Hydraulic Clamp/Thumb - Bobcat 325

[DelVin]

I’d been having some difficulty getting a new aftermarket thumb to work on my 1998 Bobcat 325 mini-ex. However, I finally have the whole thing working perfectly!

Along the way, I purchased the Operation and Maintenance Manual. On Page 83, there is one small note in the manual that says that the use of the thumb “Requires” the use of port relief valves in the auxiliary valve circuit, but the whole thing is working perfectly. So, could anyone help? Am I going to damage something by using the thumb without adding a relief valve? Or is that just if I continue to smash the bucket and thumb for an extended time?

 

[funwithfuel]

Risk of hose rupture , risk of damaging pump . In a dead head situation the oil has no where to go.

 

[DelVin]

So, that opens the door for a lot more questions….. First of all, there are several manufactures selling direct bolt on thumbs for this machine. I had thought that I only found a typical manufacturers over-warning, but if it is really required, it would seem to me to be a great opportunity for these manufactures to sell me (all buyers) some additional parts. Why wouldn’t a single one of these manufactures offer the extra pressure valve to make more money and help me/us not wreck a machine?????

Please forgive the question; I don’t have a lot of hydraulic system background, but how would the thumb deadhead any more than if I just kept pulling on a rock that’s too heady with the boom?

Finally, I own the Service Manual and the mentioned Operation and Maintenance Manual. The single footnote I mention above is the only place an auxiliary pressure relief valve is mentioned. So, the $10,000 question: how do I add a pressure relief valve, how would I get the Part Numbers for these optional parts, and where would I get the instructions to put them? It’s a 325 Serial #514014911.

 

[funwithfuel]

OK, let's consider what you have and what you're doing.
First, what kind of thumb? Is it hydraulic running off oil from the option spool? Or are we looking at a stiff arm that is rigid to the dipper which the bucket works against.
Let's assume you're using the latter . If you have a main relief setting of say 3500psi , you have that same pressure on your bucket and thumb cylinder. Let's say your bucket cylinder is 5 sq in and your thumb is 3 1/2 sq in. With each having 3500 psi the thumb is gonna lose. Now consider, the oil is mechanically locked in the cylinder and you have no port relief. The rod will bend, the piston packing will fail or the hoses will fail. Worse case you push the piston back, it cocks and gouges the barrel and sends material back to your valve. Now you've damaged the cylinder and your control valve.
The example might be a bit exaggerated, but you get the idea. The port relief purges oil in the event a greater pressure is placed against it. Considering it's a mini exc the hoses that could/would fail are staring you in the face, and aren't that far away, I'd want that purge contained,for safety sake.

 

[funwithfuel]

If you're using a stiff arm thumb, none of this matters because the bucket circuit port relief will provide protection

 

[007]

Hello Delvin,
I think you will find there is no simple answer to what you are asking.
I have had similar issues with my Kubota and have read all the threads on this site and done my own research.
I know i am going to get attacked for this post but here goes.
From what i can see most mini ex are fitted with a motor spool block on the Aux circuit and have pressure relief adjustment on the spool for both directions.
As the manufacturer does not know if a ram or motor will be used on this circuit (I am Guessing).
So as your spool is open center to tank (check your manual) it should be as simple as just adjusting your spool to the desired pressure in each direction.
But as you know im shore life is not always simple so here are some things to consider.
My machine is set to relief at 3200psi and my aux ram is rated at 3000psi, what is your pressure relief and ram on the aux circuit rated for.
Even if you set your pressures by the book you have to look at the stroke of your thumb and you will notice usually there is a position through the middle where the ram is mechanically strong.
So at the beginning and end of stroke there is huge mechanical advantage over that ram from the bucket so you might consider a pressure that protects and suits the ram or arm.
I think most guys like to place there thumb in the strong position and curl there bucket maybe some one will comment?
Some of the threads here i have read where the guys like at least 2800psi to be able to grab a larger object, depends on the thumb design and ram i would imagine.
The next most common thing i read here is people with thumbs have problems as the pressure bleeds away and the object they are holding with the thumb falls.
Which is the problem i am having right now with mine and I am having fitted a single pilot operated check valve in the closed line to my ram to hopefully correct this.
It is just a small valve in the lines which holds the pressure in the closed position.
My set up is a little unusual and i cannot over pressurize my ram but in your case if you run into trouble like me and fit some sort of check valve it will have to have pressure relief function in the valve or separately to protect your ram.

Hope this helps Cheers.

