BH75 Piston broke, twice! Design problem??

Volnavy · Apr 27, 2005

I've heard you all discussing the relief valve but am really not familiar with it. I understand relief valves, being a nuclear engineer, I am very familiar with the Three Mile Island relief valve catastrophy.

Where is it located? When does it relieve? Where does it relieve to? Can you hear it or how do you know it relieves?

I also have heard words like stall. I guess you mean the cylinder just moves no further. Like when digging a big root. You don't mean that it actually slows down the tractor, right? Remember these are not PTO driven hoes, they run directly off the tractor hydraulics.

Anyway, I just want to make sure I'm not missing anything and always interested in learning.

surfran · Apr 27, 2005

OK I got it. Didn't try right clicking on the link in the post, only once I had clicked it and it opened the new window with the Quicktime then it didn't have the option to save as. Now I have downloaded and watched. Of course then it came up in Microsoft media player and worked fine.

Wow it doesn't look like it was doing anything that would over stress it to me. Not that I have ever run backhoes but it just looks like normal moving of dirt and digging.

Hope you have it resolved with the new unit. I know what you mean about wanting to use your new toys and getting work done. Nothing irritates me more than to spend money on something new and then not be able to use it because of a problem.

Best of luck and keep us posted.

JerryG · Apr 27, 2005

It should be on the valve body on the backhoe. When a preset pressure is achieved, the fluid bypasses going back to the tank. Most times, you can hear the relief chatter. If the relief is bad, set to high or has a lot of air in the fluid it may not open.
Stall > when the cylinder stops before the end of the stroke while under pressure. On most systems, you can actually hear the engine slow to some degree. The reason is because the pump is driven by the engine and when the cylinder stalls, the pump is pumping against it's self.

Henro · Apr 27, 2005

But if there is a PRV on the backhoe, there is still one on the tractor too. So if the backhoe PRV was to not operate, it seems like the tractor PRV would...

Now I don't know if there is a separate PRV on the BH75 or not.

Something else I don't know, is if my BL4690 has a PRV on it. I suppose since it is a system separate from the tractor, with a PTO pump, it almost has to have one. I wonder if the pressure of my BL4690 hydraulic system might be different than what is provided by the B3030 for a given engine RPM?

Perhaps the 3PH hoe actually operates at less PSI. I never measured it and don't have a pressure gage on it, so I don't have any way of knowing. I do know, that I have not EVER worried about how I dig, pull or push with the hoe, and to date I have not had a problem...

That was a very interesting video. I think I operate my backhoe like that all the time. Then again, I'm not sure. I also try to angle the teeth of the bucket so it cuts as I pull it with the boom and stick movements. The more I think about it the less I am sure about... /forums/images/graemlins/blush.gif

Volnavy, I hope the new hoe works well for you. If not, perhaps you should consider applying for the position of "backhoe test pilot" with Kubota USA... /forums/images/graemlins/grin.gif

DelfinoTX · Apr 28, 2005

Looks to me, that at the exact time the bucket is being curled the operator is not using the dipper lever which effectively closes off the dipper cylinder circuit. Depending on the design of the valves there may not be a relief valve on a closed circuit. As the bucket curles in, it's exerting an outward load (away from the tractor) on the dipper arm which overcomes the strength of the dipper cylinder rod via mechanical leverage. If there were a relief valve on the closed dipper circuit to allow breakout then it would not bend. But then again, allowing it to break out would not allow you to take full advantage of the bucket curl power. Seems then that the only fix is to use a larger dipper cylinder or increase the distance between the dipper arm pivot and the cylinder rod attachment point (to balance out the mechanical leverage).

Volnavy · Apr 29, 2005

When I got home last night Kubota had my tractor back with a new backhoe attached. It looks the same as the old one, I'm not sure if there are any modifications. When it drys out here, I'll be back to ditch digging and will report back on how it went.

They picked it up Wednesday evening and had it back Thursday, you can't get faster turn around than that.

Junkman · Apr 29, 2005

Take the video from the same angle and you will instantly know if this one is any better. If it is the same, then send them both video clips and tell them to come get the machine and drop off a check for what you paid.

Steve_Miller · Apr 30, 2005

Here's my two cents worth. Too me from what I have read so far, it sounds like the cylinder rod is too small. I know this will raise a lot of questions, so I'll try and give my theories as too why. First the cylinder should be able to take whatever stresses the backhoe/hydraulics can put into it, even if it is what some people call abuse. Not all people that buy a backhoe can properly use it when they first get it, myself included. Secondly, if the rod size were to be larger this would give you more strength and less likely to bend. The sacrifice would be on the retracting side of the cylinder with a reduction in surface area thus resulting in a loss of power with the dipper arm being extended out. Not sure on the negatives of this as I would guess that most of the power is needed when extending the cylinder pulling towards the tractor. This seems to me is where the problem lies, the rod is the weak link, whether a design flaw or too much hydraulic pressure putting undue strains on the rod. If this were a 3" cylinder the force generated by the cylinder would be roughly 14000lbs @ 2000psi and 15400lbs @ 2200psi. Fire away at my theory, I always love to hear what others think as I tend to be narrow-minded some times.

