Abstract:
A fastening apparatus  10  for tightening a threaded fastener  14  into a component, the apparatus comprising a drive mechanism  12  for rotating the threaded fastener and a detector for detecting a transitional tightening stage when tightening the threaded fastener to the component, wherein the apparatus operably applies a first set of tightening conditions to the threaded fastener during a first tightening stage, and a second set of tightening conditions during a second tightening stage in response to the detection of the transitional tightening stage by the detector.

Description:
FIELD OF THE INVENTION 
     The present invention relates to fastening apparatus used to position and tighten threaded fasteners, and in particular to automated fastening apparatus. 
     BACKGROUND TO THE INVENTION 
     Known fastening apparatus apply a predetermined set of conditions to a threaded fastener being tightened. Typically a constant axial load is applied to the threaded fastener which is rotated at a speed determined by the friction generated at the threads of the threaded fastener and the torque applied to the driving bit. 
     It is an object of the present invention to provide a fastening apparatus that is capable of applying at least two distinct sets of tightening conditions to an associated threaded fastener to improve the speed and efficiency of tightening of the threaded fastener and also to improve its reliability. 
     SUMMARY OF THE INVENTION 
     Thus according to the present invention there is provided a fastening apparatus for tightening an associated threaded fastener in which the apparatus applies a first set of tightening conditions to the associated fastener, during a first tightening stage the apparatus being capable of detecting a transitional tightening stage of the associated fastener, and applying a second set of tightening conditions during a second tightening stage in response to the detection of the transitional stage. 
     According to the present invention there is also provided a fastening apparatus for tightening an associated threaded fastener in which the apparatus rotates the associated threaded fastener at a first speed during a first tightening stage and said apparatus rotates the associated threaded fastener at a second speed during a second tightening stage. The second speed may be faster or slower than the first speed. 
     According to the present invention there is also provided a fastening apparatus for tightening an associated threaded fastener in which the apparatus applies a first axial load to the associated threaded fastener during a first tightening stage and said apparatus applies a second axial load to the associated threaded fastener during a second tightening stage. The second load may be greater or smaller than the first load. 
     A further aspect of the invention provides a robotic arm for use on an assembly line, for example for car production, comprising a fastening apparatus according to the earlier aspects of the invention such as a two speed fastening device adapted to enable a threaded fastener to pierce and tap a support layer such as a sheet of aluminium. Beneficially, the robotic arm can be controlled by a programmed device such as a micro controller to move the fastening apparatus to different locations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example only with reference to the accompanying drawings in which: 
     FIG. 1 shows a section view of an apparatus according to the present invention; 
     FIG. 2 is an enlarged view of an upper part of FIG. 1; 
     FIG. 3 is an enlarged view of a mid part of FIG. 1; 
     FIG. 4 is an enlarged view of a lower part of FIG. 1; 
     FIG. 5 is a view of the apparatus taken in the direction of arrow A of FIG. 1; 
     FIG. 6 is a view taken along the line E—E of FIG. 1; 
     FIG. 7 is a view similar to FIG. 3 with the second piston fully advanced and with the first piston partially advanced; 
     FIG. 8 is a view similar to FIG. 3 with the first and second pistons fully advanced; 
     FIG. 9 is a block diagram of a control system used to control the fastening apparatus of the present invention; and 
     FIG. 10 is a part view of a modified block diagram as shown in FIG. 9 incorporating a preferred embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIGS. 1-6 there is illustrated a fastening apparatus  10  comprising a fastening holding/positioning device  11  and a fastening driving device  12 . Both are connected to a head  13 . 
     Driving device  12  comprises a motor  20  such as a servo-motor with a motor pulley  20 A, a drive belt  21  and a drive shaft  22 . Drive shaft  22  comprises a first part  24  and a second part  25 . First part  24  is rotatably mounted in head  13  by bearings  26 . Bearings  26  also axially secures first part  24  relative to head  13 . A drive shaft pulley  23  is rotatably secured on first part  24  by key  27 . The lower portion  28  of first part  24  includes an axially splined hole  29 . 
     The second part  25  of drive shaft  22  includes a fastening driving bit  30 , a mounting portion  31 , and a splined end portion  32 . Splined end portion  32  mates with the axially splined hole or bore  29  to ensure relative rotation of the first and second part cannot take place. However, the axially splined hole  29  and the splined end portion  32  do allow axial movement of the first part relative to the second part. The second part is rotatably mounted via bearings  33  in axial positioning tube  40 . Bearings  33  also ensure that second part  25  is axially fast with axial positioning tube  40 . 
     The fastening holding/positioning device  11  includes a mounting part  50  secured to head  13 , a first cylinder  51  secured to mounting part  50 , a second cylinder  52  secured to first cylinder  51  and an end cap  53  secured to second cylinder  52 . Mounting part  50  is generally cylindrical in shape with a central axial hole  60  and two lateral slots  61 ,  62 . A portion  63  of mounting part  50  acts as an end cap for the first cylinder  51 . 
