same ratio dfferent gears

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.

Hi Stoner
You are asking about gears that are the correct diameter for the size of teeth rather than these standard diameter pinions with different numbers of teeth?

Two different possibilities
1 The tooth size is changed so the diameter remains the same (for example 9:27 in 48 pitch and 12:36 in 64 pitch are approximately the same size and both the same ratio)

2 The tooth size is constant so more teeth means bigger diameter.

Taking those in turn
1 Doesn't make much differance. I have some practical experience of this - it doesn't make any noticeable differance. (Although some makes mesh better than others)

2 Bigger diameter gears are inevitably heavier. Theoretically that'll reduce acceleration and braking.
Bigger diameter gears that are delivering the same torque put less side load on the bearings Theoretically that'll reduce friction.
Both effects are probably so small so they don't matter much

Stoner,

The simple answer is that a mechanical advantage of 3 is a mechanical advantage of 3. This is whether it is achieved using 27/9 or 30/10 or whatever other combo you want. Anyone that says something different failed year 11 high school physics. Remember though that this is the AVERAGE mechanical advantage, as the each individual tooth engauges and dis-engauges this value will oscillate by a small amount both above a mechanical advantage of 3 and below a mechanical advantage of 3 (by a very small amount). The larger the tooth count the less the mechanical advantage will vary from 3.

Any one that says they can notice the difference in acceleration using a spur gear that weighs 1.2g compared to 1.1g is pulling there short one too hard. A while back it was fashionable to use hollow axles because they had less rotational inertia. I did some calculations and on the assumption that the hollow axle was 1g lighter, if you removed 0.003g from the outer edge of the rim/tyre it reduced the rotational inertia by the same amount. Smoke and mirror stuff. Not disputing the axle weighs less and that this could increase the accelleration, but the gain would not be due to less inertia!!

cheers
rick1776

QUOTE (Flange @ 25 Apr 2012, 00:28) <{POST_SNAPBACK}>Life's not all just maths , this is from experience of the way cars go

It's a good point. After all at one time (the 1930's) it was theoretically impossible for a Bumble Bee to fly (according to accepted aerodynamic theory at the time) but try telling the Bumble Bee that!

I'm not knocking the engineers - but perhaps there should be a practical experiment undertaken to test acceleration and braking using the same mechanical ratio but with different tooth counts. Maybe a long straight with a dead strip would be a good start. After the dead strip could be a separate controller - use it open to measure distance travelled after the dead strip as a test of top speed reached at the dead strip and with the brakes on to see whether the percentage shortening in that stopping distance represents a significant difference in braking power. You obviously need the same start line with the same car and ideally an on/off switch instead of a throttle.

Oh and before somebody points out the Bumble Bee thing was the result of a drunken argument between a biologist and an aerodynamicist - yes it was but the underlying problem was that the aerodynamic theory being used was appropriate to fixed wing aircraft and not rotational wings. My point here is that what works in theory starts to get muddied by the fact that materials used behave differently (flex for example), that frictional coefficients have a bearing (HAH - INADVERTENT PUN!) and that we are talking about tiny little cars that probably weigh less in total than the smallest cog in a conventional automobile gear box yet rev about 5 times faster than their giant brothers.

We all know that scale speed and wind resistance don't scale down to 1:32 in a way that natural logic would expect. I suspect the same applies to the gears we use.
I think Flange has the right approach - forget the theory - give it a go and find out what works for you.

QUOTE (stoner @ 26 Apr 2012, 08:38) <{POST_SNAPBACK}>the best ratio for a neo drag car is 14-54- 64 pitch. you can use a smaller pinion and gear to get the same ratio,s the drag boys do hundreds of gear changes to get the right combo, so why go with the bigger heavier gears, when weight is a big issue in drag cars, same as 1/32 and 1/24. what i.m getting at does a large pinion with more leverage slow a motor down, or is it such a small amount in the leverage ,not make any difference. ive seen old 16-24 pinions, why make them if theres no advantage. john

We seem to be going around in circles. If I understand you correctly one of the questions youre asking is whether there is for example, a difference between a 6.5mm 9T pinion coupled to a 19mm 27T spur compared to say a 5.5mm 9T pinion coupled to a 18mm 27T spur. On the assumption that they mesh properly then both MUST provide the same mechanical advantage which will be 3. The leverage as you call it will be the same in both cases, and that is 3. Now granted that one gear set may have a better mesh and so have less inherant losses (less friction) compared to the other. But that is a different question altogether.

The other question is, if I unerstand it correctly, why a particular ratio and not another? That really depends on the torque of the motor, the rpm, the wheel diameter, weight of car, is top speed important or is acceleration, short track, long track etc. You choose a ratio that gives the best performance for your situation. Sometimes its as simple as the physical room you have that dictates the gear sizes used.

Why make a 16T pinion? Well the motor may be a low RPM type with huge torque, so you can afford to go to a 16T with say a 36T spur. Put that in a Scaly motor with about 90-100 g.cm of torque and the car will be very sluggish.

cheers
rick1776