Rotary drill bit

A rotary drill bit comprising a head and a roller cutter rotatably carried thereon, wherein an annular sealing means is arranged in an annular chamber adjacent to a gap between the roller cutter and the head. At least one of the sealing surfaces of the chamber is generally seen V-shaped. According to the invention larger sealing pressure is obtained at two circumferentially spaced portions of the V-shaped sealing surface than at an intermediate portion thereof while simultaneously maintaining sealing pressure along the entire circumferential extent of the sealing means between said spaced portions.

BACKGROUND AND OBJECTS OF THE INVENTION 
The present invention relates to a rotary drill bit comprising a head, at 
least one roller cutter which is rotatably carried by the head upon 
bearing means, and a resilient annular sealing means disposed in an 
annular chamber for sealing the bearing means. The chamber comprises a 
first sealing surface in the roller cutter and a second sealing surface on 
the head; at least one of said sealing surfaces being generally V-shaped. 
The object of the present invention is to provide a rotary drill bit of the 
above-mentioned type in which the sealing means is squeezed in the 
V-shaped sealing surface so that relative movement occurs only at the 
opposed sealing surface. 
Another object of the invention is to provide a rotary rock drill in which 
the pressure distribution at the sealing surfaces is optimized. 
A further object of the invention is to provide a rotary rock drill in 
which, during drilling, the change in shape of the sealing means is 
facilitated in direction toward the sealing surface opposed to the 
V-shaped sealing surface.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
In FIG. 1 the lowermost portion of the head of a roller bit for rotary 
drilling in rock and earth formations is denoted 10. The bit head 10 
comprises in conventional manner three separate legs. In FIG. 1 only one 
leg is shown and denoted by 11. The lowermost portion of the leg 11 is 
formed as a bearing shaft 12 upon which a roller cutter 13 provided with 
cutting means, such as hard metal inserts, is rotatably carried over a 
bearing system comprising a radial friction bearing 14, a ball bearing 15, 
a radial friction bearing 16 and axial friction bearings 17, 18. 
Lubricant is supplied to the bearing system through passages 20, 21 from a 
lubricant reservoir 19 in the leg 11. A resilient annular sealing means 22 
is disposed in an annular chamber 23 in connection with a gap 26 between 
the roller cutter 13 and the leg 11 for sealing the bearing system, i.e. 
the sealing means 22 shall on the one hand prevent drillings and other 
impurities from entering from outside into the bearing system and on the 
other hand prevent leakage of lubricant out from the bearing system. The 
chamber 23 comprises a sealing surface 24 in the roller cutter 13 and a 
sealing surface 25 on the leg 11. At least one of the sealing surfaces 24, 
25 is generally V-shaped. The sides of the groove defined by the V-shaped 
surface can be of arbitrary shape, such as straight, slight convex or 
slight concave, provided that they have a projected surface area falling 
on the opposed sealing surface. 
According to the preferred embodiment the sealing means 22 and the V-shaped 
sealing surface 24 are mutually dimensioned such that the sealing 
pressure, i.e. the contact pressure of the sealing means, is larger at two 
circumferentially spaced portions 27, 28 of the sealing surface 24 than at 
an intermediate portion 29 thereof. This means basically that the main 
sealing occurs at the outer portions of the sealing surface 24 and means 
further that the sealing means 22 becomes squeezed in the V-shaped sealing 
surface 24 so that relative movement does occur only at the sealing 
surface 25. 
In the illustrated embodiment the sealing means 22 is an O-ring seal. The 
V-shaped sealing surface 24 is provided on the roller cutter 13 which 
means that the O-ring is squeezed or pinched in the roller cutter, thereby 
ensuring that relative movement arises only between the O-ring 22 and the 
leg 11. This is advantageous since the relative velocity becomes lower 
than compared to the relative velocity where the relative movement occurs 
between the O-ring and the roller cutter and since the circumference of 
the sealing surface where relative movement occurs becomes smaller. 
As shown in FIG. 3 the O-ring 22 and the sealing surfaces 24, 25 are 
mutually dimensioned such that a certain contact pressure, denoted by 30 
in FIG. 3, does arise also at the intermediate portion 29 of the sealing 
surface 24. The contact pressure 30, however, is considerably smaller than 
the contact pressures 31, 32 at the portions 27, 28 of the sealing surface 
24. The sealing pressure 30 both increases the active sealing surface area 
between the O-ring 22 and the roller cutter 13 and ensures that no pockets 
do arise at the portion 29, which pockets could create a pumping action so 
that lubricant would be pumped out of the bearing system. 
