Rotary shear valve with a two-pin drive shaft for liquid chromatography applications

A rotary shear valve assembly for liquid chromatography applications comprises a rotor assembly having a rotor and a drive shaft with a head portion. The rotor has a substantially planar surface with one or more rotor grooves and a pair of holes. The head portion has two pins. The pins are disposed substantially diametrically opposite of each other on a line through a center of the head portion. Each pin mates with one of the holes in the rotor. The rotor assembly can further comprise means for urging the rotor surface against the stator surface such that each rotor groove aligns with and provides a fluidic channel between two of the stator openings.

FIELD OF THE INVENTION

The invention relates generally to valve assemblies for switching pressurized fluids. More particularly, the invention relates to rotary shear valve assemblies for liquid chromatography applications.

BACKGROUND

High-pressure liquid chromatography systems, such as high performance liquid chromatography (HPLC) and ultra performance liquid chromatography (UPLC) systems, typically employ injection valves having a rotary shear seal. A force of several hundreds of pounds or more is applied between the rotor and stator to seal against pressures that can exceed 15,000 psi. The force is maintained while the rotor rotates between valve switch positions, thereby placing stringent requirements on the quality of the sealing surfaces. The injection valves are typically designed for tens of thousands of cycles without excessive wear and leakage. Liquid chromatography instrument manufacturers anticipate future instruments will require sealing pressures of injection valves to exceed 18,000 psi and still provide tens of thousands of cycles. The trend to greater operating pressures may be beyond the capabilities of design and materials of conventional injection valves. Increasing the compressive load may suffice to achieve higher sealing pressures, but this approach can result in faster wear of the rotor and stator, with an associated increased leakage and reduced valve lifetime.

SUMMARY

In one aspect, the invention features a rotary shear valve assembly comprising a rotor assembly having a rotor and a drive shaft with a head portion. The rotor has a substantially planar surface with one or more rotor grooves and a pair of holes. The head portion has two pins. The pins are disposed substantially diametrically opposite of each other on a line through a center of the head portion. Each pin mates with one of the holes in the rotor.

In another aspect, the invention features a rotary shear valve assembly comprising a rotor assembly having a drive shaft and a rotor. The drive shaft has a head portion with only two pins extending orthogonally from a distal surface of the head portion. The rotor has a substantially planar surface with one or more rotor grooves, a first hole for tightly receiving one of the two pins of the head portion, and a slot for receiving the other of the two pins.

DETAILED DESCRIPTION

Rotary shear valve assemblies described herein have a two-pin drive shaft for holding a rotor in relation to a stator. The two pins of the drive shaft bear the driving force used to rotate the rotor under a compressive load. To couple to the drive shaft, the rotor has a mating hole and a slot for receiving the two pins of the drive shaft. The size of the mating hole is designed to closely receive one of the pins of the drive shaft, whereas the slot provides sufficient clearance in one dimension to facilitate the process of slipping the rotor over the pins of the drive shaft.

Having only two drive shaft pins has advantages over conventional three-pin shafts by achieving better alignment of the rotor to the stator through tighter tolerances and by providing a truer “on center” rotation force through the rotor. Having two pins, instead of three, produces less tolerance stack-up error for positioning the rotor to the stator, there being fewer critical dimensions that can contribute to the position error, and permits less clearance between one pin of the drive shaft and the rotor's mating hole. The reduced position error improves alignment between the rotor and stator, leading to less fluidic carryover, flow restriction, and dispersion. Having two pins also enables those driving forces acting on the pins to be diametrically opposed, producing a truer rotation effect of the rotor around the axis of the drive shaft and, consequently, prolonging valve life because of a more uniform wear of the rotor and stator than what is presently produced by three-pin drive shafts.

