WO1998012497A1

WO1998012497A1 - Bearing measurement system

Info

Publication number: WO1998012497A1
Application number: PCT/GB1997/002393
Authority: WO
Inventors: Peter Dean Onyon
Original assignee: Taylor Hobson Limited
Priority date: 1996-09-20
Filing date: 1997-09-05
Publication date: 1998-03-26
Also published as: GB2317453B; GB2317453A; EP0938642A1; GB9619704D0

Abstract

The invention relates to apparatuses and methods for making measurements of bearing irregularity and/or misalignment of a component assembly, of the type which has a body part rotatably mounted to a shaft via the bearing. The assembly to be measured (26) is mounted to a turntable (14) via a conventional chuck (16). Two sets of surface profile measurements are thereafter taken, the turntable being rotated throughout 360° during each measurement set. For the first set of measurements a flat faced bar (38) is attached to the spindle (1). The flat faced bar (38) is prevented from following the rotation of turntable (14), by stop (36) mounted on stop assembly (30, 32, 34). Measurements of the deviation of an end of the flat faced bar (38) are measured by a conventional stylus (22), mounted on stylus assembly (20, 18, 24). A second set of measurements is taken without flat faced bar (28). In this measurement set the stylus directly follows a datum face of the body, such as the top or bottom face (3c, 3b). From the above two measurement sets, a number of results relating to the flatness, squareness and parallelism of a face of the body relative to the rotating axis can be calculated, analysed and displayed.

Description

BEARING MEASUREMENT SYSTEM

This invention relates to apparatuses and methods for making measurements on a component assembly comprising a drum rotatably mounted to a shaft via a bearing. In particular, but not exclusively, this invention relates to the use of surface form or profile measuring instruments in measurements of a bearing of a video drum assembly.

Apparatus such as the Applicant's commercially available Talyrond 300 range of equipment is known for the measurement of the form or profile of a surface of a component. In such apparatus, a stylus carried at one end of a support arm is mounted so as to be pivotable about a pivot axis which allows the stylus to follow the profile of a surface of the component during relative movement between the stylus and the surface. A method of achieving suitable relative movement is by rotating the component. This can be achieved by securing it to a rotatable turntable. The displacement of the stylus as it follows the profile of the surface is measured and provides a measurement of relative surface profile, i.e. it shows the relative position of each point on the surface with respect to other points on the surface. The support arm which carries the stylus is preferably movable in a direction along the axis of rotation of the turntable so as to allow a plurality of sets of measurements to be taken at different heights from the turntable for each component . It is often desirable, for quality control reasons, to be able to obtain an indication of the accuracy of a bearing. One example of an assembly where this is important is a video drum assembly. A typical video drum assembly is shown in Figure 1. The assembly comprises a drum 3 having a central cylindrical portion or collar 3d from which extends a flange 3e carrying at its periphery a drum surface 3 ' . In such a component the collar 3d of the drum 3 is rotatably mounted on a shaft 1 via a shaft bearing (not shown in Figure 1). The drum surface 3' has top and bottom datum faces 3b and 3c. The shaft bearing is typically of a ball race type. A record head drum 5, carrying a record head 7 is mounted to the shaft 1 of the video drum assembly such that the record head drum 5 rotates with shaft 1. This allows rotation of the record head drum 5 and thus the record head 7 with respect to drum 3.

On the outer surface of drum 3 there is defined a helical guide path or lead line 3a which, in use of the video drum assembly, guides a video tape such that the tape passes the record head at the correct angle to achieve the desired tape scanning pattern. Inaccuracies in the bearing between the shaft 1 and the drum 3 may result in the record head 7 not following the required path across the tape and thus may detrimentally affect the quality of both recording and subsequent playback of the tape .

One current practised approach to measuring the accuracy of video drum bearings is to use a lead line form measuring instrument. In such an approach two releasable clamps are carried by X-Y movements mounted externally to the rotatable turntable. A dummy shaft is then clamped between these two clamps. Two gauges are mounted to the turntable at different heights above it. By manipulation of the X-Y movement of the clamps and by monitoring the gauges, the axis of the dummy shaft may be aligned with the rotational axis or spindle of the turntable. Without changing their position, the clamps are thereafter released and the dummy shaft removed and replaced by a video drum assembly to be measured. The shaft of the video drum assembly is then clamped in position by the two clamps . This ensures that the shaft is aligned with the turntable axis .

