Front mower lift arm hydraulics question

[BeezFun]

I have a New Holland front mower, the schematic for the lift arm hydraulics is attached. When there is no implement on the front, even when the lift arm lever is in float position, I'm not able to push the lift arms down using my weight. I have to turn the engine off and stand on the lift arms to make them go down. Looking at attached schematic I think I understand why, but I'm not sure. To lower the arms, fluid has to enter port B on the lift cylinders, and be forced out of port A. That fluid has to go back through the oil cooler and into the reservoir. But that means the pressure on the A side of the lift cylinder is always slightly higher than on the B side, so it takes a fair bit of weight (more than a human) to make the lift arm go down. Does that seem right?


View attachment CM274 lift arm schematic.pdf

 

[J_J]

If you are using the FEL for the mower, the float mode couples the A port, the B port and the TANK, and should move freely up and down.

However,.

I see a valve in the mower circuit, and not sure what it does. Maybe flow adjuster., and maybe it is tighten down to much.

Read the manual and set that valve correctly.

 

[BeezFun]

If you are using the FEL for the mower, the float mode couples the A port, the B port and the TANK, and should move freely up and down.

It doesn't have a FEL, it's one of those front mowers like they use on golf courses. So that valve is only used to raise and lower the mower. I think that this valve is different than a "normal" FEL valve. The schematic does not show A and B connected to Tank in float mode, they are connected to the Weight Transfer Valve.

However,.
I see a valve in the mower circuit, and not sure what it does. Maybe flow adjuster., and maybe it is tighten down to much.
Read the manual and set that valve correctly.

That valve is used in float mode to provide a metered amount of lift to the mower deck so it doesn't gouge grass as it goes over bumps. I know how to adjust it, and I have it completely open.

The lift cylinders are the top two items in that schematic, each lift arm is a separate single acting cylinder. The thing I don't understand is why it appears to be designed so that the lift arms won't drop unless there's a fair bit of weight on them. The A port on the lift cylinder is always going to be at a slightly higher pressure than the B port, because the A port effectively connects to the upstream side of the oil cooler, and the B port connects to the downstream side of the oil cooler. So the pressure drop across the oil cooler will be equal to the pressure applied to side A of the lift cylinder, assuming the weight transfer valve is open all the way. I've confirmed the valve works correctly by using it to lift the implement in float mode, and then letting the implement drop when it's fully open.

 

[Farmerford]

There would be some pressure differential from the flow of the exhaust fluid through the oil cooler, but I doubt it is more than a few (less than 5?) psi. Greater pressure differential comes because it appears from the schematic that all exhaust fluid first goes to the hydrostatic transmission, probably to keep the transmisson pump and motor circuit charged with fluid. This pressure could be 100psi or more, created by a pressure regulating valve in the outflow circuit that allows the fluid not required to charge the HST to flow from the HST transmission to the sump.

 

[BeezFun]

There would be some pressure differential from the flow of the exhaust fluid through the oil cooler, but I doubt it is more than a few (less than 5?) psi. Greater pressure differential comes because it appears from the schematic that all exhaust fluid first goes to the hydrostatic transmission, probably to keep the transmisson pump and motor circuit charged with fluid. This pressure could be 100psi or more, created by a pressure regulating valve in the outflow circuit that allows the fluid not required to charge the HST to flow from the HST transmission to the sump.

You're right, that low pressure relief valve is shown to be set at 56-85psi, but I think that means that the lift cylinder ports A and B both see that same pressure, ignoring the small difference of passing through the weight transfer valve and oil cooler. I think I found the answer to my question in a paragraph in the service manual: "Oil in port P is fed oil from the rod side of the lift cylinder port A by the WEIGHT OF THE IMPLEMENT."

I must be missing something about why this system would be designed this way. It means that the only way to mount the mower is drive close to the mower, turn the engine off so the lift arms will drop, and rent a gorilla to slide the mower into the lift arms. I bought this thing used and this is the first time I took the mower off, what a pain.

 

[Farmerford]

You are correct that the HST charge pressure will appear on both sides of the piston; but the area differential favors an extension of the cylinder, which it appears will lower the arms. Assume a 2inch diameter piston with a 1 inch rod and 85 psi pressure (created by the HST charge pressure) on each side in the float mode. The rod area is roughly 3/4 square inch, so the base end of the cylinder has 3/4 square inch more area than the rod end; for two cylinders this is 1 1/2 square inches times 85 psi = 128 pounds of down force. If the cylinder does not extend with that force plus your weight on it, something must be creating significantly more pressure than the "system" pressure of 85psi on the rod end.

