Hydraulic Cylinder overflow before full extension

[SvdSinner]

In my personal garage, I have a hydraulic automotive scissor lift. Recently one of the two cylinders began having an issue. While raising the scissor lift, before would reach its full height, hydraulic fluid will shoot out of a relief valve near the end of the cylinder, and it would no longer raise. It has been several years since I've studied the various parts of hydraulic cylinders, and my foggy memory thinks this a some form of a pressure relief valve that is probably going bad. The scissor lift was from Harbor Freight, so I am on my own to fix the issue.

[ul]
[li]Is there a common mechanical breakdown that causes this? (E.G. O-Ring seals gone bad) What would you assume is causing this?[/li]
[li]With an engineering degree and plenty of tools, can I assume that I should be able to fix it by disassembling the cylinder and replacing any broken seals, etc. I've rebuilt brand name cylinders that had specific rebuild kits, by never a no-name cylinder. (I'm comfortable getting out the digital calipers and measuring the old parts.)[/li]
[li]If this requires hydraulic-specific skills/tools and I need to take it to someone, do I just ask around for a local shop that does hydraulic repairs, or is there something specific I should be inquiring about?[/li]
[/ul]

FWIW, here is the scissor lift:

 

[3DDave]

That seems odd - the force required should decrease as the leverage increases and so the internal pressure should be dropping. It's also odd that there is a relief valve on the cylinder at all - that's normally part of the pump and it looks like this unit has the fluid returning to the tank from the pressure relief valve.

So, where is it exactly leaking from?

I think there's no relief valve at the end of the cylinder. At the rod end of the cylinder there may be a relief hole. If fluid has made it past the piston seal then accumulated fluid would be forced from any vent hole. I looked at the manual for what you appear to have and I see no other relief valve mentioned.

 

[hydtools]

Going to guess that the piston seal has failed and oil is blowing past the rod seal. Filling the rod end with oil will reduce the force of the cylinder. Or the leak caused a drop in oil level in the tank and the pump is sucking air.

Ted

 

[PNachtwey]

Going to guess that the piston seal has failed and oil is blowing past the rod seal.

If so it should make noise and be very warm.


Peter Nachtwey
Delta Computer Systems

http://www.deltamotion.com

http://forum.deltamotion.com/

IFPE Hall of Fame Member

 

[3DDave]

It's only warm if it was under a lot of pressure. Apparently it did not leak so badly when the leverage was worse.

 

[hydtools]

A very slow piston seal leak would not fill the rod end very quickly. Once the rod end is full, pressure intensification could result in high velocity spray passed the rod seal. Still, the low oil level allowing air to enter the pump would stop further movement.
There is probably a vent hole in the rod end of the cylinder through which the leaked oil is being pushed.
The non-leaking cylinder would require more pressure to carry a higher portion of the load. Both cylinders would see the higher pressure being in parallel.
Ted

 

[3DDave]

A lot depends on the geometry and the location of a cylinder vent hole. If there is a lot of air the pressure would not increase much, just dribbling the fluid out. The pressure near maximum extension is the lowest operating pressure due to the change in leverage, so if the seal was going to leak it would start as soon as the lift carried the load.

 

[LittleInch]

Without a link to a manual or a drawing of the cylinder difficult to be certain but hydtools I think has the best remote diagnosis.

Now can it be fixed?

No idea as we can't see the details- a few photos would help...

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

 

[hydtools]

Need a response from OP.

Ted

 

[Compositepro]

These are single acting cylinders. Piston leakage will just slowly fill the rod end and not be visible until the rod extends almost all the way. The cylinders were probably not designed to be repairable.