Help operating a Hydro-Ax 670

I think I'd still try to disconnect the drive lines at the transmission and see if the transmission yolks turn freely or jerky. And then reattach one axle at a time. The front axles take more abuse on these machines. Maybe you have an axle that is wonky and binding or the front brakes are stuck somehow. It's great you have extra help. I usually have to monkey around by myself. Power tilt on the cab would be a nice feature. I always thought the pilot inputs and associated valving controls swashplate condition but I'm not well versed in hydraulics either.

Well, I've had the transmission out, and on blocks sitting on the shop floor. Turning the yolks took very little effort on both front or back, and and everything was smooth. I'll also say that turning the input shaft via the splines was also very smooth and took little effort while the output yokes showed very little backlash, which was a little surprising to me, considering the hours on the machine. This didn't feel any different in high or low gear. The splines on the propel motor/gearbox interface showed very little wear and also were smooth as glass where they made contact. Virtually no step where the contact/no contact areas met on the splined shaft. When the machine was pushed into the shop, there was no noise, no excessive effort required, nothing unusual, so I don't think there is anything going on in the axles. But I'm not ruling out anything at this point. Since the gearbox locks the front and back together, if there were a problem with either, it should show up any time the machine was rolling, I'd think.

I'm glad for the help too. I have serious back issues (plus a few others)and only am involved in this work formally as a consultant. Sure, I'll do some light work. I don't mind getting greasy, but my back (and broken hip from about 4 years ago) really limit how much crawling around on the floor I can do without really regretting it for a few days afterward. So I have an agreement with my customers to have a helper when I need it. I'll do electrical work, troubleshooting, design work, and other not-too-physical stuff, but heavy work I leave to the younger crowd. I did my time.

Hope this is not your situation , but pushing a hydrostatic drive machine without disconnecting the drive motor from the drive line or relieving the pressure valves can cause pump/motor damage. On the hydro ax having the transmission in neutral would be the easiest.

Yes, I know this. The gearbox shift selector that was thought to be the original problem was in neutral. Part of the problems faced and fixed up to this point were to lock it into low, and I made sure everyone involved knows that means no more pushing the machine. I know the mechanics in the shop know you can't back-drive a hydrostatic setup, but I am a little concerned with the field guys. That said, however, they are instructed not to attempt repairs in the field. So hopefully that will not become an issue. I should probably put a big red tag on the dash to remind them.

Looks like I'm in for a new propel motor. Found a source, but price is high and lead time I can't live with. The guys are pulling it tomorrow and I'll start teardown to see if I can find the problem. I'm convinced it is a hard part failure. The source I found told me the seal kit, about $530 rarely fixes anything. It's nearly always a mechanical problem. They sell all the bits and pieces so hopefully I find out what's wrong and can get the parts soon enough. I'm running out of time. I don't want a used one, but I'd take a factory reconditioned one if I could locate it on the shelf, so if anyone has suggestions on sources for that, I'd appreciate it. I can rebuild this one, unless the case is damaged or something serious like that, but I'd rather not have to. From the exploded views, it's a pretty involved motor. Guess I'll find out tomorrow. Guess I better clean off one of my workbenches this evening. it should be interesting.

At this point, I'd even consider renting a machine if I could find one, just to get out of this bind.

Well well. Upon teardown, I find a missing seal ring (or they call it a piston ring, more properly) out of 9 that is completely missing. It's about 7/8" O.D. 1/16 thick and 3/32" wide. Sometime in the past it went crunch. The bore it was is suffered some linear scoring, and what some call the swash plate (they call the valve plate, and the mating seal surface also got a few small scars from eating the bits of ring. I believe this is what gave the machine the "lurch" or "cogging" behavior when we tried to move it. One of nine cylinders produced little to nothing force-wise. Kind of like a V8 running as a V7, except a v8 has a flywheel to spin it on past the dead hole. This motor pretty much stopped when it got to that "cylinder.

I had a nice, hour long chat with the service manager about what when wrong and why, and he assisted me selecting the proper replacement parts. Nice guy. Even stayed past whistletime to help.

It certainly appears there will be some tricks to get everyting lined back up to reassemble, and given time I'm sure I could get it together, but he offered to school me on how it's done.

So I have a list of parts I am going to try to rush in so I can reassemble this beast and give it back to the shop. It's 2-3 hours to reinstall, so not too bad. My worry is not finding all the crunched up bits of ring. I was told if they weren't right there, they probably made it to the tank and they would be stuck in the gunk so not to worry. But I will. I'll flush out all the passages and fish with a magnet and blow with air....everything I can think of. It would be bad to have a loose piece get tangled up in this. But then, the clearances are <0.001 in most of these fits, as new, so even on a used motor the pieces would have to be pretty small to get anywhere. Plus if there were any in the motor, likely they would have already been through it and made it to the reservoir. I'm hoping, anyway.

