US20150136491A1

US20150136491A1 - Hydraulically Operated Polished Rod Clamp

Info

Publication number: US20150136491A1
Application number: US14/086,585
Authority: US
Inventors: Bill W. White
Original assignee: National Oilwell Varco LP
Current assignee: National Oilwell Varco LP
Priority date: 2013-11-21
Filing date: 2013-11-21
Publication date: 2015-05-21

Abstract

A polished rod clamp includes a clamp housing having a central passage passing through a tapered bore. A wedge lock is arranged in the tapered bore such that a first surface of the wedge lock is in sliding engagement with a tapered surface of the tapered bore and a second surface of the wedge lock grips a rod in the central passage in a lock position. A first reciprocating piston is arranged to apply a first force to the wedge lock that effects a relative sliding motion between the wedge lock and the tapered surface in a first direction corresponding to the lock direction of the wedge lock. A second reciprocating piston is arranged to apply a second force to the wedge lock that effects a relative sliding motion between the wedge lock and the tapered surface in a second direction that is opposite to the first direction.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

The present disclosure relates generally to a mechanism for coupling a power source to a rod string, such as a sucker rod string.
A string of sucker rods is used to couple a rod pump in a well with a reciprocating or rotary power source at the surface. The uppermost joint in the sucker rod string is known as a polished rod (or polish rod). The polished rod is machined to a high diametrical tolerance and has a polished external surface that enables a hydraulic seal to be made around the sucker rod string as the sucker rod string reciprocates or rotates. The polished rod passes through a wellhead stuffing box, which establishes a seal against the polished rod. Typically, a polished rod clamp is used to couple the polished rod to the surface power source, which may be a walking beam in the case of a reciprocating pump system or a rotary drive in the case of a progressive cavity pump system.
FIG. 1 shows a conventional polished rod clamp 10 in exploded view. The polished rod clamp 10 includes clamp blocks 12, 14 with recesses 16 (the recess in the clamp block 12 is not visible) for receiving a polished rod 18. The clamp blocks 12, 14 are secured together and around the polished rod 18 by means of high torque bolts 20. The clamp blocks 12, 14 may have mortises 22, 24 for engaging a drive shaft of a rotary drive, in the case of a progressive cavity pump system. The torque to each clamp bolt 20 is typically high. For example, for a 1.5-in diameter polished rod, the torque to each bolt can be in excess of 300 ft-lbs. For a progressive cavity pump system, the polished rod clamp will be located on top of the rotary drive, which is commonly in excess of 7 ft above the ground level in its use environment. Applying such high torque with a conventional wrench from the top of an elevating device, such as a step ladder, can be dangerous.
Other examples of polished rod clamps are based on hinged jaws that are held together by a bolt. See, for example, U.S. Pat. Nos. 2,260,544 and 3,114,188, each of which being incorporated herein by reference in their entirety.

BRIEF SUMMARY OF THE DISCLOSURE

The subject matter disclosed herein relates to a polished rod clamp for coupling a rotary power source to a rod string.
In one illustrative embodiment, the polished rod clamp comprises a clamp housing having a central passage and a tapered bore, where the central passage passes through the tapered bore. A wedge lock is arranged in the tapered bore such that a first surface of the wedge lock is in sliding engagement with a tapered surface of the tapered bore and a second surface of the wedge lock circumscribes the central passage. The second surface of the wedge lock is adapted for gripping a rod extending through the central passage in a lock position of the wedge lock. A first piston is arranged in a first cavity within the clamp housing to reciprocate within the first cavity in response to hydraulic pressure and to apply a first force to the wedge lock that effects a relative sliding motion between the wedge lock and the tapered surface in a first direction corresponding to a lock direction of the wedge lock. A second piston may be arranged in a second cavity within the clamp housing to reciprocate within the second cavity in response to hydraulic pressure and to apply a second force to the wedge lock that effects a relative sliding motion between the wedge lock and the tapered surface in a second direction opposite to the first direction.
In another illustrative embodiment, a drive system for a rod pump comprises a rod pump driver and a polished rod clamp as described above coupled to the rod pump driver. The rod pump may be a rotary pump or an axially reciprocating pump. The rod pump driver is a rotary drive, in the case of a rotary pump, or a mechanism that provides axial reciprocating motion to a rod string, such as a walking beam, in the case of an axially reciprocating pump.
In another illustrative embodiment, a method of coupling a rotary drive to a rod string comprises mounting a polished rod clamp on a drive shaft of the rotary drive such that a central passage of the polished rod clamp is aligned with a central bore of the drive shaft. The rod string is received in the aligned central bore and central passage. Hydraulic pressure is the applied to a first piston in the polished rod clamp to drive a wedge lock in the polished rod clamp to a lock position in which the wedge lock grips the rod string in the central passage.
It is to be understood that both the foregoing general description and the following detailed description are exemplary of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a description of the figures in the accompanying drawings. The figures are not necessarily to scale, and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