     First cylinder  51  comprises a first bore  64 , the walls of which constitute a first cylinder wall  65 , and a further bore  66  parallel to the first bore  64 . 
     Second cylinder  52  is generally cylindrical in shape and includes a second bore  52 A the walls of which constitute a second cylinder wall  67 . 
     End cap  53  has a central axial hole  68 , and acts to seal the second cylinder  52 . 
     The fastening holding/positioning device  11  further includes a fastening positioning shaft  70  secured axially and rotatably fast to a fastening positioning arm  71 . The end of fastening positioning arm  71  remote from fastening positioning shaft  70  includes a fastening holding device  72 . The end of fastening positioning shaft  70  remote from the fastening positioning arm  71  projects into head  13  and is connected rotationally and axially fast to a pinion  73  and a stop arm  74 . Pinion  73  engages a rack  75  which in turn is connected to air piston  76 . By applying air pressure to either port  77  or  78  the rack  75  can be caused to reciprocate resulting in a clockwise or anti-clockwise rotation of the fastening positioning shaft  70  when viewing FIG.  5 . Rotation of fastening positioning shaft  70  is limited by stop arm  74  contacting dampers  79 A or  79 B (note in FIG. 5 that stop arm  74  is shown in alternative positions at the limit of rotation in both clockwise and anti-clockwise directions). A set of dog clutch teeth  70 A is secured to the lower end of the fastening positioning shaft  70 . 
     Fastening positioning shaft  70  is located within locking tube  80 . Locking tube  80  is rotationally secured to head  13  by pin  81  but can move relative to head  13  by an amount (B+C) (see FIG.  4 ). Locking tube  80  includes a set of dog clutch teeth  83  corresponding to and facing dog clutch teeth  70 A. Locking tube  80  further includes upper abutment  80 A and lower abutment  80 B. 
     Fastening holding device  72  comprises a body  90  with two holding jaws  91 , 92  each being pivotally mounted, via respective pivots  91 A,  92 A onto the body  90 . Spring  93  (acting in the manner of an elastic band) keeps the holding jaws closed prior to fixing of the fastener but allows the holding jaws to open to allow the fastener to pass through the holding jaws as the fastener is being screwed into the supporting component. 
     Pawls  94  and  95  can pivot about respective pivots  94 A and  95 A and are spring loaded into the position shown in FIG.  1 . They allow a fastener to be loaded into the fastening holding device  72  and ensure that it does not fall out if the fastening apparatus is being used to secure an overhead component. 
     Slidably mounted within the first cylinder  51  there is a first piston  54  including a head  55  and a shaft  56 . Head  55  is sealed against first cylinder wall  65  and shaft  56  is sealed against shaft  43  (see below). 
     Axially slidably mounted in second cylinder  52  there is a second piston  41  including a head  42  and a shaft  43 . Head  42  is sealed against second cylinder wall  67  and (as mentioned above) shaft  43  is sealed against shaft  56 . Second piston shaft  43  extends beyond head  55  of first piston  54 . At end  44  of second piston shaft  43  there is axially secured an arm  45  which projects through lateral slot  61 . Arm  45  is also axially secured to end  40 A of axial positioning tube  40 . Thus, second piston  41 , arm  45 , axial positioning tube  40 , bearings  33 , second part  25  and fastening drive bit  30  are all axially secured relative to each other. 
     Mounted in lateral slot  62  are three inductive sensors  84 A,  84 B and  84 C. Mounted on arm  45  there is a conductive lug  85 . Conductive lug  85  and inductive sensors  84 A,  84 B and  84 C together act as axial positional sensors indicating the axial position of fastening drive bit  30 . 
     Head  13  can be connected to an apparatus positioning device (not shown) such as a robot arm which positions the fastening apparatus  10  as a whole relative to a corresponding component to be secured by the fastener  14 . 
     Operation of the fastening apparatus  10  is as follows; 
     With the sets of dog-clutch teeth  70 A and  83  disengaged (see below), air piston  76  is actuated such that stop arm  74  abuts damper  79 A. In this position the fastening holding device  72  is no longer aligned with the fastening drive bit  30  and it is thus possible to load a fastener  14  into the fastening holding device  72 . Air piston  76  is then actuated to rotate the fastening positioning shaft  70  clockwise when viewing FIG. 5 until stop arm  74  contacts damper  79 B. Under these circumstances fastening holding device  72  and fastener  14  are aligned with fastening drive bit  30  (see FIG.  1 ). 
