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[andythenorth]

Astound me with simple maths

I have following case:

Rear wheels

PF XL motor

20t double bevel drive

12t double bevel idler

24t driven on 2nd gen differential

(all in same plane on a 1x5 L liftarm, keeping them together, but eh, details).

Front wheels

PF XL motor

??? drive / idler / driven gear(s)

driven axle

20t double bevel drive

28t driven 3rd gen differential

(bevel and diff are orthogonal in 5x7 rect frame)

So to get approximately same rotational speed out, what gear combination do I want between the motor and the axle driving the 20t/28t diff pair?

For extra fun, what if I substitute an M or L motor for the XL on the front wheels only?

Edited May 12, 2013 by andythenorth

[piterx]

i didnt get it exactly....you're saying that you have 2 motors giving 2 different speed to your axles?

i dont know what you're building but if you put the two motors together and put a differential in-between the two axles the problem should be solved :D

but if you asked this question i reckon that you can't do that lol

[750ACE]

http://gears.sariel.pl/

[pluto7443]

http://gears.sariel.pl/

dang, you beat me to it.

[Splat]

Let's assume that the XL motor spins at 146rpm at 9v (http://www.philohome...s/motorcomp.htm).

For the rear wheels, you can ignore the idler gear, and you get 146rpm / 20 * 24 = 175.2rpm (but I don't know how you are getting the 12 tooth bevel to mate with a 24 tooth 2nd gen diff using a 1x5 liftarm...?)

For the front wheels, without any extra gearing, you have 146rpm / 20 * 28 = 204.4rpm.

Assuming that you have the same size wheels, you need to get 204.4prm back down to 175.2rpm.

175.2 / 204.4 = 0.857

The closest single gear pairing to this is 24 tooth and 28 tooth, but it is difficult to get a 28 tooth gear. You can use another 1st or 3rd gen diff, but then why use the 2nd gen diff at all? You could also use a mini turn-table, but that introduces quite a bit of friction.

The next closest is the 20 tooth and 24 tooth, which gives you a reduction of 0.833, so the front wheels would spin at about 170.3prm.

If you use a different motor on the front wheels, just find the speed that they run on Philo's page, and then substitute the values accordingly.

But... why do you have to use two different types of differentials? Is this a limitation on the number of parts that you have available?

[andythenorth]

I don't know how you are getting the 12 tooth bevel to mate with a 24 tooth 2nd gen diff using a 1x5 liftarm...?)

Pictures attached, it's a 1 stud offset that works perfectly. They are inline, not orthogonal btw

The next closest is the 20 tooth and 24 tooth, which gives you a reduction of 0.833, so the front wheels would spin at about 170.3prm.

Thanks, that will do nicely

But... why do you have to use two different types of differentials? Is this a limitation on the number of parts that you have available?

Rear axles have locking diffs, gen 2 diff is easiest way to do that. Front axles are steering, the gen 3 diff is the only one that fits in a strong structure with a sane build time.

EDIT, added pictures. This is the rear axle unit, it's for a 4 axle truck, twin-steer front axles, dual drive non-steering at the rear. I'm building for towing performance and capacity to climb small obstacles. The whole rear unit is centre-pivoted as a walking beam. It doesn't need to oscillate in more than one plane; with the diff locks in there is always at least one wheel with traction, it works pretty well.

However the truck is currently 8x4, and the unpowered front tyres won't climb any obstacle higher than the axle line, the tyres just dig in. The rear drive is strong enough that it will then try and push the front axles off the truck. Converting to 8x8 is the obvious solution, and more power is good eh?

Untitled by andythenorth, on Flickr

Untitled by andythenorth, on Flickr

Untitled by andythenorth, on Flickr

Untitled by andythenorth, on Flickr

The rear unit is insanely strong for a such simple construction, and it's easily taken apart and fixed if a gear or axle breaks (which happens).

Diff locks are pneumatic, inspired by Sariel

The whole thing is a v2 of this http://www.brickshelf.com/cgi-bin/gallery.cgi?f=307640 previous version had gearbox with 2 speeds, M-motor driven diff locks and 1 PF XL motor for drive. It was way overcomplicated, a bit gutless, and broke a lot of parts. Construction was over-engineered, and replacing broken parts was a nightmare. Pretty pleased with v2

Edited May 13, 2013 by andythenorth