If torque is your thing and you want a slow crawler, then you might want to consider this mod.
The idea was suggested on a forum by E. Nigma, and as far as I can ascertain, has not been previously
developed. I've run with the idea, modifying a steering servo from a ZipZap SE to drive a standard MT
transfer gear. A step-by-step tutorial of my version of such a modification follows below.
I've used a stock SE steering servo motor, without the potentiometer, as shown in the images.
Other ZipZap and ZipZap SE motors with differing speeds will also work. The motor you select will
determine how you make the electrical connections -- I'll leave that part of the mod to you. Likewise, any
of the other stock ZZ MT gears will work. Final performance is your choice. It would be interesting
to hear what motor and gears you choose. I wish you well.
Two items on a stock ZZ MT hinder the proper seating of the ZZ SE steering servo,
viz., the left-side PCB cover mounting screw and the motor "cradle" in the motor well. The first is
simple; just remove the PCB cover -- it will be modified and replaced once the servo is installed. The second
requires the removal of the cradle. An Xacto knife, an end mill, or a small sanding block would work.Removing the "cradle" permits the rebuilt SE steering servo to sit deep enough in the motor well to permit the pinion gear to mesh with the transfer gear. The PCB cover is removed to eliminate its interference with fitting the servo motor. It is easily modified by filing or sanding the left mounting tab at an angle as shown in the accompanying picture.
1. Change the Servo ProfileIt is necessary to remove parts of the ZZ SE steering servo to permit a good fit in the ZZ MT's motor well. Remove those items shown in the pictures on the left and right. Cut or saw the items away from the servo housing, then sand or file the areas smooth.
2. Remove the Output GearThe output gear is physically connected to the final gear located inside the servo. The output gear needs to be removed because its teeth will not properly mesh with the standard MT transfer gear.
Connect the motor leads to a 3-volt d.c. source and while the output gear is spinning, cut it down to the shaft using an Xacto knife or razor saw. Disconnect the power source, remove the output gear and put it in your parts box, then press-fit the pinion gear of your choice on the servo shaft.
Common pinion gears include the stock MT pinion (which will mesh properly with the 7-tooth MT transfer gear, a stock 6-tooth ZZ SE pinion gear, and an 8-tooth pinion cut from an MT transfer gear (see the tutorial for the fast MT gearbox).
3. Add the Front Cover/SpacerTo strengthen the servo housing and properly align the pinion gear, add a spacer to the front of the housing. Note the added strip of styrene used to brace and support the spacer, as shown in the picture on the left.
4. Add the Back SpacerThe back spacer must be thicker than the front one. Cut two pieces of styrene and cement them against eachother. When the adhesive has set, cut or sand the notch in the space, then add the spacer to the back of the housing.
5. Add the Side and BottomTo maintain "squareness" a styrene side and bottom should be added. The picture on the right shows the results of the prototype. No doubt the styrene pieces added to the servo could have been done in a much classier manner -- what you see here is the result of trial-and-error.
1. Sizing the ServoWith the motor well ready to receive the modified servo, begin test fitting the servo in place. Using a sheet of 220-grit sandpaper, remove just enough of the back spacer so the servo will fit snugly in the motor well. Then, remove material from the bottom of the servo until it will slide deep enough into the motor well to provide the desired gear mesh.
To help insure a flat surface, place the servo on a sheet of sandpaper and move the servo in a circular motion to slowly remove the styrene material.
2. Modifying the Motor ClampUsing the existing hole as a guide and a drill bit smaller than the threaded hole, drill through the styrene back as shown in the picture on the left. This hole provides a visual indicator on the back spacer for the location of a larger clearance hole to accept the original screw (see the picture on the right). Drill only through the spacer, but not into the servo itself.
When replacing the motor in the clamp, do not over tighten the screw!
Once the gear mesh has been achieved, fasten the servo in the motor well with two small machine screws: Holding the servo in place, drill two 0.039-in. dia. holes with a no. 61 drill through the back of the MT chassis and into the servo. Remove the servo and enlarge the chassis holes with a no. 55 drill. In the servo, tap the holes for 00-90 thread. Replace the servo in the motor well and secure it with two 00-90 machine screws. Although not yet tried, the holes in the chassis could possibly be vertically enlongated to permit an up and down adjustment of the gear mesh.
