Fastening device for sensors

Abstract

A sensor assembly 10 comprises a sensing device (12 in figure 1) and a fastening device 14. The fastening device comprises a shank 18 and a nut assembly 22 located around the shank. The nut assembly c

Description

This invention relates to fastening devices. The invention relates especially to fastening devices for fastening sensors to machines or other objects. BACKGROUND TO THE INVENTION Some sensors need to be mechanically fastened to an object during use. In particular, it is known to mechanically fasten vibration sensors to a machine in order to monitor vibrations, e.g. for condition monitoring or diagnostic purposes. The transmission of vibrations from the machine into the sensor is important for sensing high frequency vibrations. Layers of components on the transmission path can lead to vibration isolation, unwanted damping, and potentially additional resonances, all of which are undesirable. Screwmounting the sensor provides good transfer of mechanical vibration to the sensing device in comparison with other mounting means such as magnetic clamping. In some applications, the orientation of the sensor relative to the machinery or equipment being monitored is also important. For example, vibration sensors may be configured to sense vibrations along one or more axis, and it is important to know how each axis is aligned with respect to the machine in order to correctly analyse the measured vibrations. It is desirable to mount the sensor on the machine such that the, or each, sensing axis is correctly aligned with respect to the machine. If the, or each, sensing axis is not correctly aligned, then the sensor’s measurements will be inaccurate unless corrected by post-measurement processing. Such correction can be complicated and so it is desirable to align the sensor correctly before measurements are taken. Mounting studs have been used for vibration sensors for many years. However, early sensors had uni-axial sensing elements (z-axis) and their orientation on the stud was not important. With more modern tri-axial accelerometers, the final clamped orientation is critical and conventional studmounting methods would not guarantee alignment of the other axes. Even in cases where the bottom surface of the sensor is glued to the machine so that the orientation can be pre-set prior to curing of the epoxy, the sensor orientation will still be torque dependant (i.e. different torque settings or multiple removal / replacements, or part to part differences can cause the tightened end position to be different every time). In some applications, there is a requirement to have mounting means that is compact to fit on industry standard mounting points, for example or 025mm (or 1”) diameter. Furthermore, the size of the mounting means affects the distance between the machine and the sensing device, which should be minimized since the distance has a detrimental effect on the vibration transmission. The proximity of the sensing device to the machine will also determine the thermal transfer performance. Too large a separation between the machine and the sensing device creates a thermal lag that reduces the sensor’s ability to accurately determine changes in temperature, and may ultimately prevent the sensor from detecting faults. It would be desirable to provide a fastening device for sensors that mitigates the problems outlined above. SUMMARY OF THE INVENTION From a first aspect the invention provides a sensor assembly comprising a sensing device and a fastening device for fastening the sensing device to an object, wherein the fastening device comprises: a shank having a first external screw thread; and a nut assembly located around said shank, wherein said shank has a first end connected to, or connectable to, said sensing device and a second end connectable to, or connected to, said object, the nut assembly being located between said first and second ends, and wherein the nut assembly comprises: an inner nut having a second external screw thread; and an outer nut located around said inner nut and having a second internal screw thread that matches and is in engagement with the second external screw thread of said inner nut, wherein a first internal screw thread is provided on one or other of said inner nut and outer nut, said first internal screw thread matching and being in engagement with the first external screw thread of said shank, and wherein said first internal and external screw threads have a handedness that is opposite to a handedness of said second internal and external screw threads. From a second aspect the invention provides a method of fastening a sensor assembly of the first aspect to an object, the method comprising: rotating said outer nut in a first rotational direction to cause said nut assembly to move along said shank towards one of said object or said sensing device until said inner nut or said outer nut engages with said object or sensing device, or rotating said outer nut in a first rotational direction to cause said outer nut and said inner nut to move with respect to each other until said outer nut or said inner nut engages with one of said object or said sensing device; further rotating said outer nut in said first rotational direction to cause said outer nut to move along said inner nut, or said inner nut to move towards said outer nut, towards the other of said object or said sensing device until said outer nut or said inner nut, engages with the other of said object or said sensing device, or further rotating said outer nut in said first rotational direction to cause said inner nut to move with respect to the outer nut, or said outer nut to move with respect to said inner nut, towards the other of said object or said sensing device until said inner nut or said outer nut engages with the other of said object or said sensing device. From a third aspect the invention provides a fastening device comprising: a shank having a first external screw thread; and a nut assembly located around said shank, wherein said shank has a first end for connection to a first object and a second end for connection to a second object, the nut assembly being located between said first and second ends, and wherein the nut assembly comprises: an inner nut having a second external screw thread; and an outer nut located around said inner nut and having a second internal screw thread that matches and is in engagement with the second external screw thread of said inner nut, wherein a first internal screw thread (30) is provided on one or other of said inner nut and outer nut, said first internal screw thread (30) matching and being in engagement with the first external screw thread of said shank, and wherein said first internal and external screw threads have a handedness that is opposite to a handedness of said second internal and external screw threads. From a fourth aspect the invention provides a method of fastening a first object to a second object using a fastening device according to the third aspect, the method comprising: connecting the first end of the shank to said first object; connecting the second end of said shank to said second object; rotating said outer nut in a first rotational direction to cause said nut assembly to move along said shank towards one of said first object or said second object until said inner nut or said outer nut engages with said first object or second object, or rotating said outer nut in a first rotational direction to