The form of the lead line of the drum is then measured by causing the stylus of the form measuring instrument to follow the lead line in a conventional manner. During this measurement a pin which engages with the video drum is attached to the turntable in such a manner as to cause the rotational movement of the turntable to be transmitted to the video drum. Thus, relative movement of the drum with respect to the stylus is carried out by rotation of the turntable as the pin causes corresponding rotation of the drum.

The measurements obtained indicate the path which will be followed in practice by the tape and are indicative not only of the form of the lead line but additionally of any errors of the bearings between the shaft and the drum.

A problem with the above described method is that it is not, however, possible to isolate the different sources of error. As an example, errors in the form of the lead line cannot be isolated from the bearing errors. Furthermore, considerable skill is required of an operator in the manipulation and alignment of the two clamps with the dummy shaft and the replacing the dummy shaft with the shaft of the video drum assembly.

A method according to the invention involves taking two sets of measurement readings, only one set of which includes the errors introduced by the bearing.

In one aspect, the present invention provides a method of making measurements on a component assembly having a body rotatable about a shaft wherein the form and/or movement of a surface of the assembly is sensed when there is no or a first degree of relative rotation between the body and the shaft and the form and/or movement of a surface, possibly the same surface, is sensed when there is relative rotation or a second degree of relative rotation between the shaft and the body.

In another aspect, the present invention provides an apparatus for making measurements on a component assembly having a body rotatable about a shaft, which apparatus comprises means for sensing the form and/or movement of a surface of the assembly when there is no or a first degree of relative rotation between the body and the shaft and means for sensing the form and/or movement of a surface, possibly the same surface, when there is relative rotation or a second degree of relative rotation between the shaft and the body.

One embodiment of the invention provides a method of measuring bearing irregularity in a bearing assembly, said assembly comprising a body rotatable around a shaft, the method comprising the steps of: providing a surface form measuring instrument of the type in which the assembly to be measured is positioned on a rotatable turntable and a stylus is arranged so as to give an indication of a surface feature around said assembly; clamping one of said body or said shaft of said assembly onto said turntable; making a first measurement of a surface during which no relative movement of the drum to the shaft occurs; making a second measurement indicative of surface movement during which rotation of the measured part of the assembly is prevented and the other part rotates.

In an embodiment yet further bearing alignment measurements can be provided by calculating the difference between said first and second measurement.

In a further embodiment said step of making a second measurement of roundness during which rotation of one part of the assembly is prevented and the other rotates comprises an initial step of moving a stop into contact with the part of the assembly not clamped to the turntable.

Embodiments of the invention will now been described by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a simplified diagram of a video drum assembly and associated record head drum;

Figure 2 is a diagrammatic simplified side view of apparatus according to a first embodiment of the present invention;

Figure 3 is a block diagram illustrating processing means of an embodiment of apparatus according to the present invention;

Figure 4 is a diagrammatic perspective view of part of the apparatus shown in Figure 2 together with the video drum assembly of Figure 1, for explaining a first embodiment of a method according to the present invention;

Figure 5 is a diagrammatic perspective view of part of an alternative apparatus to that shown in Figure 4 together with the video drum assembly of Figure 1, also suitable for use in a first embodiment of a method according to the present invention;

Figure 6 is a diagrammatic perspective view of a part of alternative apparatus to that shown in Figure 2;

Figures 7a, 7b and 7c are schematic diagrams for explaining the first embodiment of a method according to the present invention;

Figures 8a, 8b and 8c are schematic diagrams for explaining a second embodiment of a method according to the present invention;

Figure 9 is a schematic diagram for explaining a third embodiment of a method according to the present invention; Figure 10 is a schematic diagram illustrating a modified version of the third embodiment of the present invention; and

Figure 11 shows sample output representing the flatness of the drum datum face produced by an apparatus or method according to the present invention;

Figure 12 shows sample output displaying parallelism of the rotating axis relative to the drum datum face produced by an apparatus or method according to the present invention; Figure 13 shows sample output displaying the flatness of the rotating axis relative to the drum datum face, the output being produced by an apparatus or method according to the present invention.

