hello all- first post here. i have recently acquired my first excavator: a Mitsubishi MM30B. pretty sure it is a grey market machine. i fabbed (most) of a thumb for the machine. everything except for the crimping and installation of the new hoses which attach to an unused valve in the big spool? stack. the thumb is made from 1/2" plate steel. the cylinder is made by Chief. 2" bore X 18" throw. rated at 3000 PSI. just over $100 on ebay... after using the thumb some, i bend the cylinder shaft. rod? the shaft/rod that extends in and out of the bore. i'm pretty sure this happened when i had the thumb stationary, and curled the bucket into the thumb while grubbing bushes. i bent it pretty bad. the thumb weldment still looks great though. so i bought another identical cylinder. did some research and found out about relief valves. so i learned (correct me if i'm wrong) that when a 3" cylinder is pushing on a 2" cylinder, the 3" will always win... unless a relief valve is used to let the 2" thumb cyl. collapse at a lower pressure than the larger bucket cylinder. so i went and thought that if i was careful, i could leave the bucket stationary while grabbing items, and use the smaller thumb cylinder to do the work thus negating the issue. while in theory this "may" work, in the real world, i bent another cylinder rod, then continued with my trail work and finally snapped the rod entirely. my questions are these: 1) can someone tell me if there is a relief valve built into the spool valve on my machine from decent pictures? 2) how hard is it to add a relief valve if there isn't one built in. 3)any rules on picture size? some forums have 150KB limits... 4)am i looking at this problem all wrong? any/all help would be greatly appreciated! adam
i think you need more posts, I believe 3 is the majic number to be able to post pics. This is to keep the spammers at bay. the relief is usually under the cap on the hydraulic valve bank. If you get to having trouble finding a relief due to grey market machine and issues like that I believe there are some hydraulic cylinders made with built in reliefs. But your on the right trail, the answer is not a larger thumb cylinder... Oh, and welcome to hef!
Stumpjumper- thanks for the reply. i'm going to try and post some pics as it looks as though the icon is ready for me to due so. just hope i don't post them too big a PO admin ect. maybe the site re-sized automatically as it looks like this forum is running on a pretty advanced software. below is the vacant plugged valve that i tied into... and here is the mostly completed thumb work: adam
Adam, there is a thread on Kubota Kx series on the compact excavators on this site. Some Kubota's do not have a relief valve on the aux circuit and you can learn to live with it. 1.Do not use both thumb and bucket @same time when picking up, set the thumb and use bucket. 2. If wanting to curl bucket after picking up back off on thumb & curl bucket at same time with practice you will get good at it. 3. One of the Heavy equipment members did put a external relief valve on his Kubota pic's in thread. 4. My thumb cylinder is about the same size as the bucket cylinder and will bend the bucket pin if both cylinders are used at same time on solid item like a rock.
jack- thanks for the advice. do you have a link to the thread? it'd probably take me forever to find it! adam
Go to Compact equipment on this site and in compact excavators search "Thumb problems KX-71 "
jack- i couldn't figure out how to search within just the compact class. couldn't find the thread. again, thanks for your foresight! adam
See the valve section next to the unused one? It has a cartridge there parallel to and above the spool with two hex sections and a screw with a slot in it for a screwdriver? Port relief valve. Hard to see but I think the section you're tapping into there has port plugs. Also notice that on top of the section there's only one plug and the other section has two? I'm guessing there's a load check on only one port of this valve. Was probably intended to run a small breaker hammer or something. If you can buy a port relief cartridge that should be sweet. You can also use something like a SUN crossport relief valve with makeup checks and a TANK LINE plumbed inline with both hoses, and the tank line plumbed to the return filter. Do not use a normal crossport relief block, as this will cause your cylinder to explode. No, not kidding. Here's a suggested model code: RDDA-LAN-FMB $239.50 You can buy SUN pretty much anywhere. The cheap approach is to plumb a three port relief valve (Cross, Prince, Brand, etc.) in the line to the butt end of the cylinder, put a tee in at the return filter, or in any line that runs directly to the filter, and plumb the outlet of the relief into the tee. By rights you should also tee a check valve into the rod end hose and connect the return to that so it can suck oil while the relief is open and the cylinder is moving. Otherwise you're likely to suck air (and dirt) into the rod end of the cylinder through the rod packing. Pretty small cylinder, and a long skinny rod, you may still have issues with it, but you won't know till you try it at different relief valve pressure settings. :my2c Hope that helps. Nice looking thumb by the way! Cheers, Lanway
your biggest problem is the geometry of the thumb at full extension.
