Cushioning Your Hard Stop

[IKE]

A little late, but something your team might want to plan during your unbag or practice day.

At Week 0 and looking over several catapults, I have observed 3 types of hard-stops for catapults that will likely lead to broken parts.

The frame stop for the catapult, where part of the catapult (usually near pivot) stops the frame of the catapult. Putting a little bit of hard rubber at this position will keep the frame from hammering itself to death.

Catch strap to frame. This uses a stiff rope, strap, or cable to stop the frame at a given position. This can put a lot of stress into the frame, pivot, or anchor point. Having a compliant bushing under the loop of the anchor or frame will help cushion the end travel here as well.

Last but certainly not least is using pneumatics as the driver/end stop. This is potentially dangerous as the extra inertia of the catapults and high speeds can cause the ends or pistons to get damaged on the pneumatic cylinders. I have heard some of them have an internal cushion to help keep from having these break the cylinders. I would highly recommend a catch strap for those catapults as well.

The physics behind this problem is similar to crash dynamics.

The spring or motorized element has a lot of power that translates into a whole bunch of kinetic energy (some in the ball, but some in the frame of the catapult.
So... KE=Kinetic energy = 1/2*M*V^2=1/2*K*x^2 (stored energy of the spring).

This helps explain why firing without a ball can be worse than with a ball. With the ball, a lot of the spring energy goes to the ball. When dry firing, all of that energy goes to the frame.

Energy though doesn't break the parts, but forces do. This is where the "cushion" comes in.
Energy also equals force time distance: or KE=F*D
So KE=1/2*M*V^2 of the frame = F*D
Where force is at the point of the stop, and distance is the deflection at that stop. As the stopping distance goes to 0, then the Force goes to infinity!

Well, not exactly. With forces approaching "infinity", all sorts of things start acting like springs. and compressing/stretching at the local level. When you get into your mechanics of material, it is known as strain. Strain them too much, and they stay permanently deflected. Permanently deflect something too many times, and it will break or stop functioning.

This is where the little block of stiff rubber comes in. Let's say that the mechanism has an inertia equavalent to about 1 kg, and is traveling at 13 m/s when it reaches end of travel. This is 1/2*1*13^2 or... about 85 J of energy. If your stopping distance was 1m, then the force would only be about 85 N (19 lbs). Not too bad. If the stopping distance was consistant, and 0.1 m, then the forces go up to 850 N (190 lbs). A little scary, but a lot of things can handle 200 lbs. Of course, 0.1 meters is about 4 inches which is a fair amount to stop something. Now, a lot of these catapults have ahrd stops. At 0.01m (1 cm or about 0.4 inches) is 8,500 N (1,190 lbs) starting to become a lot of load. 0.001m (0.040 inches) is a "hard stop", and would be 85,000 N. Since the ball weighs about 1 KG, removing the ball would double this set up. or 170,000 N (38,217 lbs). Even with 1" of cross section, a lot of aluminums will deform. The neat thing is, adding in 1/4" thick of stiff rubber will reduce these stresses immensely. For instance, this stuff: is set up for 2,500 psi. Thus, a square inch of it for two points of contact would match up reasonably well (0.040->0.25 reduces forces to 6,000 lbs.. one on each side would then be 3,000 psi). This would be some easy to apply stuff with adhesive backing. 70-90A would be my recommendation.

[nxtmonkeys]

What if one has two cables that hold the arm from going any farther? Mine does. how could I cushion that?

[IKE]

Originally Posted by nxtmonkeys

What if one has two cables that hold the arm from going any farther? Mine does. how could I cushion that?

I would recommend a rubber bushing for each. There are some other tricks you could do as well. Send me a pic, and I can offer some thoughts.

[Monochron]

We discovered the need for a reinforced stop a while back in testing. We are using a drawer slide to guarantee linear motion for our shooter and, as you might imagine, the draw slide designer did not anticipate hundreds of pounds of force pulling it out.

When we dry fired our shooter the small plastic hardstop built in to the slide shattered into pieces and the end of the slide rocketed off. We had to remove the restraints and replace them with hardened steel bolts right through the outside. Rubber may not be a bad idea to help with the force, but mounting locations are at a bit of premium on that thin slider.

[Ikillee]

Our pneumatic cylinders for the shooter is mounted on the robot through holes in the frame and coder pins. When the cylinder comes to a stop, there's stress on the frame and on the cylinder to extend farther. What is a catch strap and how will it help?

[DampRobot]

We have 6 1/8" thick rubber washers dampening our shooter as it hits the forward stop. The shooter plate still audibly rings when it stops going forward.

We've also seen the need to be really conservative with material strength in shooters, in addition to adding dampers. We originally had some 1/4" and 1/8" plate in some places that we wouldn't think would see much static load, and essentially no shock load. After seeing rather frightening deformations (15 degree or so bends in 1/8" 6061 and noticeable bowing in 3/8" 6061) we decided to beef everything up from 1/8" to 1/4" and 3/8" to 3/4". As of yet, we haven't seem any significant deformation as a result of strain, but were still not very far into the season...

[Doug G]

We tried a bunch of different things and finally decided on this...

[TheKeeg]

Our team originally had designed and built a hard stop that was linked to the winch, but bad things happened...very bad things. So, on bag day we had an idea to use the same tubing that was powering the catapult to stop it. It worked great. Now, it does oscillate a little bit after shooting, but it doesn't for long because our tubing is so strong. The robot is taking the same amount of force, but it is doing it in a much longer time period (impulse). No more exploding versa planetaries (oops did I just say that)

[nxtmonkeys]

Here is my cable hard stop. I don't have any real pictures, but I used PAINT to make this.



I'm getting a real picture of my cable put up on the portal right now.

[nxtmonkeys]

Never mind. i'l have it later.

[who716]

we started with 1/8 flat stock the goes along the length of the catapult that is covered in pipe insulation. when we were driving and shooting it on Monday the whole piece snapped of at both ends. this lasted the entire scrimmage event and 4 full days of practice, but we switched to a solid aluminum tube that it goes in the same thing.

[Teamcodeorange]

Ike,
Thanks for the helpful, thorough, and insightful post. We are interested in looking into some of the stops that you have mentioned, but the links seem to lead me to a general McMaster page of "standard rubber sheets" rather than something specific. Can we get a part number?

Thanks,
Code Orange

[OWilliamson]

We didn't use any of the rubber sheets, instead we used rubber bumpers. Here are the specific ones we used:

http://www.mcmaster.com/#93115k111/=qt6ao7

We have 4 of them on our catapult with 2 on each side that hits our hard stop. We we haven't had any issues with our frame deforming from the catapult firing as of yet, and we have shot a couple times.