 

[DelVin]

007 – Thanks, that is a helpful start. Funwithfuel-The thumb is from Showtime Fabrication, but there are several manufacturers selling similar on ebay. It’s a complete pin on hydraulic kit that attaches to the machines existing aux hydraulic quick connects. I’m not sure if this link will work through the forum, but it’s all the technical information I have:
http://www.ebay.com/itm/251328414461?ul_noapp=true

At this point, I don’t even have a suitable pressure gauge. I’m new to the tractor world, but I have a fair amount of (I believe) transferable skills. Does anyone have an actual p/n for at least the type of valve we’re discussing (preferably something that I might be able to also read manufacturers documentation on)?

 

[DelVin]

So, I called Showtime Fabrication, and they were also helpful in learning more about the issue and my machine in particular. Shortly after discussing with them I found Bobcat P/N 6680065. That’s a 2950 PSI relief valve. The thumb ram will take 3000PSI, so I’m thinking that is all the insurance needed….?

I was able to find some photos (although not very clear) online of the install for a 331, but haven’t begun to poke around my 325 yet. Apparently this part comes with bobcat clamp kits (7104321 [was 6714449]). Does anyone out there have a copy of those instructions you could post?

 

[007]

Thats good you sound like you are making progress.
Showtime Fabrication obvious make them for bobcats so did they say they fit additional relief valves or just use the existing?
The part you found was that an existing valve or an optional valve bobcat sell?
Another thing i find with mine is it moves to quick due to the high flow rate of the machine and its not adjustable like on larger machines.
I am thinking of fitting a small hole in one of the fittings to slow it down at bit.
If you are doing any of the welding on your Stick or boom do your welds correct because cracking can be an issue if not done right.

 

[DelVin]

No welding required. The whole thing just bolted on, but the more I dive into the hydraulic circuit, the more frustrating it gets. Page 2-14 of the Service Manual says the Main Relief protects the auxiliary and bucket circuits. On that same page it says that circuit should be set to 2550 PSI.

Doesn’t that imply that the thumb/bucket system will not dead-head? Could it be an issue that both bucket and thumb are on the same relief valve? And if that is the case, wouldn’t just installing a 2950 PSI valve on the thumb side do absolutely nothing?

Still looking for the installation manual for Bobcats thumb, to know what else I’m missing.

 

[007]

I am no expert on hydraulics and i don't know your machine but i think you are not reading your schematics correctly.
Before i bought my machine i read untold brochures on different machines and even the oldest machine has a couple of pumps to the more modern having 4 pumps or more.
I think my machine has three and one of those pumps is for my final drives and Aux circuit.
I think the factory pressure relief on the bucket stick and boom is set to 3500psi
And my tracks and Aux factory relief is set for 3200psi.
Maybe you are right with only one big volume pump, if that is correct none of your circuits should exceed 2550psi.
No problem being on same pressure relief valve.
But the point funwithfuel was making earlier was that a more powerful bucket cylinder could over power the thumb cylinder and could create very high pressure in the thumb cylinder and the oil has no where to go as it is locked up stream by your aux spool.
When i look at my aux spool it is different to all the rest with two pressure relief adjustments.
So if that happened on mine one of my aux spool reliefs should lift no problem.
What does your manual tell you when you look at your Hydraulic Schematic, do you have more than one relief in the system.
Can you scan that page and post it?
They usually show what control items are in each circuit.
There are some real guns on hydraulics on this site and they will help interpret what we can't figure out.
I would be surprised if you ever found a manual on fitting the thumb unless there was a factory option to use one from bobcat.
A good hydraulics shop will alter each machine to suit the hundreds of different objects that can be attached to your machine.

 

[DelVin]

I can read a wiring schematic, but unfortunately I'm still a little new to the hydraulic schematic. So, I was only referencing what is said in the manual. I’ve attached both that and the schematic you suggest. If anyone can help, that would be awesome. Find the Aux switch at #19. There is a great theory of operation in the 3-4 pages preceding the hydraulic schematic, but unfortunately, it is lacking on the auxiliary circuit.

The machine definitely has two pumps.

It is the instructions for the factory Bobcat Clamp I am trying to get. That kit P/N is (7104321 [it was 6714449]). I’m hoping that will have a better description of how the circuit functions and specifically where and how to install the Bobcat pressure relief valve (which I just found on Ebay for $30.00).

