GB2404703A - Slipping clutch with ceramic coating on bearing surface - Google Patents

Abstract

A slipping clutch assembly 25 for a printing apparatus, includes a first clutch member 28 which is attached to or integral with a moveable member 23 (eg a spindle of a spool), and a second clutch member 30, with there being a resilient biasing device 32 to bias the first and second clutch members 28, 30 together whereby when a force is applied on the moveable member 23 to tend to move the moveable member 23, the moveable member 23 is maintained stationary relative the second clutch member 30 where the force is below a threshold value, and the moveable member 23 is permitted to move relative to the second clutch member 30 where the force applied is above the threshold value, and characterised in that one of the first and second clutch members 28, 30 includes a bearing surface 36 through which the applied force is transmitted, which surface 36 is provided by coating of a ceramic material.

Description

PATENTS ACT 1977 A10900GB/DJL Title: Slipping Clutch Assembly Description of Invention This invention relates to a slipping clutch assembly and to an apparatus in which such a clutch assembly may be provided. The invention has particularly but not exclusively been developed for use in a printing apparatus which includes a spindle on which a spool is mounted, the spindle having wound upon it, ribbon which carries marking medium which is deposited on a substrate during a printing process. Thus during a printing operation, ribbon is pulled from the spool for use, either as a ribbon drive device such as a capstan roller is rotated, or a shuttle is moved, or as a ribbon take-up spool is rotated, in each case to move ribbon around a ribbon path of the apparatus, through a printing station where a print head acts to print the marking medium on the substrate. It is desirable to maintain a tension in the ribbon as it is pulled from the spool, and to prevent the spool from freely rotating thus permitting ribbon to be removed from the spool in an uncontrolled manner. It is known to provide a slipping clutch which only allows the spool to rotate and thus ribbon to be removed therefrom, when a ribbon pull force above a threshold force is applied to the ribbon. Such clutches include a first clutch member which is attached to or integral with the spindle, and a second clutch member, with there being a resilient biasing device to bias the first and second clutch members together whereby when a ribbon pull force is applied to tend to rotate the spindle, the spindle is maintained stationary relative the second clutch member where the force is below a threshold value, and the spindle is permitted rotationally to move relative to the second clutch member where the force applied is above the threshold value. The clutch members are metal plates, and in a typical arrangement, an oil impregnated felt/fibre washer is positioned between the first and second clutch members which allows the first clutch member to rotate when a sufficient ribbon pull force is applied. However the necessary minimum pull force to allow the clutch to slip and the first clutch member to rotate is highly dependent upon the quality and quantity of oil as well as the thickness of the felt/fibre washer. Generally when the washer is new, too much oil is impregnated into the felt washer with the result that as the clutch members are urged towards one another, oil is exuded from the felt washer, making a mess in the printing apparatus. Moreover, over time, the felt/fibre washer becomes dry and the clutch ceases to operate adequately. According to a first aspect of the invention we provide a slipping clutch assembly including a first clutch member which is attached to or integral with a moveable member, and a second clutch member, with there being a resilient biasing device to bias the first and second clutch members together whereby when a force is applied on the moveable member to tend to move the moveable member, the moveable member is maintained stationary relative the second clutch member where the force is below a threshold value, and the moveable member is permitted to move relative to the second clutch member where the force applied is above the threshold value, and characterised in that one of the first and second clutch members includes a bearing surface through which the applied force is transmitted, which surface is provided by coating of a ceramic material. Thus an improved clutch assembly is provided which does not require the use of an oil impregnated washer to permit of slippage between the first and second clutch members when a force exceeding the threshold value is applied to the moveable member. Moreover, because the clutch is always dry, the clutch is more durable than a slipping clutch employing the traditional felt washer. Moreover, in a conventional arrangement which utilises a felt/fibre washer, the washer