FIG. 1 shows an exploded view of a prior art polished rod clamp.

FIG. 2 shows a rotary drive coupled to a sucker rod string.

FIG. 3 shows a cross-section of a hydraulically operated polished rod clamp according to one embodiment.

FIG. 4A shows a perspective view of the wedge lock of FIG. 3.

FIG. 4B shows a top view of the wedge lock of FIG. 4A.

FIG. 5 shows a cross-section of the hydraulically operated polished rod clamp of FIG. 3 in a locking procedure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, numerous specific details may be set forth in order to provide a thorough understanding of disclosed embodiments. However, it will be clear to one skilled in the art when embodiments disclosed herein may be practiced without some or all of these specific details. In other instances, well-known features or processes may not be described in detail in the disclosed embodiments so as not to unnecessarily obscure the disclosure. In addition, like or identical reference numerals may be used to identify common or similar elements.
FIG. 2 shows an illustrative embodiment of a rotary drive 30 including a drive head 32 and a driver or motor 34. The drive head 32 includes a gear box 38, which is coupled to a drive shaft 40. The drive shaft 40 extends out of the drive head housing 42 and is coupled to an output shaft 44 of the motor 34 by a belt drive 46. The motor 34, belt drive 46, and gear box 38 rotate the drive shaft 40 with a desired torque. A polished rod clamp 36 is mounted at the top of the drive shaft 40 and is rotatable with the drive shaft 40. Extending through the polished rod clamp 36 and the drive shaft 40 is a polished rod 48, which is connected to a sucker rod string 45 in a well 47. The polished rod 48 passes through a wellhead stuffing box 43. The weight of the sucker rod string 45 is supported by activating the polished rod clamp 36 to the closed position, where the polished rod clamp 36 engages the polished rod 48. The polished rod 48 can be released by activating the polished rod clamp 36 to the open position. In one illustrative embodiment, the polished rod clamp 36 is activated to the open or closed position by hydraulic pressure.
FIG. 3 shows an illustrative embodiment of the hydraulically-activated polished rod clamp 36. The polished rod clamp 36 includes a clamp housing 50, which in one embodiment is made of an upper clamp body 52 with a flange 54 and a lower clamp body 56 with a flange 58. The upper clamp body 52 and lower clamp body 56 are secured together, e.g., by bolting or otherwise securing the flanges 54, 58 together. In one embodiment, the clamp bodies 52, 56 are constructed of a material that would enable the clamp housing 50 to withstand an internal pressure of 1,000 psi or more. An example of a suitable material is steel. The clamp housing 50 has an axial axis 60 and a central passage 62 running along the axial axis 60. The central passage 62 runs through an upper bore 64 in the upper clamp body 52, a space 66 between the upper clamp body 52 and the lower clamp body 56, a tapered bore (or frusto-conical bore) 68 in the lower clamp body 56, and a lower bore 70 in the lower clamp body 56. For illustration purposes, the polished rod 48 extends through the central passage 62. In other embodiments, the polished rod clamp 36 may be used for clamping of a tubular or cylindrical member other than a polished rod.
