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# Bevel drive efficiencies

## Bevel drive efficiencies

(OP)
I'm looking to do some analysis for transmission options that include both transverse and longitudinal configurations. I.e. with and without a 90deg bevel gear set. At this stage there is plenty of design flexibility in the longitudinal case, such as performing the main reduction at a set of bevels with a ratio of 3:1 (like a conventional final drive) or running them at lower torque closer to 1:1 and performing the final reduction as a helical gear pair.

Can anyone help with rough efficiencies for the two options? Or ideally a reference to a calculation method so I can map efficiencies from 0 to 100% of rated torque?

The idea has been inspired by a number of motorsport transmission schemes that I've seen where the 90deg drives are all configured as high-speed / low-torque with ratios close to 1:1. I'm assuming that the reason was efficiency?

### RE: Bevel drive efficiencies

murpia,

If your engine is transverse, then you'll want a parallel shaft gearbox (ie. all helicals or spurs).  If your engine is longitudinal, then you'll need at least one orthogonal gear axis.

Spiral or hypoid bevel gearsets tend to be heavier and bulkier than helicals or spurs simply due to their geometries.  If weight or package size is an issue, you'll want to do the bevel drive at the point in the drive train with the highest shaft speed possible, since that will be the point of lowest torque and will thus require the least amount of gear and bearing mass.

Theoretically, spur gears supported by ball bearings would  have the best efficiency.  But even a spiral bevel gear mesh can be very efficient if designed properly.  The difference in mechanical loss between a spiral bevel or helical mesh of similar ratio is probably only a few tenths of a percent.

Things like oil windage or churning can cause very significant losses if you don't pay attention to them.  Adverse changes in gear contact geometries can also be an issue, due to inadequate housing structural stiffness or material CTE mismatches between the gears and housings.

Regards,
Terry

### RE: Bevel drive efficiencies

Ian, I don't think there will be much in it efficiency wise if it is all properly designed.

But that is a big if. Typically production diffs are very inefficient (85% would not be unusual) because you can't make the case stiff enough to hold the hypoid gears in their correct relationship, except at one torque and temperature. I think you will face similar problems.

Having waded through two boxes of textbooks and manuals, and 15 feet of bookcases, that is the best I can offer. Sorry.

Cheers

Greg Locock

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### RE: Bevel drive efficiencies

For a given torque capacity, hypoid gears are more compact, but less efficient (significantly), than spiral bevel gears.

Running a long shaft at high speed can be problematic - you can get into natural frequencies, etc.

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