IL33974A - A mechanism for driving a rotary member and a pipette comprising this mechanism - Google Patents

Description

nt ji-ti a n 'aan nen*m aainoa las nyan j Λ mechanism for driving a rotary member and a pipette comprising this mechanism MICR0M3DIC SYSTEMS, INC.

C: 32286 This invention relates to a pawl mechanism for driving a n otary member.

In very many fields of science or technology it is necessary to be able to take up or deliver particularly small quantities of liquid, for example of the order of one nanolitre.

This is particularly the case in biology, for example when it is required to tap kidneys, in particular the kidney tubules, or in chemistry, in the field of chromatography in the gaseous phase in order to effect the injection of micro-quantities of liquid.

In certain micro-mechanisms such as, for example, those of watch parts, it is also required, to effect lubrication by the distribution of oil in particularly small quantities, arid in an identical manner and in as accurate a manner as possible during each such lubricating operation. These intake and delivery operations are at present effected by all sorts of pipettes in which the volume aspirated or discharged depends to a fairly large extent on the viscosity of the liquid being taken up or delivered, and on the duration of these operations.

An object of the invention is to obviate the above mentioned disadvantage by devizing a mechanism which will effect simple and rapid mani-pulation of a pipette of very high accuracy and capable of dealing with quantities of liquid of the order of one nanolitre.

According to the invention there is provided a pawl mechanism for driving a rotary member which comprises a toothed wheel kine-matically connected to said member, an armature rotatably mounted around said member co-axially with the rotational axis of the latter, a support mounted on the armature for pivotal movement at right angles to the rotational axis of the armature and enveloping said toothed wheel , the support on opposite sides of its pivotal axis, in line with s„ toothed wheel, and capable of being selectively brought into engagement with the teeth of the wheel by rocking the support in one direction in the case of the first pawl and in the opposite direction in the case of the second pawl, and which further comprises a brake for braking the rotary motion of the armature, a support bearing on the armature for limiting said rocking in each direction, and a member rigid with the support to enable the necessary couple to be transmitted to the mechanism firstly for rocking said support relative to the armature in one or the other direction, into a position of engagement of one of said pawls with said toothed wheel, and secondly for angularly driving the armature against the action of the brake after the support has come to bear on the armature, and hence for angularly driving the rotary member by the thrust exerted by the pawl on the toothed wheel.

In the accompanying drawings : Figure 1 is a longitudinal section through a pipette for the intake and delivery of liquid which is fitted with one form of embodiment of a pawl mechanism according to the invention; Figure la shows a detail of Figure 1 on a larger scale; Figure 2 is a cross-section of the pipette along the line II-II of Figure 1; Figure 3 is a section through part of the Figure 1 pipette, taken along the line III-III of Figure 2; Figure 4 is a perspective view of a component of the Figure 1 pipette; Figure 5 is a side view showing part of the casing for the Figure 1 pipette; Figure 6 is a diagrammatic cross-section of the pipette along the line Figures 7 to 9 are cross -sections of the pipette along the line ΙΪ-ΙΙ of Figure 1 in three different positions of certain components; Figures 10, 11, 11a , 12 and 13 illustrate diagrammatically various ways in which of the pawl mechanism fitted in the Figure 1 pipette can operate; and Figures 14 and 15 represent respectively a front elevational view and a plan view of the Figure 1 pipette mounted on a support device serving to fill the pipette.

The illustrated pipette comprises a tubular handle 1 at the top end of which is arranged a head 2 carrying firstly, three rotary rings 3, 4 and 5 and secondly, a mounting 6 for a cranked tube 7 by means of which the intake of liquid and its delivery can be effected.

This intake and delivery are controlled by rotational movement of the ring 4, in one direction for intake and in the opposite direction for delivery, the quantity of liquid which can be aspirated into or discharged from the tube being set beforehand in particularly accurate manner by suitab ly positioning the rings 3 and 5 relative to associated fixed reference points 8a. and 8b (Figure 15). For this purpose, the rings 3 and 5 respectively carry on their outer surfaces equidistant graduations 3a and 5_a in facing relationship with the reference points 8a and 8b, each graduation corresponding, in the illustrated pipette, to a difference in aspirated or discharged volume of 5 nanolitres with respect to the adjacent graduations.

