Abstract:
A rotational to linear displacement conversion mechanism comprises: an assembly including a plurality of driver discoidal elements and a plurality of driven discoidal elements mounted alternately on a common central axis to form an interleaved stack. Each discoidal element has a ramped surface, the ramped surfaces of adjacent elements being complementarily shaped and opposed so that, when in contact and completely interengaged, they form a stack of minimum length. A coupling means is provided for coupling the driver discoidal elements for rotation together about the axis by an externally applied force while permitting them to translate along the axis. The driven elements are mounted in such a way as to permit translation along the common axis while preventing rotation of the driven elements about the common axis. A rotational displacement of the driver elements by such an externally applied force causes the elements to separate by camming action of their interengaged ramp surfaces so as to produce an extension of the stack corresponding to the cumulative separations of the driver and driven elements. Finally, a resilient bias means is coupled to a driven element in such a way as to restore the assembly to its minimum length in the absence of the externally applied force.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to displacement conversion mechanisms and, in particular, to the conversion of rotary to translational displacement and to actuators employing such mechanisms. 
       BACKGROUND OF THE INVENTION 
       [0002]    Actuators for producing a mechanical displacement of a member to be driven are employed throughout industry in a wide variety of applications. These include machinery control mechanisms, including valves and linkages, robotics, prosthetics, camera optics, pumps, brakes and power tools to name but a few. The displacement required may be rotary, linear or other translational and of short or long stroke. It may be unidirectional, with a separate return mechanism such as a spring or bidirectional, including reciprocation. The choice of actuator for a particular application often depends on the environment in which it is to be used. 
         [0003]    Many forms of actuator for producing linear or other translational displacement of a driven object are known in the prior art. These include straightforward pneumatic and hydraulic piston arrangements and more recently developed devices known as “air muscles” in which inflation of a bladder causes contraction of an outer metal sheath in a manner similar to living muscle contraction. Other forms of linear actuator are electromagnetic, such as the solenoid and the voice coil motor. Such devices have limited extension capabilities. 
         [0004]    Electric motors, such as stepper or servo motors, are also convenient drivers for actuator devices but to produce linear displacements their rotary output must be transformed into a linear motion by a suitable conversion mechanism. Many such mechanisms have been employed for this purpose such as the rack and pinion mechanism and the lead screw. In the latter case, a short threaded nut is translated along a long threaded shaft rotated by the motor and is coupled to a member to be driven, such as a print carriage. By appropriate choice of thread pitch or use of additional gearing, the mechanical advantage of this type of mechanism can be increased to produce relatively large extensions for small rotations. 
         [0005]    Cam shafts and followers, biased by a return spring, are also widely used, especially in conventional engines, for producing reciprocating linear motion and similar cam follower and spring arrangements are also used in power tools to produce a reciprocating action from a conventional electric motor drive shaft. 
         [0006]    There is still scope however for a simple rotary to translational motion conversion mechanism, capable of producing large extensions for a limited angle of rotation and robust enough to be tolerant of hostile environments. The present invention offers such a mechanism. 
         [0007]    Also known in the prior art are adjustable shims or spacer arrangements for producing a desired static linear displacement by relative rotation of complementarily shaped discoidal wedges or cams. Such arrangements are described in U.S. Pat. No. 4,433,879 (J. C. Morris) for an “Adjustable Extension-Cam Shim” and in GB published patent application 2331568 (A. Szmidla) for “Wedges and arrangements thereof”. 
       DISCLOSURE OF THE INVENTION 
       [0008]    Accordingly, the invention provides a rotational to linear displacement conversion mechanism comprising: an assembly including a plurality of driver discoidal elements and a plurality of driven discoidal elements mounted alternately on a common central axis to form an interleaved stack, each discoidal element having a ramped surface, the ramped surfaces of adjacent elements being complementarily shaped and opposed so that, when in contact and completely interengaged, they form a stack of minimum length; coupling means for coupling the driver discoidal elements for rotation together about the axis by an externally applied force while permitting them to translate along the axis; said driven elements being mounted in such a way as to permit translation along the common axis while preventing rotation of the driven elements about the common axis, whereby a rotational displacement of the driver elements by such an externally applied force causes the elements to separate by camming action of their interengaged ramp surfaces so as to produce an extension of the stack corresponding to the cumulative separations of the driver and driven elements; and resilient bias means for restoring the assembly to its minimum length in the absence of the externally applied force. 
