Patent Application
20140271264
Pumps used in biotech systems and applications, in vitro diagnostics and/or analytical instruments and systems such as liquid chromatography, including high performance and ultra high pressure liquid chromatography, capillary electrophoresis, and mass spectrometry systems require high accuracy and precision. In addition such systems often involve high pressures, potentially corrosive agents, and very small sample sizes. The pumps to be used in such systems thus require exacting designs to meet these objectives.
High precision fluid dispensing systems can utilize a positive displacement pump in which a stepper motor controls the position of a piston. Upon actuation, the stepper motor armature rotates in finite angular steps, thus rotating a lead screw or threaded shaft. This rotation is converted to linear motion through a threaded nut at the central axis of the motor armature. The nut, which can be splined to preclude rotation engages the threaded shaft. As the armature turns, the rotary motion is converted to linear motion of the nut, which is transferred to a piston enclosed in the pump volume. The piston moves in or out of the volume depending on the direction of rotation. The piston volume at full stroke determines the maximum operating volume of the pump. As the piston moves into the pump volume, a volume of fluid equal to the change in the piston volume is dispensed. One example of a high precision pump is the V-series pump commercially available from IDEX Health & Science of Middleboro, Mass.
Two issues that can affect pump function and longevity are backlash and friction. Backlash is the amount of lost motion due to play or clearance between mating components in a mechanical system. For the positive displacement pumps addressed herein, the mating components are the threads of the lead screw and the mating threads on the nut. When there is a significant clearance between the mating threads, during the initial motion of the lead screw, particularly as the direction of rotation is reversed, its threads would not be in contact with the threads of the nut, causing inaccuracy in the piston motion. However, a precise fluid dispensing system such as the positive displacement pump of this disclosure relies on having little or no backlash in the drive train in order to be considered an accurate dispensing tool.
Any solution for the reduction of backlash, however, must also consider increased friction that could be a side result of the backlash solution. Increased friction can decrease axial force output translated from motor torque and can also shorten pump longevity and increase maintenance costs and down time. The conventional anti-backlash methods, either biasing the nuts apart or radial pressure, induce more friction within the threads. Methods that push the threads apart require the spring force to be greater than the pump hydrostatic force in order to keep equilibrium between the two thread halves. The radial designs load both the leading and trailing thread flanks. This creates twice the friction used in either a push or pull method.
In many applications, it is desirable to have a pump that will have an expected lie and perform accurately through one million or more cycles, and even more preferably to perform to five million or more cycles, which is the expected life of the instrumentation. There is still a need in the art, therefore, for an anti-backlash method to be used in pumps that can minimize increased friction or wearing of the threads, in order to address backlash while preserving pump longevity.
The present disclosure addresses the problems of the prior art by providing an anti-backlash nut assembly for a pump that pushes the threads together rather than pulling them apart. This method requires a smaller spring force than is required for systems that push the nut halves apart, and allows the same side of the thread flank on both nut halves to be loaded. This method thus reduces production cost and maintenance and increases pump longevity and performance when compared to prior art anti-backlash devices.
The following drawings form part of the present specification and are included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to one or more of these drawings in combination with the detailed description of specific embodiments presented herein.
The present disclosure is directed to systems and methods of reducing backlash in a positive displacement pump system including an anti-backlash nut assembly and a positive displacement pump including the nut assembly. The pump works by reciprocal action of a piston in a chamber in which the piston is withdrawn, creating vacuum and pulling liquid into the chamber and on the reverse stroke expelling the fluid. In the pump, rotary motion is converted to linear motion by a mechanism that includes motor shaft attached to an externally threaded lead screw that turns in a nut assembly. Backlash is caused by clearance between the mating threads. As described herein, the nut assembly can include a primary nut, which engages the piston and a secondary nut, which is disposed on the motor side of the assembly. The two components can be interchangeably referred to as a primary nut and a secondary nut or as nut halves. The primary nut is used to describe the nut that contacts the piston and the secondary nut is between the primary nut and the motor.
The anti-backlash mechanisms of the present disclosure reduce or eliminate backlash by removing clearance between the internal threads of the primary and secondary nuts and the external threads of the lead screw. As used herein, reducing backlash refers to reducing measurable backlash by a detectable amount, usually indicated by percent of total pump volume. Eliminating backlash refers to reducing backlash to near the limit of the appropriate volume measurement capability. With the disclosed devices and methods, the reduction or elimination of backlash is accomplished in a novel way by biasing the nuts together. In conventional anti-backlash pump assemblies, the primary and secondary nuts are pushed apart or subjected to radial pressure to reduce the clearance between the internal and external threads.
The disclosed nut assemblies provide advantages specifically for a pump system as opposed to a system with an equal load in both directions. In a positive displacement pump, aspiration is the process of drawing fluid into the piston chamber. This process draws a vacuum thus creating an opposing load to the spring tension. During the dispensing process hydrostatic force is created in the opposite direction tending to push the primary and secondary nuts together. The force on the spring is thus substantially lower during fluid aspiration compared to dispensing. In biasing the nuts apart, therefore, the spring force is required to balance the much higher dispensing hydrostatic force. For example, a typical aspiration force in a liquid chromatography application the dispensing hydrostatic force can be up to 150 times higher than the vacuum in the aspiration cycle.
The disclosed devices and methods, therefore do not require a large spring force to balance the hydrostatic load imposed on the threads during the dispensing step as is necessary with systems that bias the nuts apart from. In the disclosed devices, the necessary restoring spring force is derived by only having to keep the two threads in equilibrium during an aspiration cycle, which is at much lower pressure. Another advantage over the prior methods is uniform loading of threads with a single spring.
Certain preferred embodiments of the disclosure are shown in the attached figures.
A side view of an isolated anti-backlash nut assembly is shown in
An exploded view of the nut assembly is shown in
As described above, the positive displacement pump works by converting the rotating of the lead screw through a non-rotating nut which moves along the screw pushing the piston in or out of the fluid chamber. In
A second embodiment of the anti-backlash nut assembly is shown in
This embodiment can be better understood from the cross-section view with the lead screw 120 in place in
While not limited by size unless otherwise indicated, in certain embodiments the precision dispense positive displacement pump assemblies of the disclosure have a pump volume of from about 25 μl to about 5000 μl as used in automated chemical reactions or liquid chromatography applications, or in certain embodiments a volume of 25 μl, 50 μl, 100 μl, 250 μl, 500 μl, 1000 μl, 2500 μl or 5000 μl. Such pumps can have a pump stroke of from about 0.25 to 1.0 inch, or in certain embodiment 0.5 inches. The lead screw for such instruments can typically have a resolution of 20 turns per inch or 40 turns per inch.
The anti-backlash nut assemblies described herein can be used with various pumps including low pressure pumps with pressures no greater than 100 psi and high pressure pumps with pressures up to 1500 psi or greater. The materials can be any appropriate materials known in the art, depending on the application. In certain embodiments, the backplate can be manufactured of stainless steel and the ferrules or nuts manufactured of natural PEEK (polyether ether ketone), for example.
All of the devices and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the devices and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the devices and/or methods and in the steps or in the sequence of steps of the methods described herein without departing from the concept, spirit and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.