The handle 1 is formed by a metal sleeve 9 which is covered with a sheath 10 of plastics material and which is closed off at one end by a cap 11 secured to the sleeve by screwing, while the other end is rigid with a cup 12 forming, in co-operation with a cap 13 screwed on the cup, f t h d 2 Wi thin the sleeve 9 and the casing* of the pipette head 2 is arranged an axially movable shaft . 14 having. an annular flange 14_a \vhich is slidably mounted in an enlarged diameter portion 9a of the sleeve and which also forms a stop member for limiting movement of the shaft 14 in direction by abutment against a shoulder 9b of the sleeve, the amplitude of possible displacement of the shaft in direction F^, opposite to F ^ being limited by abutment of the end of a shaft portion 14b adjacent flange 14a. with the internal surface of the cap 11.

On the side of flange 14a opposite to portion 14b_, the shaft 14 has a cylindrical portion 14 whose length corresponds at least to the extent to which the shaft can travel axially from the moment it moves away from the shoulder 9b_ until its portion 14b comes into contact with the cap 1 1 , or vice versa; this cylindrical portion is formed with a longitudinal groove 14d_ within which is freely engaged the tip of a stud 9c_ whi ch projects into the hore of sleeve 9 and prevents any rotary displacement of shaft 14 during axial sliding movement thereof.

This cylindrical portion 14c_ of the shaft is extended by a threaded part 14e_ with which engages a nut 15a. formed at one end of a sleeve 15, the other end 15b of this sleeve constituting a bearing for guiding a cylin-drical shaft portion 14_f adjacent the threaded portion 14e_ , during axial movement of the shaft. .

The shaft 14 terminates in a cylindrical element 14g of lesser diameter than the portion 14f and from the end face of which projects a rod 14h engaged, at its free end, in the tube 7, in particular in the tube portion 7a held by the mounting 6 in co-axial relationship with the rod 14h . The diameter of rod 14h is so sized as to slide with a fairly close fit in the tube portion 7∑i when the shaft 14, of which rod 14h forms part, is moved the tube portion 7a. acting as the cylinder, and it is this rod which causes liquid to be sucked into the tube, upon movement of the shaft in direction F^, and the liquid filling the tube to be discharged there from, upon movement of the shaft in the opposite direction F ^.

It should at this point be noted that the tube 7 is secured in a fluidtight manner on the cap 13 by the mounting 6, which mounting comprises a nut 6a engaging over a threaded flange 13a. that extends radially from the cap 13, and serving to press a ring 6b towards the cap 13 inside the annular flange 13a. At one end of ring 6b is formed a cylindrical aperture portion in which is engaged a sleeve of synthetic material 6c_, passing through the nut 6a. via a passage 6d_ and surrounding the tube portion 7a over part of its length, this sleeve constituting a resistant protective element for the tube. At its other end, the ring is formed with a frusto -conical aperture portion adapted to fit over an annular gasket 6e_ of synthetic material, arranged around the base of tube 7, in contact with the cap 13. The outer surface of gasket 6e_ is also of frusto- conical shape so that upon nut 6a being tightened, gasket 6e_ is clamped by ring 6b_ thereby firmly to hold tube 7 in position. Another annular gasket, 6_f , is located in a recess 13b in cap 13.

Because the shaft 14 is prevented from rotating about itself by virtue of the stud 9 engaging in groove 14d , any rotation of sleeve 15, and thus of nut 15_a , is converted into a displacement of the shaft in direction F ^ or depending on the direction of rotation of sleeve 15 . This rotation is caused by rotatably moving the ring 4 to an extent which can be adjusted by acting on rings 3 and 5 as will now be described.

The sleeve 15 is in fact rotatably mounted with respect to the cup 12, firstly by virtue of a first ball and cage assembly 16 mounted between cup, and secondly by virtue of a second ball and cage assembly 17 arranged between two rings 18a_ and 18b of which the first, 13_a , is force-fitted in part in an annular member 19 of synthetic material, and of which the second, 18b , is mounted with light frictional enga-gement on the portion 15b of sleeve 15 and is subjected to the action of a spring washer 20 urging it against the ball and cage assembly 17.

