Rock bit face seal having anti-rotation pins

The invention provides a mechanical face seal for rotary rock bits with a novel coil spring energization system that prevents yielding of the coil springs in extreme torque conditions. Within at least one of the coil springs is a cylindrical pin designed to co-act with the spring such that the combination becomes very stiff in the radial direction as the spring extends beyond a certain amount. This prevents the high torque from yielding the springs. When the spring returns to its normal extension, the pin does not interfere with the axial compression of the spring. This combination allows the coil spring energizers of a rotary rock bit mechanical face seal to survive the occasional extreme torque event.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention provides an enhanced rotary face seal for roller cone rock
 bits. The new seal has pins which positively prevent rotation of the seal
 ring with respect to the seal ring carrier.
 2. Description of the Related Art
 Modern, premium roller cone rock bits utilize sealing systems to prevent
 the loss of lubricant from the roller cones. The seal system also prevents
 the abrasive laden drilling fluid outside the bit from entering into, and
 failing the bearing system of the rolling cones.
 There are two basic types of sealing systems in common use in rolling
 cutter drill bits. In most drill bits, an elastomeric packing ring
 provides the seal between the rolling cone and the bearing system. These
 bits utilize an elastomeric compression type sealing system, and have
 adequate performance in most drilling applications. For rock bits used in
 very severe bit applications, however, rotary mechanical face seals are
 disposed between the rolling cone and the bearing to provide the seal.
 Rotary mechanical face seals are generally made up of two flat sealing
 faces which are designed to maintain a thin film of lubricant between the
 sealing faces. As the sealing surfaces rotate relative to each other, they
 are urged together at a carefully controlled force by one or more
 energizers as shown for instance in U.S. Pat. Nos. 5,040,624, 4,838,365,
 and 3,761,145.
 Although generally more expensive than elastomer seals, mechanical face
 seals are able to assure a level of performance in rock drilling bits
 which easily justifies the higher cost. Most mechanical face seals, also
 known as rigid face seals used in rotary rock bits are made from stainless
 steels and have sealing faces which are manufactured to be flat and
 smooth. These faces mate together to form a planar, annular sealing
 interface. These seals are usually made with one or two sealing rings with
 a gradually tapered shape adjacent to the sealing interface at the
 lubricant side. This creates a diverging geometry which provides
 preferential access for lubricant to enter into the sealing interface. As
 abrasives wear the outer periphery of the sealing interface, the diverging
 geometry also facilitates inward movement of the sealing interface to
 maintain the contact width.
 Mechanical rigid face seals have become the seal of choice for rock bits
 used in the most severe drilling environments, due to the operating
 limitations of elastomers as dynamic seals. Rigid face seals are typically
 manufactured from materials which readily tolerate the thermal, chemical
 and mechanical attack of severe drilling environments. The seals provide a
 higher level of reliability than elastomer seals in rock bits and are
 capable of extremely long runs without significant loss of lubricant.
 It is important to maintain a lubricant film between the two sealing faces.
 Oftentimes in operation, however, the film becomes too thin and frictional
 contact between the sealing faces will cause high torques on the seal
 faces. These high torques can cause failure of the systems which hold the
 seal in place. For instance, if elastomeric energizers are transmitting
 the torque, they may slip. A small amount of slippage can cause excessive
 wear on the elastomer energizers, leading to an early failure.
 Even when coil spring energizers, such as shown in U.S. Pat. No. 4,838,365,
 are transmitting the torques, it is possible, under some circumstances for
 the coil spring energizers to fail. When the operating torques become too
 high, the shear forces on the coil springs can cause them to yield. Once
 any one of the springs yield, the seal assembly loses its ability to move
 in response to volume changes in the lubricant near the seal, leading to
 rapid seal failure. The key to the proper operation of rigid face seals
 for rolling cutter drill bits lies in their ability to accommodate the
 lubricant volume changes near the seal, as described in U.S. Pat. No.
