Question about "float" position

[KentT]

Mad,

I'm not sure you understand hydraulics either, given those assertations There is always fluid on both sides of the piston, period. If there were air or anything else, you'd have to bleed it out for the system to work, because it would compress (which fluids do not, the basic premise of using them). Further, the volume on one side of the piston is larger than on the other, due to the displacement of the rod, but that's a whole 'nuther issue...

The fluid flowing through hoses and fittings when the piston moves causes a momentary increase in pressure -- you say it isn't meaningful, then go try pushing or pulling a cylinder. It'll feel somewhat like trying to push or pull a shock absorber, which it is, only with larger "jets" than a shock has....

I'm tired of arguing with you -- go try pushing or pulling a cylinder for yourself. You'll feel resistance to movement.... I promise!

 

[KentT]

Assuming there would be no buffering effect of the cylinders (I think there would be some) do you think that the "dead weight" of the lift arms would be too much on the "beefed up" tubular frame of a "typical" 200# capacity tow-behind broadcast spreader (12" pneumatic tires)? My idea would be to extend and crossbrace the 2" receiver tubing back near hopper of the spreader to take torque off the thinner tubular frame of the spreader.

If too much weight, my second thought would be some sort of hinge in the male receiver tube that would allow maybe 4-5" of vertical play of the spreader with the loader arms held at a midpoint of the hinge travel. I assume that the loader arms stay pretty much where you set them when the hydraulics are pumping. Thoughts anyone?

With those 12" pneumatic tires, it will likely not cause a problem with them "digging in" which is what happens with the much smaller guide wheels on the rough-cut mower, etc.

 

[MossRoad]

I'm pretty sure Kent T. is correct. These are double acting cylinders. There is oil on each side of the piston that is inside the cylinder at all times. If any oil leaves one side of the cylinder, oil fills the other side of the cylinder. There's no way around that.

Mad's argument might be correct for a single acting cylinder, but not these double acting cylinders on the PT.

 

[MadReferee]

One last time then I will go away. When the spool is in float both work ports are connected to tank or the outlet if the loader valve does not have power beyond. That is an absolute fact and is governed by the design of the spool. When placed into float, gravity causes the loader to fall and all fluid from either side of the cylinder goes to the tank because it has no other place to go. One side of the cylinder does not siphon fluid from the tank because on most systems that is impossible. In most all loader designs I have seen, the loader lift/lower cylinders retract when lowered. No bleeding is necessary because the cylinders are self bleeding when they are used.

There is also no difference on how float will work for an open or closed center system. We are talking about work port circuits here and the open/closed center design of the pressure fluid flow has absolutely nothing to do with float which is a spool function. And yes I have pushed and pulled on cylinders when they are disconnected and the rod is moveable. When they are connected to tank it is just like being disconnected.

 

[BobRip]

One last time then I will go away. When the spool is in float both work ports are connected to tank or the outlet if the loader valve does not have power beyond. That is an absolute fact and is governed by the design of the spool. When placed into float, gravity causes the loader to fall and all fluid from either side of the cylinder goes to the tank because it has no other place to go. One side of the cylinder does not siphon fluid from the tank because on most systems that is impossible. In most all loader designs I have seen, the loader lift/lower cylinders retract when lowered. No bleeding is necessary because the cylinders are self bleeding when they are used.

There is also no difference on how float will work for an open or closed center system. We are talking about work port circuits here and the open/closed center design of the pressure fluid flow has absolutely nothing to do with float which is a spool function. And yes I have pushed and pulled on cylinders when they are disconnected and the rod is moveable. When they are connected to tank it is just like being disconnected.

I think both of you have a good understanding of hydraulics. What will happen is a dynamic loading (moving loading) which is greater when the piston is move quickly rather than slowly. This may only be a small amount of force, but theortically it is there. It takes energy to move a fluid, but I think the pressure drop or force is low so the energy required is low. I call this a draw and let's just get along.