The annular member 19 is fitted into the part of the cup 12 adjacent the mouth of the latter, and its aperture 19a. is sized to correspond to the external diameter of the sleeve portion 15b. The member 19 thus acts firstly, as a positioning member for sleeve 15 in the cup 12 of the pipette head 2, ' and therefore also as a positioning member for shaft 14 via said sleeve, and secondly, as a rotation bearing for sleeve 15.

The spring washer 20 bears on a toothed disc 21 which is force-fitted, together with a second similar. toothed disc 22, on the external surface of sleeve portion 15b , the second disc bearing on a shoulder 15ci of said sleeve.

The dimensions and arrangement of the members 17 to 22 are such that the toothed discs 21 and 22 come to lie symmetrically on opposite sides of the sectional plane II-II that divides the ring 4 into equal parts. As can be ssen from Figure 2, the teeth of discs 21 and 22 are saw-toothed in shape, the teeth of one disc being however directed in a direction opposite to those of the other.

These discs constitute in fact ratchet wheels with which co-operate a pawl 23 in the case of disc 21 and a pawl 24 in the case of disc 22 (Figures 2 and 3), which pawls project from the inner face of a ring 25 in diametrically opposite positions. The ring 25 is pivoted on a pin 26 between the two radial flanges of an annular armature 27 which is of - peripheral portion, which constitutes the base of the U, sliding over the inner surface of the cup. The 'extent to which the armature 27 can rotate is limited by the engagement- of the tip of a screw 28 extending between the armature flanges, through a peripheral slot 27a. , with either of two dowels 29a and 29b mounted between these flanges, symmetrically on opposite sides of the pin 26 (Figures 1 and 2 ).

The ring 25 has, in the vicinity of pawl 23, a peg 30 and, in the vicinity of pawl 24, a peg 31, these pegs extending in opposite directions through circular ports 27b and 27_c (Figures 2, 3 and 6 ) respectively formed in the radial flanges of armature 27.

The diameter of these ports is such that, whenever pawl 23 or 24 is brought into engagement with the associated wheel 21 or 22 by pivoting ring 25 about its pin 26, pegs 30 and 31 come to bear against the edges of ports 27b and 27c_, i. e. the left hand side of these edges (Figure 6 ) when pawl 23 co-operates with wheel 21 or the right hand side of these edge when pawl 24 engages with the teeth of wheel 22. This contact of pegs 30 and 31 with the edges of ports 27b and 27 determines the extent to which the pawls 23 and 24 penetrate between the teeth of the associated toothed disc : in fact, this penetration is never total for the pawl that is in engagement, whereas the other pawl is at that moment totally out of engagement with the teeth of its associated disc.

The armature 27 is additionally formed, in the part thereof opposite pin 26, with a peripheral slot 27_d through which passes a stud 25a rigid with ring 25 (Figures 1, 2 and 6 ). This slot 27_d has a length sufficient to enable ring 25 to pivot about pin 26 to the extent indicated above, without rod 25a. coming into engagement with armature 27.

In the side wall of cup 12 are formed three slots 123, 124 and 125 lengths corresponding substantially to the angular distance between the - dowels 29 i and 29b plus the angular distance, as viewed from the axis of the cup 12, travelled by the stud 25a of ring 25 when the latter is pivoted about pin 26, which pivotal movement is limited by the pegs 30 and 31 coming into contact with the edges of ports 27b and 27c_. These slots are contained in equidistant and parallel planes that are transverse to the longitudinal axis of the cup; a slot 126, perpendicular to the slots 123, 124 and 125, connects the central parts of the latter and the edge of cup 12. The slots 123, 124, 125 and 126 lie below the rings 3, 4 and 5 and below the cap 13 and are therefore not visible when the pipette is in the assembled state.

These slots define in the side wall of the cup 12 four tongue -like elements 12A, 12B, 12C and 12D (Figure 5) which are arranged in facing pairs, 12A and 12 C, and 12B and 12D, and which are slightly bent, before assembly, in the direction of the axis of cup 12 in such a way as to exert a slight pressure on the armature 27 (Figure 6), thereby causing a braking action on the armature when the latter is rotated in the clip, in either direction.

It should at this point be indicated how the rings 3, 4 and 5 are mounted on the side wall of cup 12.

