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?

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

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.

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

The ratchet only stops the gearbox from backdriving when it is under load…

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

What releases the dog gear…

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.

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

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/showthread.php?t=125123

Hope all the comments here help you out.

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.

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.

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.

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

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.

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-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.

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.

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!

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

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.

How about a pawl?

ratchet & pawl.jpg


ratchet & pawl.jpg

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

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.

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