The tapered bore 68 has a tapered surface 71 that circumscribes the central passage 62. A wedge lock 76 is arranged in the tapered bore 68 and is movable between a first position to lock the polished rod 48 to the lower clamp body 56 and a second position to unlock the polished rod 48 from the lower clamp body 56. In one illustrative embodiment, the wedge lock 76 has a wedge body 77 terminating in a flange 82. In one embodiment, the wedge body 77 has a frusto-conical shape, with an inner surface 78 that is generally straight and an outer surface 80 that is tapered. The inner surface 78 is for engaging the polished rod 48 in the central passage 62, and the outer surface 80 is for engaging and sliding against the tapered surface 71 of the tapered bore 68. Typically, the taper angle of the outer surface 80 will be substantially the same as the taper angle of the tapered surface 71 of the tapered bore 68. For example, the taper angle may be in a range from 3° to 6°.
As shown in FIGS. 4A and 4B, the wedge lock 76 has an integral longitudinal slot 79 that runs from the bottom of the wedge body 77 to the top of the flange 82. The longitudinal slot 79 enables tangential movement of the wedge lock 76, allowing the wedge lock 76 to conform to the mortise between the tapered surface 71 (in FIG. 3) and the polished rod 48 (in FIG. 3) in the central passage 62 (in FIG. 3), e.g., when locking the polished rod 48 to the lower clamp body 56 (in FIG. 3). Circumferentially spaced longitudinal grooves 81 are formed in the outer surface 80 of the wedge body 77 to allow even distribution of force along the circumference of the wedge body 77. Similarly, circumferentially spaced radial slots 83 are cut into the flange 82 to allow for even distribution of force along the circumference of the flange 82. The grooves 81 and slots 83 help avoid points of excessive strain along the wedge lock 76 as the wedge lock 76 expands or contracts.
Returning to FIG. 3, the wedge lock 76 is preferably made of a corrosion resistant alloy, such as stainless steel. A wear resistant coating, such as a nitride, can be applied to the outer surface 80 of the wedge body 77 that will slide against the tapered surface 71. Further, the outer surface 80 may be polished, e.g., to a minimum root mean square (RMS) finish of 16 μm, to reduce friction between the outer surface 80 and the tapered surface 71. The tapered surface 71 may be similarly treated, i.e., with a wear resistant coating and polishing. The inner surface 78 of the wedge lock 76 may be knurled or include features to enhance the friction of the wedge lock 76 on a polished rod 48.
In FIG. 3, an upper cavity 84 a is formed in the upper clamp body 52, and a lower cavity 84 b is formed in the lower clamp body 56. In one embodiment, the upper and lower cavities 84 a, 84 b are annular in shape and circumscribe the central passage 62. An upper piston 86 a is arranged in the upper cavity 84 a to reciprocate axially along axis 60, and a lower piston 86 b is arranged in the lower cavity 84 b to reciprocate axially along axis 60. In one embodiment, the upper and lower pistons 86 a, 86 b are generally cylindrical in shape. The upper piston 86 a carries seals 88 a for sealing the upper cavity 84 a. Similarly, the lower piston 86 b carries seals 88 b for sealing the lower cavity 84 b. In one embodiment, the upper piston 86 a and lower piston 86 b are arranged in opposed relation, with a space 90 in between them for receiving the flange 82 of the wedge lock 76. This allows either of the pistons to apply a force to the wedge lock 76 in other to move the wedge lock 76 to a lock position or an unlock position. The upper piston 86 a is the locking piston in that it applies force to the wedge lock 76 to move the wedge lock 76 to the lock position. The lower piston 86 b is the unlocking piston in that it applies force to the wedge lock 76 to move the wedge lock 76 to the unlock position.
In an alternate embodiment, the upper piston 86 a and lower piston 86 b may be integrated into a single piston, where the upper part of the piston can function as the upper piston 86 a and the lower part of the piston can function as the lower piston 86 b. In other words, the term “upper piston” may mean a single piston arranged in the upper cavity 84 a or an upper part of a single piston arranged in the combined cavities 84 a, 84 b. Similarly, the term “lower piston” may mean a single piston arranged in the lower cavity 84 b or a lower part of a single piston arranged in the combined cavities 84 a, 84 b.