Just for entertainment value, I'll post a couple of pics for those who haven't torn into a bent axis hydraulic motor.

Pics as promised:

That piston ring either left the motor through the case drain or it went back into the hydrostatic pump as it is a part of the closed loop drive. It would be wise IMO to remove the pump and disassemble it to check for damage and flush or replace the propel hoses. If pieces of that ring stayed in the loop, repeat failure can be instant. I would imagine your parts supplier would concur. If that machine has an access port in the tank it would also be a good idea to open it and inspect for debris. Change the filters and esp the hydrostat charge filter.

In order to clean the system properly, you really need a filtration cart. I've blown hydraulic pumps and motors in the past. When I do, I connect the lines into a loop with extra fittings and start flushing one way and then connect everything in reverse to flush the other way. A decent filtration cart will provide enough GPM to flush particulates downstream and then catch them in either the coarse (40 micron) of fine (usually 2-10 micron) filters. As per advice from a hydraulic shop, I usually flush with diesel fuel first. Then I keep a barrel of mixed diesel and hydraulic oil from former flushes. After the flushing with the second fluid, I suction everything out as well as possible and then fill with new oil transferred via the filter cart. New fluid is not always as clean as it should be.

I also regularly hook up the filtration cart to my machines and do a regular preventative maintenance on the fluids. This way I can also run my fluids longer as there is no particulate contamination. The fluids never really go bad. We are only given change intervals to get rid of contamination.

P&S, I've already gone through the pump, replaced a few parts and have pressure in excess of 6 ksi both directions after a quickie going through. Hoses are all clean. There was no damage. Two of my service advisors where I buy my parts agree that odds were that the pieces of the ring went on through and ended up in the reservoir. There is no reverse flow on this motor. The pump has two outlets, one for each direction of the motor. When there is no propel command, there is a bypass straight back to the reservoir. At any rate, the pump escaped any damage. The filters have already been changed, and the old ones tossed. I really wanted to open them up to see what they had caught, but missed that chance.
I do plan on going fishing in the reservoir and hopefully find the remains of the ring, ideally in large pieces. I do hope it has a larger access port. At this point, it seems that if I can get my hands on a set of piston rings, I'll be able to limp by this job anyway. I'll be looking for a buildable core in the mean time and will take my time and go through it with a fine tooth comb so I'll have a spare to swap in if this one bombs on me. I see a few parts with minor wear, but nothing serious, or out of service specs.

Muncher, I couldn't agree more. But at this point in building up our shop, we don't have that luxury yet. I'll just have to do the best I can. In our fleet, I'd estimate >50 pieces of equipment, with nearly all equipped with hydraulics. I'd love to have a flush cart, but it's not in the cards presently. Maybe I'll just build one. I'm sure I have all the needed components.

It would be nice to add that to our PM program.

Yes the pump has two outlets. Under operation one propel hose is the pressure hose and the other is the return hose which is basically the suction hose for the main rotating group . When the opposite direction is commanded that just reverses. That it why it is called a loop and why I am always concerned about containation when dealing with one of these type systems. There is a charge pump located inside the main pump that serves to keep the loop full by replacing oil lost from normal leakage to case drain and what oil is removed by the flushing valve on the motor , as well as providing pressure to control. The flushing valve lets oil out if the loop so it can replaced with cooler oil. With the failure on that motor you posted , you could have diagnosed internal motor failure by running the motor case drain hose into a bucket and measuring the flow out if that hose in gal per minute at neutral as the charge pump creats that flow that is going around that missing piston ring and into the case drain When the pump is in neutral, the pressure is equal in both propel hoses and that pressure is from the setting of the charge pressure relief valve. When the pump is in neutral there is no flow in the loop, it is static. I wish you luck patch repairing that 51 Saeur Danfoss motor. I have not had much luck fixing those things with out buying the whole rotating group which as far as I know is the only way to replace the pistons and that dog bone is tricky to get in right with those rollers. This is likely not an issue on what you are dealing with here but I have also found internal leakage between the valve plate and the head where the valve plate had worn into the head. Keep in mind that I have no formal training on hydraulics , just on the job training and I am only trying to help out.

P&S, I appreciate the explanation, clarification, and suggestion. OJT is something that there is no substitute for. I tried to locate some sort of spec on what the flow from the case drain should be but could turn up nothing.

You are quite correct in that the only way to replace the pistons (in particular) is to buy the output shaft. The pistons are held in place by pressed in rings on the end opposite the cylinder. If I had to, I could build the tooling needed to remove and replace them, but they are not available as separate items.

