Power losses helical/SC gears

[Asklepios]

Can anyone help me ,by estimating the power that helical gears will consume over a S/C gearbox set up please?

[Asklepios]

Not even a rough guess?

 

Surely somebody has questioned the assertion of increased noise for reduced power loss ,(although something must be producing the noise between the gears, so maybe I have stumbled on a modern myth!)

[Joe 90]

As you've asked for a guess, here goes. A modern helical box probably has losses of 7% to 8%. The losses on older boxes is higher as they have less sophisticated gear teeth profiles and are manufactured to poorer tolerances. A straight cut gear set might reduce those losses by 2% to 3%.

 

99,000 miles so far

[scooby dooby doo]

*** pet hate rant alert ***

 

presumably you mean a helical loses 7% and a straight cut loses 5% 🤔 That's a 28% reduction (according to my mental arithmetic).

 

ah ❗ I feel better now, sorry about that

 

so its worth perhaps 3bhp on an average seven...

 

HOOPY 500 kg R706KGU

 

Edited by - hoopy on 18 Jan 2003 09:41:11

[Asklepios]

Thanks Oliver and Hoopy

[stevefoster]

So what about a semi heli gear then?

 

 

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[R2D2]

I would be surprised that the difference in power losses between a helical and spur gear was as much as three percent.

 

The difference in power losses between single stage helical and spur industrial gearbox is mainly due to the thrust bearing needed in the helical design, but even if this is taken into account the difference is much less than 3%.

 

In an automotive box the thrust bearings are always used and their losses are more a function of any pre-load rather than side thrust.

 

If you can gain a 3% improvement due to a spur gears then factory BDA engined escorts would have gained at least 8 BHP by replacing the gears in their ZF boxes with "straight cuts".

 

I really think that straight cuts have been produced for competiton cars because they are cheaper and easier to make. I think the power arguement is not really justified.

 

In some ways the spur gear is weaker and more prone to failure at the root as bending stresses are higher and scuffing loads are certainly higher. Scuffing loads are certainly a cause of failure and need to be taken into account in the design of individual gears.

 

Automotive gears have used an involute tooth profile for quite a long time now and I can't see this changing too much in the future.

 

The "semi-helical" box should really be described as a helical box with a shallow helix angle, I don't see why it has much advantage over a standard angle helix other tahn it may be cheaper to make.

 

If it isn't cheaper why bother?

 

Edited by - chris flavell on 20 Jan 2003 11:32:38

 

Edited by - chris flavell on 20 Jan 2003 11:34:07

[Joe 90]

As I said, I'm guessing on the numbers. I do know that straight cut gears have lower losses because there is less relative sliding motion between the teeth contact patches. The reason why not all race cars are fitted with them (Chris's Escort BDA example) may be dictated by regulations that insist on the use of production gears.

 

99,000 miles so far