cause said outer nut and said inner nut to move with respect to each other until said outer nut or said inner nut engages with one of said first object or said second object; further rotating said outer nut in said first rotational direction to cause said outer nut to move along said inner nut, or said inner nut to move towards said outer nut, towards the other of said first object or second object until said outer nut or said inner nut, engages with said other of the first object or second object, or further rotating said outer nut in said first rotational direction to cause said inner nut to move with respect to the outer nut, or said outer nut to move with respect to said inner nut, towards the other of said first object or said second object until said inner nut or said outer nut engages with the other of said first object or said second object. From a fifth aspect the invention provides a method of tightening the fastening device of a sensor assembly of the first aspect or a fastening device according to the third aspect, the method comprising: rotating the outer nut in a first rotational direction causing the nut assembly to move along the shank in a first direction and causing the inner and outer nuts to move away from each other and into engagement with a respective one of the objects being fastened by the fastening device. From a sixth aspect the invention provides a method of untightening the fastening device of a sensor assembly of the first aspect, or the fastening device according to the second aspect, the method comprising; rotating the outer nut in a second rotational direction causing the nut assembly to move along the shank in a second direction and causing the inner and outer nuts to move towards each other. Typically, said first internal and external screw threads are right-handed and said second internal and external screw threads are left-handed. Alternatively, said first internal and external screw threads may be left-handed and said second internal and external screw threads may be right-handed. In some embodiments, the first end of said shank is releasably connected to said sensing device. The first end may have an external screw thread, conveniently said first external screw thread, and said sensing device comprises a socket for receiving said first end, said socket having a matching screw thread. Typically, said sensing device comprises a base, said first end of said shank being connected to said base, and wherein, preferably, said base is formed from metal. The socket may be formed in said base. Typically, said sensing device has at least one sensing axis. At least one of said at least one axis typically extends perpendicularly to a longitudinal axis of said shank. Said at least one axis optionally comprises first and second mutually perpendicular axes, each of which is perpendicular to the longitudinal axis of said shank. Typically, said at least one axis comprises an axis that is parallel or coincident with the longitudinal axis of said shank. In preferred embodiments, said sensing device comprises at least one vibration sensor. Typically, said at least one vibration sensor is configured to sense vibrations along at least one axis. Preferably, said sensing device has a body and at least one alignment aid is provided on said body, wherein, preferably, the, or each, alignment aid comprises a marking, formation or other visual and / or tactile aid. Optionally, said at least one alignment aid is indicative of a direction of said at least one axis. Typically, the shank is, or is part of, a bolt or screw, for example a grub screw. The shank may be at least partly, preferably wholly, formed from metal. The shank is typically cylindrical or substantially cylindrical. Optionally, said sensing device comprises at least one temperature sensor. In typical embodiments, said inner nut, outer nut and shank are co-axial with each other. In preferred embodiments, said second end of said shank is releasably connectable to said object. The second end of the shank may have an external screw thread, conveniently said first external 5 screw thread. Alternatively, said second end of the shank is fixed to or integrally formed with said object. Preferably, said inner nut has an outer end that is level with or projects beyond a corresponding outer end of said outer nut, and wherein, preferably, said outer ends face away from the sensing device. Preferably, said first internal screw thread is provided on said inner nut. Optionally, the first end of the shank is fixed to or integrally formed with said sensing device. In preferred embodiments, the sensing device comprises a tri-axial vibration sensor, and may be used for industrial condition monitoring. In preferred embodiments, the fastening device provides mechanical connection between the sensing device and the machine or other object to which it is fastened. Advantageously, the fastening device is configured to allow the sensing device’s orientation to be aligned to an external reference or axis, after which the fastening device can be tightened, preferably with a simple twisting action, without upsetting the alignment of the sensor. Preferably, the main threaded screw continuously extends into the machine (or other object) and into the sensing device to provide a solid and continuous engagement. In preferred embodiments, the same twisting action that causes the fastening device to tighten causes the inner and outer nuts of the nut assembly to move apart to apply a tensioning force on the main threaded screw. This separation of the inner and outer nuts is caused by having a reverse thread on the inner and outer nuts that causes the inner and outer nuts to move apart axially. Advantageously, the fastening device applies a clamping force in both the direction of the machine and also in the direction of the sensor to provide a tensioning axial force on the main threaded screw. This provides a dual clamping force from a single twisting input. In preferred embodiments, bi-directional motion of the inner and outer nuts is achieved by rotating the outer nut to move the nut assembly towards the machine / object. During the rotation, the inner nut engages with the object, and continued rotation of the outer nut in the same direction causes the inner and outer nuts to separate as a result of the reverse thread, causing the outer nut to move towards and into engagement with the sensor. This dual direction movement causes an axial tensioning on the screw. Preferred embodiments of the invention offer some or all of the following advantages: • Provides continuous mechanical connection by the main screw, or shank, between the mounting surface of the machine, or other object, and the sensor to improve the transmission of vibrations; • Allows for sensor orientation adjustment before clamping for aligning axes of measurement; • Allows for a simple and intuitive one-directional twisting (rotational) motion to tighten the fastening device; • A compact design suitable for connecting to, for example, a 25mm diameter mounting point and also for thermal proximity. