pressure difference in hydraulic motors

[greenerdreams]

Hello, It has been a while since i posted but i often lurk to get my questions answered, can't find what i am looking for so maybe there are some geniuses out there to help me.
I have a vermeer chipper that uses vertical shaft hydraulic feed wheels. Lately they seem to get weaker the longer i use the machine. I added a pressure gauge to see what is happening and i have the spec inlet pressure of approx 2500psi. when i run the feeders in forward the pressure to the dual motors is roughly 1000psi no load, in reverse it is almost zero. I am wondering if it is possible for the motors to have worn on the normal rotation(IN) and still be fresh on the rarely used reverse rotation(out) or maybe there is just a restriction in the valve that is not allowing unrestricted return flow.
My hydraulic knowledge is pretty limited but i am an engineer minded person(like most on here), I just don't know where to continue with my troubleshooting.
BTW, it seems when the system starts to loose power, the fluid is usually very hot, my guess would be the restriction causing the pressure is also overheating the hyd fluid.
i have considered swapping motors to effectively reverse the standard direction of each to see if the pressure issue switches from in circuit to out circuit.
filter has been changed but i have not checked the reservoir strainer yet.

 

[Industrial Toys]

I'm not guessing that a motor can wear out one direction only. I might be totally wrong but there may be valves and flow control to the infeed circuit. IF I was designing that, I would want to be able to control the feed pressure, but would want max force in reverse,

I found Vermeer really good to call up and pick their brains. Unlike JD or Kubota, who always say, bring it in!!

 

[Xfaxman]

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I have a vermeer chipper that uses vertical shaft hydraulic feed wheels. Lately they seem to get weaker the longer i use the machine. I added a pressure gauge to see what is happening and i have the spec inlet pressure of approx 2500psi. when i run the feeders in forward the pressure to the dual motors is roughly 1000psi no load, in reverse it is almost zero.
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Not sure where you measured the pressure, did you move the gauge to the other hose to check the pressure in reverse?

Does everything that turns in forward, also turn in reverse?

 

[oldnslo]

Greenerdreams,
A couple of questions
1) do the feed rolls run the same speed in forward and reverse?
2) Do you know if these are geroter / geroter style motors? I.e. low speed high torque

running slower with hot oil does indicate the oil is leaking or bypassing somewhere. The question is what is causing this heat?

If you have an infrared heat gun take temperatures on the different components in the circuit to see if one heats up quickly compared to the rest of the system. That might help determine if it is a failing motor or some other component

 

[greenerdreams]

Greenerdreams,
A couple of questions
1) do the feed rolls run the same speed in forward and reverse?
2) Do you know if these are geroter / geroter style motors? I.e. low speed high torque

running slower with hot oil does indicate the oil is leaking or bypassing somewhere. The question is what is causing this heat?

If you have an infrared heat gun take temperatures on the different components in the circuit to see if one heats up quickly compared to the rest of the system. That might help determine if it is a failing motor or some other component

thanks for the input so far. I had some time and did a little more work. changed fluid, looked good before but now is new. tank pickup was totally clean. changed the motors to see if any difference with each running opposite with no change. what i did notice, is when unhooking the valve control linkage, the valve is very touchy as to where it will operate in both forward and reverse, not sure if there is supposed to be detents but if there is they are not working anymore. I am now wondering if the valve is just very worn and leaking internally.

to answer the questions, the feeder can run same speed forward and reverse, they are very low speed high torque motors. I checked pressure at the input line to the valve.
i do think i will call vermeer to see if they can be of any help as well.

 

[Xfaxman]

Your "slowing down when hot" problem sounds like hydraulic fluid, instead of motor oil, in a gerotor motor.

Many years ago I bought a Case 446 tractor at an auction. It sold cheaper than expected with a new Linamar replacement engine. It mowed great for a half hour, then would slow to a crawl. I drained the hydraulic fluid, put in the recommended motor oil and it operated like a new machine.

 

[ddbackhoe]

Your "slowing down when hot" problem sounds like hydraulic fluid, instead of motor oil, in a gerotor motor.

Many years ago I bought a Case 446 tractor at an auction. It sold cheaper than expected with a new Linamar replacement engine. It mowed great for a half hour, then would slow to a crawl. I drained the hydraulic fluid, put in the recommended motor oil and it operated like a new machine.

Ahhh, wrong spec fluid. One transforms properties quicker (ie heat loss, viscosity, etc) than the other? Isnt non-detergent motor oil somewhat interchangeable with ISO (hydraulic) specs? 10W30 = ISO68 , etc?

 

[ericm979]

Somewhere I found a chart that shows the viscosity of motor oil and hydraulic fluid. Turns out that the motor oil spec has a wider range than the ISO spec, which is why there's not a 1:1 correspondence between motor oil and hydraulic oil. Here's some useful links I found (below)

a multi grade oil would help if the oil temp is high (see the last link with efficiency vs temp chart). But since it used to run fine and it doesn't now I'd talk to the manufacturer and see if they can provide advice for trouble shooting. Unless maybe you're using it differently the fluid should not be getting very hot. Dumb idea- is the level in the tank low? The tank is also the cooler in a lot of small hydraulic applications. A typical rule of thumb is to have 2x the pump gpm in the tank.