It should, of course, be understood that the drawings are not to scale and that like parts are referred to by like reference numerals.

Referring now to the drawings, Figure 2 shows very schematically a metrological instrument for the measurement of surface form or profile. Mounted upon a workbench or surface 12 is a turntable 14 for supporting the component 26 to be measured. In the figure the workbench 12 lies in an r-θ plane, the origin of which lies on the axis of rotation of the turntable 14. The turntable 14 is preferably supported on a high accuracy air bearing spindle (not shown) as described in published patent application GB 2178805 the whole contents of which are incorporated herein by reference. Also mounted on the workbench 12 is a support column 24 which carries a carriage 18 movable along the support column 24 in a Z direction. A support arm assembly 20 is mounted to the carriage 18. A free end of the support arm assembly 20 carries stylus 22 having a tip 22a which is arranged to contact a surface of the component 26. Typically, the stylus tip 22a will have a radius in the range 1mm to 4mm. When the component 26 is rotated on the turntable 14 the stylus tip 22a follows the surface and movement of the stylus 22 is measured, for example, by using an inductive means such as a LVDT (linear variable differential transducer) or by use of an interferometric system.

The metrological instrument as described thus far is based on, for example, the Talyrond 300 manufactured by the Applicant.

The component 26 to be measured may be a video drum assembly such as that shown in Figure 1. This is positioned on the turntable 14 and either the shaft 1 or, as in the arrangement shown, the drum collar 3d is fixed securely to the turntable 14 by means such as a chuck, clamp or vice 16 mounted at the spindle axis by means of, for example, a well known screwthread mounting (not shown) . Alternatively the clamp may be an integral part of the turntable_* The rotational axis (Z axis) of the component is aligned with the rotational axis of the turntable using appropriate known techniques, for example, as discussed in published patent application EP 0240150, the whole contents of which are herein incorporated by reference. Although initial alignment of the axis of the component 26 and the turntable 14 is preferable, it is possible to compensate for inexact alignment during processing of the measurement data.

The stylus 22, support arm assembly 20 and carriage 18 are preferably adapted such that the stylus 22 may be used for taking measurements of the body of the component or may be moved up or down so as to take measurements of the top and/or bottom surfaces of the component. In arranging the apparatus such that measurements can be taken at the top and/or bottom surfaces it may be necessary to reverse the orientation of the stylus 22 and support arm assembly 20 in relation to the carriage 18. Alternatively, a dual directional biased stylus such as provided in the Talymin 4 stylus set widely commercially available from the Applicant can be used.

The apparatus embodying the invention shown in Figure 2 is further provided with a second support column 30 also provided with a carriage 32 moveable in the Z direction. The carriage 32 carries a support arm assembly 34 which carries stop 36 which cooperates with a bar 38 mounted in use of the apparatus to the shaft 1 for reasons which will be explained later. It will be appreciated that as an alternative to the above a single support column could carry both stylus 22 and stop 36.

Figure 3 illustrates processing means suitable for use in apparatus according to the present invention. The operation of the instrument is controlled by processor 50, having read only memory (ROM) 52 and random access memory (RAM) 54 and a fixed or removable data storage device such as a hard disk or rotatable optical disk drive 55. A series of instructions for automatic control of the measuring instrument may be stored in ROM 52 although alternatively these instructions could be stored in a part of the RAM 54 and loaded thereto from the fixed or removable storage medium 55. The use of processor 50 may also allow processing to be conducted in order to correct any inexact alignment of the axis of the shaft with respect to the axis of turntable rotation, for example in the manner described in published patent application GB 2294327. The RAM 54 or disc drive 55 may be used for storage of measurements made by the instrument. Also provided are a visual display unit (VDU) 56 to facilitate user interface and to allow display of the results; keyboard 58 and pointing device 60 to allow user input and printer 68. Input to the processor 50 is provided by three transducers 62, 64, 66 connected to the carriage 18, stylus 22 and turntable 14. The combination of signals provided by these transducers provides the necessary indication of the stylus position. Additional transducers (not shown) may also be provided to monitor the position of the stop 30.

