BACKGROUND 
     In the production of equipment, components of the equipment are tested to determine that the component is working properly. One such test is to determine if a drive mechanism is rotating the component properly based on the input when the drive mechanism has a load that simulates real life conditions. For example, a drive mechanism rotates a shaft to control or manipulate an air-loaded control surface, such as an air foil or rudder. 
     One such technique of testing is to secure a torsion bar to the end of the drive mechanism. The torsion bar is held to simulate loading of the surface. 
     Traditionally, the torsion bar has been clamped either with a disc brake design or high-pressure hydraulic collets. Both of these techniques have shortcomings. 
     The disc brake design requires pads that attach to the torsion bar. The pads are clamped with a brake caliper similar to wheels on a vehicle. However in that the pads are extruded radially from the torsion bar, the pads add rotational inertia resulting in not accurately simulating the loading of the surface. This increased rotational inertia is not desired during performance testing. In addition, the brake calipers are typically driven electrically and therefore have additional electrical requirements. 
     The high-pressure hydraulic collets use hydraulics to compress a collet onto the torsion bar. Either a hand pump or high-powered electric pump is typically used to provide the pressure for the hydraulics. The hand pump has many shortcomings including the additional time required by a test technician to create the needed pressure. In addition, the requirement to test components quickly, i.e., a high volume production, necessitates the need for quick connections. These quick connects and fluid leakage from collets adds a concern. The leakage could create spills that are potential safety hazards. In addition, leakage on the torsion bar clamping area would create a decrease in clamping friction requiring more clamping load. Likewise, the need for electricity for the pump makes it more difficult to test components quickly and efficiently. 
     SUMMARY 
     Unfortunately, there are deficiencies to the above-described testing apparatus including concerns with using hydraulics and/or electrical requirements. Furthermore the addition of pads on the torsion bar increases rotational inertia and therefore results in not properly simulating the load. 
     In contrast to the above-described conventional testing approaches, improved techniques are directed to testing using a pneumatic collet assembly that does not require hydraulics and does not require pads or other items that will influence the rotational inertia improperly. 
     One embodiment is directed to a pneumatic assembly which includes a piston disposed in a housing. A bore within the piston receives the collet. The collet has a base and fingers. The fingers of the collet define a center channel or bore configured to receive a torsion bar. The central channel extends along a longitudinal axis. The ball bearings are disposed between the piston and the fingers of the collet. The piston is configured to move between a first position along the longitudinal axis and a second position along the longitudinal axis in response to pressure changes within the housing. The piston increases force against the ball bearings to urge the fingers of the collet against the torsion bar to clamp the torsion bar. 
     The housing defines a chamber having a clamping port and an unclamping port. The ports are connected to a source of pressurized air; such as shop air. The piston has an annular ring that encircles the bore of the piston and divides the chamber. The annular ring of the piston has a pair of face walls. The first face wall is in communication with the clamping port and the second face wall is in communication with the unclamping port. The piston moves between the first position and the second position by the ports allowing pressurized air into and out of the chamber divided by the annular ring of the piston. 
     In some arrangements of the pneumatic collet, each finger of the collet has a finger ball channel. The finger ball channels of the fingers are angled such that the finger channels are further from the longitudinal axis at the distal end than at the base. In addition, the bore of the piston has a plurality of piston ball channels. The collet is received in the bore of the piston such that the finger channel of each finger of the collet is aligned with a piston ball channel of the piston. The piston ball channels of the piston are angled such that the piston channels are further from the longitudinal axis at an end closer to the distal end of the fingers of the collet. The ball bearings are interposed in the ball channels between the collet and the piston. 
     In one arrangement, the aligned channels of the collet and the piston are at an angle of between 1 and 2 degrees from that of the longitudinal axis. In addition, the aligned channels of the collet and the piston are parallel to each other. 
