32 replies to this topic

[dc-65x]

I need some help with gearing a twin rear axle setup like this one by Derek Cooper:
 
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It's a pinion on the motor meshing with a crown gear on the first rear axle. On the outside of that axle bearing is a spur gear. Then that spur gear meshes with a pinion on an idler shaft. There is another pinion on the idler shaft that meshes with a spur on the second rear axle.
 
I want both rear axle to turn at the same speed. Is that what is happening here?
 
If so, does that mean the actual gear ratio for the first rear axle is being set by the motors pinion and crown gear as is normal. And for the second rear axle I can use any number of teeth on the other gears as long as both spurs on the axles and both pinions on the idler shaft are the same?

You "Gearologists", please help

[Lone Wolf]

Now that's scary. To simplify things I would use a variation on the 1-1 car method of checking gear ratio. I would mark both rear tires with a line at the intersection where they meet closest. Then I would rotate the tires one full revolution by means of the crown. If the lines line up you have a ratio match. Of course both rear tires would have to be exact or very close in diameter or just mark the rims to be sure.

[dc-65x]

Hi Joe,

 

I will do as you suggest to confirm my build.  But, I sure hate to go to all the work if I'm doing it wrong in the first place

 

I tried to PM Derek on Slotforum about his car but sadly he is ill and couldn't help.

[Pablo]

Holy cow, man. That is a brain teaser. 

 

Where is my doggone calculator?

[Tex]

Now, that there is some scratchin' and buildin'! VERY cool!

[SlotStox#53]

Wow, that's a gearing headache!! Good luck getting it worked out, Rick. 

Is this your next build?

-Paul

[slotcarone]

Very easy!!! Not the pinion on the motor and the crown that it mates with but the two other sets of gears have to match each other and you will have the same speed on both axles. It doesn't matter what the ratio is as long as they match.

[John Streisguth]

Beat me to it, Mike... what he said!!

[dc-65x]

Okie-dokie, thanks, guys. So just double checking to make sure I've got it "Korrect".

 

I set up the motor pinion and crown gears as usual. Then I can use whatever size spur and mating pinon gears as long as both sets are the same.

[slotbaker]

Yep, that'll do it, or how about keeping it real simple by using same tooth count for all three pinions, and the same tooth count for all three gears?

[dc-65x]

Thanks, Steve, for the help.

 

I was thinking about using 48p for the crown and pinion but going with 64p for the spur gears and their pinions for their smoothness. That would mean a different tooth count. Not sure yet...

 

Thanks again!

[Pablo]

Wow, I really disagree. but I'm not afraid to be wrong. 

My take is, every single time you "reduce" a ratio, via a gear reduction, the ratio changes, and gets higher.. 

In this case, the initial motor/crown gear reduction is about 3 to one.

Then, a spur, outboard of the bracket, reduces the overall ratio even further. For argument's sake, lets assume it is about 3 to one.

Now we have a six to one ratio in respect to the motor. If we keep reducing things, we end up with a very high gear ratio.

 

Am I wrong so far?   I submit to you, the goal of having all four wheels turn at the same rate is only going to be accomplished via idler gears that do NOT reduce the original ratio between motor and crown.

 

That being said, this setup is impossible due to space and clearance limitations imposed by slot car needs.

 

That is my theory. If I am wrong, it won't be the first time. 

[Duffy]

Not real sure what Pablo's going for here; sometimes we overthink ourselves into a conundrum and can't dig ourselves out....and I freely acknowledge I am one fair dinkum overthinker.
 
But, look at it this way:
Your motor drives the first set of wheels. Whatever ratio you have there, ALL that's happening is the motor's driving the rear wheels. Leave that alone now...
So, now there are two sets of rear wheels. Same diameter & ev'thing. What you want is to get them moving the same rate and that's all. Don't complicate it with what the motor's giving you, you've already got past that: just isolate the two axles, and you do that by having spurs of the same size and idler gears (the little transfer shaft between the rear wheels) of the same size. Pitch don't matter, ratio don't matter as long as the two spurs and the two idlers are the same.
& then you're okay.
 
Make sense? Or did I just overTALK us into even more puzzlement?
 
Duf
 
EDIT: Just making sure... Katz and Dub say it simplest, and then the rest of us go all geeky. But, Pablo, look: sure, you see a "reduction" or - what's the opposite of reduction? Abduction? Seduction? Anyway, look at the spur-to-idler ratio as ONE going IN, then the same going OUT. If the spurs and idlers are the same, it don't matter if it's reduction or ducktails in there.

Try a spur of 36 to a 12 idler, the idler shaft will spin at a 4:1 ratio... then go over to the other side to its 12 to 36, and it's 1:4. See" 4 equals 4.