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which: Figure 1 is a perspective view of a sensor assembly embodying one aspect of the invention and including a fastening device embodying another aspect of the invention, the sensor assembly being shown fastened to an object; Figure 2 is a cut-away perspective view of the sensor assembly of Figure 1; Figure 3 is a sectioned side view of the sensor assembly of Figure 1 with the fastening device shown in an untightened state; Figure 4 is a sectioned side view of the sensor assembly of Figure 1 with the fastening device shown in an intermediate state; Figure 5 is a sectioned side view of the sensor assembly of Figure 1 with the fastening device shown in a tightened state; Figure 6 is a plan view of the sensor assembly of Figure 1; and Figure 7 is a perspective view of the sensor assembly of Figure 1 with Its housing removed. DETAILED DESCRIPTION OF THE DRAWINGS Referring now to the drawings there is shown, generally indicated as 10, a sensor assembly embodying one aspect of the invention. The sensor assembly 10 comprises a sensing device 12 and a fastening device 14 for fastening the sensing device 12 to an object 16. The object 16 may be part of a machine, an apparatus or a component, or more generally any object to which it is desired to fasten the sensor assembly 10. In preferred embodiments, the sensing device 12 comprises at least one sensor 15 (only one sensor 15 is shown In Figure 2 for the purpose of illustration), preferably at least one vibration sensor. The, or each, vibration sensor 15 may be of any conventional type, for example comprising one or more accelerometer, piezoelectric sensor or MEMs vibration sensing device. The sensing device 12 may be of a type that senses vibrations along a single axis. Preferably, however, the sensing device 12 is configured to sense vibrations along more than one axis, in particular more than one mutually perpendicular axis. It is particularly preferred that the sensing device 12 is configured to sense vibrations along three mutually perpendicular axis (i.e. the X, Y and Z axes), which is commonly referred to as tri-axial vibration sensing and such sensors are commonly referred to as tri-axial vibration sensors or tri-axial accelerometers. This may be achieved in any conventional manner, for example using a respective vibration sensor oriented to sense vibrations along a respective axis, or using a single vibration sensor that is capable of sensing vibrations along multiple axes. In embodiments in which the sensing device 12 is configured to monitor, or measure, vibrations, the object 16 may for example be, or be part of, a motor, a pump, fan, engine, gearbox, compressor, robot, sieve, screen, vehicle, transformer, electrical equipment or any machine, device or apparatus that vibrates or rotates, or has a component that vibrates or rotates. More generally the object may be anything to which it is desired or required to mount the sensing device 12. In alternative embodiments, the sensing device 12 may comprise any other type(s) of sensor or measurement device, especially sensors that require alignment, e.g. a motion detector, magnetometer, gyroscope, inertia sensor, optical sensor or camera, the alignment typically being with respect to one or more sensing axis. Alternatively still, the fastening device 14 may be used with devices other than sensing devices, especially those that require alignment, for example a directional antenna or directional transmitter or receiver, wherein the alignment may be in respect of one or more axis, e.g. a line of sight, or with any other object (especially those that require alignment) that requires fastening to another object. It will be understood that, depending on the embodiment, the object 16 may be anything to which it is desired to fasten the relevant device or other object to. In typical embodiments, the sensing device 12 comprises a body 40 which includes a base 42. The body 40 may comprise a housing 44 in which the, or each, sensor is located. Typically, the sensor(s) 15 are fixed to, or fixed with respect to, the base 42. For example, with reference to Figure 7 which shows the sensing device 12 with the housing 44 removed, the sensor(s) 15 may be provided on a substrate 17 (e.g. a printed circuit board), and the substrate 17 may be fixed to the base 42. The housing 44 may contain other components (not shown) of the sensing device 12, e.g. a battery, a wireless transmitter, a wireless receiver, and / or a processor, as applicable, and / or may contain a temperature sensor 19, depending on the embodiment, any of which may be provided on the substrate 17 as is convenient. Preferably, the vibration sensor(s) 15 and / or the temperature sensor 19, as applicable, are located at or adjacent the base 42 to facilitate transmission of vibrations or heat, as applicable, to the sensors 15, 19. The base 42 is preferably solid and may be made of metal or other thermally conductive material. In the example of Figure 7, a lower part of a face of the substrate 17 is fixed to a face 43 of the base 42. This may be achieved by forming a recess or cutout portion in the base 42 in order to define the face 43. The sensor(s) 15 and or the temperature sensor 19 are advantageously provided at or adjacent the lower part of the substrate 17, preferably such that they overlap at least party with the base 42 in the vertical direction. Typically, the reverse face of the substrate 17 is fixed to the base 42, while the sensors 15, 19, as applicable, are provided 8 on the obverse face. A support structure 45 is typically provided for supporting the substrate 17 with respect to the base 42 and / or the housing 44. The fastening device 14 comprises a shank 18 having an external screw thread 20, and a nut assembly 22 located around the shank 18. The shank 18 has a first end 24 that is connected to, or connectable to, the sensing device 12, and a second end 26 for connection to the object 16. The nut assembly 22 is located between the first and second ends 24, 26. In preferred embodiments, the shank 18 is a bolt or screw, for example a grub screw, or is part of a bolt or screw. Typically, the shank 18 is at least partly, preferably wholly, formed from metal (e.g. steel) or other thermally conductive material. Preferably, the shank 18 is cylindrical or substantially cylindrical in shape, although alternatively it may be tapered. In preferred embodiments, the first end 24 of the shank 18 is releasably connectable to the sensing device 12, preferably to the base 42. The base 42 may be shaped to define a socket 25 for receiving the first end 24. The first end 24 may have an external screw thread, which is conveniently part of the screw thread 20, although a separate screw thread could alternatively be provided at the first end 24. The socket 25 may be correspondingly screw-threaded for releasable connection with the first end 24. Alternatively, the shank 18 and sensing device 12 may comprise any other conventional means for releasably connecting with each other. Alternatively still, the shank 18, in particular the first end 24, may be permanently fixed to, or integrally formed with, the sensing device 12, preferably with or to the base 42. In preferred embodiments, the second end 26 of the shank 18 is releasably connectable to the object 16. The object 16 may be shaped to define a socket 27 for receiving the second end 26. The second end 26 may have an external screw thread, which is conveniently part of the screw thread 20, although a separate screw thread could alternatively be provided at the second end 26. The socket 27 may be correspondingly screw-threaded for releasable connection with the second end 26. Alternatively, the shank 18 and object 16 may comprise any other conventional means for releasably connecting with each other. Alternatively still, the shank 18, in