Bobcat 322 Dipper Arm Dropping

[mpb]

Hi all, newbie here.

Ive got a Bobcat 322 that I use on my small holding.

The wiper seal on the dipper arm started leaking, so I went to the local engineering factors and they ordered me a set of seals. I fitted the new seals and reassembled the ram, and topped up the hydraulic fluid.

I used the machine for about an hour with out any problems. Then the dipper arm started dropping by its self. If you hold it out straight and let go of the control the arm drops to a neutral position.

I took the ram back apart to check that the seals hadn’t moved etc, but everything looked ok. I’ve bled as much air as I can out of the ram.

Im now at a bit of a loss as to what the fault can be, every other function seems to operate as it should.

Many thanks

Martin

 

[Shovelhands]

When you reassembled the ram, was there much resistance when you push the rod into the barrel?

 

[mpb]

When you reassembled the ram, was there much resistance when you push the rod into the barrel?

Seemed to have quite a bit of resistance when it went back together

 

[Shovelhands]

Seemed to have quite a bit of resistance when it went back together

That’s good then, the main seal needs to be a tight fit in the barrel. Assuming the piston is not coming off the rod, as you’ve checked this and you still have the fault? It sounds like the ram is ok. Is it possible that you’ve got something in the pipes when you took the ram off? A foreign particle that is now lodged in the relief valve possibly? Bloody long shot, but if the ram is ok then it’s got to be something in the spool block.
This machine hasn’t got a hose burst protection valve has it?

 

[V8Druid]

to be absolutely sure of the ram ... extend the dipper fully out and sit it on the floor .... drop the hoses off and cap both ram ports and hoses ... then lift the boom and see if the dipper stays put ... you will then have definitely isolated the ram as the fault
assume you have the piston seals on the right way around ??

 

[mpb]

That’s good then, the main seal needs to be a tight fit in the barrel. Assuming the piston is not coming off the rod, as you’ve checked this and you still have the fault? It sounds like the ram is ok. Is it possible that you’ve got something in the pipes when you took the ram off? A foreign particle that is now lodged in the relief valve possibly? Bloody long shot, but if the ram is ok then it’s got to be something in the spool block.
This machine hasn’t got a hose burst protection valve has it?

Many thanks Shovellhands.

I take it the relief valves just unscrew out of the spool block, then give them a clean with brake clean.

Ill pick some more hydraulic oil up tomorrow, then if I get back early from work I’ll have a look.

 

[Shovelhands]

Many thanks Shovellhands.

I take it the relief valves just unscrew out of the spool block, then give them a clean with brake clean.

Ill pick some more hydraulic oil up tomorrow, then if I get back early from work I’ll have a look.

Well yes, but as Druid says, you want to be sure the ram is ok. After all, the ram is the bit you’ve been working on and you didn’t have this issue before you worked on the ram.
I don’t know your machine at all, so couldn’t say how easy it is to get the the valve block or how accessible the ARVs are.

 

[mpb]

Sorry I missed Druids post. That makes a lot of sense. I’ve just ordered some plugs and caps to give that ago.

I’ll let you know how I get on.

many thanks

Martin

 

[V8Druid]

Sorry I missed Druids post. That makes a lot of sense. I’ve just ordered some plugs and caps to give that ago.

I’ll let you know how I get on.

many thanks

Martin

if the dipper drops, you have a couple of choices ... piston seals on backwards / scored / damaged / turned slightly ... piston not on tight enough, or piston to rod seal damaged/missing (if it had one) ... seal and back up joints incorrectly aligned, or worst case - bore damaged

 

[Bri963]

The seals may be a red herring. If there is an internal leak past the rod seals, the stick will only drop a little bit then stop when pressures equalise both sides of the seal. More likely leaking past the spool, or possibly the line relief.

 

[V8Druid]

The seals may be a red herring. If there is an internal leak past the rod seals, the stick will only drop a little bit then stop when pressures equalise both sides of the seal. More likely leaking past the spool, or possibly the line relief.

a leaking seal would also act like a damper with the weight of the dipper bearing on it Bri, until it reached a neutral position

 

[Bri963]

if The stick is falling all the way the oil has to be escaping the circuit via line relief or spools.

 

[V8Druid]

if The stick is falling all the way the oil has to be escaping the circuit via line relief or spools.

sorry Bri ... I disagree .... if the oil is bypassing the seal/piston, it's simply transferring from one side of the piston to the other ... the overall volume in the cylinder doesn't change, just the oil is displaced as the rod / piston moves through it ... same way as a hydraulic damper works

 

[Bri963]

Common mistake, Gra. Both sides of the piston seal are not equal volumes, you’re forgetting the volume of the rod Itself which you have to take into account. When the rod is fully extended, there’s more oil in the cylinder than when it’s fully retracted, unless it’s something like a steering ram. So, for equal pressure and flow, the cylinder will extend more slowly, but with greater force, and retract more quickly with less force. You can’t compress oil, so if it can’t get out of the rod end or valves, it’ll reach hydrostatic equilibrium.

