Sundstrand Piston to Piston Pressure readings

[pfrederi 17,881]

Since there is a wealth of knowledge here now on Sundstrands thought i would ask for the collective wisdom. My D200 is reasonably strong but the hydro heats up after and hour or so of use. When I get the chance to put her on deadline i will swap out the hydro motor and then the pump with ones I have worked over. But in the mean time I am a bit confused about some pressure readings (and what conclusions i should draw from them).

I am running Dexron ATF and the filter is new.

Cold my charge pump pressure is only about 85. After the temp gage gets up above 120 the pressure improves to 100. In both instances the pressure dos not change when I am moving forward or back. Cold operating the 3 pt to full down jumps the pressure to about 550...warmed up a bit it hits a consistent 600. As the pressure doesn't change when I am moving am i looking at a weak charge pump or are the relief valves (particularly the implement relief) too weak...e.g. should I look at adding shim to the implement relief???

[coldone 35]

Got to ask why you are running ATF in the hydro?

[JamesBe1 82]

A couple of questions that may lead us in the right direction:

If the charge pump was weak, wouldn't the pressure decrease when the viscosity decreases (hot fluid)?

Also, could a worn valve seat or spring operate poorly when cold and better when warm?

@Coldone. He's probably running atf since some of the hydros came that way from the factory. It's a real pain to try to switch back to motor oil, so most people just stick with what they got. I'm wonder what everyone's preference would be given the choice. Myself, I like the commonness of motor oil, but I might be inclined to choose atf or hydraulic fluid for the additives to deal with moisture in the system.

[pfrederi 17,881]

ATF was the original recommendation and is what was in theD200 and running in my Electro12 and my Charger. (Heck I still use 40w motor oil on my L107 transmission that is what wheel horse put in it when my Dad bought in 1967 and has been used in there ever since with no issues.)

People running motor oil also seem to have issues.

James you might be on to something about the relief valves. The pump it self has no problem maintaining pressure when I am moving.

I am thinking I can pull the 2 relief valves with out having to remove the pump. will have to look at that later today.

[coldone 35]

Thanks for the refresher. I had read that atf was in there but forgot.

Yes the two releif valves can be pulled without pulling the pump. You have to remove the battery, battery supports, and side panels to get acess. Just do one side panel at a time or else the dash becomes unsupported and is a pain to get it all to line back up. When removing the implement relief be carefull not to drop the shims, they can be hard to find.

[wheeledhorseman 574]

Here's just a few thoughts Paul.

Cold my charge pump pressure is only about 85. After the temp gage gets up above 120 the pressure improves to 100.

The range quoted for 3/4 throttle is 70 - 150 PSI you seem to be in this range. The fact that even Sunstrand are no more specific than that tends to make me think that there are variables involved that they don't explain. The critical low point I recall is not to fall below 50 PSI.

The fact that the pressure rises when the hydro warms up would be odd in terms of oil getting thinner I'm not sure about the properties of ATF though. It might also not be strange in terms of the amount of oil / ATF 'weeping' from the hydro pump and motor. Although the circuit between the hydro pump and motor is a kind of 'sealed' or closed loop piston to piston system, some fluid is lost from it by design, I believe it is called 'clearance flow'. It is this loss that has to be made up by the charge pump. I guess that clearance flow is critical to insure that moving surfaces are well lubed and to ensure that fluid in the piston - piston loop does get changed and filtered. With the differential expansion between the various metals used in the pump / motor it seems possible that the gaps between surfaces that alow clearance flow may well change with temperature and affect charge pressure. To my mind there must be reasons why Sundstrand only ever talked in terms of acceptable pressure ranges.

My D200 is reasonably strong but the hydro heats up after and hour or so of use.

