Kubota Hydraulic cylinder with broken piston rod

[ptsg]

I don’t think a good shop would do that repair. The rod already broke once under the optimal conditions. A weld will compromise the heat treat and make the rod weaker than it started.

The rod didn't broke, it simply stripped the threads, plus that area is not hardened.

 

[MTGreen]

I don't understand people that call themselves "machinists". If they already had it chucked up to turn down why didn't they just single point the threads with a carbide threading tool? Basic Machining 101.
I don't do this for a living but I have enough experience to avoid using dies to cut new threads, especially for a relatively large 16-18mm thread. IMO dies are for thread repair, tiny threads, or if you can't find a way to chuck it in the lathe

Even for a marginally incompetent "machinist" who can't hit the right thread depth, he/she could single point half thread depth then use a die - at least that way it will be straight and proper depth

 

[rScotty]

I'd do what I think you did.... Which is get enough clearance to slide the nut onto the shaft with a bit of shaft protruding and weld the far end of the nut - wrappng on enough wetted heat sink rags to protect the shaft seals.
It won't come off, and if you ever need it off you are not much worse off than now. Cut the weld off and you can weld it again. Olosing about a quarter inch of rod each time. Good for years is my bet.

I'd probably want to change all the hydrauic filters just in case.
rScotty

 

[4570Man]

The rod didn't broke, it simply stripped the threads, plus that area is not hardened.

It’s probably 1045 which is twice as hard as mild steel. And the repair very well might work. It’s probably what I would do if I was performing the labor myself. But I wouldn’t want to pay a shop to preform a less than optimal repair when a new rod is only $400.

 

[pmsmechanic]

It’s probably 1045 which is twice as hard as mild steel. And the repair very well might work. It’s probably what I would do if I was performing the labor myself. But I wouldn’t want to pay a shop to preform a less than optimal repair when a new rod is only $400.

I'm thinking that a competent machine shop could build a replacement rod for a lot less than $400.

 

[ptsg]

I would have 0 issues if the shop would build the thread diameter with weld and then re-cut the thread on the lathe with a good fit. Looks to me the OEM part had too loose of a thread fit and stripped the threads overtime, so any repair that provides a good threaded fit will only be better.

 

[jwmorris]

I haven’t messed with a Kubota cylinder but I have drilled and tapped lots of holes down the center of Thompson shafting (hardened 60-65C on the outside), it’s the outside thats hard, the center is softer, thats why you see more bent rods than ones that break in two, like they would be if fully hardened.

 

[LD1]

Im thinking either overtorque or under torque from the factory.

Overtorque and weaken the threads to the point this was inevatible.

Or undertorque and the nut loosened, and the slop overtime....even if small....the piston bouncing back and forth basically hammered the nut off.

When I rebuild a cylinder with a new rod.....it gets red loctite on the nut, torqued.....then I stake the nut/threads for extra assurance

 

[repete]

I had a rod bent in one of my backhoe cylinders and it cost me about $95 for new rod stock plus $40 for a seal kit.

I cut the "pin" end off the old rod and since I had a lathe I was able to thread the rod end and turn a bevel end with centering pin on the other end and put a hole in the "pin" end for rod centering and alignment and then welded the head back on.

 

[ejnichol]

If you haven’t already screwed up the cylinder barrel and contaminated the system with metal shards you dodged a bullet. I’d do a careful inspection to make sure that’s not happened and buy a new rod and go on.

I siphoned all the oil out of cylinder and filtered it. Took a high powered diving light and examined cylinder down to bottom and saw nothing. Took a wooden dowel wrapped it with clean white tshirt and swabbed out cylinder. I inspected tshirt area where in contact with bore and found no metal flakes that were visible.

Also inspected seals on piston for signs of embedded metal and saw none.

I will do a change out of fluid, clean screen and replace filter and end of season. What ever damage once done by debri probably occurred when I continued fel work with machine and quit using coupled hoe.

Does anyone know if fluid returned from cylinders get filtered before going though hydrostat or after?

I'd do what I think you did.... Which is get enough clearance to slide the nut onto the shaft with a bit of shaft protruding and weld the far end of the nut - wrappng on enough wetted heat sink rags to protect the shaft seals.
It won't come off, and if you ever need it off you are not much worse off than now. Cut the weld off and you can weld it again. Olosing about a quarter inch of rod each time. Good for years is my bet.

This is basically what I did.

Only thing added was 2 or 3 specific size/type washers between nut and piston top and install a new 14 mm nut snug . I needed for rod to protrude from nut about 3/16 to 1/4" inch to net me a nice weld shoulder and allow me to grind off weld and remove nut to reclaim piston if I did buy a new rod one day.