I have a Kubota KX040-4 Mini Excavator with a hydraulic thumb. I've been using it for about 60 hours to pick up logs, rocks, etc., with some that are quite heavy. There have been quite a few times I've picked something up with the bucket fully extended and then extended the thumb to grab it. The problem is when I try to curl my bucket to get under it. The bucket curls okay, but I have a hard time coordinating the electric rocker switch on the thumb so it retracts at the same rate the bucket curls. Several times the bucket curl has forced the thumb to retract. My question is am I hurting the hydraulics by forcing the thumb to retract? I'm assuming it is pushing fluid past a bypass for the thumb. I am having trouble doing it another way. What are your thoughts? Am I going to hurt my hydraulic system?
Most thumb valves I have seen have a relief block so you cant damage it. If its a home made job then it may not have.
I was hoping for that. It's factory Kubota.
It shouldn't be a problem then.
I will say that forcing oil over a relief for extended periods can create heat. I wouldn't do it anymore than necessary.
Once you get the hang of the operation you should have no issues.
I am sure trying not to do it. It doesn't happen too often because I try to position my machine before grabbing on. I'm getting ready to build a stone wall with 1,500 lb rocks and worried about dropping one because of losing thumb pressure.
How old is this machine?
Its a 2018 model.
Is it under warranty?
Yes. I bought the insurance. I accidentally overstated the weight per rock just now. I got 14.1 tons delivered and they came on 8 pallets with about 4 rocks each, so the rocks would average around 450 lbs. There are some that are a lot bigger and some that are smaller. At any rate the wall is off the edge of a slope and worries me to get to and build safely. I'll creepy-crawly my way into it and back out before things go south.
Thumb pressures as a rule are set at 2,100 PSI. It is normal to grab onto something with the bucket open and thumb extended and then rolling the bucket into the thumb to maintain the grip and not drop the object. Having said that, if the thumb is set up properly there will be a point where the bucket won't move the thumb anymore because of the mechanical advantage of the thumb over the bucket and linkage. Most operators will set the thumb in a position that is handy to them and then use the bucket curl for actually grabbing on. You will pick up all the nuances before too long.
Good day The thumb is very popular in America however in Europe you will see more Selecta grabs which completely detach when not in use. If you ask the majority of excavator manufactures they will explain that the arm is not designed to support a cylinder on the back side. While no doubt sales have driven the demand made by the market my statement will reach many by objection, the principle of the bucket cylinder forcing against the thumb cylinder is a bad idea whilst I am sure many have port relief valves installed the flow from a cylinder being forced inwards will generate more flow that small port valves are capable of taking. Kind regards Uffex
Thank you. I value your information. I am in the situation where I have to use the tool I have available. I’ll do the best I can to not abuse the relief valves. I wish I had a Selecta Grab but I don’t. Thanks again.
Uffex, could you provide a image of Slecta grab please? I have no idea what that is. Thanks
If the thumb bracket is welded on properly, you will likely never have a problem with the thumb or the stick due to the engineering. Komatsu recommended for years welding a doubler plate to the bottom of the stick and for a few years the Sumitomo line (Link-Belt/Case) made people weld doubler plates on all four sides of the stick. That doesn't seem to be the case anymore for either line.
Good day Mother Deuce Attached as requested independent open and closing - less likely to foul the boom - when removed aids fuel consumption and visibility - runs from standard option hydraulics - rotating version available if you need - favoured and used in demolition so its up to the job - Opps beginning to sound like a salesman which I am not. Kind regards Uffex
If you like to see these working there are many on you tube KR Uffex
Thanks Uffex! I am familiar with the tool and have used something similar. I didn’t recognize the trade name. We are in the U.S. a little more likely it seems to use what we have on hand, rather then invest in “proper” purpose built attachments. It is a mentality that racers call “run what ya brung” and it permeates a lot of what we do. Using YouTube operator Duck Girl as an example with her trailer load of attachments. From a job performance standpoint, I envy her and would love to have those tools at my disposal. However, if someone actually showed up and delivered that attachment trailer to me... I would probably pass out in the seat from shock! In the meantime, I will be pounding along doing it the way we do it until someone can produce a cost analysis to support the attachments and then find a way to get management to read it.
Those are mostly known as clam shells over here. They used to be used a lot in the woods for road maintenance. They are expensive to purchase and maintain, especially if it has a rotation function. The open/close function can be done with the auxiliary valve but a rotate circuit has to be added to the machine for the function. They are also heavy and don't work for underground utilities. For the most part they have been rejected by the construction industry here except for specific applications.
Just found this thread and am in the same position. Picked up an older KX121-3 and going to be building a boulder wall shortly. When the seller delivered it I hopped in and was monkeying around with it...I had the thumb extended and curled the (empty) bucket into it. He warned me not to do that or I would/could bend the cylinder by putting all that curl force on it while fully extended. So I have been trying to figure out as I move these boulders around how to curl the bucket once I have the stone picked up...have had a heck of a time trying to back off the thumb at the same rate the bucket is curling. That is the takeaway from this thread though? that thats what you need to do if you plan on curling when you have something picked up? Or was the takeaway that most modern thumbs will have a relief block so dont worry about it? Or does the relief block not mean you dont need to back off the thumb while curling bucket, but just means you dont need ot be super conservative with doing so (which would result in dropping object)? How would I know if my thumb has a relief block?
All modern(and most older) factory thumbs will have overpressure reliefs built in, it's usually when someone added a thumb to a machine and just used an existing AUX valve or similar that they didn't bother to install/convert correctly that you run into trouble.
Good day Bubbles We have made information regarding Thumb installations it would appear from your statements you are looking for the version connected to work in unision with the bucket. All cylinders require port relief valves one solution would be to install a external high capacity port relief to the thumb positive side the pressure could be set as to prevent over pressure by mechanical force. Kind regards Uffex
On my previous 121-3 and current 57-4, I’ve never worried about pushing the thumb in with the bucket, never had a problem, and never expect to have a problem. Coming on 1500 hours between the pair, primarily doing tree work.
On my Tak if I grab something with the thumb, then curl bucket, then easily release the thumb they move I unison and keep a good hold. I love my thumb despite what europe says its powerful though and takes some "gentle touch".
On excavators that only have a single aux circuit that runs the thumb and valves off that to run other attachments like hoe pacs, breakers and mulchers that circuit has to have the psi set low enough as John C states to allow the thumb to give when over pressured by the bucket. That setting is not automatic, it needs to be set that way. Some attachments, like mulchers and mowers prefer a higher psi setting to keep from going over relief and 2K is pretty low. So a machine that leaves the factory without a thumb may be set to a higher psi setting on the aux. If you put a thumb on the machine, the psi on the aux. needs to be set so you don't break the thumb cylinder if you over pressure the bucket. Some machines have an additional circuit and this is ideal in my view, one circuit powers the thumb and set to the appropriate psi for thumb work. The additional circuit runs attachments with a higher psi setting on the relief so you can get the most out of your attachments. Reading through some of this it seemed to me that some suggested that the thumb circuit will automatically go over relief, it can if it is set low enough to achieve that, but you want to confirm that before you turn your thumb cylinder into a pretzel. As far as thumbs themselves, I am pretty particular on the style and build of a thumb on the ones I buy. There are some really poor designs out there in my opinion, especially in the area of the claw design. I like the progressive link (prolink) style thumbs. Allows for greater thumb articulation, especially helpful doing rock work but I like it for everything. Wouldn't have a thumb without it. As far as learning to feather the thumb and the bucket at the same time, it takes some time to master that, but with practice it becomes easy.