     The channels of the collet and the channels of the pistons in the pneumatic collet assembly are semi-cylindrical. The channels have a diameter substantially equal to the diameter of the ball bearings. The ball bearings transfer load to the channels along a line. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a schematic showing a test fixture having a plurality of pneumatic collets; 
         FIG. 2  is an exploded view of the pneumatic collet; 
         FIG. 3  is a sectional view of the pneumatic collet with a torsion bar extending through the pneumatic collet; 
         FIG. 4  is an isometric view of the piston; 
         FIG. 5  is an isometric view of the collet; 
         FIG. 6  is a sectional view of the pneumatic collet taken along the line  6 - 6  of  FIG. 2 ; 
         FIG. 7  is a sectional view of the pneumatic collet taken along the line  7 - 7  of  FIG. 6 ; and 
         FIG. 8  is view of the relationship of the ball bearing with the piston and the collet in the unloaded position and in a loaded position in phantom. 
     
    
    
     DETAILED DESCRIPTION 
     An improved pneumatic collet assembly utilizes a piston which moves between a first position along a longitudinal axis which forces ball bearings and collet fingers toward a torsion bar to clamp the torsion bar in place and a second position along the longitudinal axis which releases the torsion bar. The use of an air-controlled piston and ball bearings to move the collet fingers enables tight clamping of the torsion bar as well as easy release with minimal piston stroke and manageable amounts of air pressure. Accordingly, conventional approaches to clamping torsion bars using disk brakes or extremely high amounts of pressure are unnecessary. 
     When referring to the drawing in the description which follows, like numerals indicate like elements.  FIG. 1  shows a pneumatic collet  20  as part of a test stand  22 . Improvements to a test stand  22  include having a plurality of pneumatic collets  20  that are operated by pneumatic or shop air to move a piston within the collet to force a plurality of ball bearings radially inward to force a plurality of fingers of the collet to clamp on a torsion bar therein having the benefit of tight clamping of the torsion bar and easy release with minimum stroke of the piston and avoiding shortcomings associated with hydraulics, additional electrical requirements, and adding items that result in added rotational inertia. 
     Referring to  FIG. 1 , the test stand  22  has a plurality of pneumatic collets  20  for testing of a drive mechanism  24 , particularly a torsional drive mechanism. The drive mechanism  24  in normal operation is for the moving of an air-load surface  26 , such as an aileron, shown in phantom. The aileron  26  is connected to a shaft  28  that is rotated by the drive mechanism  24 . The test stand  22  has a torsion bar  30  that is secured to the shaft  28 . The torsion bar  30  is extended through at least one pneumatic collet  20  (three shown in  FIG. 1 ). 
     Referring to  FIG. 2 , an exploded view of the pneumatic collet  20  along a longitudinal axis  38  is shown. The pneumatic collet  20  has a housing  40 , a piston  42 , a collet  44 , a ball cage  46 , a plurality of ball bearings  48 , and a sleeve  50 . In addition, the pneumatic collet  20  has a plurality of sealing rings  52   a  and  52   b ,  54 , and  56  for sealing a chamber as described in more detail below. The pneumatic collet  20  also has a cover  58 . The pneumatic collet  20  has a plurality of fasteners securing components of the pneumatic collet  20  as described below. 
     Referring to  FIG. 3 , a sectional view of the pneumatic collet  20  taken along the line  3 - 3  of  FIG. 1  is shown. Generally, the piston  42  is moved, to the left in the figure, to move the ball bearings  48  to have the collet  44  clamp the torsion bar  30  to stop rotation of the torsion bar  30  at the location of the pneumatic collet  20  as explained below. 
     The housing  40  has a first cylindrical bore  60  and a second cylindrical bore  62  which has a smaller diameter and extends deeper than the first cylindrical bore  60 . The bores  60  and  62  are co-axial along the longitudinal axis  38 . The second cylindrical bore  62  ends in a base  64  of the housing  40  having an annular ring  66  and a raised center portion  68 . The base  64  has an opening  70  co-axial with the bores  60  and  62  along the longitudinal axis  38 . 
     The first cylindrical bore  60  defines a cylindrical piston riding wall  72  on the housing  40  and a piston face wall  74 . The second cylindrical bore  62  defines a second or inner cylindrical piston riding wall  76 . 