Due to clearances in the bearing system the roller cutter 13 might have a 
radial movability up to 0.2 mm and an axial movability up to 0.4 mm; said 
values increasing upon wear of the bearings. If the O-ring 22 was not 
squeezed in the roller cutter 13 (or alternatively in a groove in the leg 
11) then either the entire O-ring would slide sidewards or slide sidewards 
at the unloaded side of the drill bit and be tortured at the loaded side 
thereof. Such an axial relative movement or "wobbling", respectively, of 
the O-ring might, besides the negative influence on the life of the 
O-ring, cause impurities to be fed into the bearing system and lubricant 
to be pumped out thereof. 
The surface being most difficult to seal against is the one where relative 
movement occurs, i.e. against the sealing surface 25 on the leg 11. 
According to the preferred embodiment it is ensured that the nominal 
sealing pressure at the sealing surface 25, denoted by 33 in FIG. 3, is 
larger than the sealing pressures 31, 32. 
In the illustrated embodiment the V-shaped sealing surface 24 has a 
confined angle, .alpha.+.beta. in FIG. 4, smaller than 70.degree., and the 
circumferentially spaced portions 27, 28 of the sealing surface 24 are 
planar and form angles .alpha. and .beta. of substantially equal size with 
the normal to the sealing surface. The intermediate portion 29 of the 
sealing surface 24 is curved. The radius of curvature R is smaller than 
the radius D/2 of the O-ring in relaxed condition and the curved portion 
29 is tangential to the portions 27,28. Further, the largest extension H 
of the chamber 23 in radial direction is between 90% and 150% of the 
diameter D of the O-ring in relaxed condition. In the illustrated 
embodiment the largest radial extension of the portion of the roller 
cutter 13 nearest to the gap 26 is larger than R (1-sin .beta.), where 
.beta. is the angle between the portion 28 and the normal to the sealing 
surface 25; said radial extension being defined as T in FIG. 4 and being 
measured from the bottom of the portion 29 toward the sealing surface 25. 
The largest extension of the chamber 23 in axial direction from the normal 
to the sealing surface 25, defined as A and B in FIG. 4, is in the same 
size in both directions. Due to the illustrated and preferred embodiment 
the change in shape of the O-ring 22 in direction toward the sealing 
surface 25 is facilitated. Further, the O-ring 22 becomes axially centered 
approximately in the middle of the chamber 23, thus ensuring that there is 
provided space for allowing axial deformation of the O-ring in both 
directions when the chamber 23 is changed in shape radially. 
In FIG. 5 the principle is illustrated for an optimized building-in of the 
O-ring 22. Basically the O-ring can be considered as being squeezed or 
pinched between three surfaces, namely the surfaces 34,35 in the roller 
cutter 13 and the surface 36 on the leg 11. If, for the sake of 
simplicity, the triangle, where the surfaces 34,35,36 each forms a part of 
a side of the triangle, is considered to be equilateral then the centre 37 
of the O-ring 22 remains in the same position during the squeeze of the 
O-ring. Thus, the shape of the O-ring has to be changed. The "squeezed" 
portions 38,39,40 push away material, thereby causing the O-ring to bulge 
or expand at the portions 41,42,43. The deformation force, i.e. the 
sealing pressure, can be considered as depending on the size of the 
deformation and of the deformation stiffness. The deformation stiffness or 
the form factor f in its turn depends on, among other things, the number 
of deformation positions. Thus, three deformation positions creates a form 
factor which is larger than that obtained by two deformation positions; in 
the order of say 50% larger. Due to the fact that the radial compression 
for a certain sealing force becomes smaller with a 
three-position-deformation than with a two-position-deformation the heat 
generation at the movable sealing surface of the sealing ring becomes 
smaller, which favourably affects the life of the sealing ring, especially 
at high number of revolutions of the drill bit. 
As above-said in connection with the description of the preferred 
embodiment the O-ring 22 is not allowed to fill all the deformation space 
23. Due to the restricted radius R the bulging against the leg 11 will be, 
relatively seen, larger and the compression against the surfaces 27,28 in 
the roller cutter 13 will be, relatively seen, smaller. This means that 
the sealing pressure against the leg 11 increases and that the peaks of 
the sealing pressure against the surfaces 27,28 decrease, thus 
concentrating the sealing force on the surface being most difficult to 
seal against, i.e. the surface 25 where relative movement occurs. 
The above advantages described in connection with an O-ring seal can also 
be achieved by means of sealing means having other cross section than the 
circular one. Specifically, the sealing means can be formed with a 
half-circular outer surface intended to be mounted in the roller cutter 
and formed with a somewhat larger extension in radial than in axial 
direction.