Bobcat 331 Excavator Hydraulic Thumb problems

[Modn]

My Father has a 2002 Bobcat 331 Excavator. It did not come with a factory thumb, but he insisted to buy an aftermarket one with 1-1/2" ram, although it does have the factory Auxiliary ports with a thumb rocker switch. His problem is he has bent the ram on the thumb multiple times. I always tell him to use the thumb only for pressure as it isn't as big a ram as the bucket. I have read all I can and purchased a service manual to figure out how to cut the relief valve down to 1800# instead of the 2900# it is set for now so the bucket will harmlessly push it back in if it overcomes it rather than bending the cylinder. According to the service manual if I adjust the pressure down it will also affect the left hand travel, arm, and bucket. I assumed the Aux. would have been on it's own circuit.....am I missing something? Please correct me if I'm wrong, but the manual does not break down the Aux. circuit on its own either. Although I will state....he may be engaging both at the same time, which is a problem, he says he's not of course! If I can reduce the thumb to 1800#, will that still be an issue?

 

[Jabonee]

Sorry,but its all on the same circuit. You can't lower the pressure without effecting your other functions.Whats needed is a port relief in the AUX valve. 331's only have port reliefs in the boom and dipper valves.

 

[Modn]

Okay, at least I read it right. Back to the drawing board I guess. Thanks for the reply! Maybe install an adjustable relief valve? I assume on the pressure/engage side? Then pipe it back to the tank when it dumps the fluid?

 

[Jabonee]

I'm assuming the thumb cylinder is getting bent when the bucket is curled against it? If so you are correct but in this instance its called a port relief.Its the same as a relief valve but it works on the trapped oil side of the hyd. circuit. It could be plumbed it but you would have a real Rube Goldburg out there on the boom.

 

[Modn]

That is correct...in a perfect world I'm looking for the bucket to push the thumb back in. Only thing I can find is a 3 port adjustable relief valve Item # 9-7901 from Surplus Center.....I was planning on putting it in where all the hydraulic valves are located and put it between the pressure hose and Hydraulic block and piping the blow off back to the tank......Sounds like what your saying is that isn't what I'm looking for.

 

[Jabonee]

You may be on the right track. Post the units SN# so I can look close at the AUX plumbing. I'm assuming the base end of the thumb cylinder faces the thumb which means the relief has to be on the base side of the AUX circuit.You would not plumb the hose from the relief valve back to tank but back into the rod side of the AUX circuit with a check valve or orifice. That set up would provide anti-cav/replenishing to rod side. Remember if your squeezing oil out of the base end you need to be putting some in the rod side.:thumbsup:

 

[Modn]

SN#232513435. Not right at the machine, but pretty positive the base of the cylinder is hard mounted to the dipper and the extending part of the cylinder is attached to the actual thumb.

 

[Jabonee]

OK, still the same.Base end . The relief needs to be on the hose that the oil is being squeezed into from the thumb cylinder. I looked at your control valve set up. It may be a tight fit but if your able to get run tee's on your AUX control valve where the hoses connect you should be able to stuff the rest of the plumbing under there.

 

[Modn]

Sweet!! I sure do appreciate your help on this!!! Much appreciated!! So here we go just so I have it correct. Tee into the hose that powers the ram to engage thumb with the 3 port valve(base of ram). The 3rd port (the escaping over pressured fluid)should plumb into the other hose going to the thumb but should have a check valve into it so the return pressure when retracting doesn't bleed back in? Is my thinking correct?

 

[SSdoxie]

Jabonee is correct that there is not a port relief in that auxiliary valve section, but there a simple port relief installation for this. Below is quick photo snip taken from the Installation Instructions for a Bobcat Thumb. For this serial number order Port Relief part number 6680065 and install it in place of the anti-cavitation plug the is in valve now.

 

[Jabonee]

Correct. The check will prevent oil going into your new valve from the rod side when opening is commanded.:thumbsup:

 

[Jabonee]

Jabonee is correct that there is not a port relief in that auxiliary valve section, but there a simple port relief installation for this. Below is quick photo snip taken from the Installation Instructions for a Bobcat Thumb. For this serial number order Port Relief part number 6680065 and install it in place of the anti-cavitation plug the is in valve now.

