Ratchet Switch under load
 

Our team is trying to use a ratchet to release stretched surgical tubing (like I imagine a lot of teams are doing). We pull back our shooter using a winch and then shift into neutral so our motor shaft is no longer connected to the winch shaft. As the winch tries to unwind the ratchet holds it in place (ratchet acts as a hard stop).

The problem that we have however, is that nothing we have tried has been able to flip the ratchet's directional switch while it is under load. I have seen a couple other teams using this idea and so I am a bit confused as how they released the surgical tubing.

So to you other teams we are successfully releasing surgical tubing, how are you doing it under load?

Re: Ratchet Switch under load
 

Some teams are putting the ratchet on the motor side of the clutch, instead of on the load side.

Re: Ratchet Switch under load
 

In our setup (and if I am correct, most of the other team's winch setups), the direction of the ratchet never changes. The ratchet only stops the gearbox from backdriving when it is under load. What releases the dog gear is a pneumatic cylinder- in our case, we have a hefty cylinder of somewhere in the 1.5 inch diameter range, on a lever around 14 inches long to multiply the force. This is way overkill, but it works flawlessly, and we can engage and release the gearbox by hand if we need to.

If my description was hard to understand, check out the Andymark robot in 3 days team's videos on how they did their winch system. We aren't using the same shooter design as they are, but our retraction and release systems are quite similar.

Re: Ratchet Switch under load
 

The ratchet is on the motor side of the dog clutch, instead of on the load side like the OPs post.

...and the motor has been turned off to keep the motor from overheating.

Since the ratchet is on the opposite side of the dog clutch from the winch, disconnecting the clutch allows the winch to unwind without moving the ratchet.

Re: Ratchet Switch under load
 

Exactly. Make sure your ratchet is on the motor side of the shifting block, not the output shaft side.

Re: Ratchet Switch under load
 

You need to install the ratchet (anti back drive) on the motor side and then use a quick release drive dog device. Please see post #14 for what we did in this thread;

http://www.chiefdelphi.com/forums/sh...d.php?t=125123

Hope all the comments here help you out.

Re: Ratchet Switch under load
 

Ah, I see where I misunderstanding the other designs now, it is as Ether said.

We were trying to be clever and remove the need for shifting into a neutral gear back attaching the ratchet to the motor shaft and using a worm drive in our winch. That way the ratchet would wind up the winch and the worm drive would give us our hard stop. Then to fire we only have to switch the ratchet's clutch and it would easily unwind.

Unless we can find a ratchet that is very easy to actuate under load though . . . then it looks like we will need a transmission.
Unfortunately all the Vex and AM transmissions are out of stock right now!
Hopefully we can find something similar soon.

Re: Ratchet Switch under load
 

I think you are still misunderstanding. If you have the ratchet on the axel directly driven by the CIMs, there's no need to switch the ratchet's clutch. When the dog gear disengages, the output shaft will spin freely regardless of the ratchet or not. This is because the output shaft is driven directly from the shaft with the dog gear, not the shaft driven directly by the CIMs.

Re: Ratchet Switch under load
 

No I understand. If you see, I said that we were trying to do this assembly WITHOUT a shifting gearbox. It is kind of hard to get your head around, but we were planning on actually rotating our ratchet with our motor shaft. The ratchet would rotate the winch shaft and then to release we only had to switch the ratchet's clutch to reverse. When firing the ratchet and motor would stay stationary while the winch axel would spin inside the ratchet.

It is incredibly simply. The only problem is finding a ratchet that can switch under load.

Re: Ratchet Switch under load
 

Oh ok. Sorry I guess I'm the one who misunderstood:D

Re: Ratchet Switch under load
 

Make one, or have one made for you. We plasma cut a ratchet out of 1/4" mild steel. The ratchet works extremely well and can switch while under load. We obviously expect the ratchet to wear down rather quickly because of the material and switching under load. We plan on having a few replacements made out of a more durable material by our primary sponsor.

Re: Ratchet Switch under load
 

What? Worm gears don't unwind "easily" as a general rule of thumb. They don't backdrive well, generally speaking; you'd have to power the motor in reverse at top speed. See "window motor". There are worm gears that do backdrive, but they're not all that easy to find.

Re: Ratchet Switch under load
 

Re-read the second post I made describing it. The window motor is not directly attached to the winch, it would stay stationary during unwinding.

If we had infinite resources that is probably exactly what we would do. Sadly we don't have any contacts in companies/people with the equipment to do this. That is pretty impressing that you have that, being on a second year team yourself.

Re: Ratchet Switch under load
 

Now you're clear. The first couple of description posts were very unclear as to where the ratchet was, except that it was on the winch side of the system; the second seems to say that there's a worm drive between the ratchet and the winch--and then talks about "easily" spinning the winch! Now I understand what you're saying: window motor/worm drive--> ratchet --> winch.


To be honest, I can't think of a single ratchet that will switch easily under load. What I might do in your case would be a small 3-gear setup, where one gear is on the motor shaft (through the ratchet, though if you've still got the worm gear that won't be needed), one gear is on the winch shaft, and one gear is on an arm that drops in to connect the other two--a pneumatically-powered arm, if possible. Pull the arm out to unlock the gearbox. I've seen something like that work in LEGO before... You'd want to have the stroke to put the gear back in be slow, possibly at really low pressure, with the motor turning slowly, though.