 

[MossRoad]

One last time then I will go away. When the spool is in float both work ports are connected to tank or the outlet if the loader valve does not have power beyond. That is an absolute fact and is governed by the design of the spool. When placed into float, gravity causes the loader to fall and all fluid from either side of the cylinder goes to the tank because it has no other place to go. One side of the cylinder does not siphon fluid from the tank because on most systems that is impossible. In most all loader designs I have seen, the loader lift/lower cylinders retract when lowered. No bleeding is necessary because the cylinders are self bleeding when they are used.

There is also no difference on how float will work for an open or closed center system. We are talking about work port circuits here and the open/closed center design of the pressure fluid flow has absolutely nothing to do with float which is a spool function. And yes I have pushed and pulled on cylinders when they are disconnected and the rod is moveable. When they are connected to tank it is just like being disconnected.

When the spool is in float all fluid from both sides of the cylinder is free to return to the tank. That doesn't mean it does.

To test this, take your FEL and raise the bucket all the way up and shut off the engine. With the engine off, shove the valve to the float position and drop the FEL to the ground. Now go disconnect the hydraulic cylinder and remove it from the tractor. Operate the cylinder one full stroke by hand and see if you don't get a big old puddle of hydraulic fluid on your garage floor. By your reasoning, there should be no fluid in that cylinder because it all drained back to the tank. Yet one side of the cylinder is completely full of hydraulic fluid. Why?

 

[KentT]

Yet one side of the cylinder is completely full of hydraulic fluid. Why?

 

[MadReferee]

When the spool is in float all fluid from both sides of the cylinder is free to return to the tank. That doesn't mean it does.

To test this, take your FEL and raise the bucket all the way up and shut off the engine. With the engine off, shove the valve to the float position and drop the FEL to the ground. Now go disconnect the hydraulic cylinder and remove it from the tractor. Operate the cylinder one full stroke by hand and see if you don't get a big old puddle of hydraulic fluid on your garage floor. By your reasoning, there should be no fluid in that cylinder because it all drained back to the tank. Yet one side of the cylinder is completely full of hydraulic fluid. Why?

I never said that ALL of the fluid returns to tank. What I said was that (a) fluid does not transfer from one side to the other, and (b) fluid is not siphoned from the tank into one side of the cylinder. You will find that one side is not completely full, in fact it will be only have a small amount of fluid in it. In any case, on most loaders the cylinders are not fully retracted when resting on the ground so there is bound to be some fluid left. Furthermore, the pressure of the fluid returning to the tank is commonly called neutral pressure, and for a good reason. There is really not enough pressure there to operate a cylinder. That's one of the reasons on valves without power beyond that any valve further downstream may or may not work properly when both valves are used simultaneously.

 

[MossRoad]

Yes, fluid will travel from one side of the piston to the other via the open return port, and no, it will not be siphoned, it will be sucked under pressure.

For example, let's say the cylinder is fully retracted and has two ports: port A is on the left and port B is on the right.

As the cylinder extends under pressure, the valve is directing fluid into port A. This causes the fluid on the B side to exit the cylinder and go out the return port on the valve back to the tank through the return hose.

Likewise, when the valve is operated in the opposite direction, fluid under pressure enters port B, which forces fluid out port A and through the return port on the valve, back to the tank through the return hose.

Now if we put the valve in float, both ports A & B are connected to the return port. That means they are also connected together, similar to a Y, like this:

​

There is nothing preventing fluid from going from point A to point B when the valve is in float. Therefore, when the cylinder extends and contracts due to the movement of the implement going up and down with the contours of the ground, fluid is free to travel to and from the A and B sides of the cylinder as well as back from the return, as there is nothing to prevent this.

 

[SPYDERLK]

Assuming fluid is returned to the tank below the level in the tank, the cylinder will try to suck it back. The thing is, unless the cylinder moves very slowly, the fluid will not be able to move back fast enuf under atmospheric pressure. A vacuum bubble will form and air may leak in to fill it, but this effect should be pretty slight. More likely is that the return to tank is not submerged much, or at all, and air is sucked back. Note that altho effectively connected together the 2 sides are not the same volume and fluid must go to and from the tank to keep both sides full. Im thinking the bubble comes from the tank.
Larry