Motor Assembly
Motors are expensive, especially the kind needed to drive a moderate sized van de Graaff generator.  Although I try to use common off-the-shelf commercially available parts, I made an exception in this case.  I decided to use a motor from a discarded chain saw. I imagine others who read this will probably scavenge useable motors from their junk pile as well, so this is just an example of how I modified a motor to drive a van de Graaff.

The chain saw motor is AC/DC, series wound, has adequate starting and running torque, and if needed, a continuously controllable speed. Other motors usually in this category are vacuum cleaner motors, and kitchen blender and mixer motors. These are all probably at least a half-horsepower. A commercial juicer motor is shown below. Note the left-hand threads on the shaft.

The chain saw motor had a drive sproket attached to the shaft. I carefully heated the sprocket up with a blow torch (burned grease is visible on the gear). Then I gave a gear tooth a good whack (in the direction of normal rotation) with a
piece of flat stock and a hammer.
The gear easily came right off. Note the left-hand thread on the motor shaft. This is a very common scheme for attaching gears and sprockets to shafts for consumer appliances. As torque is applied to the load, the union becomes tighter.  There is no worry about set screws coming loose.

The next problem was determining how to get this little stub to drive something.  I searched for a suitable coupling but could not find one. So I decided to make my own. I ordered some 1018 3/4" steel  bar stock and a left-handed  7/16"-14 tap from Enco.  I  cut off a short piece of the bar stock and then drilled and tapped it in a lathe.  It fit and extended the motor shaft stub perfectly.  It is shown as 3/4" in diameter but the other end can be turned down to a diameter that will fit a pulley.Plan view of the motor. Motor is geared down about 5:1 . A different set of drive pulleys can be used if necessary for speed adjustment.

Elevation view of the motor. Motor mounted securely on a plate that attaches to the angle iron frame of the van de Graaff.

Motor and the inside view of the custom-built shroud. The inside of the shroud has been coated with silicone I adhesive, a layer of heavy paper, and a final coat of silicone. The motor still has the chain saw wiring. The shroud covers and protects the motor.

This is how the motor drives the lower belt rollers.  The steel pulleys are from MISUMI USA Inc.  ( ) as is the polyurethane round belt (MBT10-690) The drive pulley uses a larger shaft bore than the driven pulley.  The drive pulley is mounted so that the collar is on the outside because of  extra shaft length. The threaded rod at the right is for adjusting belt tension. The rollers are geared so that the outsides of the belts are moving upwards and the insides are both going down through the center. The gears are normally covered by the shield shown above.

The stainless steel gears are expensive and noisy; an alternative scheme could use two pulleys and a figure eight belt.

I used factory belts (from Misumi) for reliability. However, it is possible to make your own for testing or quick repair purposes.  The example in the photo shows half a joint of  a 3/8" OD x 1/4" ID piece of tubing to be used as a pulley belt. The ends will be spliced together with a short section of  1/4" OD x 0.170" IDvinyl tubing and a generous coating of vinyl glue.  The piece shown was only used for testing of the splice;  it held up well under tension.
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