 4,516,641. In non-rock bit applications, rigid face seals do not have to
 deal with this peculiar volume compensation problem. The unique design
 requirements for rock bit volume compensating rigid face seals are such
 that they are in a unique class of rigid face seals. There are many
 superficial similarities between volume compensating rigid face seals for
 rock bits and non-rock bit face seals. However, the diverging design
 requirements of the two groups tend to make them non-analogous.
 For example, coil springs are often used in rigid face seal applications
 other than for rolling cutter drill bits. To prevent torque from being
 applied to the springs, a plurality of pins are often interspersed with
 the coil springs. These pins allow free axial movement of the seal faces
 while transmitting all the face torque. It is undesirable in these designs
 for the springs to carry any part of the face torque because torque
 loading can profoundly affect the springs' ability to energize the seal
 faces. The wire coil of the spring may bind or `hang` against the comer of
 the spring bore, changing the springs' force/deflection characteristics.
 A non-rock bit face seal design incorporating coil springs for energizers
 and pins for torque transmission is disclosed in U.S. Pat. No. 4,261,582,
 herein incorporated by reference. The pins and their mating bores
 typically have diameters sized such that all the torque load is
 transmitted from the seal rings through the pins. No torque is carried by
 the coil springs. Even in designs where pins and springs are combined, as
 shown in U.S. Pat. Nos. 4,215,870 and 5,080,378, the components are
 arranged such that the springs never transmit any of the torque load from
 the seal ring.
 In all the prior non-rock bit mechanical face seal designs known to the
 inventor of the present invention, great care is taken to assure that no
 torque is carried by coil spring energizers.
 In rock bits, however, coil spring energizers are able to successfully
 carry torque. The events which normally lead to high face torques in
 volume compensating face seals in rock bits also tend to relieve the coil
 springs of their energization duties during these events. This happens
 because the pressure force on the seal face during an `onward loading`
 volume compensating event causes the springs to extend and also causes the
 axial face load to increase. Under this condition, the force contribution
 from the spring is not necessary for an effective seal of the seal faces.
 As soon as the event is past, the face torque rapidly decreases as the
 springs retract. When the pressures are finally balanced, the spring
 returns to its centered position.
 The coil spring is designed such that the thickness of the wire in the coil
 is greater than the gap between the seal and the cutter bore when the
 spring is in its centered position. This prevents the wire from `hanging`
 on the lip of the spring cavity in the cutter from normal operating
 torques.
 However, it has been observed that face torques may sometimes exceed 200
 inch-pounds in these bits. At this torque level, the wire in coils can
 yield, effectively disabling the spring as an energizer. The present
 invention provides a means to prevent this spring yielding.
 SUMMARY OF THE INVENTION
 The present invention provides a rigid face seal for rotary rock bits with
 a novel coil spring energization system that prevents yielding of the coil
 springs in extreme torque conditions. Within at least one of the coil
 springs is a cylindrical pin designed to co-act with the spring such that
 the combination becomes very stiff in the radial direction as the spring
 extends beyond a certain amount. This prevents the high torque from
 yielding the springs. When the spring returns to its normal extension, the
 pin does not interfere with the axial compression of the spring. This
 combination allows the coil spring energizers of a rotary rock bit rigid
 face seal to survive the occasional extreme torque event.
 As the rigid face seal assembly operates within the rolling cutter rock
 bit, the seal assembly moves axially with respect to the roller cutter,
 causing the coil springs to extend or compress. The coil spring is
 vulnerable to yielding only when it is extended enough to leave one loop
 of the coil unsupported. Because the coil spring often extends enough to
 leave a coil loop unsupported, the torque on the seal faces transmitted
 through the coil springs can cause the coil springs to yield.
 The inside diameter of a coil springs tends to get smaller as the spring
 extends and grow larger as it compresses. In the present invention, a
 cylindrical pin is installed within a coil spring energizer for a rigid
 face mechanical seal for rolling cutter drill bits. The pin and spring are
 sized such that as the spring extends and the seal face torque increases,
 the spring and pin co-act to be very stiff radially. When the spring
 returns to its original extension, the pin is released, allowing the
 spring to operate unencumbered by the pin.