lantraxco- i also live in Portland, OR! thanks a ton for the very helpful information. the largest problem with fixing my machine is it is currently (and probably won't leave) my ranch in Bickleton, WA. so if i understand your explanation correctly, ideally i'd be able to fit a port relief valve into the spool and call it done?? it'd be really nice to not have to "T" into a tank return line. but if it has to happen, it will. the "SUN crossport relief valve" that you gave the model number to looks as though it would also just thread into the end of the spool yet need a return line on the other end of it to go back to the filter? then option three looks cheaper ($80) plus hoses and check a valve. i think i understand this one. basically a relief valve on the butt side of the cyl. with a check valve on the rod side to recycle butt side fluid if needed? i'm obviously not a hydraulic expert! any detailed explanations are greatly appreciated! yea, it is a pretty long skinny cyl. had plans to use a 3" bore x 16" agg. style, but it just wouldn't fit. next time up, i'll have to get better pics of the valve and pull the side plug to get thread type ect. @cutting edge: i'm curious as to why you say this? sure the clamping force isn't all the great at full extension, but that's something i expected with the angles ect. i don't see how that would yield bent rods on cyl.s though. adam
That comment came probably from seeing that the rod hits the pivot pin, or appears to at full extension. If the rod contacts the pivot pin and extends at all it with bend the rod, and if it bends the rod in the slightest bit, when you use it the next time, it would most likely bend a lot more then. to test this, leave cylinder hang from top mount, extend cylinder fully, then try to attach the thumb, if you cannot attach due to the rod hitting the pivot before the holes line up at the thumb, then that's a problem. To correct, either change thumb geometry by moving brackets out, or shorten the cylinder rod.
It looks like the picture was taken during the mock up stage, and the cylinder appears to simply be hanging vertical. Perhaps another image posted of the finished set up would help...
full extension of the thumb may be the problem. i left 3/16" of an inch between the rod and the pin sleeve. the pin is 1-5/8" and the pin sleeve is some pipe i found that had a pit of slack when slid over the pin. i put a zirc on the pin sleeve as a means to grease the joint. i could easily remove this pin sleeve and manually grease the pin. the pin is held to the stick knuckles and thumb knuckles with welded locking shaft collars. that is the collars are welded to the knuckles and nothing else. loosening the set screws and the pin slides right out. i do see how when extending with full force how that 3/16" gap could go away due to flex and or a bit of over extension of the cyl. even a slight collision could put a slight bend in the rod as stumpjumper suggested. then the next time full force is applied, noodle city on the rod! by removing the pin sleeve, i'd get closer to 1/2" clearance. does this sound like enough? if it isn't i could always grind a notch in the pin and use the various set screws on the shaft collars to make the pin not rotate, but the thumb shaft collars rotate on the pin. this could easily give me 1" of clearance. keep in mind this machine is a dedicated "ranch hoe" at my vacation spot and will be used 10-12 times a year with minimal hours per time. also keep in mind i only have a total of about 3k tied up in this machine, so i'm willing to work with a bit of quirkyness. some pics so you can see how things were done ect. i also included a pic of the chinese nameplate on the machine that a friend of mine translated for anyone else that may be curious.
On the port relief cartridge, well, I think so. I don't know for sure that the valve section has the makeup check for the other port, but it wouldn't be the end of the world if it didn't. The Sun valve, I don't know if the model number I gave you leads to a picture of the complete unit, it's a manifold that both hoses coming off the valve would connect to, and then two hoses come off to connect to the thumb lines, and one more port to connect to the tank return. Option three, I think you've got it, yeah the check valve tees into the rod end line, the other end of the check is connected to the tank return, just allows oil to be sucked into the rod end to fill it whenever the butt end relief opens to allow the cylinder to be pushed back. Cheers, Lanway
One more suggestion, for two reasons: Usually these off the shelf cylinders have very little stack length so at full extension the rod will c**k easily and the rod and piston will c**k and bind. The other thing I think I see is you're trying for too much throw and the angle of the rod relative to the two pins for the thumb is extremely tight. Effective force I believe is cylinder force multiplied by the sin of the acute angle, so as a guess I would say you're only getting 15-20% of the push delivered to the thumb. Put another way, the tight angle kind of multiplies the force transferred to the cylinder like four or five times. I would probably take the cylinder apart, pull the piston off and find some good heavy steel pipe that would just slide over the rod. Cut about a six inch piece and leave it between the gland and the piston. A lot less chance of the cylinder sticking and also reduces your final angle at full extension. Only negative is you will have less curl of course. :my2c
lantraxco- first off, thanks for the more detailed response about relief valves! that's a wonderful idea! adding the pipe within the cylinder would accomplish multiple good things! 1) supporting the rod at the base and seal over a greater distance while at (the new) full extension. 2) limit travel to eliminate potential clearance issues between the rod and the pin sleeve. 3) limit hard angles where the thumb cyl. is most vulnerable. 4) i don't have big welding equipment at my ranch, so its an easy thing i can do in town with the new cyl. once it arrives at my door step in the next few days. i think the other key to success, aside from a relief valve, has to do with the foot control. the guy who did the hoses also did the foot control. it's located almost under the seat behind my right foot. i have to rock my heel back and forth to operate it. i was never pleased with it, but it does work. if i hadn't had bent cyl. issues, i probably would have lived with it. i think reworking it so its directly in front of the left foot with an easy front to back rocking motion will give much better finite control, thus further reducing the accidental binding/bucket curl against it issue! i feel like if the foot pedal for the thumb control is very easy to use is key! if all that doesn't do it, then i'll look into a relief valve. adam
i came home to yet another new cyl. sitting on my front porch. tomorrow after work will be putting a chunk o pipe inside the cyl. and trying it out of the machine next week! adam