Do you use a synthetic transaxle/hydraulic oil?

[tomrscott]

cowboydoc,

I appeciate your thoughtful response, but that advice becomes complicated by the fact that in the 790, the transaxle and hydraulics are one and the same 4 gallon system. The former is no more sealed than the latter.

Still, I wonder if the synthetic might not help keep the contamination down, more slippery, help the seals work better, etc.? It might also help keep what contamination does occur in suspension better, "attaching to" and "lubricating" the contaminant particles as it were, until they can be filtered out? I guess if contamination is, to some degree, a fact of life, the other part of the equation is, which lubricant protects best against the effects of the contamination?

Speaking of contamination of the hydraulics, specifically through the cylinder rods and seals, I've seen one site, a general discussion of hydraulics care, that had a recomendation for rod boots made of fabric wth velcro. It seems like this might keep most of the crud away from the rods and seals and extend their life. Does anyone use these? Has anyone ever done any controlled tests to show how much this helps? I can see something like a Gore-Tex material that would keep moisture and dirt out, and could be taken off and washed periodically. Seems like that might help quite a bit.

Thanks!

 

[JerryG]

I don't think that a synthetic will be of any benefit in a 790, being that it is a straight gear tractor. If it had a hydro, power-shuttle or GST, it would be a benefit for the clutchs to work better at low temperatures.

 

[BGL990]

tomrscott:
Sorry if this is a bit off topic, but it sound like you might have the answer to a question: I'm planning to install the power beyond kit on my 990 this summer and add the valves and plumbing for some remotes myself.

Do you need to connect the low pressure tank return from the valve block? I was under the impression that most backhoes that plug into the PB outlets just have the two connections - they just "complete the loop" like the loopback hose that would otherwise connect the ports. If you just keep the tank return port in the valve block plugged, is that OK?

 

[cowboydoc]

<font color="red"> I wonder if the synthetic might not help keep the contamination down, more slippery, help the seals work better, etc.? It might also help keep what contamination does occur in suspension better, "attaching to" and "lubricating" the contaminant particles as it were, until they can be filtered out? I guess if contamination is, to some degree, a fact of life, the other part of the equation is, which lubricant protects best against the effects of the contamination?
</font>

From what I have read I personally don't think that synthetics would help much more than a good quality hydraulic oil. But that is my personal opinion there. We go through hydraulic oil by the 55 gallon drums so we are always looking to cut costs by doing the proper maintenance to keep repairs down. We have tryed synthetics in the passed and the oil samples were no different than with regular hydraulic oil.

One thing that is for sure it can't hurt to use synthetic at all. I just don't know that it is worth the cost when you have to change it out every year anyway. If I only had one tractor I may do it. But as much as we buy it just isn't cost effective.

 

[tomrscott]

bgl990,

Not a problem, not off-topic at all.

Humm, interesting question, this depends a good deal on how the BH valve is designed and plumbed, but yes, my understanding is that with an open center system, if you trace the fluid flow through the valve, when you are driving one side of a double acting cylinder, the other side will get returned to the T port which should connect to a low-pressure return to tank port. That outgoing flow has to go somewhere. The trouble with porting it to a pressurized PB port is that then you will have a hard time doing any work because the outgoing flow is pushing against pressure.

I suppose you could have a system with pressure "P" and low-pressure return to sump "T" lines and no power beyond "PB." In this case the BH would be in parallel with any other hydraulic systems, so I guess this requires a "closed center" system and some sort of pressure compensation somewhere in the system so that there is still pressure available when you need it. In this case, if you weren't doing work, no fluid would flow in the BH system, but when you divert pressure flow to do work on one side of a cylinder the other side could return to tank. But I don't see how a double acting cylinder can do work without a low pressure flow attached to the back side of the piston. If you have equal pressure on both sides of a piston, it can't do any work.

Hydraulics depends on two basic principles, !) fluid is relatively uncompressable, and 2) fluid will equalize any pressure differential on two sides of a piston by moving the piston.

If you have an open center system with power beyond, I think you always need three system connections, not counting the cylinder work lines, P, PB, and T, though they go by various names.

Perhaps someone with greater expertise will jump in here if I'm off base, which is entirely possible, but that is my understanding. I have an engineering background, but am pretty new to tractor hydraulics.

 

[BGL990]

Thanks. It does make sense that you need the T port returned directly to the tank. The only thing that confuses me a bit is that I've never heard of a third connection brought out with a quick connect for the standard power beyond connections for the typical removeable backhoe as is often found on these sort of tractors. I don't have such a hoe, nor have I inspected the connections for one closely, nor do I have the PB kit for my 990 yet, so maybe I'm just wrong. The pictures I've seen, though, look like there is just 1 supply and 1 return hose to the entire hoe, so ....

I just thought of one possibility - that the PB connections brought out to the back of the tractor are actually AFTER everything else in the system, so one is pressure out and the other is essentially a return to tank. If that's the case then the hoe (or whatever else you have plugged in) would use the P and T ports on it's valve block and the PB port on the valve would be plugged (I think ...) I had been assuming that the PB connections were after the pump/loader SCV but before the 3pt hitch, requiring both sides of the PB loop to withstand high pressure.

Anyone else with a PB kit and backhoe care to offer any insight?

 

[tomrscott]

BGL990,

Yeah, now that you mention it, that does make sense. But you're right, it would have to be the last thing in an open center circuit, so then you'd essentially have a P and T line and no PB, but the PB and T lines from the SCV would have to combine into one line.

The fluid has to have someplace to go when you aren't doing anything with it, so the open center has to be there, and then when you divert some to a cylinder, the outflow from the cylinder which would come out the T port has to go someplace too.

 

[cowboydoc]

On my backhoe, 4600 tractor and 48 hoe, my power beyond kit is completely separate from the rear valve and the loader valves. I have two ports on the back of the tractor for the backhoe. One is located up higher than the other, but both are below the remotes for the tractor. The top port is for oil pressure in and the bottom one is the dump to return the oil back.

 

[JerryG]

Here is a schematic of how mine is set up. I don't know if it will answer any of your questions, but it might.

 

[BGL990]

Thanks! That schematic is exactly the sort of thing I was looking for. Makes sense now - the backhoe is the last thing in the circuit, at least as far as it is concerned. Somehow I had it in my mind that the PB kit intercepted the flow somewhere in the middle of the loop, so both sides of it would be under pressure when using, say, the 3pt hitch on the tractor. That isn't the case in your schematic, and I suspect all our compact tractors would be set up similarly.

Most valves I've seen have the option to simply plug the PB port if you aren't using it, so in my case I'll only need to plumb the Pressure in and Tank return line from the remote valve block to the PB kit connectors.

Thanks again everyone!
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