Ports 92 formed in the upper clamp body 52 are connected to the upper cavity 84 a to allow hydraulic fluid to be fed into or removed from the upper cavity 84 a. Similarly, ports 94 formed in the lower clamp body 56 are connected to the lower cavity 84 b to allow hydraulic fluid to be fed into or removed from the lower cavity 84 b. In one embodiment, the hydraulic fluid used in operating the polished rod clamp 36 is grease. Grease is chosen because it is readily available in the well operation environment and can be injected easily into either of the cavities 84 a, 84 b using a grease gun. However, other types of hydraulic fluids and other methods of injecting the hydraulic fluid into the cavities 84 a, 84 b may be used. In the embodiment shown in FIG. 3, plug fittings 93, 95 are mounted in the ports 92, 94, respectively. These plugs can be selectively replaced with grease or lubricant fittings to allow grease to be injected into either of the cavities 84 a, 84 b, i.e., if grease is the hydraulic fluid used in operating the polished rod clamp. For other types of hydraulic fluids, the plugs may be replaced with suitable fittings to enable injection of the hydraulic fluid into the cavities 84 a, 84 b. It is also possible to use valves instead of plugs for bleeding of fluid.
In one illustrative embodiment, a method of operating the polished rod clamp 36 includes mounting the polished rod clamp 36 on a drive head of a rotary drive as shown in FIG. 2. The lower clamp body 56 includes a mortise 89 (also see FIG. 3) for mating the polished rod clamp 36 to the top of the drive shaft 40 of the drive head 32. In FIG. 2, the polished rod 48 extends through the central passage 66 of the polished rod clamp 36 and through the central bore 41 of the drive shaft 40. The polished rod 48 also extends through the wellhead stuffing box 43 into the well 47. The sucker rod string 45 is connected to the bottom of the polished rod 48. Initially, the polished rod 48 is not locked in the polished rod clamp 36 or the polished rod clamp 36 is not in the lock position. In this case, a lifting device (not shown), such as a crane, is used to initially support the weight of the polished rod 48 and the sucker rod string 45 connected to the polished rod 48.
In one embodiment, as shown in FIG. 5, to lock the polished rod 48 in the polished rod clamp 36, a plug fitting 93 is placed in a port 92 of the upper clamp body 52 and a grease fitting 93 a is placed in another port 92 of the upper clamp body 52 so that the upper cavity 84 a is sealed. For the purpose of describing this locking procedure, it is assumed that there are only two ports 92 in the upper clamp body 52, although it is possible for the upper clamp body 52 to have more than two ports 92. Plug fittings are removed from the ports 94 of the lower clamp body 56 so that the lower cavity 84 b is unsealed. With the upper cavity 84 a sealed and the lower cavity 84 b unsealed, a grease gun is used to extrude grease into the grease fitting, as shown by arrow 96. The grease flows into the upper cavity 84 a, filling and pressurizing the upper cavity 84 a. The hydraulic pressure in the upper cavity 84 a acts on the upper piston 86 a and moves the upper piston 86 a downwardly. The downward motion of the upper piston 86 a forces the wedge lock 76 downwardly into the tapered bore 68. As the wedge lock 76 moves downwardly, its outer surface 80 slides along the tapered surface 71 of the tapered bore 68. Also, the wedge lock 76, being flexible, radially contracts into the mortise between the tapered surface 71 and the polished rod 48 to grip the polished rod 48. When the wedge lock 76 can no longer move