BH75 Piston broke, twice! Design problem??

[Volnavy]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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

Volfandt