DRIVE MECHANISM

Abstract: A lead screw drive comprises a lead screw (10) which is rotated by a motor. A traveller (40) is clamped at one end to a rod (14) parallel to the lead screw, the rod in turn engaging the plunger of a medical syringe. The other end of the traveller (40) has a U-shaped engaging element (42) with the two parallel arms (44, 46) thereof having grooves on their inner surfaces for engaging with the lead screw thread. The grooves also extend upwardly and downwardly of the region of engagement with the lead screw thread, and radial movement of the engaging element (42) therefore moves the region of contact with the lead screw without causing disengagement. The device is particularly suitable for use with a medical syringe pump, to prevent inadvertent disengagement of the engaging element from the lead screw.

Description: DRIVE MECHANISM The present invention relates to lead screw drive mechanisms such as are used to convert rotational to linear movement, and particularly, but not exclusively, to such mechanisms used with medical syringe pumps. Such drives are commonly used to convert rotational movement imparted to a male threaded part to linear motion of a female threaded part, which engages the male part and is driven linearly along the male part as the male part rotates. One application of such a drive mechanism is in medical syringe pumps such as are commonly used for infusing solutions of a drug into the body of a patient. In such pumps, the body of a replaceable syringe containing the solution to be infused is held clamped in a suitable cradle or frame which mounts the syringe drive components and a linear force is applied to the plunger of the syringe to move a piston within the syringe body in order to expel the solution. In such pumps the force is commonly applied to the plunger by means of a pusher attached to a running nut (the female part) which engages a rotating lead screw (the male part) driven by an electric motor. It is a common requirement in such systems to be able to readily disengage the running nut from the lead screw to enable the pusher engaging the plunger to be displaced, for example, to enable the syringe to be changed without the need to rotate the lead screw until the running nut, and the linked pusher, reaches the desired position upon the lead screw. One method of doing this is to employ a split- or half-nut arrangement as the running nut, with means such as a manually operated mechanical linkage which enables the split- or half-nut either to operatively engage the lead screw, or to be displaced from that position until thread contact is lost, to disengage from it. With a half-nut arrangement the thread of the half-nut thus engages the lead screw over some 180 degrees or less, and with the split-nut arrangement, each of the two half-nuts engages the lead screw over some 180 degrees or less, to permit engagement and disengagement with the lead screw. A shortcoming of such arrangements is that for commonly-used threadforms the half-nut or half-nuts tend to be forced out of engagement with the lead screw when driven against a resisting force such as a syringe operating against a back-pressure. Disengagement of the half-nut (s) may then occur due to the inclined face of the threading of the half-nut (s) producing a radial displacing force on the half-nut (s) when axial force is applied to them from the lead-screw. To combat such shortcomings the use of special thread-forms e.g. so-called buttress threads with one inclined and one straight face, has been proposed. However as a group these special thread-forms have various disadvantages, including high manufacturing cost as some cannot be manufactured by the conventional rolling process, limitation in some cases to a relatively coarse pitch, and, such as in the case of the buttress thread, unsuitability for bidirectional drives. Alternatively, complicated mechanisms have been used to constrain the half-nut or half-nuts from being displaced from the lead screw, such mechanisms in some cases incorporating detector arrangements to detect and signal engagement or disengagement of the half-nut (s) . It is an object of the present invention to provide an improved mechanism of the general type described in which the disadvantage of the conventional half-nut or half-nuts is avoided by use of an element which replaces the conventional half-nut (s). It is a further object of the invention to provide a syringe pump incorporating such an improved mechanism. According to one aspect the invention consists in a lead screw drive comprising a lead screw and a disengagable lead-screw-engaging element, the engaging element having a drive portion upon a face thereof for driving engagement with the lead screw, characterised in that the drive portion on the engaging element extends beyond the region of contact with the lead screw, such that driving engagement is maintained in the event of displacement of the engaging element with respect to the lead screw in a direction generally parallel with the said face. The advantages of using a half-nut arrangement, particularly the case of disengagement from the lead screw when desired, are thus maintained, whilst the disadvantages of the half-nut are overcome by using a thread on the engaging element which extends beyond the region of contact with the lead screw. Preferably, the engaging element is substantially U- shaped, the drive portion then preferably comprising at least one thread element. Thread elements may be located on each of the parallel faces of the engaging element. The thread element may comprise one or more grooves and, additionally or alternatively, one or more protrusions located on the engaging element. The engaging element may further comprise a stop on the base portion bridging the two parallel faces. Alternatively, the drive portion may be a substantially continuous thread between the parallel faces of the engaging element and its base portion. The invention is particularly suited for use in a medical syringe pump. Accordingly, the engaging element may preferably be connected to a rod via a traveller, the rod being arranged parallel to the