Following up on the discussion of using the pump as a brake, what is the worst case scenario when the pump swash plate returns to neutral or even reverse, but the load continues in its original motion? Is the pump internally protected against this event? (This condition would be intermittent.)
The pump in question is a Sundstrand 36 series with integral charge pump.
The wheels are White RE (230) series Roller Stator.
Thanks for your insights.

By the way, there will be no slip available for the wheels - solid rubber on textured concrete, heavy load, 3 mph.
Bud, you mentioned cross-port relief valves, is this normally built into the pump or added to the plumbing externally?

The Cross Port Reliefs can be in the Pump or Motor or even a package like from Sun Hydraulics that would be mounted and connected somewhere between the working flow lines. Most I'm familiar with have them in the Motor since that is closest to the component that is trying to act like a pump when the Hydroatatic pummp is being destroked and/or centered to no flow.

These Reliefs are the only protection from Over Pressure the Closed Loop Circuitt has in a standard circuit.

They can also relieve pump flow to the opposite line when the motor can't turn for any reason and the pump is still on stroke. When this occurs the small amount of oil in the closed loop HEATS UP ALMOST INSTANTLY and is not a good situation.

Most Hydroststatic pump manufacturers offer an option called a Pressure Override Valve (POR) It is set 150-200 PSI lower than the Cross Port Reliefs and automaticlly destrokes the pump, regardless of operator input, when pressure tries to excede its setting.

quote:

Following up on the discussion of using the pump as a brake, what is the worst case scenario when the pump swash plate returns to neutral or even reverse, but the load continues in its original motion? Is the pump internally protected against this event? (This condition would be intermittent.)

When the pump flow is reduced or stopped the Motor acts as a pump and if the flow it produces while the vehicle is decelerating had no place to go something would have to give. Often it is wheel slip but if not the Cross Port Relief opens and sends oil to the opposite line. Needless to say while the Cross Port relief is bypassing the motor is being resisted by high back pressure and is decelerating the load just as fast as the pump could accelerate it when in the travelling mode.

Bud,
thanks for your clear description of the overload protection on a closed hydrostatic system. I am trying to trouble shoot a system that has experienced an overload and is damaged. It is a hydrostatic pump (Hydromarine S23) connected to a motor (Volvo type 78 i think) which drives a propshaft in my boat. The overload came as a result of wrapping a crab pot line around the prop sufficient to kill the perkins 4-108 running the pump. I can now power the system to about 2300 rpm and then the pump destrokes (same in forward and reverse). The max recommended pressure in the system is 2000psi and at 2300 rpm, I'm getting about 2100psi. I am having trouble finding someone who can work on the system. I am trying to figure out if the problem is in the motor or pump. I am fairly certain it is in the motor because the pump is putting out the max recommended pressure. I think this pressure is too high for the rpm at this point because the system is rated at 3000 rpm. It may be that the pressure override valve is destroking the pump. Is there a way to test this? Could a restricted filter cause the above mentioned problem by limiting fluid flow to the feed pump?
Is there a procedure to safely ascertain that it is indeed the motor?
thanks for the help.

I have only diagnosed a couple of Hydro-Static Drives since 99% of my work has been Industrial where Hydr0-Statics are not used extinsively.

If I were checking this one I would disconnect the lines and the motor and install a high pressure shutoff valve and a Bi-Directional Relief Valve in parallel with it. Then I would connect the two Pump lines to the block with the Shut-off and Relief and try the pump under load by closing the shutoff and forcing oil to bypass through the relief. A Flow Meter in the lines would be a way to check pump flow at no and high pressure.

If the pump proves satisfactory I would be looking at replacing the motor.

On the other hand your problem could be a bad hose that was damaged from the high pressure during the fouled motor stalling.

Maybe Kevin could offer some advice since he works a lot with hydro-Static drives.

Bud,
thanks for the suggestion. How do I get this post to Kevin to see if he can offer additional.

thanks

Buzz

Kevin is usually on here at least once a day so you should hear from him soon. If not go to the Member List and Private Message (PM) him.

You say the system only runs @2300r.p.m. but rated for 3000r.p.m...Is the prime mover making rated speed?Can you remove the driver from the driven easily and run the engine up to confirm (with a master tacho/timing light)rated speed?Those perkins engines are they mechanical linkage to the fuel pump?Take it off and give it full fuel with no load,and then load it up.A pressure override valve or limiting valve or governing valve what ever part of the world you are from call it what you will,should be adjustable as the ones that I have worked on all have been(mind you these are Rexroth pumps and Denison.)

Back from a great weekend off!

If I understand correctly, you stopped increasing engine rpm at 2300 because you reached high pressure, not because the engine would not rvev further? A prop load is similar to fan or centrifigual pump, smoothly increasing torque with rpm.

I have no experience with this pump, but it sounds to me that the pump is acting just fine. The pressure override is limiting pump stroke as it should when pressure reaches the 2000-2100 psi adjustment. Bud's pump test procedure would confirm that. I would focus on the load being too high. I think somewhere there is extra drag on the system.

My first guess is that some of the line or cable or string is still wound in the shaft causing a high torque requirement. Check around the motor shaft seal area if it is exposed to the water. On lawn and farm equipment, grass and string or twine fibers get in there and either heat up and destroy seals, or work their way under lip seals and destroy them.

If motor is remote, can you disconnect it from the line shaft and run the system? Then you should see I assume 3000 rpm with only maybe 200-500 psi freespin pressure. That would also confirmn the load issues.

It is possible the motor shaft got bent, or side loaded, from the prop shaft loads.


k

Wow, thanks for the responses. I have been up to my ___ in alligators for a few weeks now. I put the boat in the yard $20 per day, and so I have been hard at it since. I didn't have my notifications set up right to let me know that other had responded. I apologize. I pulled the motor out and took it to somebody who sounded like they knew the motor. They said it is obsolete but may still be able to get parts. They had some trouble getting the flange off the front and had to cut it off. Yikes. I haven't heard back on the condition of the shaft or bearings. they said the back end looked like new (33 years old). To address your questions. The perkins will rev beyond the 2300 rpm. That is not the problem. The pump gets destroked at that speed. The pressure is at the max recommended operating (2000 psi). Kevin, the motor shaft and seal are not exposed to the water. The only bearing other than the motor bearings, is the "cutlass" bearing. This is a rubber bearing, 8 feet from the motor, and 3 inches in front of the prop. It seems to be operating normally as I have pulled the shaft back and turned the shaft by hand. I should have disconnected the prop shaft from the motor and run it as you suggested. Had I been back to the forum, I would have seen your post and done that. I did change out the filter prior to putting the boat in the yard, and the motor was making what seemed to me to be excessive noise. I put a dowel on the front end and could hear clicking noises which I imagined could be bearings. The motor also puked out about a quart of oil in about a 5 minute run, so I knew it had to come out and be gone through. that was a chore.. Took me 2 days to get it out. (I'm slooooow). I cut the old oil filter open found some metal inside, about the size of a pin head. Maybe 3 or 4 pieces in 1/4 of the pleats. It is non ferrous and easily breaks in two with my fingernail. I don't know what it is. The pump is housed in a marine grade aluminum housing. That concerns me. I hope that the pump is not damaged. I am waiting to hear the condition of the shaft and bearings from the motor. If I can find someone to hook up a test rig on the pump, as Bud suggested, I will do it. I can't seem to find anybody who is able and willing to mess with it. thanks to everybody for your input. I am getting quite an education.