The ring 5 is maintained in engagement with a shoulder 12b of the cup by a ring 8B (Figures 1 and 3) which carries the reference point 8b and which is angularly locked by a screw, not shown, engaging with the cup Opposite this ring 8B is a similar ring 8A which carries the reference point 8a and which is angularly locked by a screw, also not shown and rigid with the cup, this ring 8A being separated from ring 8B by an intermediate ring 8C locked angularly by the screw 28 and by two other screws 28a and 28b.

These rings 8A and 8B moreover form a support for the^rotatin ' ring 4 which is in addition locked, axially between the rings 8A and 8B by engagement of an internal portion 4a. between their opposite faces (Figures 1 and 3 ). . · The ring 3, which bears on ring 8A (Figures 1 and 3), is held in position on the cup 12 by the cap 13.

To each of the graduations 3a. and 5a_ on the rotary rings 3 and 5, there corresponds on the fixed rings 8A and 8B a hemispherical recess 32 made in the faces of these rings opposite their associated rotary rings 3 and 5 (Figure la.); the recesses in each series are equidistantly arranged over an arc of a circle and are adapted to cooperate with a micro-ball 33 subjected to a spring 35 and slidably mounted in a corresponding recess 34 formed in both of the rings 3 and 5 opposite the median points of the graduations 3a. and 5a. . The two series of recesses 32 pro-vided in rings 8 A and 8B ensure exact positioning of each graduation on rings 3 and 5 opposite the corresponding reference point 8a or 8b by penetration of th corresponding micro-ball 33 into the appropriate one of the corresponding series of recesses 32.

The ring 8C is slotted at 80 in the part thereof facing the slot 124, over an angular length corresponding to that of said slot (Figures 1 and 2 ).

The stud- 25_a which is rigid with ring 25 and which extends not only through the slot 27c of armature 27 but also through the slot 80 of ring 8C and the slot 124 of cup 12, projects into a recess 4b of ring 4 with which it is therefore in engagement.

It follows that any rotary displacement imparted to the ring 4 causes stud 25a as well as ring 25 and armature 27 to be driven in the same direction, the extent of the displacement of ring 4 being limited by the abutment of the ro ectin ti of screw 28 a ainst one or other of the The diameter of this projecting tip is the determining factor for exactly adjusting the maximum amount of liquid the pipette can aspirate or discharge during a single actuation of- ring 4. This quantity depends on the axial distance travelled by the rod 14h of shaft 14 within the portion 7a of tube 7, such distance being directly related to the angular displacement of the nut 15a , i. e. of the sleeve 15 carrying the toothed discs 21 and 22. The drive of armature 27 and of ring 25 carrying pawls 23 and 24, by means of which discs 21 and 22 are rotated, is dependent on the ring 4, Figures 7 and 8 show the way in which these discs are rotated.

In Figure 7, which is a view similar to that of Figure 2 but with the cup 12 and the ring 8 shifted angularly through 90° in an anti-clockwise direction, the ring 4 has been moved to an extreme left position such that the dowel 29a. of armature 27 contacts the tip of stop screw 28, ring 25 being pivoted about pin 26 into a position in which pawl 23 comes into engagement with the teeth of disc 21 whereas pawl 24 is disengaged from disc 22.

When ring 4 is turned in direction F^ (Figure 7) it is only ring 25 which is at first caused to move, by pivoting about pin 26, because the armature 27 is held in place by the braking action which is exerted on its outside surface by the tongue-like elements 12 A, 12B, 12C and 12D of cup 12. The extent to which the ring 25 can pivot is governed by the size of the ports 27b_ and 27£ which are formed in armature 27 and in which are engaged pegs 30 and 31. Thus the ring 25 will pivot for as long as the pegs, upon moving away from the edge portions of the ports. with which they are in contact, as shown, say, in Figure 7, do not come into contact with another portion of the port edges situated approximately The pawl 24 of ring 25 will then mesh with the toothed disc 22 whereas pawl 23 will then be remote from- the disc 21 with which it meshed previously.

It is upon completion of this change-over and upon continued rotation of the ring 4 in direction F^, that the sleeve 15 comes to be turned by the pawl 24 which is now in meshing engagement with disc 22. Thus, because the ring 25 can no longer pivot within armature 27 as it is abutting against the latter via pegs 30 and 31, continued rotation of the ring 4 in direction F^ causes the armature 27 to turn inside cup 12 in opposition to the braking action exerted by the tongue -like elements 12A, 12B, 12 C and 12D, i. e. it also causes the ring 25 and its pawl 24, and therefore the toothed disc 22 and the sleeve 15 to which this disc is secured, and in the final resort the nut 15a , to turn by a corresponding amount.