Another way to think of it is to picture a ram with no seals around the piston, but good seals in the cap, and the connections blanked off, or even linked by a hose. Now picture it fully extended with the head end full of oil. If you now apply an external load to retract the cylinder, you aren’t just forcing oil past the cylinder, you’re also reducing the internal volume available to a set volume of oil by trying to force the rod into the same space. The only way the rod can be pushed in is if an equal volume of oil can escape, which in our hypothetical ram can only be through the cap seal or leaking through bad connections.

if the hoses were linked to a spool valve, the ram still couldn’t drop unless the spool was operated or leaking, or ditto a line relief.

Now you’re going to say we’re talking about a stick ram which is being pulled out by gravity rather than compressed, and so may be cavitating, but pulling the stick all the way in and seeing if it drops will prove off the ram seals. If it still drops, it’s not the seals, or at least not just the seals.

 

[Shovelhands]

Common mistake, Gra. Both sides of the piston seal are not equal volumes, you’re forgetting the volume of the rod Itself which you have to take into account. When the rod is fully extended, there’s more oil in the cylinder than when it’s fully retracted, unless it’s something like a steering ram. You can’t compress oil, so if it can’t get out of the rod end or valves, it’ll reach hydrostatic equilibrium.

Agreed, but this ram will be fully retracted when the dipper is fully extended. So there will be less oil in the rod side, there should be more than enough room for all the oil to transfer from one side to the other. If it was a lift ram, working in the other direction, then it would be a different story.

 

[Bri963]

Agreed, but this ram will be fully retracted when the dipper is fully extended. So there will be less oil in the rod side, there should be more than enough room for all the oil to transfer from one side to the other. If it was a lift ram, working in the other direction, then it would be a different story.

Re-read my last paragraph, I covered that. Also, if you’d got a ram full of air/vacuum, trying to pull the stick all the way in from vertical, you’d see a delay in movement until you’d re-primed the cylinder.

 

[Shovelhands]

Re-read my last paragraph, I covered that.

Well I read it before you edited it. Sorry, my mistake

 

[Bri963]

Well I read it before you edited it. Sorry, my mistake

No problem, mate. Too much time on my hands today, been doing a lot of editing. Sorry if I came across a bit smart-a, comes from thinking, which hurts.

 

[V8Druid]

Common mistake, Gra. Both sides of the piston seal are not equal volumes, you’re forgetting the volume of the rod Itself which you have to take into account. When the rod is fully extended, there’s more oil in the cylinder than when it’s fully retracted, unless it’s something like a steering ram. So, for equal pressure and flow, the cylinder will extend more slowly, but with greater force, and retract more quickly with less force. You can’t compress oil, so if it can’t get out of the rod end or valves, it’ll reach hydrostatic equilibrium.

Another way to think of it is to picture a ram with no seals around the piston, but good seals in the cap, and the connections blanked off, or even linked by a hose. Now picture it fully extended with the head end full of oil. If you now apply an external load to retract the cylinder, you aren’t just forcing oil past the cylinder, you’re also reducing the internal volume available to a set volume of oil by trying to force the rod into the same space. The only way the rod can be pushed in is if an equal volume of oil can escape, which in our hypothetical ram can only be through the cap seal or leaking through bad connections.

if the hoses were linked to a spool valve, the ram still couldn’t drop unless the spool was operated or leaking, or ditto a line relief.

Now you’re going to say we’re talking about a stick ram which is being pulled out by gravity rather than compressed, and so may be cavitating, but pulling the stick all the way in and seeing if it drops will prove off the ram seals. If it still drops, it’s not the seals, or at least not just the seals.

agreed with the ram fully extended Bri, but still disagree with it fully closed, holding the dipper up ... it will pass sufficient fluid to prove the seal fault, before any induced vacuum stops the motion

 

[Bri963]

That’s it though, Gra. You pull the stick in to prove off the valve, if it falls down from the ram-extended, stick-in position it’s not the seals. Been through all of this before on a twenty four tonne machine. Had the relevant service manager say exactly the same thing as you, but i got their fitter (warranty job) to pull the spool anyway. The bore of the spool block was scored end to end From machining debris That had never been cleaned out at manufacture. New spool block fitted (at their expense) no more dropping of the stick.