D2cat
I hope we can continue to explore this topic as it helps me learn and re think my opinions.
This first photo is a mistake I made years ago which resulted in the rear casting on an old Nuffield tractor being broken out by the 3 pt cylinder push rod.
This marked up illustration is my attempt to explain what went wrong which, in my case, was the falling out of a small roll pin inside the rear hydraulic unit.
There was no way for me to know this pin had come out as the lift arms worked great. Normally, the push rod which presses on the upper lift arms to raise the hitch assembly, is bearing against the dished top of the hydraulic cylinder piston and at the lift arm end pushing into a cupped depression. The loose pivoting feature of the push rod is in many tractors because the cylinder push rod is not pushing in a straight line like a normal hydraulic cylinder would do. The designers want to avoid any bending moment on the push rod.
The arms normally stay down under their own weight and although the pin has come out life goes on.
I was trying to connect an implement and the lift arms were too low. I just lifted them by hand a few inches, connected the implement, got into the tractor and started to lift the implement. Because of the missing roll pin, the push rod had dropped out of position when I lifted the lower arms.
In its new position, the push rod was against the rear casting. Under the pressure of the piston against the push rod, the push rod forced a piece of the casting out of its way with a loud crack.
Another point.
The links between the lower and upper lift arms have ways to adjust the relative height of the two lower arms. On some tractors I know, on one side the link to lift the lower arm has a slot so there is several inches of up movement possible without the upper arm moving.
Here are parts illustrations from two other tractors. My point in including them is to support my position that the 3 pt system cannot hold back or down the arms because in many designs the push rod is loose and cannot pull back or resist the arms moving up.
Dave
