HIGH ASPECT RATIO THREADFORM

Cross-Reference to Related Application

This application claims priority to provisional application 60/835,600, filed August 4, 2006.

Field of the Invention:

This invention relates in general to threadforms, and in particular to threadform for a gate valve stem, the threads having a high aspect ratio.

Description of the Prior Art:

A variety of equipment or tools used in oil field drilling and production have threaded members that rotate relative to each other while under a load. For example, gate valves are often used for oil and gas production control equipment. A gate valve has a body with a cavity. A flow passage intersects the cavity. A gate is moved between open and closed positions relative to the flow passages. The gate valve has to be able to move from a closed position to an open position while under the pressure of the flowline, which may be quite high.

The gate is moved by a stem, which may be either a rising stem or non-rising stem. A rising stem moves axially as it rotates. With a non-rising type, the stem remains axially stationary while being rotated. The stem is threaded in both types. A high flowline pressure acts against contact area of the threads, creating friction. The flowline pressure also forces the gate tightly against the seat, creating friction. These frictional forces must be overcome in order to move the gate from the closed to the

open position. Increasing the diameter of the gate valve bore further increases the frictional forces.

A variety of thread forms have been utilized in the prior art for valve stems and other threadforms under high loads when rotated against each other. In one type, the threads appear in cross-section as frusto-conical shapes, with flat crest and flat roots between the crest. This type of thread, referred to as a modified ACME ₅ is typically hardened and then coated with a dry lubricant.

The modified ACME thread has worked well but under high pressure, the thread undergoes non-uniform contact pressure between the flanks of threads of the drive bushing and the stem. The non-uniform contact pressure overloads the surface treatments and lubricants, resulting in inefficiency and occasionally galling. This is particularly a problem with higher pressure and larger bore valves.

In the prior art, stem and drive bushing threads were designed to have shear strengths that matched the tensile strengths of the stem and drive bushing. The tensile strength of the stem is a function of its diameter, and when the diameter is made larger, the thread bearing area also increases because of the larger diameter. The thread bearing area is the contacting surfaces between threads. In the prior art, the thread profile for thread forms for valve stems were essentially fixed in height or radial dimension, and did not change with an increase or decrease in the diameter of the valve stem. As a result, even though the thread bearing area would increase with an increase in thread diameter, it would not increase at the same rate. The thread bearing stress can more than double when going from a stem diameter of 0.5" to 2.0".

This analysis is based on a threadform axial engagement length equal to one

diameter of the stem, which is a typical industry guideline. Making the thread engagement length longer does not really increase the thread bearing area because the threads along the length of the stem do not load up evenly. Additional threads along the length of the valve stem may be of essentially no benefit.

Summary of the Invention:

The thread form of this invention is configured to reduce contact stress by using a high aspect ratio. That is, the threads have a ratio of thread height over the thread base thickness that is higher than in the known prior art for high load threadforms. Preferably this ratio comprises at least 1.7. Also, the ratio of the thread height to the thread base thickness is at least one-half the natural logarithm of 0.388 divided by a nominal thread diameter of the threads. Preferably the threads are of triangular configuration, each having a rounded crest and rounded root. Preferably, the threads have between them an included angle less than 50 degrees.

The stiffness is held to a maximum so as to provide flexibility and greater contact area. In the preferred embodiment, the thread stiffness is not greater than the pitch of the threads times the included thread angle times a constant C times the natural logarithm of a constant B divided by the pitch diameter of the threads.

Brief Description of the Drawings:

Figure 1 is a sectional view of a gate valve having a stem threadform in accordance with this invention.

Figure 2 is an enlarged sectional view of a portion of the threads of the valve stem of the gate valve of Figure 1.

Figure 3 is an enlarged sectional view of the valve stem threads of Figure 2, shown engaging the drive bushing threads of the gate valve of Figure 1.

Detailed Description of the Invention:

Referring to Figure 1, valve 11 has a body 13 containing a cavity 15. A flow passage 17 intersects cavity 15. A valve seat 19 is located at the intersection of each portion of flow passage 17 with cavity 15. A gate 21 having a hole 22 through it moves from an open position shown in Figure 1 to a closed position. In this embodiment, when in a closed position, gate 21 is located lower within the cavity so that portions of gate 21 block flow through passage 17. Gate 21 is shown as a split gate, having two separate slabs. Alternately, gate 21 could have a single slab.