         [0009]    Such devices are very compact and rugged and, in contrast to the prior art devices of U.S. Pat. No. 4,433,879 and GB 2331568 which are essentially static and have no guide system or return mechanism, are suitable for many dynamic precision applications such as positioning actuators or measured stroke fluid delivery devices, such as syringes for medication or for fuel dispensers. Reciprocation may also be produced by continuous rotation and used in pump applications. 
         [0010]    Using a stack of elements allows for a much greater, cumulative extension for a given rotation and is made possible by the coupling of the driver elements for rotation while allowing their linear separation. 
         [0011]    This is preferably implemented by providing at least the driven discoidal elements intermediate the ends of the stack with axially aligned bores, each driver element having a projection extending axially from one face which extends through the bore of its adjacent driven disc and locates in a recess in a proximate driver element in keyed, slideable engagement therewith so that rotational drive force can be transmitted between driver elements while allowing relative sliding motion in an axial direction. 
         [0012]    Preferably each said driver element recess is part of a bore through the driver element and said projection is preferably part of at least one rib formed on the inner surface of the bore of its corresponding driver element, which rib projects outwardly from its driver element discoidal portion and engages at least one complementarily oriented rib portion in the bore of the proximate driver element to provide said keyed slideable engagement. 
         [0013]    Although other arrangements would be possible, one preferred arrangement is for the bore in each intermediate driver element to be provided with two diametrically opposite ribs each extending over a 90 degree arc of the bore, said ribs being keyed into engagement with a similar pair of ribs in a proximate driver element oriented at 90 degrees to the first mentioned pair of ribs. 
         [0014]    The preferred way of preventing rotation of the driven elements is to provide them each with a plurality of peripheral lugs, the mechanism further including grooved guide means surrounding the stack in which the lugs locate to prevent rotation. 
         [0015]    Preferably, a driver element is located at one end of the stack and has an outer surface adapted to be coupled to an external drive and an inner ramped surface and a driven element is located at the opposite end of the stack and has an outer surface adapted to deliver a translational load force and an inner ramped surface, intermediate driver and driven elements having ramped surfaces on both sides. Preferably the end driver element is fixedly mounted on an outwardly extending axial shaft, threaded externally for coupling to the external drive. 
         [0016]    In such an arrangement, it is preferred that the mechanism includes a housing assembly for the stack, comprising a cylindrical cover to one end of which the terminal driven element is fixed, the other end of the cover terminating in a slotted flange. The housing assembly further comprises a fixed cage structure surrounding the cylindrical cover and being formed with a plurality of guide legs extending in the axial direction and passing through the slots in the flange of the cylindrical cover to constrain it to linear movement. Additionally, the cylindrical cover is provided with external grooves and the guide legs are provided with internal grooves in both of which said peripheral lugs locate, in operation, to restrain the driven discs against rotation while permitting translation. 
         [0017]    Another preferred feature is that the resilient bias means is a coil spring trapped between the flange of the cylindrical cover and an end of the cage. 
         [0018]    Another preferred feature is that the driver and driven elements each have a plurality of ramps per ramped surface, distributed circumferentially at evenly spaced positions. This enables an even greater ratio of displacement to angle of rotation than would be the case with a single 360° ramp. 
         [0019]    For a single stroke application, it is preferred that the camming ramp surfaces are planar, rising at a relatively shallow angle to the plane of the discoidal elements and alternating with relatively steep return surfaces. 
         [0020]    For a continuously rotated application, both the rising and falling surfaces of the ramps could be at the same angle or the ramped surfaces are smoothly undulating in form without discontinuity at the peaks. The latter arrangement is the more compact, in its unextended state. 
         [0021]    For single stroke applications, the mechanism may include a stop for preventing rotation of the driver element beyond the arc defined by the ramp surface. 
         [0022]    When provided with a drive mechanism for rotatably driving such driver elements, the displacement conversion mechanism becomes an actuator. The drive mechanism may be a motor or a manually operated crank. A continuously rotated driver element will produce a reciprocating linear output. 
