The On-Line Magazine of Rideable Model Railroading

  NUMBER 164


© December 21, 2011   

 ©Discover Live Steam. This material may not be published, rewritten, or redistributed without written permission.


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Building a Freelance Electric Locomotive


by Al Crowley

It all started with a discussion at the local track about the simplest way to build a powered locomotive. The biggest issue was the axle bearing journal boxes. Most of the in frame designs required a milling machine to cut the slots that guided the journal box and allowed it to slide up and down with a spring action due to the track irregularities.

After a rather extensive discussion and a pot of coffee I suggested a 4 piece bolted construction housing. The parts were the outer cover, a ,030 shim plate, a center bearing holder and a rear cover. I volunteered to build a prototype system to prove the concept. After some time on a CAD program I had a set of rough drawings to work from and began with the bearing housings as described below.

The front and rear covers are identical except the front cover is drilled for the #8-32 assembly screws and the rear cover is tapped for them. The .030 shim is about .015 smaller on each side than the center bearing holder.

Blanks for front center and rear of bearing housing. Click to enlarge

The center bearing holder on my prototype was cut from 3/8 thick cold rolled steel 1 ¼ wide by 1 3/8 high. The 4 holes were drilled to clear the #8-32 screws and the shim was punched to clear the screws. The covers were 1 3/8 x 2 inch long cold rolled steel. When everything was screwed together the unit formed an “H” section that had .030 side clearance in the 3/8 thick side frame. 

At this point a cutoff saw and drill press were the only power tools used and the only critical dimensions were controlled by the standard material sizes and the accuracy of the hole center layout for the drill press.

Once the bearing housings were assembled the hole for the needle bearing was positioned and marked for drilling and reaming. This was done with the covers, shim, and bearing holder bolted together. The bearing was reamed .6875 to be a press fit for a standard ½ inch I.D. Torrington bearing. 

With the bearing housings completed the side frames were laid out with the 3 pockets for the axles spaced 12 inches apart and 1.281 wide on 3/8 x 4 x 42 inch long cold rolled steel. The slots were cut 2.25 deep again using the horizontal/vertical band saw set for the vertical position. This takes some time since the cut needs to be as smooth as possible and exactly on the line. Once the pocket was cut out a few strokes of the file left a smooth surface and the .030 clearance on the sides and faces leave room for the housings to move around to compensate for track variations.

The next step was to cut the 2 ends and 3 top plates from 3/8 x 4 cold rolled steel and drill and tap the holes for assembling the frame. Everything was checked and one last disassembly of the frame was done to drill the spring guides holes in the bottom of the pockets to keep the springs in place. ¼ roll pins were used that were about ¼ inch above the bearing housing when the locomotive was fully assembled on the track with the motors and batteries in place. The springs I used were set to keep the bearing housing 1/8 above the retainer plate when the locomotive was sitting on level track. 

Frame sitting on top of table saw with bearing housings, end and top plates. The body mock up is in the back ground.

The motors were (2) 250 watt 24 volt DC scooter motors wired in parallel to a 60 amp controller. One chain ran around the drive sprockets and an idler to allow tensioning the chain, The power was provided by 2 deep cycle marine batteries and reversing was done using 2 single pole double throw 30 amp automotive relays wired to work as a DPDT (double pole, double throw) relay.

The first trip to the track about 6 weeks after the coffee pot discussion proved the concept was workable. The only problem was the sprocket ratio needed to be revised to increase the torque and reduce the top speed. 12 MPH on 7.5 gauge track is FAST. That is what happens when you reverse one of the sprocket sets. Once the speed was down to a more reasonable 5 MPH the locomotive would pull 6 cars and 3 riders around the track and over the 2.3 % ruling grade with out exceeding the motor load current.

Locomotive with temporary body   structure. Now that everything has been thoroughly tested the final body  and details will be added.  Click to enlarge.

The final step was to mock up a body that looks like a Davenport or similar yard switcher and finalize the mechanical system by putting some hours on the drive to see where there are problems. So far the only derailments have come from trying to run over a 2 in diameter Oak branch that fell out of a tree around a blind curve and a broken rail at  switch frog. The engine is stable and with the 3 axle design will track amazingly well. 

Based of the success of the design I am starting construction of a riding car for behind the engine to look like a calf unit and have 2 more 250 watt drive motors and 2 more batteries.

The calf should be done by the end of the month and with the 4 motor set up should have all the power necessary for any yard or mainline job. The build for the 2 prototype units will be 8 months of spare time or about 280 hours so far. The final cabs and details will add another 100 hours.

The locomotive was built to prove that a functional unit could be built with a minimum of machinery for under $1000.00. This amount will be very close to the final cost of the locomotive after taking out the cost of the extra sprockets and steel that I purchased for use in the powered calf unit that I am building.

The final cost for the calf unit will be less as it will only have 2 axles. That is less sprockets, chain, axle and wheel set required. The frame is also a lighter construction since the on the calf since that is where the engineer will ride.  

If anyone is interested in more information or details of the frame and drive I have a set of drawings and a list of parts available for a $10.00 handling fee. I can be contacted at the following E mail: crowley1127@yahoo,com


Written by Al Crowley


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