The present invention relates to a screw-thread arrangement, and to a conversion arrangement for mounting optical units on cameras, particularly though not exclusively on film or television cameras.

Accordingly, the present invention provides a screw-thread arrangement for enabling two co-axial members to be rotated relative to one another to move said members axially one relative to another, the arrangement comprising a screw-thread formed on a radial surface of a first of said members; a follower supported on a second of said members; and means biassing said follower into engagement with said screw-thread to cause said follower to follow said screw-thread on relative rotation of said members to move said members axially relative to one another.

The present invention is further described hereinafter by way of example, with reference to the accompanying drawings in which:

Figure 1 is a longitudinal section through a conversion arrangement for an optical unit for use with a camera, the arrangement incorporating a screw-thread arrangement according to one embodiment of the present invention, with certain parts omitted for clarity;

Figure 2 is an enlarged sectional view of a first portion of the

arrangement of Figure 1;

Figure 3 is an enlarged sectional view of a second portion of the arrangement of Figure 1; and

Figure 4 is a view in the direction of arrow A of Figure 3 with certain parts omitted for clarity.

Referring now to the drawing, this shows a section through a conversion arrangement 10 which is intended for use with various types of optical unit. The conversion arrangement enables the optical unit to be mounted on to a camera such as a film or television camera. The conversion arrangement is fitted within the camera body, which is to the left as seen in the drawing, whilst the optical unit is contained within the conversion arrangement, facing to the right.

The conversion arrangement 10 comprises a mounting eansor member A, a carrier on optical unit support B, a focusing gear C, and an iris drive gear D.

The mounting member A is dimensioned so as to fit on the front of a camera body and has a mounting adaptor 12 of suitable form and a mounting body 14. The adaptor 12 engages with the mounting arrangement on the camera to secure the conversion arrangement 10 in position. The mounting body also has an annular recess 18 which opens axially away from the camera body to receive a radially inwardly extending flange 20 of a focusing

ring 22 of the focusing gear. The flange 20 is formed at the camera body end of the focusing ring 22 and is retained in the recess 18 by the action of a thrust ring 24 and retaining ring

26, this being described further below.

The optical unit is contained in the support B and is secured by a locking ring 34. The optical unit support B is co-axially mounted within the mounting member A so as to be axially slidable relative thereto but is prevented from rotating relative to the mounting member A. The optical unit support B is located in the mounting body 14 by a support bearing 36 at the rear, left hand end of the conversion arrangement 10 as seen in the drawing, and by a support ring 38 at the front of the arrangement 10. Both rings 34, 38 are adjustable, the ring 38 being adjustable by means of three equi-angularly spaced screws (not shown) and the ring 36 being adjustable by means of a clamp ring 40. The latter has a radially outer screw-thread which engages a screw-thread formed on a radially inner surface of the mounting body 14, enabling the clamp ring 40 to be screwed axially of the mounting body. The support bearing 36 has a radially outer frusto-conical bearing surface which bears on a correspondingly shaped radially inner surface of the mounting body 14.

The focusing ring 22, during manufacture of the arrangement 10, is provided with a cylindrical internal bore 42. An internal screw-thread, or camtrack, is then provided by means of stainless steel wire 44 which is formed into a coil on the inner surface of the bore 42. The pitch of the thread is controlled by two end

shoe plates 46 and pitch shims 48 with the assembly being held in place by a clamp ring 49. The shoe plates are generally annular, each having a respective axially inwardly directed surface 50 which is machined or contoured to the desired pitch helix, together with an endstop to abut the wire ends. Each pitch shim 48 is conveniently in the form of a split ring which fits between turns of the wire to ensure a uniform spacing between turns. The wire 44 may conveniently be formed in 1, 2 or more lengths. The wire is conveniently made from stainless steel or other suitable resilient material and the coil of wire has a Bree or "rest" diameter which is slightly greater than that of the surface 42 so that the wire coil resiles radially outwardly against the focusing ring. The wire may form a single or multi-start thread.

An annular extension 28 of the optical unit support B supports two followers or movement segments 30 (Figure 3) which are generally 180 degrees apart. The arrangement of each movement segment 30 is the same and therefore only one of the movement segments is described below in detail. It will be appreciated that more than two such segments may be used.

