Reducing Friction in Shooter

[Dr.Gusta]

My team used a winch based shooter using sergical tubing. A VEX 2 speed gearbox would pull back the tubing then pneumaticly shifts into a neutral gear (neutral gear was created by removing the "balls" form the high speed stage. The issue we are having is it takes about 14lbs to un spool the rope while in the neutral gear this greatly reduces our acceleration on the shooter. Oh and just to let you know we bagged our electronics and drive base but held back our manipulator mechanism.

[Chris is me]

All things considered, if you only need 14 pounds to release your mechanism you're in good shape. The 3/4" bore piston at 60 PSI pushes with 26.5 lbs of force. It pulls with somewhat less force since the piston rod takes away some surface area.

Are you having trouble shifting the piston under load? Does it take too long? An easy fix would be moving the gear to the end of the shifter so that you push to shift to neutral. If you still need more force, it is possible to get creative and mount a bigger piston to the mechanism, but be warned that the Vex shifter components (bearing and e-clips) cannot handle a ton more than 30 pounds of force.

One last thing: are you sure it's the gearbox that's slowing you down?

[mott]

Originally Posted by Chris is me

All things considered, if you only need 14 pounds to release your mechanism you're in good shape. The 3/4" bore piston at 60 PSI pushes with 26.5 lbs of force. It pulls with somewhat less force since the piston rod takes away some surface area.

Are you having trouble shifting the piston under load? Does it take too long? An easy fix would be moving the gear to the end of the shifter so that you push to shift to neutral. If you still need more force, it is possible to get creative and mount a bigger piston to the mechanism, but be warned that the Vex shifter components (bearing and e-clips) cannot handle a ton more than 30 pounds of force.

One last thing: are you sure it's the gearbox that's slowing you down?

Chris, I think what he means is that once into Neutral, the resistance in the launcher system consumes 14lbs of his available shooting force, thus reducing the energy remaining to put into the ball.

OP, is this correct?

If so, can you describe your system in a little more detail? What material are your "moving part" surfaces made of, etc.?

[Dr.Gusta]

I think you miss understand. It shifts normally but the issue is the force it takes to actually pull the rope off of the spool when it is in neutral gear is 14lbs so their is 14lbs of resistance when the sergical tubing retracts. Yes the friction is in the gear
Box we are 100% sure.

[JamesCH95]

Sounds like something got assembled incorrectly and/or something is damaged. I would suggest taking the gearbox apart and checking everything very carefully. Clean off grease etc. to get a good look at everything and see what's wearing or binding.

I would also make 110% sure that you're actually getting into neutral and disengaging completely.

Things to check:
1) All bearings rolling smoothly
2) Abnormal wear on gears (a little black discoloration is okay)
3) Any wear on the sides of gears (very bad)
4) Wave-washers installed in the proper locations
5) Gearbox housing distortion
6) Any side-loading on the output shaft
7) Any slight misalignment in how the gearbox is mouted

Pics would go a long way, especially of disassembled components.

[Ether]

Originally Posted by Dr.Gusta

It shifts normally but the issue is the force it takes to actually pull the rope off of the spool when it is in neutral gear is 14lbs

Did you measure this, or is it the result of a calculation based on the observed shooting distance (or lack thereof)?

What is the radius of the winch roller?

Are there any gear ratios (including sprockets and/or pulleys) between your gearbox output and the winch roller?

[colin340]

i'm going to take a shot in the dark and say the problem is you have 3 bearing on the shaft or have bent the output shaft.

Also try measuring the 14lbs number with no air in the system side load on this type of bearings is not fun

[Dr.Gusta]

Originally Posted by Ether

Did you measure this, or is it the result of a calculation based on the observed shooting distance (or lack thereof)?

What is the radius of the winch roller?

Are there any gear ratios (including sprockets and/or pulleys) between your gearbox output and the winch roller?