This movement in direction F^ of the armature 27 within cup 12 ceases as soon as the dowel 29b which is carried by the armature abuts against the tip of stop screw 28 (Figure 9 ).

As has already been described, the nut 15a. of sleeve 15 is in engagement with the threaded part 14e_ of shaft 14 which is prevented from rotating by engagement of stud 9 in groove 14_d so that, upon sleeve 15 being turned, the shaft 14 is moved axially by a distance which is dependent on the pitch of the screw thread on the part 14_e and on the angular movement of the nut 15c_, and in a direction which is dependent on the hand of the screw thread and on the direction in which this nut is rotated.

In the illustrated mechanism, the hand of the thread on the part 14e_ of shaft 14 has been so chosen that, when the ring 4 is turned in causing the rod-cum-piston 14h to move in the tube 7 in a manner such as to effect discharge of the liquid contained in this tube.

By way of example, if the toothed discs 21 and 22 each comprise fifty teeth, the nut 15a_ and the threaded .part 14e_ of shaft 14 are so dimen sioned that one revolution of the nut causes the rod-cum-piston 14h to move axially 500 ^ m, and the diameter of the tube portion 7a and of the rod-cum-piston 14h is 0. 8 mm, then when the toothed disc 21 or 22 is rotated by a distance equivalent to the pitch of its teeth, the axial movement of the rod-cum -piston 14h in tube 7 is equivalent to a decrease or an increase in volume of the space comprised between the free end of 3 the tube and the rod-cum-piston equal to 5 nanolitres (0. 005 mm ).

Obviously, rotation of the ring 4 in a direction opposite to F^, once this ring and the members that are kinematically rigid therewith have come to occupy the position shown in Figure 9, will bring about displa¬ cement of the shaft 14 in direction F^, i. e. movement of the rod-cum- piston 14h in the tube 7 such as to cause liquid to be sucked into the tube.

In a manner similar to that which has been described with reference to Figures 7 to 9, rotation of the ring 4 in a direction opposite to F ^ will firstly cause ring 25 to pivot about pin 26 tending to free pawl 24 from the toothed disc 22 and to bring pawl 23 into engagement with disc 21 , the armature 27 then rotating in the cup 12 until dowel 29_a abuts the tip of stop screw 28. At the end of this rotational movement of the rinjr 4 in a direction opposite to F ^ the various components of the mechanism come to occupy the position shown in Figure 7.

It should at this stage be pointed out that, in the illustrated embo¬ diment, the angular length of the free space lying between the dowels 29a. and 29b_, on the one hand, and the head of the stop screw 28, on the of the teeth on disc 21 and 22, so that the maximum volume of liquid capable of being discharged or aspirated by the pipette through turning the ring 4 from the Figure 7 position to the Figure 9 position, or vice- versa, is in fact equal to 50 nanolitres, as stated earlier. This volume can be calibrated in a very accurate manner by selecting a stop screw 28 having a tip of suitable width.

As mentioned, each graduation 3a. oroa on the rings 3 and 5 corresponds to a difference' in volume aspirated or discharged by the pipette of 5 nanolitres in relation to the adjacent graduations, all ten graduations 3∑ι or 5_a being equivalent to a total volume of 50 nanolitres. These graduations moreover extend over an arcuate length that is indicative of the extent to which the armature 27 is able to rotate in the cup . 12, such rotation being limited, as described, by the dowels 29a and 29b abutting against the tip of screw 28, It is in fact by acting on the rings 3 and 5 that it is possible exactly to set the quantity of liquid which the illustrated pipette will discharge or aspirate in the course of one rotary movement of- armature 27, in one direction or the other, the amplitude of such rotary movement being limited by abutment of the screw 28 with one or other of the dowels 29_a and 29b .

In the present instance, the ring 3 serves to adjust the quantity of liquid being aspirated while the ring 5 serves to set the amount of liquid being discharged. For these purposes the rings -3 and 5 are res pectively connected by studs 36a. and 37a., extending through the slots 123 and 125 of cup 12, to annular cams 36 and 37 that are rotatably mounted within the cup 12.