 In its broadest form the invention is a rolling cutter rock drill bit for
 drilling boreholes into the earth with at least one rolling cutter mounted
 upon a cantilevered bearing shaft. A lubricant is disposed between the
 rolling cutter and the cantilevered bearing shaft. A rigid face seal
 assembly is mounted between the rolling cutter and the bearing journal to
 seal the lubricant within the rolling cutter. The rigid face seal assembly
 is made with at least one rigid seal ring and a plurality of coil spring
 energizers disposed upon the seal ring. A cylindrical pin is disposed
 within one or more of the coil spring energizers. The outside diameter of
 the pin is just slightly smaller than the inside diameter of the coil
 spring when the face seal is assembled at equilibrium. This allows the
 spring and the pin to be independent of each other. However, when the
 spring extends and the torque on the seal faces increase, the pin and
 spring co-act to become stiff and transmit very high torques without
 damage to the coil spring.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERED EMBODIMENTS
 Referring now to the drawings in more detail, and particularly to FIGS. 1
 and 2. A rolling cutter rock drilling bit 10 includes a body 12 with a
 plurality of leg portions 14. A rolling cutter rock drilling bit 10 is
 also commonly called a rock bit, a rolling cutter drill bit or an oilfield
 drill bit. A cantilevered bearing shaft 16 formed on each leg 14 extends
 inwardly and downwardly. A rolling cutter 18 is rotatably mounted upon the
 shaft 16. Attached to the rolling cutter 18 are hard, wear resistant
 cutting inserts 20 which engage the earth to effect a drilling action and
 cause rotation of the rolling cutter 18. A friction bearing member 36 is
 mounted between the bearing shaft 16 and a mating bearing cavity 38 formed
 in the cutter 18. This friction bearing 36 is designed to carry the radial
 loads imposed upon the cutter 18 during drilling. A retention bearing
 member 42 is mounted in the cutter 18 to retain the cutter 18 upon the
 bearing shaft 16 during drilling.
 Internal passageways 22, 24, & 26, as well as a reservoir 28 and bearing
 area 30 of the leg 14, are filled with lubricant (not shown) during bit
 assembly. The lubricant helps reduce bearing friction and wear during bit
 operation and is dynamically sealed within the cutter 18 by a rigid face
 seal assembly 32.
 The pressure balancing diaphragm 34 equalizes the pressure between the
 drilling fluid and the lubricant and typically has a built in pressure
 relief means which releases lubricant into the drilling fluid when a
 predetermined pressure differential is reached. This is intended to
 protect the bearing seal 32 and pressure balancing diaphragm 34 against
 unintended rupture or damage.
 Referring now to FIG. 3, the mechanical rigid face seal assembly 32 is
 comprised of two seal rings 42, 44 which are preferably formed of AISI
 440C (UNS S44004) stainless steel, although many other materials are also
 suitable. Seal ring 42 is sealed with the bearing shaft 16 and also
 energized against its mating seal ring 44 by an elastomer ring 48. Since
 seal ring 42 does not rotate with respect to the bearing shaft 16 under
 normal operating conditions it is considered the stationary seal ring.
 The rotating seal ring 44 is mounted within the cutter 18. This ring 44 is
 energized by a number of coil springs 46. An elastomer seal 50 prevents
 fluids from bypassing the rotating seal ring 44 while allowing the seal
 ring 44 to move axially.
 A least one pin 50, but preferably two pins 50 are disposed within a
 corresponding coil spring. If more than one pin is utilized it is
 preferred they be placed within springs that are symmetrically arranged in
 a diametrically opposed manner on the rigid face seal assembly 32 so that
 the torque forces are evenly transmitted from the seal ring 44, through
 the springs 46 with pins 50.
 The following dimensions are typical for a coil spring energized rigid face
 seal for a 121/4 inch diameter rolling cutter drill bit. Other bit sizes
 may have differently sized sealing elements, so the dimensional and
 physical properties indicated are presented below for example only.
