same ratio dfferent gears

6260 Views 64 Replies 19 Participants Last post by stoner, May 21, 2012

stoner

Discussion Starter · Apr 24, 2012

hy would some one like to explain the different combo,s, and why. take a 3-1 ratio, the standard is 9-27. going to extremes, what about 7-21 or 11-33 there must be a difference and i,d like to know. a euro sport runs 6-1 6-36 why. john

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John Cahill

· #3 · Apr 24, 2012 (Edited by Moderator)

hi, John
I believe what you're asking is what about the same ratio but different tooth counts. More than anything I believe that is based on the tooth size (and therefore stress/loading as well as ease of machining with a
quality profile). Having established a tooth size, then the number of teeth determines the spacing of the two gears, as well as ground clearance. Obviously an 11 tooth pinion is a larger diameter than a 7 tooth with
the same tooth size, etc. There are other issues such as flywheel effect if you go to extremes on size. Does this address your real point, or do you have something else in mind?

with best regards,
John

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John Cahill

· #21 · Apr 25, 2012 (Edited by Moderator)

QUOTE (Ember @ 24 Apr 2012, 09:16) <{POST_SNAPBACK}>I don't really think that's the question Mr Cahill, sorry.

Is there an engineer in the house?

Embs

hi, Embs
Are you sure about that? *grin* I believe I introduced energy effects, profile efficiency, stress and geometry and questioned how deep Stoner wanted to go into this before introducing other issues.
As one of several Mechanical Engineers here, I could have moved further into the weeds, but I'm still skeptical that John wanted any more info than first presented..or whether it would be helpful.
Energy losses from profile are pretty trivial, for example, in comparison to differences in material deformation and hysteresis (plastic, brass). I still think rotational inertia and overall geometry
(clearance and spacing) are the main issues, unless we're talking high performance cars and high rpm motors where efficiency becomes more of an issue. That notwithstanding, as has already
been said...mechanical advantage is mechanical advantage. It's not the ratios but rather the ancillary issues that predominate here.

I don't believe John/Stoner has yet clarified his intentions...I want to be careful not to hijack his thread.

John

PS with respect to rubbing, there shouldn't be any if the tooth profile is correct and not deforming under load (and endplay limited, ball bearings etc)...but that's a big if....definitely an issue separating higher quality/precision cars from...others.

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John Cahill

· #43 · Apr 26, 2012 (Edited by Moderator)

I believe we are on the edge of answering the original questions. Resolved (I believe): 1) any 3:1 ratio gives the same mechanical advantage and 2) clearly there can be issues with inefficiency
based on the quality of the gear (Involute) profile and material deformation etc which should be minimized; quality counts. I think the other issue crying out for clarification is what is commonly
called "flywheel effect"....that each gear has a mass moment of inertia which effects how much resistance it has to start turning as well as it's propensity to continue spinning once you are up to
speed and now trying to slow down. Beyond that there are also gyroscopic effects as well. These may sound inconsequential if you are used to contemporary Scalex cars for instance with small
plastic gears, but now consider vintage brass Spur gears that are 3/32" thick and almost 1" diameter. So, as has already been said, it depends on the application. I believe this is what rick1776 was
addressing when he talked about motor torque and trying to turn a 16t pinion/36 tooth spur gear pair.

So for a given ratio, I think clearly one would want to select the smallest and lightest combination of gears with good profile if one wants to stop and start briskly.

John

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John Cahill

· #49 · Apr 27, 2012

hi, 300SLR
yes, I agree completely...so for example. I would prefer a Slot it Spur gear which is aluminum, thin and precisely profiled....over a vintage, massive brass gear
with generally good quality profiling. I'm not suggesting that size should dominate all other variables, just that it be in the mix.

good thread, guys!

John

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John Cahill

· #63 · May 20, 2012

re: "all gears work on a sliding action thats how the pinion moves them"

with respect, that's not true...and that's one of the points I was trying to make earlier on. Any gear that's designed with the classic Involute profile (such as a sidewinder with pinion and spur,
as well as properly shaped contrates on inline setups) are specifically shaped to eliminate frictional losses from sliding contact. That's the whole point of the profile...it's by definition, the shape
which allows only "normal" reaction forces at all points of contact. You might think of it as "rolling" contact at all times.

That's why I'm saying that weight (inertia) is one problem...but going too small to save weight and sacrificing a well-formed gear profile isn't good either. There's a balance
that involves lightness, quality of profile and material stiffness for optimum performance. Sorry to repeat, but something like a thin, Aluminum, precise slot-it gear is a good example.

John

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