Overcenter valve control system and method for drilling

A drill feed control system for controlling the force exerted on a drill string having substantial weight. A feed cylinder, which has a first end and a second end, is connected to the drill string. Fluid is pumped from a reservoir to displace the feed cylinder. A first conduit is connected to the first end while a second conduit is attached to the second end. A variable displacement pump displaces fluid from the reservoir, within the first conduit, to the first end of the feed cylinder. A feed pressure control regulates the pressure of the variable displacement pump. An overcenter valve, which is connected to the second conduit, restricts the fluid exiting the second conduit. A relief valve controls the flow of fluid to a pilot conduit which extends between the first conduit and the overcenter valve. An orifice or check valve to release the pressure is connected to the pilot conduit such the the pressure in the pressure conduit will respond to a pressure increase or decrease in the first conduit. The pressure in the pilot conduit and the second conduit interact to open the overcenter valve.

BACKGROUND OF THE INVENTION 
This invention relates generally to an overcenter valve, and more 
particularly to a valve which may be precisely operated to control the 
feed force for a drill. 
FIGS. 1 and 2 show the prior art pertinent to the instant invention. 
Displacement of a cylinder 10 causes motion in drill string 14, as is well 
known in drilling operations. A variable displacement pump 16 supplies the 
fluid to operate the feed cylinder 10 via a four way valve 18 and a first 
conduit 20. 
The four way valve 18 controls the direction of flow of the fluid which is 
supplied to the feed cylinder 10. A feed pressure control 22 affects the 
pressure at which the pump 16 acts. A reservoir 17 contains the hydraulic 
fluid which is used by the pump 16 in extending the feed cylinder 10. 
While the first conduit 20 is in fluid communication with a first end 26 of 
the feed cylinder 10, an overcenter valve 24 is in fluid communication 
with a second end 27 of the feed cylinder 10 via a second conduit 28. The 
overcenter valve 24 (also known as a counterbalance or a holding valve) 
effects the change in feed force exerted by the feed cylinder 10 as a 
certain feed pressure is exerted by the pump 16. A graph for this 
relationship is shown in FIG. 2. 
The feed pressure required to open the overcenter valve 24 can be adjusted 
by a set spring 25. The two pressures interacting to open the valve 24 are 
the pressure exerted through a second conduit 28, and the pressure exerted 
through a pilot line 30 which is directly connected to the first conduit 
20. 
When overcenter valve 24 opens, fluid in a second conduit 28 is permitted 
to travel through the valve. An initial gradient 46 in the FIG. 2 feed 
force v. feed pressure graph, occurs as the valve 24, is holding pressure 
against the cylinder 10 rod end 27. A shallower gradient 48 occurs when 
the valve 24 opens due to the influence of pressure in 30 alone, so that 
no hold back pressure at 28 is generated. 
The steeper the gradient of the graph, the less the change in feed pressure 
(as controlled by the operator) being necessary to produce a similar 
change in the feed force. In other words, the feed force is more sensitive 
to changes in the feed pressure. This situation can be analogized to an 
inexpensive transistor radio in which a minor change in the volume control 
produces an excessive change in the volume of the radio. 
When starting to drill a hole, a small, controllable feed force is 
desirable to assure the hole gets started straight. Excessive downward 
force when starting a hole can result in a crooked hole and/or severe 
strain in the drill string 14 attached to the feed cylinder 10. It is thus 
greatly preferable to be able to more precisely control the feed force by 
a change in the feed pressure than occurs in the prior systems. 
The forgoing illustrates limitations known to exist in present devices and 
methods. Thus, it is apparent that it would be advantageous to provide an 
alternative directed to overcoming one or more of the limitations set 
forth above. Accordingly, a suitable alternative is provided including 
features more fully disclosed hereinafter. 
SUMMARY OF THE INVENTION 
In one aspect of the present invention, this is accomplished by providing a 
drill feed control system comprising a feed cylinder having a first end 
and a second end. A first conduit is connected to the first end and a 
second conduit is connected to the second end. A reservoir is included in 
the drill feed control system. Variable displacement pump means displace 
fluid from the reservoir to the first end of the feed cylinder. Feed 
pressure control means control the pressure applied from the variable 
displacement pump. An overcenter valve is in fluid communication with the 
second conduit. A pilot conduit communicates fluid in the first conduit to 
the overcenter valve. Pilot conduit pressure control means hold pressure 
in the pilot conduit constant for a predetermined range of variable 
displacement pump pressures, and increase the pilot conduit pressure at a 
controlled rate for increases of the variable displacement pump pressures 
above the predetermined range. 
The forgoing and other aspects will become apparent from the following 
detailed description of the invention when considered in conjunction with 
the accompanying drawing. It is to be expressly understood, however, that 
the drawing figures are not intended as a definition of the invention, but 
are for the purpose of illustration only.