View attachment 447707

You got my brain smoking a little SSDOXIE. Service and parts don't seem to jive. JIC schematic shows port reliefs for AUX valve. Disassemble and reassemble in the service Manuel do not. I found the pt# you posted. I'ts listed as a port relief for the extendable arm. No value listed but I'm sure its fixed.This may be the easier solution but it may require removing or disassembling the AUX valve to install it.

 

[SSdoxie]

No disassembly of valve required. Just remove the plug cap currently in valve and install. If he has the hoses connected to correct coupler on the boom, the this plug goes in the top side of the valve as it sets in the machine.
It is shown in the installation instructions for the clamp (thumb) Bobcat sells for that machine. Here is clip from next page of instruction, shows better where it goes.


The only 331 models that did not have provision in the valve for port relief were very early models; all those with 5519 as first 4 digits of serial number and 521913973 and below. All others will accept simple port relief installation in not already factory equipped.

 

[Modn]

No disassembly of valve required. Just remove the plug cap currently in valve and install. If he has the hoses connected to correct coupler on the boom, the this plug goes in the top side of the valve as it sets in the machine.
It is shown in the installation instructions for the clamp (thumb) Bobcat sells for that machine. Here is clip from next page of instruction, shows better where it goes.
View attachment 447778

The only 331 models that did not have provision in the valve for port relief were very early models; all those with 5519 as first 4 digits of serial number and 521913973 and below. All others will accept simple port relief installation in not already factory equipped.

Is it adjustable or is it already set to the correct pressure to be overcome by the bucket. So, it's as easy as basically removing the existing plug with the part # you provided? You had me scared on the 1st post as it did look like I was going to have to remove the valve assembly!!

Update: Just went and looked and can't seem to find any plug per say. This is what I have, which is close, but doesn't look like the pic where it looks like plugs are on the top.

 

[SSdoxie]

Not sure if it is adjustable or not, I think it is. Your current problem is not what relief pressure is, but that there is no relief at all when the auxiliary (thumb) control is in neutral. The fluid is locked in the cylinder so when the bucket pushes against it the weakest point is the extended cylinder rod which bends.

The last photo I posted shows a port reliefs in the valve sections just to the right of auxiliary, these appear to have jam nuts on top of the cap so are adjustable. Part number for the Auxiliary relief is same as the port relief for the dipper arm cylinder which is second to right of auxiliary valve.

 

[Modn]

Not sure if it is adjustable or not, I think it is. Your current problem is not what relief pressure is, but that there is no relief at all when the auxiliary (thumb) control is in neutral. The fluid is locked in the cylinder so when the bucket pushes against it the weakest point is the extended cylinder rod which bends.

The last photo I posted shows a port reliefs in the valve sections just to the right of auxiliary, these appear to have jam nuts on top of the cap so are adjustable. Part number for the Auxiliary relief is same as the port relief for the dipper arm cylinder which is second to right of auxiliary valve.

Take a look at the pic I posted.....I don't see anything that resembles jamb nuts on caps.....do i have too early a SN to use that part#? Sorry for the ignorance and I surely appreciate the help!!

 

[SSdoxie]

OK, a little more research on my part. The port reliefs on your valve are located behind the spool caps (with the small hydraulic lines) and recessed in the valve casting, end cap does not stick above the valve casting. The part number I gave is correct and is preset to 2950 psi.

 

[Modn]

Sweet!! Good info, and I surely appreciate it!!

 

[Modn]

OK, a little more research on my part. The port reliefs on your valve are located behind the spool caps (with the small hydraulic lines) and recessed in the valve casting, end cap does not stick above the valve casting. The part number I gave is correct and is preset to 2950 psi.

So I looked where you described and found what seems to be an adjustable piece rather than just a plug. Of course I'm assuming it is adjustable just due to what seems to be a set screw type on top. Any thoughts?

 

[Jabonee]

Looks correct. I wonder if it has a mate on the bottom side?