Why would this Mashima motor falter from time to time?

[Captain Kernow]

I'm a bit puzzled by this. I've built a High Level High Flier gear box (54:1), no problems with that, went together fine and seems nice and free running. It's attached to a Mashima 1420, which is just a little bit 'coggy' when starting up, but seems to run smoothly enough at low revs for most of the time. However, every 1.5 - 2 secs or so, at slow speed, it falters, causing the loco itself to 'falter'.

I don't think the gear box is the problem, and I don't currently have an alternative motor to try on it, but are 1420s prone to occasional problems of this nature? It's not a problem at higher revs, but this is all about getting the loco to run smoothly at low speeds!

Many thanks.

[Horsetan]

How did it run before you installed it into the gearbox?

[Jol Wilkinson]

Tim,

it's difficult to see why the motor would falter regularly although a dry joint somewhere, coupled with a bit of binding in the loco motion and not evident/obvious at higher speed, might cause a momentary reduction of current/power.

Could there be heating/cooling of something in the motor causing a regular increase/decrease in friction, although I can't see what would cause it to happen over such a short period.

I always assume any poor running on my locos is down to something other than the motor. I can understand a gradual slowing through heating/friction, but not something as rythmic as this.

Jol

[Philip Hall]

Tim,

I had one one a while back (a Mashima, think it was a 14 - something) which did something like this, although not quite so regularly. I was pointed in the direction of the rear bearing, which can sometimes move inwards, causing binding against the commutator. In my case it didn't solve the problem, so in the end that nice Mr Gibbon exchanged it as a rogue, which completely solved the problem.

I would try it 'completely light' first, and if you hold the motor in your fingers as it revolves under power you will soon feel any stuttering even if it's not that visible.

Philip

[Captain Kernow]

Thanks all for advice so far. Just to explain, the faltering happens when there is no load, apart from the gearbox itself. It's harder to discern any problem with the motor running completely by itself, yet the gear box also seems very free running. I'm planning to try another motor on it (once obtained), which will be used for another project if the original 1420 is found to be OK.

I'll try it on it's own again, as Philip suggests, and see what happens.


Edit - I'm having a hard time understanding how the free-running gearbox could be the source of the problem. I've built quite a few of these before and this one seemed no different. I've tried sticking an axle through the drive bearings of just the gearbox and putting a wheel on the end of it, so as to be able to watch the rotation easily, and the 'faltering' doesn't seem to happen in the same place each rotation...

However, what could be the problem with the gearbox to cause this, I wonder?

I originally thought it was the motion, so I disassembled that, then found it was doing it with just the coupling rods on, so they came off, only to find the above...

[Horsetan]

Right, so if the motor spins fine regardless of how few volts you give it, and the geartrain appears to spin well by hand, and faltering only happens after you've connected the gearbox to the motor, that sounds to me as if:

- the motor shaft (or the initial worm) might imperceptibly be that bit off-centre. What happens if you were to change just the initial worm-and-wheel for a fresh set?

[Jol Wilkinson]

I can't see why just the worm being off centre would cause it. However, if both the worm and the worm gear were off centre, then there would be a cyclic variation in meshing accuracy as they both rotate. A bent motor shaft would have the same effect as an off centre worm.

While theoretically possible I would imagine that this would be most unlikely with HL gears (I haven't yet built any of the three samples I have as I find I always end up using LRM motor mounts and gears).

As Horsetan suggests, changing the gears would be the only way to prove/disprove that.

Jol

[Tim V]

I suppose the gears are clean? No bits of dust/dirt?

I have a gear where the teeth are not correctly moulded. It was the worm, fine in one direction, but juddered in the other.

Check the gears under a loupe.

[Captain Kernow]

Thanks again, folks. I'll check the gears again, although I have looked at them under magnification, and they seem fine.

I will have the option of trying another worm and motor in a few days, when my order to HL has been processed (only posted this weekend). The original brass worm is more or less permanently attached to the Mashima 1420 shaft now, by virtue of the superglue used to attach it. I wouldn't like to try forcing it off, too much risk of permanent damage...

However, I've been 'smoothing' the motor on it's own this morning, it does seem to run smoother, and have been progressively adding the gearbox (minus final drive gear), plus (after a further hour or so in each direction), the final drive gear wheel plus an axle (I've put a new axle in place, as the original one had score marks along it where I'd forced the brass final drive gear. I've also removed a tiny amount of material from the inside bore of the brass gear). The ensemble is again under test on the bench. Either tonight or tomorrow I may re-assemble the basic chassis and try running the loco for a few hours on a circle of track...

[Captain Kernow]

Further and extensive running in of the motor on it's own and then the motor/gearbox combination have yielded results, and it is now much smoother. The chassis itself has now been running in for an extended period, which has also brought about improvements.