tends to compress in use, which can lead to the threshold force required before slippage occurs between the first and second clutch members, varying in use as the distance between the first and second clutch members varies. In the assembly of the invention, the distance between the first and second clutch members is more consistently maintained, as a felt/fabric washer is not required. Preferably the ceramic coating is provided by ceramic particles, for example particles having a maximum dimension in the order of 1 Microm. Although the thickness of the coating is not crucial to the performance of the invention, preferably the coating has a thickness of at least 50Microm. The coating may be deposited on an underlying surface of the respective clutch member by for example only, an electrochemical micro-arc oxidation process such as that known as a microplasmic oxidation process. An example of such a process is described in US patent number 6197178. Thus the clutch member, or at least the underlying surface of the clutch member to which the coating is applied may conveniently be made of aluminium or magnesium, titanium or zirconium, or an alloy thereof. The bearing surface of the coated clutch member may be plane, or may be profiled having for examples only a plurality of radially extending grooves, and or a plurality of arcuate grooves which extend from the centre of the surface to a periphery thereof, and/or a plurality of discrete recesses. Desirably, between the first and second clutch members there is provided a washer made of a polymer bearing material which bears on the ceramic coating. The washer is preferably secured relative to the clutch member which is biased towards the clutch member with the ceramic coating. In one example, the ceramic coating is provided on the bearing surface of the second clutch member and the washer, where provided, may be secured relative to the first clutch member. The invention is particularly but not exclusively applicable where the first clutch member is moveable about a rotational axis relative to the second clutch member. Thus conveniently the resilient biasing of the biasing device may act axially of the assembly to bias the clutch members together. The resilient biasing device may thus be a spring, such as a coil spring which acts to pull the first and second clutch members together. However the invention is applicable to other slipping clutch arrangements where the first and second clutch members may relatively move other than rotationally, e.g. linearly. Where the moveable member is rotatable, the moveable member may be a spindle which in use carries a spool which has wound upon it, a supply of a flexible elongate material such as a ribbon or filament. Thus the force which is applied to tend to move the moveable member may be exerted on the moveable member as the wound material is pulled from the spool for use. Thus the clutch acts to maintain a tension in the wound material as is desirable where for example only, the material is a ribbon carrying marking medium which passes along a ribbon path including a printing station where a print head is operative to remove marking medium from the ribbon and deposit the marking medium onto a substrate in operation. Pixels of the marking medium may be melted by heating elements of the print head, whereby the pixels may be removed from the ribbon, the pixels solidifying when contacting the substrate to provide a marking. According to a second aspect of the invention we provide a printing apparatus including a spindle which in use carries a spool having wound thereon a ribbon carrying marking medium, and a slipping clutch assembly including a first clutch member which is attached to or integral with the spindle, and a second clutch member, with there being a resilient biasing device to bias the first and second clutch members together whereby when a force is applied on the spindle as ribbon is pulled from the spool which tends rotationally to move the spindle, the spindle is maintained stationary relative the second clutch member where the force is below a threshold value, and the spindle is permitted rotationally to move relative to the second clutch member where the force applied is above the threshold value,and characterised in that one of the first and second clutch members includes a bearing surface through which the applied force is transmitted, which surface is provided by coating of a ceramic material. The clutch of the apparatus of the second aspect of the invention may have any of the features of the clutch assembly of the first aspect of the invention. Embodiments of the invention will now be described with reference to the accompanying drawings in which:- FIGURE 1 is an illustrative side view of a clutch assembly in accordance with the invention; FIGURE 2 is an illustrative view of a printing apparatus which may include the clutch assembly of figure 1. FIGURES 3a to 3c