Help..Hyd. cylinder repair$$$

Took the stick cylinder off the backhoe on my Cat 931B to see what happened inside. It's a 4" bore x 34" stroke cylinder. It uses a large bolt that threads into the rod to hold the piston on. It was too big for me to work on in the field so I took it in. Bad news! The bolt came out and took 2/3's of the internal threads with it.

That cylinder always creeped a little so I think maybe the bolt was a little loose. The guy from the shop said that can happen over time and the shock loading causes the bolt to loosen off. They don't make heli-coils large enough to fix it. The rod itself has a .058" bend in it that they could probably straighten but might leave a small mark on the rod.

They said they could try put a new bolt in with loctite and hope the remaining threads hold. They also said they could just leave the bend in the rod. I think it's worth trying to straighten the rod. That part's minor as far as I'm concerned. I hoping one of the experts on here has ran into this and has a cost effective solution. Welding up the threads would would cause greater problems. They can make me a new rod but it would cost close to $1000! The seal kit isn't bad at just under $100 but for everything including honing the barrel and testing it, I'd be looking at around $1600!!! Anyone have a solution that's not so expensive? This shop specializes in heavy equipment repair but it's not cheap. $142/hr. and $152/hr. for the machine shop. This is in Alberta where you don't find $50 or $75/hr. shops.

What does CAT get for a rebuilt cylinder?

Don't know what Cat would charge but the rod for this particular cylinder has been discontinued and is no longer available from Cat. I called Finning to try and get a comparison.

Find a good machine shop and have them drill and tap the shaft for the next size bolt. Buy a good grade 8 bolt and loctite in.

I,M like Russ take and drill out the end where you can tap to the next size. I am thinking you are dealing with a metric bolt so might not be hard to go up one size. I would let them try and make the shaft straight. My place uses some brass blocks and does the pressing. Usually no damage.

I would get the rod straightened first. A shop that knows what they are doing will not damage the shaft. Then bore out the end for the next size larger bolt. then tap out cut the new threads in the lathe. Boring will give you a much straighter hole and better size control than a drill. Bore out piston to fit new bolt. Get an L9 or similar high strength bolt,
use fine threads. The bolt will be much stronger than the shaft so the thread length should be a least 1 1/2 times the bolt diameter. Be sure to torque the bolt to manufactures spec. Properly torqued bolts don't come lose! Under torqued ones do! Lock tight is ok but is not substitute for torque!

I think it's a coin toss as to wether or not you should straighten the rod. It'd be better if it was straight but honestly... if it's off a bit on that length and diameter it's not a big deal. That's probably what is responsible for the creep in the cylinder tho... as it scuffs the piston seals and the packing when it's bent.
As far as the bolt goes... I'd bore the whole works just enough larger to take the next size thread that it will hold. I'd probably bore it deeper into the rod as well... then rethread and Loctite the whole works.
There's nothing cheap about toys for big boys but mabey you get out of this for 500 instead of 1600...

Rod

Have to disagree that properly torqued bolts can't come loose. Some applications call for loctite no matter how tight you put them. Sprocket bolts on high performance motorcycles are one example. Some guys use the red loctite on them. Have seen many a broken hub from the bolts coming loose without loctite. I don't know if this cylinder has ever been apart for new seals but if it was may explain the bolt pulling out or coming loose. I have thought about using the next larger size bolt but I'm not sure what size the original bolt is. It's got to be pretty big for a 4" bore cylinder though. The stick on a backhoe has nothing to stop it except the cylinder piston. There was no o-ring on the bolt and if it was just a tad loose could cause enough oil to leak that it creeped.

I know they're not cheap but I wasn't expecting over $1000 let alone $1600. They have a machine shop so I'll look into boring the piston bigger and using a bigger bolt in the rod. If it can be done, I'd think it would be cheaper than $960 some bucks to make a new rod.

Look up- Surplus Center- on the net. They carry a lot of different cylinders. They have a web site. Can't remember what state they are in right off.

Give the guys at SHP a call. Granted they are in SC, but even with shipping I can't see nearly $1000 in a new rod.

That said, if there are still a few good threads in the rod why not have them chamfer the end of the rod around the hole, and then install a stud into the threaded hole. With the chamfer there would then be room to put a good weld bead around the stud to insure it stays in place. Many rods are initially turned with a male thread on the end -vs- using a bolt so it's a well proven design. They may, or may not need to countersink the piston to make room for the nut, but with the stud both threaded (even a little bit) and welded into the rod, I don't think it would ever gove you anymore problems...if it can be done that way.
SHP

I thought about that too. They said the welding would cause distortion and it would have to be machined and re-chrome plated. I'll see if it can be drilled and tapped for the next bigger size bolt.

Stick welding:

If your shop there can straighten the shaft WITHOUT DAMAGING IT, then have them do so. As far as the bolt & stripped hole, have them bore & tap for the next size larger bolt. Do N O T weld on the shaft.