A first method of making measurements on a video drum assembly using the apparatus shown in Figure 2 will now be explained with reference to Figures 4, 5, 6, 7a and 7b, which show the video drum assembly 26 and part of the apparatus on enlarged scale.

Figure 4 shows the video drum assembly to be measured and its association with the flat faced bar 38 and stop 36. As discussed above, the flat faced bar 38 is attached to the shaft 1. The attachment must be such that the flat faced bar 38 does not rotate with respect to the shaft 1. The flat face bar 38 preferably extends both sides of the shaft in a direction perpendicular to the shaft so that the shaft 1 is not unbalanced. One end of the flat face- bar 38 is formed with thinned section defining a surface 40 for receiving the stylus tip 22a (not shown in Figure 4) of the measuring instrument. This stylus receiving surface 40 is preferably a distance from the shaft axis approximately equal to the distance of the drum face 3c from the shaft axis. The other end of the flat faced bar 42 is formed plane or otherwise adapted such that rotation can be prevented by means of the stop 36 as will be explained later.

In the arrangement shown in Figure 4, the flat face bar is attached by a grub screw (not shown) although the attachment could be made by any suitable means .

An alternative method of attachment will be now be described with reference to Figure 5. In the arrangement shown in Figure 5, the flat face bar 38 is formed with a recess 38a towards its centre which receives the shaft 1. There is furthermore a location spring 41 secured to the flat face bar which frictionally engages the shaft 1 in order to retain the shaft in the recess. The location spring 41 is preferably formed on its inner side (not visible in the Figure) with a semi-resilient pad to facilitate centring and to avoid metal-on-metal contact. The flat face bar 38 is also formed with a cap plate 39 (shown dotted in Figure 5) such that the bar 38 is suitably located on the shaft 1 above the shaft housing portion 3d of the drum 3. A ball 35 (shown dotted) may additionally be provided to enable said location.

In Figure 5 an alternative arrangement of the stop is used compared to that shown in Figure 4. In this alternative arrangement the flat face bar 38 is provided with a radially extending part or stop pin 33 at the stylus receiving end. The stop pin 33 engages with stop bar 36 rather than there being direct engagement between the stop bar 36 and the unformed end of the flat faced bar 42.

Advantageous.!,y this enables the stop bar 36 and the stylus 22 to be mounted to the same carriage. An example of such a carriage arrangement is shown in Figure 6. The apparatus is similar to that of figure 2 except that a single support column is provided. In the arrangement shown in Figure 6, however, the position of the stop 36 is manually adjustable, so as to properly engage with stop pin 33, by adjustment of knob 31 which allows movement of the carriage 34 which holds the stop bar 36. It will also be noted that the rotation of the drum in the arrangement shown in Figures 5 and 6, because of the location of stop bar 36, is in the opposite direction to that shown in Figure 4 , the rotational direction being inessential to performance of the invention. It will furthermore be appreciated that the flat face bar arrangement of Figure 5 could alternatively be used in the dual support column arrangement previously described with reference to Figure 2. Figures 7a to 7c are schematic cross-sections on an enlarged scale of a part of the apparatus showing the video drum assembly mounted on the turntable 14 and with the bar 38 fixed in place during various stages of a method embodying the invention. In figures 7a to 7c, bearings 10 of the bearing assembly between the shaft 1 and the drum collar 3d can be seen.

As discussed above, the drum 3 is secured to the turntable 14 by the clamp or chuck 16. The clamp 16 is fixed to the turntable 14 and may either be a separate or an integral part of the turntable 14. The fixture arrangement is such, however, that movement of the shaft 1 is not prevented.

