GB2145795A

GB2145795A - Improvements in or relating to a mechanical transmission

Info

Publication number: GB2145795A
Application number: GB08323517A
Authority: GB
Inventors: Ernst Linder
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-09-01
Filing date: 1983-09-01
Publication date: 1985-04-03
Also published as: GB2145795B; GB8323517D0

Abstract

A mechanical transmission which transfers power from a driving device, such as a motor 20, to a driven object, such as a piston 21 in an aspirator, includes an elongate transmission element 23 formed as a single (or double) spiral of tightly wound wire. The transmission element 23 can follow a non-linear path with a minimum of guidance 29,34. The transmission element 23 may be connected to the piston 21 by a magnetic coupling 31. <IMAGE>

Description

SPECIFICATION Improvements in or relating to a mechanical transmission The present invention relates to a mechanical transmission and more particularly, although not exclusively, to a mechanical transmission for use in a device such as a small aspirator.

In certain situations there is a need to transmit movement from a driving device, such as a geared electric motor, to an element that must move linearly. It is often necessary to drive the member that moves linearly in both linear directions.

One specific example of a device in which such a mechanical transmission may be used is a small aspirator. An aspirator consists of a cylinder having a tight-fitting piston that is linearly movable within the cylinder. Upon an appropriate movement of the piston air is drawn into the cylinder through an inlet at one end of the cylinder, and pressed out through an appropriate aperture provided at the other end of the cylinder. In an aspirator the piston is driven so that it moves at a very low speed. Thus air is continuously drawn into the cylinder from the surrounding atmosphere over a period of time which may be, for example, four hours. The air can subsequently be analysed, for example in a gas chromatograph, and thus quantative measurements can be obtained as to the amount of certain chemical compounds within the cylinder.It is thus possible to determine the amounts of the chemical compounds that a person carrying the aspirator has breathed into his iungs during the period of time in which the sample of air was taken by the aspirator.

It hasbeen proposed, in a previous aspirator, to provide a motor which, via an appropriate gear train, rotated a threaded rod. The threaded rod passed through a threaded bracket which, upon rotation of the threaded rod, moved up or down. The threaded bracket was connected to the piston. The disadvantage of this arrangement was that the apparatus, in its entirety, was over twice as long as the piston stroke.

The above-described prior-proposed aspirator was unwieldly, and it was found desirable to make the aspirator shorter so that it would easily fit into an outside pocket on the clothing of the person carrying the aspirator. One such shorter aspirator is described in Swedish Patent No. 411,147, which corresponds to U.S.A. Patent No. 4,281,544. In this prior proposed aspirator the motor drives a shaft on which a drum is mounted. A cord is connected to the periphery of the drum. The motor and drum are arranged at the side of the sampling cylinder, and the cord passes around a pulley located in the bottom of the sampling cylinder, and continues up to the piston. As the drum rotates the piston is pulled down the cylinder at a predetermined speed, thus drawing air into the cylinder.

Simultaneously the cord becomes rolled up on the drum.

The prevent the cord from becoming slack, and so that the piston can be returned to its initial position at the end of a cycle, a spring is provided, within the cylinder, which engages the underside of the piston. The spring becomes compressed as the piston is pulled down by the cord. The described aspirator is only slightly longer than the length of the piston stroke, and may thus be fabricated to have a convenient size.

A disadvantage of this prior proposed aspirator, however, is that the force required to compress the spring increases as the piston is moved downwards. Thus, as the spring becomes compressed, a greater force is required to compress the spring further. This gives rise to a mechanical strain, and also leads to an unnecessarily high power consumption. This leads to a relatively rapid drain on the battery that is provided to drive the motor. In addition, when the piston is released at the end of a cycle the piston is forced up rapidly by the spring. Since a considerable tensional force is present in the spring, the piston is forced upwardly extremely rapidly. The piston is normally make of glass, and thus in these prior proposed devices the pistons are frequently damaged.A further disadvantage of the prior proposed device is that the increasing resistance to piston movement provided by the spring affects the uniformity of the speed of the piston movement. Thus the piston tends to move more slowly towards the end of a cycle, and thus air is not drawn into the cylinder at a precisely constant rate throughout the whole of a sampling cycle. This can, of course, lead to inaccuracies.

According to this invention there is provided a mechanical transmission adapted to transfer power from a driving device to an object and adapted to drive the object in two opposed directions, the transmission including a transmission element consisting of an elongate tightly wound spiral element formed from a wire, said transmission element being adapted to follow a non-linear path.

