Axle Hung Traction Motor

Intro.

I'm modelling BR 3rd rail electrics in P4, my chosen period is around the time of the Bournemouth electrification. I was never happy with the old "pancake" type motor bogies and a big motor inside the coach was even less appealing. What I wanted was to get away from the worm drive and the motor shaft at 90 degrees to the axle. I also wanted the same number of motors per unit as the prototype, i.e. 2 car unit 2 motors, 4 car unit 4 motors. I first toyed with the idea of using small Mashima motors axle hung but this would be expensive and far too powerfull. It was two things that helped me down this route. Ted Scannell of CLAG built an axle hung traction motor using a belt drive. I did some sketches with belt drives but wasn't entirely happy with the arrangement. I wanted to make it more compact. It was browsing the NWSL web site that I found something called a "Magic Carpet" for 0 scale American street cars. This used a small can motor with a plastic housing  and a three stage gear train. This was what I was looking for.

The Motor Blocks.                                                                                                             

                                                                                                        
  
 The original intention was to use cast iron. The reason for this was the motor block would act as a bearing for the axle, and it would act as a heatsink for the motors. An early prototype was made in iron, but all I have available is rough offcuts and there was a lot of work required to get it square and to size. A change to steel was made because it is readily available in sizes that suit the motor block, which is 12.7mm square by 8mm thick. Brass bearings are fitted and protude to fix the wheel back to back. The block on the right has been fitted with motor which is a Nigel Lawton Micro Motor. These motors measure 6.03mm in diameter, so a 6mm hole is far too small, and a custom reamer had to be made. This was after the first one was opened out with a file, and the commutator was push out the back of the motor.

The Gears 




  These gears are made by Didel in Switzerland. The pinion is a 12t and has to be cut in half. The two spur gears are 36/12t, and the 12t has to be reduced in thickness, with one of the bores being opened out to suit suit the axle diameter. The final drive gear is a 36/12t with 12t cut off and secured to an Alan Gibson wheel and pin point axle with 14BA countersunk screws.          

Assembled and ready for testing.                                                                                                             


    
  The brass bearings are 3mm long to reduce friction. and the clearances are very tight. Too loose and everything wobbles around with the risk of the gears becoming displaced and jamming. The motor shaft is only just clear of the wheel back. Too tight and the motor would be severly overloaded. This view shows the Gibson wheels before the backs were faced flushed.

Testing under power.




  Running at the full 6 volts before any grease was applied. The second motor was greased on assembly. They were briefly run up to 12 volts, just to see what happened. They run faster and didn't get much warmer. This testing also gave a bit of running in which reduced the starting voltage. After prolonged running at 6 volts the motor blocks were just warm, which was very encoraging as it showed that the heat-sinking of the block was working.  When I received my first two motors I ran one at 6 volts and held the motor in my fingers, it got a lot hotter than this set-up.                                                                                                              

The Motor Bogie



                                                                                                                
   Two motors assembled in a heavily modified Masokits pinpoint bogie. The motors are wired in series for testing and fitted with temporary pick-ups. Load testing will use a Bachmann Mk1 using DC power. It is intended to carry out full testing using an Ian Kirk 2 Bil set, DCC and live axle/bogie pick-up. I'm going to try a parallel/series switching arrangement using a function and relay on the DCC.

Load Test.




Under a Bachmann Mk1 for load testing. All my coaches are fitted with Masokits pinpoint bogies so they are a straight swap, hence the temporary pick-ups. The bogie was quite happy moving this coach and another Bachmann Mk1. Control was very good, with slow running and a reasonable top speed. It was a little noisey but this is possibly because one of the final drive gears is slightly eccentric. A drilling jig will need to be made to correct this. Also it was found that the motor pinnion on the same unit wasn't in full engagement with it's spur gear. A minor adjustment to the motor in the block fixed this.


Graham Hickson
gray@ukfsn.org