particular the second end 26, may be permanently fixed to, or integrally formed with, the object 16. The nut assembly 22 comprises an inner nut 28 and an outer nut 34 located around the inner nut 28. The inner nut 28 has an internal screw thread 30 that matches the external screw thread 20 of the shank 18 to allow screw-threaded engagement between the inner nut 28 and shank 18 such that rotation of the inner nut 28 with respect to the shank 18 about the longitudinal axis of the shank 18 causes the inner nut 28 to move axially along the shank 18 (the direction of the axial movement depending on the direction of the rotation). The inner nut 28 also has an external screw thread 32. The outer nut 34 has an internal screw thread 36 that matches the external screw thread 32 of the inner nut 28 to allow screw-threaded engagement between the outer nut 34 and the inner nut 28. When mated, the screw threads 36, 32 cause relative rotation of the inner and outer nuts 28, 34 about their central axis to be translated into relative movement of the nuts 28, 34 along their central axis. In particular, rotation of the outer nut 34 about its central axis with respect to the inner nut 28 causes the outer nut 34 to move axially with respect to the inner nut 28, or otherwise causes relative axial movement between the inner nut 28 and outer nut 34. The direction of the axial movement depends on the direction of the rotation of the outer nut 34. The axial movement of the outer nut 34 with respect to the inner nut 28 is along the central axis of the nuts 28, 34, which coincides with the central axis of the screw threads 32, 36. In preferred embodiments, the inner nut 28, outer nut 34 and shank 18 are coaxially arranged with each other such that axial movement of the inner nut 28 with respect to the shank 18, and axial movement of the outer nut 34 with respect to the inner nut 28 are in the same axial direction. The nuts 28, 34 may be formed from metal or any other suitable material, e.g. nylon or other suitable plastics. Matching internal and external screw threads can be described as being male and female, wherein assembling a male-threaded fastener to a matching female-threaded one is called mating. Typically, external threads are considered to be male while internal threads are considered to be female. When the nut assembly 22 is located around the shank 18, the internal screw thread 30 of the inner nut 28 mates with the external screw thread 20 of the shank 18. As such the nut assembly 22 can move axially with respect to the shank 18 upon rotation of the nut assembly 22. The internal screw thread 36 of the outer nut 34 mates with the external screw thread 32 of the inner nut 28 to allow relative axial movement between the inner and outer nuts 28, 34. The helix of a screw thread can turn in either one of two directions, clockwise or anti-clockwise, when viewed along the central axis of the helix. The direction of the screw thread is commonly referred to as its handedness. Screw threads are commonly described as being right-handed or left-handed depending on the direction of the screw thread. Conventionally, screw threads having a direction that, when viewed along the central axis, causes the threaded item to move away from a notional viewer when rotated in a clockwise direction and to move towards the viewer when rotated in an anticlockwise direction are said to be right-handed, while screw threads having the opposition direction (and causing the opposite movement) are said to be left-handed. The internal screw thread 30 of the inner nut 28 and the external screw thread 20 of the shank 18 have a handedness that is opposite to the handedness of the internal screw thread 36 of the outer nut 34 and the external screw thread 32 of the inner nut 28. In preferred embodiments, the internal screw thread 30 of the inner nut 28 and the external screw thread 20 of the shank are right-handed, while the internal screw thread 36 of the outer nut 34 and the external screw thread 32 of the inner nut 28 are left-handed. Alternatively, the internal screw thread 30 of the inner nut 28 and the external screw thread 20 of the shank are left-handed, while the internal screw thread 36 of the outer nut 34 and the external screw thread 32 of the inner nut 28 are right-handed. In response to the outer nut 34 being rotated in a given direction, the opposite handedness of the respective matching screw threads causes the inner nut 28 to move in one axial direction with respect to the shank 18, and causes the outer nut 34 to move in the opposite axial direction with respect to the inner nut 28. These relative movements may occur simultaneously, but typically, one or other of these relative movements occurs at a time, i.e. in response to the outer nut 34 being rotated in a given direction, either the inner nut 28 moves in one axial direction with respect to the shank 18, or the outer nut 34 moves in the opposite axial direction with respect to the inner nut 28. Which of these relative movements occurs at a given time may depend on any one or more of a number of factors depending on the embodiment, including for example the respective friction (e.g. static friction) between the respective matching screw threads, the relative torque applied at the respective matching screw threads and / or on whether or not one or other of the relative movements is prevented (e.g. by engagement with the object 16 or sensing device 12). For example, in typical embodiments when the outer nut 34 is rotated, the torque applied at the screw threads 20, 30 may be greater than the torque applied at the screw threads 32, 36 and so, from the state shown in Figure 3, the nut assembly 22 tends to move along the shank 18 first. This arrangement may be facilitated by ensuring that the frictional force (e.g. the static frictional force) between the screw threads 32, 36 is greater than or equal to the frictional force between the screw threads 20, 30. Optionally, the configuration is such that the friction between the outer nut 34 and inner nut 28 (i.e. between the respective screw threads 36, 32) is greater than the friction between the inner nut 28 and the shank 18 (i.e. between the respective screw threads 30, 20). This may be achieved in any conventional manner, e.g. by appropriate configuration of the respective screw threads such as selection of the respective thread angle or pitch angle of the screw threads, and / or by selection of the material from which the respective screw threads are made. It will be understood that the order in which the relative movements takes place is not critical, and the fastening device 14 may be configured to cause the relative movements to take place in either order or simultaneously. In typical embodiments, the configuration is such that, if neither of the inner or outer nuts 28, 34 is obstructed, then both nuts 28, 34 move together along the shank 18 (at least during tightening), but if either nut 28, 34 is obstructed, then relative movement of the nuts 28, 34 occurs. The configuration of the respective mating screw threads 30, 20; 36, 32 (e.g. in terms of thread angle, pitch angle and other relevant parameters) may be determined empirically or by calculation to achieve the desired operation of the fastening device 14. In preferred embodiments, the pitch of the threads 36, 32 is larger than the pitch of the threads 30, 20. This has the effect of increasing the rate at which the nuts 28, 34 move relative to each other in comparison