12v 80s Train Motor 7865: brass vs plastic gears?

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Posted (edited)

Hi all.

I have about 5-6 open (now "screw-able" ) 7865s and on two of their shafts I have brass bevel wheels (and on the motor shaft brass worm gears in all cases). All other shafts have white plastic bevel wheels.

I never saw any photos of brass bevels I think, and could not g**gle it.

I didn't pay much attention and had one brass and one plastic shaft combined in one motor. After a large number of rounds I noticed that one shaft was rotating freely. Opened up and damn! the worm gear running on the brass bevel was completely f*cked, nothing left of the spindle. The bevel looks undamaged. So, the brass bevel is a worm-killer. But ... it must have come out of one of my motors, I definitely have not bought them and swapped them on. I must have cross-swapped the brass shafts out of that source motor into the other motors at some point. I *believe* the brass ones may have come out of my very first motor (with fixed, non-removable pin on top), received in 1983 with my beloved 7740 under the tree. That one lost a worm gear in abt. 2014 when my little one tested her strength against the train, holding it in place while it was at full ahead. (After that my misery ("addiction" in psychology I guess) started,  as a replacement Motor had to be sourced, doubling the track length when coming with a lot of extra track . . . don't ask how many more meters, trains and motors there are today, just don't ask please (and don't tell my wife!) :-))) )

The other brass bevel shaft sat in a red motor safely stored and hardly ever run since I fixed it - luckily. Will swap that one out for a plastic one immediately to not risk more worm gears.

The question is: which types of 7850 came with brass bevels, and is there a difference then in the worm gears that came with them? I'm afraid there might be a mismatch from plastic bevel matching worm gears versus those that originally were in that brass bevel motor. I can't check it any more, as that original motor took damage to the commutator- fixed ring plate later on, and donated it's remaining worm gear to another motor. "All a mess" , sorry.

Also, does anyone have the correct specs for the bevels (plastic ones more importantly) and the worm gears? I never saw any modulus or ratio figures. Bevels have 18 teeth in both variants, and overall dims are easily measured. But detailed teeth and worm geometries not. It is not a modulus of 0.4 on the worms, I know cause I guesstimate that and got hold of two worms with that ratio, but they are different.

If we had those numbers...

Or, if we had alternative similar matching pairing of bevels and worms available from the hobby train sector... swapping worms and bevels off the lego shafts is doable. Only the parts need to be found.

Anyone of you experts?

Have a great day!


The white one has obviously suffered and isn't usable any longer. Not sure when and where that happened.


Edited by DoVoMonOgel
added pics plus caption

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It is fairly small so you could even try letting e.g. Shapeways print a metal alternative. 

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I wouldn't want bevel gears from metal ever again given my findings, as I believe the pairing brass-worm-gear vs. plastic-bevel-gear is "safer". If one would have suitable plastic bevels available from other sources (not sure if 3D printing is fine enough to produce such delicate structures with sufficient precision?), they would be the "sacrificial" part in the combo and are (relatively) easy to replace. Replacing worm gears is way more complex and risky. If the motor itself drops or you slip with your tools you may damage the commutator disc, and that's more or less it then (there is a guide in German here on how to steal such a disk (and commutator) from a different kind of motor, but that's quite a job). So the motor itself with the brass worm gears is the "holy grail" to be protected by all means I'd say.

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For the quest for the worm and wheel geometry: I took some measurements of the plastic gear wheel and the brass worm  the other day.

*Please* do scrutinize, question, correct! I'm not that familiar with worms :-)

Gear Wheel
(plastic, white; won't deal with the brass ones much more as I somewhat fear them now)

  • n = 18 teeth
  • D = 8 mm (outside / addendum / tip diameter)
  • d = 3,2mm (inner diameter on shaft; measured the shaft itself; believe there's not much difference as the actual transmission is by tiny teeth on the shaft onto which the plastic worm wheel is pressed)
  • face width of the teeth: 4 mm
  • base width of the wheel on the shaft:  8 mm
  • Modulus: m = Outside Diameter/(Teeth+2) = 8/(18+2) = 8/20 = 2/5 = 0,4 => this should then also be the modulus for the worm, which is harder to measure accurately

Brass Worm

  • 3-start worm (3 worms/screws/teeth on the cylinder); can be seen best when looking along the shaft axis at the flat face of the worm gear
  • L = 6 mm
  • D = 5 mm (outside diameter)
  • d = 2 mm (inner diameter; likely rather 1.95mm given the interference fit; I measured the shaft outer diameter)
  • approx. 1,66 windings for each worm along the 6 mm length (not relevant I guess)
  • Modulus? 0,4 as for the worm wheel above, right? Otherwise they wouldn't fit.