The cam 36 is placed between one radial flange of armature 27 and the annular member 19, while cam 37 engages the other radial flan e of armature 27 and bears on a shoulder 12c rovided inside the The profiles of cams 36 and 37 are identical : in fact these are two similar components whose working surfaces are mounted in facing relationship. Figure 4 shows one such cam which consists of a ring a. of synthetic material, from the radially outer surface of which projects a stud b , denoted previously by reference 36a. , in the case of cam 36, and 37a in the case of the other cam 37 (in Figures 1 and 2 ). The diameter i of the opening in ring a. is slightly greater than the external diameter of rings 18a and 18b and of the ball and cage assembly 17.

The ring a has over a part of its thickness a recess c_ of which one portion,^, termed the "high" portion of the cam, has a cylindrical surface of diameter _d corresponding very exactly to the distance separating the pegs 30 and 31 provided on the ring 25, plus double the width of one of these pegs. The other portion, c^, of recess also has a cylin- d2 drical surface but the radius of this other portion is greater than — j- by an amount at least equal to the diameter of the ports 27b and 27£ in the armature 27. The angular extent of the cam recess portion , which will be termed the "hollow" of the cam, corresponds at least to the length of the arc lying between the dowels 29a and 29b provided on the armature 27.

The portions c , and £_ of recess are connected to one another by J. ' two inclined surfaces _f ^ and^-- the ends of each one of which are angu¬ larly distant from one another by a length corresponding substantially to the angular displacement, as viewed from the axis of shaft 14, of the pegs 30 and 31 when the ring 25 is moved for example from one extreme position (Figure 7) to the other (Figure 8).

As shown in Figures 1 and 3, the peg 31 carried by the ring 25 projects into the recess £ of cam 36, while peg 30 projects into the each cam has one end is positioned substantially diametrically- opposite the associated stub b.

It will now be explained how the cams 36 and 37 enable the amount of liquid to be discharged or aspirated by the illustrated pipette to be s-et in various ways.

To facilitate this explanation, reference will now be made to Figures 10 to 13 which are diagrammatic representations of the relevant components of the mechanism, each component being shown in each figure in a number of different operative positions identified, where appropriate, by indicia °, ' , " and so on, indicium0 being indicative of a starting position.

In the following description, reference numerals bearing any one of these indica will at times identify the component as such in a particular position and at other times simply identify a position.

Before considering a number- of possible working cases, it should further be explained that, in Figure 10, the straight line 27A connecting the dowels 29 a and 29¾ as well as the pin ¾o° serves only to indicate that these three components are all rigid with the armature 27 (not shown) and that, in operation, they move along a circular path when the armature rotates within the cup 12 as has been described. The path followed by the pin 26 coincides in Figure 10 with the representation of the ring 25°.

Case 1 : Suction nil, discharge 50 nanolitres (total).

In this case, the position of the cams 36 and 37 is that shown in Figure 10, At the beginning of the discharge operation, the ring 25 is ■in the position 25°, shown by a thick continuous line ·, and the armature which carries it and which is not shown, is so positioned that the dowel 29a . is in contact with the screw 28. The relative position of the cams o o 7 ' ■ 36 and 37 is such that the two pegs 30 and 31 are each in contact with one end part of the "high" portion of the corresponding cam.

If the stud 25 Subsequent movement of the stud 25'a in direction causes the armature 27 to rotate in . the cup 12 until dowel 29b comes into contact with the screw 28 at 29k with pin 26 and d o w e l 29a being moved into positions 26' and 29'a..

The ring 25 then occupies position 25" shown by a chain-dotted line, v/ith the stud 25a occupying position 25 a and with the pawl 24 being at 24" . T is pawl 24 has moved through an angle corresponding to ten times the pitch of the teeth on toothed disc 22 which is equivalent, as described above, to a movement of the pipette shaft 14 in direction F ^ over a distance equal to a discharge of a quantity of liquid, equal to 50 nanolitres. it in If the stud 25a. is now moved from position 25 a to position 25a in a direction opposite to F ^, the ring 25 will pivot about pin 26' in the same direction, by virtue of the fact that the armature 27 is braked by the tongue-like elements 12A to 12D, until the peg 31, then at 31 " , comes into engagement at 31 '" with the "high" portion of cam 36. By virtue of the diameter (3 that has been chosen for this "high" portion (Fi ure 4 the rin 25 is then held b e 31 in a central osition relative 1 to the toothed discs 21 and 22, coinciding with that shown by the thick continuous line, such that neither -pawl 23 nor pawl 24 can come into engagement with the associated toothed disc. Thus, in the angular position given to the cam 36 in Figure 10, the disc 21 can never be driven when the stud 25a_ is moved in a direction opposite to F · " the pipette therefore aspirates nothing.