lead screw and being connected to a pusher for pushing a plunger of a syringe into a syringe barrel. To minimise frictional losses during displacement of the traveller, the or each face of the engaging element is preferably arcuate about the centre of the rod. This also facilitates engagement and disengagement of the engaging element from the lead screw when desired. An embodiment of the present invention will be described with reference to the accompanying drawings of which: Figure 1 shows a perspective view of the operative parts of a prior art lead screw drive, using a traveller incorporating a conventional single half-nut and employed in a syringe pump; Figure 2 shows a perspective view of the traveller used in the drive of Figure 1; Figure 3 shows a replacement traveller for the worm screw drive of Figure 1 in accordance with the invention; Figure 4 shows an end-view and a top-view of the traveller of Figure 3 with an indication of the position which would be taken-up by the lead screw of the drive; and Figure 5 shows a perspective view of the operative parts of a lead screw drive, using the replacement traveller of Figure 3, employed in a syringe pump. Referring to Figure 1 (prior art) , a lead screw 10 is engaged by traveller 12, which may be clamped upon rod 14, arranged parallel to lead screw 10 and attached at one end to a syringe pusher 16. An extension 18 of traveller 12 comprises a half-nut portion 20, which is threaded complementarily to, and engages with, lead screw 10. The extension 18 further comprises a lug 22, engaged by and pivotally attached by means of complementary lugs 24, to arm 26. Clamping of traveller 12 upon rod 14 is arranged by means of a screw (not shown) passing through the countersunk clearance bore 28 into a nut carried in an aligned bore in the opposed extension 30 of traveller 12. The traveller 12 of Figure 1 is shown in greater detail in Figure 2. In operation a charged syringe of appropriate size is mounted upon the syringe pump frame or cradle upon which the syringe drive mechanism is itself mounted, with the syringe plunger engaged with pusher 16, the body of the syringe being positively engaged elsewhere upon the frame. To facilitate engagement and disengagement of the syringe plunger from pusher 16, traveller 12 may be disengaged from lead screw 10 by pivoting it about the axis of rod 14 by means including arm 26, so that half- nut 20 is removed from lead screw 10. Rod 14 and pusher 16 can then be axially moved to the position in which the syringe plunger is engaged within, or disengaged from, pusher 16. With pusher 16 in the desired position, traveller 12 may be returned to the driving condition with half-nut 20 in engagement upon lead screw 10. In operation lead screw 10 is rotated about its axis by means of an electric motor, causing linear movement of traveller 12, and hence pusher 16, causing the plunger of the charged syringe to move toward the syringe barrel, thereby expelling solution from the syringe. Under conditions of axial load, such as will occur with a charged syringe driven against a back-pressure due, for example, to an occlusion, there is a tendency for the half-nut element 20 to be thrown off the lead screw 10, owing to the radial forces generated upon the inclined faces of the screw threads within half-nut 20, as it is driven by rotation of lead screw 10. When this occurs, traveller 12 ceases to move as does pusher 16, causing movement of the syringe plunger to cease and with it the infusion. Whilst a certain amount of force may be applied to traveller 12 by means of member 26, forming part of a manually operated drive engaging/disengagement mechanism, in a direction counter to that tending to lift half-nut 20 from lead screw 10, sufficient force to ensure engagement under all load conditions would cause excessive friction and wear upon the components of the drive, and would lead to a requirement for a far greater rotational force e.g. a more powerful electric motor, to drive lead screw 10. By substituting traveller 12 of the drive described with reference to Figures 1 and 2 with a replacement traveller 40, as shown in Figures 3 and 4, in which the lead screw engaging element 42 is no longer in the form of a conventional half-nut, engaging element 42 will remain in contact with lead screw 10 despite the generation of radial forces due to the inclined faces of the screw threads of engaging element 42 being acted upon under conditions of axial load by lead screw 10. Engaging element 42 is of generally U-shaped cross- section, of greater depth than the diameter of the lead screw 10, and, in the present embodiment, with one arm 44 extending downwardly further than the other, 46. Both inner faces of the arms of the U are, at least in part, grooved on their inner faces complementarily to the lead screw, with the grooves on one inner face advanced half a pitch of the lead screw thread pitch relative to the grooves upon other. The grooves on the inner faces of engaging element 42 constitute means for engaging lead screw 10 but extend both upwardly and downwardly of the region of engagement with the thread of lead screw 10. An unthreaded plastic insert 48 is positioned in the bridge of the U of engaging element 42, against which the uppermost part of engaging element 42 may run when traveller 40 is in the most clockwise position it is able to adopt . Alternatively the grooves may extend into the base of the U, and may be continuous through the base from one face of the U to the other. In the present embodiment both faces of the U are, at least in their lower parts, arcuate about the centre of the aperture 34 of traveller 40. In all other respects engaging element 40 is identical to the prior art engaging element 12 of Figures 1 and 2. When engaging element 42 is fitted into the drive system illustrated in Figure 1 in place of prior art engaging element 12, any radial forces generated due to the action of the lead screw 10 upon the inclined faces of the grooves upon the walls of engaging element 42 will tend to rotate the lead screw 10 anti-clockwise about the axis of rod 14 to which it is fastened. However as the faces of