with the rate at which the inner nut 28 moves with respect to the shank 18. It may also create higher friction between the inner and outer nuts in comparison with the friction between the inner nut and the shank 18. In preferred embodiments, the nut assembly 22 is configurable between a contracted state (Figure 3) and an extended state (Figure 5). In the contracted state, the inner and outer nuts 28, 34 are relatively contracted with respect to each other in the axial direction in comparison with the extended state in which they are relatively extended with respect to each other in the axial direction. In particular, the inner and outer nuts 28, 34 overlap more in the axial direction in the contracted state than they do in the extended state. The mated screw threads 36, 32 allow the outer and inner nuts 34, 28 to move towards or away from each other along their central axis in response to relative rotation about their central axis. The nut assembly 22 is configurable from the contracted state to the extended state by relative rotation of the inner and outer nuts 28, 34 such that the inner and outer nuts 28, 34 move axially away from each other, while configuration from the extended state to the contracted state is effected by relative rotation of the inner and outer nuts 28, 34 such that the inner and outer nuts 28, 34 move axially towards each other. Preferably, the inner and outer nuts 28, 34 are each shaped to define mutually abutting surfaces 46, 48 to limit the movement of the inner and outer nuts 28, 34 towards each other. In preferred embodiments, the surfaces 46, 48 abut with each other when the nut assembly 22 is in the contracted state. The surfaces 46, 48 may be provided by any suitable formation(s) provided on the respective nuts 28, 34, typically respective formations that overlap radially such that they can engage depending on the relative axial position of the inner and outer nut 28, 34. For example, the abutment surface 46 of the inner nut 28 may be provided on an annular flange 50 that may for example be located at a leading end 52 of the inner nut 28. The abutment surface 48 may be provided around the aperture of the outer nut 34. In preferred embodiments, the leading, or outer, end 52 of the inner nut, i.e. the end that faces outwardly of the nut assembly 22, is level with, or projects beyond, a corresponding leading, or outer, end 56 of the outer nut 34, including when the nut assembly 22 is in the contracted state. In preferred embodiments, the ends 52, 56 face away from the sensing device 12 when the nut assembly 22 is mounted on the shank 18. Operation of the preferred embodiment is now described with particular reference to Figures 3 to 5. In Figure 3, the first end of the shank 18 is connected to the sensing device 12 and the second end 26 is connected to the object 16. The nut assembly 22 is located around the shank 18 between the object 16 and the sensing device 12. In Figure 3, the nut assembly 22 is in an untightened state in which it does not axially tension the shank 18. Accordingly, a user (not shown) is able to rotate the sensing device 12 about the longitudinal axis of the shank 18 in order to establish a desired alignment of the sensing device 12. In preferred embodiments, this rotation of the sensing device 12 is facilitated by the screw threaded connection between the sensing device 12 and the shank 18, and / or by the screw threaded connection between the shank 18 and the object 16. In alternative embodiments there may not be a screw-threaded connection between the shank 18 and the sensing device 12 (e.g. if the shank 18 is permanently fixed to or integrally formed with the sensing device 12), in which case the rotation of the sensing device 12 may be facilitated by the screw-threaded connection between the shank 18 and the object 16. Alternatively still there may not be a screw-threaded connection between the shank 18 and object 16 (e.g. if the shank 18 is permanently fixed to or integrally formed with the object 16), in which case the rotation of the sensing device 12 may be facilitated by the screw-threaded connection between the shank 18 and the sensing device 12. Alternatively, either one or both of the sensing device 12 and the object 16 may be connected to the shank 18 by connecting means that allows rotation of the sensing device 12. As shown in Figure 3, in the untightened state, the nut assembly 22 is preferably spaced apart from both the sensing device 12 and the object 16. In preferred embodiments, the configuration is such that rotation of the outer nut 34 in the first rotational direction to tighten the fastening device 14 causes the inner and outer nuts 28, 34 initially to move together along the shaft 18 until there is an obstruction, after which continued rotation of the outer nut 34 causes relative movement of the nuts 28, 34 to take place. Figure 4 shows the nut assembly 22 in an intermediate state in which the tightening, or tensioning, process has begun. From the untightened state of Figure 3, the user rotates the outer nut 34 to cause the nut assembly 22, i.e. both the inner and outer nuts 28, 34, to move axially along the shank 18 towards and into engagement with the object 16. The inner nut 28 (in particular its leading end 52) engages with the object 16, which prevents further axial movement of the inner nut 28 along the shank 18. Further rotation of the outer nut 34 in the same rotational direction causes the outer nut 34 to move axially with respect to the inner nut 28 in the opposite direction, i.e. away from the object 18, by means of the opposite, or reverse, handedness of the screw threads 36, 32 with respect to the screw threads 20, 30. As shown in Figure 5, the further rotation of the outer nut 34 is continued until the outer nut 24 engages with the sensing device 12. After this engagement, further rotation of the outer nut 24 in the same rotational direction serves to tighten the fastening device 14 whereby the outer nut 34 exerts a clamping force (upwardly as viewed in Figure 5) on the sensing device 12, and the inner nut 28 exerts an opposite clamping force (downwardly as viewed in Figure 5) on the object 16 thereby tensioning the shank 18 and fastening the sensing device 12 to the object 16. There is no need for the shank 18 to rotate during the tightening process and so the desired alignment of the sensing device 12 can be maintained. Advantageously, the fastening device 14, in particular the nut assembly 22, allows the user to hold the sensing device 12 in the desired alignment with one hand while tightening the nut assembly 22 with the other hand. The sensing device 12 preferably includes one or more alignment aid 60 (Figure 6) to facilitate alignment of the sensing device 12. The, or each, alignment aid 60 may comprise one or more marking, formation or other visual and / or tactile item, which may conveniently be provided on the body of the sensing device 12. By way of example Figure 6 shows first and second alignment aids 60A, 60B in the form of markings that indicate the direction of a respective sensing axis (the X and Y axes in this example) of the sensing device 12. The markings 60A, 60B are provided on the top end of the body 40 in this example but could alternatively be provided in any other convenient location. In embodiments where the sensing device 12 comprises a tri-axial vibration sensor, the alignment aids 60A, 60B may be aligned with, or otherwise mapped with respect to, the X and Y axis of