The object may be driven in two opposed directions along a predetermined linear path or in two opposed directions along a predetermined curved path.

The elongate element is, at least effectively, bendable in at least one direction to permit the elongate element to follow the non-linear path.

Preferably, the coil diameter and wire thickness of the elongate element are such that no internal or external guidance of the member is required in the straight lengths thereof to avoid sideways bending of the element when a compressive or pushing force is applied to move the object. Preferably the spiral element has such a prestressing in the coiling, and such a wire thickness, that the spiral is not stretched out with the traction or pulling force that is normally applied to the elongate ele ment to move the object.Thus it will be appreciated that as the object is moved in one direction, with the elongate element applying a pushing force, the elongate element does not tend to bend or buckle in the straight parts of its path, even though it is not guided in those straight parts, and when a pulling force is applied to the object the elongate element does not tend to stretch out. The size of the maximum pushing force that can be applied without buckling depends partly upon the length of the straight part of the path, and partly upon the outer diameter of the spiral.

The maximum force that can be withstood without buckling depends upon the length, the diameter and the material in the wire. The maximum pulling force that can be withstood without extension of the coil depends upon the material of the wire, the diameter of the wire and the prestressing present in the coil.

Preferably, the elongate element is guided by guide means provided either inside or outside the spiral where the element is to follow a curved path. The guide means may be constituted by rollers adapted to engage the exterior of the elongate element. Alternatively there may be an external tubular guide element, that is to say a tubular guide element that surrounds the elongate spiral element, the elongate spiral element passing through the central passage of the external tubular guide element. Alternatively again the guide means may be an internal rod or tubular guide element located within the centre of the coiled elongate member. The guide means may be coated with a material having a low coefficient of friction.

In order to enhance the properties of the elongate element it may be desirable to provide a cord or wire arranged inside the spiral and fastened to one or both ends of the spiral.

Such a cord or wire may assist when the elongate element has to provide a pulling force.

The spiral may be single spiral, but it may be advantageous to use a double spiral, with one spiral member surrounding or being formed on the outside of another spiral member. This makes it possible for the elongate element to provide an increased pulling force without becoming elongated.

In one embodiment of the mechanical transmission in accordance with the invention one end of said elongate element is connected to a movable member, the movable member having a threaded portion adapted to engage the threaded exterior of an elongate threaded member, there being means to rotate the elongate threaded member to move the movable member axially of the elongate member.

Thus a driving force may be applied to the elongate element.

In an alternative embodiment of the inven tion said driving device may consist of a drum around which the elongate element is ar ranged to be wound up, the arrangement being such that the object is moved whenever the elongate element is wound on to or un wound from the drum.

A mechanical transmission in accordance with the invention may be of particular use in fabricating an aspirator, and thus the present invention also relates to an aspirator incorpo rating a mechanical transmission of the type described in the preceding paragraphs, the aspirator including a cylinder having a piston axially movable therein said piston constitut ing the said object. Such an aspirator may incorporate a geared direct current motor or a stepping motor as the prime mover. In such an aspirator it is preferred that a closely wound spiral elongate element is used between the driving device and the piston, the spiral element being guided by rollers or abutting faces being provided with a surface of a material with a low coefficient of friction.

In a preferred embodiment one end of the elongate element is connected to a movable member which has a threaded portion adapted to engage the threaded exterior of an elongate threaded member which is rotated by an appropriate motor to move the movable member axial of the elongate member. The movable member is mounted on an appropriate guide rod to ensure that the movable member moves axially. Preferably means are provided to release the piston from the driving device to enable the piston to move freely.

Such a means may comprise an operating lever or the like. In a preferred embodiment the operating lever disengages the movable member which is threadedly engaged with the elongate threaded member.

As has been mentioned, the guide members may constitute a rod or a tube located in the interior of the coil elongate member. The outer diameter of such a rod or tube should, of course, be slightly smaller than the diameter of the spiral element. The advantage of using a tube instead of a rod is that a string or electric wire can be threaded through such a tube. One advantage of guiding the spiral element with an internal guide member is that the resultant sliding friction is low.

In an aspirator of the type in which the driving device consists of a motor driven drum no space need to be provided for an upwardly moving part of the elongate element when the object is being driven downwardly, and when the spiral member has been wound onto the drum the spiral member will of itself provide a force tending to move the spiral element to the unwound position. Thus this inherent unwinding force may be sufficient to return the object to its initial position when the drum is disengaged from the motor.