are illustrative perspective views showing alternative surface configurations for one of the clutch members of the assembly. Referring to figure 2 of the drawings, a printing apparatus 10 includes a base 9 on which is mounted a ribbon supply spool 11 which has wound upon it, a supply of a carrier ribbon 12 which carries on it marking medium for use in printing operations as hereinafter described. The ribbon 12 is guided around a ribbon path by guide rollers 13, 14 to a ribbon take-up spool 16, through a printing station 18 where a print head 20 is provided. The printing apparatus 10 shown is a so called thermal printer in which the print head has a plurality of heating elements 21 arranged in an array, which are selectively energised by a controller, to melt pixels of the marking medium on the carrier ribbon 12, and deposit the pixels of marking medium on a substrate 22, in this example, a label conveyed to the printing station 18 on a backing web 23.The printing apparatus 10 may be of the kind in which in use the print head 20 is held stationary at the printing station 18 while the label 22 or other substrate moves past the print head 20 whilst the heating elements 21 are selectively energised, or of the kind in which the label 22 is held stationary at the printing station 18 whilst the print head 20 relatively moves along the label 22, e.g. in the direct of arrow A, whilst the heating elements 21 are selectively energised. However the invention may be applied to other printing apparatus types, for example where both the substrate 22 moves through the printing station 18 during printing and the print head 20 moves. In this example, movement of the ribbon 12 along the ribbon path is achieved by rotating the take-up spool 16 with a motor M which may be directly coupled to the spool 16 or connected to the spool 16 by a transmission. However the invention may be applied where another drive arrangement is provided to move the ribbon 12, such as for examples only, a capstan or "Rogers" roller, or a shuttle. In each case as the ribbon 12 is moved along the ribbon path, ribbon is pulled from the supply spool 11 which is mounted on a spindle 23, to rotate with the spindle 23, the spindle 23 being a moveable member of a slipping clutch assembly 25 in accordance with the first aspect of the invention. By virtue of the slipping clutch assembly 25, the ribbon 12 may not freely be pulled from the supply spool 11, as rotation of the supply spool 11 on the spindle 23 is resisted, at least until a threshold pull force is applied. In this way, a tension is maintained in the ribbon 12 around the ribbon path, as is required by the printing apparatus 10. Referring now to figure 1, the slipping clutch assembly 25 includes a first clutch member 28 which in this example is a clutch plate which is integral with, but may be attached to the moveable member provided by the spindle 23, and a second clutch member 30 which is fixed relative to the base 9 . The clutch members 28, 30 are urged together by a resilient biasing device 32 provided by a coil spring 31 which is wound about an elongate pin 33 which extends within the spindle 23 and mounts the spindle 23 to the base 9. The pin 33 around which the spring 31 is wound, extends through a floating stop 37 provided within the spindle 23, on which stop 37 one end of the spring 31 bears. The pin 33 end is threaded and receives a nut 39 on the opposite side of the stop 37 to the spring 31. The pin 33 extends through the first and second clutch members 28, 30 and at the other end is longitudinally fixed relative to the second clutch member 30, the end of the spring 31 remote from the stop 37 bearing on a blind end of a hollow H in the spindle 23 through which the spring 31 and pin 33 extend. Thus the spring 31 is compressed by an amount which may be adjusted by adjusting the position of the nut 39, to provide a biasing force to bias the first and second clutch members 28, 30 towards one another. The spring 31 acts along an axis of rotation B of the spindle 23 and first clutch member 28. Of course, any other suitable kind of resilient biasing device may be employed instead of the device 32 described. Between the clutch members 28, 30 in this example, there is provided a washer 35 which is made of a suitable polymeric bearing material. An example of a suitable polymeric bearing material is a thermoplastic alloy, for example as sold under the trade name "Iglide", which includes a fibre matrix and solid lubricants. The washer 35 is in this example secured relative to the first clutch member 28 thus to rotate with the first clutch member 28, by means of one or more securing pins 36. Alternatively the washer 35 may be adhered to the first clutch member 28, or otherwise secured to rotate with the first clutch member 28. In accordance with the invention, the second clutch member 30 has a bearing surface 36 which is urged towards the first clutch member 