Hydraulic problem

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Dale_F
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Hydraulic problem

Post by Dale_F » Sun Dec 17, 2017 3:47 pm

I am helping a friend with an electric / hydraulic problem. The locomotive has an electric motor, controlled by a speed control. The Electric motor turns a hydraulic pump (Eaton V10 vane pump), which goes thru a solenoid used for forward / reverse / bypass to the motors. I have attached a flow diagram that hopefully explains the circuit.

The problem is the pump almost seems like it loses prime - since it does not start pumping oil as soon as the electric motor/pump starts. It might take 5 or even 10 seconds to start pumping. Once it starts, the loco runs fine - fast, slow, fast again - all OK. But if you stop, then it may, or may not start immediately start pumping oil. It should in no way lose prime since the bottom of the return tank is above the motor! It is constantly flooded.
hydraulic.JPG
One thing I noticed is if I was coming down grade and cut back the throttle, causing the motor to stop, with the loco coasting, when I applied power, the problem was very noticeable. What happened when I was coasting is the motors were still turning, but the pump was not. The motors must have been moving oil, but from where and to where is unknown.

My goal is to start the pump up and have the loco start moving. It should not seem like I need to prime the pump for 5 to 10 seconds before the pump starts moving oil.

I doubt this explanation makes a lot of sense, but I need help from someone when hydraulic circuit experience. Please ask questions, but most importantly, please help!
Thanks.
Dale.

rkcarguy
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Re: Hydraulic problem

Post by rkcarguy » Sun Dec 17, 2017 4:14 pm

Check valve has failed? This could allow a siphon effect to partially drain the system.

Dale_F
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Re: Hydraulic problem

Post by Dale_F » Sun Dec 17, 2017 4:22 pm

The check valve has been checked several times and is clean and working correctly.

John Hasler
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Re: Hydraulic problem

Post by John Hasler » Sun Dec 17, 2017 5:00 pm

When you were coasting the motors were pulling a vacuum on everything from the last motor in the loop back to (and including) the pump. When you restart the motor you're running dry until everything fills up again (unless leakage refilled the system while you were stopped). Add a check valve in parallel with the pump so that the motors can idle.

Dale_F
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Re: Hydraulic problem

Post by Dale_F » Sun Dec 17, 2017 5:11 pm

John, I added a path in red to the drawing. Is this where you meant I should add the check valve???
hydraulic2.jpg
Thanks.
Dale.

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Dave_C
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Re: Hydraulic problem

Post by Dave_C » Sun Dec 17, 2017 6:55 pm

I think your red line position will work. Here is most likely what is happening:

Hydraulic fluids act like solids which is why they work as they due under pressure. They work the same way under vacuum! They remain as a solid unless you pull a low enough vacuum to cause the oil to vaporize. (not likely here)

What I think is happening is that your pump seal is a one lip pressure seal. It seals great on the hydraulic pressure side but when the train free wheels it draws a suction against the pump. This lets the seal pass air into the system and then it takes some time to purge out the air which goes to the tank and is vented out.

Adding the check valve as drawn above should stop the problem as the train can free wheel and just draw hydraulic fluid from the tank instead of introducing air into the system.

Dave C.
I learn something new every day! Problem is I forget two.

John Hasler
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Re: Hydraulic problem

Post by John Hasler » Mon Dec 18, 2017 12:23 am

In this context "vacuum" just means pressure below atmospheric. I just meant that the motors pull a vacuum which results in the system filling with low pressure gas, which can come from air/volatiles/water in the oil and/or air leaking in. This is very hard on your pump and motors. You always want at least a little positive pressure throughout your system at all times.

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BigDumbDinosaur
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Re: Hydraulic problem

Post by BigDumbDinosaur » Mon Dec 18, 2017 1:22 am

Dale_F wrote:I am helping a friend with an electric / hydraulic problem...
Observations:
  1. In your application, if I correctly understand how it is being operated, power is being cut to the motor ("prime mover") that drives the pump when it is desired to drift the locomotive. The fact that the reverser valve maintains a closed circuit is likely the root cause of your problem, as the momentum of the locomotive is overhauling the pump—in power transmission lingo, "overhaul" is what happens when power is cut and the load drives the propulsion system.