         [0023]    Such an output from a displacement mechanism including a rotatable crank for rotatably driving the driver elements is eminently suitable for a hand pump application which would require a sealed casing for enclosing the mechanism and forming a pump chamber containing a one way inlet means for permitting fluid to be drawn into the pump chamber as the mechanism contracts and an outlet for enabling fluid to be expelled from the pump chamber as the mechanism extends one way, as the crank is rotated continuously. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The invention will now be further described, by way of example only, with reference to preferred embodiments thereof as illustrated in the accompanying drawings, in which: 
           [0025]      FIG. 1  is a side elevation of an unextended stepped disc stack illustrating the principles underlying a displacement mechanism according to the invention; 
           [0026]      FIG. 2  is an end elevation of one of the discs making up the stack of  FIG. 1 ; 
           [0027]      FIG. 3  is a side elevation of the disc stack of  FIG. 1  in a partially extended state; 
           [0028]      FIG. 4  is a side elevation of the disc stack of  FIG. 1  in a fully extended state; 
           [0029]      FIG. 5  is an exploded view of an assembly of driver and driven stepped discs forming part of a displacement mechanism according to the invention; 
           [0030]      FIG. 6  is an enlarged exploded view of two driver and one driven disc at one end of the assembly of  FIG. 5 ; 
           [0031]      FIG. 7  is an isometric perspective view of the assembly of  FIG. 5  in its unextended state; 
           [0032]      FIG. 8  is an isometric perspective view of the assembly of  FIG. 5  in a partially extended state; 
           [0033]      FIG. 9  is an isometric perspective view of the assembly of  FIG. 5  in its fully extended state; 
           [0034]      FIG. 10  is a side elevation of a portion of an unextended undulating disc stack illustrating the principles of an alternative displacement mechanism according to the invention; 
           [0035]      FIG. 11  is an isometric perspective view of one of the undulating discs making up the stack of  FIG. 10 ; 
           [0036]      FIG. 12  is a side elevation of the stack of  FIG. 10  in a partially extended state; 
           [0037]      FIG. 13  is a side elevation of the stack of  FIG. 10  in a fully extended state; 
           [0038]      FIG. 14  is an exploded view of an assembly of driver and driven undulating discs forming part of a displacement mechanism according to the invention; 
           [0039]      FIG. 15  is an enlarged exploded view of driver and driven discs at one end of the assembly of  FIG. 14 ; 
           [0040]      FIG. 16  is an isometric perspective view of the assembly of  FIG. 14  in its unextended state; 
           [0041]      FIG. 17  is an isometric perspective view of the assembly of  FIG. 14  in a partially extended state; 
           [0042]      FIG. 18  is an isometric perspective view of the assembly of  FIG. 14  in its fully extended state; 
           [0043]      FIG. 19  is an exploded perspective view of an actuator and displacement mechanism according to the invention including either the stepped disc assembly of  FIGS. 5 to 9  or the undulating disc assembly of  FIGS. 14 to 18 ; 
           [0044]      FIG. 20  shows the actuator and displacement mechanism of  FIG. 19  in its unextended state; 
           [0045]      FIG. 21  shows the actuator and displacement mechanism of  FIG. 19  in a partially extended state; 
           [0046]      FIG. 22  shows the actuator and displacement mechanism of  FIG. 19  in its fully extended state; 
           [0047]      FIG. 23  is an exploded perspective view of a modification for a pump application of the actuator and displacement mechanism illustrated in  FIG. 19 ; 
           [0048]      FIG. 24  is an exploded view of a pump employing the assembly of  FIGS. 14 to 18 ; 
           [0049]      FIG. 25  shows the assembled pump of  FIG. 24  on its inlet stroke; and 
           [0050]      FIG. 26  shows the assembled pump of  FIG. 24  on its outlet stroke. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0051]    In  FIGS. 1 to 14 , the principles of operation of one form of displacement mechanism according to the invention will now be described. This mechanism involves a stacked assembly  100  of discoidal elements, also referred to as discs, of which three, numbered  101 ,  102  and  103 , are shown in  FIG. 1 . The discs are not planar but are relieved to provide four planar ramp surfaces in four sectors on opposite sides, surrounding a central bore  104 , those on one side being offset by 45° from those on the other side. Four of the ramped surfaces  105 - 108  are seen in the end elevation of the mechanism from the left hand side, looking at disc  101 , as shown in  FIG. 2 . The visible edges of the ramped surfaces on the assembly  100  are drawn as continuous lines in  FIGS. 1 to 4  whereas the invisible edges are dashed. The edges of the discs nearest the viewer are hatched for illustrative purposes only. Each ramped surface terminates in a steep return step, such as steps  109 - 112  in the case of the outer face of disc  101 . 
