Position Control --> Hydraulics. How does it work?

[noobfarmer]

My tractor has a open center hydraulic system. I understand how the remotes work. I push or pull the lever and the attached cylinder actuates in or out until I release the lever. However, I don't understand how the 3-point hydraulics are controlled. Let's say I have the position control at a 1 position (lowest) and raise it to a 7 position (out of 10). How does the valve know to raise the implement only part way and then stop? This must be a different mechanical action entirely. Right?

Thanks in advance for an explanation!

 

[BukitCase]

Jeff, actually I think you just described DRAFT control - tractors that have it have a type of sensing that works off the toplink, so that when the toplink is pulled on harder, the hitch will raise a bit for a short time, then when the tension on the toplink lessens the hitch will lower again, back to whatever POSITION the hitch has been set for. Draft control is normally only used for ground engaging implements such as plows, etc -

Most tractors have POSITION control on the hitch, whether they have DRAFT control or not - compacts as a rule are less likely to have draft control.

Position control works by moving a balance beam of sorts inside the hitch housing - when you move the position lever, you disturb the setting of that balance beam, and when you LOWER the position control, the hydraulic controls then dump fluid out of the internal cylinder that raises the hitch, therefore allowing the hitch to drop.

When the hitch drops enough to re-establish neutral setting on the balance beam, the position valve is closed, and no more fluid is dumped so the hitch stops lowering. Raising the control disturbs the balance again, and it ADDS fluid to the internal hydraulic cylinder, raising the hitch.

A typical problem with position controls is if the internal cylinder's seal wears out - when that happens, the hitch will drop due to the leak - then it gets low enough that balance is disturbed, fluid is pumped into the cylinder, it raises til it satisfies the position control and stops - then it bleeds down again, and repeats. The more weight on the hitch at the time, the faster this happens and the more the hitch bounces, or chatters.

Usually there is also a "speed" control on the hitch, normally found under the seat - this is used to taylor the hitch's reaction time for different implements, and if it is "slowed down" TOO much it can lock the hitch in position. A fair amount of times people think their hitch is broken, when it's just been played with by their kids.

That's my understanding of the different hitch controls, HTH... Steve

 

[Tx Jim]

My tractor has a open center hydraulic system. I understand how the remotes work. I push or pull the lever and the attached cylinder actuates in or out until I release the lever. However, I don't understand how the 3-point hydraulics are controlled. Let's say I have the position control at a 1 position (lowest) and raise it to a 7 position (out of 10). How does the valve know to raise the implement only part way and then stop? This must be a different mechanical action entirely. Right?

Thanks in advance for an explanation!

When position control handle is moved from #1 to #7 location linkage opens hyd pressure valve to send hyd pressure to RS piston then when rockshaft(3 pt) reaches set height an internal or external mechanism(depending on tractor design) called feedback rod moves linkage to close pressure valve causing 3 pt to stop moving at set height.

 

[aircommuter]

What you are asking about is lift height control, Tx Jim has described that. Another understanding of it is that unlike your other levers which are connected with direct linkage the lift control is connected through additional lnkage which is connected to the hitch arms inside. Draft control keeps that position by way of more linkage that is controlled by the tractors horizontal angle so if the front wheels go up and down over terrain it adjusts the valve.

 

[BukitCase]

Draft on one of my tractors is accomplished by blocking/un-blocking a heavy U-shaped spring bar that, when it's allowed to move, counteracts the position control - but this is one built by UTB, and there are a few other things that aren't done in the "usual" way... Steve