Couplings for well pumping components

1. A sucker rod string, comprising:

a sucker rod including an end having a pin with an external thread;

a valve rod bushing including a first end having a pin with an external thread and a second end having a counterbore with an internal thread, wherein the valve rod bushing connects to a downhole pump; and

a coupling comprising a core having a first end, a central portion, and a second end, the first end and the second end each having an end surface, wherein the first end tapers linearly inwards from the central portion to the first end surface; and

wherein the coupling is made from a spinodally-hardened copper-nickel-tin alloy that has a sliding coefficient of friction of 0.175±10% to less than 0.4 when measured against carbon steel, and wherein a threaded bore runs entirely through the core from the first end to the second end;

wherein the threaded bore at the first end of the coupling is complementary with the external thread of the valve rod bushing, and the threaded bore at the second end of the coupling is complementary with the external thread of the sucker rod.

2. The sucker rod string of claim 1 , wherein the second end surface has a rounded edge, or wherein the second end tapers linearly inwards from the central portion to the second end surface.

3. The sucker rod string of claim 1 , wherein an outer diameter of the sucker rod is greater than an outer diameter of the valve rod bushing.

4. The sucker rod string of claim 1 , wherein an outer diameter of the coupling is greater than both an outer diameter of the sucker rod and an outer diameter of the valve rod bushing.

5. The sucker rod string of claim 1 , wherein the first end surface abuts a shoulder portion of the valve rod bushing.

6. The sucker rod string of claim 1 , wherein the second end surface abuts a shoulder portion of the sucker rod.

7. The sucker rod string of claim 1 , wherein the alloy comprises from 8 to 20 wt % nickel, and from 5 to 11 wt % tin, the remaining balance being copper, wherein the alloy has a 0.2% offset yield strength of 75 ksi to 150 ksi.

8. The sucker rod string of claim 1 , wherein the alloy comprises 14.5 wt % to 15.5 wt % nickel, and 7.5 wt % to 8.5% tin, the remaining balance being copper.

9. The sucker rod string of claim 1 , wherein the alloy has a 0.2% offset yield strength of 95 ksi to 150 ksi and a Charpy V-notch impact energy of 22 ft-lbs to 30 ft-lbs at room temperature.

10. A coupling for a sucker rod string, comprising:

a core having a first end, a central portion, and a second end, the first end and the second end each having an end surface,

wherein (A) the first end tapers linearly inwards from the central portion to the first end surface, and (B) the second end surface has a rounded edge, and (C) wherein a diameter of the first end surface is less than a diameter of the second end surface; and

wherein the coupling is made from a spinodally-hardened copper-nickel-tin alloy and has a sliding coefficient of friction of 0.175±10% to less than 0.4 when measured against carbon steel.

11. The coupling of claim 10 , further comprising a threaded bore running entirely through the core from the first end to the second end.

12. The coupling of claim 11 , wherein the threads of the bore have a Rockwell C hardness (HRC) of 20±10% to 40±10%.

13. The coupling of claim 10 , where the first end and the second end each also includes a counterbore.

14. The coupling of claim 10 , wherein the alloy has a 0.2% offset yield strength of 75 ksi to 150 ksi.

15. The coupling of claim 10 , wherein the alloy comprises 14.5 wt % to 15.5 wt % nickel, and 7.5 wt % to 8.5% tin, the remaining balance being copper.

16. The coupling of claim 10 , wherein the alloy has a 0.2% offset yield strength of 85 ksi to 150 ksi.

17. The coupling of claim 10 , wherein the alloy has a 0.2% offset yield strength of 95 ksi to 150 ksi and a Charpy V-notch impact energy of 22 ft-lbs to 30 ft-lbs at room temperature.

18. A method of extracting a fluid from a well, comprising:

operatively connecting a downhole pump to a motor using a sucker rod string; and

operating the downhole pump using the sucker rod string to extract fluid from the well;

wherein the downhole pump includes a valve rod bushing including a first end having a pin with an external thread and a second end having a counterbore with an internal thread;

wherein a sucker coupling connects the valve rod bushing to the sucker rod string; and

wherein the sucker coupling comprises:

a core having a first end, a central portion, and a second end, the first end and the second end each having an end surface,

wherein (A) the first end tapers linearly inwards from the central portion to the first end surface, and (B) either (i) the second end surface has a rounded edge or (ii) the second end tapers linearly inwards from the central portion to the second end surface; and

wherein the coupling is made from a spinodally-hardened copper-nickel-tin alloy and has a sliding coefficient of friction of 0.175±10% to less than 0.4 when measured against carbon steel.

19. The method of claim 18 , wherein a mean time between failure (MTBF) of the sucker coupling is three to five times greater than the MTBF of a coupling made from L80 carbon steel.

20. A pump system comprising:

a downhole pump;

a power source for powering the downhole pump; and

a rod string located between the downhole pump and the power source;

wherein the rod string comprises:

a sucker rod including an end having a pin with an external thread;

a valve rod bushing including a first end having a pin with an external thread and a second end having a counterbore with an internal thread, the valve rod bushing being connected to the downhole pump;

a coupling comprising a core having a first end, a central portion, and a second end, the first end and the second end each having an end surface, wherein the first end tapers linearly inwards from the central portion to the first end surface, and either (i) the second end surface has a rounded edge or (ii) the second end tapers linearly inwards from the central portion to the second end surface; and wherein the coupling is made from a spinodally-hardened copper-nickel-tin alloy that has a sliding coefficient of friction of 0.175±10% to less than 0.4 when measured against carbon steel, and wherein a threaded bore runs entirely through the core from the first end to the second end;

wherein the threaded bore at the first end of the coupling is complementary with the external thread of the valve rod bushing, and the threaded bore at the second end of the coupling is complementary with the external thread of the sucker rod.