Hydraulics 101

metalspinner · May 04, 2006, 10:12:29 PM

My shop crane doesn't lift anymore. :( It pumps up then settles down and doesn't go as high as it should. I'm guessing it's low on fluid. Is that possible? If so, how does one fill it up?
Thanks!

scsmith42 · May 04, 2006, 10:14:53 PM

You may be losing the seal on the cylinder. If so, unless it's an older "high quality" jack, it's usually cheaper to buy a replacement at Northern or Surplus Center.

Re filling it - there is usually a rubber stopper that goes into the cylinder - about halfway up or so. Good luck - mine always have had to be replaced.

wiam · May 04, 2006, 10:23:26 PM

Most any hydraulic jack that i have added fluid to make it go did not have that problem. It would go up a ways and then just stop. I think you have seal issues.

Will

bitternut · May 04, 2006, 10:29:01 PM

Why do you think it is low on oil? Is there oil leaking that is visible? If there isn't any visible leaks then you have an internal leak as scsmith suggested and the cylinder probably needs rebuilding or replacement.

metalspinner · May 04, 2006, 10:52:01 PM

Tomorrow I will examine it closely and take a couple of pics to show. I bought this at a pawn shop several years ago to move my lathe and it has worked very well until lately. It is much stouter than the cheapies in the catalogues. The label rates it at 8 tons.

DanG · May 04, 2006, 11:08:27 PM

Filling it up with oil would sure be a cheap way to find out. ;)

scsmith42 · May 05, 2006, 06:49:34 AM

Ditto DanG and the others.

If it's an older, heavy duty jack, if the oil fill doesn't help then take it to a hydraulic shop and have it rebuilt. The old ones are actually much better than most of what's available today.

metalspinner · May 08, 2006, 06:00:16 PM

I think I diagnosed the problem. When the boom is under load then released, oil shoots out of this hole in the middle of the cylinder. It looks as though a plug is missing. Can I refill through this same hole? Should the crane be in the up or down position? How d I know how much oil to put in? And of course, what sort of store can I get a replacement plug? Thanks everyone!

wiam · May 08, 2006, 09:48:49 PM

That hole is for filling. Crane should be all the way down. A parts store may have a plug. I have widdled a wooden plug before when in a bind. Problem is my temporary fixes are usually not. :

If the cylinder will extend all the way you have enough.

Will

scsmith42 · May 08, 2006, 09:50:07 PM

That's the fill hole. There is usually a little rubber plug that goes into it - you can find a replacement at a hydraulic shop (or you might try the surplus center).

To calculate how much oil to use, you will need to determine the displacement volume of the jack. The formula for determining volume is pi r squared x height.

So, first measure the diameter of the rod. Let's say that it's 1.25". The radius is 1/2 of the diameter, or .750. .750 squared is .5625. Next, check the length when it's totally extended. Let's say that it's 16". Therefore, 3.14 x ..5625 x 16 = 28.26 cubic inches.

To convert from Cubic Inches to quarts, multiply times .01732.

Using our example of a 1.250 diameter shaft, 16" stroke jack, it would be 28.26 cubic inches x .01732 = .489, or roughly one half of a quart of hydraulic jack oil. Be sure to dump out all of the old fluid when you refill it, so that you don't overfill it (other than a few ounces for a safety margin).

That's the good news - here's the bad. Every jack that I've had that popped the fill plug out had a problem with a seal somewhere inside. You can try filling it with oil, but this may not fix it.

Once you fill it with oil, you'll need to bleed the air out of the jack. To do this, extend the cylinder several inches, then turn the jack over (so that the valve is up), open the relief valve and quickly push down on the jack to compress the shaft. Repeat this a few times until all of the air is purged.

Good luck!

metalspinner · May 08, 2006, 10:18:53 PM

Thanks wiam and scsmith!
I knew I would get the answers I needed here.

I will give it a shot this week....let you know how it turns out.

DanG · May 08, 2006, 10:35:50 PM

Don't want to bust no bubbles here, but you may not be outta tha woods yet. Could be that you have an internal leak that caused pressure in a place there shouldn't be none, and thats what happened to your plug. Proceed with testing, but with some degree of pessimism. Thataway, if you get a surprise, it'll be a good one. ;D

wiam · May 09, 2006, 06:34:16 AM

I agree that there could be a problem that caused the plug to be out, but I have had jacks that worked fine after replacing oil and plug. I think that the rubber plug just got bumped out in my case.

Will