         [0052]    In  FIG. 1 , the assembly  100  is in its unextended state and the ramped surfaces of the three discs are snugly interengaged in complementary fashion to take up the minimum space. If discs  101  and  103  are rotated in the direction of the arrow shown in  FIG. 2 , while the intermediate disc  102  is restrained against rotation, the camming action of the opposed ramped surfaces forces the discs to separate, as shown in  FIG. 3 . The maximum displacement is achieved, as shown in  FIG. 4 , after a rotation of 45°, when the stepped return surfaces at the end of the opposed ramp surfaces coincide. The maximum displacement is equal to double the height of the ramps multiplied by the number of disc-to-disc interfaces and the rotation needed to achieve it depends on the number of sectors per disc. So in this example, four sectors require a rotation of 45° to achieve maximum displacement. 
         [0053]    How this principle is applied to a practical mechanism is illustrated in  FIGS. 5 to 9 . In  FIGS. 1 to 4 , no distinction was made between the discs, except for the implied restraint against rotation of disc  102 . In a practical application, it is necessary to design driver and driven discs differently. In fact, in the assembly  120  of  FIG. 5 , there are several types of each disc. These consist of an input driver disc  121 , identical intermediate driven discs  122  alternating with identical driver discs  123  and  124 , and terminating in an output driven disc  125 . The driver discs  123  and  124  are structurally identical but discs  123  are in a first orientation while discs  124  are oriented at 90° to discs  123 . All the discs are stacked together in engagement with each other on a common axis. 
         [0054]    Torque to rotate the input driver disc  121  is provided by way of an integral threaded shaft  126  by means not shown in this drawing, such as a motor or a manual crank. In order for the mechanism to extend, the drive torque must be transmitted from input driver disc  121  to all of the driver discs  123  and  124 . Also the driven discs  122  must be restrained against rotation. This restraint is achieved by means of four projecting lugs  127  on each driven disc which can locate in an external spline or similar channels, not shown in this drawing. 
         [0055]    The communication of the drive torque cannot be by fixed linkage because the separation between the driver discs increases as the assembly extends and they move outwardly along the axis. Communication of the torque from driver disc to driver disc is thus effected by a system of projecting ribs  128 ,  129  which consist of internal raised portions, formed within keyhole bores  131  and  132  within central bosses  133  of the driver discs, and external prong-like portions. The external prongs pass through bores  130  in the driven discs and engage in the keyhole bores  131 ,  132  of adjacent driver discs. The prongs  128  and bores  132  on driver discs  123  are identical to the prongs  129  and keyhole bores  131  on driver discs  124 , the only difference being their relative orientation of 90° to each other in the assembly stack. 
         [0056]    Each projecting rib extends over 90° of arc so that its extending prong portions actually key into the spaces between the ribs in the central bore of the next driver disc. Thus the driver discs  123 ,  124  are keyed for rotation together and with the input driver disc  121  by means of the engagement of the pronged extensions of ribs  128 ,  129  with the internal portions of the ribs  128 ,  129  within keyhole bores  131 ,  132  of the next driver disc. At the same time, this arrangement of prongs and keyholes allows them to slide relative to each other in the axial direction, thereby enabling the assembly to extend. 
         [0057]      FIGS. 7 ,  8  and  9  show the assembly  120  in its unextended, partially extended and fully extended states, respectively, the fully extended state again being achieved after a rotation of 90°. 
         [0058]    Another form of displacement mechanism according to the further aspect of the invention is illustrated in principle in  FIGS. 10 to 13  and a practical implementation is shown in  FIGS. 14 to 18 .  FIGS. 10 ,  12  and  13  show a stacked assembly  150  of three discoidal elements  151 ,  152  and  153 . For clarity, the outer edges of the discs are shown cross hatched in  FIGS. 10 ,  12  and  13 . The operation of the mechanism is very similar to that of the mechanism of  FIGS. 1 to 4 , the principal difference lying in the relief of the faces of the discoidal elements. 
         [0059]    By way of example, one of the elements  152  is shown in perspective in  FIG. 11  in the initial orientation that it has in  FIG. 10 . It can be seen, by noting the intersection of the disc at various points with three dashed reference circles, that instead of a ramped surface, the disc has smooth out-of-plane undulations, surrounding a central bore,  154 . Looking at the visible face of disc  152  from the right in  FIG. 11 , these undulations consist of three ridges,  155 , 156  and  157 , interspersed with three valleys,  158 ,  159  and  160 . On the reverse face, the ridges become valleys and vice versa. It should be noted that, although, in  FIG. 11 , the discs do not appear circular but waisted, this is an effect of the undulations on the perspective view and is caused by the fact that the ridges, such as  155 , are raised with respect to the neighbouring valleys,  158  and  160 . The vertical projection of a disc onto a plane is actually still a circle. 