In certain cases it can be desirable that a mechanical transmission, when used in an aspirator, be capable of transmitting a traction force that would be great enough to stretch or elongate the spiral element, even though such a force is not normally required. It is in these situations where a cord or wire will be provided which will be parallel to or inside the spiral element and extending between the driving device and the object. Thus, the wire or cord will be directly or indirectly connected to one or both.ends of the spiral. In such an arrangement any excess traction force required will be provided by the cord, whilst the compressive or pushing force will be taken up by the spiral.

In an aspirator in accordance with the invention it may well prove desirable to connect the object to the transmission agent via a threshold force coupling, that is to say a coupling that will become disengaged when a force exceeding a predetermined force is applied to the coupling. Such a coupling may comprise a magnetic coupling. When such a coupling is used if, for some reason, the object jams whilst it is moving, the force applied to the object through the elongate element via the coupling would increase, and the threshold coupling will then release. In this way the elongate element is not subjected to a sufficiently great force to permanently deform the elongate element.

To enable an operator either to know when the threshold coupling is being released from the piston, or to enable the power supply to the driving motor to be terminated at that time, it is preferred that an electric contact function is provided by the threshold coupling which provides an output signal if the coupling is released. The electric contact function may be provided by means of an insulated wire which contacts the piston simultaneously with the magnetic coupling device.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view of a pocketsized aspirator of a prior-proposed type, parts of the aspirator being shown as being transparent to facilitate illustration of the arrangement; Figure 2 is a diagrammatic cross sectional view of an aspirator incorporating a transmission in accordance with the present invention; Figure 3 is a diagrammatic cross sectional view of part of a modified aspirator incorporating an alternative embodiment of the invention, only those parts of the aspirator that have been modified being illustrated;; Figure 4 is a further illustration of another embodiment of a drive in accordance with the invention, and Figure 5 shows an example of a coupling that may be utilised with a drive in accordance with this invention.

Fig. 1 of the drawings illustrates a pocketsized aspirator that has been proposed previously. The aspirator comprises a housing 1 within which a cylinder 2 is defined. One end of the cylinder is open to the atmosphere through an appropriate port 3. A piston member 4 is provided which is axially movable within the cylinder, the piston being a substantially air-tight fit within the cylinder. The piston is biassed upwardly by means of a compression spring 5 which is located within the cylinder, the compression spring engaging the underside of the piston 4 and also engaging the lowermost part of the cylinder 2. A wire or fine chain is connected to the underside of the piston 4 and extends axially through the cylinder 2 to a pulley 7. The wire 6 passes round the pulley 7 and extends substantially horizontally to a drum 8.The drum 8 has a vertical axis and is mounted for rotation on a vertical shaft which terminates with a horizontal gear wheel 9. A relatively broad gearwheel 10 engages the periphery of the gearwheel 9 and also engages the periphery of the gearwheel 11 which is rotationally driven by a motor 12. A space 13 is provided within the housing to accommodate a battery. An upper part 14 of the housing carries a cover 1 5 that may be removed by means of a knob 1 6 to enable batteries to be inserted into the housing appropriately. A further knob 1 7 protrudes through the upper part 1 4 provided on the housing, the knob being mounted on the end of the shaft on which the gearwheel 10 is mounted.The knob 1 7 may be pulled, thus moving the gearwheel 10 axially, effectively disconnecting the gearwheels 9 and 11.

In operation of the device the motor 1 2 is actuated, causing the gearwheel 11 to rotate.

With the gearwheel 10 in the position illustrated, the gearwheel 10 is driven rotationally by the gearwheel 11, thus rotating the gearwheel 9 and causing the drum 8 to rotate.

The cord 6 is wound onto the drum 8 and the piston 4 is gradually pulled down the cylinder 2, drawing air into the cylinder through the inlet port 3. The spring 5 becomes compressed. As the spring becomes compressed more power is required from the motor 1 2 to move the piston downwardly. The piston thus tends to move more slowly, and consequently air is not evenly drawn into the cylinder. At the end of an operational cycle the port 3 is connected to an inlet port of a gas chromatograph and the knob 1 7 is pulled, thus disconnecting the gearwheels 9 and 11. The spring 5 then forces the piston 4 up the cylinder 2, simultaneously causing the wire or chain 6 to be unwound from the drum 8.The spring 5 may exert a considerable force on the piston 4, driving the piston up the cylinder rapidly, so that the piston 4, which is usually made of glass, may break.

