closed loop...really?

[marrt]

I've read several times on this forum that the PT uses a "closed loop" drive system. I asked terry about this once and he denied it. In any case, how could it be true? The hose coming from the filter is huge. If it was only feeding the charge pump, and only make up fluid at that, why so big? The case drain hoses on the wheel motors are tiny (1845). It don't make no sense...as my dad used to say.

 

[SnowRidge]

My PT-425 has a closed loop drive, in my opinion. The fluid from the variable volume pump goes to the wheel motors and right back to the variable volume pump. The case drain for the variable volume pump goes back to the hydraulic reservoir via the hydraulic cooler.

The charge pump in the variable volume pump sucks oil from the reservoir through the hydraulic filter to make up for the case drain losses in the variable volume pump. When mowing, the hot oil in the tank gets into the drive circuit pretty darn fast. The charge pump appears to be moving quite a bit of oil out of the tank. Obviously, the charge pump pumps more oil than the variable volume pump needs, or it would starve.

The PT-425 wheel motors have internal drains, if they have any at all. They have no external drain lines.

 

[Bob999]

The charge pump is designed to maintain supply fluid available to the variable displacement drive pump at all times. If fluid is not maintained by the charge pump at all times the variable displacement pump can suffer damage. The excess fluid provided by the charge pump flows through the hydraulic oil cooler and back to the sump. My understanding is that the hydraulic filter and the lines connecting it to the sump and to the charge pump are large to allow free flow of fluid at a very low pressure even when the fluid is cold because if there is not free flow there may be damage to the drive pump. The hydraulic filter and its connecting lines operate under vacuum because the charge pump is pulling oil from the sump through the filter.

 

[MossRoad]

Isn't there some debate regarding the use of suction side filters VS return line filters?

 

[SnowRidge]

Isn't there some debate regarding the use of suction side filters VS return line filters?

Yes, return line filters are considered safer because suction filters "may" cause cavitation, especially if they become partially clogged. But this is mainly with open loop systems. Suction filters are also available with bypass valves built in, which are supposed to protect against cavitation.

Personally, I think it is all a crap shoot, and we need Heart singing Magic Man in the background as an ode to the all knowing hydraulic guy who will fix our errant machines after we thoroughly hose them up .

 

[ponytug]

Following up on SnowRidge's comments, if we had return line filters on all of the connections that can be made/broken, we would have much less dirt/debris entering the system.

And, yes, if you have a suction line filter, you really want to make sure that you change it before it ever develops much of a pressure differential. Cold starts have got to be tough.

On the other hand, there is a horsepower/heating issue with a return line PTO filter, which has a pressure drop across it, which creates heat, and uses horsepower...

I admire the external filters that folks have built here. I think that they are a good idea. I have considered a cheapo alternative of a high pressure filter and some PTO QAs that could be hooked up for some filtering during non-PTO, non-HP limited activities.

All the best,

Peter

Isn't there some debate regarding the use of suction side filters VS return line filters?

 

[Bob999]

Isn't there some debate regarding the use of suction side filters VS return line filters?

I presume that is a rhetorical question.

There are tradeoffs in all equipment design. The variable displacement pump is very sensitive to foreign matter in the oil supply. Putting a filter on the supply provides about as high confidence that the oil supply will be clean as possible. The downside is the potential for cavitation. Return side filters don't cause cavitation--but they don't provide the same assurance of clean oil supply.

 

[marrt]

Bob999, your explanation finally makes sense to me. Thanks. You also added some information that I hadn't seen before...about excess fluid from the charge pump. It sounds like the charge pump is moving a lot of fluid. When I had a 425, I once cranked it in cold weather to let it warm up. I was amazed at how fast the oil reservoir got warm, even though the tractor was not moving. This suggested to me that a lot of oil was being moved and was under pressure to some degree. I guess the gear pumps are always working and moving oil around too.

A couple years ago, I bought a high pressure filter to add to the PTO. I never put it on because I couldn't figure out how to easily mount it. It would probably be a nice addition though.

 

[Bob999]

A couple years ago, I bought a high pressure filter to add to the PTO. I never put it on because I couldn't figure out how to easily mount it. It would probably be a nice addition though.

I mounted a high pressure filter in the return line on the mower. I fabricated a bracket to mount it to the mower. I used a high pressure filter because I was concerned that the quick connect at the tractor provides a significant amount of resistance and didn't want a low pressure filter failing from excess pressure. Because the mower is the primary attachment I use, as measured by hours, I get filtering on the PTO circuit most of the time.

 

[J_J]

The charge pump is a small pump, normally pumping about 10 percent of the gpm's of the VSP pump. The main purpose of the charge pump is to keep fluid at some pressure, about 320 psi if I remember correctly supplied to the VSP. because some fluid is lost through hydraulic wheel motors, and is also used to help cool the case side of the VSP pump. I believe the motor case lines come back to the pump case drain, or meet somewhere and then goes to the cooler. The charge pump doesn't need to be large, because it is only a makeup pump, making up what is sent out of the wheel motors case drains and pump case drain. Another thought here is that not all wheel motors have case drains, so maybe the charge pumps on that system might be smaller. My main thought here is that it is not pumping a lot of fluid all the time, like when sitting still, and therefore it is pumping less, and filtering less, just enough to keep the charge pump side pressurized.

In my system, the PTO and the steering pumps are always running. They will build up heat just sitting there with the engine running. If relief valves come on a lot they generate heat. Attachment motors working hard, generate heat.

Someone mentioned about putting a high pressure filter in the attachment return line. I suggest a better place would be in the return outlet on the PTO block, and that would catch any debris from any attachment. There should be a gage in the line before the filter to give you an indication when the high pressure filter might be getting clogged. If you do this , I would note down the normal operating psi of the attachment. If it starts to run higher, it might be time to install a new one. Yes, you could put one on each attachment, but it will get expensive.

If the 2 gpm charge pump was pumping fluid at full capacity, it could filter 20 gal of hydraulic fluid in 10 min. This can not happen in our system, because we are not filtering fluid in a linear fashion. Contaminants are not injected in a linear fashion, but random, like when plugging in the quick disconnects, or opening the tank cap. Otherwise, we don't filter the entire tank of fluid and then we quit, what we are doing is to keep mixing filtered fluid in with unfiltered, and I don't know the math to compute the total filtering sequence since the oil is circulating in a continuous loop. A visual concept would be to put some kind of dye in the hyd fluid, and run it through a filter to remove the dye, and see how long it takes to completely filter the dye from the 20 gal.