I'm not sure what you're reading into this. I don't think Sundstrand mention it in any of the WH versions of their manuals but it gets a mention in their generic series manuals for piston - piston hydros. Heat exchange to keep the hydro cool (i.e. pump fins, manifold, and tarnny case) must be capable of dissipating up to 25% of the input horsepower to the transmission. That my friends is how amazingly inefficient a piston-piston hydrostatic transmission is, so they do get hot. The temperature they reach will depend on how hard the transmission / engine are working and the ambient air temperature that cools it. I did about 2.5 hours continuous mowing yesterday with my C-120 auto on two fields that are on a slight slope. It was a reasonably warm day, the grass was sort of average length but not short, and the tranny case got pretty hot. It's interesting in a way that, as a C-120 doesn't have an oil temp gage, you just don't give it a thought. As I've never taken any pressure test readings on it either I don't give what might or might not be quite correct a thought either. It just works, and quite hard at that.

It was interesting that my son was mowing alongside me on the C-100 manual and the tranny case on a manual gearbox was barely warm, just a little above air temperature to touch.

Re the big debate over ATF vs OIL - I use oil in both my hydros basically because it's what was originally specified by WH for them. With the C-120 I had to drain the ATF that a PO had filled it with. Couldn't get every drop of ATF out but changed it again after 10 hours. It works fine, perhaps slightly quieter with oil over the ATF but that's about it so I guess either will do.

The generic Sundstrand manuals of the period gave guidance to application designers as to suitable fluids which should be chosen for their viscosity, oxidation, thermal stability, shear stability, low temperature fluidity, rust inhibition, anti-foam, water compatibility, filtration limitations and anti-wear properties. They didn't actually specify a particular fluid - that, it would appear, was down to Wheel Horse for example. They mention that ant-wear hydraulic oil, ATF type F, agricultural hydraulic transmission fluid have been used successfully and that if fire retardant properties are required then Pydraul 312. So my take on all of this is that WH went initially with ATF but then decided 10w30 or 10w40 did a better job for some reason.

Sorry for another lengthy ramble based mainly on reading, a bit of experience (though not a lot) and continually trying to understand more about the mysteries of these hydros. Feel free to disagree if I'm wrong on any of this - it's how we improve our collective understanding.

Andy

[pfrederi 17,881]

Andy: Your knowledge is impressive. I was thinking that teh range of acceptable values was due in part to the use of a rather simplistic ball and spring pressure release. After ll what are the manufacturing tolerance for the spring in terms of compression. On the lift side they used shims to get an acceptable pressure level. One wonders if they were actually pressure tested and shims added subtracted at the factory or only after a customer complained. In my case as the lift will pick up a rear mount mower with out any issue I am not overly concerned about the 600 psi reading

Maybe my problem is jus one of normal heat that can't be dissipated...why WH later decided to add a cooling fan.

I have mounted a small 4" pancake fan but it is only rated at 100cfm. I have not figured out how to mount the larger fan used on later D's.. Between th custom seat spring mounting column, the 3 point and rear PTO there isn't enough room..

[wheeledhorseman 574]

Paul, I just wish there was somebody out there in RS land who has the knowledge and experience to chip in with you've got these bits right but........

Such a guy (halbert) did exist on the old RS site though he didn't make many posts and doesn't seem to have joined up again after the old site crashed. Last night I came across a post he made when I was searching for terms like 'clearance flow' on RS. Didn't come up with anything on the new site but Google took me to the old site which seems to have been archived by forumer.com into a text only resource with adverts interspersed with the content. I checked the same thread on the new RS site but sadly his lengthy description of how a Sunstrand works got reduced to a few lines in the migration. So here it is again for reference.

In the thread 'Playing with a D-160', Henry wrote......

Just for the record: There are two different pumping elements within the one Sundstrand pump. Both are driven off the same input shaft.

1.) There is the main hydrostatic pumping element composed of a variable displacement, axial piston type pump. This pump can be varied from zero to maximum displacement on both sides of zero--one direction is forward, the other is reverse. This pump is referred to as a "closed loop pump", meaning that the oil moves from the appropriate discharge side of the pump, through the motor (which is a fixed displacement, axial piston motor) and then back to the other side of the pump in a closed loop. Depending on which direction of travel (forward or reverse) you are going, the side between the pump discharge and the motor inlet is called the "high pressure" side of the loop or sometimes the "drive side". The other side is referred to as the "low pressure" side or the "return side".

