Derek,
Just sent another reply, but wanted to comment on one or two things here.
Loss in gear mesh is due to friction, not "metal bending". Let's hope the
teeth aren't bending much or we're in real trouble! Actually, there is
always some deflection but bending does not dissipate energy -- bending is
what springs do, which are energy STORAGE devices not energy ABSORBING
devices. The energy put in to bending the gear tooth is returned to the
system as the tooth unloads & returns to its "normal" position. Think of a
sling shot, if that helps.
Otherwise you're on the right track. Oil "churning" (usually called
windage) losses are proportional to speed, oil shearing proportional to
speed & load.
Perhaps surprisingly, very little of the loss actually occurs at the gear
mesh itself -- typically 1.5% of transmitted power per mesh. Most of the
losses are due to the bearings, and windage. This is because, in involute
profile gear teeth, the relative motion is actually a rolling motion, NOT a
sliding motion (true for spur or "straight cut", & helical). The losses
are typically 5% or so, if you include the radial & thrust bearings that
support that gear mesh (1.5% mesh loss + 3.5% bearing loss). Bevel gears
have a combination of rolling & sliding motion, hence are less efficient
(typically 2.5 - 3% loss per mesh; 6 - 8% when you add in the supporting
bearings) and worm gears are the worst, all sliding, that's why we don't see
them much on cars.
Regards,
Mark
>From: Derek Harling <derek.lola@home.com>
>Reply-To: Derek Harling <derek.lola@home.com>
>To: vintage-race@autox.team.net
>CC: S800Racer@aol.com, brian@uunet.ca, ralph@cloverleaf-auto.com
>Subject: Re: horsepower
>Date: Mon, 13 Nov 2000 14:09:39 -0500
>
>I would suspect that the truth is somewhere between your respective answers
>- in
>other words the total loss is partially depended on hp/torque transmitted
>and
>partially independent. No doubt it is also dependent on speed.
>
>The losses occur because of metal bending, mainly in the gear teeth, which
>I
>suspect is very dependent on torque transmitted, on oil shearing (dependent
>on
>torque and speed?), oil churning (speed?) - and other things.
>
>Lets hear it from a gearing/transmisison expert.
>
>Derek
>
>S800Racer@aol.com wrote:
>
> > In a message dated 11/13/00 9:16:09 AM, brian@uunet.ca writes:
> >
> > << How about 15% loss for a Hewland( rear wheel drive, all straight cut
> > gears), up to 20% loss for typical rear wheel drive helical trans, >>
> >
> > I've often heard the 15 - 20 % "rule of thumb". Is this true
>regardless
> > of HP? If a 100hp vehicle lost 25hp, would a 500hp vehicle lose 125hp?
> > Wouldn't it take roughly the same # of HP to drive similar Trans & rear
>end
> > gears regardless of the HP of the engine?
> >
> > Doug Meis
> >
> > P.S. Would a hand count necessarily yield a better result? ;-)
>
> I dunno - you'll have to ask the countess!
>
>
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