I have a hydrostatic drive in a boat and I am trying to trouble shoot the pump. I thought my problem was in the motor attached to the pump and removed it for service. Turns out the mechanic can find nothing wrong with it. So I'm back to looking at the pump. I posted about a month ago and several people were kind enough to respond. See previous post: overrunning load danger
I didn't mean to hijack that thread. Anyway Bud T gave me a reasonable test rig for the pump. He suggested a high pressure shut off valve, a bi-directional relief valve in parallel, and a flow meter. I can find no one near me with the equipment who is willing to do the job. What I need to know is....
1. Is this something I can do myself? I have reasonable mechanical knowledge.
2. What would the specs for the relief valve be? the pump is rated at 38cc per revolution and max recommended continuous operating pressure of 2000psi. I think the rpm of the pump would be the same as the prime mover (the engine is directly coupled to) which would be 3000 rpm which equates to 114 liters/min.
3. Would it be reasonable to put the motor back in, leave it uncoupled to the prop shaft, and run the system. I could rig up a tach on the motor I think, and measure it's rpm. from that I can calculate it's fluid flow and I already have a pressure gauge on the system. If the pump is bad could this damage the motor? I have been running the system with the damage for about 50 hours and there is no visible damage in the motor. Of course the pump could start puking metal at any time. An inspection of the filter showed some small aluminum flakes. The filter had been on about 500 hours total time. the pump is housed in a cast aluminum housing. Normal flakes or signs of damage?
Just to quickly reiterate the problem is the pump destrokes prior to reaching full operating rpm. It is at 2150 psi or so when it destrokes to 2000 psi. The prime mover will rev higher. But the pump is at it's max recommended pressure at a reduces rpm (2300). It should go to 3000 rpm.
thanks to all who are willing to share their knowledge and help.
buzz

If you found aluminum flakes the pump is prob scrap. Take it apart and have a look, if the pump body is all scraped up where the gears sit (I assume its gears) call Josh Cosford!!!

Buzz;
Do you have a full part number and manufacturer of the pump and the motor? Also do you have or can you get a Schematic dawing of the ciruit with component part numbers? Without that informationeveryone is shooting in the dark.

If it actually is a HYDRO-STATIC pump and is rated at 2,000 PSI it will only "destroke" if it has a Pressue Override valve. If there is a Relief Valve in the circuit set for 2,000 PSI the pump is not necessarily Destroking but sending excess flow through the Relief Valve. The circuit is doing exactly what it was designed to do but you are wanting to run it above its rating.

It would be a big help to have part numbers and manufactuer names so an educated answer can be given. Not only a BIG HELP but absolutely necessary.

Hey all,
thanks for the replies. E. West, the pump is not a gear pump but a variable displacement piston pump. Still not good to have alum flakes though. Who is Josh Crawford.

Bud,
The pump is a Hydromarine S23. I cannot find the company on the internet. Most of the documentation that came with the boat is in Italian (the boat was made in Italy. The drive system was engineered by Nanni Diesel. It looks to me, from some of the documentation that came with the boat, that the Hydromarine company took what appears from some documents to be a Saur/sundstrand series 18 pump and marinized it. I have a Saur Italiana Catalogo Part
Di Ricambio (parts manual)for models 15-18 but the models don't look exactly like mine (remember they marinized it). Anyway, whatever model (can't find an id plate) They put it in a marine alloy alum casting. It has several coolers cast with the same case. I have two Hydromarine manuals (if you can call them that) that have some information on the pump. There are some very crude schematics and maybe even one of them is missing (loose leaf pages). The boat is 33 years old and the system worked fine when I bought it 10 years ago. I can assure you that the system is not performing as it did before the damage. I used to be able to run at 3000 rpm with a boat speed of around 7 knots. Now I cannot rev higher than 2300 without the pump destroking. I say destroking because what happens is the selector (attached to the pump via cable) moves toward the neutral position from it's full on detent. The cable attaches to a lever which is directly attached to the swashplate. It does the same in forward or reverse. I hope this info helps. I have been trying to investigate this problem for about 3 years now. As I said in the beginning. The motor (volvo type 78) is out of the boat and has been gone through. Nothing was found that would explain a system reaching max pressure before full volume (revolutions). I need to measure the pump output. Bud, you suggested in the other thread a test rig comprising a shut off valve, and in parallel, a bi-directional relief valve, and a flow meter. As I understand the test procedure, the system would be run up in rpm (assuming it will attain full rpm) and then the shut off valve would be slowly closed to bypass through the relief valve. The relief valve is substituting for the motor no? then the flow through the relief valve is observed. I would expect to see full volume (38cc/rev) at 3000 rpm at 2000 psi. Man, that's a lot of fluid going through there. question: how do I spec out the relief valve. the rest of the stuff in this test rig I can figure out.

Buzz;
It sounds like you have a FLOW RESTRICTION from some source thats pressure high enough to force the unbalaned Swash Plate back to center.

Over the years I have been surprised many times by the types of restrictions I've found in circuits that have run for years and suddenly have a problem and no one admits to having done anything.

One possibility (guess) that I have come across several times is a Hose Liner has dislodged and restricts the oil stream when it tries to go above a certain flow. However, your circuit may not have any hoses.

Tore into the plumbing of a circuit that had run for 7 years and now had a slowdown problem after it ran 2-3 hours. Found a 1/4" Allen Pipe Plug that had been rattling in the plumbing so long that it was almost the shape of a Ball Bearing and was retarding flow. Never understood why it stayed in place until the oil got warm but removing the culprit got the machine back to full production.

I don't believe anyone can diagnose your problem from afar. If it were my machine I would be looking for a consultant. Sometime that is a trusted/trained Fluid Power supplier, Like so many salesman I did that for years to get the business.

i think you dont need to buy an extra relief valve or flow meter because flow meters for use in pressure lines are very expensive.

each hydrostatic pumps used in close loop circuits are equipped with crossline relief valves in both output a/b ports. you can try only buying two pressure gauges or probably 3 and connect it to A/B ports (there are plugs that you can open and connect a gauge into it. connect a pressure gauge on to the charge pump ( it is a gear pump attached at the end of the piston pump and watch the pressures in to it, charge pump relief valves are set only to very low pressure, just enough to compensate for the void created in the A/B ports. charge pumps are commonly the source of stroking/destroking pilot pressure.

also make provision to observe motor case drain flow. as far as i can remember when i was working this thing, i think hydrostatic close loop pump case drain is internal.

But as BUD T say check the line hoses first , try the easiest way.

but you have to observe the performance of your unit in just its about to destroke(the gauges reading and motor case drain flow). higher case drain flow means pump or motor components clearance fits are worn out

But the only way the pump destroke is because the cylinder that holds the swash plate is being activated! try connecting 1 pressure gauge on each side too. i think the pilot pressure neede to destroke the pump is not that high. if i were to do it i can put 1 hand pump on each pilot ports and try to manipulate it to destroke or not to destroke

there is also a mechanical stroking/destroking limiter which you could try screwing in and out, but please with some common sense

when adjusting any lever in hydraulic components, dont be scared... just slowly and nice and easy OK!!!