I have a platform that is hydraulically supported, depending on what is on it loads can be as high as 5000 pounds the platform itself weighs 1200 pounds. Most of the time all it supports is me while I am working on whatever is parked over it. I have 6 horizontal pins to lock it at floor level they are mechanically actuated, however when lifting a vehicle I often can't raise it high enough safely to engage the pins, so it is sitting on the hydraulics, a few inches above the floor. Anyway, my problem is leakdown, it drops 1" every 20 minutes and that is excessive if I am using a trans jack or doing fabrication. I currently am using a double acting cylinder in single acting mode, pressure is applied to the shaft end. There is no leakage from the unused port on the cylinder. I have a hydraulic power unit with a 4(?) way solenoid valve, it has two solenoids one for up and one for down, down, in my case is gravity powered. I took this valve apart and all appears well, my question is should a spool-type valve like this leak much? Should I replace it, or just put a solenoid valve on the hose going to the cylinder to shut it off? Thanks in advance!
Interesting setup. Some leakage will always occur through a spool valve. How much depends on the pressure acting on it, and that pressure depends on the diameter of the cylinder and the load, but 1" over 20 min doesn't sound excessive. Standard practise dictates you shouldn't work under anything supported on hydraulics, but if you're set on doing it this way I would suggest putting a needle type shutoff valve in the line to the cylinder.
In the case of the picture, the car is lifted only 2" above the floor, for weighing, or tire or brake work if I have total hydraulic failure, it only falls 2", In any event I would never be under it unless it was sitting on the floor. In that case if I have total hydraulic failure, the platform I am standing on would drop as much as 6 feet. Is there a valve I could put on the cylinder to prevent rapid drop it something fails?
Yes, you can get hose burst valves which prevent drop in event of, well, hose burst. But these won't stop gradual drift.
There are manual butterfly type shut off valves that block all flow. We use them all the time on excavator thumb circuits that also can use a breaker or compactor.
Ok, So this uses a 2.75 inch cylinder with a 1-1/4 rod, and it is the rod side I'm using. So is the general feeling that the leak down rate I'm experiencing will not be eliminated by a new spool/solenoid valve? Is it common to put an additional solenoid valve on the cylinder to eliminate any drift? Thanks for all the input, everyone
You can use a low leakage poppet type solenoid valve, activating the solenoid unseats the poppet and allows flow, when power is removed flow is blocked in both directions. Personally if the power unit is where you have easy access, I would just put a high pressure ball valve right where the hose connects to the control valve. You should have a needle check valve inline which will allow it to lift as fast as the pump pushes oil but restricts return flow with the needle valve, you can install it at the cylinder if you think a hose failure may ever happen.