Rewinding armatures - questions

[narrowminded]

Definitely less friction with a Delrin on brass worm.  It's due to the materials but also the inherently high friction in worm gears.  And with these light loads brass is fine, evidence all of the locos we've seen that never wore out a worm gear set using brass on Delrin. 

The standard best and lowest friction mate to Delrin would be a hardened steel worm but again, for this light load application I don't think you could find much difference.  And worms are great for getting a lot of reduction in a small space but they are also pretty bad for the friction they generate.  Spurs are the best.

There are design principle booklets that you should be able to find with a search.  Boston gear used to have a handbook that I used to use regularly as a quick reference with a lot of good engineering information of this sort as well as detail beyond what you are likely to want.

Here's a quote from a regular Boston Gear catalog that offers some affirmation to what is being said here, the first sentence in particular.  "The high rate of sliding friction that takes place at the mesh of the Worm and Gear results in a more complex method of rating these Gears as opposed to the other Gear types.  Material factors, friction factors and velocity factors must all be considered and applied to reflect a realistic durability rating."   Throw in brass on brass and it makes for a pretty poor latchup.

So, your observations and suspicions are confirmed when all is considered.  

[nickelplate759]

What sort of Sagami did you use?  What size? 
I have one of those, purchased in the late 1980s.  I note from Spookshow's review that when Rivarossi took over production from Rowa, they changed the internal gears from plastic to metal.  So mine would have the metal gears, and it's silky smooth.  It can't run particularly slow, because like all engines of that era, it is geared too high.  But it certainly isn't noisy or rough-running at its lower speeds.

If you used an open-frame Sagami, or one of their 10mm cans, I would suggest you try a different motor.  Some of the open-frame ones were okay, some were not.  And the 10mm size were very weak motors.  Nothing like the 12mm black can motors they made, which were quite good.

It's a 14x20 Sagami that I bought from NWSL years ago.   Maybe "a little rough" is overstating the issue. It is noisy - maybe I got the worm mesh too tight?   I have a TCS M4 decoder in the engine, and slow speed is actually good for an engine of this vintage.

[peteski]

Thanks for the bearing details Max.
Man, that old RR armature looks really nice - like it was brand new.  If the commutator was also clean, then it would look like really new armature.

[mmagliaro]

Thanks for the bearing details Max.
Man, that old RR armature looks really nice - like it was brand new.  If the commutator was also clean, then it would look like really new armature.

YES!  I've got about 20 of these taken apart, and nearly all the armatures look spotless like that, and the commutators look awful (That's a good one in the photo).   These things break down because the comm slots load up, the motor gets hotter, the brushes fall apart and load up the comm more, and then he comm melts.  I don't think I've seen any where the coils look melted or in any way damaged.

[peteski]

Yes, those commutators are terrible!  They are just formed form copper sheet metal. Very imprecise, with large gaps between segments.  Like something one woudl expect to find in a cheap toy.

[peteski]

Or just change this topic to "Rebuilding Rivarossi"

Well, this is more about getting different performance from any permanent motor electric motor by changing the armature winding (Max just used Rivarossi  motor as an example) I woudl think that the "rewinding armatures" might stay as its  own topic.

[mmagliaro]

I did some RPM measurements.  I hand-wound another one of those armatures so I would have one here to
test with.  I only did 425 turns per pole instead of 460 this time.  It was easier to get them to fit.

Stock Rivarossi Motor (best one I have after trying a bunch)
Min starting RPM 4400  (starts at about 4.2 volts)
Max RPM @12v  18,500

My rebuilt Rivarossi with 3-pole skew-wound arm and Neodymium magnet ring:
Min starting RPM 1800 (at 2.5 volts)
Max RPM @12V 13,500

Rebuilt Rivarossi, with hand-rewound armature, 425 turns of  #42 wire per pole:
Min starting RPM 1250 RPM (2.5 volts)
Max RPM @12v  9,600

Given the performance we have seen, these numbers don't surprise me.   

Observations:
1.
The rewound armature, even though it has a much higher resistance than the factory-wound one, still starts at about the same voltage.  So it is doing what we hoped: making more torque at lower voltages - enough to start turning at a lower speed.  Interestingly, the current draw is about the same as the NON rewound armature!  About 35 mA.  I don't really understand that.  More windings, same voltage, you'd think the current would be lower.  It must have something to do with how the magnetic field affects the current draw at a lower RPM, but that's way beyond my education and training level with electric motors.

2.
The stock Rivarossi motor's starting speed and voltage are erratic (which I think we all know).  Sometimes it starts at 4.2, sometimes at 5.6, etc.  The starting speed is therefore all over the place, anywere from 4000 to 6000 RPM.  "4400" was an average, but this would be very frustrating to operate in a loco.  By contast, the new armature starts up within about 0.2 volts of the same number every time, and USUALLY dead-on the same.  If I rebuild one and it starts up at 2.4v, then it pretty much starts at 2.4 every time.

3.
With the stock Rivarossi, the cogging is so bad you can feel it in your hand just turning the armature with your fingers.  It's like the detents on a volume control knob - cog, cog, cog.  3 times per revolution.  The replacement armature and magnet do not do this.

[peteski]

The rewound armature, even though it has a much higher resistance than the factory-wound one, still starts at about the same voltage.  So it is doing what we hoped: making more torque at lower voltages - enough to start turning at a lower speed.  Interestingly, the current draw is about the same as the NON rewound armature!  About 35 mA.  I don't really understand that.  More windings, same voltage, you'd think the current would be lower.  It must have something to do with how the magnetic field affects the current draw at a lower RPM, but that's way beyond my education and training level with electric motors.

Yes, permanent-magnet DC motors are also electrical generators since the armature coils are moving across magnetic field from the permanent magnets.  That is BEMF.  The BEMF voltage is of the opposite polarity from n the voltage that is powering the motor when the motor is being used as a motor, not a generator.  There is a rather complex interaction between those 2 voltages (The BEMF actually reduces the current consumed by the motor). Things get even more complex when the motor is not free running, but operates under load.  I understand this interaction, but never got into it much deeper.

I'm also not sure about more torque being produced by the rewound armature.  The stronger magnet you installed likely increases the torque, but not sure how much of that can be contributed to more wire turns on the poles.  Too bad we don't have a good way to measure the torque of those motors at various RPMs.