I disagree that a quick drop when you move the valve to "lower" shows no restriction in an "open" weight transfer valve; when the control valve is shifted to "lower" the exhaust fluid from the rod end of the cylinders flows around the weight transfer valve. And the pressure differential on the piston should favor the drop, not oppose it.I believe that comes from the restriction in the weight transfer valve that must remain even when the valve appears to be fully open. It would not be unsual if the weight transfer valve had restrictions, either deliberate, or just because of design, that did not dissapear after the valve were opened. If you look closely at the weight transfer valve, there appears to be a restrictive orifice where the fluid enters the valve. Even when the engine is at idle, the pump flow goes through the valve, and the valve pressure acts on the rod side of the piston, which tends to hold the arms up. If the pressure drop on an open weight transfer valve were only 100psi, that would create 300 # force to extend each 2 inch cylinder.

 

[BeezFun]

You are correct that the HST charge pressure will appear on both sides of the piston; but the area differential favors an extension of the cylinder, which it appears will lower the arms.

Yes, thanks, I didn't think about that.

I disagree that a quick drop when you move the valve to "lower" shows no restriction in an "open" weight transfer valve; when the control valve is shifted to "lower" the exhaust fluid from the rod end of the cylinders flows around the weight transfer valve. And the pressure differential on the piston should favor the drop, not oppose it.I believe that comes from the restriction in the weight transfer valve that must remain even when the valve appears to be fully open. It would not be unsual if the weight transfer valve had restrictions, either deliberate, or just because of design, that did not dissapear after the valve were opened. If you look closely at the weight transfer valve, there appears to be a restrictive orifice where the fluid enters the valve. Even when the engine is at idle, the pump flow goes through the valve, and the valve pressure acts on the rod side of the piston, which tends to hold the arms up. If the pressure drop on an open weight transfer valve were only 100psi, that would create 300 # force to extend each 2 inch cylinder.

Yes, thanks for that thoughtful analysis. I agree with you, so now back to my original problem, why don't the arms go down when there is no implement on the front? Even my weight by itself is not enough to lower the arm when it's in float. Once I turn the engine off, both arms bleed down to the lowest position. I'm going to spend more time on this with the implement on and off to see if I'm mistaken about what's going on.
thanks

 

[BeezFun]

Here's what I observe:
In float mode with weight adjustment valve fully open it takes about 50# on each lift arm to make them go down.
In float mode with weight adjustment valve fully closed no amount of weight will make the lift arms go down.

I can't explain the behavior, other than to guess that there may be an obstruction in the weight adjustment valve that's reducing the maximum flow.

 

[Farmerford]

I think you are correct; pressure on the rod side of the piston is created by the restriction to flow in the weight transfer valve even when it is fully opened. The think we don't know is whether that restriction is designed into the valve, perhaps by an orifice in the flow path mentioned above, or whether the restriction results from the fact that even fully opened the valve has a significant restriction to flow. If you dissasembled the valve you should be able to see any orifice restriction, and perhaps drill it out. If the valve itself is causing the restriction, you will have to install some sort of bypass around the valve that is available to the fluid when you want the arms to drop to attach the mower. A globe valve would do that, but whether it is easy or hard to install one depends on where the weight transfer valve is located and what kind of fittings it has. I suspect it is connected with hard lines and banjo fittings, which are the most difficult to modify. But slip on compression fittings are available; if you cut the lines going into and out of the weight transfer valve and insert tees. it should not be hard with hoses to install a globe valve between the tees to create a bypass circuit.

 

[BeezFun]

If the valve itself is causing the restriction, you will have to install some sort of bypass around the valve that is available to the fluid when you want the arms to drop to attach the mower. A globe valve would do that, but whether it is easy or hard to install one depends on where the weight transfer valve is located and what kind of fittings it has. I suspect it is connected with hard lines and banjo fittings, which are the most difficult to modify.

Your guess is right, it's plumbed in hard in a very congested spot. I played around this morning, best approach is to let the arms bleed down with engine off, then as I approach the implement slowly raise them until implement slides on the arm. Not the greatest but it works. Once the implement is on everything works as I would expect.
Although many things on this machine are designed and built like a tank, the ergonomics were designed for a contortionist yoga instructor, and an assistant.