# Strength of screws

Our team wants to try a variable tensioner for our catapult by putting rubber bands through one standoff, then connecting more standoffs to a gear, shown below. The motor will drive the gear to pull the rubber bands. Edit: HS shafts will be used of course!

The problem is that there is friction between the standoffs and the rubber bands, so when the arm is released, only the rubber bands that are not touching a lot of standoffs will contract, not making use of all the force.
What we think might work is to replace the standoffs with screws and spacers on them, so the rubber bands can slide freely. We just don’t know if the screws will be strong enough to hold the force, or if there is a better way to do this.

It should work. The lateral movement of the nylon spacers (along the axis of the bolt) will be more than that of the standoffs. But it should be OK I would think.

Nice reading on bolts and properties. The typical failure is at the head of the bolt. You can read about he effects of different forces starting on page 18.

You can get variable force from variation in the stretch distance, while keeping the same rubber bands.
Instead of winding the rubber bands up on the drum,
wind up a rope attached to the end of the rubber bands.

I second this. This tends to be the common way of stretching rubber bands using a drum like this. One example that comes to mind is 118’s catapult tensioner in Nothing But Net.

I don’t think 1 rope will be strong enough for this year’s catapult. Btw, 118’s catapult looks like a slip gear, but I can see a drum spinning to pull a rope. Can someone explain how it works?

The drum spinning to pull a rope is the tensioner. The rope is attached to rubber bands, which are used to propel the catapult. By varying the stretch distance of the rubber bands, the force exerted by the rubber bands increases or decreases accordingly.

If you have the right sized screws, you can get them to jam/tighten against each other within that standoff before tightening against the gears allowing them to rotate freely.

Or you can drill a standoff sized hole >1/8" in a nylon spacer and just have it soon around the standoff as an axle.

I would use the string to rubberband solution. I used this system on my nothing but net robot and it worked well. We used a worm gear so the motor doesnt have to work to keep the tension. The problem with worm gears are that it creates such a high ratio it is often too slow. We tried gearing it up after the worm gear but this only caused skipping in the gearbox.