Backhoe BH77 broke - defective part?

[beckmurph]

TwinCam88 this is not pointed at you:
I have to throw the BS card on the link posted to the broken bh77.
Look at the wear on the bucket. There is no way you can wear the paint off a Kubota bucket in 3 hours of use.
Then look at picture of the 3200 in his heated garage. That picture is PhotoShopped:ashamed:.

 

[nspec]

That's a very common setup on the curved boom backhoes. All of the larger machines (Deere, CAT) use a similar design with two cylinders meeting at one pin. Personally, I don't think it looks like a very good idea. I'd much rather have independent pins.

And to think, here I was wishing I had the curved boom with the cylinder on the outside - I had just pulled my cylinder from inside my boom (BH75A) to replace a hose. I'm sorta glad now. Two identical failures like that on brand new backhoes is most definitely NOT coincidence. I wouldn't stop fighting until I got a new backhoe. Shouldn't happen - period.

 

[nspec]

Beckmurph: I have to respectfully disagree with the B.S. call. The garage pic is probably a picture of the tractor when new. The bucket certainly shows paint wear consistant with a few hours use. The paint on the sides of the bucket is intact - just dirty. The wear is on the back/bottom. My bucket looked very similar to that after just a short time. You can see the teeth still look sharp and new.

 

[SixHoeBob]

Looking at the picture of the other broken pin with the bent pin flanges, I would be very concerned. Are your boom flanges bent, or did you see the broken pin in time?
Obviously the pin diameter is too small. The tempering might also be wrong. I would demand a lifetime warranty on the pin, cylinder eyes, boom with full coverage for parts and labor. Have the warranty notarized or whatever it takes, to make it a binding legal document. If Kubota will not stand behind their product, then return it for a full refund. An identical pin will eventually break in the same fashion. Once the standard warranty has expired you will spend several thousand when the boom and cylinders are mangled.
After spending several thousand you will have the same setup, that will break again in the same fashion.
Everybody is building curved boom hoes, only because they look cool and modern. Looks like a poor design to me. The dipper cylinder is often times pushing on the center of the pin that is spanning a wide distance. Sand box engineering tells us that the pin only flex so many times before it breaks.

 

[Verticaltrx]

The B26 uses the exact same cylinder/boom arrangement, and I have neither seen nor heard of any failures with them (I rent them often for my business). Now granted, they are commercial duty machines, but the engineering behind it is the same, you'd think Kubota would have had it figured out by now.

And I also have to say, they aren't just building them that way because the 'look cool.' The curved boom design really is beneficial when you are digging at near maximum depth or trenching at a fair depth. The digging geometry is much better in those situations. Also better for loading trucks.

 

[SixHoeBob]

I am going to stick with they are built to look cool. Cat came out with their abortion of a curved boom, and now all loader and hoe booms are curved. It is a sales gimmick. Turn the pressure up or increase the cylinder diameter and anything will have more digging power. Of course it has to be able to handle the higher forces. The best backhoe main boom design is like what is found on a JD310,A,B,C,D. The best extendahoe is like what is found on a Case 580K,L,M.
Best loader boom on an industrial TLB is JD310C,D or Cat 416 etc.
Best CUT loader design is like Kubota 2850, 3410,L35.

 

[wdchyd]

Looking at the picture of the other broken pin with the bent pin flanges, I would be very concerned. Are your boom flanges bent, or did you see the broken pin in time?
Obviously the pin diameter is too small. The tempering might also be wrong. I would demand a lifetime warranty on the pin, cylinder eyes, boom with full coverage for parts and labor. Have the warranty notarized or whatever it takes, to make it a binding legal document. If Kubota will not stand behind their product, then return it for a full refund. An identical pin will eventually break in the same fashion. Once the standard warranty has expired you will spend several thousand when the boom and cylinders are mangled.
After spending several thousand you will have the same setup, that will break again in the same fashion.
Everybody is building curved boom hoes, only because they look cool and modern. Looks like a poor design to me. The dipper cylinder is often times pushing on the center of the pin that is spanning a wide distance. Sand box engineering tells us that the pin only flex so many times before it breaks.

I agree....Sand box engineering also dictates that a machine is only as good as its weakest part......You found it......

I wonder if the yield and tensile strength was not enough for premature failure...there are many grades of steel with different methods of hardening for strength.....could have been an oversight with these pins or weak pin bosses welded into the structure/sideplates......

After this is repaired I'd be putting on a dial indicator in a vulnerable spot to check for flexing under strain while digging or simulating digging......

If it is flexing then it would show inferior/undersized materials used in manufacturing the BH

 

[SixHoeBob]

The easiest design solution is a 50% larger diameter pin. The better solution would be to have separate pins for each cylinder, but then there is not enough room on top of the boom for both cylinders. CAT exaggerates the curve of the boom to create enough room for separate pins. Early model Fords actually had the first "curved" booms with a common pin if you look closely.

The best solution is the triangular boxed booms as I stated earlier. Kubota has already done the engineering on them.

The pin bosses are not meant to keep the pin from flexing. More SBE.
Why don't these engineers call me with their prototype designs?

 

[nspec]

Curved boom being a sales gimmick or not (not), that is besides the point. Both CAT and Deere (in addition to Kubota) are joining the boom and dipperstick cylinders with one pin. Case does not. Works for them. Kubota has a problem here and needs to fix it. Vaguely reminiscent of the bending cylinders on the BH75.

Put another way; I don't care what kind of abuse you give a machine - there is no way it should ever fail like that. That is either a design failure or a manufacturing defect plain and simple.

 

[TwinCam88]

Here's some pics of the defective/broken pin. I've contacted the dealer that I purchased the tractor from and they are sending me another pin for it. I'm just a little worried that this may be a bigger issue than just replacing the pin now. It looks to me like it may have actually bent the frame that holds the cylinder...maybe torqued/pushed it out a little (see pic 3). Not sure how I'd get this pulled back in. I'm also concerned that it may have damaged the inside of the pin sleeve...perhaps scuffed it which could lead to additional wear later?

Feeling pretty sick about this right now...