Case 2 : Suction 15 nanolitres (partial), discharge 50 nanolitres (total).

This case is illustrated by Figure 11 for the "discharge" aspect and by Figure 11 a. for the "suction" aspect.

In relation to the case described with reference to Figure 10, the cam 36 has been slightly shifted in a clockwise direction (position 36° ) by an amount corresponding to three graduations of the ring 3, whereas cam 37 has not moved (position 37°). As can be seen, in the starting position 25° of the ring 25, the peg 31° is engaged in the "hollow" of cam 36°. Upon the stud 25 a. being moved in direction F^, ring 25° first pivots about pin 26° by virtue of the fact that the armature 27 (not shown) is braked by the resilient tongue-like elements 12A to 12D of the cup 12. This pivotal motion proceeds until the pegs 30° and 31°, which were, in these positions, as shown in Figure 7, bearing on a part of the edges of the ports 27b and 27c_, come to occupy positions 30' and 31' where they bear, as shown in Figure 8, on a part of the edges of these ports opposite the first-mentioned part. The ring 25 is then in position 25' , indicated by a thin continuous line, and the stud 25_a is in position 25a (Figure 11), It will be seen that, under these conditions, the pawl 24 is in engagement with the toothed disc 22, the peg 30 having been able to move into the "hollow" of cam 37 to occupy position 30' . It is from then on that the disc 22 and the sleeve 15 to which it is secured come of the movement of the ring 25 in direction F^, the pivotal pin 26 of this ring following the curvilinear .path which is imposed thereto by the rotational movement of the armature 27 within the cup 12, until the peg 29b comes to bear on stop screw 28.

As will be seen, this phase in the operation of the sleeve driving mechanism comprised by the illustrated pipette is practically identical to the discharge phase described with reference to Figure 10.

At the end of this phase, the ring 25 occupies position 25" , shown as a chain-dotted line in Figure 11, the toothed disc 22 having been turned through an angle corresponding to ten times the pitch of its teeth. This same position 25" is again to be found in Figure 11a. , indicated by a thick continuous line.

To cause the pipette to aspirate liquid, it suffices to return the ring 25 from position 25" to its initial position 25° in Figure 1 1, i. e. position 25Vl in Figure 11a' indicated as a dotted line, by moving the stud 25a in a direction F^ opposite to F^.

Because the armature 27 is braked by the tongue-like elements 12 A, 12B, 12C and 12D, the ring 25 starts off by pivoting about its pin 26 but this pivotal motion is limited by the engagement of peg 31 with the "high" portion of cam 36 (position 31 '" ), the ring being then in position 25'" in which neither pawl 23 nor pawl 24 engages with its associated toothed disc 21 or 22. These discs and the sleeve 15 are therefore not rotably driven during much of the movement of the stud 25_a in direction F^.

IV But upon the peg 31 reaching position 31 , at which time the stud 25_a occupies position 25¾ , the peg penetrates into the "hollow" of cam 36 (position 31V) and the ring 25 pivots further anticlockwise about pivot IV V 26 to occupy position 25 indicated by a chain-dotted line. The pawl 23 then comes into engagement with the toothed disc 21 (position 23 of v this pawl and 25a of stud 25a). v Upon continuing to move stud 25a in direction F , pawl 23 drives disc 21 angularly in the same direction until dowel 29a meets the stop- screw 28 (the position 25V1 of ring 25, as stated earlier, coinciding with position 25° in Figure 11 ). The amplitude of this movement is equivalent to an angular length equal to three times the pitch of the teeth of disc 21 thereby causing the shaft 14 to move in direction F^ (suction by a dis tance corresponding to an increase of 3 nanolitres in the volume of the free space in the tube 7.

Case 3 : Suction 50 nanolitres- (total), discharge 50 nanolitres (toral).