downwardly, the polished rod 48 has been locked in the polished rod clamp 36. The upper piston 86 a may engage the lower piston 86 b as the upper piston 86 a moves downwardly. By unsealing the lower cavity 84 b, fluid in the lower cavity 84 b will be able to flow out through the ports 94, allowing the lower piston 86 b to move downwardly. After the polished rod 48 is locked in the polished rod clamp 36, plug fittings are inserted in the ports 94 in the lower clamp body 56. With the polished rod 48 locked in the polished rod clamp 36, the weight of the polished rod 48 and sucker rod string (45 in FIG. 2) connected to the polished rod can be transferred to the polished rod clamp 36 and the lifting device can be disconnected from the polished rod 48.
Returning to FIG. 2, with the polished rod 48 locked in place, rotary motion and torque from the rotary drive 30 can be transferred to the polished rod 48 via the polished rod clamp 36. As the polished rod 48 rotates, the sucker rod string 45 connected to the polished rod 48 will also rotate, thereby powering the pump (not shown) in the well 47.
To unlock the polished rod 48 from the polished rod clamp 36, a lifting device is again connected to the polished rod 48. Then, the plug fitting 93 and grease fitting 93 a shown in FIG. 5 are removed from the ports 92 of the upper clamp body 52 so that the upper cavity 84 a is unsealed. A plug fitting is inserted in one of the ports 94 of the lower clamp body 56, and a grease fitting is inserted in another of the ports 94 of the lower clamp body 56. This seals the lower cavity 84 b. Grease is extruded into the grease fitting located in one of the ports 94 with a grease gun. As the grease is extruded into the grease fitting, the lower cavity 84 b will be filled with and pressurized by the grease, which would force the lower piston 86 b upwardly. The upward motion of the lower piston 86 b will force the wedge lock 76 upwardly in a direction out of the tapered bore 68. As the wedge lock 76 moves upwardly, the outer surface 80 of the wedge lock 76 slides up the tapered surface 71 of the tapered bore 68 and the wedge lock 76 expands to release the polished rod 48. When the wedge lock 76 can no longer move upwardly, the polished rod 48 has been unlocked from the polished rod clamp 36. The lower piston 86 b may engage the upper piston 86 a as it moves upwardly. By unsealing the upper cavity 84 a, the upper piston 86 a will be able to move upwardly, pushing grease out of the upper cavity 84 a.
Grease has been used as an example of hydraulic fluid to operate the polished rod clamp 36. However, it should be clear that other types of hydraulic fluids could be used. It is also possible to connect the ports in the upper clamp body and lower clamp body to a fluid control system that can be operated automatically or remotely to supply hydraulic fluid to either of the upper and lower cavities to lock the polished rod in the polished rod clamp or unlock the polished rod from the polished rod clamp.
The rotary drive 30 of FIG. 2 is just one example of a drive that could be coupled to a rod string using the polished rod clamp 36. Other types of rotary drives may similarly be coupled to a rod string using the polished rod clamp 36. In other words, the polished rod clamp 36 can be used with any rotary drive having a drive shaft on which the polished rod clamp can be mounted.
The polished rod clamp described above can be applied to conventional axially reciprocating pumps as well. The polished rod clamp can be coupled to a mechanism for providing axial reciprocating motion to the rod string and pump, such as a walking beam mechanism. An example of a walking beam is shown in U.S. Patent Application Publication No. 2013/0037257, which is incorporated herein by reference in its entirety.
While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.