 

[007]

I had a quick look and i think i can follow generally whats happening.
There are check valves dotted hear and there as they are using fluid from both pumps on tasks to speed the machine up.
There are relief valves used as well as your main relief valve you mentioned.
What the manual is trying to do is load up both pumps to set the main relief at full flow.
By lifting your blade and opening your bucket is a circuit on each pump which has no relief valve and that reflects the pressure back on the main relief valve.
Aux switch 19 selects Motor or Cylinder on your Aux circuit.
I can't see any relief valves on your aux circuits so you are supplying generally 2550psi.
I say generally because you have cracking pressure and full flow pressure on relief valves but are close to each other, and the system pressure will vary depending on engine rpm and how multi tasking you are on the controls.
From what i can tell your Aux spool is not protecting your thumb ram from excessive pressure being applied to the ram.
If it was me i would compare rams on each side of the stick and factor in the mechanical advantage on both sides and if they were close to each other i would not fit a relief valve.
Remember if you fit a relief its one more place for fluid to bleed away under pressure as i believe they all leak a certain amount.
And you might start getting the problem i have.
Remember if you abuse your thumb in its weakest position the relief valve wont save your ram from damage.
Hope I helped Cheers

 

[funwithfuel]

Sorry for the delay, I was out of signal reach. If you look at your main control valve illustration, you will see working valves , bucket boom arm and blade have additional valving adjacent to the delivery ports. these are your port reliefs . You'll see that travel does not. Travel relief is incorporated into the travel motors , the same within the aux spool, it has blank ports. They don't know what you are going to implement as an option. As mentioned before, the main relief protects your pump(s) the port relief protects your circuit. Remember, you would ideally want to protect both A&B ports .

 

[funwithfuel]

Looking at your schematic, item 15 in second valve bank would be your main relief item 19 is a bi-directional return valve. In hammer mode, the return oil runs straight back to tank, in shear it returns through bi-directional and back to MCV through port B in a closed loop. You'll notice the absence of port relief as illustrated in items 30-35. The strange thing is the photo shows no port reliefs for travel but the schematic does.

 

[melli]

One thing I would check for those with issues, is your main relief pressure...I had the situation where I slowly starting losing ability to hold onto boulders...until I couldn't. Turns out main relief was off just a bit (which meant bucket was below thumb relief). Want bucket relief higher than thumb.
And as far as having a relief on aux (thumb), you definitely need one or you'll bend something.
I posted on here how I made a pressure kit to check all pressures on all circuits. Was a couple hundred bucks...well worth it. Now, I can dial in all pressures, and even mod my machine (331). And the manual has a chart of all pressures, so it was easy peasy to adjust.

 

[Tim-in-ND]

I am facing same issue as original poster - thumb from Showtime Fabrication and not sure if I have pressure relief valves in the aux hydraulic circuit.
I have a 2002 Bobcat 331 SN 232512491.
Talked to the local dealer and they looked up the thumb kit for this model and they list a SN range where pressure relief valve kit is needed for safety when installing a thumb.
For future reference for the next person who finds this thread, here are the SN ranges per the Bobcat manual (6900367enUS 12-16 T) where pressure relief valves need to be added. Highly suggest talking to your local dealer to verify what is needed for your machine S/N.

Install the relief valve P/N 6674259 in the control valve for the following models as follows:
325: S/N 232311001 - 232312387
331: S/N 232511001 - 232512782
334: S/N 232411001 - 232412130 and 232611001 - 232612158

My 331 is in the above range so I've got to figure out how to solve.

Also from the sheets printed by the dealer:
NOTE: For model
331 (S/N 234313000 & Above)
334 (S/N 234513000 & Above)
430 (S/N 56911001 & Above & 563011001 & Above)
All 335, 337, 341, and ALL 435 Excavators, the relief valves are factory installed in the control valve.

Hope this helps next person
Tim

 

[Gumboot]

One thing I would check for those with issues, is your main relief pressure...I had the situation where I slowly starting losing ability to hold onto boulders...until I couldn't. Turns out main relief was off just a bit (which meant bucket was below thumb relief). Want bucket relief higher than thumb.
And as far as having a relief on aux (thumb), you definitely need one or you'll bend something.
I posted on here how I made a pressure kit to check all pressures on all circuits. Was a couple hundred bucks...well worth it. Now, I can dial in all pressures, and even mod my machine (331). And the manual has a chart of all pressures, so it was easy peasy to adjust.

Hi Melli, couldn't find your post on assembling a pressure kit; Could you supply post title? Thanks, Gumboot