Biased lubricant compensator for an earth boring drill bit

An improved pressure compensator for earth boring drill bits for minimizing the pressure differential between the lubricant and the well bore. A flexible diaphragm is located in the lubricant reservoir for separating the lubricant from the borehole fluid. The diaphragm is positioned in the reservoir so that a substantial void space exists on the borehole fluid side of the reservoir. A retainer holds the diaphragm in this position, requiring a positive lubricant pressure to stretch the diaphragm in order to fill the void space. For lubricating the bit, the lubricant is introduced under pressure sufficient to stretch the diaphragm. The lubricant pressure is then released, allowing the compensator to return to its original shape which in turn causes the excess lubricant to be expelled. This assures complete filling and a desired void space.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to earth boring drill bits and in 
particular to an improved pressure compensator in an earth boring drill 
bit. 
2. Description of the Prior Art 
Earth penetrating tools, including the rotatable cutter-type earth boring 
drill bit, commonly used lubrication systems that include a pressure 
compensator to limit the pressure differential between the lubricant and 
the pressure in the wellbore. A typical type includes a flexible diaphragm 
located in the lubricant reservoir and separating the lubricant from the 
borehole fluid. A portion of the diaphragm moves in response to the 
pressure differential across it tending to equalize the pressure 
differential between the lubricant reservoir pressure and the borehole 
fluid pressure. 
Temperature increase in the lubricant as the bit is lowered into the well 
and due to friction heat while rotating causes expansion of the lubricant. 
Temperature increase may also cause the lubricant to evolve gaseous 
hydrocarbons. If the drill bit is of the type having positive seals 
between the cutter and bearing shaft that prevent egress of lubricant, 
thermal increase results in a pressure build-up in the system that can 
cause the diaphragm to be damaged as it is pressed against the port 
leading to the exterior. Excess internal pressure build-up can also damage 
the seal between the cutter and bearing shaft. A drill bit that contains 
positive seals is shown in U.S. Pat. No. 3,476,195, while a drill bit 
having the type of seals that release lubricant at a selected pressure is 
shown in U.S. Pat. No. 3,230,020. 
One device used to reduce the internal pressure build-up in a positively 
sealed bit is a pressure relief valve, such as shown in U.S. Pat. No. 
3,476,195 and in U.S. Pat. No. 3,942,596. Since failure of a pressure 
relief valve can destroy the effectiveness of the entire lubrication 
system, it is advantageous to eliminate the valve if internal pressure 
increases can be minimized by other means. 
One way to minimize pressure build-up in a positively sealed bit without a 
pressure relief valve is to allow room in the reservoir for expansion. 
Since a relatively high pumping pressure is required to assure complete 
filling, one means for allowing room for expansion is to remove lubricant 
after the pressure is released. Another means for allowing expansion space 
is to use an extra thick compensator cap for filling, replacing it with a 
thinner cap for use, thereby leaving a void space in the reservoir as 
shown in U.S. Pat. No. 3,917,028. While these techniques are successful, 
improvements are desired. 
SUMMARY OF THE INVENTION 
It is a general object of this invention to provide an improved lubricant 
system for sealed earth boring drill bits and a method of filling the same 
with lubricant. 
It is a further object of this invention to provide an improved means for 
reducing internal pressure build-up in the lubricant system of a sealed 
earth boring drill bit. 
It is a further object of this invention to provide an improved means for 
reducing internal pressure build-up in the lubricant system of a sealed 
earth boring drill bit that allows complete filling of lubricant, and does 
not require a pressure relief valve. 
