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[MOC/WIP] PF Reduction Gears

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As I posted in one of my other threads (from several upon several months ago) my Emerald Northern is set up to be powered by two L-Motors. I am wanting to get back into designing larger locomotives with more power. I plan on not only having the true driving wheels be powered, but also the wheels in the tender. Doing something called [Math] I figured out that the smaller LEGO train wheels have half of the circumference of the larger wheels and therefore spin twice as fast. I figured that instead of speeding up the larger wheels that it would be easier to make the smaller wheels spin slower.

Here is the current L-Motor setup in the Emerald Northern:

01_zpsu33j97to.png

This setup is using standard 12-tooth gears with a 1:1 gearing. To achieve a 1:2 ratio I came up with the following idea, which would be mounted in the tender:

02_zpsza9ytosn.png

This site, http://gears.sariel.pl/ was shown to be by Cale Leipart a while ago. The 1:2 reduction is achieved by the 12-tooth gear that is attached directly to the L-Motor to be driving a larger 24-tooth gear. According to my [Math] and [Theories] this should work. However, any additional input or corrections would be most helpful!

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You should also take into consideration that different motors might behave differently. Best would be to have some servo (loop control) principe, but will be difficult in LEGO I think.

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If both axles turn at the same rate, the larger wheels will cover more distance than the smaller wheels. (This is why fast passenger steam locomotives had huge drivers.) To make both wheels cover the same distance, you would need to slow down the large wheels or speed up the small wheels. Slowing down the small wheels will just make the problem worse.

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If both axles turn at the same rate, the larger wheels will cover more distance than the smaller wheels. (This is why fast passenger steam locomotives had huge drivers.) To make both wheels cover the same distance, you would need to slow down the large wheels or speed up the small wheels. Slowing down the small wheels will just make the problem worse.

So my math was backwards, okay. For some reason the large driver versus smaller driver did not occur to me.

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Worse, I think the math is wrong. The official small wheels have a "nominal" diameter of 17.6mm; measuring with my calipers yields values closer to 17.9mm. Doubling that gives 35.2mm or 35.8, both significantly larger than the lego drivers, even accounting for the added thickness of the rubber band. Those values are also smaller than the Big Ben Bricks XL drivers, which have a nominal diameter of 36.8mm. The nominal sizes lead to the inconvenient ratio of 19:11. Since those are both prime numbers for which there don't exist Lego gears, a design with this exact ratio is does not exist, so this approach isn't going to work ... or is it?


What happens if you use a close but not exact ratio?

  • If there isn't enough friction, one set of wheels will slip and be pulled along by the other set. That means they basically contribute nothing to traction or pulling power, so there's no point to the design.
  • If there is enough friction, the two sets of wheels will be forced to turn at the same speed. This works, but since at least one of the motors is being forced to turn at a non-ideal speed, you will be wasting some electrical power.


In fact, there is a mechanism that allows driving multiple sets of outputs at different speeds -- the differential, which is used in cars to drive the left and right wheels at different speeds of the same output when the car is making a turn. It's also seen in four-wheel-drive vehicles connecting the front and rear axles. Here's an example:

diffdemo.png

When power is supplied to the grey differential cage, this mechanism will power the blue and red axles without slipping, even though they are turning at different speeds, regardless of what the ratio between their speeds is. However, the axles are at the wrong height for the chassis to be level -- solving this is left as an exercise for the reader. You'll also have to figure out how to pass the power between locomotive and tender.


At this point, I have to ask what your objective is, since the right course of action will depend on if you want to pull light loads fast or heavy loads slowly.

About half a year ago, Commander Wolf and I ran some tests to figure out how much power our locomotives put out. One of the conclusions we reached is that pulling power is usually limited by traction, and not power supplied by the motor. Traction can be improved by using better tires, increasing the weight of the locomotive, or concentrating the weight on the powered wheels. The third option can be achieved by adding more powered wheels, or by distributing the weight differently -- one way to do that might be to only power the wheels in the tender, and then move all the PF gear there as well (to get the weight from the battery). You can then fill extra gaps in the tender with weight to get even more traction. Hiding all the PF gear in the locomotive is an option as well, but tends to be much more difficult.

Edited by jtlan

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I think the large end of the diff will scrape against switches and such.

The diff can be mounted higher up and you can use u-joints to connect the drive shafts.

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I think the large end of the diff will scrape against switches and such.

solving this is left as an exercise for the reader.

:wink:

Edited by jtlan

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This is the same problem faced by the designers of real steam engines. In the end they decided to power only 1 size of wheels, except for a few unusual locos.

The key has always been adhesion weight - getting weight over the driving wheels to put traction on the rails.

A 4-8-0 scores better than a 4-6-2 because the rearmost wheels are driving wheels. The rear pony truck (-2) should not take much weight because this removes it from the driving wheels. In fact in a LEGO loco it should take almost no weight at all.

In my Co-Co diesel locos I have put the weight on the driving motor axles of each bogie and let the trailing pair of wheels float so that the weight of the loco is adhesion weight.

Mark

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