Garden Railway Forum

Hi.
Can anybody throw some light on why some of my 16mm scratch-built electric diesel's in one direction it's very noisy but when switched to the opposite direction it's quiet, I use MFA motors and the usual brass worm and drive gears? Thanks, Frank.

Hi Frank,
Do you run your locos mostly in one direction and only occasionally in the other? I'm wondering if the problem is a function of 'wear' or 'running in'.
There are more knowledgeable people on here with experience of electric motor running who I am sure will chime in currently, but more information may be helpful to them also.

Hi Frank, you are not alone in this matter.
All of my locomotives are battery operated with a variety of electric motors and many of them suffer from the same condition. Whether it is the meshing of the gears or some other rubbing/grinding I have never discovered. No doubt someone has done a study.
Interesting you mention it regarding scratch-built models, does it not happen on others too?

Hello Phil.
Thanks for your helpful reply. I took another long look at the worm gear on the motor shaft and could see slight movement when the motor is switched between forward and reverse so I moved the worm gear further along the shaft from the motor and no more noise Perhaps some motors shafts move more than others when put in forward and reverse you get what you pay for I shall investigate more. Hope this helps other enthusiasts. Frank.

A worm gear although it will run on a 90 degree flat drive gear, because of the angle of the worm gear is cut actually runs much smoother when set at the same angle as the worm "tooth". I found this out by printing a rail car from Thingiverse and it took me a while to work out why he had offset the motor at a slight angle, the result, perfect silent running backwards and forwards. It makes sense when you think about it.

Just to conform I have exactly the same problem with most of my electric locos and most of those are on brand new gears. I will be trying my own offset motor at some point on a future loco to see if I get the same results.

frank3447 wrote: ↑Sun Apr 10, 2022 10:47 am I took another long look at the worm gear on the motor shaft and could see slight movement when the motor is switched between forward and reverse so I moved the worm gear further along the shaft from the motor and no more noise.

As you found, end float in the motor shaft is the usual cause of more noise in one direction than the other. Wear and/or poor meshing is more likely to make the gears noisy in both directions.

Worm gear sets need to have thrust control on the worm shaft so that there is minimal longitudinal movement of the worm, but it is still able to rotate freely.

Worm drives generate high end thrust on the worm shaft. Most of the small motors used in models have no control of longitudinal movement of the armature shaft and the bearings aren't designed to take end loading. So this has to be done with some form of thrust control on the worm. I checked a MFA-Como 385 motor I have in stock as a spare and the armature shaft has about half a mm of end float.

Also worms are meant to mesh with a matching worm wheel, which has the teeth cut at an angle and hollowed to wrap partially around the worm. They aren't intended to be used with straight cut spur gears as often found in cheap toy drives, which leads to high tooth loading and rapid wear. Worm drives aren't particularly efficient as the teeth have to slide against each other, rather than the rolling contact of properly made spur gears, so some power is lost in the worm drive.

I've lived with worm drives in HO scale models due to the compact size, but I try to use gearboxes if at all possible. In large scale space isn't such an issue and I prefer to use gear motors with closed gearboxes and mitre gear final drives. My battery electric models are quiet enough out on the track, that the metal wheels of the train running on metal rails make more noise than the loco.

Regards,
Graeme

frank3447 wrote: ↑Sun Apr 10, 2022 10:47 am Hello Phil.
Thanks for your helpful reply. I took another long look at the worm gear on the motor shaft and could see slight movement when the motor is switched between forward and reverse so I moved the worm gear further along the shaft from the motor and no more noise Perhaps some motors shafts move more than others when put in forward and reverse you get what you pay for I shall investigate more. Hope this helps other enthusiasts. Frank.

Glad you managed to sort it Frank. And like I suggested, loads of other helpful advice from our good members too.