Dipper and boom both dropping - 3CX

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Dipper and boom both dropping - 3CX

Post #1 by UnclejakeNZ » Sun Oct 13, 2024 5:44 am

Hi folks, and sorry for another request for advice.

There's hydraulic oil leaks from pretty much every ram in the loader, stabiliser and excavator in my 1988 3CX, but apart from one stabiliser leg - the leaks haven't gotten much worse over the last year.

What has 'suddenly' started to happen is both the excavator boom and dipper arm are 'falling down' on their own. With an empty wide bucket raised off the ground a metre, and engine at normal working RPM, the bucket will be on the ground again in about 5 seconds.

The machine didn't show this behaviour a few weeks ago and it seems to be a combination of both boom and dipper sag. The only other (possibly) relevant symptom I can think of is that I've never been able to get much boom power when I'm also using the dipper, but that problem seems to have gotten worse in the last week or so.

Hydraulic filters are all fresh, hydraulic level is probably a bit low, but digging power is still good. I'm guessing a crook valve somewhere is the cause of the boom and dipper drop, but I'm not sure which valve to disassemble for a look. Any advice would be appreciated

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Re: Dipper and boom both dropping - 3CX

Post #2 by Billy26F5 » Sun Oct 13, 2024 10:35 am

Not a 3CX expert but sounds like either ram seals or a valve spool is worn. The latter case is a big job as they're a lapped fit. Ram seal kits are widely available.
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Re: Dipper and boom both dropping - 3CX

Post #3 by gecko.cx » Sun Oct 13, 2024 12:53 pm

I'd put my money on the piston seal having just disintergrated.
Changing the seals is not nearly as bad as it looks.

If you have a 4in1 bucket (and the jaw rams are leaking), they are the lightest and easiest to get at - great for practice.
Last edited by gecko.cx on Mon Oct 14, 2024 11:32 am, edited 1 time in total.


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Re: Dipper and boom both dropping - 3CX

Post #4 by essexpete » Mon Oct 14, 2024 1:56 am

Can you lift the rear of the machine using the boom and dipper and then does it creep down? If it does I think the problem may be with the spools. With sound spools, the rods cannot retract with leaking piston seals as the free volume inside the cylinder is decreasing and oil cannot be compressed. (assuming there is not a significant external leak).
Last edited by essexpete on Mon Oct 14, 2024 12:37 pm, edited 2 times in total.


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Re: Dipper and boom both dropping - 3CX

Post #5 by UnclejakeNZ » Mon Oct 14, 2024 12:07 pm

essexpete wrote:Can you lift the rear of the machine using the boom and dipper and then does it creep down? If it does I think the problem may be with the spools. The rods cannot retract with leaking piston seals as the free volume inside the cylinder is decreasing and oil cannot be compressed.


Excellent diagnostic test. I'll get back to you by Friday night after trying that out.

The front loader bucket ram seals leak way worse than the excavator ram seals.... but the loader bucket stays up (sort of).


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Re: Dipper and boom both dropping - 3CX

Post #6 by UnclejakeNZ » Thu Oct 17, 2024 6:37 am

Interestingly: the machine largely held itself up. It very slowly dropped but it was an extremely slow drop. In this shot all four wheels are off the ground... resulting in a fairly obvious hydraulic fluid puddle under the left loader ram.... but no obvious drips from any of the excavator rams (although I know they all weep)

The other symptom I forgot to mention is how fast the boom responds to my input on the down action. It's terrifyingly fast going down, but very slow going up.


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Re: Dipper and boom both dropping - 3CX

Post #7 by essexpete » Thu Oct 17, 2024 9:33 am

Probably worn piston seals then. A closing ram will usually move more quickly than an opening ram due to the quantity of oil moving so that rather points to piston seals as well. As the spool is diverting oil to the rod side of the piston the spool will also allow oil on the opposite side of the piston to return to tank. So some of the oil pushing on the rod side will be going past the piston and returning to the tank. In the absence of check valves the weight of the boom and dipper will also play a part in the rate of descent.


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Re: Dipper and boom both dropping - 3CX

Post #8 by mechman » Thu Oct 17, 2024 3:03 pm

unclejakenz
I would go for the the piston seals, once they're done that's a problem eliminated. If the problem still exists then it could be the spool block.
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Re: Dipper and boom both dropping - 3CX

Post #9 by UnclejakeNZ » Thu Oct 17, 2024 9:55 pm

Thanks gents. Not what I have expected to hear.... but you are all making perfect sense so I'll plan piston seal replacements


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Re: Dipper and boom both dropping - 3CX

Post #10 by poitch » Sun Nov 03, 2024 3:30 pm

Hi Uncle Jake. you can test the seals to see if they are leaking internally. youtube how to test cylinder seals. lots on there for that can explain it better than i can. one other thing can be the check valves on the valve chest. they are on the bottom of the valve block. you can swap them around. so swap the lifting one with the lowering one and see if the issue remains. or swap the boom holding valve with the bucket one. See if the issue remains. its quicker to swap these than do a boom cylinder seal.


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