engaging element 42 carrying the grooves extend downwardly of the initial region of contact, any radial movement of the engaging element 42 will merely move the region of contact whilst maintaining the grooves in contact with the thread of lead screw 10 and traveller 40 will continue to be driven along lead screw 10 without becoming disengaged. By providing the extension to the outer arm 44 of engaging element 42, driving contact would be maintained between the lead screw 10 and engaging element 42 well beyond the limits of any radial movement likely to be produced by the axial drive forces due to lead screw 10. Making the inner walls of engaging element 42 arcuate about the centre of aperture 34 and separating them by a distance to accommodate the lead screw 10 ensures that frictional losses do not increase as a result of radial displacement of traveller 40 due to forces generated by the drive, and additionally enables the engaging element 42 to be more readily engaged and disengaged from lead screw 10 by pivotal movement under the control of the linkage comprising arm 26, the extended outer arm 44 ensuring a positive lead-in upon engagement . Traveller 40 is inexpensive to manufacture in comparison to the cost of providing a complex half-nut retaining mechanism, with or without any associated disengagement detector (s) . However, should it be required the detection of engagement or disengagement of engaging element 42 from the lead screw 10 is simpler to achieve than with a conventional half-nut mechanism as a result of the considerably greater movement needed to take the traveller 40 from an engaged to a disengaged position under control of the linkage comprising arm 26: in systems employing a conventional half-nut or half-nuts the difference between engagement and disengagement can be as little as the depth of threading on the half- nut (s) . A further advantage of the engaging element 42 over a conventional half-nut is that any unintentional disengaging forces such as, for example may be transmitted to traveller 40 by sideways movement of the pusher 16, are unlikely to disengage engaging element 42 from the lead screw, again due to the extended contact provided between lead screw 10 and the grooved walls of engaging element 42. Although the use of the modified drive system in accordance with the invention has been described in relation to, and has particular relevance to, a medical syringe drive system, it will be appreciated that it is not limited to such an application, and may be employed in other drive systems in which radial pressures upon the half-nut or nuts of the traveller could lead to undesired displacement of the half-nut or half-nuts from the lead screw. Whilst the traveller of the specific embodiment is shown as having both walls of engaging element 42 grooved to engage lead screw 10, either one or both walls may be so grooved, as may the base of the U. Similarly whilst it is preferred, for the reasons stated, to arrange for both walls to be arcuate about the same centre when both are grooved, if only one is grooved to engage the lead screw, that wall only may be arcuate. Alternatively neither wall need be arcuate, whether the lead screw engages grooves upon one or both walls, provided the increase of friction between grooves and lead screw with radial displacement can be tolerated in a particular application.

Claims: CLAIMS 1. A lead screw drive comprising a lead screw (10) and a disengagable lead-screw-engaging element (42) , the engaging element (42) having a drive portion upon a face thereof for driving engagement with the lead screw (10) , characterised in that the drive portion on the engaging element (42) extends beyond the region of contact with the lead screw (10) , such that driving engagement is maintained in the event of displacement of the engaging element (42) with respect to the lead screw (10) in a direction generally parallel with the said face. 2. A lead screw drive as claimed in claim 1, in which the engaging element (42) is substantially U-shaped. 3. A lead screw drive as claimed in claim 1 or claim 2, in which the drive portion comprises at least one thread element . 4. A lead screw drive as claimed in claim 3, in which the drive portion comprises thread elements located on each of a pair of generally parallel faces (44,46) of the engaging element (42) . 5. A lead screw drive as claimed in claim 3 or claim 4, in which the engaging element (42) further comprises a stop (48) on a base portion on the engaging element (42) bridging the two parallel faces (44,46) . 6. A lead screw drive as claimed in claim 3 or claim 4, in which the or each thread element is substantially continuous between the parallel faces (44,46) of the engaging element and a base portion thereof. 7. A lead screw drive as claimed in any one of the preceding claims, in which the engaging element (42) is connected to a rod (14) via a traveller (40) , the rod (14) being arranged parallel to the lead screw (10) and being connected to a pusher (16) for pushing a plunger of a syringe into a syringe barrel. 8. A lead screw drive as claimed in claim 7, in which the face of the engaging element (42) is substantially arcuate about the centre of the rod (14) . 9. A lead screw drive as claimed in claim 7 when dependent upon claim 4 in which each of the generally parallel faces (44,46) is arcuate about the centre of the rod (14) . 10. A medical syringe pump including a lead screw drive as claimed in any one of the preceding claims. 11. A lead screw drive constructed and arranged substantially as specifically described with reference to and as shown in Figures 3, 4 and 5 of the accompanying drawings. 12. A medical syringe pump constructed and arranged substantially as specifically described with reference to and as shown in Figures 3 , 4 and 5 of the accompanying drawings .

Inventors: COBB ANTHONY RICHARD, ZAIDI SYED MUJAHID, COBB, ANTHONY, RICHARD, ZAIDI, SYED, MUJAHID
Applicants: COBB ANTHONY RICHARD, GRASEBY MEDICAL LTD, ZAIDI SYED MUJAHID, COBB, ANTHONY, RICHARD, GRASEBY MEDICAL LIMITED, ZAIDI, SYED, MUJAHID
Application year: 1996
Main classification: A61M5
IPC codes: A61M5, F16H25