Fig. 2 illustrates one embodiment of the invention in the form of a transmission that is incorporated in a pocket-sized aspirator. A battery powered motor is provided which drives a horizontal gearwheel 20. A force is transmitted from the gearwheel 20 to a piston 21 that is axially movable within a removable cylinder 22 by means which incorporate an elongate transmission element 23 in the form of a tightly coiled spiral of wire. One end 25 of the element 23, is fastened to a movable member 27. The movable member is siidably movable axially along a bar 26, and in the embodiment illustrated the member 27 has an aperture that accommodates the bar 26. The member 27 also has a screw threaded bore that accommodates an elongate externally threaded member 28 that extends parallel to the bar 26.The member 28 is rotatable about its longitudinal axis and is provided, at one end, with a gearwheel that meshingly engages the horizontal gearwheel 20 that has been mentioned above, to provide a drive to rotate the threaded member 28 about its axis. Alternative means, such as a driven cog belt, may be provided for rotating the externally screw threaded member 28.

It will be understood that, in the illustrated embodiment, when the electric motor is actuated the screw threaded member 28 is rotated and, depending upon the sense of rotation, the member 27 is driven upwardly or downwardly.

The elongate element 23 that interconnects the driven element 27 and the piston 21 follows a substantially "U" shaped path. Thus as the driven element 27 moves downwardly the piston moves upwardly and vice versa.

The elongate element 23 may pass through a guide 29 which may be a member defining a cavity or a tubular member. The interior surface of the cavity or the tubular may have a low coefficient of friction, and thus this surface may be coated with nylon or polytetrafluoroethylene as sold under the Registered Trade Mark "TEFLON". Alternatively the guide may be constituted by rollers 34, which again minimise any friction between the elongate member and the guide.

Fig. 3 illustrates a further type of guide which may be used as an alternative to or (as shown in Fig. 2) in conjunction with the guides described above. The guide illustrated in Fig. 3 constitutes a guiding rod (or tube) 35 which has a vertical portion extending parallel with the axis of the threaded member 38 (28 in Fig. 2). The lower end of the vertical portion merges into an arcuate portion that defines the path to be followed by the elongate element. The rod 35 extends through the hollow centre of the coiled elongate member 40 (23 in Fig. 2) thus guiding the elongate member.

The guide rod 35 may, at its upper end 36, be fastened to the solid casing of the aspirator or may, as shown in Fig. 2, not be fastened to the casing. In each embodiment a lever, such as the lever illustrated at 33, may be provided. The lower is arranged so that if it is lifted up and rotated a coupling (not shown) in the movable member 27 disengages the member 27 from the screw 28, enabling the member 27 with the attached elongate element 23 to be manually pushed down, whereupon the piston is driven up to its starting position at the top of the cylinder 22.

The piston 21 illustrated in Fig. 2 consists of a plunger member having an iron sheet 32 secured to its underside. The end of the elongate member 23 that is connected to the piston is constituted by a magnet 31, and thus there is a magnetic coupling between the end of the elongate member 23 and the piston 21. This magnetic coupling will be described below in greater detail.

In operation of the aspirator described in Figs. 2 and 3, initially the aspirator will be in a condition where the movable member 27 is in an elevated position and the magnetic member 31 is in a lowermost position. A cylinder 22 is then mounted in position with the open lower end of the cylinder aligned with the piston 21. The lever 33 is then operated to move the movable member 27 downwardly, thus forcing the piston 21 upwardly. The tightly coiled elongate member 23 has such characteristics that the member 23 does not tend to bow or buckle in the straight unguided portion that is located within the cylinder 22 during this operation.

Of course, during this particular operation any air within the space 24 is driven out of the cylinder through the nozzle 30. The electric motor is then actuated, and the movable member 27 moves upwardly, thus gradually drawing the piston 21 downwardly. The magnetic connection between the magnet 31 and the iron plate 32 is sufficient for this purpose.

Air is thus drawn into the cylinder through the nozzle 30. When the piston has reached its lowermost position the nozzle 30 is closed and the entire cylinder is removed. The piston will, during this operation, become disengaged from the magnet 31. The cylinder may then be taken to the gas chromatographic apparatus, and a fresh cylinder may be mounted in position on the apparatus. The air present in the removed cylinder will then be discharged into, and analysed by, the gas chromatographic apparatus. The operational cycle may then be repeated.

Fig. 4 illustrates an alternative mechanism for pulling down a piston 45. In this mechanism a drum 42 is provided which rotates about a horizontal axis 40. An elongate element 43 constituted by a tightly wound coil becomes wound up on the drum 42 as it rotates, thus drawing the piston 45 down wardly.