 The rotating seal ring 44 is forced against the stationary ring 42 by a
 series of twelve coil spring energizers 46, spaced around the
 circumference of the ring 44 to apply load at discrete locations. Each
 coil spring 46 is about 0.175 inches outside diameter and 0.111 inches
 inside diameter in its free state. In the assembled position as shown in
 FIG. 3, the outside diameter of the spring 46 grows slightly to about
 0.179 inches, and the inside diameter grows to about 0.115 inches. The
 springs 46 are compressed so that each spring exerts about 7.5 pounds
 force onto the rotating ring 44 at assembly. Since there are twelve
 springs, the assembled face load is about 90 pounds.
 The wire of the coil spring 46 is about 0.032 inches in diameter, and the
 clearance gap 54 between the cutter body 18 and the seal ring 44 is
 nominally 0.025 inches at assembly. Therefore, during normal operation,
 the wire of the coil spring 46 cannot be moved through the clearance gap
 54 and cause yielding of the spring 46.
 The recesses 47 in the rotating seal ring 44 are each about 0.188 inches in
 diameter and about 0.142 inches in depth, not counting the drill point.
 The recesses 52 in the cutter 18 are each also about 0.188 inches in
 diameter and about 0.340 inches in depth not counting the drill point. At
 least some of the recesses 52 in the cutter 18 are aligned generally
 axially with recesses 47 in the rotating seal ring 44. A plurality of the
 coil springs 46 are disposed within the aligned recesses 47, 52. The
 combined depth of the ring recess 47 and the cutter recess 52 is about
 0.482 inches.
 The pins 50 each have a diameter D of about 0.112 inches and are about
 0.462 inches in length. The exact length of the pin is not critical
 provided that it is somewhat longer than the sum of depth of recess 52 and
 the gap 54 and less than the combined depth of the two recesses 47 and 52
 so the pin 50 does not stop the seal ring 44 from contacting the cutter
 body 18 when the springs 46 are fully compressed. The pins 50 are smooth
 and have rounded ends so that they do not interfere with the normal axial
 movement of the springs 46 during normal operation.
 The centers of the coil spring energizers 46 are positioned at a diameter
 of about 3.218 inches, which is smaller than the 3.350 inch innermost
 diameter of the sealing interface.
 As can be appreciated by the above dimensional data, as the spring 46
 extends during extraordinary volume compensation events while in operation
 it can grip upon the pin 50. This helps the spring 46 and pin 50 to co-act
 as a single element when extreme volume compensation movement of the seal
 assembly 32 causes maximum extension of the spring 46. This event is also
 typically a high torque event.
 During high torque events, the co-action of the spring 46 and pin 50 allows
 high torques to be transmitted through the spring 46 without damage. When
 the springs are not extended (as in normal operation), the wire of the
 springs 46 is too large to pass into the clearance gap 54 between the seal
 ring 44 and the cutter 18. There is sufficient clearance between the pin
 50 and the inside diameter of the coil spring 46 in normal operation so
 that the spring 46 and the pin 50 do not interact.
 Although only one pin is required to successfully practice this invention,
 it is preferred to have two pins positioned in diametrically opposite
 locations on the seal ring 44 so that the torque will be transmitted
 evenly. However, it is contemplated that any symmetrical arrangement of
 coil springs 46 with pins 50 inside would be effective in transmitting the
 torque without damage.
 The advantages of the embodiments of this invention are that the mechanical
 seal ring 44 mounted within the cutter 18 is positively prevented from
 rotation within the cutter 18 without damage to the spring 46 and without
 affecting its axial movement in normal operation.
 It would be readily apparent to one skilled in the art that there are many
 other combinations of seal rings 44 and coil spring energizers 46 and pins
 50 which can be made and yet do not depart from the scope of the present
 invention. For instance a single energizer face seal could be manufactured
 with coil springs 46 and pins 50 which would effectively transmit high
 torques without allowing yielding of the coil springs.
 Whereas the present invention has been described in particular relation to
 the drawings attached hereto, it should be understood that other and
 further modifications apart from those shown or suggested herein, may be
 made within the scope and spirit of the present invention.