Pumps – Motor driven – Electric or magnetic motor
Patent
1991-09-26
1993-04-13
Bertsch, Richard A.
Pumps
Motor driven
Electric or magnetic motor
74 8915, F04B 3500, F04B 1300, F16H 2920
Patent
active
052018512
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pump and metering apparatus having improved flow stability.
2. Description of Background Art
For certain analytical techniques, e.g. within medical research, access to pumps capable of low, stable flows is required. A conventional type of such a pump basically consists of a syringe and a means for actuating the syringe plunger. The latter device usually comprises a rotatably mounted screw, along which a runner engaging the screw and actuating the syringe plunger is moved by rotating the screw through either a stepping motor or a DC motor and a gear-wheel transmission. Factors which directly will influence the precision and flow stability of such a pump device are backlashes and wobbles in gear-wheel transmissions, slip-stick phenomenons of the syringe plunger, temporary absorption of energy in connections between driving motor and screw, and elongations of the pump chassis. While the flow stability achievable by such a pump device is completely sufficient for many purposes, there has recently, e.g. in biosensor technology, arisen a need for pump devices having improved flow stability performance.
While several of the above enumerated disturbing factors may be reduced or eliminated relatively easily, e.g. by replacing a DC motor and necessary transmission with a stepping motor and/or avoiding elastic couplings for eliminating slip-sticks, it was found to be more difficult to overcome the disturbances in the form of flow ripple caused by wobbles of the screw, either due to it not being completely straight or not being completely aligned with the motor axis. Such wobbles cause the runner threads to travel up and down on the thread flank of the screw, and also very small wobbles have been found to give unacceptable flow disturbances in cases where the requirements of flow stability are high.
It is known to use so-called ball screws to avoid travelling on the screw thread flank. Such ball screws are, however, relatively expensive, and the biasing of the balls must also be continuously adjusted in accordance with the wearing of the balls for them to contact the thread flank all the time.
SUMMARY AND OBJECTS OF THE INVENTION
The object of the present invention is to provide an improved pump and metering apparatus, in which the above mentioned problem of flow instability due to wobbles of the screw has been eliminated and which may thus be made to fulfil very high requirements concerning flow stability.
According to the invention this is achieved by a pump and metering apparatus, which supported in a chassis or frame has a liquid container with a movable plunger therein, by means, of which liquid may be pressed out from or drawn into the container. The plunger is connected to a runner means which through a nut portion thereof is in thread engagement with a screw rotatably mounted in the frame and connected to a driving motor. The rotation of the screw caused by the driving motor brings about a longitudinal movement of the runner and thereby a corresponding displacement of the plunger in the container. The invention is characterized in that the above mentioned nut portion is arranged to be movable in the radial direction of the screw relatively to the remaining runner structure. Hereby self-centering of the runner means nut portion on the centre axis of the screw is achieved, whereby all tendencies to travelling on the thread flank of the screw, e.g. due to a not completely straight screw or the screw being inclined in relation to the motor axis, are eliminated such that excellent flow stability is achieved.
In the present context radial movability means movability in all radial directions with respect to the screw axis. Such radial movability of the nut portion of the runner means, simultaneously with rigidity in the axial direction of the screw, may be achieved in various ways within the scope of the invention; the term nut portion is herein to be understood in a wide sense and may also constitute the major part of or sub
Bertsch Richard A.
McAndrews Roland
Pharacia Biosensor AB
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