same ratio dfferent gears

QUOTE (stoner @ 27 Apr 2012, 02:25) <{POST_SNAPBACK}>that wasn,t the question rick take a 14t pinion 9mm in dia and mate it to a 42 gear 3-1 then take a 6t pinion 5mm in dia and mate it to an 18t gear. whats the advantage, does a big pinion drop motor torque or does a small pinion increase motor torque.pretend that wheel, ground clearance isnt an issue. why would you go for the different pinions and gears to achieve the same ratio 3-1. there must be a very good reason judging by the racers and scratch builders setups. i hope this is a bit clearer. john

But you sked two questions, and Rick answered both in really simple language.

Q1 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.

A1 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.

Q2 a euro sport runs 6-1 6-36 why.

A2 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.

What amazes me, is that this same question comes up here and on other forums quite often..... I guess a lot of folk have trouble getting their heads around there being "several methods of removing the pelt from the moggie"
Gearing ratio is gearing ratio, it is a multiplier (or reducer) of force as required to acheive the objective - just the same principle and kids on a see saw (teeter totter for the americans), light kid, use longer side out from the hinge-point to equal heavier kid closer to the centre. - That explains why choose different gearing ratios..... Q2

The only thing to add would be that since most of our gears in RTR racing are based on approximate 48 pitch, give-or-take a smidgeon, different tooth number combinations are needed to get the physical "fit" in the cars for the different motor arrangements. - inline, anglewinder, sidewinder.
ie
- for low to ground anglewinder cars you may be using say 8:24 in gears because that is what for instance Slot.it make to fit for the low GT/LMP cars,
- while an inline may be 9:27,
- and a sidewinder setup with 6.5mm pinion may be 12:36, simply because you need to use such 6.5mm and 18 or 19mm diameter spur gear sizes/diameters to make space for the motor and axle shafts to physically fit alongside each other.

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

QUOTE (stoner @ 26 Apr 2012, 15:25) <{POST_SNAPBACK}>that wasn,t the question rick take a 14t pinion 9mm in dia and mate it to a 42 gear 3-1 then take a 6t pinion 5mm in dia and mate it to an 18t gear. whats the advantage, does a big pinion drop motor torque or does a small pinion increase motor torque.pretend that wheel, ground clearance isnt an issue. why would you go for the different pinions and gears to achieve the same ratio 3-1. there must be a very good reason judging by the racers and scratch builders setups. i hope this is a bit clearer. john

Not sure if youre winding me up John? Ill assume your not. Both 42/14 = 3 and 18/6 = 3 have the same mechanical advantage (you calling it leverage). That is for example if the motor produced 100 g.cm of torque then at the rear axle the torque would be multiplied by 3, so it would have 300 g.cm available. What is the penalty for having a mechanical advantage? The speed will be reduced by 3.

So youre question is why bother making or using 42/14 if 18/6 does the same job? Right? A couple of reasons spring to mind. The physical space you have to play with. Maybe the bigger bulkier gears wont fit, or vice versa. Another reason, the bigger gears will tend to give a nicer smoother mesh and as a result have less frictional losses. Try for example getting a 12T spur to mesh nicely with a 4T pinion!! 12/4 = 3 so you get the same mechanical advantage but the mesh would be horrible. Going to the other extreme 300T spur and 100T pinion. The size would be huge and the weight would start to be significant by then. The third advantage is that when I say the mechanical advatage is 3, that is the AVERAGE ove one complete rotation of the gears. The instantaneous advantage will fluctuate slightly so that at any one point in time it might the advanteg will be 2.95, then 3.05 etc. This is due to the way the teeth engage and disengage. But the average over one rotation will always be 3!! This fluctuation becomes less pronounced the more teeth we have. So 36/12 = 3 fluctuates less than 27/9 = 3. But we are starting to split hairs at this stage.

With respect to the typical ratio of say 3:1 that is used for a lot of slot cars, assuming the mesh is good, then I really dont think there is a lot of difference between 27/9, 30/10, 33/11, 36/12. They should give the same result. Assuming each meshes well the frictional losses will be about the same, there will be very little difference in the weight so inertial losses will be about the same etc. It then comes down to personal preference and what will physically fit.

OK if I still havenet answered your question Im officially giving up.

cheers
rick1776

QUOTE (stoner @ 27 Apr 2012, 08:48) <{POST_SNAPBACK}>you go for the biggest gear set that will fit into the car and you achieve better gear mesh and less frictional loss.
Yes that's right, so is
QUOTE (John Cahill @ 26 Apr 2012, 22:38) <{POST_SNAPBACK}>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.
So you've got two effects - one recommends bigger gears, the other recommends the opposite.
So which to choose?
The answer is choose the best compromise, which may be different in different applications.
(Can take a bit of getting your head round that!)