FIG. 1Ashows a side view of an embodiment of a rotary shear valve assembly10including a stator12secured to one end of a housing14and a drive shaft clamp16at the opposite end of the housing14.FIG. 1Bshows a cross-sectional view of the rotary shear valve assembly10taken along line A-A inFIG. 1A,FIG. 1Cshows an exploded view of the rotary shear valve assembly10, andFIG. 1Dshows the rotary shear valve assembly10from the end with the drive shaft clamp16. Mounting screws18secure the stator12to a flange20of the housing14. The housing14substantially encloses a rotor assembly22comprised of a disk-shaped rotor24, a drive shaft26with a head portion28, four springs30grouped in two sets of two separated and flanked by washers32, a spacer34, a thrust bearing36sandwiched between bearing washers38, and, optionally, a shim40.

The rotor24is coupled to the head portion28of the rotor assembly22. Extending orthogonally from the distal face of the head portion28are two pins42-1,42-2that enter corresponding openings (FIG. 4A) in the rotor24. A substantially planar surface44of the rotor24abuts an opposing surface46of the stator12. In addition, the rotor24sits on a raised region or dais48of the head portion28. The dais48concentrates the force applied to the rotor and is preferably smaller than the base surface of the rotor24, so that the rotor24may slightly teeter on the dais48to facilitate complete contact between the rotor and stator surfaces44,46. In various embodiments, the rotor surface44is made of a PEEK or carbon-reinforced PEEK material and the stator surface46is made of a metallic material coated with a layer of DLC (Diamond-like Carbon) that dramatically reduces the friction between the stator and rotor surfaces. These combinations of materials have been demonstrated to achieve effective sealing for tens of thousands of cycles of valve rotation.

The drive shaft26extends through an opening at the base of the housing14. The end of the drive shaft26extends into an opening50of the drive shaft clamp16, which is appropriately shaped to closely receive a notched end (FIG. 3A) of the drive shaft. The end of the drive shaft26has a notch. A threaded screw52passes through pincers54, which tightens the opening50about the drive shaft's end to hold the drive shaft26securely. When secured properly, the end of the drive shaft26is almost flush with the plane of the clamp16. Alignment grooves56,58on the housing14and clamp16, respectively, are used to position these units appropriately for coupling the clamp16to the draft shaft26. A drive mechanism (not shown) couples to holes60in the clamp16in order to provide a rotating force about a central axis62(FIG. 1B).

The compression of the springs30translates to an axial force to the rotor24, urging the rotor surface44against the stator surface46and maintaining a fluidic seal at the interface of these surfaces44,46. In one embodiment, the springs30are clover springs. Other types of springs can be used, for example, Belleville washers, without departing from the principles described herein. In one embodiment, the compressive load achieved by the springs30is approximately 600 lbs. and is designed to produce a seal between the rotor and stator that can prevent leakage at fluidic pressures at least as great as 20,000 psi. For example, in UPLC instruments, the fluidic pressure typically ranges between 15,000 psi and 20,000 psi. The springs30maintain the applied force applied throughout the rotation of the drive shaft26and the rotor24.

The spacer34serves to separate the thrust bearing36and bearing washers38from the spring stack comprised of the springs30and spring washers32. The thrust bearing36and bearing washers38facilitate rotation of the drive shaft. The shim40is used to achieve the desired amount of compression along the axis of the draft shaft, with additional shims being added to the drive shaft until the compressive load produced by the springs30reaches the desired target of, for example, approximately 600 lbs.

FIG. 2shows a top view of the rotary shear valve assembly10from the end with the stator12. The stator12has six ports70, each extending to an opening at the contact surface46of the stator. Each port70couples to a fluidic tube or channel (not shown), by which fluid flows to or from the rotary shear valve assembly10. Rotation of the rotor24with respect to the stator12changes the connectivity of the ports70, as described in more detail below. The stator12also has a guide hole72for receiving an alignment pin64(FIG. 1C) extending from the leading raised ring of the housing14.

