BH75 Piston broke, twice! Design problem??

[JoelD]

WOW, thanks for the video post, amazing footage.

In my opinion, as someone who's spent many hours on small backhoes. A backhoe should never bend a piston during opperation.

Hook a chain to the hoe, drive at 15 mph trying to pop something out of the ground, then something's gonna give.

But during opperation, a cyl. should never bend. That would mean either geometry or bypass valve are bad.

Joel

 

[PERCTK]

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.

WOW!!! That looks like something is not right. I saw your other posts about the thumb that you had installed.
Do you think that the thumb had anything to do with this lastest issue.
My father-in-law was out recently at a Kubota dealer looking at a L2800 & a B3030 both with an FEL and a BH. The dealer had both the BH76 and BH75. The BH76 had a factory installed thumb bracket while the BH75 did not. The salesman told him that the BH76 was engineered to support a thumb while the BH75 was not, and that he would most likely void his warranty if he would add an aftermarket thumb on the BH75.
I will pass on your info to him, maybe it will help with his decision.
Does anyone know if you can tell if the cylinder is big or small when looking at one on a dealers lot? I would not want to have this issue with my father-in law.

Let us know what Kubota says about your cylinder. I hope they can find out what caused this.

 

[MessickFarmEqu]

In my opinion, as someone who's spent many hours on small backhoes. A backhoe should never bend a piston during opperation.

Bent cylinders are actually very common, even on full size equipment.

 

[dodge man]

I've watched the OP's video several times, and when the rod bends, its when he is moving the bucket away from the machine and my guess is it's pushing down also. That is what is causing it to bend. The problem is, he isn't pushing down hard enough to even lift the machine. I can't tell you how many bigger backhoes I have seen reposition themselves by using that technique, they push down hard enough to lift the back of the machine up and push it forward. They will lift the tires clear off the ground. The OP is not putting anywhere that kind of force on his hoe. I've seen alot of heavy equipment used over the years, construction grade stuff, and I can't say bent cylinders are common, although I can't say it doesn't happen. I think the problem is the backhoes for these compact tractors are just not going to hold up as well as a construction grade TLB would.

 

[MacPDaddy]

I got the tractor back tonight, new bigger cylinder installed, originally a 28mm, now it's a 32mm (and while that doesn't sound like a big change, it really looks bigger)

WOW!!! That looks like something is not right. I saw your other posts about the thumb that you had installed.
Do you think that the thumb had anything to do with this lastest issue.

No, the tractor was taken in and the dealer checked my job, got some nice compliments.

Kubota IS following these posts
I got called on the carpet for my thumb project as detailed in my other thread, hence the trip in to the dealer for a check up.
For those of you that haven't seen my thumb:
http://www.tractorbynet.com/forums/attachments/128216-kubota-bh75-damaged-bolt-thumb.html
(my pic links are broken, I am working with TBN.com to fix them).

In the end it was well worth getting called out for, the dealer went over everything done with the thumb project. Roger, my fab man, really did a nice job. All pressures, stops, and pin locations were checked. So, other than the drive, I benefitted with a full backhoe check-up and the new cylinder installed at the original agreed cost.

Some advice for what it's worth:
POSTING: Don't be a Cisco Fatty!! Hold your comments until you get your issue resolved! Companies are reading these posts as I was a little surprised/flattered to find out.

BH75: I still like my BH75 backhoe, dug for 4 years with no cylinder issues, but I never dug close in hard ground either. The smaller dipper cylinder is on pre-2006 hoes, so a current model should be fine. Just be aware that digging close to the tractor is a potential danger zone. I do think that the design should be able to handle anything that normal digging should throw at it. Reviewing the other posts, the larger cylinder appears to solve the problem.

THUMBS: Be careful, I see many posts on adding thumbs to the BH75, I think your better off buying a hoe with the thumb option if needed.
The stock dipper arm is only 1/8" thick steel, not designed for aftermarket bolt-on thumbs which create a heck of a bending force on the arm.
If you decide to do it anyway, as many have, just be careful!!
If your interested, take a look at my thumb thread to see what I went through.
Do NOT expect Kubota to repair your mess (I will side with them on that, I made my mess and I fixed my mess when I tried a bolt-on thumb).
Just so you believe me, here is a picture of a cross section of my original stock dipper arm:
(My current dipper arm is made of 1/4" steel)

DEALER DEALER!!! Be sure to buy from a good dealer, I can't emphasize this enough. Don't expect Kubota to step up if your dealer isn't going to help you, they won't. I want to strongly thank my sales and service guys for helping me with this issue, they both even managed to slide some humor into this. I will refrain from comments about Kubota corporation, I need to cool down about them... I am such a solid Kubota customer with a F2400 and a L3400 both in the garage, how Kubota talked to me really hurt that (the $300 hurt too).

For reference, my broken cylinder pictures are below along with a new picture showing the ground I was digging in when it happened.
(That is a plastic gas line in the ditch, one reason I didn't notice the initial bend, all my attention was on NOT hitting that line!)

 

[kubob]

did anyone have an approximate serial number range that had the smaller cylinders and what the actual size of original cylinder was and what is the size of the new cylinder

 

[leonz]

Good evening,


Found your initial posting and I wanted to eplain
why what happened to your dipper cylinder happened.


What you are dealing with as far as your back hoe
is concerned are a number of issues.


1. When you are dipping in you are attempting to break
the vacuum suction of the dirt and clay created by
its natural compaction from rain and water in the ground.

2.
a.The dipper cylinder as is has no flow control regulator
valve or pilot operated relief valves on the rod end or
barrel end of the cylinder barrel.

b.The dipper cylinder itself does not have a remote
relief circuit to regulate pressures on one cylinder port.

c. The relief valve on the valve body if it has one is a
single cartridge for the entire valve body

d. The hydraulic system is simply providing fluid power to
the circuit that is diverting the oil to that circuit and
has no method of relieving it self thus creating the
broken rods, barrels etc.

e. The fluid power system is delivering energy and the rod
breaks or bends or the cylinder rod is under such pressure
in retract the oil pressure is forcing the piston and rod to
the point it will fail.

 

[leonz]

continuing what I was explaining:


The rod and piston force the packing gland out the end of the barrel and make it egg shaped and ruining the cylinder entirely.

its not a cylinder defect it is simply a case of no control of the cylinder or being able to relieve its pressure and the cylinder loses.


I will add more as I do not want to be thrown out again

 

[leonz]

The dipper cylinder needs a stop tube on the rod end of the cylinder equal to 1 inch in length for every foot of extended cylinder rod.

the stop tube must be nearly the same diameter of the piston to create a larger load bearing surface than just the piston and stuffing box.

the cylinder barrel requires two needle valves to restrict and control fluid flow of the dipper cylinder to slow and regulate the speed and this must be done or you will continue to have problems as the dipper cylinder has no remote relief circuit.



The larger cylinder will have the same problem unless two needle valves are installed on the cylinder ports to control flow and speed, and a stop tube are installed on the rod end of the barrel.

 

[Sodo]

Support block on dipper stick at gland?

This looks like a fairly important thread as I am planning to go have a look at a BH75 today.

Lets say a guy buys an early BH75 from ~2006, and it has the smaller 28mm rod (not the 32mm upgrade). Is it feasible put a "block" on the dipper stick that the gland will bear on? If the cylinder was extended 100% and the block had 1/32" clearance to the gland, then if it flexed 1/32" the block may prevent a permanent bend.