Hydraulic Cylinder Sealing

Unless you're running a non-mineral based hydraulic fluid (water, vegetable oil) the manufacturer's seals should be fine.

Depending on the brake's design and age, I would suspect differing reasons for the machine to be stuck after sitting for a time. If the machine has only one centrally-located cylinder and is older, I would suspect the brake head is dropping at one end or the other and binding up in the guides. Adjusting and/or lubricating the guides should help the problem. If sufficiently worn, they may need replaced. Check the head with a level just after stopping the machine and then again just before re-start.

If you have a twin-cylinder machine, I would again look at one end or the other dropping due to cylinder leak-back. Again check with a level. If the leak-back is through the control valve, install a simple piloted check valve (about $20) on each cylinder. If the leak-back is at the cylinder piston, have the cylinder rebuilt to manufacturer's spec. Depending on the size of the cylinder, most shops can do same-day for $100-$150.

If there is moisture in your hydraulic oil, the interior chrome coating of your cylinder may be corroded and pitted. In that case, you will be damaging your piston seal and could be hanging up on a bad spot upon stop. If you suspect moisture (check your hydraulic tank fill location, if you see a brown/tan goo any where near the top of the tank or at your max fluid level, it is emulsified water/oil), pull the cylinder and have a shop tear it down and inspect. Depending on the cylinder size and cost, you can have the bore refinished or just replace the entire cylinder. Also flush your entire hydraulic system and replace the fluid. Moisture attracts moisture and you need to get it all out or the problem will recur.

Hope this gives you some things to look for. Let me know if I can help further or if you come across any other symptoms. Good luck!

When the cylinder is stuck, How do you get it moving again?

What kind of Seals are on fhe Piston? Rod?

Is the cylinder always stopped in the same position?

If so what position is that? Fully extended? Fully Retracted? Mid Stroke?

Have you tried stopping it and letting it set in different positions

What is your operating pressure?

A smooth cylinder bore such as produced by some Burnishng tools can cause "Stiction" when used with Resilient seals but it usually shows up also in chatter as the cylinder is moving through a cycle. Cross Hatch honing usually takes care of that type of stiction.

Some brake presses that have dual cylinders have a single overcentre valve installed as a load hold valve. This type of device will make the pressure required to lower the press higher than that required to lift it. If the pump output (under load) starts to drop off through inevitable ware, the first symptom will be that you can't lower the press. Ok, somebody may have reset overcentre valve! Could place a flowmeter in system and load pump (after the pressure relief valve) and if the flow drops off while generating pressure then you know what to do!

Good luck!

There is the possibility that air is seeping into the pressure side of the cylinder while the press sits idle. If your seals are leaking oil on the pressure side, that would be a good sign that this is the case, but not necessarily. The seals might seal just fine against oil pressure, but relax under vacuum and allow air to enter the cylinder. When air is trapped in a cylinder, it will compress as you actuate the system and oil is pumped into the cylinder. The piston will not move until the pressure builds up and exceeds the force needed to move the piston (against static friction in the seals, the machine, and the workpiece being pressed). What this will look like is that the piston is sticking. After a few cycles, the air will be forced out the return line and vented out of the system, and it will work fine.

Is the cylinder located higher than the oil reservoir such that gravity will tend to drain oil from the cylinder and draw a vacuum on the cylinder when not in use? Place a tee in the pressure line to the cylinder and attach an oil pressure gauge to it (use parts rated to the max working pressure of the system- not brass or sched 40 black iron). Watch the rate of pressure increase on the gauge as you operate the system after a few hours of idle time and compare to normal operating conditions. If you notice a slow building of pressure the first time, and quicker cycles after that, you probably have air in the cylinder -- so rebuild it. Additionally, the check valves/control valves might be leaking internally, allowing the vacuum to build as oil drains through the lines, so you may want to rebuild them too.

I hope this helps.

I agree with some of the previous replys. I would go in the following order.

1. check the valve(s), manually verify they are shifting. IF Yes then

2. Tear the cylinder down and look for bore roughness. If the seal is semi-soft elastomer it could be conforming to a rough area and grabbing it when you then want it to slide. For Stiction theory to apply -the seal material is an important factor - what is is made of? teflon with retainer rings, delrin, metal? Does the piston have wear bands?

Also, What do you do to get it moving again? that IS an important point in troubleshooting.

I do not agree with the vacuum theory. pressure from the pump would replace the "vacuum" with pressure -ie force to move the piston.

You did not mention drifting as part of the problem...but If the cylinder drifts down fluid is either moving around the piston seals, valve is leaking or rod seals are leaking.

Rod seal leaks are easy to spot.... is there fluid on the rod? otherwise>>>

While the cylinder is under load use ball valves on the cylinder ports to verify a piston seal is bad. Safety first, turn off the pump before shuting the valves. Over pressure can occur with large dia rods if you pressurize cap and shut off rod end port.

Hope this helps,

Tim

Mechanical Engineer