DETAILED DESCRIPTION 
Referring now to the drawings, FIGS. 3, 4, 5 and 6 illustrate embodiments 
of the control system for an overcenter valve of the instant invention. 
Similar elements to those existing in FIGS. 1 and 2, as described in the 
background of the instant invention, are similarly numbered throughout. 
A relief valve 32, which may be adjusted by a relief valve pressure setting 
34, controls the pressure at which the valve 32 permits pressure from the 
first conduit 20 to enter a pilot line or conduit 30. The pressure in the 
pilot conduit 30 interacts with the pressure in the second conduit 28 to 
open the overcenter valve 24. The pressure in the pilot conduit 30 
responds to, but often is not identical with the pressure in the first 
conduit 20, as follows. 
Once the relief valve 32 opens by the first conduit 20 pressure meeting the 
relief valve pressure setting 34, the pressure in the pilot conduit 30 is 
increased. However, the pressure in the first conduit 20 will remain 
greater than the pressure in pilot conduit 30 by the relief valve pressure 
setting 34 which depends upon the physical configuration of the valve 32. 
A relief orifice 36 permits a slow flow of fluid in the pilot conduit 30 to 
escape from the pilot conduit. This amount of fluid can easily be replaced 
by the pump 16. The relief valve and orifice interact to keep the pressure 
in the pilot conduit 30 responsive to the pressure change in the first 
conduit 20. The pressure in the pilot conduit 30 thus will not remain 
excessive after the pressure in the first conduit 20 has dropped. 
In another embodiment, shown in FIG. 5, a check valve 38 is connected 
between the pilot conduit 30 and the first conduit 20. The check valve 38 
will release fluid from conduit 30 when the pressure in the pilot conduit 
30 equals or exceeds the pressure in the first conduit 20. This is another 
device which keeps the pressure change in the pilot conduit 30 responsive 
to pressure in the first conduit 20. 
In yet a third embodiment of this invention, as shown in FIG. 6, a shuttle 
valve 40 is utilized to control the pressure in the pilot conduit 30. A 
remote constant pressure source 42 supplies a constant pressure to the 
pilot conduit 30, and seals off the pressure in the first conduit 20 by 
valve sealing member 41 until such time as t he pressure in the first 
conduit 20 exceeds the pressure in the constant pressure source 42. When 
the first conduit 20 pressure exceeds the constant pressure source 42 
pressure, the sealing member 41 in shuttle valve 40 will be displaced 
downward into position 41' to seal off the constant pressure source 42, 
and the pressure in the pilot conduit 30 will equal the pressure in the 
first conduit 20. While the pilot conduit pressures in this embodiment 
will usually exceed the pressures in the previous embodiments, the feed 
force vs feed pressure graph will display a similar outline to the prior 
embodiments, and this embodiment will display similar precise control of 
feed force by affecting feed pressure to that displayed in the previous 
embodiments. 
The two characteristics which are shared by all of the above embodiments to 
the instant invention are: 
1) The pilot pressure is held constant for a predetermined range of system 
feed pressures. 
2) At pressures above the aforementioned feed pressure range, the pilot 
pressures will increase at a predetermined threshold and a controlled 
rate. 
The relief valve 32 does not open at all until the pressure setting 34 is 
reached. Before the valve 32 opens, the only pressure acting to open the 
overcenter valve 24 is the pressure in the second conduit 28. The gradient 
of the feed force vs feed pressure graph is therefor shallower than in the 
prior art until such time as sufficient pressure is achieved in the first 
conduit 20 to cause the relief valve 32 to open. The feed force of the 
feed cylinder 10 is in this manner more precisely controlled (in the lower 
feed force ranges) by altering the feed pressure in the instant invention 
as compared with the prior art. 
In drilling applications, the lower feed force ranges (which correlate with 
a lower graph portion 44) are typically used for hammer drilling type 
operations, while the higher ranges (which correlate with a higher feed 
force graph portion 48) are typically used for rotary drills. 
The position of a middle, steeper portion 48 of the FIG. 4 graph can be 
altered by changing the adjustments in the system. The operator can thus, 
depending upon the type of drilling which is being done, choose suitable 
adjustments such that the middle (and less controllable) feed force 
portion 48 of the graph will not be utilized when the operator is 
operating the drill. 
While this invention has been illustrated and described in accordance with 
a preferred embodiment, it is recognized that variations and changes may 
be made therein without departing from the invention as set forth in the 
claims.