We just wrap the rope around the peice of 1/2" hex shaft. We use the high torque third stage offered by VEX for their ball shifter and I measured it with a fish scale attached to the rope and unspooling the rope by pulling on the scale. It read out 14lbs consistently.

[Ether]

Originally Posted by Dr.Gusta

We just wrap the rope around the peice of 1/2" hex shaft. We use the high torque third stage offered by VEX for their ball shifter and I measured it with a fish scale attached to the rope and unspooling the rope by pulling on the scale. It read out 14lbs consistently.

Here's what I think you are saying. Correct me if it's wrong:

There's a substantial speed reduction gear stage that's still connected to the winch when you release it.

You are pulling with 14lbs at a radius of 1/4"

How is the 1/2" hex shaft supported?

[mott]

Originally Posted by Dr.Gusta

We just wrap the rope around the peice of 1/2" hex shaft. We use the high torque third stage offered by VEX for their ball shifter and I measured it with a fish scale attached to the rope and unspooling the rope by pulling on the scale. It read out 14lbs consistently.

Hearing this, the one immediate suggestion might be to try to use as large of a "drum" as you can to wind up your rope. Obviously, this would impact your ratios but unspooling a tiny drum is harder than unspooling a larger LIGHTWEIGHT drum.

[Conor Ryan]

With all misconceceptions that everyone is working through, I have a vauge idea what we are trying to accomplish.

So you have a couple of options:
1) Go with a larger diameter spool. 1/4" is really small, 2" probably will do the trick. You want to minimize the amount of revolutions you do on the spool.

2) Tie some surgical tubing to "Spring Load" your spool. The idea is that as you wind the winch back, you pull on the surgical tubing more. Spring force is translated to the unraveling of the spool without using the force to catapult the ball.

3) Make sure you follow the 45 pound withholding allowance rules.

[mott]

Originally Posted by Conor Ryan

2) Tie some surgical tubing to "Spring Load" your spool. The idea is that as you wind the winch back, you pull on the surgical tubing more. Spring force is translated to the unraveling of the spool without using the force to catapult the ball.

Isn't this the same as having more spring power on the shooting mechanism itself?

Maybe I'm misunderstanding this suggestion...

This is a reasonable solution if the drawback motor/gearbox has additional pulling capacity over what they are currently using but I assumed not.

[Conor Ryan]

Originally Posted by mott

Isn't this the same as having more spring power on the shooting mechanism itself?

Maybe I'm misunderstanding this suggestion...

This is a reasonable solution if the drawback motor/gearbox has additional pulling capacity over what they are currently using but I assumed not.

The main difference is you anchor it to the frame rather than the catapult arm. This allows you use one set of spring force to overcome the intertia of the shooting arm, and another set of spring force to overcome the intertia of the winch being wound back.

All in all, this is fairly useless unless you can move to a larger winch spool diameter, 1/2" is not enough you should go to at least 1", ideally 2"

[mott]

Originally Posted by Conor Ryan

The main difference is you anchor it to the frame rather than the catapult arm. This allows you use one set of spring force to overcome the intertia of the shooting arm, and another set of spring force to overcome the intertia of the winch being wound back.

Ok, I understand what you're saying but assuming your overall "load" on the motor/gearbox is limited to some maximum, wouldn't you be better off attaching ALL of the springs directly to the launcher so that every bit of energy not needed to overcome the inertia of the winch would be available to put into the shooting arm (instead of being wasted pulling on a frame member)?

[Chris is me]

Originally Posted by Conor Ryan

All in all, this is fairly useless unless you can move to a larger winch spool diameter, 1/2" is not enough you should go to at least 1", ideally 2"

This is probably obvious, but I just wanted to make clear to the OP - increasing the winch drum diameter will increase your speed at the expense of torque. Think of it like the opposite of a gear reduction. That said, the greater diameter the winch drum is, the greater torque your spring force will put on the output shaft upon release.

A quick and dirty way to make a bigger winch is to throw some hex hubs on the shaft, the kind normally used for sprockets and wheels.