Building and Powering AAR A Trucks

Building and Powering AAR A Trucks

by Daris A Nevil

5 March 2015

A little over 12 years ago I purchased a set of castings from MDM Locomotive Works to build a pair of AAR A trucks. I am really impressed with this kit, as it includes everything except the motors. It includes all screws and hardware, flame cut parts for brakes, wheels, axle material, even a set of Clippard air brake cylinders. It's really a shame that no one has taken over Jim Murray's fine business.

I have worked on this kit off and on since I purchased it. I had never quite been sure how I was going to power the trucks. I wanted electric driven axles, as I know how to design and wire the electric circuits. Recently (well, about a year ago), I decided to put more effort into constructing the trucks, partially in hopes that an epiphany would come to regarding what motors to use and how to mount them.

This plan has worked. I am nearing completing of the trucks, including the electric motor drives. The motor design is simple but effective, and may be used on other truck styles or even single axle trucks, such as on Plymouth switches. This article will illustrate the method and materials I used.

AAR A Trucks


First, a little background on the AAR A Diesel Truck.

Martin P. Blomberg filed a patent application for the truck design on 6 July 1937, and the patent was issued on 15 November 1938. Blomberg worked for General Motors Electro Motive Division at the time.

The AAR A trucks were designed for switcher service where the more complex high-speed passenger locomotive trucks were not needed. The truck uses drop equalizers but does not have swing hangers as used on the Blomberg B and others. A combination of coil springs, and leaf springs, positioned fore-and-aft inside the drop equalizers, give a good balance of damping. The wheelbase is 8 feet 0 inches.

Electric Motor
I finally settled on an electric motor for the trucks. I chose the MY-1016 scooter motor, which is commonly available on eBay and other scooter supply sites on the Internet. The motor has a convenient four-bolt motor mount and includes an 11-tooth sprocket for #25 chain. It is rated at 24 volts D.C. with maximum power consumption of 250 watts (about 1/3 horsepower). The best part is that the motor is very affordable. They can be had for less than $40 each.

A single MY-1016 motor drives my mini-F9 diesel, which has plenty of power for hauling. I believe that four of these motors, one per axle, will provide more than enough pulling power for my target diesel.



Bill of Materials
This section provides a list of materials used for mounting the motor to the axles, and for the chain drive system.

Each wheel set (pair of wheels and an axle), use the following components. The quantities required are shown in parenthesis.


 * (1) Electric Motor - MY-1016
 * (2) Pillow Block - UCP204-12
 * The Big Bearing Store
 * (1) Martin SPK Roller B - 25B48
 * Martin Sprocket & Gear Inc
 * McMaster Carr, Part #2737T32
 * (1) Taper Pins - #2 by 2-3/4 inch
 * (4) Motor mount bushings
 * (4) M6-1.00 x 40 Hex Cap Screws
 * (4) M6-1.00 Nylon Insert Lock Nuts
 * (1) 5" x 5" Tee-brace (Home Depot), modified as shown below
 * (1) Everbuilt #304 1/2 inch pipe U-bolt (Home Depot)
 * (1) 5/8 inch outside diameter rubber hose (such as for car coolant system), 1 inch long

The only custom part in the list above is the "Motor mount bushing". I turned these bushings out of 3/4 inch round bar CRS. They help stabilize the pillow blocks and provide a solid foundation for the heads of the bolts securing the motor to the block.

I ordered the large sprockets directly from Martin Sprocket. They came with a 1/2 inch bore and no key or set screw. The MDM truck kit came with 1/2 inch axle stock, but I prefer 3/4 inch and substituted using 3/4 inch round CRS. The sprockets were drilled and bored from 1/2 inch to 3/4 inch, then placed on the axles in the correct location and taper drilled all the way through. The sprocket was locked into place by driving the taper pin and cutting off the excess.

I later found the equivalent sprocket from McMaster-Carr. The next wheel-sets I build will use the McMaster-Carr sprocket, as it is already bored for a 3/4 inch axle and includes a 1/8 inch keyway and set screw. It is the same price as the Martin sprocket (I'm guessing the McMaster-Carr sprocket is also made by Martin).