Gate 21 has a drive bushing 23 that is engaged by a stem 25. Stem 25 extends out of cavity 15 and has stem threads 27 that engage mating threads in drive bushing 23. Stem 25 is shown to be a non-rising type; when rotated, it remains axially stationary and causes translational movement of gate 21 between open and closed • positions. Alternately, stem 25 coiild be a rising type.

Stem 25 extends out of a valve bonnet 28 that is secured to body 13. Bonnet 28 contains a stem packing 29 to seal against pressure, and bearings 31 to reduce friction. In this embodiment, a handle 33 is shown for rotating stem 25. In many cases, an actuator will be employed to cause rotation of stem 25, the actuator either

being hydraulically or electrically driven.

Referring to Figures 2 and 3, stem threads 27 engage bushing threads 35 that are identical in configuration. Each stem thread 27 has a load flank 37 and an unloaded flank 39. Flanks 37, 39 have an included angle a between them that is preferably no more than 50 degrees. In one example, included angle a is approximately 29 degrees.

Stem load flank 37 engages a bushing load flank 41 to prevent bushing 23 from moving axially out of cavity 15 (Figure 1) in response to pressure. Stem unloaded flank 39 is normally spaced from bushing unloaded flank 43 by a clearance as shown in Figure 3, when valve 11 is underpressure.

Each stem thread 27 has a stem root 45 separating it from an adjacent stem thread 27. Root 45 is rounded, formed at a radius in this embodiment. Also, each stem thread 27 has a stem crest 47 that is a reverse image of stem root 45. Stem crest 47 is also rounded, formed at a single radius.

Each bushing thread 35 has a bushing root 49 that is identical to stem root 45. Each bushing thread 35 has a bushing crest 51 that is identical to stem crest 47. While in the loaded position under pressure, shown in Figure 3, bushing root 49 is spaced from stem crest 47 by a clearance. This results in a thread bearing area Ba (Fig. 3).

Stem threads 27 have an aspect ratio much higher than any known prior art for high pressure gate valve stem threads. The aspect ratio is defined as height Th divided by the thickness Tt of thread 27 at its base. In this example, thickness Tt is equal to the pitch, which is the axial distance between two adjoining threads 27. The

height Th of the threads is measured from root 45 to crest 47. This measurement is radial or perpendicular to the measurement of thickness Tt. In the preferred embodiment, thread height Th over the thread base thickness Tt is at least equal to one-half the natural logarithm of 0.388 divided by a nominal thread diameter of the threads. The thread diameter Td is measured from crest 47 on one side to crest 47 at a point 180 degrees away.

This high aspect ratio relationship was derived to provide more flexibility in thread 27 to reduce the contact pressure in bearing area Ba and distribute the load more uniformly. This relationship results in a ratio that is at least 1.7 for the smallest diameter anticipated for valve stem 27, which is 0.5". Larger valve stems will not exceed the value of 1.7 in the preferred embodiment. (Is the bearing area Ba larger than a prior art modified ACME?)

The thread stiffness can be expressed in another manner. In this embodiment, threads 27, 35 have a maximum thread stiffness that no more than the pitch of the threads, which is the same as the thickness Tt in this example, times the included thread angle a, times a constant C, times the natural logarithm of a constant B divided by the pitch diameter Td of the threads. The constant C and the constant B depend upon the material of stem 25 and drive bushing 23. (We need to explain how to derive C and B and give an example)

Threads 27, 35 are designed for the capability of being rotated in high load applications. The contact pressure of threads 27, 35 is less than in one prior art gate valve. The thread bearing area Ba for each thread 27, 35 is greater than the thread bearing area Ba for the prior art modified ACME threadform.

The invention has significant advantages. The high aspect ratio for the threads makes them more flexible, which reduces both the contact pressure on the thread flanks and distributes the load more uniformly over the flanks. Further, the increased thread flexibility helps to distribute the load among the total threads engaged. Also, the high aspect ratio increases the contact area of the threads, further reducing the contact stress. The reduction in stress reduces the chance of wearing away of friction coatings and reduces the possibility of galling.

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 without departing from the scope of the invention.