In accordance with these objects, an improved lubricant system for an earth 
boring drill bit is provided that utilizes bias means to bias the 
diaphragm in its retaining cavity to a position with a substantial void 
space between the diaphragm and reservoir on the borehole fluid side of 
the diaphragm. The bias means preferably includes a retainer ring that 
retains the diaphragm in an intermediate position so that it must stretch 
in order to fill the void space on the borehole fluid side of the 
diaphragm. As the lubricant expands, the diaphragm stretches, increasing 
the volume on the lubricant side of the reservoir, and reducing pressure 
build-up. To assure complete filling, the lubricant is pumped into the 
reservoir to a pressure sufficient to stretch the diaphragm and fill 
substantially all of the void space. The pressure is then released, 
allowing the diaphragm to return to its normal state, and expelling 
lubricant.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The numeral 11, in FIG. 1 of the drawings, designates a lubricated, 
rotatable, cutter-type earth boring drill bit having a body formed in 
three head sections 13, of which one is shown. Each section contains a 
depending bearing pin or shaft 15 that supports a rotatable cutter 17 
having earth disintegrating teeth 19. Lubricant passages 21 supply 
lubricant to the bearing surfaces between the bearing pin 15 and cutter 
17. An O-ring seal 23 prevents borehole fluid from entering the bearing 
surface, and also prevents lubricant from leaking past to the borehole. A 
seal of this type is shown and described in U.S. Pat. 3,397,928. A 
pressure compensator system 25 helps provide lubricant through passage 21 
to the bearing pin 15, and limits the pressure differential across seal 
23. 
Referring to FIG. 2, the pressure compensator system 25 is located in a 
reservoir or recess 27. Reservoir 27 has a base 29 and sidewalls 31 that 
are substantially cylindrical and stepped. Lubricant passage 21 intersects 
the sidewalls 31, and a borehole fluid passage, or vent hole 33, 
intersects base 29. One end of the borehole fluid passage 33 is located in 
the center of base 29, while the other end is located at the exterior of 
the bit in the crotch area, to provide means for borehole fluid to enter 
the lower portion of reservoir 27. 
A diaphragm 35, normally of oil resistant rubber or nitrile rubber 
compound, is inserted in the lubricant reservoir 27, separating lubricant 
from the borehole fluid that enters passage 33. As shown in FIG. 4, 
diaphragm 35 is generally cup-shaped with a mouth 37, sidewalls 39, and a 
bottom 41. A portion of the sidewalls 39 are foldable to allow the bottom 
41 to fold inside the sidewalls 39 so as to be positioned as shown in the 
drawing. Bottom 41 moves along the longitudinal axis of the reservoir 27 
in response to pressure differential. The mouth 37 contains an outwardly 
protruding lip 42 that seats against the reservoir base 29 at the 
intersection with the reservoir sidewall 31, to seal borehole fluid from 
the lubricant. A metal disc 43 is bonded to the borehole side of the 
diaphragm bottom 41 to strengthen the bottom and prevent damage to the 
diaphragm when it is pressed against the entrance to passage 33. The 
centering button 45 assures that disc 43 will be centered over passage 33. 
A retainer means comprising a rigid metal band or retainer ring 47 is 
bonded to a portion of the diaphragm sidewalls 39, and bears against the 
upper edge of the lip 42. A portion of ring 47 extends upward within 
sidewalls 39 for a selected distance, serving as stiffening means for 
stiffening a portion of the sidewalls to prevent them from folding. A 
channel or groove 49 is formed in ring 47 near the edge that contacts lip 
42. An O-ring 51 (FIG. 2) is located in groove 49, providing a secondary 
seal against borehole fluid from entering the lubricant side of reservoir 
27. The height of ring 47 is selected to place the diaphragm bottom 41 a 
selected distance from the entrance to passage 33 when the sidewalls 39 
are folded as far as possible as shown in FIG. 4. Consequently, ring 47 
serves as bias means for preventing the diaphragm bottom 41 from coming 
into contact with the entrance to the passage, requiring the folded 
portion of the sidewalls 39 to stretch in order to fill the void space 52 
on the borehole fluid side of the reservoir. 
Referring to FIGS. 2 and 3, a protector member 53 encloses the diaphragm 35 
to limit expansion. The protector member 53 is generally cup-shaped, with 
its mouth bearing downwardly against channel 49 of ring 47. The bottom of 
the protector member contains an aperture 55 for the passage of lubricant. 