This is the case shown in Figure 12 in which the two cams 36 and 37 are so positioned angularly that their "hollows" occupy diametrically opposite positions with respect to the axis of the toothed discs 21 and 22.

During the discharge phase (the position of the components of the mechanism being successively identified by the indicia °, ' and " with the stud 25_a moving in direction F^) the driving mechanism operates in the same way as in the preceding case since the position of cam 37 has not changed.

During the suction phase (the position of the components being successively identified by the indicia " , '"; and ° with the stud 25a moving in direction F ) the mechanism operates in the same way as during discharge, it being however inder stood that the part played by the peg 31, the pawl 24 and the disc 22 during discharge is now being played by the peg 30, the pawl 23 and the disc 21 for suction purposes. Case 4 : Suction nil, delivery nil.

This is the case shown in Figure 13 in which the two cams 36 and 37 are so positioned angularly that their "hollows" are in overlapping associated toothed discs since the pegs 31 and 32 always move in contact - with the "high" portion of the two cams and are never able to penetrate into their "hollows".

The driving mechanism for the sleeve 15 is placed in this position. when it is desired to fill the whole of tube 7 with liquid intended to be distributed in several shots by the pipette, in very accurate and particularly small quantities.

For this purpose, the sleeve 15 carries on its part 15_c adjacent the ball and cage assembly 16 a toothed wheel 40 (Figures 1 and 14) that projects partially outside the cup 12 through a slot 120 formed in the latter, which wheel can be driven from outside the pipette, for example by making use of the device shown in Figures 14 and 15.

This device has a frame comprising a base 41 and an upstanding member 42 solid with this base and forming a support for the pipette.

To this end, member 42 carries two lugs 43 and 44 having facing ends that constitute slideways between which is arranged the pipette, by engagement in parallel grooves 121 and 122 formed on the external surface of the cup 12 (Figure 15). On the member 42 there is also fixed a pillar-like element 45 forming a support for a curved and removable clamping strap serving to hold the pipette in position when it is placed between the lugs 43 and 44.

This upstanding member 42 further carries a boss 47 on which is rotatably mounted a toothed disc 48 to which is secured a stem 49 provided with a milled knob 50 enabling a user to rotate the disc 48 with his fingers. This disc meshes with a wheel 51 which is kinematically connected to a toothed disc 52 through a friction coupling, not shown, this disc 52 being so placed that it comes into meshing engagement with 4 f whe the latter is mounted on the device ■( The . base 41 is formed at its end opposite the member 42 with a circular recess 41_a adapted to receive a container, such as the one outlined in broken lines and referenced 53, which contains the liquid with which the pipette is to be filled, the tube 7 of this pipette being then dipped at its end into the container (Figure 14).

This filling operation is effected in the following manner : once the cams 36 and 37 have been placed as shown diagrammatically in Figure 13, the knob 50 is turned in direction F _. (Figure 14) thereby to move the shaft 14 of the pipette in direction F^ (Figure 1 ).

This movement continues until the circular flange 14a_ of shaft 14 abuts against the shoulder 9b of the sleeve. Even if the user carries on turning the knob 50 in direction F no force is then transmitted to the pipette : because the toothed disc 52 is locked angularly by abutment of the flange 14_a on the shoulder 9b , the kinematic connection between this disc and the wheel 51 is interrupted by virtue of the fact that the friction coupling interposed between these components then comes to slip.

Once the shaft 14 comes to be located in this extreme position, after moving in direction F^, the knob 50 is turned in direction Fg thereby causing the shaft 14 to move in direction F^ and the rod-cum-piston 14h to aspirate into the tube 7 a certain quantity of the liquid filling the container 53. Movement of the shaft 14 in direction F^ is interrupted by abutment of its part 14b against the cap 11, and due to the presence of a friction coupling between wheel 51 and disc 52. Once filled, the illustrated pipette can be removed from the filling apparatus and is ready for use.

In a variant, not shown, the pipette may itself have means for driving the toothed disc 40. These means may for example consist of an internally toothed wheel which meshes with the disc 40 and which is rotatably mounted Although the above driving mechanism has only been described with reference to its application in a pipette, clearly such a mechanism may have other applications, some quite 'different from that described. Moreover, the driving mechanism may, according to a variant, com prise only one toothed disc instead of two in which case the pawls 23

Claims (1)

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