If you can quantify exactly how much efficiency changes with gear size and you can quantify exactly how much acceleration is changed by the inertia effects of gear size then there's a neat mathematical way of working out what is best.
Or you can try it and see which works out best.
In my experience, slot racers generally try it rather than attempt to calculate it.

Dont worry about me John, I was having a bad hair day. If youre mostly into vintage 1/24th stuff then a lot of the gears that were used were from "where ever" people could source something. Im also mostly into 1/24th stuff of late. I prefer to use brass pinions with delrin (plastic) spur gears. The wear rate will be too high with metal to metal gears in 1/24th. Take extreme care to get the gear backlash to nothing more than a bee's dick otherwise youre asking for trouble. The cars are usually 150-220g in weight so they have a lot of inertia. Heavy cars with back lash and metal on metal gears will kill the gears real quick.

Also with the vintage stuff inertia starts to have an affect. The difference in performnce between a gear weighing 1.1g and 1.2g is unmeasurable (in my opinion). Howe ever the diference between 1.1g and 8-10g is measurable. Scale auto look like they make nice spur gears in plastic. I would not use their plastic pinions though. They are marginal. I use a lot of MJK Engineering stuff, especially their stainless steel chassis, they are really quite simple in design but work very well. Their gears are OK with a bit of work. Below is an MJK Eng Can-Am chassis kit.







cheers
rick1776

Thanks smurfen. Yes the lexan bodies are typically painted from the inside. Cant help but get a nice looking finish, even if you hand paint it!! The injector stacks were painted on the outside. The black lines were done using a fine tipped marker pen. The interior base coat is applied on the under surface and then you paint the driver on the top surface. Decals need to be applied on the inner surface which is a real pain. Next time I will use peel and stick vinyl.

Heres a couple of more pics of my car, the 2011 season Can Am championship winner, as well as some of the other guys in the club. Lot stiffer comp this year and I doubt I'll retain the title.









cheers
rick1776

Stoner, great question!

QUOTE (rick1776 @ 25 Apr 2012, 00:08) <{POST_SNAPBACK}>Flange are you saying that 27/9 = 3 will have better braking than 33/11 = 3 ?? I have to disagree with you on that

Mmmmm! Methinks it's not quite as simple as all that.

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

I am with Flange on that one, and I think Abarth Mike also touched on it at the start of the thread.

QUOTE (Ember @ 25 Apr 2012, 22:11) <{POST_SNAPBACK}>Of course, there is always the problem of observation being coloured by expectation, unless you went through double blind testing.

I think Flange is the better judge because he didn't start with any expectations, he just observed the car behaviour.

As for me, I think the car with 10/30 is better than 8/24 every time (below 8 tooth you get meshing issues which might interfere with the process (assuming 48 pitch gears)).

It's all to do with mechanical advantage I believe (I like that word, for the first time in many many years I have learnt a new English expression).

Joel

PS: You don't have to believe me, just look it up.
PPS: This thread is nearly as good as the ones about big motor cables, but both are far far behind scale speed.
PPPS: Actually, not sure why it would be better under braking.

hy rico, just rereading the thread. all gears work on a sliding action thats how the pinion moves them. as the pinion revolves, the tooth thats been pushing on the big gear slides of, the next tooth thats engaged should just be starting to push. with a pinion of 5.5mm dia and different tooth counts,, look at the profile of the teeth. you go from a v shape on the 8t to a waisted profile with a little flatted ball on top in the 10t pinion, for clearance. this allows many different teeth sizes in a given diameter. im talking about inline motors, but the same applies, to a [fixed pinion and gear centres] on an anglewinder and sidewinder. down to the reason i was leading upto. gears are cut on a hob with the cutter set at a right angle to the centre of the gear so if you want a 1/8 offset you just move the cutter down 1/8 and cut the gears. this will give a perfect gearmesh if set up correctly at the 1/8 offset. it wont work if you try to set it it up in a straight line. the 1mm offset on slot it gears are a bit of a fudge up, it just happens that a slot it gear[inline] thats been cut on the centreline will work nearly as well if its been set up with a 1mm offset, if you tried to offset it anymore than 1mm you,d run into trouble pretty fast. end of my one finger typing marathon. john