28, which bearing surface 36 is provided by a ceramic coating on the underlying second clutch member surface. The ceramic coating is provided by ceramic particles, preferably having a maximum size in the order of 1Microm, and which are deposited on the underlying second clutch member 30 surface, for example by an electrochemical micro-arc oxidation process such as that known as a microplasmic oxidation process Such a process is essentially involves applying a controlled high voltage AC power to the second clutch member 30 submerged in an electrolytic bath of proprietary composition including ceramic particles. Due to the high voltage and high current, intense plasma is created by microarcing at the surface of the second clutch member 30, this plasma oxidising the surface resulting in ceramic particles being deposited thereon. Such technology is applicable only for coating surfaces of certain metals; in the present example, the second clutch member 30 is made of aluminium or a suitable aluminium alloy which lends itself to the electrochemical micro-arc oxidation process. During the coating process, an outer soft coating is formed, which is required to be buffed off to leave an extremely hard, polished, ceramic coating, which may have a hardness of over 1300 kg/mm . Thus the polished ceramic coating is resistive to erosive wear, and has a low coefficient of friction which makes it an ideal material for a dry bearing surface 36 of the second clutch member 30. The surface of the ceramic coating may be plane, but preferably is profiled, as indicated by one or more of the examples in figures 3a to 3c. In figure 3a the ceramic bearing surface 3b is provided with a plurality of radially extending grooves G1; in figure 3b arcuate grooves G2 which extend from the centre of the surface 16 to the circumference are provided; in figure 3c the surface 16 is provided with a plurality of discrete recesses R, which may be through openings through the ceramic coating and also if desired, the underlying second bearing member 30. The thickness of the ceramic coating layer is not crucial to the invention, but may have a minimum thickness in the order of 50Microm. By adjusting the coil spring 31 compression, by adjusting the position of the nut 39 along the pin 33, the minimum or threshold force required to rotate the moveable member, i.e. the spindle 23 relative to the second clutch member 30 may be adjusted. Of course, the threshold force needs to be adjusted to a minimum required to maintain a slight tension in the ribbon 12 of the printing apparatus 10, or else, if the ribbon 12 is over-tensioned, this can lead to ribbon breakage where the ribbon is a thin lamina and not overly strong material. Various modifications not indicated above may be made without departing from the scope of the invention. For example, the polymeric washer 35 may not be required where the first clutch member 28 has a surface 40 to which the bearing surface 36 of the second clutch member 30 is urged, which is made of a suitable material and to a suitable surface finish appropriate to provide an acceptable bearing surface for bearing on the ceramic coated bearing surface 36 of the second clutch member 30. Where a washer 35 is provided, this need not be made of a polymeric material, but may be made from another "dry" material which does not suffer from the problems identified above relating to conventional oil impregnated felt washers. Moreover the washer 35 where provided, need not be fixed to the first clutch member 28, although if free to rotate, the surface 40 of the first clutch member 28 which is urged towards the second clutch member 30, may need to be an appropriately finished bearing surface so that relative rotation or other relative movement between the first and second clutch members 28, 30 does not occur when a force below the threshold force desired to maintain a tension in the ribbon 12 is applied to the spindle 23. In the example described, although the ceramic coating has been described as being applied to the underlying surface of the second clutch member 30, in another example, the ceramic coating may be applied to an underlying surface 40 of the first clutch member 28, in which case, the washer 35, where provided, may be fixed to the second clutch member 30 as desired. Although the invention has been described particularly in relation to a printing apparatus, the invention may be applied in other applications where a where a slipping clutch is required, for example where an elongate flexible ribbon, tape, or filament is wound on a spool carried by, or integral with a spindle or other moveable member which is attached to or integral with a one of first and second clutch members. For example the invention may be applied to a bagging or wrapping machine where bagging/wrapping material is pulled from a reel for packaging purposes. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims (22)