The two spurs could be on just one side, with just one idler; maybe simpler, if you aren't worried about your centerline balance - and you might visualize the whole mechanism a little easier that way.
 
Short: equal gears on wheels, driving an idler. Go from there. -df

[James Wendel]

Sure, Duff. You are making sense. You just boiled down what the others have said. Everybody is right here... except Pablo. 

[Duffy]

Like I said, KatzenDub (oughta be a '30s comedy duo, huh?) just laid it out. I provided the obfuscation/illuminata. We all have our place here in the food chain.

[dc-65x]

Thanks, Duffy.

 

The reason I posted here is that I have the same gut feeling as Pablo. But, I knew Derek's car worked and it looked like he was using the same size idler and spur gears. I still have the same gut feeling that I'll be putting in a further gear reduction. 

 

So, my plan is to stop beating my head against the wall to understand. Instead I'm just going to learn and accept what's what.

Thanks again guys. Hmmmm..."Formula III For Fun"... hmmmmm

[slotcarone]

Hey Duff,

 

12 tooth and a 36 tooth - 3:1 ratio!!!

[Duffy]

Like my second edit: the thing's easier to visualize if you play with ONE idler between two spurs of equal size.

 

And, Mike... yup, right, and it took from 9:30 PM to now for somebody to Do The Math AAAAND it's one of Our Generation. Ohhhh wot is this world coming to!

 

Duf

[Steve Okeefe]

Rick,

 

Where is a good mechanic when you need one? These machinists have got the whole mess flummoxed up like kittens in a ball of yarn.

 

Yes, it will work just fine. The rule of thumb is that an even number of shafts meshed together changes the direction of rotation but an odd number keeps it the same - no matter how many shafts are in the gear train.  You have three shafts here; the two axles and the idler, so the direction of rotation on both axles is the same, and the idler turns the other way.

 

About gear ratios; if the RATIO of both gear sets (axle 1 to idler and idler to axle 2) is the same (regardless of the pitch or number of teeth involved), the rotaional speed of the two axles will also be the same. Period. Paragraph.

 

The only ratio you need to be concerned about is the one between the motor shaft and axle 1.

 

The other rule of thumb is that the efficiency of the whole gear train is inversely proportional to the number of gears it includes. You've got waaay too many gears here.

 

Rookies... 

 

Steve

[Pablo]

I came to my senses about 2 AM. I was totally wrong.

The first set of idler gears are reverse reduction, and the second set reduces it back to its original state.

As long as both sets of idler gear ratios are the same. Then you could fine tune it with the mark and measure idea.

[Steve Okeefe]

Pablo,

 

Bingo!

 

"You are a smart man Dr. Floyd, you will know what to do."

 

As an adjunct to what I said about gear ratios and how pitch and number of teeth doesn't matter, you could even use 48 pitch gears for the first set (axle 1 to idler) and 64 pitch gears for the second set (idler to axle 2) or vice versa, as long as the ratios of the two sets are the same. That would really confuse 'em, eh?

 

From a design perspective, I would consider attempting to use 1:1 ratios on the two gear sets to avoid having the idler shaft running at something akin to motor shaft RPM. Then you'd be looking at having to balance the idler shaft and its gears as a unit. 

 

Better, I'd go with something the Duf suggested; one pinion (same tooth count as the spurs) mounted between the two axle spurs.

 

Steve

[Cheater]

Just for some additional insight, if you have a train of spur gears, say five of them perhaps on the front of some tall gasoline motor joining the overhead cam with the crankshaft, the ratio is only determined by the tooth counts of first and last gears in the train. The tooth counts of the three intermediate gears have no effect on the overall gear ratio.
 
Rick, if you want the two axles to turn at the same RPM, the only thing you have to do is make sure the two spurs involved in transmitting rotation between the two axles are the same tooth count. That's it. The tooth count of the idler pinion is inconsequential as far as the gear ratio between the axles is concerned.

[dc-65x]

Thanks again for everyone's help, it's really appreciated!
 
Here's a link to Derek's car over on Slotforum:
 
Six-wheel slot cars
 
I've got something a bit different in mind but I need to mock it up first.
 
Let the games begin...

[MarcusPHagen]

It would be interesting to see if the motor shaft could be dropped sufficiently below the axles to allow an extended shaft with two pinions driving a crown gear on each axle. This would require larger tires, but would eliminate the spur gear train.

As an alternative to larger tires, a smaller motor such as one of Russell Sheldon's HO rewinds could lower the motor shaft.

At that point, though, I would prefer to use the smaller motor to drive the sidewinder gear train, and eliminate the inline crown gears.

Marcus

[Phil Hackett]

Yes. The two sets of wheels are turning the same speed as long as the gears are the same...
 
1/2 * 2/1 = 1