2.) The other pump is a smaller, fixed displacement, pump called the "charge pump". The specific type of pump in this instance is called a "gerotor" pump (as opposed to say, a "spur-gear" type pump). The purpose of the charge pump is two-fold:

A.) It supplies "makeup" or "charge" oil to the closed loop piston circuit. This means that because of normal leakage (called, "clearance flow")* from the closed loop to the case of the pump, some means must exist to keep the closed loop full of oil. This is done through a system of check valves that direct charge pump flow to be available into the "low pressure" side of the closed loop system. The "clearance flow" from the case of the pump is discharged from the pump case to the reservoir through a "case drain" port. This case drain flow also carries heat away from the pump and motor as well. "Clearance flow" from the motor element of the closed loop passes to the motor case and out of the case drain of the motor into the reservoir as well. It too carries heat away from the closed loop circuit. The charge pump draws oil from the reservoir through a suction strainer.

B.) The charge pump is also arranged so it can make available its flow and pressure for the "implement circuit" of the system. This is typically a mid-lift cylinder (for say a mower deck) and/or a rear lift cylinder (for say a three-point lift). This type of system is called an "open loop" system, meaning that it supplies pressurized fluid flow through a directional control valve to the implement cylinder and returns the discharge flow from the implement cylinder back through the directional valve to the system reservoir. A system of built-in check valves, etc. assures that "makeup" flow and pressure are prioritized for the closed loop (which is normally very low flow volume) and the remainder is then available for the "implement circuit". The "reservoir", in most cases, is actually the case of the gear reducer for the traction drive.

CLEARANCE FLOW AND IMPLEMENT VALVE RETURN OIL FLOW PATH:

Oil returning from the implement valve is passed to the case of the pump where it combines with the pump clearance flow and any flow that passes over the charge pump and/or implement relief valves (charge pump and implement relief valves are located on a block at the rear of the pump housing). This oil then passes from the pump case to the motor case, where it combines with motor clearance flow. Finally this ‘spent’ oil passes through a return filter (typically an automotive, spin-on type) and hence to the reservoir (typically the traction drive gear case).

Also, be aware that there are those two different relief valves that control the pressure of the "charge pump". When in normal driving mode, where the implement circuit is not being used, the charge pump pressure is controlled by a "charge pump relief valve". When the charge pump flow is called on to provide flow and pressure to the implement circuit, it is controlled by a higher pressure, "implement relief valve". This is why the sound of the pump changes when an implement cylinder reaches the end of its stroke. Since there is no more "stroke movement" of the cylinder, the oil pumped by the charge pump must pass over the higher pressure implement relief valve in order to get back to the reservoir (remember that the charge pump is a "fixed delivery" pump; meaning that it is going to try to pump a given volume of oil at a given drive RPM). You will hear the sound change most audibly when lowering an implement. The implement flow sees very little resistance because the load is being lowered as opposed to being lifted. When the cylinder "bottoms out", the charge pump flow jumps up to implement relief pressure because it must flow over the implement relief valve (it is going to try to go somewhere even if it has to rupture a hose or shear off a pump drive shaft, so a relief valve is provided to limit how high the pressure can climb when the flow is "dead-headed").

*Clearance flow is the leakage or slippage caused by the oil that escapes between closely fit pump and / or motor elements. An example is the oil that slips between the pistons and the piston bores of the pump rotating group (typically a rotating cylinder block or barrel with nine pistons). This slippage occurs from the high pressure side to the low pressure side of the minute clearances involved. When this occurs, heat energy is released.