Claims

What is claimed is:

1. A polished rod clamp, comprising:

a clamp housing having a central passage and a tapered bore, wherein the central passage passes through the tapered bore;

a wedge lock arranged in the tapered bore such that a first surface of the wedge lock is in sliding engagement with a tapered surface of the tapered bore and a second surface of the wedge lock circumscribes the central passage, the second surface being adapted for gripping a rod extending through the central passage in a lock position of the wedge lock; and

a first piston arranged in a first cavity within the clamp housing to reciprocate within the first cavity in response to hydraulic pressure and to apply a first force to the wedge lock that effects a relative sliding motion between the wedge lock and the tapered surface in a first direction corresponding to a lock direction of the wedge lock.

2. The polished rod of claim 1, further comprising a second piston arranged within a second cavity in the clamp housing to reciprocate within the second cavity in response to hydraulic pressure and to apply a second force to the wedge lock that effects a relative motion between the wedge lock and the tapered surface in a second direction opposite to the first direction.

3. The polished rod clamp of claim 1, wherein the wedge lock comprises a wedge body terminating in a flange.

4. The polished rod clamp of claim 3, wherein a longitudinal slot is integrally formed with the wedge body and flange such that the wedge lock is flexible to expand or contract.

5. The polished rod clamp of claim 3, wherein a plurality of circumferentially spaced longitudinal grooves is formed in the wedge body and a plurality of circumferentially spaced radial slots is formed in the flange to facilitate distribution of force along a circumference of the wedge lock.

6. The polished rod clamp of claim 2, wherein the first piston and second piston are arranged in opposing relation within the clamp housing.

7. The polished rod clamp of claim 6, wherein the wedge lock has a flange received in between the first piston and the second piston such that the first piston and the second piston are configured to apply the first force and the second force, respectively, to the flange.

8. The polished rod clamp of claim 1, further comprising at least a first port formed in the clamp housing for communication of hydraulic fluid to the first cavity.

9. The polished rod clamp of claim 8, further comprising a first fitting removably mounted in the at least a first port, wherein the first fitting is selected from the group consisting of grease fitting, plug, and valve.

10. The polished rod clamp of claim 8, further comprising at least a second port formed in the clamp housing for communication of hydraulic fluid to the second cavity.

11. The polished rod claim of claim 10, further comprising a second fitting removably mounted in the at least a second port, wherein the at least a second fitting is selected from the group consisting of grease fitting, plug, and valve.

12. A drive system for a rod pump, comprising:

a rod pump driver;

a clamp housing coupled to the rod pump driver, the clamp housing having a central passage and a tapered bore, wherein the central passage passes through the tapered bore;

a wedge lock arranged in the tapered bore such that a first surface of the wedge lock is in sliding engagement with a tapered surface of the tapered bore and a second surface of the wedge lock circumscribes the central passage, the second surface being adapted for gripping a rod extending through the central passage; and

a first piston arranged to reciprocate in a first cavity within the clamp housing in response to hydraulic pressure and to apply a first force to the wedge lock that effects a relative sliding motion between the wedge lock and the tapered surface in a first direction corresponding to a lock direction of the clamp.

13. The drive system of claim 12, further comprising a second piston arranged to reciprocate in a second cavity within the clamp housing in response to hydraulic pressure and to apply a second force to the wedge lock that effects a relative motion between the wedge lock and the tapered surface in a second direction corresponding to an unlock direction of the clamp.

14. The drive system of claim 12, wherein the rod pump is a rotary pump and the rod pump driver comprises a rotary drive having a drive shaft with a central bore, wherein the clamp housing is mounted on the drive shaft such that the central passage is aligned with the central bore.

15. The drive system of claim 12, wherein the rod pump is an axially reciprocating pump and the rod pump driver is a mechanism for providing axial reciprocating motion to a rod string.

16. A method of coupling a rotary drive to a rod string, comprising:

mounting a polished rod clamp on a drive shaft of a rotary drive such that a central passage of the polished rod clamp is aligned with a central bore of the drive shaft;

receiving the rod string in the aligned central bore and central passage; and

applying hydraulic pressure to a first piston in the polished rod clamp to drive a wedge lock in the polished rod clamp to a lock position in which the wedge lock grips the rod string in the central passage.

17. The method of claim 16, further comprising applying hydraulic pressure to a second piston in the polished rod clamp to drive the wedge lock to an unlock position in which the wedge lock releases the rod string in the central passage.

18. The method of claim 17, wherein applying hydraulic pressure to the first piston results in sliding of a surface of the wedge lock along a tapered surface in the polished rod clamp in a first direction.

19. The method of claim 18, wherein applying hydraulic pressure to the second piston results in sliding of the surface of the wedge lock along the tapered surface in the polished rod clamp in a second direction opposite to the first direction.

20. The method of claim 19, wherein the first piston is arranged to reciprocate in a first cavity in the polished rod clamp, and wherein applying hydraulic pressure to the first piston comprises injecting grease into the first cavity to pressurize the first cavity.

21. The method of claim 20, wherein the second piston is arranged to reciprocate in a second cavity in the polished rod clamp, and wherein applying hydraulic pressurize to the second piston comprises injecting grease into the second cavity to pressurize the second cavity.