As the elongate element 43 is not rigid, it may tend to extend elastically if the force applied to the elongate member exceeds a certain limit. Thus, if for any reason the piston becomes jammed, the elongate member could become extended which is clearly undesirable.

Consequently, it is preferred that the coupling between the elongate member 23 and the member to be moved, i.e. the piston 21, is constructed in such a way that the connection between these two elements will break when the force applied to the connection exceeds a predetermined threshold value. Fig. 5 thus illustrates an arrangement in which an iron plate 32 is secured to the interior of the piston 21 whilst a magnetic member 31 is connected to the end of the tightly wound spiral elongate element 23. It is preferred, as can be seen in Fig. 5, that a contact spring 50 is arranged in a cavity within the magnet 31. The contact spring 50 is connected to an electric wire (not shown) and forms part of an electric circuit that includes the plate 32.

Thus, when the magnet is magnetically attached to the iron plate 32 the circuit is closed, by the contact spring 50. However, when the magnet is separated from the iron plate, the circuit is broken. When the circuit is broken an appropriate indication may be given by an audible or visual alarm device, and/or the electric power may be disconnected from the driving motor.

It is to be appreciated that in embodiments where a greater traction force is required then the spiral itself can provide, it may be practicable to complement the elongate spiral element with a pull wire, a gear belt or some similar means.

Claims

1. A mechanical transmission adapted to transfer power from a driving device to an object and adapted to drive the object in two opposed directions, the transmission including a transmission element consisting of an elongate tightly wound spiral element formed from a wire, said transmission element being adapted to follow a non-linear path.

2. A mechanical transmission according to claim 1, wherein the coil diameter and wire thickness of the elongate element are such that no internal or external guidance of the member is required in the straight lengths thereof to avoid sideways bending of the element when a compressive force is applied to move the object.

3. A mechanical transmission according to claim 1 or claim 2, wherein the spiral element has such a prestressing in the coiling and such a wire thickness that the spiral is not stretched out with the traction force that is normally applied to the elongate element to move the object.

4. A mechanical transmission according to any one of the preceeding claims, wherein said elongate element is guided by guide means provided either inside or outside the spiral where the element is to follow a curved path.

5. A mechanical transmission according to claim 4, wherein the guides are constituted by rollers, or by external tubular guide elements, or by an internal rod or tubular guide element.

6. A mechanical transmission according to any one of the preceeding claims, wherein a cord or wire is provided which is parallel to or inside the spiral, and fastened to one or both ends of the spiral.

7. A mechanical transmission according to any one of the preceeding claims, wherein the spiral is a double spiral.

8. A mechanical transmission according to any one of the preceeding claims, wherein one end of said elongate element is connected to a movable member, the movable member having a threaded portion adapted to engage the threaded exterior of an elongate threaded member, means being provided to rotate the elongate threaded member to move the movable member axially of the elongate member.

9. A mechanical transmission according to any one of claims 1 to 7, wherein said driving device consists of a motor driven drum around which the elongate element is arranged to be wound up, the arrangement being such that the object is moved whenever the elongate element is wound onto or unwound from the drum.

10. A mechanical transmission according to any one of the preceeding claims, wherein the elongate member is connected to the object by a threshold coupling.

11. An aspirator incorporating a mechanical transmission according to any one of the preceding claims, wherein the aspirator includes a cylinder having a piston axially movable therein, said piston constituting the said object.

1 2. An aspirator according to claim 11, wherein one end of the elongate element is connected to the piston by a threshold coupling.

1 3. An aspirator according to claim 12, wherein an electric contact function is provided by the threshold coupling which provides an output signal if the coupling is released.

1 4. An aspirator according to claim 10 or 11, wherein means are provided for releasing said piston from the driving device to enable the piston to move freely.

1 5. A mechanical transmission substantially as herein described with reference to and as shown in Fig. 2 of the accompanying drawings.

1 6. A mechanical transmission substantially as herein described with reference to and as shown in Fig. 3 of the accompanying drawings.

1 7. A mechanical transmission substantially as herein described with reference to and as shown in Fig. 4 of the acompanying drawings.

1 8. An aspirator incorporating a mechanical transmission substantially as herein described with reference to Fig. 2, 3 or 4 of the accompanying drawings.

1 9. A mechanical transmission as claimed in any one of claims 1 5 to 18, incorporating a magnetic coupling substantially as herein described with reference to and shown in Fig. 5 of the accompanying drawings.

20. Any novel feature or combination of features disclosed herewith.