Floating a pull down cylinder during breakout

[Doug Hanson]

Any one with suggestions on floating a pulldown cylinder on a core drill during the breakout process? Pull down cylinder is rod up, Drill string is being converted from bayonet box to threaded connection. Must achieve a pressure balance to support the drill string while allowing the unthreading action of the connections to push the top head drive up and not cause the threads to pile up. Load of the string varies with working depth.
Any ideas?

[Doug Hanson]

No suggestions... this is what I came up with. This is the manifold, there is a directional control valve (not shown) that selects between the two pressure reducing valves. The lowest set reducing valve is also a relieving valve. This sets the pressure for making the connection, which allows the draw works cylinder to be pulled down.
When the directional control valve is shifted in the opposite direction, the higher reducing valve is used, supporting the load and allowing the draw works cylinder to be forced into extension without causing excessive force on the threaded connection.

[Doug Hanson]

I had to make a couple of revisions to the drawing, so here is a schematic of the manifold, and another schematic of it's application. The manifold was machined and put into operation, everything seems to work good. This was kind of an unusual application. Basically a dual setting ,variable, backpressure/boost pressure valve.