     Furthermore the dog-clutch teeth  70 A and  83  are aligned such they can engage. Air pressure is evacuated from annular volume  47  allowing the springs  82  (only one shown) to push the locking tube  80  downwards when viewing FIG. 1, and ultimately for the dog clutch teeth  70 A and  83  to fully engage and rotationally secure fastening positioning shaft  70  relative to locking tube  80  and hence (via pins  81 ) relative to fastening driving bit  30 . FIG. 4 shows the axial movement of locking tube  80  in progress since abutment  80 B has moved off away from end cap  53  by an amount B, but the locking tube  80  still has to move axially by an amount C before the dog-clutch teeth are fully engaged. 
     Fastener  14  comprises a head  15  and a fixing portion  16  of length L (see FIG.  4 ). Fixing portion  16  comprises a piercing portion  17  of length L 1  a parallel sided non-threaded portion  18  of length L 2  and a threaded portion  19  of length L 3 . 
     The apparatus positioning device positions the fastening apparatus such that the fastener holding device  72  with fastener  14  is presented proximate the component to be fixed  1  which in turn lies proximate to a component to which it is to be fixed, the supporting component  2  (see FIG.  4 ). In this case the components to be fixed  1  has a pre-drilled hole  3  through which the fastener  14  will pass and the supporting component  2  is a sheet metal component with no corresponding pre-drilled hole. 
     The motor  20  is started and runs at a first rotational speed which causes the fastening driving bit  30  to rotate. Air pressure P is admitted simultaneously into the annular volume  57  above the first piston  54  and into the annular volume  46  above the further piston head  42 . 
     The pressure in annular volume  57  causes piston  54  to move downwards when viewing FIG.  1 . This causes shaft  56  of piston  54  to act on head  42  of the second piston  41  causing the second piston  41  and all other components axially secured thereto including the fastening driving bit  30  to also move downwards. Fastening drive bit  30  therefore is caused to engage head  15  of fastener  14  which then rotates at the first speed. 
     The piercing portion  17  of fastener  14  is forced against supporting component  2  by a toad dependent upon the air pressure P admitted into annular volumes  57  and  46  and upon the effective area A 1  of the first piston  54  (A 1 =pi [D 1   2 −D 3   2 ]/4, where D 1  is the diameter of the first cylinder  51  and D 3  is the diameter of shaft  43 ) and the initial effective area A 2  of the second piston (A 2 =pi [D 2   2 −D 4   2 ]/4 where D 2  is the diameter of the second cylinder and D 3  is the diameter of shaft  48 ). 
     The fastener continues to advance and pierces the supporting component  2  whereupon the first piston head  55  contacts abutment  58  axially secured to first cylinder  51  (see FIG.  7 ). This restricts further movement of first piston  54  but the same air pressure P which has also been admitted into annular volume  46  of further cylinder  52  acts on the new effective area A 3  of the second piston (A 3 =pi [D 2   2 −D 3   2 ]/4). Note that since in this second stage of tightening the shaft  56  no longer contacts the head  42  the effective area of the second piston  41  changes from A 2  to A 3  (see FIG.  8 ). Thus the axial load applied to the fastener  14  by the fastening driving bit  30  is initially P (A 1 +A 2 ) and once piston head  55  has contacted abutment  58  the axial load reduced to P (A 3 ). 
     Note that in a preferred embodiment separate valves can control the air being fed to annular volumes  46  and  57 . In particular such separate valves can admit air at different pressures and at different times, dependent upon the application. Typically the air pressure admitted to annular volume  57  might be 6 bar and the air pressure admitted to annular volume  46  might be 2 bar. 
     Inductive sensor  84 B and conductive lug  85  are arranged such that a signal is produced in the inductive sensor  84 B just as the first piston  54  contacts abutment  58 . This signal is processed so as to cause a reduction in speed of motor  20  such that the drive shaft  21  now rotates at a second, in this case slower speed, whilst the parallel sided non-threaded portion  18  of fastener  14  passes through the just pierced supporting component  2 . A further reduction in speed is effected when the threaded portion  19  of fastener  14  starts to tap a thread in the supporting component  2 . By knowing the number of threads on the threaded portion  19  and the speed at which the fastener is being tightened, it is possible to effect a further reduction in speed of tightening as the fastener approaches a fully tightened condition. The motor can be stopped when the tightening torque reaches a pre-determined final tightening level so as not to over-tighten the fastener and damage the supporting component  2 . 
     Typical speeds of rotation of the fastener might be 9,000 revs per minute during piercing, reducing to 3,300 revs whilst the parallel sided portion of the fastener is passing through the hole and further reduction to 1,500 revs per minute whilst the threaded portion self-taps supporting component  2  and finally reducing to 500 revs per minute when the fastener is say 5 revolutions (or thread turns) from being fully tightened. 
     Induction sensor  84 C is arranged as a back-up sensor in the event that the final tightening torque is not achieved. 