the vibration 13 sensor(s) 15 respectively, or may be aligned with a respective discrete axis of the sensor(s) internally. Typically, the Z axis extends along or parallel with the longitudinal axis of the shank 18. In order to untighten the fastening device 14, the outer nut 34 may be rotated in the opposite rotational direction to release the tension in the shank 18 and to cause the outer nut 34 to disengage from the sensing device 12. Initially, the nut assembly 22 is prevented from moving along the shank 18 (because the outer nut 34 is engaged with the sensing device 12 in this example) and so rotating the outer nut 34 in the opposite direction causes the inner and outer nuts 28, 34 to move towards each other, which releases the tension in the shank 18. In the illustrated embodiment, the configuration is such that the outer nut 34 moves towards the inner nut 28 until it is prevented from moving further (by abutment of surfaces 46, 48 in this example) after which further rotation of the outer nut 34 in the second direction causes the nut assembly 22 to move along the shank 18, i.e. the inner and outer nuts 28, 24 move together (away from the object 16 in the illustrated embodiment). It will be apparent that the nut assembly 22 may be used in a reverse orientation to the one shown in the drawings and described herein. In the reverse orientation, the outer ends 52, 56 of the inner and outer nuts 28, 34 face towards the sensing device 12. To tighten the fastening device 14, the outer nut 34 is first rotated such that the nut assembly 22 moves towards and into engagement with the sensing device 12, whereupon further rotation of the outer nut 34 in the same direction causes the outer nut 34 to separate from the inner nut 28 and move into engagement with the object 16, and still further rotation in the same direction causes the fastening device 14 to tighten and create tension in the shank 18. In alternative embodiments (not illustrated) the internal screw thread 30 for engaging with the external screw thread 20 of the shank 18 is provided on the outer nut 34 rather than on the inner nut 28. The arrangement of the inner and outer nuts and of the respective screw threads is such that, during the tightening process, rotation of the outer nut causes the nut assembly to move towards either the sensor or object until the outer nut engages with the sensor or object, whereupon further rotation of the outer nut in the same direction cause the inner and outer nuts to separate, in particular such that the inner nut moves into engagement with the other one or the sensor or object, whereupon further rotation of the outer nut in the same direction causes the fastening device to apply tension to the shank. The fastening device can be untightened or released by rotating the outer nut in the opposite direction, causing the inner and outer nuts to move towards each other. In alternative embodiments (not illustrated), the configuration is such that, starting in the untightened state as for example shown in Figure 3, relative movement between the inner and outer nuts occurs before movement of the inner nut with respect to the shank. For example the configuration may be such that the friction between the outer nut 34 and inner nut 28 (i.e. between the respective screw threads 36, 32) is less than the friction between the inner nut 28 and the shank 18 (i.e. between the respective screw threads 30, 20). This may be achieved in any conventional manner, e.g. by appropriate selection of the respective thread angle of the screw threads and / or selection of materials. In such embodiments, starting in the untightened state and with the nut assembly in the contracted state, initial rotation of the outer nut 34 causes relative movement between the inner and outer nuts 34, in particular causing the outer nut 34 to move axially with respect to the inner nut 28 (towards and into engagement with one of the sensing device 12 and the object 16 depending on the orientation of the nut assembly), further rotation of the outer nut 34 in the same direction causes the inner nut 28 to move axially away from the outer nut 34 (towards and into engagement with the other of the sensing device 12 and the object 16 depending on the orientation of the nut assembly), and still further rotation of the outer nut 34 in the same direction causes the fastening device to tighten and create tension in the shank 18. To untighten the fastening device, the outer nut is rotated in the opposite direction to cause the inner and outer nuts to move towards each other and along the shank. More generally, either one of the inner or outer nuts 28, 34 has a threaded engagement with the shank 18, and the inner and outer nuts have a threaded engagement with each other, the respective threaded engagements having an opposite handedness. This arrangement causes the fastening device to be operable in the preferred manner, namely: to tighten the fastening device the outer nut is rotated in a first direction causing the nut assembly 22 to move along the shank in one direction and causing the inner and outer nuts to move away from each other; and to untighten the fastener the outer nut is rotated in the opposite direction causing the nut assembly 22 to move along the shank in the opposite direction and causing the inner and outer nuts to move towards each other. In preferred embodiments the configuration is such that, when tightening the fastening device, the nut assembly first moves along the shank until it engages with one or other of the objects being fastened, and then the inner and outer nuts move apart until each of the inner and outer nuts engages with one or other of the objects being fastened, creating tension in the shank. In preferred embodiments the configuration is such that, when untightening the fastening device, the inner and outer nuts towards each other to release the tension in the shank, and then the nut assembly moves along the shank. Preferred embodiments of the fastening device 14 offer some or all of the following advantages: • provides continuous mechanical connection by the shank 18 between the object 16 (e.g. a mounting surface of a machine) and the sensing device 12 to improve the transmission of vibrations (in embodiments where the sensing device 12 comprises one or more vibration sensor). Preferred embodiments of the fastening device 14 provide improved vibration and temperature transfer through the shank 18 to the sensing elements, giving improved accuracies of measurements. • Allows for sensor orientation adjustment, or alignment, before the fastening device 14 is tightened, e.g. to align one or more axis of measurement; • Allows for a simple and intuitive one-directional twisting (rotational) motion to tighten the fastening device 14; • The fastening device, in particular the nut assembly 22, is compact and suitable for connecting to, for example, a 25mm diameter mounting point, and also to facilitate conduction of heat by reducing heat loss. Optionally, a mild to strong adhesive (thread-lock) product can be used to provide a semi-permanent 5 installation (removable with a suitable break-away torque). Optionally, a tool (e.g. a wrench) can be used to apply the tightening torque to the outer nut 34. However, manual tightening can be performed, especially when accessibility is poor. The invention Is not limited to the embodiments described herein which may be modified or varied 10 without departing from the scope of the invention.