As shown schematically in Figure 7a, after the component 26 has been mounted as described above, the stop 36 is lowered or otherwise moved into engagement with the plane end 42 of the flat face bar 38. Such engagement prevents the rotation of the flat faced bar 38 and thus the shaft 1 when the turntable 14 and thus the drum 3 is rotated. The stylus tip 22a is then brought into contact with the stylus receiving portion 40 of the flat face bar 38 in conventional manner in a direction as shown by arrow A. The turntable 14 is then revolved through one revolution during which the stylus tip 22a follows the displacements of the surface 40. The displacement of the stylus 22 is measured by the stylus transducer 64 and together with readings from the carriage transducer 62 and turntable transducer 66 is logged by the processing means during the revolution so that a first set of measurements of the displacement of the stylus 22 is obtained.

As the flat face bar 38 is stopped, it will be appreciated that for this first set of measurements the drum 3 rotates whilst the shaft 1, the flat faced bar 38 and the stylus 22 remain in a constant angular position, but due to the bearings may move radially and/or axially relative to the drum 3 as the drum 3 rotates relative to the shaft 1. A set of measurement which include the errors introduced by the bearings is thus produced.

A second set of measurements is thereafter taken in a manner which will be described with reference to Figure 7b. As shown, the stop 36 is preferably removed from contact with the flat face bar 38. The position of the stylus 22 is adjusted by means of the carriage 18 so that the stylus tip 22a contacts drum datum face 3b as shown by the arrow B. As will be appreciated by those skilled in the art, this may require manual repositioning of the stylus to enable it to contact the drum datum face 3b. Alternatively, it is possible to use a bidirectionally biased stylus such as the Talymin 4. A second set of readings or measurements of the displacement of the stylus 22 as it follows the datum face 3b are logged by the processing means for one revolution of the turntable 14 and thus of the drum datum face 3b. It will be appreciated that in this measurement it is not relevant whether the shaft 1 rotates because the drum 3 and thus the drum datum face 3b is driven directly by the turntable 14. This second set of readings indicates the form or shape of the datum face 3b of the drum 3a and the alignment of the drum 3 with respect to the spindle axis, but will not include the errors associated with the drum bearings .

It will of course be appreciated that the two measurements could be performed in the reverse order.

The results thus obtained are a first set of measurements which include the errors associated with the bearings and a second set of datum measurements, being those taken directly from the drum. The processor is arranged to determine the difference between the two set to provide an indication of the errors in the bearing, being that which it is desired to measure. The first and second sets of measurements and, optionally, the difference values are stored by the processing means, for example in the disc drive 55 and may, if required, be displayed by the VDU 56 and/or printed out by printer 68 and/or transferred via a remote link to another processing means or storage device.

It is preferable to take the second set of readings with reference to drum datum face 3b as the typical shape of a video drum assembly means that good contact can be made between this edge and the tip of the stylus 22a. Also this avoids having to move the bar 38 after the first set of measurements.

However, an alternative method of taking the second set of measurements in the first embodiment will now be described with reference to Figure 7c. In this method the second set of readings are taken so that the stylus follows the upper_* surface 3c of the drum rather than the drum datum surface 3b as shown by arrow B'. In Figure 7c the flat faced bar 38 has been removed to prevent interference with the stylus. However, this is not essential and instead the bar could merely be rotated so that it does not interfere with contact between the stylus 22 and the upper face of the drum which is being used as the drum datum face 3c. This alternative method reduces the distance through which the stylus 22 has to be moved by the carriage 18 between the two sets of measurements and so lessens any inaccuracies resulting from this movement.

From the two readings A and B or A and B ' , a number of relevant measurements can be calculated and displayed.

Figure 11 shows a typical display of the flatness of the drum datum face 3b or 3c, that is to say the second set of measurements. The circular drum is shown projected as an oval, angles around the oval corresponding to angles around the drum. The flatness of each point around the drum is represented by the length of the vertical lines. The flatness can be directly calculated from the displacement of the stylus 22 during the second measurement set described above, being that in which the stylus 22 is in direct contact with the drum datum face. Also displayed by the apparatus is certain additional statistical information. This includes the radius (R), the runout (in this case 31.2μm) and the flatness (in this case O.lOμm). The flatness value is essentially the minimum distance between two planes, both of which are parallel to a best fit plane, the two planes totally enclosing the measured profile data. The best fit plane is preferably calculated as the plane in which the least square errors of the profile data is at a minimum, although other ways in which the best fit plane could be calculated will be apparent to those^, in the art .