The movement segment 30 is generally arcuate in length and has a groove 32 of generally U-section (e.g. part circular cross section) formed on its radially outer surface, the groove being dimensioned to receive part of the circumference of the wire 44 as can be seen in the drawing. As is shown in Figure 3 the groove 32 has two side walls 33 which taper away from one another

outwardly of the groove.

The inner diameter of each shim 48 and shoe plate 46 is arranged to be slightly less than the fitted centre line diameter of the wire 44. This is to ensure that the wire, shims, and shoe plates are correctly retained when the clamp ring 49 is tightened. The outer radial diameter of the movement segment 30 is slightly less than the inner diameters of the shims 48 and shoe plates 46. The movement segment 30 contacts the wire 44 on its side walls (conveniently machined at 30 degrees to the vertical centreline) , at contact points on the wire which are conveniently at -90 degrees to each other. The contacting side walls extend just beyond the points of contact with the wire 44. The groove 32 has substantially the same radius of curvature about the axis of the bore 42 as the wire 44 in order to provide a firm and positive engagement but, in a preferred form, the movement segment 30 contacts the wire only at or near its extreme ends.

The movement segment 30 is supported on the annular extension 28 of the optical unit support B in a suitable slot or opening 41 formed in the mounting body 14, preventing rotational movement of the movement segment about the axis of the arrangement when the focusing ring is rotated. During assembly of the arrangement the initial radial position of the movement segment 30 is set by one or more shim washers 52 which are secured in position in a groove or recess in the optical unit support B by means of one or more screws 54.

Further biasing of the movement segment 30 into positive engagement with the wire 44 is effected by a wire spring 56 which locates in facing grooves or slots 58, 60 in the radially inner surface of the movement segment 30 and in the annular extension

28. The spring bias which the spring 56 applies to the movement segment 30 is adjusted by means of a cam pin 74 which locates in an axial slot 76 of the extension 28. The slot 76 is generally cylindrical to accommodate a cylindrical shank 66 of the cam pin

74, the shank carrying an eccentrically mounted cam 68 at one end which acts on the spring 56.

As can be seen from Figure 3 the shank 66 has a slot provided at its opposite end by means of which the cam pin can be rotated to rotate the eccentrically mounted cam 68 and increase or decrease the biasing force of the spring 56 on the movement segment 30. Such adjustment may be performed during assembly of the arrangement or subsequently by a user since access to the cam pin is from the front of the arrangement and does not require removal of any parts other than a lens cap. As will be appreciated, the fine adjustment of the spring biasing is conveniently used to provide the desired torque of focusing movement, i.e. light or heavy or somewhere in between. The spring biasing also compensates for wear which may occur in the movement segment or wire 44.

As will be appreciated, because the optical unit support B is prevented from rotating relative to the mounting body 14, rotation of the focusing ring 22, causes the optical unit support

B to move axially of the camera body which is secured to the adaptor 12. The very positive engagement of the movement segments 30 with the focusing ring 22 eliminates any backlash in the relative movement of the focusing ring and the optical unit support B and therefore in focusing of the optical unit.

In a modification (not shown in the drawings) each movement segment 30 may be supported on a pivotal bearing allowing the segment to pivot laterally in the direction of the axis of the arrangement. The pivotal bearing may be in the form of a radial ^' pin secured to the annular extension 28 and extending into a- hole or bore in the centre of the movement segment 30. This allows the movement segment 30 freely to follow the wire 44 and also enables the pitch of the wire coils to be varied along the bore 42 of the focusing ring 22. This variation in pitch can be achieved by gradually increasing the thickness of the pitch shims 48. The movement segment 30 may conveniently be made of plastics material.

It will be appreciated that the movement segments can be used with normal, machined screw-threads on the inner surface of the focusing ring 22 which has the advantages of providing automatic wear compensation to avoid backlash.