Can Engine Oils Replace Hydraulic Oils?
SAE-to-ISO Oil Conversion Chart | Blain's Farm & Fleet Blog
Hydraulic Fluid Chart Hydraulic Oil Table and Cross Reference

 

[greenerdreams]

Somewhere I found a chart that shows the viscosity of motor oil and hydraulic fluid. Turns out that the motor oil spec has a wider range than the ISO spec, which is why there's not a 1:1 correspondence between motor oil and hydraulic oil. Here's some useful links I found (below)

a multi grade oil would help if the oil temp is high (see the last link with efficiency vs temp chart). But since it used to run fine and it doesn't now I'd talk to the manufacturer and see if they can provide advice for trouble shooting. Unless maybe you're using it differently the fluid should not be getting very hot. Dumb idea- is the level in the tank low? The tank is also the cooler in a lot of small hydraulic applications. A typical rule of thumb is to have 2x the pump gpm in the tank.

Can Engine Oils Replace Hydraulic Oils?
SAE-to-ISO Oil Conversion Chart | Blain's Farm & Fleet Blog
Hydraulic Fluid Chart Hydraulic Oil Table and Cross Reference

thanks for the input, i was trying to determine correct fluid type via the manual, it states vermeer vmf or philips hg. I cannot come up with a cross reference for either on the net. I went with napa recommendation of aw46. I am not the original owner of the machine so i do not know what was in it before. The problem has occurred prior to changing fluid but since i don't know what was in it i can't rule out fluid. It still doesn't account for pressure diff between forward and reverse. fluid level is correct at the dipstick

 

[oldnslo]

thanks for the input so far. I had some time and did a little more work. changed fluid, looked good before but now is new. tank pickup was totally clean. changed the motors to see if any difference with each running opposite with no change. what i did notice, is when unhooking the valve control linkage, the valve is very touchy as to where it will operate in both forward and reverse, not sure if there is supposed to be detents but if there is they are not working anymore. I am now wondering if the valve is just very worn and leaking internally.

to answer the questions, the feeder can run same speed forward and reverse, they are very low speed high torque motors. I checked pressure at the input line to the valve.
i do think i will call vermeer to see if they can be of any help as well.

This may be a dumb question but, is the reversing valve before or after the valve that you installed the gauge in? Reason for asking is that on a unit I rented last year I believe the reversing valve fed the speed control valve so that reverse was full speed and forward was at a reduced speed. I could also be wrong on that thought.

I would be surprised if something in the motors would cause that large of a pressure difference from forward to reverse. Do you have schematic for this machine?

I suspect that AW46 oil would be OK unless you are in real warm environment.

good luck on finding the problem.

 

[MHarryE]

ISO 46 is kind of thin for your low speed high torque app. It’s 40 to 50 cSt or the low end of SAE 20 engine oil (35 to 75 cSt). A 15W-40 engine oil is good for your application although in a harsh hydraulic app the viscosity enhancers will shear down over time so you end up with 15W-30. The common farm hydraulic fluids are 55 cSt at 40 and 9 cSt at 100 so its normal to have speed loss with temperature rise due to the viscosity drop.

 

[greenerdreams]

the valve is after the speed control, i tested pressure at the inlet before the speed control. I am understanding now that the aw46 is possibly not the best product but i can't find a crossover chart for the recommended vermeer vhf or philips hg. the interesting thing is that after maybe less than an hours use the motors will lose so much power that they will stall on almost any size branch(when fully operational it can take over a 12" tree easily). the more i mess with the control valve the more i think it may be the majority of the problem via large internal leaking. I would try a rebuild but getting parts has been a problem. I am going to try to identify the model so i can maybe get a replacement from surplus center for cheap just to see what happens.

 

[ddbackhoe]

the valve is after the speed control, i tested pressure at the inlet before the speed control. I am understanding now that the aw46 is possibly not the best product but i can't find a crossover chart for the recommended vermeer vhf or philips hg. the interesting thing is that after maybe less than an hours use the motors will lose so much power that they will stall on almost any size branch(when fully operational it can take over a 12" tree easily). the more i mess with the control valve the more i think it may be the majority of the problem via large internal leaking. I would try a rebuild but getting parts has been a problem. I am going to try to identify the model so i can maybe get a replacement from surplus center for cheap just to see what happens.

This site has more info than you really want but it is a very good one.

Hydraulic Systems and Fluid Selection
Choosing the Right Hydraulic Fluid

 

[oldnslo]

the valve is after the speed control, i tested pressure at the inlet before the speed control. I am understanding now that the aw46 is possibly not the best product but i can't find a crossover chart for the recommended vermeer vhf or philips hg. the interesting thing is that after maybe less than an hours use the motors will lose so much power that they will stall on almost any size branch(when fully operational it can take over a 12" tree easily). the more i mess with the control valve the more i think it may be the majority of the problem via large internal leaking. I would try a rebuild but getting parts has been a problem. I am going to try to identify the model so i can maybe get a replacement from surplus center for cheap just to see what happens.

Greener,
I presume this unit worked well for you in the past and this problem is something new or has it never worked well?

before you try and rebuild the valve can you either take a couple more pressure readings or temperature readings to try and identify the source of the heat.

I suggest starting with the system cold and monitoring the temperature of the different components. If the valve shows rapid temperature climb Vs the other components that is where I would start looking for problems & or perform more tests to further isolate the problem area. Possible some linkage is not shifting the valve one way, did a spring keeper come loose, etc..

Another option is install multiple test points or gauges in the system to monitor pressure losses. I.e. one at the inlet to the valve in question and one at the port feeding the motors.