Hydraulic internal leak.

[Smallfry]

I have an old (1972) Unimog, that is fitted with a similarly old HIAB crane that FiL had fitted in the early nineties.

It has its own hydraulic system, independent from the Unimog, apart from its pump being driven off the main gearbox. The problem is, that the main lift ram drops during operation. Not fast, more slow, but not not really slow. So, for example, I wanted to lift a ton bag of sand etc, I can position the arm, but by the time I have walked over to hook it up, it will have dropped 8 to ten inches.

With the bag of sand not too much of a problem, but if I were trying to lift something that would be easily damaged, it is !

I see there are a few knowledgeable people on here by Chismac's recent post, so what is the most likely place to be looking for the fault ? There are NO external leaks, so should I be looking at the ram itself, or the hydraulic block ?

The hydraulic block has two spare services, only four of the six are used, so could I swap the pipes over to one of these, or is it likely that the characteristics would be vastly different ? The block is one piece, and not a sandwich of slices

 

[Ton-up]

I would look at the ram seals first.

I've done this on a forklift by holding up the carriage, blocking the flow and return on the valve block, and then letting the weight back onto it. Carriage still dropped and seals were indeed knackered!

 

[Smallfry]

I would look at the ram seals first.

I've done this on a forklift by holding up the carriage, blocking the flow and return on the valve block, and then letting the weight back onto it. Carriage still dropped and seals were indeed knackered!

I guess the forklift has a single acting ram ? This thing has feed at both ends of the ram, so I guess its double acting ? I can't see why though.

I cannot see a way of blanking off the lift without releasing pressure from the ram though.

 

[m_c]

Start with the easy checks.
Take the rod hose of the ram (that's assuming the ram extends to lift) with the crane raised.
If oil comes out the hose port on the ram as the ram contracts under load, then the ram piston seal has failed.

If no oil comes out, put the hose back on, drop the load, then try swapping both hosed to a different slice, and see if the crane is still dropping.

 

[Smallfry]

Start with the easy checks.
Take the rod hose of the ram (that's assuming the ram extends to lift) with the crane raised.
If oil comes out the hose port on the ram as the ram contracts under load, then the ram piston seal has failed.

If no oil comes out, put the hose back on, drop the load, then try swapping both hosed to a different slice, and see if the crane is still dropping.

OK Thank you. I will give that a try ! Yes, ram extends no problem.

Just to be clear though, surely if I disconnect the hose, oil will come out anyway ? I hate oil leaks !

I have the fuel tank off at the moment, so once I get it back together that is !

 

[m_c]

It's hydraulics. It's going to be messy

Just make sure you have some suitably placed containers to catch oil, ideally with an assistant nearby to help get them to where they're needed.

How old is the crane?
And do you know if it has check valves fitted to the rams?

 

[Pete.]

Just to be clear though, surely if I disconnect the hose, oil will come out anyway ? I hate oil leaks !

If you remove the rod end hose the ram should not drop and only the oil that's dribbling out from removing the hose should come out. If the ram is sinking and pushing oil past the seal it will have to spew out the rod end port because as the ram sinls the rod starts using up space inside the ram casing that is full of oil.

Great caution required when doing any of this.

 

[m_c]

Just to clarify my question about the check valve, if they're fitted, then the spool may connect the ram to Tank/Return when in the neutral position, and rely solely on the check valve to hold the ram.
If that's the case, if you disconnect the hose with the pump running, you may get a fair bit of oil working it's way back up the hose, as the return/tank line will usually still have a bit pressure in it, especially if there's a return line filter fitted.
Also, I just had the thought that the tank might be pressurised, which will also push oil out even with the pump stopped.

I'd still do the above test, but with the pump stopped, and the tank vented if applicable.

If check valves are fitted, the check valve itself might be the problem.
They do occasionally fail, or could have a bit dirt stopping it from fully closing.

 

[Smallfry]

It's hydraulics. It's going to be messy

Just make sure you have some suitably placed containers to catch oil, ideally with an assistant nearby to help get them to where they're needed.

How old is the crane?
And do you know if it has check valves fitted to the rams?

Assistants ? Ha, they will all be too busy when needed. I'm sure you know that.

The crane is a HIAB 650A. I can't find much, if any info on it on the net. Seventies, early eighties at a guess.

Check valves ??? Wibble wibble wibble. No idea

 

[Smallfry]

If you remove the rod end hose the ram should not drop and only the oil that's dribbling out from removing the hose should come out. If the ram is sinking and pushing oil past the seal it will have to spew out the rod end port because as the ram sinls the rod starts using up space inside the ram casing that is full of oil.

Great caution required when doing any of this.

Caution yes. The thing frightens me a bit to be honest. The ram rod is about three inches diameter, and the whole thing is mostly inside the arm. Its going to be heavy, and I see plenty of potential for crushed fingers.

 

[Smallfry]

Just to clarify my question about the check valve, if they're fitted, then the spool may connect the ram to Tank/Return when in the neutral position, and rely solely on the check valve to hold the ram.
If that's the case, if you disconnect the hose with the pump running, you may get a fair bit of oil working it's way back up the hose, as the return/tank line will usually still have a bit pressure in it, especially if there's a return line filter fitted.
Also, I just had the thought that the tank might be pressurised, which will also push oil out even with the pump stopped.

I'd still do the above test, but with the pump stopped, and the tank vented if applicable.

If check valves are fitted, the check valve itself might be the problem.
They do occasionally fail, or could have a bit dirt stopping it from fully closing.

I don't know what a check valve would look like. Would it be a part of the ram, or something fixed to it ? ISTR some of the hoses go to a junction ? block on the mast. Four in and four out I think. Will look in the morning.

Could be dirt, as I have no idea when the oil or filter was changed last, FiL died 12 years ago, and was ill with COPD for a few years before that, so certainly not since then.

 

[chrismac]

I have just found out check valves can be fitted in the spool block like below, I think these are top end spool valve types,

or they can be fitted as additional blocks like these

And I can confirm my simple task of swapping my spool blocks over was very messy!!!
Good luck.

 

[m_c]

I don't know what a check valve would look like. Would it be a part of the ram, or something fixed to it ? ISTR some of the hoses go to a junction ? block on the mast. Four in and four out I think. Will look in the morning.

Could be dirt, as I have no idea when the oil or filter was changed last, FiL died 12 years ago, and was ill with COPD for a few years before that, so certainly not since then.

A check valve would usually be fitted onto the ram itself, with steel pipes running from each end of the ram to the valve block, although on a crane ram, they're usually mounted over the cylinder end port, with a steel pipe running to the rod end port.
The predecessor to them was a burst valve, which would be fitted inline with the hose, which simply snaps shut and stops oil flow if it senses too much oil flow (i.e. is a hose bursts).

70s/80s I'd guess the most it will likely have is burst valves, but my experience with cranes starts well after that era, and burst valves were things I only ever seen mentioned on training courses, although they are still fitted to some applications.