Claims

1. A sensor assembly (10) comprising a sensing device (12) and a fastening device (14) for fastening the sensing device to an object (16), wherein the fastening device comprises:a shank (18) having a first external screw thread (20); anda nut assembly (22) located around said shank,wherein said shank has a first end (24) connected to, or connectable to, said sensing device and a second end (26) connectable to, or connected to, said object, the nut assembly being located between said first and second ends, and wherein the nut assembly comprises:an inner nut (28) having a second external screw thread (32); andan outer nut (34) located around said inner nut and having a second internal screw thread (36) that matches and is in engagement with the second external screw thread of said inner nut, wherein a first internal screw thread (30) is provided on one or other of said inner nut and outer nut, said first internal screw thread (30) matching and being in engagement with the first external screw thread of said shank, and wherein said first internal and external screw threads have a handedness that is opposite to a handedness of said second internal and external screw threads.

2. The sensor assembly of claim 1, wherein said first internal and external screw threads are right-handed and said second internal and external screw threads are left-handed, or wherein said first internal and external screw threads are left-handed and said second internal and external screw threads are right-handed.

3. The sensor assembly of any preceding claim, wherein said first end of said shank is releasably connected to said sensing device, and wherein, preferably, said first end has an external screw thread, conveniently said first external screw thread, and said sensing device comprises a socket for receiving said first end, said socket having a matching screw thread.

4. The sensor assembly as claimed in any preceding claim, wherein said sensing device comprises a base, said first end of said shank being connected to said base, and wherein, preferably, said base is formed from metal.