1. A slipping clutch assembly including a first clutch member which is attached to or integral with a moveable member, and a second clutch member, with there being a resilient biasing device to bias the first and second clutch members together whereby when a force is applied on the moveable member to tend to move the moveable member, the moveable member is maintained stationary relative the second clutch member where the force is below a threshold value, and the moveable member is permitted to move relative to the second clutch member where the force applied is above the threshold value, and characterised in that one of the first and second clutch members includes a bearing surface through which the applied force is transmitted, which surface is provided by coating of a ceramic material.

2. A clutch assembly according to claim 1 wherein the clutch is dry.

3. A clutch assembly according to claim 1 or claim 2 wherein the ceramic coating is provided by ceramic particles.

4. A clutch assembly according to claim 3 wherein the ceramic particles have a mesh size in the order of 1Microm.

5. A clutch assembly according to any one of the preceding claims wherein the coating has a thickness of at least 50Microm.

6. A clutch assembly according to any one of the preceding claims wherein the coating is deposited on an underlying surface of the respective clutch member by an electrochemical micro-arc oxidation process.

7. A clutch assembly according to claim 6 wherein the underlying surface of the clutch member to which the coating is applied is made of aluminium, magnesium, titanium or zirconium, or an alloy thereof.

8. A clutch assembly according to any one of the preceding claims in which the bearing surface is plane.

9. A clutch assembly according to any one of claims 1 to 7 where the bearing surface is profiled having one or more of a plurality of radially extending grooves, arcuate grooves which extend from the centre of the surface to a periphery thereof, and a plurality of discrete recesses.

10. A clutch assembly according to any one of the preceding claims wherein between the first and second clutch members there is provided a washer made of a polymer material which bears on the ceramic coating.

11. A clutch assembly according to claim 10 wherein the washer is secured relative to the clutch member which is biased towards the clutch member with the ceramic coating.

12. A clutch assembly according to any one of the preceding claims wherein the ceramic coating is provided on the bearing surface of the second clutch member.

13. A clutch assembly according to claim 12 where dependant upon any one of claims 8 and 9 wherein the washer is secured relative to the first clutch member.

14. A clutch assembly according to any one of the preceding claims wherein the first clutch member is moveable about a rotational axis relative to the second clutch member.

15. A clutch assembly according to claim 14 wherein the resilient biasing of the biasing device acts axially of the assembly to bias the clutch members together.

16. A clutch assembly according to claim 14 or claim 15 wherein the resilient biasing device is a spring which acts to pull the first and second clutch members together.

17. A clutch assembly according to claim 14 or claim 15 wherein the moveable member is a spindle which in use carries a spool which has wound upon it, a supply of a flexible elongate material, the force which is applied to tend to move the moveable member being exerted on the moveable member as the wound material is pulled from the spool for use.

18. A clutch assembly according to claim 17 wherein the material is a ribbon carrying marking medium which passes along a ribbon path including a printing station where a print head is operative to remove marking medium from the ribbon and deposit the marking medium onto a substrate in operation.

19. A clutch assembly substantially as hereinbefore described with reference to and/or as shown in the accompanying drawings.

20. A printing apparatus including a spindle which in use carries a spool having wound thereon a ribbon carrying marking medium, and a slipping clutch assembly including a first clutch member which is attached to or integral with the spindle, and a second clutch member, with there being a resilient biasing device to bias the first and second clutch members together whereby when a force is applied on the spindle as ribbon is pulled from the spool which tends rotationally to move the spindle, the spindle is maintained stationary relative the second clutch member where the force is below a threshold value, and the spindle is permitted rotationally to move relative to the second clutch member where the force applied is above the threshold value, and characterised in that one of the first and second clutch members includes a bearing surface through which the applied force is transmitted, which surface is provided by coating of a ceramic material.

21. An apparatus according to claim 20 wherein the clutch assembly has any of the features of the clutch of any one of claims 1 to 19.

22. Any novel feature or novel combination of features described herein and/or as shown in the accompanying drawings.