Re: Ratchet Switch under load
 

Our head coach is the schools welding teacher so we have access to his welding lab. We happen to have 2 students who are great welders so they manged to cut the winch out. Good luck solving your winch problem!

Re: Ratchet Switch under load
 

Huh, I didn't read it like that, but as long as you get the concept now, cool :)

Having a ratchet device machined might be the only solution, but we are looking into industrial grade ones that might shift easier under load. Money is a bit of an issue, but I imagine any ratchet we found would be cheaper than a transmission.

Re: Ratchet Switch under load
 

How about a pawl?

Re: Ratchet Switch under load
 

Have you tried reversing the motor while you release the ratchet? This may work depending on how quick your winch is.

Re: Ratchet Switch under load
 

From what I can tell no motors that are legal for FRC would be able to reverse like this. There are also weird alternatives like holding your tubing with a hardstop while you unwind your winch, but we haven't gone down that road just yet.

A pawl and ratchet gear is an interesting idea. Making a housing that will rotate the two of them together might be a little tricky but actuating the pawl should be as simple as actuating a ratchet switch on its own.

Re: Ratchet Switch under load
 

I don't understand why reversing the motor would be illegal.

I also don't understand why reversing the motor would be illegal.

If you have the power to pull your winch, simply pull it slightly more (in the direction used to store power in your launcher), hold it there while releasing your pawl or switching the ratchet clutch. Then release the motor.

Re: Ratchet Switch under load
 

"no motors that are legal for FRC would be able to reverse like this"
As in no motors that are legal to use in FRC are capable of this.

Because of how slow we need to gear our motor to be able to pull back with 100+ lb. of force, that same motor will not be able to unwind at the very high speed of our shooter.

We have not calculated this, but empirical evidence seems to confirm it. I would be surprised if there are FRC motors that wouldn't slow the rate of your shot if you left it mechanically connected to the winch system.

Re: Ratchet Switch under load
 

He doesn't mean reverse as the shot motion. He means continue to drive it so the natural motion of the ratchet/pawl causes the pawl to jump one tooth. If the pawl is actively being pulled on as well during this moment it can disengage.

Re: Ratchet Switch under load
 

Yes, but if your motor is connected to your winch shaft throughout the shooting movement then your shooter will be forced to pull your motor as it moves forward. This will be a considerable source of drag. This is why most designs use some sort of shifter which has a neutral gear.

If you know of another way to remove the drag from the system though, that would be great.

It sounds to me like they are using the motor/gearbox to turn the ratchet itself. I.e, taking the place of your hand if you were to use it as a hand tool.

And then using the lever to disconnect the motor/gearbox from the winch. The ratchet is acting as both the ratchet/pawl and the "shift into neutral" options.

To relieve the load on the lever, you simply need to drive the motor slightly forward, then actuate the clutch lever on the ratchet while releasing the motor. It will relieve any load the ratchet is holding momentarily (long enough to flip the switch) while changing directions/freespinning back the partial tooth distance.

To ensure disengagement, power on whatever you're using to flip the switch, then pulse the winch motor several times at 5-10 hz, full power, for a couple milliseconds each time. So effectively a very slow pwm at low power. It should sound like the motor/gearbox is clicking. You may have to play with your on vs off times and frequency to get it to work every time.

Re: Ratchet Switch under load
 

I'm sorry, you're right. I was thinking that the ratchet pawl combo also disengages the motor, but there was no basis for this thought! My apologies.

What it does buy you is the ability to move the disengage to a different portion of the system while the ratchet is taking load, meaning less force is required to pull a dog or whatever you're using (then fire with the ratchet). This is two actuations for the same thing, but could be the right choice for some teams.

Re: Ratchet Switch under load
 

This could work I think. The only issue is the concept of a hard-stop. In order to be able to reverse the motor and relieve the tubing load on the ratchet something else will have to be taking the tubing load for that short amount of time. Because the motor cannot spin as fast as the tubing wants to shoot, we can't just reverse it real fast and try to flip the ratchet switch.

Honestly though, that just shifts the problem of releasing under load to whatever is taking the load while you reverse your motor. So many the real questions should be "what will hold and release the tubing effectively". And for that I think some kind of simple pneumatic hook would do the job best.

Re: Ratchet Switch under load
 

We made a video of how our ratcheting, shift to neutral gearbox works. What was not describe in this video is the mechanism that disengages the dog gear. This was accomplished by a 1.5" bore, 6" stroke cylinder on a 12" lever arm. The dog is re-engaged by the internal spring.

http://www.youtube.com/watch?v=xx3I2...u8SDSA&index=5

I'm not sure if this video will be helpful but it may depict what others were describing.

I'm not certain you would need to relieve the loading (you may need to)after reducing it by cycling the motor.

If you do, perhaps pushing a high friction material into the winch spool, acting like a disc brake would provide sufficient relief without necessarily holding the entire load with another device.