Since you obviously are familiar with these motors, let me ask about the solenoid mounted on the valve segment end of the motor. Which direction is the normal operating position, "up" or "down"? Or is this F and R? I realize this changes the displacement and hence the flow, but the machine this is going on has been really cut up as far as controls, wiring, and lots of hosing. Assuming the solenoid is not ever energized, where does that leave the valve segment? I would not think it should just float. Or is this something that is (or I should say ought to be) normally switched manually from the operator position? It does have the connector and at least that part of the harness, but of course, following it back to the control is practically impossible. I can get power to it if needed, and switch it if that is required or desired. I suspect that one of the rocker switches on the panel did control it. Also, the nut that I imagine held the coil in place is gone, so if it is intended to be reversible, that's not possible at present and I'll have to make a nut. I'm learning that getting parts for this Hydro Ax is nearly impossible.

I've been warned about the dogbone and reassembly, and that if I call this particular service manager he will give me some tricks, but I can see that it will be awkward, to say the least. I see a bit of frustration in my future.

Well, a little update to the saga. I was able to get the rings overnighted to me. I cleaned everything up, polished all the scratches that were there. Really was not bad at all. Took a few tries to get the pistons and dogbone all aligned when assembling the cylinder block. The old arthritic hands and fingers just don't cooperate like they used to. But I did get it together and delivered. After a few test runs around the shop they decided to go take down a handful of trees and see how well it sawed and grabbed. It did very well. Unfortunately it has been raining on and off for a week or two and the operator drove into a soft spot and sunk it to the frame. End of tests. I left the scene at this point. The plan was to anchor an old milsurp 6x to a tree and winch it out. I don't know the details, but it seems to have propel issues again. I have a feeling they just pulled it out with it not running and not in neutral. That makes me think there is now some motor damage again.
So I'm on the hunt for another motor (used, but running hopefully), since getting parts seems to be a difficult problem to overcome, although I did find what appears to be a good supplier here in-state and within reasonable driving distance should the need become desperate. They say they stock a lot of Danfoss parts, among other makes. I just hate that the damage could be from something as simple as dragging the machine out of the mud. I had already told everyone involved that the machine could not be moved with it in gear, as I had locked the transfer case in low. They would not have been able to drop the driveshafts very easily, because it was sitting on the belly pan and sinking further. But they could have opened the lines or removed the locking collar I had out on the transfer case and taken it out of gear....or something. But the pressure was on, and I'm betting no one gave it any thought. I'll have to find out. They will be pulling the motor on Monday.

To Be Continued......

I'm kind of bummed about it. It was looking like a success story.

Tony, you will likely find that same ring gone from the same piston. I have tried the same thing in the past , thought it was fixed for a little while. The solonoid on the motor is either off or on. Just can't remember which is turtle and which is rabbit. It only affects ground speed . All directional control is from the pump .

Very possible. But a new factor is a blown hydraulic hose. There is oil sprayed everywhere. I have been told that it acts smooth and normal moving forward, but not reverse. I theorize that a blown hose would not supply normal pressure to one side of the motor, and if it were the "reverse hose", it might act like it is described. In the morning, I have a guy who will spend some time with our pressure washer getting the mud off it and I'll knock the cab over for him to clean the oil spray down. It's a long shot, I know. It was moving fine until it sunk. Turns out they dragged it out with a track hoe, but I still don't know if it was running and they were trying to walk it out at the same time. No way to know when the hose blew, if that's what it is. Just have to wait until it's all cleaned up now.

Meanwhile, I've found a reconditioned motor in stock that I can hotshot in if I have to, but negotiations are underway with an individual willing to come in and just do the cutting and stacking. He has a larger machine anyway, so it would probably help get back on schedule or closer. So the pressure to fix it may ease off so I can do a more thorough job of failure analysis.

So meanwhile, I move priorities to the Volvo Artic. Manifold repaired, rebuilt turbo. Runs great. Now they are telling me something is up with the transmission. They found what they thought was a relay that had had a meltdown, but it was a flasher unit. Don't know of any connection with the transmission to the flashers. Or for a fact that it served that function on this truck. No docs available. Wish I had a schematic. At least on the artic, we have a Terex and a Cat as backups. We have no backup for the Hydro Ax. It will get fixed, it just moved down the list for now.

Oh, P&S, on that solenoid, it controls where the valve segment is positioned by means of the servo piston. At one extreme, the motor is at minimum displacement, and there is an adjustable stop bolt to determine that (called minimum displacement limiter). Factory set. At minimum displacement, the available torque is at its lowest, but the motor speed is at its highest. I guess that would be the default position for transport, and the solenoid would be energized. Otherwise, the solenoid is not energized and the hydraulic controls move that servo piston and hence the valve segment, varying both the speed and torque.

That's basically the operating description in the technical documents I have downloaded from Danfoss.

I don't want to be a bother, but I am trying to diagnose a prentice 2764 hydro ax and sure could use an operators and/or service manual. I need to remove the battery and I cant get the cab jacked up. There seem to be some pins that I have to rotate but it's a Monday and I seem to stupid to figure it out.

I have learned that if one turns the battery isolation switch on it works better. Operator error. Now I just need to figure out how to operate it.