5. The sensor assembly of claim 4 when dependent on claim 3, wherein said socket is formed in said base.

6. The sensor assembly of any preceding claim, wherein said sensing device comprises at least one vibration sensor.

7. The sensor assembly of any preceding claim, wherein said sensing device has at least one sensing axis.

8. The sensor assembly of claim 7 when dependent on claim 6, wherein said at least one vibration sensor is configured to sense vibrations along at least one axis.

9. The sensor assembly of any preceding claim, wherein said sensing device has a body and wherein at least one alignment aid is provided on said body, and wherein, preferably, the, or each, alignment aid comprises a marking, formation or other visual and / or tactile aid.

10. The sensor assembly of claim 9 when dependent on claim 7 or 8, wherein said at least one alignment aid is indicative of a direction of said at least one axis.

11. The sensor assembly of claim 7, 8 or 10, wherein at least one of said at least one axis extends perpendicularly to a longitudinal axis of said shank, and wherein, optionally, said at least one axis comprises first and second mutually perpendicular axes, each of which is perpendicular to the longitudinal axis of said shank.

12. The sensor assembly of any one of claims 7, 8 or 10 to 11, wherein said at least one axis comprises an axis that is parallel or coincident with the longitudinal axis of said shank.

13. The sensor assembly of any preceding claim, wherein said shank is, or is part of, a bolt or screw, for example a grub screw, and / or wherein said shank is cylindrical or substantially cylindrical.

14. The sensor assembly of any preceding claim, wherein said shank is at least partly, preferably wholly, formed from metal.

15. The sensor assembly of any preceding claim, wherein said inner nut, outer nut and shank are co-axial with each other.

16. The sensor assembly of any preceding claim, wherein said second end of said shank is releasably connectable to said object, and wherein, optionally, said second end has an external screw thread, conveniently said first external screw thread.

17. The sensor assembly of any preceding claim, wherein said inner nut has an outer end that is level with or projects beyond a corresponding outer end of said outer nut, and wherein, preferably, said outer ends face away from the sensing device.

18. The sensor assembly of any preceding claim, wherein said first internal screw thread is provided on said inner nut.

19. The sensor assembly of any one of claims 1,2, 4 or 6 to 18, wherein said first end of the shank is fixed to or integrally formed with said sensing device.

20. The sensor assembly of any one of claims 1 to 15, wherein said second end of the shank is fixed to or integrally formed with said object.

21. A method of fastening a sensor assembly as claimed in any preceding claim to an object, the method comprising:rotating said outer nut in a first rotational direction to cause said nut assembly to move along said shank towards one of said object or said sensing device until said inner nut or said outer nut engages with said object or sensing device, or rotating said outer nut in a first rotational direction to cause said outer nut and said inner nut to move with respect to each other until said outer nut or said inner nut engages with one of said object or said sensing device;further rotating said outer nut in said first rotational direction to cause said outer nut to move along said inner nut, or said inner nut to move towards said outer nut, towards the other of said object or said sensing device until said outer nut or said inner nut, engages with the other of said object or said sensing device, or further rotating said outer nut in said first rotational direction to cause said inner nut to move with respect to the outer nut, or said outer nut to move with respect to said inner nut, towards the other of said object or said sensing device until said inner nut or said outer nut engages with the other of said object or said sensing device.

22. A fastening device comprising:a shank having a first external screw thread; anda nut assembly located around said shank, wherein said shank has a first end for connection to a first object and a second end for connection to a second object, the nut assembly being located between said first and second ends, and wherein the nut assembly comprises:an inner nut having a second external screw thread; andan outer nut located around said inner nut and having a second internal screw thread that matches and is in engagement with the second external screw thread of said inner nut, wherein a first internal screw thread (30) is provided on one or other of said inner nut and outer nut, said first internal screw thread (30) matching and being in engagement with the first external screw thread of said shank, and wherein said first internal and external screw threads have a handedness that is opposite to a handedness of said second internal and external screw threads.

23. A method of fastening a first object to a second object using a fastening device as claimed in claim 22, the method comprising:connecting the first end of the shank to said first object;connecting the second end of said shank to said second object;rotating said outer nut in a first rotational direction to cause said nut assembly to move along said shank towards one of said first object or said second object until said inner nut or said outer nut engages with said first object or second object, or rotating said outer nut in a first rotational directionto cause said outer nut and said inner nut to move with respect to each other until said outer nut or said inner nut engages with one of said first object or said second object;further rotating said outer nut in said first rotational direction to cause said outer nut to move along said inner nut, or said inner nut to move towards said outer nut, towards the other of said first object or second object until said outer nut or said inner nut, engages with said other of the first object or second object, or further rotating said outer nut in said first rotational direction to cause said inner nut to move with respect to the outer nut, or said outer nut to move with respect to said inner nut, towards the other of said first object or said second object until said inner nut or said outer nut engages with the other of said first object or said second object.

24. A method of tightening the fastening device of a sensor assembly as claimed in any one of claims 1 to 20, or the fastening device as claimed in claim 22, the method comprising: rotating the outer nut in a first rotational direction causing the nut assembly to move along the shank in a first direction and causing the inner and outer nuts to move away from each other and into engagement with a respective one of the objects being fastened by the fastening device.

25. A method of untightening the fastening device of a sensor assembly as claimed in any one of claims 1 to 20, or the fastening device as claimed in claim 22, the method comprising; rotating the outer nut in a second rotational direction causing the nut assembly to move along the shank in a second direction and causing the inner and outer nuts to move towards each other.

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