Hydraulic filtration

Bob999 · Aug 20, 2003

Power Trac only filters the suction line supply for the variable displacement pump. There are no filters in the PTO pump circuit or the auxiliary pump circuit. I don't know a lot about hydraulic systems but it seems like it would be beneficial to have additional filters in the system.

1. Has anyone opened up the hydraulic reservoir and determined whether there are strainers installed on the suction lines?

2. What are the considerations to install a return line filter on the PTO circuit? If one were installed could it be installed on the outlet port of the hydraulic motor of the mower?

wasabi · Aug 20, 2003

Working on maintaining our PT is my first real foray into messing with hydraulics, so I'm by no means an expert. That said, I too think some additional filtering would be beneficial. I ran the idea up the flag pole with Terry at PT and he cautioned that additional filters could slow the flow.

My summary conclusion is to plumb in two way valves below the control console along with an extra filter to create an easily toggled "purge system" for periodic bleeding and cleansing. I have designed it but not yet installed it.

Others with more hydraulic experience may have better ideas.

Sabi

smartguyz · Aug 20, 2003

Hi Bob,

I'm not sure I understand. I know there are two pumps - one that runs the wheels, and one that runs everything else. I realize that you're saying that there is no filter going to 'goes into' side of the other pump, but eventually, doesn't it all circulate through the same reservoir? Don't both of these pumps draw from the same reservoir? Is this reservoir not filtered? Why do we need another filter?

I'm confused.

-Rob /forums/images/graemlins/blush.gif

Bob999 · Aug 20, 2003

I think there are actually three pumps, and two of them have no filtering so far as I know.

You are correct that there is a common reservoir and that fluid from that common supply is subject to filtering when it passes through the suction line filter for the variable volume pump--the one that powers the wheel motors.

However the greatest flow is to the PTO pump--8, 12, 15 gals per minute depending on the model. This flow is, as I understand it, totally unfiltered, and I am wondering if it would be beneficial to have a filter in this circuit.

The third pump powers the lift and steering circuits and has very little flow.

PT uses a suction line filter for the variable volume pump because this design pump is very suseptible to damage from foreign materal. However suction line filters are not generally used because they can cause cavitation which can quickly destroy a pump. I assume that the frequent filter changes that PT requires are designed to change the filter before it loads up and causes cavitation.

The more typical approach is to use a return line filter and that is the approach I was wondering about for the PTO circuit.

smartguyz · Aug 20, 2003

Hi Bob,

Thanks for the clarification. I guess my next series of questions would be:

1) Is the average flow rate for the variable pump sufficient to cause an adequate overall filtration of the hydraulic oil?

2) For example, how many times per hour does this typically get filtered?

3) Would this amount be sufficient to catch any foreign material and keep it from damaging any of the systems? If this is so, then I would assume that additional filtering is superfluous.

4) Is the flow through the filter continuous, regardless of whether the wheel motors are in motion?

I suppose this series of questions simply shows that I am not completely familiar with all the design aspects of my little tractor.

Thanks,

Rob /forums/images/graemlins/smile.gif

Sedgewood · Aug 21, 2003

Bob999: <font color="red"> 1. Has anyone opened up the hydraulic reservoir and determined whether there are strainers installed on the suction lines? </font>

To answer the easier question here, no, there are no strainers on the suction lines. I had my 1845's cover off last week to check on things in the reservoir and to reseal the leaking cover while the system is empty. The suction line ports are just pipe couplings welded into the side of the tank with nothing attached to the inside end.

Here is a picture from outside - it never occurred to me to get one of the inside of the tank. It's just a welded up tank with bits of slag lying about - kinda like some of you have found with the welded fuel tanks /forums/images/graemlins/tongue.gif The largest port - in the center - feeds the PTO pump. Why it's so large (1-1/2 inch) I don't know: the pump output hose is 3/4 inch and 3/4 not being readily available to me in a 3000 psi tight bend radius hose, my new hose from the RPS valve to the quick connect is 5/8 inch, which my supplier says is more than adequate for 15 gpm.

I too have been wondering about the lack of filtration on the other two pumps and my thought is that the oil all goes through the variable volume pump on often enough to catch all but the most recently introduced crud. The biggest sources of contaminants, assuming no component is eating itself, I would think are at the quick connects and the fill cap.

Other concerns might be that there is no filtration on the breather, and no water vapor control. Water in the oil can evaporate at high temps and cause cavitation.

35 hoses installed - 2 to go,
Sedgewood

MossRoad · Aug 21, 2003

I think the variable volume pump is always sucking fluid, even if you aren't moving. It is the movement of the treadle pedal that changes the angle of the swash plate to divert fluid to make it go forwards or backwards. In the Neutral position, the fluid just passes through the pump and not through the wheel motors. So the vairable volume pump is always acting as a filter pump as well.

Does this sound correct?

Bob999 · Aug 21, 2003

My understanding of the operation of variable volume pumps is that they do not pump when the swash plate is in the neutral position--hence the name "variable volume". If you look at the diagram of a variable volume pump you will see it has several pistons that are driven by the swash plate. If the swash plate is in "neutral" the pistons are not actuated and no fluid is pumped. As the swash plate is moved the pistons begin to move and pump.

If the above explanation is correct then the filtration only occurs when the PT treadle is actuated, and the amount of fluid filtered is proportional to the displacement of the treadle.

It appears that the PT designers concluded (or assumed) that there is sufficient flow through the variable volume pump--thus they did not put any strainers (thank you Sedgewood) or other filters in the system.

The recent reports of significant foriegn material in the fuel tanks and hydraulic tanks got me to thinking that additional filtration my be desirable. I haven't found any definitive answers and I have not found any guides to making the evaluation.---Hence the decision to post the question here.

MossRoad · Aug 21, 2003

I agree, however, the motor is still spinning the pump. I think the fluid just passes through the pump when the treadle is in neutral. So fluid is always moving through it, it is just not being directed towards the wheel motors. I know fliud is moving through it because in the winter, I start it up and let it run at about 1/3 throttle for 10 minnutes. When I come back, the fluid is toasty warm. So I'm assuming the fluid has been pushed under pressure through something for that 10 minutes.

smartguyz · Aug 21, 2003

David,

I agree with you. I don't know the exact details of where it goes, but I have the exact same experience. I have simply started the engine, and then let it sit a high idle for 20 minutes while I went off to do something else. When I return, the coolant fan is running - this means that the temp sensor has been activated because of hitting the trip point for the temperature sensor. So, if it is not being moved under pressure, where is it going? The pump IS turning, so the oil is going somewhere.

Anyone know?

-Rob /forums/images/graemlins/grin.gif