Hydraulic filtration

[Bob999]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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

 

[Bob999]

Mossroad and Smartguyz:

My understanding is that the PTO pump and the Auxiliary pump are always pumping--thus the phenemon you report. Also the variable displacement pump is always spinning--all of the pumps are driven by the same shaft--but it does not pump fluid in "neutral".

In the case of the PTO pump there is an electrically actuated switch that controls whether the pump simply circulates back to the reservoir, thereby consuming little power, or flows to the attachment. PT cautions against actuating the PTO switch without having an attachment because if there is no attachment then pressure quickly builds up and the safety valve is actuated.

The auxiliary pump is "always on"--that is why you can steer or lift the arms at any time the motor is running.

But, I continue to believe, the variable displacement pump is not pumping any fluid when the treadle is centered.

 

[MossRoad]

OK, here's a way too cool link to how a variable volume pump works.
http://science.howstuffworks.com/backhoe-loader8.htm It even has a movie of a cutaway pump in action. When the swash plate is even, no fluid is pumping. So what is heating up the hydraulic fluid while idling? Hmmm...

 

[MossRoad]

Also, I just posted this link in the related topics forum. It describes how skid steers work. There is some good information for everyone in there. Check it out.

 

[Bob999]

<font color="red"> 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? </font>

I think that the fluid flow for the cooler circuit is taken from either the PTO or Auxiliary pump circuit. I do not believe that it is taken from the wheel drive circuit (variable displacement pump). I went to my PT manual to find the answer and found that no diagram in my book includes the cooler!!!! I do have what appears to be a complete diagram of the wheel motor circuit and the cooler is not shown.

Perhaps the manual for the 425 does include this.

 

[Sedgewood]

Bob999: <font color="red"> I think that the fluid flow for the cooler circuit is taken from either the PTO or Auxiliary pump circuit. I do not believe that it is taken from the wheel drive circuit (variable displacement pump).</font> Actually it is taken from the variable displacement pump here.

<font color="red"> I went to my PT manual to find the answer and found that no diagram in my book includes the cooler!!!! </font> Sounds like you might be missing a page of pretty lousy drawings - it's on drawing H077x2, Power Take-Off Circuit, even though it's not connected to the PTO! - go figger. (BTW, for the curious, the cooler fan thermostat is attached to the aluminum block of the cooler bypass valve here - well sort of - it doesn't actually touch the block - it stands off about an eighth of an inch for some reason) Now that I've become so intimate with the plumbing of my 1845 I'm going to try and draw up something a bit easier to follow (and correct a few mistakes while I'm at it). I'll post the results on my PT wiki when I get it done.

Up to his ears in hoses,
Sedgewood

 

[Bob999]

<font color="red"> Sounds like you might be missing a page of pretty lousy drawings </font>

I double checked--no page H077x2--but it is listed in the index!!

Thanks for the correct information.

I had previously found that the quick attach circuit is not shown in my manual and asked Terry about it. He confirmed it was not there but iqnored my request for a copy of it.

Taking the cooling flow from the variable volume pump means (I think) that there is no flow in this circuit when the machine is sitting still. If that is true then there is a potential for significant heat build up in the hydraulic oil if the unit is left sitting, but running, for a significnat period of time. Of course on the 1845 you have to be sitting in the seat, or defeat the seat switch, to have the machine running when you are not in the seat.

 

[Sedgewood]

I put together a quick hydraulic cooler circuit and posted it here. it's pretty rudimentary and I'll improve on it when I get some time.

<font color="red">I double checked--no page H077x2--but it is listed in the index!! </font>

I'd be happy to scan any pages you're missing and email them to you.

<font color="red">Taking the cooling flow from the variable volume pump means (I think) that there is no flow in this circuit when the machine is sitting still. If that is true then there is a potential for significant heat build up in the hydraulic oil if the unit is left sitting, but running, for a significnat period of time</font>

I'm don't know how the oil flows through the variable volume pump but it's possible there is a continuous flow to the cooler from there.

<font color="red">Of course on the 1845 you have to be sitting in the seat, or defeat the seat switch, to have the machine running when you are not in the seat. </font>

Some of us without tilt seats don't have that little problem - we just get to hang on tighter on slopes /forums/images/graemlins/tongue.gif

Sedgewood

 

[duane]

Bob999,

The PTs actually have 4 pumps. The Tandem PTO/lift steering pump, and the variable volume pump with suction charge pump. The charge pump provides suction flow to the variable volume pump. (when you change your hydraulic filter, the jumper line to the reservoir is discharging the charge pump discharge to purge any air) The charge pump is always pumping oil. In a non tredle position, the charge pump recircs all oil flow back through the oil cooler to the reservoir.

Duane

 

[MossRoad]

<font color="blue"> The PTs actually have 4 pumps. The Tandem PTO/lift steering pump, and the variable volume pump with suction charge pump. The charge pump provides suction flow to the variable volume pump. (when you change your hydraulic filter, the jumper line to the reservoir is discharging the charge pump discharge to purge any air) The charge pump is always pumping oil. In a non tredle position, the charge pump recircs all oil flow back through the oil cooler to the reservoir. </font>
Duane's Da Man!!! Thanks for the explaination /forums/images/graemlins/laugh.gif

 

[Bob999]

Duane,

Thank you.

Bob