         [0060]      FIG. 10  shows the assembly in its unextended state with the discs  151 - 153  in a relative rotational orientation which takes up the minimum space. In this orientation, the discs interfit snugly with their undulating surfaces in full contact so that the ridges and valleys of each disc surface nestle in the valleys and ridges respectively of an adjacent disc surface. In this example, it is assumed that all the discs or at least discs  151  and  153  can move axially. It is further assumed that disc  152  can be rotated while discs  151  and  153  are restrained against rotation. 
         [0061]    The effect of rotation of disc  152 , as shown in  FIG. 12  is to drive the discs  151  and  153  away from disc  152  by camming action, as the rising slopes of the opposed surfaces bear on each other. Note the new position of ridge  155  of driver disc  152 , corresponding to a rotation of 30°. Ultimately, after a total rotation of 60°, as shown in  FIG. 15 , the assembly is fully extended with the ridges of the undulating disc surfaces aligned. 
         [0062]    A practical assembly  161 , operating according to the principles of  FIGS. 10 to 13  is shown in exploded perspective view in  FIG. 14  and a portion of the assembly is shown enlarged in  FIG. 15 . Similarly to the stepped disc version, the undulating disc stack is made up of a unique input driver portion, connected directly to threaded input drive shaft  162 . The input driver portion is relieved on its inner face similarly to driver discs,  164   
         [0063]    The driver discs  164  are all identical but have successively different orientations in the stack. Each drive has a central bore  165 . Identical driven discs  166  are located between each pair of driver discs. The stack terminates in a driven output disc  167 , seen on the right in  FIG. 14 . This has a relieved inner face but a blank outer face for transmitting linear output force. 
         [0064]    Drive is communicated from the input drive shaft  162 , via its driver portion to the driver discs  164  which are able to separate axially, by means of a system of prongs and keyholes similar to that of  FIGS. 5 and 6 . However, because of the lack of depth of those discs, it is necessary to have 4 pairs of shorter prongs  169  instead of the two shown on the stepped type. These are shown schematically in dashed outline in  FIGS. 14 and 15 . As can be seen from the orientation of the prongs in the drawing, successive driver discs are rotated by 90° with respect to the next driver disc. The prongs pass through central bores in the driven discs and key into correspondingly shaped recesses in the bores  165  of other driver discs and of the input driver portion on shaft  162 . Because the discs are so thin, the prongs actually pass through and key into more than a single neighbouring driver disc in the stack. 
         [0065]    The driven discs  166  are each restrained against rotation by a system of four lugs  168 , located 90° apart on the circumference of the driven discs. These engage in splined external channels, not shown in this drawing. As the driver discs are rotated, the assembly expands owing to the camming action between driver and driven discs. 
         [0066]    The assembly  161  is shown in  FIGS. 16 ,  17  and  18  in its unextended, partially extended and fully extended states, respectively. In comparison with the stepped disc assembly  120  of  FIGS. 7 to 9 , the extension is the same for the same amount of relief but it will be noted that the discs of assembly  161  can be packed much more closely in their unextended state. Thus a much more compact actuator can be produced using the undulating discs or else a much greater extension can be used by packing more discs into the same initial length assembly. These illustrations show how fewer undulating discs achieve the same offset as the stepped version and could possibly achieve an offset of 200% of their initial unextended length. 
         [0067]    Turning now to  FIGS. 19 to 22 , the incorporation of the assemblies  120  or  161  into a complete rotary to linear displacement mechanism in an actuator will be described.  FIG. 19  is an exploded view of the actuator, which has a common structure capable of accommodating either the stepped disc assembly  120  or the undulating disc assembly  161 , both of which are shown in their unextended state. In fully assembled form in  FIGS. 20 to 22 , only the stepped disc version is shown but it will be understood that it is interchangeable with the undulating disc version. However, the following description will refer only to the stepped disc version, for brevity. 
         [0068]    An annular base plate  170  supports the moveable portions of the actuator by means of two bearing races  171  and  172  in which the drive shaft  126  is mounted for rotation. A drive mechanism  173  comprises a hub  174 , threaded onto shaft  126  which hub is itself rotated by a crank  175 . The drive mechanism  173  could equally well be an electric motor such as a stepper motor or servo motor. 
