BACKGROUND: I have a Hydra-Mac 14C Turbo that I had to replace the failed "lock-up" hydraulic/charge pump assembly last year. With this repair, I had to replace the shaft in the front hydrostatic pump due to a piece of the sheered off shaft from the hydraulic/charge pump assembly had fused itself into it. Inspection of the internals of the front hydrostatic pump looked great. Unfortunately, I found out the "hard-way" that my new replacement hydraulic/charge pump assembly did not incorporate an internal relief valve for the charge pump. The "hard-way" meaning, upon start-up, it immediately blew the charge oil supply filter clean off the machine!!! Researching my dilemma, I learned the hydraulic/charge pump assembly with internal charge relief was NLA, so I fitted an external relief valve, installed a pressure gauge, and set the relief valve pop-off pressure to the Hydra-Mac factory recommended pressure @ 110-120#. I verified that the fitted external relief valve port was at least as large as the original internal relief valve porting. Note all fluid, filters, strainers, are clean, fluid in it is Case Hytran Ultra. This machine's hydraulic/charge filters are under positive pressure, (not under negative pressure like in older Hydra-Macs), point being air leaks are unlikely. All pumps and motors are Cessna/Eaton. PROBLEM/CONCERN: Operationally the machine works great. But when loading the hydrostatic system(s), such as when taking a big cut or spin the machine around, the charge oil pressure drops off significantly. The pressure drops more as the oil heats up. (note this particular machine was fitted with the optional larger hydraulic oil cooling). If I push the machine hard enough, when warm, the pressure drops to zero, but recovers immediately when I back off the load. I don't think this problem is isolated to either side as the pressure drops the same while making either a hard left or right. FINALLY TO THE QUESTIONS: Is this normal? I would not have ever noticed this if I hadn't installed the pressure gauge to set the relief valve pop-off pressure. Is there a hydraulic oil that holds it viscosity better than Case Hytran Ultra when heated? I didn't get an reading exact reading on my fluid temperature, but by feel, it's doesn't appear to be "overly hot". Any suggestions on likely culprit(s)? Sorry for the long post, thanks in advance , Joe
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Totally normal for your charge pressure to drop off when you are working it, but usually it goes to say 90psi. Charge pressure is used to replace *wasted* oil in your hydrostatic system. Your pumps and motors have internal leakage that when under load will leak more, this oil will pass back through the oil cooler and back to the tank, fresh cool oil is drawn back into the system to help keep things cool. As the oil warms up it thins out, thinner oil passes through the gaps in the pumps and motors easier so your pressure will drop quicker when warm. From memory, your hydraulics are happy to run till they get to around 100C, i think my temp switch turns on at about 120C
You seem to know your stuff after adding an external relief, you sure have done your home work!.
Back to charge pressure, on Bobcat machines you can lift the arms up and drive around, when the charge pressure falls, lower the arms quickly and your charge pressure should come back up. I don't know if thats just because of the hydraulic circuitry bobcat uses or if they are all setup the same.
I hope some of that made sense.
You seem to know your stuff after adding an external relief, you sure have done your home work!.
Back to charge pressure, on Bobcat machines you can lift the arms up and drive around, when the charge pressure falls, lower the arms quickly and your charge pressure should come back up. I don't know if thats just because of the hydraulic circuitry bobcat uses or if they are all setup the same.
I hope some of that made sense.
Thanks Tazza, all of it makes sense. Guess I need to get temperature readings of the oil with an infrared thermometer and go from there. This machine doesn't have any oil temperature control except an oil cooler bypass that consists of a fixed orifice fitting of about 1/8". The probable purpose of this bypass is to prevent oil cooler from getting over-pressurized with cold oil. Perhaps I should configure a way to close the bypass for warm oil, to force more oil to flow thru the cooler. The pressure drop to zero when the hydra-static system is heavily loaded still worries me, I wondering if worn hydra-static components is causing the excessive extreme low pressure. Is there any test for worn components other than dismantling/inspection? Sounds like Bobcat shares the hydraulic system for the charge function. The only shared portion in the Hydra-Mac is the suction line from the tank to the charge and hydraulic pump. Even all the case drains on all the hydra-static components are piped independently back to the oil tank. I will try the drop boom trick anyway. Thanks for the kudos regarding the external relief valve addition. I spent considerable time selecting the external valve in the McMaster-Carr catalog. Thanks again for the reply, Joe
I have never done it myself, but you should be able to open the case drain fitting on the motor and plug the hose that goes back to the tank. Run the machine and see how much flow you have out of the motor, there should be a little flow but there shouldn't be a big squirt coming from it. Your charge pump needs to replace this amount of oil that goes back to the tank.
