garageFab.cc

I've been trying to work on Contraptor despite recent inspiration shortages. They probably have to do with things not working as expected.. which can be frustrating at times. Anyway, I have been able to get the belt drive to work, but there are a couple of issues, one of which seems to be resolved and the other one will need extensive testing again.

The first issue is the torsional stiffness of the transmission between the motor and the belt drive pulley. I had been using the rubber fuel hose to connect the coupling nut on the pulley shaft to the drilled-through coupling nut held by compression on the motor shaft. So far, with the leadscrew drive it has been working well. When I started testing belt drive, I quickly discovered that direction reversals lose about 1/32" or more of the movement. After trying standard remedies like lowering the speed, increasing motor current etc, I found that the rubber hose was twisting. It would not have been noticeable with the leadscrew drive, but in my setup of the belt drive the twist of just 5° is equivalent to 0.027" of motion. I bolted a couple of angles with the motor onto the sliding element to simulate some weight, and further testing showed that the lost movement increases with the load on the sliding element. I think it's due to combination of friction and inertia. To fix the twisting, I replaced the rubber hose and the coupling nuts with the same mechanism that couples 1/4" ACME screw to 10-24 threaded rod - nylon spacers. Only instead of filing D-shaped notches on the shafts, I drilled 1/8" holes for 6-32 set screws. This worked pretty well as the motor seems to be strong enough to move it around. Initially, I did not drill deep enough hole in the motor shaft and was puzzled when the problem reduced but did not disappear. After fixing that, there still remains a subtle flex in the entire chain: [motor shaft-set screw-nylon spacer-set screw-threaded rod-set screw-XL pulley], but I think it's bearable. We'll see how it's going to be with the weight of an entire stage. Possible remedies - upgrade the motors, and/or implement acceleration/deceleration. The change to stiff transmission also prompted modification to the motor mount - the motor mounting holes are now 1/4", to accomodate the alignment needs.

The second problem is finding good design of the sliding element. Current working version uses Teflon pads only, has reasonable friction and doesn't bind, but it also has noticeable amount of play. The play is especially bad with twisting/torsional load - which is a prime scenario for the belt drive - moving a pen or a paper cutter or such. Torsional play can be easily fixed by adding a second rail, but I want to see if there are other ways to deal with it. So far the few ideas that I had, had some drawbacks. I guess I need to keep looking..

In other news - a positive development: puncturing hole centers in the template label with a needle or push pin prior to sticking them (see the picture) helps to center-punch and drill accurately, and results in superior holes.

Another video of belt drive in action, this time with sliding element (2x):