The flatness angle (in this case 288.1 degrees) relates to the direction of the maximum "uphill" gradient of the best fit plane and is the angle θ subtended by that gradient direction on the r-θ plane of the turntable 14 relative to a reference zero. The gradient is measured with respect to a plane perpendicular to the Z axis which in this case is the plane in which the spindle axis lies.

The display also shows the squareness of the drum datum face (in this case 31.15 μ ) and the squareness angle. The squareness value relates to the minimum distance between two planes which are perpendicular to a defined reference axis (in this case the spindle axis) and which totally enclose the measured profile data. It will be appreciated that this value differs from the flatness value in that the two parallel planes enclosing the measured profile data must lie perpendicular to a defined axis, rather than being related to a best fit plane of the profile values.

The squareness angle (shown below the squareness value) is the angle subtended on the turntable r-θ plane, relative to the reference zero, of the direction of the maximum "uphill" gradient of the measured data best fit plane and is measured relative to a defined reference axis. As, in this case, the defined reference axis is the spindle axis, the squareness angle is the same as the flatness angle. Figure 12 shows a typical display of the perpendicularity or squareness of the rotating axis relative to the drum datum face. The display is arranged in the same manner as that of Figure 11 above. The set of measurements taken when relative movement between the drum and the shaft occurs, are shown displayed in this figure with reference to (i.e. by subtracting) the position of the calculated best fit plane of the drum datum face. Additionally, in this display, a parallelism measurement (in this case 11.40μm) is displayed. Parallelism is a similar (but not identical) measurement to squareness and relates to the minimum distance between two planes which are parallel to a defined reference plane and which totally enclose the measured profile values. Thus the difference between parallelism and squareness is that parallelism is calculated with reference to a reference plane whereas squareness is calculated with reference to a reference axis . Additionally, the parallelism angle (shown below the parallelism value) of 50.1° is displayed which is the angle subtended on the turntable r-θ plane of the direction of the maximum "uphill" gradient of the measured data best fit plane relative to the defined reference plane. As already stated, in Figure 12, the defined reference plane is the best fit plane of the drum datum face. Figure 13 shows a typical display of the deviations from a best fit flatness plane of the rotating axis. The figure also shows additional statistical information relative to the drum datum face. The set of measurements taken when relative movement between the drum and the shaft occurs are shown displayed in this figure and any lack of parallelism or squareness is ignored. In other words, the display is only concerned with the measurements set where relative movement occurs and relates, therefore, to bearing irregularities only. The statistical information, is the same as that shown in Figure 12 and is calculated in the same manner.

It will be apparent to one skilled in the art that in both Figures 12 and 13 the analysis and hence the display have been performed with respect to a datum face rather than a datum axis. However, the results could have been performed with respect to a datum axis in which case the statistical information would be in terms of squareness rather than parallelism. Other displays are possible using either or both sets of measurements, together with additional measurements, for example, the roundness of the drum. Furthermore, by adaption of the plane of movement of the stylus, it is possible to measure and display the accuracy of the bearings in the radial rather than the axial direction. Yet further in excess of one revolution could be performed by the turntable which may allow a measurement of the precession of the bearings to be made, analysed and displayed. It may also be possible to effect rotation at different speeds to see the effect of the speed of rotation on the bearings .

A second embodiment of a method according to the present invention will now be described with reference to Figures 8a and 8b. In this embodiment although measurements similar to the first embodiment are taken the arrangement of the video drum assembly during measurement differs in that the clamp or vice 16 is adapted so as to allow the mounting of the drum 3 in an upside-down position in comparison with the first embodiment. In the same manner as the first embodiment, a flat faced bar 38 is attached to the shaft 1 in such manner that no significant relative movement between the two occurs .

A first set of measurements is then taken as shown in Figure 8a with the flat face bar 38 stopped by the stop 36 and the stylus tip 22a pointing in the direction indicated by the arrow A to follow movement of the surface 40 caused by the bearing as the drum rotates relative to the shaft. Again in this first set of measurements the drum 3 will rotate with the turntable 14 and the shaft 1 will remain stationary.

