Dual Motor Conversions

This page explains how to convert the single motor gearboxes to dual motor gearboxes.

Use Parts from Two Skil Screwdrivers for a Drive System

This modification came from Rich Upton and he says it takes quite a bit of fiddling and creation of new parts. Here is a brief description of what he did:

First, I ordered some 9 tooth nylatron sprockets from Stock Drive Products in New Hyde Park N.J. and obtained some 1/4" steel chain from a local surplus place. Next, I obtained two refurbished Skil Twist Extras from the Skil factory service center, they're cheaper, and took out the motor-planetary gearbox units. Then, I punched out the drive dogs and got down to the basic splined output shafts. I dumped out these gears and put the shaft back on each. On two of the Nylatron gears I cut off the portion containing the allen screw in order to make as short a sprocket as possible since the output shafts are so short. In addition, I found tubing with a wall thickness of .011" and made shims for the shafts since they are undersized, e.i., short of the standard 1/4" diameter. Using an arbor press I mounted the sprockets on the shafts. Next, with a vise, I drilled through the sprockets, shims and shafts and hammered in 1/16" rolled pins. Later, an interested buddy made up two Lexan mounts for the Twist Super drive units. Remember, I didn't use the stock outer screwdriver shells. The Lexan cradle mounts provide for a plastic cable tie to be passed around the gearbox to really lock them down since the simple horizontal screw and bolt sets that clamp the gearbox and motor to each other doesn't prevent the unit from exploding out of it. However, since this was just a prototype it didn't allow for the tremendous torque! The drive units are mounted to aluminum plates that are each in turn mounted to the King Tiger hull in three places on stand offs. The base plates have elongated slots for chain tightening, i.e., these plates move, not the motor mounts. I used some surplus 1/4" bore flanged bearings on 1/4" steel shafts to go with the 9-tooth sprockets. One end of each shaft has been turned down to 5mm so it inserts into the brass Tamiya fitting that accepts the tank's sprockets. Since the o.d. of this fitting is 8 mm I had to buy matching bearings. Each shaft is located by two aluminum bearing blocks of my own creation. So what you have then on each side is this: the inboard bearing is the 8 mm bore, of course, followed by the 9-tooth sprocket, then shaft collar, nylon washer, 1/4" bearing in block, second nylon washer and final shaft collar. The tank's sprockets have to be displaced slightly back and upwards to accomodate the 9-tooth sprocket with chain. I couldn't find a smaller size. However, I could've used .1475 pitch chain and sprockets and not have had that problem, but I will do that with my next "Twister" tank.

Expect to spend a couple weeks of free time to do the conversion. I have had problems with the track breaking. Here is the reason: with the old clutches when a rock got jammed in the idler, etc. the clutches would slip. With this chain drive there is nothing to give. I have decided that since the screwdrivers used 3.6 V batteries I shouldn't run them at 7.2 or even 6 V. Even at 1:1 ratio the King Tiger doesn't behave in an authentic manner. Believe me, at 7.2 it will climb any angle until it loses traction! Later, I will have the tank running slower with less track breakage. BTW this conversion saves space. Basically the space the clutch would have taken up is saved for other things. The new Sherman type gearboxes take up a tad less room, but I know my system is more robust with its bearings and planetary drives. One Further improvements I am contemplating is attaching a flexible coupling to the sprockets to absorb some of the shock on take off, and maybe mounting the gearbox assemblies to powerful springs to also smooth out the torque shock. I think though, that if I just reduce the voltage by half that these changes may not be necessary. I did try using 3/32" aluminum tubing in the final drive sprockets, but they sheared off too easily. I replaced them with 3/32" annealed brass tubing and I found that the brass was too stout to shear. Perhaps if I go to 3.6 volts the aluminum might be suitable after all. This would mean driving responsibly since replacing the shear pin would mean taking off the track, sprocket, withdrawing the shaft and so on. If I'm in the middle of a run, I would be prone to put the tank aside and grab another one!

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