Simple hydraulic cylinder keeps retracting (gradually) after pump is turned off, agaist load.

[Andres9292]

Hi,

I have a cylinder that is only using the small chamber. When I turn on the hydraulic unit, the oil fills the small chamber and the cylinder retracts. Using a system of pulleys, an elevator cabin goes from the first floor to the second floor when the cylinder retracts. If I want the cabin to go down to the second floor, I open a valve that lets the oil from the cylinder go back to the tank, the weight of the cabin pushes the oil out of the small chamber and the cylinder expands as the cabin goes down to the first floor.

The problem Im having is that apparently, after leaving the cabin in the second floor for a while, the cabin rises between 2-4 inches. This only happens when the cabin is in the second floor, but doesnt occur if the cabin is in the first floor. At first I thought it might be because the oil in the cylinder might be expanding due to temperature changes between morning and afternoon temperatures since the cylinder is currently exposed to the sun. I made some calculations and it kind of checked out. But it looks like this might not be it since there hasnt been as much of drastic changes in temperature these days anymore. Also, if the oil is already hot at the middle of the day, and then I make the cabin go down (returning the oil to the tank) and then send it to the 2nd floor again (fillling the chamber with already hot oil, the tank is pretty small) the cabin still gradually rises. Once you stop in the 2nd floor, you can hear the cabin rise a tiny amount every 10-15 seconds when the cylinder´s seal unsticks from the wall of the cylinder and moves half a milimeter or so.

We have also tried to rid the system from any air bubbles that might be trapped in the cylinder or hydraulic unit. But other than that we dont know what else could be causing the issue.

If you have any ideas on what might be going on or if you can suggest what to check for I would really apreciate it.

Thanks

 

[3DDave]

It would be less difficult to understand with a sketch showing the hydraulics and the way the cylinder is connected to the cabin.

 

[Andres9292]

The oil enters the chamber in the top part of the cylinder

 

[3DDave]

Almost certainly thermal expansion of the fluid. It usually comes from a cool tank, but even if the oil is already warm, the sun load can add more heat to the oil causing it to expand. Check the cylinder temperature with a non-contact thermometer.

Air could be a problem, but it's so compressible that you would notice that the cabin seemed like it was on soft springs for that amount of motion for it to be the source.

Adding a shade or thermal insulation to the cylinder would decrease the effect.

 

[73lafuite]

Hello,

In a steel enclosure the volume of oil increases by 1% for 15 ° C. If you are on the 1st floor there is no oil in the cylinder, so no variation in position depending on the temperature. On the other hand, on the 2nd stage there is oil in the cylinder. So the stroke increases by 1% for 15 ° C.

Be careful if you go to the cylinder stop and the oil is trapped, the pressure increases by 150bar for 15 ° C.

Cordially

 

[LittleInch]

As well as temperature which is your main suspect, how is the oil in the cylinder isolated from the hydraulic pump?

Is there any way for high pressure oil to be leaking past your isolation valve and slowly pushing the cylinder down?

Can you post the hydraulic system schematic?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[akkamaan]

What are the inside cylinder diameter, the piston rod diameter, and the length of the piston stroke?
What is the pump pressure to lift and hold the elevator cabin?
How far away is the pump unit (the directional valve) from the cylinder ie the length of the hydraulic line?
What is the inside diameter of the hydraulic line?
Is the cylinder fully retracted on the 3rd floor?
Is the cylinder pressureless (ie fully extended) on the 1st floor?

2"-4" lift of the cabin is x2 ie 1"-2" on the cylinder piston