    This is not a good application for a vane pump. Vane pumps are primarily meant for continuous run applications, not start-and-stop, as the vane design tends to lose prime when the pump is stopped. Furthermore, overhauling a vane pump may cause the vanes to unseat from the ring (or the slide to reverse, if a variable displacement pump) due to the generation of a partial vacuum on the outlet. The result is a rapid loss of prime.

    In order for your propulsion system to function as desired, a open-center, three-position valve is required to set up an alternate path for oil flow when the pump is not being driven. That valve must have an open center in neutral so the locomotive can drift without overhauling the pump. When the prime mover is stopped the valve must be shifted to neutral, which means the traction motors will then freely circulate oil between themselves through the valve, rather than attempt to maintain the closed loop flow that was occurring while the prime mover was running.

    In addition to the valving change, I recommend you replace the vane pump with a gear pump of slightly lower displacement. Unlike a vane pump, a gear pump is not dependent on rotation to maintain a seal between inlet and outlet, and will not lose prime should a partial vacuum be pulled on its outlet (note that allowing that to happen may cause pump damage).
  2. The return line check valve is unnecessary in a properly-designed hydrostatic power transmission system—it serves absolutely no purpose except to add undesirable restriction. The return line should always be unobstructed and of sufficient size to prevent the formation of back pressure. The point on the reservoir at which the return line is connected should always be below the oil level to prevent aeration and foaming. Likewise, the pump inlet should be well below the running oil level to prevent damaging inlet cavitation.
If possible, please post some high resolution photos of the piping, valve, pump and reservoir.
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makinsmoke
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Re: Hydraulic problem

Post by makinsmoke » Mon Dec 18, 2017 8:49 am

I agree with changing the controls to a three way
valve.

Most gas hydraulic applications in our hobby allow the hydraulic pump to run continuously.
Rpm and flow is controlled by engine or in your case motor rpm. The direction of flow is controlled separately.

Introducing air into the system downstream of the tank can also cause cavitation which can and will severely damage or destroy hydraulic components, especially the pump.

im wondering why the system was not built as all electric if that is the main power supply. Hydraulic motors are usually used when the prime mover is an internal combustion engine/hydraulic pump combo.

Dale_F
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Re: Hydraulic problem

Post by Dale_F » Mon Dec 18, 2017 9:55 am

I want to clarify that the solenoid is an electrically operated, 3 position valve. My choices are Fwd, Back and center where the oil from the pump just returns to the tank.

The check valve on the return line was added by me in hopes that somehow the oil would be running backwards thru the f/r valve, "losing prime". It made no difference, but I was guessing, and it kind of made sense at the time.

The pump is below the bottom of the tank, and within a foot's distance. I will take photos the best I can later today, when the garage is lighter and warmer.

This loco is pretty much "as delivered", built by one of the suppliers to the hobby. I will not name the company here. So this is the way it has always been. I do not know the reasons why it was built this way.

I can switch to neutral when running, but that means I need to switch every time I throttle back - where the flow thru the motor is less than the pumps are passing. But I am not sure that is a good option. It sure would be hard to remember to do.

Given this setup, what do I need to do to get it running - or is there anything that can be done? Would replacing the vane pump with a geared pump work? The Eaton v10 is rated at 5gpm. Can you suggest a vendor or part number?

And thanks for all the help.
Dale F

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cbrew
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Re: Hydraulic problem

Post by cbrew » Mon Dec 18, 2017 10:17 am

I may have missed it, but one thing that is not accounted for is the V10 and V20 pumps have a min rpm. which is 200 rpm.
In this application, Maybe a fixed displacement piston or gear pump would have been a better pick.
If it is not live steam. its not worth it.

Dale_F
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Re: Hydraulic problem

Post by Dale_F » Mon Dec 18, 2017 10:25 am

Chris - you are correct. There is no provision for a min rpm -- or idle speed in this setup. To go, you run the electric motor when turns the pump. When you stop, the pump stops. No idle speed.

I see a trend towards replacing the vane pump with a geared pump (if 2 votes is a trend!). I will keep "voting" open in case someone has a cheaper, less messy option!

Seriously - thanks to everyone who has read this, and to those who have offered help.

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