case hardness of 60 HRc min core 35-40 HRc
case depth .015-.025 effective case depth after grinding

just recommendations you will need to do in depth analysis
down load the data sheet.

My recommendation.


RE: Transmission power rating

this is probably obvious, but-
look at performance motorcycles with similar transmission styles.

When people hop the motors, do they beef the transmissions? how?
do the transmissions fail? (yes, they probably do.)
so,how do they fail?

lots of people out there building and breaking bikes. Look at bikes similar to the ones you want to build...

btw - what is it?

Jay Maechtlen

RE: Transmission power rating

Quote (BobSP)

Thanks Steve and Terry. i don't have a specific design, i was looking into the possibility of making some modifications to increase power in a motorcycle transmission, that's why i wanted to know where's the limit. Also, anyone knows what steel grade is used for the gears?


OK, so it's a motorcycle gearbox. Spur gears and dog rings. Unless it's a very high end bike, the gears are probably induction hardened and made from something like carbon or low alloy steels with a carbon content between 0.40% and 0.50% (ie. 1040,1050,4140, etc.). If you're going to put more HP through the gear meshes, the most like failure mode will be tooth scoring in the lower gear ratios. There are only a couple things you can do with the existing gears to improve their performance. To improve the tooth bending fatigue life you could try shot peening the root fillet areas. To improve the scoring limit you could try ISF of the tooth flanks. Neither of these processes is outrageously expensive, and both processes are fairly low risk.

If you want to upgrade your spur gears, as mfgenggear noted the best commercially available carburizing gear alloy is AMS 6265 (double vacuum melt 9310). However, since you probably can't afford the high cost of finish grinding carburized gears, a better choice might might be the other material suggested by mfgenggear, Nitralloy N or Nitralloy 135. The Nitralloy gears can be rough machined, quenched and tempered, shaped, shaved to correct for any nitride distortions, gas nitrided, then finish honed to improve the surface finish and add slight face/profile mods like crowning . The nitrided case depth is less than a carburized case, so you will have to make allowance for this when you design your gears.

Lastly, depending upon the particular model of bike in question, there are some very nice aftermarket gearsets available from companies like Quaife in the UK.

Hope that helps.

RE: Transmission power rating

The biggest mistake I see people make when overdriving a transmission is to make no account for shock loading.

It's been said before but can be said again that torque is the real rating of a transmission. In most cases you can trade some lifetime in exchange for higher loading.

However application of the load can lead to spikes in torque. In sportscar applications one popular thing to do is harden the drivetrain mounts and use a puck clutch without marceling. While this type of clutch can handle more torque, it increases the shock loading. So keep an eye on that, if you find you're shearing transmission gear teeth then adjust the compliance in the system.

RE: Transmission power rating


nice info

if it can be built it can be calculated.
if it can be calculated it can be built.

RE: Transmission power rating

As a side line I worked with a race bike project years ago.
Lubrication failure was a major problem. The oil gets too thin/breaks down at high temp and load.
You can't just use heavier oil because it creates more drag and more heat.
They precision ground (so that they actually mated) and polished the gears (extrude hone I believe) and moved to thinner synthetic oils.
This lowered internal heating enough that then fatigue became the issue.
Selected gears were upgraded to materials suggested above and give special surface treatment.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

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