Use Caterpillar GREEN Cement INSTEAD of Locktite. With Locktite, even after it's "set" the bond can still be broken. With "Cat. Green", after it's "set" the bond is PERMANENT; The only thing that will get two parts apart that have been bonded with Cat. Green is a CUTTING TORCH. - I know, BTDT!

Doc (Sr. Underground Top Millwright / Mine Maint. Mechanic)

Sounds to me like you need to find another shop. While I've never done the exact repair I was describing, I have replaced the eye on the other end on several rods over the years. With that type of repair there is ALOT more welding involved as you have to turn the end of the rod down to, basically, a pencil point and then build back up from there. Other than chucking the assembly back in the lathe to knock off any burrs, high spots in the weld, and really just to neaten/pretty up the look of the job, I've never had to do anything else.

Being on the very end of the rod there isn't really anything that's going to distort. I mean if you were doing alot of welding in the middle of the rod I could understand it wanting to distort, but not on the end. Heck a couple of wet rags laid over the rod 6 inches from the weld and they'd be lucky to even get those 6 inches warm no more weld than it would take to do the job.

As far as the chrome goes putting as much weld on one as it takes to replace the eye, like I have done, has never gotten any of the rods I've worked on hot enough to cause any problems with the chrome coming off. Heck,it's chemically bonded to the surface so it really doesn't care wether it's hot or cold, it's not going to come off just because the rod gets hot. Like I said, a few wet rags and even that isn't going to happen. I don't know what kind of BS they are feeding you there.

I don't know, maybe I'm just old school, but I see and hear more people being told that a repair isn't possible nowdays than the law allows. Sad thing is I believe it's 'the law' that is causing all of the problems since the majority of these repair shops are more concerned about covering their a$$es by telling the customer that they HAVE to replace/make a new part rather than repair the old one for less money. Granted there are times when repair or replacement of a part is the correct thing and makes sense both monetarily and in the context of the repair, but way too often nowdays I am seeing new parts thrown at machines for no real reason.

You have to be careful not to take too much if you go to the next size. How big is it? We have helicoils here at the shop well over 1-1/2" Problem is helicoils remove material as well and will weaken the rod.


There could be an issue with the plating getting too hot since it is an internal thread. A new chrome rod is probably around $100. It would need machined and to have the rod end cut off the old and welded on the new, but not a big deal. Other thing that could be done is to thread a stud in it, weld around the base of the stud plus a plug weld if you have room and use a nut to fasten the piston on.

Everyone wants you to buy new stuff and not fix the old . It's ridiculous , I have 5 Case backhoe cylinders and can't sell them to the dealer or local machine shop for rebuilding , they all tell me to scrap them . What about the backyard guy whom can't afford new -- someday he will call me. -- good luck you may find a good used one on the net .

I can understand why a dealer or machine shop doesn't want to buy your old cylinders. They aren't in the business of buying used parts. Advertise them for what they're for, somebody with an older backhoe may need one.

have you put rod on v blocks and used a dial indicator to check how much rod is bent? a lot of bent shafts can be cold straightened in a press on v blocks using a dial indicator if you have the patinence and knowledge









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They said they can bore and tap it for the next size bolt. That said it will only save me about $500. They should be able to straighten and just polish the rod. Still going to cost about $1100 to have the cylinder rebuilt which is a lot. I checked with another hydraulic shop and they said the price wasn't really out of line. The guy there said .058 is a pretty good bend on a rod and the shaft will be weaker after straightening. He said .020 is about the most they should bend before replacing. I've seen cylinders bent a lot more be straightened and never have a problem. $500 saved is still $500. I don't think it will be weaker enough to even worry about. I think if it was bent over 1/8" maybe it might lose enough strength to think about replacing.

A credible machine shop would know that straightening a cylinder ram takes time. It can only be moved a few thousands at a time and held in that position, sometimes for up to twelve hours depending on the diameter, before moving it further. Back in the 60"s and 70"s I had the opportunity to have worked in a shop that repaired everything except the crack of dawn, and we straightened a lot of hydraulic cylinder rams. Big ones, small ones, short and long. We did some that local heavy duty shops had tried and failed. It just takes time.

Never had an issue with piston retainment that I can recall.

A point of interest: when I left the business our shop rate was $12.50 per hour and no fancy add on"s like "shop supplies" either.

Why not just drill and tap the existing hole deeper and use a longer bolt. Possibly a grade 8. Just a thought. Good luck. Dan

(quoted from post at 16:56:30 08/06/13) They don't make heli-coils large enough to fix it. The rod itself has a .058" bend in it that they could probably straighten but might leave a small mark on the rod.

Click to expand...

These inserts are big enough and are superior to helicoils. Too bad you can't just move up in size a bit and tap her for a metric bolt. That CAT green sealant is good stuff.


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