FIG. 3Ashows an isometric view of an embodiment of the rotor assembly22, including the head portion28of the drive shaft26. The head portion28has a generally disk-like shape with the dowel pins42-1,42-2(generally,42) and the dais48extending from a surface thereof. The pins42are diametrically opposite of each other; that is, considering the pins42to be endpoints of an arc on the circumference of this circle having its center at the center of the dais48, the arc defined by the pins is semicircular (i.e., 180 degrees). These pins42enable torque transfer, and thus, rotation of the rotor assembly22as the drive shaft26rotates about the rotational axis62. In one embodiment, the pins42are equal in length and pin42-1has a larger diameter than pin42-2. Having one pin larger than the other pin provides a keying feature that ensures only one orientation by which the head portion28can couple to the rotor24. Corresponding through-holes in the rotor24slideably receive the pins42in order to mount and align the rotor24relative to the drive shaft26.

Also shown, the drive shaft26has a first portion26-1(adjacent the head portion28) with a greater diameter than a second portion26-2. At the end of the drive shaft26is a notch80, sized to fit closely into the opening50(FIG. 1C) of the drive shaft clamp16.FIG. 3Bshows the rotor assembly22with the various springs30(here, e.g., clover springs) and washers32slipped over the drive shaft26(and uncompressed). For each set of two, the concave sides of the two clover springs30face the same direction. In addition, the concave sides of the two clover springs30in each set face in the direction of the concave sides of the two clover springs30in the other set. Preferably, the two springs in each set are in alignment with each other during assembly, although the two sets need not be in alignment with each other.

FIG. 3Cshows an end view of the leading face of the head portion28with the two pins42-1,42-2and centrally located dais48. In one embodiment, each pin42extends approximately 0.16 inches from a surface of the head portion28, pin42-1having an approximately 0.109 inch diameter, pin42-2having an approximately 0.093 inch diameter, and the centers of the pins being 0.500 inches apart, with each pin being 0.250 inches from the center of the dais48. In addition, in this embodiment, the dais48is raised approximately 0.012 inches from the surface of the head portion28. Other pin sizes and locations can be employed without departing from the principles described herein.

FIG. 4Ashows an isometric view of the disk-like shaped rotor24with a set of rotor grooves90disposed centrally on the contact surface44of the rotor24. The length and position of the grooves90in the rotor surface44align the grooves90for coupling to various ports70of the stator12to other ports70of the stator12when the rotor24and stator12are in particular rotational alignments. In this embodiment, there are three rotor grooves (the rotor shear valve assembly being configured as an injection valve). Other embodiments can have one, two, or more than three rotor grooves, for use in other types of valves, such as vent valves and column manager valves.

In addition, the rotor24has two diametrically opposite openings92-1,92-2(corresponding to the two pins of the drive shaft). The opening92-1, referred to as a mating hole, is adapted to receive the smaller pin42-2of the rotor assembly22closely with tight tolerance. In one embodiment, the mating hole92-1has a diameter of approximately 0.095 inches for closely receiving the 0.093 diameter embodiment of the smaller pin42-2. The opening92-2is an elliptically shaped slot adapted to receive the larger pin42-1of the two pins, with a greater measure of tolerance along the direction of the major axis of the slot than along the minor axis. In one embodiment, the minor axis of the slot92-2is approximately 0.110 inches wide for receiving the 0.109 diameter embodiment of the larger pin42-1. The rotor24can slide onto the pins42of the head portion28without pressing. The ends of the pins42within the holes92of the rotor are approximately flush with the contact surface44of the rotor.

FIG. 4Bshows a top view of the rotor24with a cross-sectional line A-A bisecting the openings92-1,92-2and center point94of the rotor and passing though the ends of two of the rotor grooves. The center point94is the center of rotation of the rotor24. In one embodiment, the center point94has an approximately 0.008 inch diameter. Detail region96encircles the rotor grooves90and center point94.

FIG. 4Cshows a cross-section of the rotor24taken along the line A-A ofFIG. 4B. In the cross-section, each opening92-1,92-2extends entirely through the rotor24. In addition, to provide a sense of scale, the center point94and rotor grooves90appear as dark dots immediately below the surface44of the rotor24. Detail region98surrounds the center point94and the one of the rotor grooves90.

