Building and Powering AAR A Trucks
Building and Powering AAR A Trucks
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 was never quite sure how 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 regarding what motors to use and how to mount them.
This plan has worked. I am nearing completion 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 switchers. 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.
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.
- Voltage: 24Vdc
- Rated Speed: 2650-2900 Rpm
- Rated Current: 16.0-16.4A
- Output: 300 Watt
- Sprocket: Removable Bolt Sprocket
- Case Length: 4-1/8" (105 mm)
- Case Diameter: 3-15/16" (100 mm)
- Drive Shaft Length: 1" (25 mm)
- Drive Shaft Diameter: 5/16" (8 mm)
- Bolt Hole Distance (Adjacent): 4-1/8" (105 mm)
- Bolt Hole Distance (Cross Bracket): 2-3/16" (56 mm)
- Motor rotation when red lead is positive: clockwise
1. There are different versions of the MY1016, each with a different power rating and slightly different size. There is a 350 watt version with a slightly longer body.
2. The mounting holes are threaded for metric bolts. If you'd rather use English bolts you can chase the threads with a 1/4-20 tap. Be sure to use plenty of cutting oil.
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
- (1) Martin SPK Roller B - 25B48
- #25 Roller Chain
- (1) #25 Connecting Link
- (1) #25 Offset Link
- (1) Taper Pins - #2 by 2-3/4 inch
- (4) Motor mount bushings
- (4) M6-1.00 x 40 Hex Cap Screws (Lowes)
- (4) M6-1.00 Nylon Insert Lock Nuts (Lowes)
- (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
- (1) Roller Chain Breaker
The only custom parts in the list above are the "Motor mount bushing" and the "Tee brace". I turned the 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 pillow block.
The "Tee brace" parts are modified stock Tee braces used in building construction. They prevent the motor from spinning on the axle.
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).
Use a Chain Breaker to remove the unwanted link. Refer to the video Splitting a Chain with a Chain Breaker
These pillow blocks were selected because they include set screws that prevent the blocks from sliding laterally on the axle. This photo shows one of the set screws. Make sure the chain is aligned between the motor sprocket and the axle sprocket, then tighten the Allen set screws on both bearings. Note that there are two set screws per bearing. Also note that the set screws are on the opposite side from the sprockets.
Test fitting the T-brace along with the U-bolt before cutting the extra length off the T-brace. The motor T-brace is attached to the motor pillow block using two nylon lock nuts. Likewise the U-bolt nuts are replaced with nylon lock nuts. This allows the tension on the rubber hose to be adjusted. The U-bolt/rubber tube configuration allows the motor to rotate on the axis as the wheelset moves up and down over the road.
The motor is secured to the outside frame using the modified Tee brace along with the UBolt and a section of rubber tubing. The combination allows the motor to move up and down with the shifting height of the axle. The flat bracket supplied with the UBolt is added on top of the Tee brace for added strength.
The factory connectors were cut off from the feed wires to the motor. Standard fork terminals were crimped onto the ends of the wires.
Note that the motors are facing opposite directions (see Photo 1). This will require swapping the leads on one of the motors so that the wheels will turn in the same direction.
- AAR Type A Switcher Truck
- AAR Type A Truck, Archive.org
- "Railway Cab Truck", U.S. Patent 2,137,704
- "Installation and Maintenance of Roller Chains" (PDF)
- "Splitting a Chain with a Chain Breaker", YouTube.com
- Alco S-4 Truck Motors, Chaski.org
- Powerpole Connectors, Anderson Power Products
- PWRcrimp Crimp Tools, West Mountain Radio