Confused about wheel motor closed loop circuit

[marrt]

As discussed numerous times, the wheel motor circuit is closed loop. So, when replacing a wheel motor, air can be introduced to the circuit. I assume there's a way for the air to be automatically purged by the VDP, probably by the charge pump circuit. Does anyone know how this works?

I once asked Terry if I should fill the wheel motor with oil when replacing. He said yes. But, he also said the pump would fill it with oil even if I didn't add the oil first. That makes sense, but how is the air removed from the closed loop? I'm asking all this because I'm replacing my four wheel motors. When done, I will then filter the circuit.

BTW, I happen to have an old "service manual" for the 18xx series. There's an image showing how to filter this circuit by connecting a filter directly to the VDP with a return going to the PTO return. Seems much more difficult than connecting the filter at a wheel motor. But I wonder if this is done to avoid introducing air into the circuit.

I'll snap a picture of that page if anyone is interested.

 

[MossRoad]

I'm guessing the air gets purged in the tram pump and by the charge pump pressurizing things. I'll have to read up on it. Your machine has three hoses on each wheel motor, correct? There's an IN, an OUT (really not in and out, as they are opposite when you reverse direction) and a case drain on the third hose, correct? It's my understanding the case drain just returns anything bypassed past the seals in the pump, but I could be mistaken. Can the case drains also move air back to the tank? Are the case drains part of the closed loop of the tram circuit, or do they drain back to the tank? Man, I gotta take a hydraulics basic course! :confused2:

 

[SpringHollow]

I certainly would be interested in a PDF of the service manual if one is available. I assume Carl's and others would be as well. I never such a thing existed.

Ken

 

[oldnslo]

Some wheel motors have a built in flushing valve that takes oil from the low pressure side of the circuit and dumps it into the motor housing to cool and flush the motor bearings. This oil comes out the case drain port. And air in the system will also be worked out over time through this flushing valve. If your motors have this flushing valve allow the system to run in neutral for a couple of minutes and then try to drive forward very slowly under no load for a couple of minutes and then go very slowly in reverse. This will help purge the air out through the flushing valve from both side of the circuit.


Filtering a closed loop system is a challenge since most filters will not allow reverse oil flow and you cannot dump one side of the loop to tank or you will destroy the main HST pump. Some companies makes filters designed for hydrostatic use but they are expensive. Best solution is to make 100% sure all of the hoses are clean as well as all of the fittings.

Does your machine have a charge pump filter? Some are on the inlet and others can be mounted right on the pump. These filter the oil going into the closed loop circuit and are used on most HST systems.

 

[woodlandfarms]

Filtration (IMO) is weak point of the PT. For most of us it is one main filter attached to the Tank and then the inlet side of the tram pump (which is running fulltime but no clue how much oil is really being recycled). I have on my machine a secondary High Pressure filter that is on the lift dump tilt steering circuit. I beleive that this filter is only on the slope machines and that they only did this for a period of time (slope mowers now are back to only one filter).

 

[marrt]

Here's some instructions I received:

Personally, I'm not a fan of connecting directly to the pump. That would be advantageous/necessary after a major failure. However, in my case, I just want to filter the old oil in the closed loop as preventative maintenance.

 

[marrt]

Does your machine have a charge pump filter? Some are on the inlet and others can be mounted right on the pump. These filter the oil going into the closed loop circuit and are used on most HST systems.

Yes, it has a suction side filter...but it's only 10 micron and filters the make up oil in the closed loop.

To that point however, this tractor does have case drains on all wheel motors. So, it's possible the closed loop does get new oil over time.

This wouldn't be true for my 425 though.

Regarding the filter, I built a filter using check valves that's reversible.

 

[SpringHollow]

Thanks for the images. I should ask Terry one of these days if they still have copies. Even if out of date for my model, some of it would be applicable I would think.

Ken

 

[marrt]

Thanks for the images. I should ask Terry one of these days if they still have copies. Even if out of date for my model, some of it would be applicable I would think.

Ken

Ken,

Sorry...I didn't respond to your comment. The "service manual" is mostly just a bunch of letters to their dealers (when they had dealers) covering specific topics...most, if not all, we have discussed here. I will look at it again and see if there's anything useful.

Tom

 

[ponytug]

Did you make a bi-directional filter like this?
PFHB Bi-Directional Flow Pressure Filter

All the best,

Peter

Yes, it has a suction side filter...but it's only 10 micron and filters the make up oil in the closed loop.

To that point however, this tractor does have case drains on all wheel motors. So, it's possible the closed loop does get new oil over time.

This wouldn't be true for my 425 though.

Regarding the filter, I built a filter using check valves that's reversible.