         [0069]    The assembly  120  is housed in a cylindrical piston-like cover  180  which is of the same length as the unextended assembly  120 . At its open end, the cover  180  terminates in a flange  181 , provided with four slots  182 . These slots locate slideably on the exterior of four guide legs  183 , secured to the base plate  170  at one end and bolted to a collar  184  at the other end to form a cage for the piston cover  180 . The cover  180  is free to move axially and to protrude through the collar  184  when driven by the expanded disc assembly. The other end of the piston cover is bolted to an end plate  185 , for delivering the output of the actuator. To restore the actuator to its original state, that is, with the assembly contracted, a return spring  190  is located between the piston cover flange  181  and the collar  184  to provide a resilient bias against expansion. The return spring is a compression spring and bears on the flange  181  and the collar  184 . 
         [0070]    In order to prevent the driven discs of the disc assembly from rotating, the lugs  127  of the driven discs in assembly  120  locate in narrow channels  191  running along the piston cover  180  in an axial direction. Since, however, in its expanded state the disc assembly is much longer than the cover  180 , the guide legs  183  are also provided with further internal grooves  192 , aligned with grooves  191  on the piston cover. These grooves  191  and  192  ensure that the lugs  127  of driven discs  122  are always engaged to prevent rotation. 
         [0071]    The operation of the actuator can be better understood by looking at  FIGS. 20 to 22 . In  FIG. 20 , the actuator is in its unextended state. Operation of the crank  175  in the direction of the arrow rotates the hub  174  and drive shaft  126  to cause expansion of the disc assembly  120 . This forces the piston cover  180  outwardly against the action of the return spring  190 , guided by guide legs  183 , as shown in  FIG. 21 . In  FIG. 22 , the piston  180  is fully extended. 
         [0072]    If the described actuator is to be used in applications requiring a single stroke, such as precision positioning or dispensing of a measured volume of fluid, then it is desirable to limit the travel to prevent the discs overshooting their maximum displacement. 
         [0073]    The displacement of the actuator of  FIGS. 19 to 22  is limited by the action of the cover flange  181  fully compressing the spring  190  against the collar  184  as shown in  FIG. 22 . This stops rotation of the stepped discs beyond their maximum displacement, which would result in an abrupt return as return steps  106  of adjacent discs slipped over each other. If the piston cover were slightly longer, it would be possible to drive the mechanism with a continuously rotating input and produce a reciprocating motion. Clearly, this would be smoother if the undulating disc assembly were used, as this has equally smooth stroke and return slopes but the return stroke is faster with the stepped version. 
         [0074]    In comparing the two types of disc, the major advantage of the undulating version is that it is particularly compact when unextended and therefore is more suitable for applications having a limited space. 
         [0075]    A variant of the assembly of  FIGS. 19 to 22  which is more suitable for a pump application is shown in  FIG. 23 . This is largely identical to  FIG. 19 , identical parts being identically numbered, but includes a larger base plate  197 , a secondary piston cover  193 , in place of end cap  185  and an outer casing  194  in which sits an O-ring  195 . The secondary piston cover  193  slides up past the O-ring, secured in the recess at the end of the outer casing  194  and is restrained from over extension by a flange  196  at the foot of the outer piston cover. The secondary piston cover is thus able to pump fluid without leakage from the cylinder formed by the outer casing. 
         [0076]      FIG. 24  illustrates the application of the undulating disc assembly as described in  FIGS. 14 to 18  to a pump. The assembly  161  is mounted in a pump barrel  200  and driven against a compression spring  201  at the end of the barrel having an outlet valve  202 . The inside of the barrel is splined or grooved to constrain the driven discs of the assembly to linear movement only, by engagement of lugs  168  with the grooves. 
         [0077]    The assembly is driven, in a similar manner to  FIG. 19  by means of a crank handle  203  and hub  204 . The hub  204  and the input drive shaft  162  are mounted in bearings  205  located in a threaded end cap  206  at the opposite end of the barrel to the outlet valve. Because the barrel is long enough to permit the discs of assembly  161  to rotate continuously, the assembly expands and contracts in reciprocating fashion to produce the pumping action. A disc  207  acts as a one way seal to permit air or other pumped fluid to be drawn into the outlet end of the pump barrel. 
         [0078]    Although the stepped disc mechanism could also be used, the undulating version offers a smoother reciprocating action, albeit with a slower return action. 
         [0079]      FIG. 25  shows the assembled pump at one extreme of its inlet stroke, with the assembly  161  fully contracted.  FIG. 26  shows the assembled pump at the extreme of its outlet stroke, with the assembly  161  fully extended.