     When the final tightening torque is achieved or back-up induction sensor  84 C is operated, a signal is generated which is processed to cause annular volumes  46  and  57  to be vented and a pressure to be applied to annular volume  47  which forces the first piston  54  and second piston  41  upwards, resulting in the upper end of second piston  41  contacting the upper abutment  80 A of the locking tube  80 , which in turn is also caused to move upwards against the action of springs  82 . Ultimately this upward movement stops when the lower abutment  80 B of locking tube  80  contacts end cap  53 . It will be noted that the upward movement of, in particular, locking tube  80  disengages the dog-clutch teeth  70 A and  83 , thus allowing rotation of the fastening positioning shaft  70  to allow re-loading of a further fastener  14  (see below). 
     Inductive sensor  84 A is positioned so as to recognise when the first piston  54  is fully retracted, whereupon a signal is generated to activate air piston  76  to rotate fastening positioning shaft  70  to a position where another fastener  14  can be loaded into the fastening holding device  72 . The apparatus positioning device (not shown) then moves the fastening apparatus  10  to another position so that the new fastener  14  can be used to fix either the same component  1  or a further component, as the case may be. 
     It can be seen that the apparatus  10  applies a first set of tightening conditions to fastener  14  ie a first rotational speed and an axial load proportional to P (A 1 +A 2 ). Upon detecting a transitional tightening stage eg after the fastener has moved axially by a predetermined amount or the fastener requires a predetermined torque to turn it, a further set of tightening conditions is then applied to the fastener eg a further rotational speed and an axial load proportional to P (A 3 ). 
     In further embodiments of the invention, depending upon the type of fastener intended to be used, it is possible to apply two (or more) independent axial loads to the fastener whilst only applying one rotational speed. Also it is possible to use two (or more) speeds of rotation of the fastener whilst only applying a single axial load. 
     The inductive sensor  84 B in conjunction with the conductive lug  85  act as axial positional sensors which can detect transitional tightening stages of the fastener  14 . Further embodiments of the invention can use other sorts of positioning sensors such as optical sensors. 
     Consideration of the fastener  14  shows that there are at least five distinguishable tightening stages: 
     a) The piercing of supporting component  2  by piercing portion  17 . 
     b) The stage when the parallel sided section  18  passes through the just pierced supporting component  2 . 
     c) The tapping of the just pierced supporting component  2  by the initial threads on the threaded portion  19  of fastener  14 . 
     d) The further threading of the fastener into the just tapped hole of the supporting component  2 . 
     e) The final torquing up of the threaded fastener. 
     By measuring the torque requirement on the fastening driving bit  30  (by using a torque sensing device e.g. in the case of a driving bit being tightened by a servo-motor, the monitoring of the current required by the servo-motor indicates the driving bit torque) it is possible to distinguish transition points between stages of tightening. For example, the initial tapping of the just pierced hole (sub-paragraph c above) requires a greater torque than the further threading of the fastener into the just tapped hole (sub-paragraph d above). 
     Furthermore the maximum torque required to tap the hole (sub-paragraph c above) can be greater than the torque required to finally tighten the fastener (sub-paragraph e above). Thus by using different detecting methods to determine at which stage of tightening the fastener is at, accurate tightening can be achieved. In the present case by using axial positioning techniques (sensor  84 B) and by knowing the speed of rotation and number of threads, the end of the high torque tapping phase of tightening (sub-paragraph c above) can be detected and then final tightening (sub-paragraph e above) can be carried out by torque measurement. 
     Fastener  14  initially requires a relatively high rotational speed and axial load to ensure efficient piercing of the support component  2 . However, once the hole has been pierced it is advantageous to reduce the rotation speed and axial load so as to ensure better self tapping of the support component  2  and also to ensure that the fastener is not over tightened resulting in stripping of threads of the support component. 
     It should be noted that relative movement between locking tube  80  and the second piston  41  is only in an axial direction, no rotational movement occurs between these components and this results in an improved service life of the seals between locking tube  80  and the second piston  41 . 
     FIG. 9 shows a block diagram which summarises a typical control system used in conjunction with the fastening apparatus of the present invention. The controller  100 , such on a programmable logic control or plc, can receive inputs from inductive sensors  84 A,  84 B and  84 C, and also a measure of the torque requirement of drive bit  30  via measurement of the servo-motor current. The controller can then output signals to valve  101  to control air piston  76 , valve  102  which admits air into and vents air from annular volumes  46  and  57 , valve  103  which admits air to and vents air from annular volume  47 , positioning device  104  which controls the position of the fastening apparatus and also to servo-motor  20  to vary the speed of rotation of the drive bit  30 . 
     FIG. 10 shows the valve  102  of FIG. 9 having been replaced by two separate valves  102 A and  102 B each admitting and venting air to respective annular volumes  46  and  57 . This arrangement allows for different pressures to be admitted to the different annular volumes and also allows the air to be admitted and/or vented at different times.