To obtain information regarding the form of the drum datum face, a second set of readings is thereafter taken with, as shown in Figure 8b, the stylus 22 arranged to take readings in the direction indicated by the arrow B, i.e. with the stylus tip 22a in contact with the datum edge of the drum 3c.

As an alternative, it is possible, as shown by Figure 8c, to take the second set of readings in the same direction as that of the first set as shown by arrow B' in Figure 8c, ie . with the stylus tip 22a in contact with the drum datum surface 3b. Again, although the flat faced bar 38 is shown removed in this figure, for clarity, it will be appreciated that this is merely for clarity in the diagram and it need not be removed but only rotated so as to prevent it interfering with the stylus 22. As described above, this alternative lessens the distance through which the stylus needs to be moved between the first and second set of measurements. The measurements are thereafter processed and displayed in a similar manner to that described in the first embodiment.

Figure 9 shows a view similar to Figure 7a of an alternative arrangement in which the clamp 16 is adapted to hold the shaft 1 and not the drum collar 3d. In this manner, the flat faced bar used in the first and second embodiments is unnecessary.

In this embodiment, the stylus tip 22a is positioned so as to take measurements in the direction of the arrow C in the figure so that the stylus tip 22a is in contact with drum datum face 3b. During the first set of readings, a pin is employed to contact a hole which is conventionally formed in the flange of the drum 3e so as to prevent drum rotation when the turntable 14 rotates. This pin 40 may be, as shown schematically in Figure 9, mounted to the arm 34 in place of the stop 36. This will thus result in relative movement between the drum 3 and the shaft 1 upon the shaft 1 being rotated by the turntable 14 of the measuring instrument. After the first set of readings has been completed, the stop 36 is removed. A second set of measurements is thereafter taken in which, due to the friction of the bearings and the slow speed at which the turntable 14 is rotated, the drum 3 rotates along with the shaft 1. An advantage of this embodiment is that it is possible to use a single stylus position for both sets of measurements .

As an alternative in the third embodiment the stylus tip 22a can be positioned on the upper face 3c of the drum 3.

It is also possible, in this embodiment, to introduce means (for example, a clamp clamping the shaft to the drum collar) , for use during the second set of readings, adapted so as to prevent any relative movement between the shaft 1 and the drum 3. Such means may only be necessary if the friction of the bearings is insufficient to prevent any relative movement between the two parts during the second set of readings when the pin 40 is removed. Figure 10 shows a modified version of the method illustrated by Figure 9. In this alternative method the shaft 1 is clamped in such a manner that the drum 3 is in the upside-down position when compared to that shown in Figure 9. The stylus 22 is positioned in the position shown by the arrow D, so that the stylus tip 22a is in contact with drum datum face 3b. It will be appreciated, however, that as a further alternative, the stylus could be positioned on the underside of the drum against datum face 3c. In both cases, readings are taken in the same manner as that described above in relation to the third embodiment .

Other modifications or variations will be apparent to those skilled in the art.

Claims

1. An apparatus for making measurements of a component assembly having a body rotatably mounted to a shaft via a bearing, the apparatus comprising: sensing means for sensing a surface of the component assembly in order to determine information representative of the form of said surface; means for effecting relative rotation between a surface of the component assembly and the sensing means so that the sensing means follows the surface to provide a first set of measurements; means for enabling rotation of only one of said shaft and said body relative to the sensing means so that the sensing means follows a surface of the other of said shaft and said body to provide a second set of measurements; and processing means for using the first and second set of measurements to determine information about the bearing.

2. An apparatus according to claim 1 wherein said rotation enabling means comprises means for preventing rotation of the other of the shaft and body of the component assembly.

3. An apparatus according to either of claims 1 or 2 wherein: said rotation effecting means comprise a turntable including clamping means for clamping said shaft of the component assembly to the turntable for rotation therewith.

4. An apparatus according to any preceding claim wherein said rotation effecting means further comprise a pin mountable between the body and the shaft and adapted to prevent any relative rotation therebetween.

