Backhoe Backhoe cylinder problem? (w/ video)

   / Backhoe cylinder problem? (w/ video) #1  

caspar3259

Bronze Member
Joined
Apr 5, 2011
Messages
78
Location
Forest Ranch, CA
Tractor
Kubota L3540
I think I have a problem with one cylinder on my Kubota BH77 backhoe, but maybe it's working correctly and my expectations are wrong.

Here's a short video I made yesterday to demonstrate the "problem":
Kubota Backhoe BH77 Cylinder Problem? - YouTube

What do you think? Is it working correctly or is there a problem?

Should the cylinder compress that easily? Should a cylinder develop "slack" that allows the bucket to bounce?

thanks for any and all input!

-Jeff
 
   / Backhoe cylinder problem? (w/ video) #2  
It looks like you have a brand new hoe, yes?

If so, you almost certainly have air in the cylinder. Fully extend the cylinder and HOLD the control to keep it extended for 5 full seconds. Then fully retract the cylinder and HOLD the control to keep it retracted for 5 full seconds. You should hear the air going past the pressure relief valve and should see foam in the fluid reservoir. You will need to check the fluid level, top it off and if there are bubbles in the fluid wait a bit for them to go away then repeat until the fluid level stays the same. I would do this with every cylinder.

If it's NOT a new hoe and it was previously working correctly, then there are other things to look into.
 
   / Backhoe cylinder problem? (w/ video)
  • Thread Starter
#3  
It looks like you have a brand new hoe, yes?

If so, you almost certainly have air in the cylinder.

Thanks for the reply.

The hoe is new-ish... it was new in July and now has 8-10 hours of digging on it. I have already done the "purge the air" procedure with no difference.

The dealer has also worked on the backhoe twice already: the first problem was it would not hold it's position and would droop. He said the fix was "cleaned some assembly gunk out of the relief valve". Second problem was the dipperstick had no power and moved VERY slowly. He said the fix for that was "replaced a valve".

The dealer has not responded to this last problem. So I'm hoping to learn as much about it as possible so I can "make my case" to him.

-Jeff
 
   / Backhoe cylinder problem? (w/ video) #4  
if you pull on the dipper and bucket at the same time does it do the same thing?

If it does I would think the piston is leaking.

If not might be a control issue.
 
   / Backhoe cylinder problem? (w/ video) #5  
Definitely not right. I used the dipper and boom inconjunction with the curl cylinder and managed to bend the rod on the curl cylinder. In other words, when working properly, the dipper shoudl not collapse.

This might be better on the hydraulics forum, but I suspect a control valve issue. The fuild is not bypassing the seals of the cylinder, it is being evacuated from the cylinder, hence the slack. Something is goofy.
 
   / Backhoe cylinder problem? (w/ video) #6  
The more I think about it, There must be air in the dipper cylinder. If there was fluid it wouldn't move like that when it comes off the stump.

The bounce is caused by compressed air expanding.

JMHO
 
   / Backhoe cylinder problem? (w/ video) #7  
Thanks for the reply.

The hoe is new-ish... it was new in July and now has 8-10 hours of digging on it. I have already done the "purge the air" procedure with no difference.

The dealer has also worked on the backhoe twice already: the first problem was it would not hold it's position and would droop. He said the fix was "cleaned some assembly gunk out of the relief valve". Second problem was the dipperstick had no power and moved VERY slowly. He said the fix for that was "replaced a valve".

The dealer has not responded to this last problem. So I'm hoping to learn as much about it as possible so I can "make my case" to him.

-Jeff


Couple comments. Obviously the hoe has issues. Equally obviously, the dealers is about as incompetent as they come. You probably figured that out, but I thought you might like to hear it from someone else.

Looking at the video it looked like the dipper stick cylinder "sprung back" when tension was removed. If it's fluid bypassing, that CAN'T happen, as in impossible. Fluid doesn't compress - air compresses. You still have air in the cylinder because the cylinder bounced back out. (unless the hose was swelling like 3x it's diameter....). Time to get "aggressive". Pull back that lever and hold it full back for 1 full minute with the engine at PTO RPM's. Use a watch. Then push it full forward for 1 full minute. You will be bypassing the fluid across the relief valve the whole time and the fluid will be heating up -- keep an eye on that. Actually, a hand. The fluid will get hot, that's ok as long as it stays under 300F. Normal operating temps are 140-180F. Short excursions to 200-250 are not an issue at all.

I suspect that there are multiple issues.

You REALLY need to spend 20-30 bucks and buy a pressure gauge that reads up to 3500 or so and the adapters to fit to the working ports of the hoe control valve. Check that you are actually reaching the operational spec pressure. Some systems set the pressure at the control valve, but some Kubota's set it in a block on the tractor by changing shims. You can get them pretty cheap. IIRC they are like 150psi per mm. On new systems going 150-200 psi over spec is ok as the relief spring will quickly weaken and settle down that much -- then hold it there for a long while. You might have to take off a hose and bring it to a hydraulic shop to get the correct adapter - or else visit the dealer. But I don't have much faith that your dealer can find his butt with both hands...

If you have a cylinder that is bypassing, you can test for that pretty easy (but not neatly!). Fully extend the cylinder, then REMOVE the hose from the cylinder on the retract side. Then with the hose still off, move the control to extend the cylinder. You will probably get a shot of fluid out as the cylinder moves a bit, then you should get just a few drips to a very minor dribble. That dribble will get worse over time as the cylinder and seals wear. Seals can hold in one direction and leak in the other, so you have to repeat with the cylinder retracted.

If there is no air in the cylinder and the cylinder is not internally bypassing (and visually not externally from your vid), then the only place left is the control valve. If the relief valve is set correctly and not opening early (detected by a low gauge pressure) then there may be internal bypassing in the valve.

Hope this helps.
 
   / Backhoe cylinder problem? (w/ video) #8  
That cyl definitely is not working correctly. That cyl should be in hyd lock with the spool for that cyl in neutral. If it moves, there is air in the cyl, especially if the cyl rod extends after being pressed in.

I agree with the air in cyl. Cycle the dipper cyl several times and see if it still does it. You could also put a gage in the cyl circuit and read the pressure.

Check and see if you have air in all the cyl. Possible suction leak on the pump.
 
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   / Backhoe cylinder problem? (w/ video)
  • Thread Starter
#9  
Thanks for all the suggestions and explanations. I'll try the "aggressive" air purge and see if that makes any difference. If the problem continues (or comes back later), I'll go back to the dealer and ask him to make it right.

-Jeff
 
 
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