They hydraulics seems to be of a weird design if it restricts oil to the cooler, maybe it also helps maintain charge pressure? I know bobcat machines have their own charge pressure relief valves on the pumps. My 753 has one in the pump but also has a poppet/relief that holds back pressure to 400 PSI, when i removed the poppet due to a noise it was generating i lost my charge pressure. I still don't understand how it works.... but its there and if i remove it it doesn't drive properly and beeps at me!.
That's a good idea to check case drain flows, I also can compare flow rates of each hydrostatic pump and motor. To clarify the orifice location I mention in the earlier post, it does not restrict the flow to the oil cooler, it bypass a portion of it. Another-words, it is plumbed in parallel to the cooler. Sounds like the charge system on your machine is more sophisticated than mine. A low pressure alarm is a nice feature. Thanks again for the replies.
A low charge pressure switch would be easy to install as you know where your change pressure line is. You could also install a temperature sender if you wanted to know fluid temperatures.
Agreed. A oil temperature gauge install is definitely on the list. The careful monitoring of this temperature is especially important when I got the backhoe attachment on it as that seems to add lots more heat to the oil. This machine already has the factory installed charge light switch, but it's slow to react to the drastic pressure fluctuations that the gauge shows. As far as the temperature gauge, I may incorporate a shortcut I did on some other machinery where-as it wasn't practical to mount another gauge due to gauge panel space limitations. I obtained another sender that matched the water temperature gauge, installed it in the oil, wired it and the water temperature sender thru a SPDT push button switch to the water temperature gauge. I wired it so it would default back to read water temperature(water temp is more important IMO), but when you pushed the button, you would get a oil temperature reading.
sterlclan
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It "could" be that your drive pumps are getting tired..........My machine had the same symptoms plus wicked groaning and loss of power . good news is the repair wasn't too hard bad news it requires removal of the pump unit. do you loose power as well or just charge pressure?
Good point, if you lap the valve plate (hopefully its removable) and rotating group face you should get it back to almost factory condition.
I was told that they are not all the same, an 843 for example, the pump has a groove where the rotating group runs and is not replaceable. Seems like a silly idea to me making it run on the valve section and when its worn out the whole thing needs dumping.
For the most part, I just loose charge pressure and get more groaning when under load with warm oil. No noticeable loss in power though. I had one of the hydrostatic pumps apart to replace it's shaft due to the broken off piece of the hydraulic pump was "fused" in it. I didn't observe any significant scoring inside. You are probably on to something, might be time to take the hydrostatic pumps to a hydraulic shop. Does the hydrostatic motors on the drives typically wear out at the same time? Thanks, Joe
The motors don't seem to wear the same, if they are geroler style they seem to hold up pretty well. The pump however has the rotating group that runs on a thin film of oil then usually a brass valve plate. There is only a little bit of oil between the steel and the brass plate. They do seem to get scored over time, but only very fine marks. It takes a fair amount to cause any real troubles. You can use a sheet of glass and a few sheets of wet and dry sand paper to get the scores out.
skidsteer.ca
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Condor
Find out how hot your oil is getting first. See if that temp is consistant through the loader, especially at your cooler inlet. It may not be flowing enough oil with your reworked pump scheme. Is the new pump rough the same volume per revoulution? The more oil you force over relief valves the more heat your will create. A oversize pump would agrivate this. On the other side ones thats too small would have a harder time replaceing the lost oil as the system warms up.
Oil on my Bobcat ant New holland loaders never gets over 130 f (our ambient temps are seldom over 85 f, not real hot up here, so that may be a factor) and you can hold you hand on the hoses for short periods without getting burnt. Now the acceptable limit is much higher then this, (say 230f) but the cooler you can make the system run the better.
Check you case drain flow, thats the best indication of the wheel motor conditions. How high can you adjust your charge pressure? Maybe you can compensate some for the drop as the oil warms by setting it up to say 150? if your filters will stand it.
A backhoe should not heat your oil much, unless your bottoming the cylinders out and holding the valve on, forcing oil over the relief. Otherwise hoe work is farely low demand imo.