A cap 57 is secured in the upper end of reservoir 27 by a snap ring 59. Cap 
57 is sealed by an O-ring 61 between it and the reservoir sidewalls 31. 
Referring to FIG. 3, the lower surface of cap 57 contains a recessed 
channel 63 for the passage of lubricant through the aperture 55. An axial 
filling passage 65 may be located in cap 57 for the introduction of 
lubricant. A threaded plug 67 (FIG. 1) seals passage 65 after the 
reservoir has been filled with lubricant. 
In the lubricating operation, the bit and pressure compensator are 
assembled prior to introducing lubricant. Then the reservoir 27 and 
passage 21 are preferably evacuated of all the air and gases through 
nozzle 69, which screws into passage 65. While retaining the vacuum, 
lubricant is then introduced through nozzle 69. The lubricant is pumped 
into the reservoir 27 to a pressure of approximately 275 psi (pounds per 
square inch) to assure complete filling. Details of a satisfactory 
evacuation and lubrication system are shown in U.S. Pat. No. 4,019,785. 
Since the lubricant system is positively sealed, the foldable portion of 
sidewalls 39 will distend, filling substantially all of the void space in 
the borehole fluid side of the diaphragm. Then pumping pressure is removed 
by releasing the quick disconnect coupling 71 of nozzle 69. This allows 
the lubricant pressure to be vented to atmosphere, and the diaphragm to 
return to its folded, unstretched state, as shown in the drawings. Grease 
equal to the volume of the void space 52 will be expelled out of the 
nozzle 69. The nozzle 69 is then removed and the plug 67 screwed in place 
while the lubricant is at atmospheric pressure. 
In the drilling operation, as the bit descends into the well, borehole 
fluid will enter passage 33 and fill the void space 52 of the reservoir on 
the borehole fluid side of the diaphragm 35. As hydrostatic pressure 
increases, the diaphragm bottom 41 will move farther from the entrance to 
the borehole fluid passage 33, tending to balance the lubricant pressure 
with the hydrostatic pressure. Also, the geothermal temperature increase 
as the bit is lowered will heat the lubricant in the reservoir, causing it 
to expand. This expansion pushes the diaphragm bottom 41 toward the 
entrance of passage 33, stretching the folded portion of the diaphragm 
sidewalls 39. Once drilling begins, heat generated by bearing friction and 
pressure variations will cause the diaphragm to respond accordingly. 
A suitable amount of void space on the borehole fluid side to allow for 
thermal expansion, yet assure sufficient lubricant, is approximately 
5%-25% of the volume of the lubricant that fills the reservoir, passages, 
and spaces in the bearings. A desirable diaphragm for a 77/8inch bit has 
its metal disc 43 approximately 7/16 inch from the entrance to passage 33 
when fully folded but unstretched, providing a void space of approximately 
18% of the volume of lubricant. A lubricant pressure of approximately 22 
psi is required to stretch the diaphragm until the disc 43 contacts 
passage 33. Laboratory tests were conducted with gradual heating of a 
drill bit constructed in accordance with this invention to 305 degrees F. 
The lubricant pressure increased to approximately 15 psi at this 
temperature. 
It should be apparent that an invention having significant improvements has 
been provided. The pressure compensator reduces pressure build-up in the 
lubricant due to thermal increase, yet does not require a pressure relief 
valve. Complete filling is assured by overfilling the reservoir, then 
allowing the excess grease to be expelled. A selected amount of void space 
in the reservoir on the borehole fluid side of the diaphragm allows room 
for thermal expansion. While the diaphragm is under varying degrees of 
stretch, the force required to stretch the diaphragm exerts a pressure on 
seal 23. This positive pressure tends to prevent leakage of borehole fluid 
across seal 23 into the bit. 
While the invention has been shown in only one of its forms, it should be 
apparent to those skilled in the art that it is not so limited but is 
susceptible to various changes and modifications without departing from 
the spirit thereof. For example, the lubricant dispensing nozzle may be 
formed integrally with a cap that is removed after filling and replaced by 
a solid cap for operation.