Powering Models - Part 2: Direct-Drive
In this feature, we'll demonstrate a simple way to create motorized four-wheel direct-drive chassis. The model shown in the photos was built using Merkur parts and the Merkur two-speed DC electric motor, although this technique can be adapted to virtually any system or model design.
Unlike many construction systems of the past, most modern brands utilize small battery powered electric motors rather than more powerful AC motors, and Merkur is no exception. Their motor is housed - typically - in a plastic case that also encloses an integrated gearbox. Conveniently, the motor's driveshaft extends a half-inch or so from both sides of the housing, making the design shown on the photos on this page possible.
One of the simplest ways to use any motor is to attach a pulley to its main driveshaft and another pulley to part of a model that you want to move, then connect them with a drive belt made from string, rubber bands, or something similar. Many of the models that you can see in the Model Gallery here at Girders &Gears use this technique, including two older Merkur designs that I adapted to use the motor (see the Arched Eccentric Press and the Upright Iron Planer). The main shortcoming of this method is the tendency for the drive belt to slip on the pulleys (one solution to this problem is to use a rubber o-ring for a belt, as seen in the photo shown at right).
Another common method involves "direct-drive", i.e., attaching the motor shaft directly to the movable portion of the model via another shaft, or series of connected shafts. Gears can be used to help control speed or to change the angle of motion. This technique is the one I used in the motorized chassis design shown below. It features a frame that can be readily adapted to any height, width, or length desired. By adding a few brackets in the right places virtually any body design - car, truck, jeep, etc. - can be fitted onto it. As it happens, I have designed this chassis to the scale dimensions (wheelbase and wheel track) of a military HUMVEE, using the large hollow Merkur tires with hub bosses to determine the correct measurements of the frame (incidentally, these tires have the precise scale dimensions of actual HUMVEE tires!).
This four-wheel drive chassis utilizes two fixed (unsuspended), rigid axles of equal length (70mm). The motor is mounted upside down in the center of the frame with its shaft oriented parallel to the frame's long axis. A 40mm shaft is attached to the front end of the motor shaft with a #1049 coupling. A #1046 pinion gear mounted on the other end of the 40mm shaft meshes with a #1057 crown gear mounted on the front axle. This arrangement is repeated between the rear end of the motor shaft and the rear axle. The only difference between the front and rear gearing is the orientation of the crown gears, which sit on opposite sides of the pinion gears (see photo below). This is necessary for the two axles to rotate in the same direction when the motor is under power. The motor can be set to run at low or high speed with a minor adjustment of the #1049 couplings. I set mine on high, but it runs great at either speed!
Underside of chassis showing motor and drivetrain.
The motor is suspended upside down in a framework made from 2 5x5 flanged plates and 1 5x5 flat plate; it is held in place by small bosses molded onto the motor housing that fit into the holes in the plates.
In the next installment, I'll show you a variation on this design that is much more complex, although it is only two-wheel drive: it features steerable wheels, independent front suspension, and coil springs at all four wheels. Watch for it in Powering Merkur Models - Part 3, coming soon...