My profession, until I retired, was "fluid power engineer". I worked in the fluid power (hydraulic and pneumatic) business for about 35 years and I taught in that field at the local community college level for several years. I would be glad to help anybody understand and troubleshoot these systems. I expect nothing in return other than a simple thanks--provided, of course, I didn't help you blow up something! LOL

WARNING: I tend to get sort of long-winded at times.

The Sundstrand and Wheel Horse manuals do a pretty good job of presenting this information too. I encourage you to get a manual and read through it for more explanations as well as pictures and diagrams of various pieces of gear.

Good tractoring,

Henry

It's reassuring to know that I'm on the right track with some of the things according to the above but there's plenty of things that I'm sure Henry would have been able to clear up for us if only he were still an active member of RS. So Henry, if you do browse the forum from time to time but as a guest these days, please get in touch by joining up again - we need your knowledge and offer to answer questions.

On the lift side they used shims to get an acceptable pressure level. One wonders if they were actually pressure tested and shims added subtracted at the factory or only after a customer complained.

The idea of using shims in the pressure relief valves on pumps was common at one time - my WW2 Willys jeep has shims in the oil pump valve but by the 60s as far as I know springs were presumably made accurately enough for this practice to have pretty much dissapeared from automobiles. I'm guessing that a standard factory set of shims were fitted, the pump tested, and most pumps passed within accepable limits. In a few cases shims were altered to achieve this. Again, this is a logical guess rather than actual known fact.

Andy

[pfrederi 17,881]

Andy: Thank you for finding that narrative.

I need the FEL for awhile longer...she doesn't overheat in that use, short cycles to load the wagon then a rest period while I unload the wagon and spread the dirt.

Once I get the project done i am going to start with pulling the hydro motor and replacing it

[meadowfield 2,595]

really useful post recovered there Andy, good to fill the Sundstrand techy gaps with and a great summary to add to a more simplified block diagram !

maybe I/we should do a really simple schematic - I mean way simpler than the Sundstrand manual...

i.e. (very simplistic) and not necessarily accurate, but you get the idea

can maybe indicate pressure/flow too??

mark

[JamesBe1 82]

Wow, such a wealth of great information. I need to take a nap now to process it all. At least my understanding of how the system works was on target. There was a bit there about charge relief that I will have to ponder a bit when my ears stop bleeding.

Thankyou Andy for adding that great info, as well as your explanation of your understanding of the system. Over the years, I've learned that it's good to have a different take. Sometimes, the same information in different words make all the difference in the world. (I've noticed this as a characteristic of great teachers who say the same things several times in different words).

And thanx to Mark for the nice graphic. As they say, a picture is worth a thousand words (in this case, perhaps a bit more). I had to save that diagram for future reference.

I sure wish we could get Henry to join in the discussion. We could learn a lot from the likes of him. Would be great also if some of the Wheel Horse designer/engineers from the old days would join too. You'd think that after all these years, that one or two might have found their way here.

Correct me if I am wrong, but the pressure increase on heat up could be the result of decreased clearances between parts due to their thermal expansions. Does that make sense? Maybe? Perhaps in some alternate universe? Kinda Sorta?

[meadowfield 2,595]

James, hopefully it's a start and we can agree on how complex the diagram is...

Heat build up in any fluid or mechanics is always due to friction. This can be from pressurisation and friction of fluid - i.e. oil, or friction from metal on metal.

The transmission will always have some minute leakage from the charge pump onwards as unless there is the required clearance the pistons will not move, nor will any of the other parts. In an ideal world there are no leaks and zero clearance, but even with the best of manufacturing this is impossible - however the improvement in modern tooling allows for smaller clearances with the benefit of less power lost as heat !

Your excessive heat build up has to be through friction, in the form of excessive losses in either the pump or motor. The first place to start is to see if the charge pump is weak through wear or lack of oil, secondly to look for excessives losses in the motor or through the relief valves cutting in too soon...

Hope this helps, agree we need an expert - Henry sure knows his stuff, in the meantime hopefully between us we can make a good attempt at simplifying the system and hopefully finding the fault.

Mark