hydraulic pumps -- question

tony wrote: (clip) it never stops.. nor becomes difficult to turn. is there backflow through these pumps thats the only thing i can imagine going on here. thoughts? ^^^^^^^^^^^^ Tony, if it is a positive displacement pump, such as a gear pump, then it should become almost impossible to turn under the conditions you describe, unless it has a pressure relief valve, or it is very worn.

Tony, It depends on many things, such as: Is it a positive displacement or centrifugal pump (most hydraulic pumps are positive displacement)

Is there a built-in relief or bypass valve (many have this)

Your auto power steering pump is probably a vane type pump with a built-in bypass valve, this would explain the symptions you noted. It is a positive displacement type pump, the built-in bypass allows it to continue turning when the output is blocked.

Steve

All gear pumps will leak internally to some extent. They have to to provide internal lubrication. A piston pump shouldn't leak.

Mark Rand RTFM

the pump i'm using is a "typical" hydraulic pump.. gear pump, i believe... looking inside with a flashlight reveals gears, for what thats worth.

could it be that i'm turning it in the wrong direction? (of course, i've tried both.. is there some convention for which way a hydraulic pump should be driven? there are no indications on the body)

it is used, but was sold to me as "working"

i am concerned that either a) it is not working, or b) i need to attain a min RPM for it to start moving oil

is "b" likely?

-tony

forgot one thing... this is the same type of pump usually used on log-splitters. (only smaller)

are pumps of this nature usually equipped with built in relief valves? and, assuming it might have a built in relief.. shouldnt there be a pressure limit before the valve opens? -- specifically, a limit much higher than i could possibly obtain by turning it by hand?

unless it is worn, as mentioned, i wouldnt expect to be able to turn it for long before it locked up.

which never happens. back to the starting line..

-tony

B is close. The manufacturer will give a minimum speed to attain the normal working pressure (eg. 1000rpm to reach 180bar on some 5000rpm gear pumps I looked at recently).

Mark Rand RTFM

Tony, you're looking at it from the theoretical point of view, but not the practical one. Gear pumps are not built to precise enough tolerances to prevent some fluid leakage past the sides of the gears and around the tips of their "teeth". They don't have the same "blocked" characteristics displayed by hydraulic cylinders, which use low leakage elastomeric seals.

A hydraulic cylinder in decent shape with it's port blocked off will go "rock hard" if there's no air bubbles in the fluid, because the leakage will be close to zero, and the fluid is essentially incompressible.

Assuming there's no pressure relief valve, if you did drive that gear pump at 1000 rpm with a motor and blocked it off, you *would* get the squaling belt and overloaded motor you mentioned.

Jeff

You don't ever want to cap off a pump that's running with out a pressure relief valve. I know a guy who did that. He needed to move a tractor only a few feet and he just capped off the end of the hose. There wasn't a built in pressure relief on the pump and it blew up on him. It took a chunk out of his leg and took months to heal. He was lucky it didn't kill him.

Richard W.

i wont be using this pump to power anything, per se. it is something of a homebrew speed-limiter.

(this came up a month or so ago in a post re: a conveyor)

the pump is (will be) driven. belt driven. the idea is that it will pump oil (running around 300rpm) through a restriction. anything faster than 300rpm, it will be slowed down (by hydraulic resistance through the restriction).. anything slower should be okay.

to test this, i capped the pump (tube) off altogether.. figuring that, once the line was filled, the pump should be almost impossible to turn. slowly turning out my valve should allow oil to pass through at only a certain rate. in this way, i would adjust for my 300rpm requirement.

maybe not a conventional application of a hydraulic pump. but its what i had around. like i said, it wont be powered by a 1000hp diesel engine, so the restriction (or bolted tube) shouldnt present such a danger.

problem is, my pump isnt working the way i think it would have.

-tony

Tony - Remove the bolts and remove the top plate of the pump then note the wear in the housing where the OD of the gears run. If the wear pattern is beyond the half way point (draw an imaginary line through the two gear shafts to the OD of the gear) then it is worn beyond practical use and will need repair. Also check the wear plates in the cover and behind the gears for excessive wear or scratches. Gear pumps will leak around the gears to some degree which would make it tough to check in the manner you're trying. Good luck.

I am not surprised by that. You aren't using it for what it was built to do. As others have said there is to much internal leaking past the gears and bearings. That isn't a problem if you are running it at the manufacturers recommended speed and pressure. The internal leaks are small at the pumps rated speed and pressure. When you run slower then it can make a big difference. You say you are using it for a speed limiter. They use something like a roller hydraulic pump for lowering life boats. Only the oil is trapped inside and there are no ports. It's used to slow the drop of life boats. They are not made for continuous use, because the oil can over heat and there are no provisions for oil cooling.

Richard W.

More thoughts:-

Assuming the wear is not excessive, you could try gearing the pump up to get sufficient speed at the pump when your rig is doing 300rpm.

It may also be reasonable to run the pump with a more viscous oil in order to generate higher pressure at lower speeds.

Don't forget that you will need cooling if this is for more than transient use.