Transmission would then be 18(Wheel)/3(Worms) = 6 => 6 Turns of the higher speed motor shaft giving one turn of the wheel(s).

Can anyone crosscheck, verify, confirm or correct this please so that we don't have wrong numbers in the world.

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This is a very interesting and useful thread for everyone (including myself) who repairs 12v motors. I never saw brass gears on the axle in any opened motors, including the 3 opened ones I own (2 type 1's and a type 2). One thing to mention is, if this leads to manufacturing replacement gears, how are we to get them on to the axles? I haven't been able to get the original gears off unless they've been cracked (which has been the case with a few of mine). The axle has grooves in it which lock the gear in place - the gear can't slide off the axle and it can't rotate on the axle.

I will have to take a look at my motor parts later and confirm measurements for the gears. I know they changed the gears in the motors from type 1 to type 2 motors- the type 2 motors have thicker gears to mesh with the worms better.

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Removal is easy, if you have reason to remove, then the wheel is already damaged in one way or another, so cracking it off is just fine.

Getting it on might be a question of well-controlled force and maybe some cooling (freezer for 10 Minutes?) of the shaft and maybe careful warming (60C?) of the new wheel.

"Interference Fit" should already do the rest of the job. When the replacement gears are e.g. Nylon and their bore is 1.95mm with the shaft being 2.00, the nylon would sit tight alread and the groves might add to the fit.

First hit on g**gle tells me we don't have to reinvent the wheel here, literally :-) and given Bühler made the groves for us no need to roughen the surface.

Still not found any source for the worms. Not sure if what shapeways could do is the right level of precision after reading about their min / max dimensions for printed + polished brass. Needs to be polished to ensure it runs smooth against the (nylon?) wheel. Maybe we have to head over to some other model builder forums for some input...RC racing or train guys maybe.


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Would AliExpress have them?  They seem to have all sorts of gears for toy and RC motors.  I was able to find something that matched the pinion gear in my fired LEGO PF M-motors.

Resin 3D printers may have the resolution to produce.  You may have to use a tough resin.

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Have not found anything on ali yet. 1 lead (and some 2 lead) worms, but not 3.

For the interface of plastic gear wheel on shaft, I measured again on the one shaft I have without a gear:

The shaft is 3.25 mm where it has no teeth. And on the teeth I measure 3.45 mm. So they are actually teeth with groves in between, not only groves.

Hence, the bore in the gear should be 3.25 (-0.00/+0.05) I'd say. Then it would slip on easily along the shaft, and the teeth can cut into the bore to give perfect grip. If this cracks the wheel quickly, one can carefully pre-extend the hole slightly, or source gears with a 3.30mm bore next.

Need to find someone who can make those wheels. Have some guys with "normal" 3D printers around, but will need to give them a model file to start with.

What I can't measure is the helical angle of the teeth, it's not a "straight" "spur" gear. Not sure how to work that out accurately, will have to dig into it later.

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Could this work?
-Roll the gear on an ink pad and roll it on a piece to paper to get an imprint of the teeths.
-Draw longer lines and measure with protractor for angle.

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Did as dr_spock suggested. Not a very accurate way of getting the numbers, as the "source lines" printed by rolling the gear are so short. It *might* be 15degrees going by the worm. Is that a common value!? Going by a plastic and a brass gear wheel, I'm not entirely sure, might as well be something like 18 degrees. Will have to dig deeper/harder into worms and gears and common "pairs". Read somewhere (not sure I got that right, though) that angles aren't necessarily matching to allow (or prevent) blocking and/or optimize the line of contact between worm and gear. So might there in fact be some few degrees "delta" intentionally? Any experts around?

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