V-Groove Belt Electric Motor Drivetrain

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Precision DC motors can be very expensive -- particularly for larger motors. Although larger motors often have ball bearings on the output shaft, it isn’t a good idea to subject them to non-rotational loads and physical shocks if it can be avoided. One method of isolating a motor from wheel impacts is to transfer the motor power via a belt drive.

This project uses a nylon 2L-section v-belt pulley with a 5/16-inch diameter finished bore aluminum hub (McMaster-Carr #3060K12). It will connect to the motor with a mating 10-inch neoprene v-belt (McMaster-Carr #7881K11, trade size 2L100). The 5/16-inch diameter bore was selected because it is almost a perfect fit for the 8 mm Faulhaber gearmotor output shaft.

Drilling the Pulley Hub

The pulley will be attached to the wheel with three screws. The aluminum hub of the pulley is solid and thick. It has plenty of material for drilling screw holes.

Left: A paper template is cut out from a CAD drawing. Right: The template is centered on the pulley with a 5/16-inch drill shank.

Left: A paper template is cut out from a CAD drawing. Right: The template is centered on the pulley with a 5/16-inch drill shank.

A hole pattern is designed on a computer using a drawing program (Microsoft Visio). The design is printed out and then cut out with scissors and a razor blade.

Using the plain end (shank) of a 5/16-inch drill, the paper design is centered on the pulley hub and taped in place.

Left: A parallel block and v-block position the pulley in the vise. Right: After tightening the vise, the parallel can be slid aside to make room for the drill.

Left: A parallel block and v-block position the pulley in the vise. Right: After tightening the vise, the parallel can be slid aside to make room for the drill.

The pulley is held in a milling vise by placing it between a v-block and the v-groove in the moving jaw of the vise. Although the vise v-groove may theoretically be enough to hold the pulley in place, the pulley feels more secure when held by the wider v-block.

To ensure the pulley is flat, place a parallel block underneath during positioning. After the vise is tightened, the parallel block should be pulled away to allow room for the drill to break through the hub. Even better, place a matching pair of parallels under the sides of the pulley with a gap in between for the drill.

A paper template is a visual guide for lining up the drill.

A paper template is a visual guide for lining up the drill.

Line up the drill visually using the attached paper template as a guide. Gently lower the spinning drill until it just scratches the surface of the paper. Based on the mark in relation to the cross hair, the position of the drill can be adjusted to be sufficiently centered.

Of course, professional machinists cringe at the thought of using a taped piece of paper for alignment. After all, the paper will shift and tear during machining. And, it is difficult to ensure that a piece of paper is properly aligned in the first place. However, you can see for yourself that the results are fairly good, especially considering the loose tolerances of hobbyist-size milling/drilling machines.

Using a #4-40 tap to thread a hole in the pulley.

Using a #4-40 tap to thread a hole in the pulley.

A hand tap is a tool for adding screw threads to holes. Using a little bit a lubricant, tap each hole on the aluminum hub of the pulley.

Machining the Wheel

The wheel for this project is a 5-inch diameter soft rubber wheel (#2243T23). This part was selected because it is inexpensive ($5), non-marking (critical for indoors), and has a 5/16-inch center hole (just like the pulley hub). This is a great, solid wheel.

The side of the wheel is convex and needs to be machined flat for the pulley to sit flush against it.

The side of the wheel is convex and needs to be machined flat for the pulley to sit flush against it.

The side of wheel has a curved bulge that prevents the pulley from sitting flush against it. Although I suppose it could be used as-is, the gap would cause stress on the screws.

Since the wheel needs to be machined for the screw holes anyway, might as well cut the convex side flat. In fact, if you aren’t going to use flat head screws on the other side of the wheel, you’d better cut both sides of the wheel flat.

A wheel can be held firmly on a milling table with step blocks or with locking c-clamps against a wood block in a vise.

A wheel can be held firmly on a milling table with step blocks or with locking c-clamps against a wood block in a vise.

If you have a set of step blocks, the wheel isn’t too difficult to hold on the milling table. The parallels underneath the wheel need to have a gap in the middle to allow the bulge on the other side to float.

Alternatively, the wheel can be clamped to a piece of wood stock held in a milling vise. An advantage to this approach is that the milling vise doesn’t need to be removed from the table -- and then realigned when returned to the table.

Wheel being drilled for flat head screws.

Wheel being drilled for flat head screws.

The paper template for the three screw holes is applied to the wheel using the same technique as was used for the pulley hub. A 5/16-inch drill shank centers the template in place.

For appearance sake, the screw holes are countersunk for flat head screws. First an 82° countersink drilled a hole. Then it was replaced with a 1/8-inch diameter drill to drill all the way through the wheel. This process was repeated for each hole.

Swapping drill bits can be a pain. But, if all three holes are drilled with the countersink only, it will be difficult to accurately re-center the drill over each hole when you switch to the 1/8-inch drill.

V-Belt Pulley Drive System

The belt driven wheel is almost complete.

Flathead screws on one side of the wheel pass through and thread into the pulley on the other side of the wheel.

Flathead screws on one side of the wheel pass through and thread into the pulley on the other side of the wheel.

The wheel and pulley are connected together using three flat-head screws. As you can see in the photograph, the pulley is well centered with respect to the wheel, and the pair are flush together. Slight misalignment is tolerable since the wheel is going to have a bit of play on the axle, and the belt can pick up some slack.

A belt-driven motor system on an aluminum frame robot.

A belt-driven motor system on an aluminum frame robot.

The motor screws are loosened, the belt is installed, the motor is pushed away from the wheel until the belt is firm, and the motor screws are then tightened. The angle stock bracket on the previous page had slots milled in it so that the tension of the belt could be adjusted by sliding the motor towards or away from the pulley.

The wheel rides on a 5/16-inch diameter phosphor bronze axle (#88555K133) located between two flange mounted bronze sleeve bearings (#5912K51). This provides a smooth, low-friction surface that has adequate strength.

This setup works well for light loads. However, with a medium-size child on board (yes, you heard me) the aluminum frame flexes too much and the belt loses tension. Of course, even with a beefed-up frame, the motor may be a bit underpowered for such a load anyway.

Conclusion

Well, there you are -- three different motor setups to add to the collection. Of course, all required some machining. But, unless you’re able to acquire a motor, mount, and wheel as a set, you’re going to have to machine some parts to connect them together.

Good luck and enjoy!