Hydraulic Theory Question

The bigger the dia. of the cyl. the slower the speed.

Bob, that certainly seems to be what my research says. A 2" cyl piston will move 4 times as fast as a 4" cyl piston (ratio of the squares of the radii).

Cylinder speed boils down to pumped volume in gpm and the swept volume of the cylinder or cylinders supplied by that pumped volume. So two cylinders plumbed in parallel will move half as fast as a single cylinder of the same diameter.

Two two inch cylinders will move twice as fast as a single 4 inch cylinder.

Cylinder speed can be slowed by a restriction in the hydraulic line. By an orifice or a valve only partly opened, or by friction in a small diameter line. On my loader (red loader on a green tractor) I installed a flow restrictor needle valve in the supply to the loader valve to make loader control easier. It didn't need 22 gpm on 2-1/2" lift and bucket cylinders, 5 is plenty. Same thing for the backhoe I can mount with the same hydraulic manifold. The flow restriction allows heavy hands on the valves, saves having to barely open the valves to prevent jerky operation.

That four inch cylinder on the bucket probably isn't rated for the 3100 psi of the AC pump.

Gerald J.

a 4in cylinder, is more than twice as big as a 2in cylinder so will take more than twice as much oil to fill it for example a 2in cylinder with a 10 in stoke has roughly 31 cu in inside to fill with oil where a 4 in cylinder has 125.6 cu in to fill with oil , so thats roughly 4x a much oil to pump to do the same speed as  2in cylinder . a 2 in cylinder has 6200# of forse at 2000psi where a 4 in cylinder has 25,000 # of force thats a big diffeence.

actually, if I'm not mistaken, two 2.5 inch cylinders would have 9.8 square inches, and one four inch cylinder would have 12.6 square inches For a given stroke, the four inch would take 25% more fluid than two 2.5 inchers. Or, for a given stroke and pump, a four incher is going to take longer than two two and a half inch cylinders, twenty five percent longer.

Edited by LouSWPA - 14 May 2010 at 7:53am

Yah, a vane pump probably can't do 3000 psi.

You have to look at the leverage on the bucket and cylinder mounts too. Certainly a smaller cylinder will be faster (and then takes more finesse at the controls to keep from banging it around), but the smaller force times the leverage might not be enough to do some jobs, like hold up a bucket full of gravel. Though that bucket of gravel might have to be moderated to keep the back wheels of the tractor on the ground. Another leverage effect.

I have two buckets for my loader, one the original snow/manure bucket and a shorter one I had made for heavy dirt work. On my big green tractor now, the snow bucket tipped down won't move the tractor, but the short bucket will. That's really handy for hitching, I can just park with the bucket pointing down and use that for the parking brake and then to inch the tractor to hitch pins I don't have to climb on the tractor, just reach up and run the bucket to rock the tractor a few inches. And when the first step on the tractor was 30" off the ground thats a real saver. I've fixed that step. A smaller pair of bucket cylinders wouldn't make that work with the short bucket.

There are many trade offs in selecting pump volume, cylinder diameter, cylinder mounts, and cylinder stroke.

Gerald J.

Kcgrain has math correct, when you double the diameter the volume is 4x greater. Mark L