Let us know what you find
Ken
Hi, it's been awhile since I posted on this and thanks to all who helped so far. To update, my oil temperature is staying very cool, staying below 125F. I did some case drain flow tests, which were all at idle, with warm oil. At idle, it maintains full charge pressure with no load. Unfortunately, since both hydrostatic pumps are "stacked" together, they both share the same case drain. At idle, the case drain from the pair of hydrostatic pumps took about 50 seconds to pass about 2 quarts. I was able to test case drain flow to each hydrostatic motor independently, each one took over 3 minutes to fill the hose to my jug, indicating I had almost zero case drain flow from the motors. One could conclude problems with the hydrostatic pumps. Now my delimma is obtaining parts for these old obsolete Cessna hydrostatic pumps. I have contacted Eaton Tech support (who bought out Cessna) who stated there are no parts available. I have been working with a Hydramac vendor who got me rotating assemblies, seals and gaskets, but as of this date, is having trouble obtaining the correct plates that mate up to the rotating assemblies. The vendor is still attempting to locate these plates but it been months. Please note, the wear plates are not replacable in these pumps, the rotating assembly mates up to the pump housing plate. Before disassembly of the machine, I am research my options. I understand there it's possible to re-finish my existing plates by surface grinding, lapping, and other processes unknown to me. Can anyone suggest a shop that can perform this service? Thanks so much, Joe
Anything is possible. The 843 machines had the same basic setup if i understand it correctly. There were no wear plates like the newer pumps have. The rotating groups ran directally onto the valve section of the pump. When scored people think they are no good, this is not true. I talked to Levi at LPS http://loaderpartssource.com/ he has the ability to get these machined back to factory spec. Ground, hardened and ground again. Contact him, i'm sure he can get that sorted out for you if you have wear in this section. He may be able to hook you up with seals and parts too. He has yet to fail me on parts i was in need of.
The case drain flow from the pump may be correct, the pumps i know of have a case drain port, but this is at charge pressure i think. I'd crack the pump open and see if there are any obvious issues.
Good luck.
I have heard of getting this surface ground and lapped but had no idea that it had to be hardened again. Great information. I will get these pumps apart and contact Levi. To clarify the case drain vs the charge line in my case, the case drain is located on the main pump housing, it captures the internal leakage and returns it back to the tank. In my case since the front and rear pumps are stacked together, only the rear pump has a fitting and line for case drain purposes, but it drains the internal leakage of both pumps (the front pump internal leakage flows thru into the rear pump housing) The charge pressure on this machine is plumbed into the valve section of each pump. Thanks again for the help, Joe
Got the machine apart, after capping off the main pump ports, tested each hydrostatic pump with compressed air in the charge oil port. This clearly determined it was the rear pump, as it passed lots of air, the front pump was almost none. Dissasembled pump, found the wear plate not too bad, (wear plate is not replacable on this one) but noted some wear on the swash plate. In the endeavor to determine the cuprit, I clamped the swash plate, rotating assembly, and wear plate together, and introduced 20# compressed air to the charge port. The leak became obvious at the swash plate. This was further verified by spraying WD-40 on the suspect leak spots, where-as it bubbled at the swash plate the most. I created an album showing this named HydraMac Novice Hydrostatic Air Test. Sorry for the fuzzy pictures. Any thoughts? Am I way off base?
The end of the pistons you have shoes, they should have a small hole in the end to lubricate the shoes where they contact the swash plate. Is this the area you are noting the bubbles?
Can you feel or see any wear on the valve plate surface that the rotating group runs on? If the valve plate isn't worn, the face of the rotating group could be worn then, causing leakage.
I have no idea if i'm thinking of the same parts as you are though. I was told the swash plate is the part that tilts back and forth to vairy the distance the pistons can move back and forth. The valve plate and wear plates are put together, this is where the rotating group rotates on to create a seal. Yours doen't have the wear plate, so ignore that part.
Most of the wear should be between the rotating group and valve/wear plate. The shoes only have a tiny hole for lubrication, i wouldn't worry about any leakage you have back there.
What I am calling the swash plate is the part that tilts back and forth by the operator control. The area where the most leakage is is where the shoes contact the swash plate. Although smooth, the swash plate does have smooth depressions which I'm guessing is the source of the leak. Although there is some wear on the rotating group and the wear plate, it seemed minimal. Also noted very few bubbles in that area. Thanks for the reply. Joe
I would concentrate on the face of the rotating group and where it runs. Even small amounts of wear cause problems here, if you can feel it with your fingernail, its too much.