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.