Steve /forums/images/graemlins/confused.gif

Volnavy · May 1, 2005

Well, I dug for two hours and had no problems. I don't notice and differences, it still digs very well. Maybe they made some slight adjustments on the new one they switched out. I've still got about 3-4 hours of digging left, but it seems to do a good job without breaking!

If anyone else had any problems, just reference the Knoxville TN problems and hopefully they be successful with yours too.

Volfandt · May 1, 2005

I saw the new post & thought, nahhh, not again! Two hrs on it now, thats good! Hope Kubota got it right now.

Volfandt

BX2230Dude · May 2, 2005

After reading several posts about bent cylinders on the BH75 and BH90 and comments that the user was digging too close to the tractor, or using the wrong digging angle on the bucket, etc., I thought I would post a link to a book I found helpful when learning how to operate a TLB. I read an earlier edition of the book and it was full of useful information, photos, diagrams, etc. Should be required reading for TLB owners.

TLB Book

KLS59 · May 2, 2005

Recently bent the bucket piston/cylinder on my BH90, which is attached to an L5030. Gave good service for a few months, then bent when digging a stump. Was it me or was it a poor design? How can I get a replacement, and where? I've read it's possible to replace this part with non-Kubota parts? If so, who can I contact? And, by the way, I need this thing like yesterday... Thanks for any info.

Junkman · May 3, 2005

You might start by filling out your bio. Without knowing what part of the country you are located in, the best that I can do is suggest that you look in the yellow pages for hydraulic dealers and start calling to see who sells what. Take the old cylinder to the dealer that you select and they can either repair or make you a new cylinder. It isn't going to be cheap, but that is the cost of owning machinery. If the tractor is less than 1 year old, contact your selling dealer about a warranty claim. With the rash of bent/broken pistons, you never know how they will deal with this one. Chances are, if you have problems, mention that you know of others that have also have had bent pistons, and you were wondering if there is a manufacturing defect in the product. Can't hurt to ask... /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

kubotabilly · May 3, 2005

Junkman,
I was at my dealer yesterday and a gentleman pulls in with a Kubota 4690 backhoe for repair. The repair was a bent cylinder. The same cylinder that the others have been bending. I asked him what had happened and he said, "same thing as before". I said WHAT /forums/images/graemlins/confused.gif. "Yea, this is the third cylinder that I bent and Kubota wanted me to bring the hoe in for some testing". To make a long story short, the problem was in the relief valve to that cylinder. One side of the valve was popping off at the recommended 2300-2400 psi and the other side was not popping off until over 5000 psi /forums/images/graemlins/shocked.gif. The cylinder was not made for that kind of pressure and as a result, bends. It seems to be a design error or a assembly line error, no one really said, but Kubota is footing the bill. I thought this info may help anyone else with this problem.

Bill

MacPDaddy · Jul 13, 2009

Here are some pictures of a BH75 that ran flawlessly through Saturday.
It was purchased December 2005, I am attempting to have Kubota replace the cylinder based on your information.
I appreciate everyone posting on this.

bigballer · Jul 14, 2009

hi mac- what size is your cylinder? should be a stamp on the outside of it. i have an '07 BH-75 with a 32mm cylinder and haven't had that issue. i've dug alot with it (over 100 stumps and close to 1000' of trenching).. best of luck to you, let us know how kubota handles this. by the way, what is the cost of a replacement cylinder?

MessickFarmEqu · Jul 14, 2009

the rest of this thread was from 2005. That shows how wide spread of a problem this is. If you ask nicely you can probably get Kubota to chip it and cover some of your parts, or throw that larger cylinder on there.

rdsaustintx · Jul 14, 2009

MacP,

Man those photos are impressive (and scary). Amazing it took so long to fail. Judging from the dates on the thread it was a frequent problem, but got resolved years ago.

When Messick posts, smart owners listen. The dealers that participate here are great guys.

Russell in Texas

Sparkyca · Jul 15, 2009

Been around backhoes a long time, and one thing that seems common on bent cylinders is 'Stumps'. If you hook the hoe over a stump and then pull forward with a 4wd tractor there is no relief built into the system and the pressure you will generate with the tractive effort is in the 10's of thousands of PSI. I guarantee something will bend.

rdsaustintx · Jul 15, 2009

Sparkyca said:
Been around backhoes a long time, and one thing that seems common on bent cylinders is 'Stumps'. If you hook the hoe over a stump and then pull forward with a 4wd tractor there is no relief built into the system and the pressure you will generate with the tractive effort is in the 10's of thousands of PSI. I guarantee something will bend.

Ahhh. I understand that. Similar to when you hit a vertical object with an FEL bucket in dump position. Mechanical advantage at the cylinder is huge.

But curling under a stump too close to the tractor? This pretty much describes how I chew through roots or limestone. Get in close, get teeth under a ledge and use the curl force (and leverage) to force a break. Dipper and curl cylinders are at relief and it pulls straight down on the tractor. So hard you can see the frame flex. Thought that was the right way to do it?

The operator books sounds great, but I don't have time to order and read it.