FIG. 4Dshow a view of the detail region96ofFIG. 4B, including the three rotor grooves90-1,90-2, and90-3(generally,90). The rotor grooves90are arcuate in shape, and reside equidistant from the central point94. In one embodiment, the rotor groove90-1forms an approximately 74 degree arc and rotor grooves90-1and90-2form 60 degree arcs, each groove being approximately 0.008 inches in width.FIG. 4Eshows a view of detail region98ofFIG. 4C, the rotor groove90-3(representative of all grooves90) being a shallow channel and the center point being a shallow hemisphere formed in the surface44of the rotor24. In one embodiment, the depths of the grooves90and center point94are approximately 0.008 inches.

FIG. 5Aand FIG. B are diagrams illustrating proper alignment and misalignment, respectively, between stator ports70and rotor grooves90. InFIG. 5A, each rotor groove90-1,90-2, and90-3fully encircles two of the stator ports70, thereby providing a fluidic channel between those stator ports. InFIG. 5B, a 0.002-inch misalignment110between the rotor24and the stator12can cause regions where the contact surface44of the rotor24covers a portion of a stator ports70. Such misalignment leads to partial flow blockage, fluidic carryover, increased dispersion, uneven wear of the rotor and stator, and, consequently, reduced valve life. The tolerances of the two-pin draft shaft can improve rotor and stator alignment over conventional three-pin draft shafts.

FIG. 6AandFIG. 6Bare diagrams comparing the dimensioning of the two-pin drive shaft with that of a conventional three-pin drive shaft. For dimensioning the two-pin drive shaft, shown inFIG. 6A, the length of the slot92-2makes one of the two dimensions non-critical, in particular, the y-axis distance between the centers of the two pins42. After the smaller pin42-2is in alignment with the mating hole92-1, the length of the slot92-2provides a measure of tolerance for where the larger pin42-1can enter. In contrast, for the three-pin drive shaft shown inFIG. 6B, the vertical (y-axis) and horizontal (x-axis) distances between the centers of each pair of pins become critical dimensions that must be machined precisely to ensure alignment between the pins42and the holes92. Thus, in comparison with the three-pin drive shaft, the two-pin drive shaft requires less through-hole clearance for the mating hole to receive the smaller pin, and leads to fewer critical position dimensions and less positional error.

FIG. 7AandFIG. 7Bare diagrams comparing the forces applied to the pins of the two-pin drive shaft with those applied to a three-pin drive shaft. For the two-pin drive shaft shown inFIG. 7A, the forces F1, F2on the pins42-1,42-2, respectively, are diametrically opposite and equal, having a true “on-center” rotation effect. In contrast, for the three-pin drive shaft shown inFIG. 7B, only two of the three pins of the three-pin drive shaft bear most of the load because of positional errors; the force F3does not act directly on the remaining pin. Consequently, the forces F1and F2applied to the other pins, in effect, carry the third pin. Thus, the forces acting upon the three pins are not equal for each pin, nor are the forces acting symmetrically about the center100of the rotor. This can lead to unequal rotor wear. Thus, in comparison with the three-pin drive shaft, the two-pin drive shaft leads to more uniform rotor wear and symmetrical internal rotor stresses, which leads to prolonged valve life.

The rotary shear valve assemblies described herein can be employed in a variety of high-pressure applications, examples of which include, but are not limited to, HPLC (High Performance Liquid Chromatography), UPLC (Ultra Performance Liquid Chromatography), analytical chemistry, and In-vitro Diagnostics (IVD). With respect to liquid chromatography applications, the rotary shear valve assemblies can be adapted for use as an injector valve assembly of a sample manger, a vent valve assembly of a pump system, and a column-manager valve assembly of a column manager.

While the invention has been shown and described with reference to specific preferred embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the following claims. For example, in other embodiments, the two pins of the drive shaft and corresponding openings in the rotor may not be diametrically opposite each other; that is, the aforementioned arc defined by the pins (and corresponding rotor openings) can be other than 180 degrees.