5. An apparatus according to claim 2 wherein: said rotation effecting means comprises a turntable including clamping means for clamping said body of the component assembly so as to prevent any relative rotation therebetween; and said rotation enabling means comprises a member mountable to the shaft so as to extend transversely to the axis of the shaft and so as to be fixed for rotation with the shaft and means for preventing rotation of the member .

6. An apparatus according to claim 5, wherein a surface of the member is shaped to define a sensing portion, the portion being adapted to be sensed by the sensing means.

7. An apparatus according to either of claims 5 or 6 , wherein the member is adapted to extend transversely of the shaft so that the sensing surface of the member is positioned in use adjacent to the periphery of the body.

8. An apparatus according to any preceding claim wherein said sensing means comprise a stylus adapted to contact the surface to be measured and means for determining the deflection of the stylus.

9. An apparatus according to any one of the preceding claims adapted to make measurements on a component assembly in the form of a video drum assembly.

10. A method of making measurements of a component assembly having a body rotatably mounted to a shaft via a bearing, the method comprising the steps of: fixing one of said body and said shaft to a support rotatable relative to surface sensing means; making a first set of measurements by using said sensing means to follow a surface of the assembly while rotating the support relative to the sensing means; and making a second set of measurements by using said sensing means to follow a surface of the other of said body and shaft while both rotating the support relative to the sensing means and preventing rotation of the other of said body and shaft of the assembly.

11. A method of measuring bearing irregularity and/or mis-alignment in a component having a body rotatably mounted to a shaft via a bearing, the method comprising the steps of: fixing said body to a turntable rotatable relative to sensing means; attaching a- bar to said shaft so that said bar extends transversely of the shaft axis and is secured to move with the shaft, the bar having portion adapted to allow a measurement to be taken by the sensing means; making a first set of measurements with said sensing following a surface of the body while said body is rotated by the turntable relative to the sensing means; and making a second set of measurements with said sensing means following movement of the measuring portion of the bar while the body of the assembly is rotated by the turntable and rotation of the bar is prevented.

12. A method of measuring bearing irregularity and/or mis-alignment in a component assembly having a body rotatable around a shaft via a bearing, the method comprising the steps of: fixing said shaft onto a turntable rotatable relative to a sensing means; making a first set of measurements with said sensing means following a surface of the body while the assembly is rotated relative to said sensing means; and making a second set of measurements with said sensing means following a surface of the body while the turntable is rotating such as to rotate the shaft of the assembly but rotation of the body is prevented.

13. A method of measuring bearing irregularity and/or mis-alignment in a component assembly having a body rotatable around a shaft via a bearing, the method comprising the steps of: positioning the assembly on a turntable rotatable relative to a stylus to enable a surface of the component to be moved past the stylus; making a first measurement of a surface using the stylus during which the turntable is rotated and movement of the surface relative to the stylus includes movement of the shaft relative to the body; and making a second measurement of the movement of a surface using the stylus during which the turntable is rotated and movement of the surface relative to the stylus does not include movement of the shaft relative to the body.

14. A method according to any one of claims 10 to 13 further comprising a step of determining the difference between said first and second measurements.

15. A method according to any one of claims 10, 12, 13 or 14 wherein said surface used for said first measurement is the same as said surface used for said second measurement.

16. A method according to any one of claims 10 to 15 which comprises making the measurement using a video drum assembly as the component assembly.

17. An apparatus substantially as hereinbefore described with reference to or as shown in Figures 2 to 8 of the accompanying drawings .

18. A computer usable medium storing computer readable instructions for causing a processor in a computer controlled metrological instrument to cause measurements to be made by said instrument of a component assembly having a body rotatably mounted to a shaft via a bearing, the instructions comprising instructions for: causing, when one of said body and said shaft is fixed to a support rotatable relative to surface sensing means, a first set of measurements to be made as said sensing means follows a surface of the assembly while the support is rotated relative to the sensing means; and a second set of measurements to be made as said sensing means follows a surface of the other of said body and shaft while the support rotates relative to the sensing means and rotation of the other of said body and shaft of the assembly is prevented.