Hows does a float circuit work

[woodlandfarms]

I have looked on the net and can't find what I am looking for.

When I put my bucket into float mode, what happens? Does the pressure that is going into the control valve get dumped back to the tank and the up down circuit just get connected together or is something else happening?

 

[Tx Jim]

When the spool on hyd control valve is pushed forward and locked in detent both "up & down lands on the spool are connected" to allow oil flow to travel in/out of both hyd hose ports of the boom cylinders so pistons are free to move back & forth in cylinder tubes.

 

[Industrial Toys]

I guess you mean put the boom into float, not the bucket, but Jim explained it well.

 

[J_J]

I have looked on the net and can't find what I am looking for.

When I put my bucket into float mode, what happens? Does the pressure that is going into the control valve get dumped back to the tank and the up down circuit just get connected together or is something else happening?

The float position allow the work ports/cyl fluid to flow to tank.

Fluid is still flowing through the valve for other spools.

There is no pressure until a cyl develops it.

Your lift arms will follow the conture of the ground. The movement of the bucket up/down will force fluid in and out of the lift cyl.

 

[Tx Jim]

J_J
I'M NOT ARGUING BUT ASKING. Are you positive the oil returns to tank when valve is placed in float or merely is displaced to the opposite side of each piston in each cylinder? If oil flows to tank then wouldn't each cylinder that had been placed in float experience some "lag time" to replenish depleted oil from float position??
Thanks,Jim

 

[murphy1244]

He said allows which is correct.

 

[J_J]

What I am thinking is that the cyls are acting like a pump and either push or pull fluid as needed.

If it is going to float, the fluid has to come from the tank.

If the flow was coming from the pump, you would have flow going to both sides of the cyl.

 

[CJONE]

J_J
I'M NOT ARGUING BUT ASKING. Are you positive the oil returns to tank when valve is placed in float or merely is displaced to the opposite side of each piston in each cylinder? If oil flows to tank then wouldn't each cylinder that had been placed in float experience some "lag time" to replenish depleted oil from float position??
Thanks,Jim

Both A and B lines are open to tank or the return circuit. So the fluid will either come from tank line in a power beyond valve or the pump excess / tank line in a non power beyond set up. You also use some oil from the cylinder to fill the other end of the cylinder from displacement like you said but that is very small. Remember because of the rod size being smaller than the bore upon extending you would have to have make up oil to fill the bottom and that comes from the return/tank cavity. CJ

 

[Tx Jim]

But if both lands on the spool valve were opened and ""pump pressure remained shut off"" couldn't oil flow from one side of cylinder piston to the other side of piston allowing rod to move in/out(vice-verse) without much oil going back to tank?? I understand that rod side of cyl holds less oil than the other side of the piston in the cylinder. I'm not stating no oil travels to tank but I think some of the oil doesn't while valve is in float. CJONE I was typing I guess while you posted. The difference in oil capacity compared from one side of piston to the other would be determined by cyl rod diameter.

 

[beemerphile]

You can float a cylinder by opening the work ports and the tank port with the pump port blocked, or you can open all four ports. With the pump port blocked, power is available to downstream valves. With four ports open, the flow returns to tank and downstream valves are disabled.

Which one is used depends on what the control engineer wants to accomplish.

 

[Industrial Toys]

Seems to me that if the cylinders in that floating circuit were moving back and forth, both plumbed to the tank at that instant, you would get air in that circuit in short order. You can't suck oil out of the return port.

Could be wrong, what do I know?

 

[beemerphile]

Seems to me that if the cylinders in that floating circuit were moving back and forth, both plumbed to the tank at that instant, you would get air in that circuit in short order. You can't suck oil out of the return port.

Could be wrong, what do I know?

Bear in mind that in use, the loader is skimming the ground. You are not putting it in float and then extending and contracting it multiple times to the limits of its travel. So the amount of oil exchange is minimal. Also, as has been stated in this thread earlier, some of the oil is supplied by the opposing side of the cylinder. Pushing up on the loader could create a bit of air because the rod side of the cylinder will not provide enough oil to fill it. When the loader sinks, the opposite is true and excess oil is sent to tank. But the amount that it moves in the normal course of intended use is not so much and it recovers quickly when power is again applied.

Using four ports open in the float control instead of pump blocked (three ports open) would flood the circuit and keep it full at the expense of downstream functionality. Having the ability to trim the curl on a bucket while floating the loader is handy enough that it would make sense to use the less rigorous but fully functional scheme of blocking the pump port. Having the tank return line below the fill level in the tank would also prevent ingesting air into the system even with the blocked pump scheme.

 

[Industrial Toys]

It does make me wonder how the return oil enters the tank. The displacement of the one side of a cylinder is different from the other, so just allowing it to circulate from one side to the other wouldn't work, right? It might explain, why a poor mans floating top link I built didn't work very well. Just a cylinder filled with oil and the two ports connected via a valve. I tried a solenoid valve which didn't work and thought it was just the orfice size, but a high pressure ball valve didn't work either!

 

[beemerphile]

It does make me wonder how the return oil enters the tank. The displacement of the one side of a cylinder is different from the other, so just allowing it to circulate from one side to the other wouldn't work, right? It might explain, why a poor mans floating top link I built didn't work very well. Just a cylinder filled with oil and the two ports connected via a valve. I tried a solenoid valve which didn't work and thought it was just the orfice size, but a high pressure ball valve didn't work either!

Your poor man's system would have probably worked with an accumulator tank in the circuit. As far as where the oil enters the tank on a tractor, I'd like to know as well. I don't know much about tractor design, but with industrial hydraulics we always returned the oil below the tank level among other reasons to minimize aeration which could lead to cavitation. Cavitation leads to noise and dead pumps. I'm thinking that tractor hydraulics would have the same issues to contend with and would expect the oil to enter the tank below the fill level and as far from the pump suction as possible. You can't go by where the tank return line enters because it could have internal piping.

 

[Industrial Toys]

I guess I could have used two cylinders, one being turned around!

I am just trying to think about operating the bucket circuit on both my JD and Kubota loaders while the boom is in float. I'm almost thinking that something weird happens, meaning some interaction between the two circuits. Maybe I'm wrong.