[FW] Hydraulic Crusher Bot

[maddox10]

What stroke is wished for?

[Giles]

Anything between 100mm and 140mm would fit well into the current design. We currently have two 80mm ones (as seen in video) and two 140mm ones. Wouldn't really want anything longer than the 140's.

[overkill]

The 20mm rams, what stroke are those? If the stroke is longer than 40, the forces impared in robot combat will bend the rods.

What about if the rams are pulling in, rather than pushing out?

[maddox10]

Then the rod size will detract from the power. Bore 20mm, rod size 12mm

1*1*pi*120bar=376.8kg of force pushing.

Pulling it's (1*1*pi*120) - (.75*.75*pi*120)= 165,85 kg


But in tension steel is better than in compression. One of the reasons we have longer suspension bridges than single arch bridges.

[overkill]

OK, so if the bore cross section is smaller, then the ram will be moving faster for the same pump flow - couldn't the leverage ratio on the jaws be changed to make up for the lower force while keeping the closing speed about the same?

[maddox10]

Yep, speed and torque are interchangable, with only the mechanical losses interacting.

But what isn't interchangable is the total available force. A retracting ram has less force to mutate from speed to power or the other way around.

[Giles]

Another thing to think about is reach.

If you have a slow, high force cylinder, then you can increase the speed with a 3:1 lever ratio, which also increases the reach of the claw.
If you have a fast, low force cylinder, then you can increase torque by using a 1:3 lever ratio, which will reduce reach, and for the most part not be as useful.

So ideally, a slow, powerful cylinder on a decent lever ratio would be the thing to aim for.

I was looking at the build thread for Cobalt, who uses a really nice custom made titanium cylinder. I don't have anywhere near the tools or skills to make something like that. So its looking like im going to need to go with one of the heavy steel cylinders out there. Or stick with my two thin ones and hope that between them they can bare the load :P

[Eventorizon]

The guts of Cobalt are now inside inside Little Nipper, you could ask the Big Nipper team what ratio/force/pressure they use. I know they swapped the brushed pump motor to a Scorpion HK4035 to up the power/speed of the pump, maybe you can do something similar.

[overkill]

While I love anything made of titanium, 7075 aluminium is probably just as good for a light weight cylinder and much easier to fabricate.

[overkill]

Yep, speed and torque are interchangable, with only the mechanical losses interacting.

But what isn't interchangable is the total available force. A retracting ram has less force to mutate from speed to power or the other way around.

Good point. So that would mean that you could have a larger, possibly hollow rod that would give you a faster return stroke, assuming the crusher doesn't need nearly as much power to get the point out of the opponent. Would I be right in saying that for the same weight, a wider hollow rod will be stiffer than a smaller diameter solid rod? Sorry for all the questions, I am looking around for something a bit unusual for a sportsman weapon for next year.

[Eventorizon]

Would I be right in saying that for the same weight, a wider hollow rod will be stiffer than a smaller diameter solid rod?

Yes If the weigh was identical, the wider hollow rod would be stiffer. But I think (since the cross sectional area would actually be the same, just distributed differently) they would have identical compressive and tensile strengths.

[maddox10]

A wider, thinner walled tube is stiffer and tougher in compression, weight for weight compared to a solid rod or thicker walled tube.

TAN uses a D60 d50 rod since day 1 for its 100mm bore ram. Just to give an example.

[Kenny]

I Still have the 32mm Bore Cylinder from Cobalt if your still looking for one, Gear pumps have a motor matched for the correct flow and pressure for the size of the pump, you can overspeed and create Cavitation so don't overvolt (https://en.wikipedia.org/wiki/Cavitation)

[Giles]

Hi Kenny,
Potentially interested in the cylinder, yes, havn't found anything else to fit the job yet. Just been continuing with other parts of the build in the mean time. Do you have any pictures of it? And how much you might want for it? PM me if you like.