The radially inner flange 20 of the focusing ring 22 is maintained in engagement with the mounting body 14 by the thrust ring 24 and retaining ring 26. The thrust ring is provided with a gap and has its surface facing the retaining ring 26 machined

at a preselected angle with the abutting surface of the retaining ring 26 machined at the same angle. This angle lies in the range substantially 55 degrees to 80 degrees and preferably 70 degrees to the axis of the focusing ring.

The thrust ring 24 is retained between the radial flange 20 of the focusing ring 22 and the retaining ring 26, the latter being screwed onto a cylindrical extension of the mounting body 14 which forms a wall of the recess 18. Rotation of the focusing ring 22 and "therefore the flange 20 causes the thrust ring to contract very slightly, the abutting surfaces of the thrust ring 24 and retaining ring 26 sliding relative to one another. This slight contraction of the thrust ring 24 allows ^' smooth rotation of the focusing ring 22 without introducing any axial play in the focusing ring 22 relative to the mounting body 14. However, once rotation ceases, the thrust ring 24 resiles into its rest position, thus providing a very positive location of the flange 20 and therefore of the optical unit support B whilst avoiding introducing any backlash between the focusing ring 22 and the mounting body 14 and therefore in focusing of the optical unit. To assist smooth rotation of the focusing ring 22, PTFE washers 78 may be inserted between the flange 20 and thrust ring 24 and between the flange 20 and the mounting body 14.

The position of the retaining ring 26 can also be adjusted to provide the desired torque of focusing movement, i.e. light or heavy or somewhere in between.

It will also be appreciated that the design and operation of the thrust ring 24 automatically provides compensation for any wear which might take place in the thrust ring itself.

Although the thrust ring 24 is preferably of resilient material, a circlip 25 may conveniently be provided in an annular groove in a radially inner surface of the thrust ring 24 in order to assist in biasing the latter into firm engagement with the flange 20 and retaining ring 26.

The radially inner surface of the flange 20 of the focusing ring 22 is machined to a close tolerance to bear against the surface of the mounting body 14 and prevent any radial play in the ring 22.

In an alternative embodiment, the thrust ring 24 is replaced by a plurality of arcuate segments which are held in a generally circular arrangement by a circlip. Each wedge may be provided with a groove in its surface abutting the retaining ring 26 which engages with a corresponding peg or projection on the retaining ring 26 to allow radial movement of each arcuate segment but to prevent any angular movement about the axis of the mechanism.

The focusing ring 22 is provided with gear teeth on a radially outer surface to enable the ring to be rotated by the camera focusing drive.

The iris drive gear D is independent of the focusing gear C. In

practice, the iris actuating mechanism is generally located towards the rear of an optical unit. Therefore, since the iris drive is positioned well into the body of the camera whilst the iris drive gear D is placed towards the front of the conversion arrangement 10, a remote connection is required between the two.

The iris drive gear D has an iris movement gear 74 which is provided on a radially outer surface with gear teeth to enable the gear to be rotatably driven by the camera iris drive. The gear 74 rotates on a closely machined diameter on the mounting body 14, axial displacement of the iris gear being controlled for example by a flange at the rear and a front reice ring 70 at the front.

A bracket 76 is secured at one end to the iris gear 74 whilst its other end engages in an axial slot machined in an iris carriage (not shown) of the arrangement.

Port holes 72 arranged in the mounting body 14 and optical unit support B allow access for the bracket 76 to operate the iris carriage and thus the iris in the optical unit, over the full working range of the optical unit.

Rotary movement of the iris gear 74 can therefore be transmitted to the iris of the optical unit via the bracket 76 and iris carriage without it being affected by any movement of the focusing ring 22 or movement of the optical unit support B.

It will be appreciated that although the screw-thread arrangement using the wire coil 44 is described above with reference to a camera lens conversion arrangement, it is not restricted to such use. In addition, the wire 44 may be of circular, square or any other suitable cross-section.

The wire 44 may also form a screw-thread in a conical, tapered or varying diameter bore or on a correspondingly shaped outer surface.

The biassing force of the follower biassing means bore spring may be adjusted by means of a tapered screw in place of the adjusting cam pin 74.

In place of the end stop means 46 and spacer means 48, a groove may be machined in the bore of the first member or focusing ring 22, in which the wire 44 sits.