     The sleeve  50  has a cylindrical bore  80  and a pair of annular planar walls  82  and  84 . The outer annular wall  82  engages a front wall  78  of the housing  40 . The front wall  78  of the housing  40  has an annular groove  86  for receiving the sealing ring  54  for sealing of a portion of the chamber  88 . In the embodiment shown, the remainder of the chamber  88  is open to the atmosphere. The sleeve  50  is secured to the housing  40  with a plurality of fasteners in the embodiment shown. The inner annular ring  84  defines a piston face wall, an outer head wall. The piston face wall  84  will theoretically limit the travel of the piston  42 , however the interaction of the ball bearings  48  with the piston  42  and the collet  44  results in the clamping of the torsion bar  30 ; this clamping of the torsion bar  30  will stop the movement of the ball bearings  48  and will limit the travel of the piston  42 . 
     Referring to  FIGS. 3 and 4 , the piston  42  has a base  90  having a bore  92 . The piston  42  has an annular ring  94  that extends radially outward from the base  90 . The annular ring  94  has an outer cylindrical wall  96  and a pair of annular piston heads  98 . The base  90  of the piston  42  has a cylindrical outer wall  100 . 
     Referring to  FIG. 3 , the outer cylindrical wall  96  of the annular ring  94  of the piston  42  has a groove  102  for receiving the sealing ring, piston sealing ring  56 . The piston sealing ring  56  makes a sliding engagement with the piston riding wall  72  of the first cylindrical bore  60  of the housing. The inner piston riding wall  76  of the housing and the cylindrical wall  104  of the bore  80  of the sleeve  50  each have an annular groove  106  and  108 , respectively for receiving the sealing rings  52   a  and  52   b , respectively. The piston  42  has a plurality of ball channels  110  where the bore  92  of the base  90  is enlarged such that each ball bearing  48  is carried in part of the ball channel  110 , as best seen in  FIG. 4 . 
     The housing  40  and the sleeve  50  define the chamber  88  in which the piston moves laterally along the longitudinal axis  38 . The chamber  88  has an annular chamber  112  defined by the piston rising wall  72  and the piston face wall  74  of the housing  40 , the piston face wall  84  of the sleeve  50  and the cylindrical outer wall  100  of the base  90  of the piston. The annular ring  94  of the piston  42  is located within the annular chamber  112  and has the annular piston heads  98  that are moved by the air as described below. The sealing rings  52   a ,  52   b , and  54  assist in sealing the annular chamber  112 . 
     Referring to  FIGS. 3 and 5 , the collet  44  has a base  116 , a stem  118  projecting from the base  116 , to the right in  FIG. 3 , and a plurality of fingers  120  extending from the base  116 , as best seen in  FIG. 5 . Referring to  FIG. 3 , the stem  118  of the collet  44  extends through the opening  70  in the base  64  of the housing  40 . A washer  122  and the nut  124  encircle the stem  118  of the collet  44  to secure the collet  44  to the housing  40 . The collet  44  is limited from rotating relative to the housing  40 . The collet  44  is positioned along the longitudinal axis  38 . 
     The collet  44  has a bore  126  extending along the longitudinal axis  38  of the pneumatic collet  20 . The bore  126  has a larger diameter portion  128  and a smaller diameter portion  130 . The torsion bar  30  extends through the bore  126  in the collet  44 . The fingers  120 , at the distal end, where the bore  126  has the smaller diameter portion  130  is where the collet  44  will engage the torsion bar  30  as explained below. 
     In the embodiment shown, the collet  44  has six collet fingers  120 . Each finger  120  projects from the base  116  and extends radially outward from the bore  126  and the longitudinal axis  38 . Each of the collet fingers  120  has a ball channel  132  on the cylindrical outer wall  134  of the collet  44 . 
     Referring to  FIG. 3 , the cover  58  is secured to the sleeve  50  by a plurality of fasteners. The cover  58  protects the cylindrical bore  80  of the sleeve  50 . The cover  58  has an opening  138  that is centered along the longitudinal axis  38 . The torsion bar  30  extends through the opening  138  when inserted into the pneumatic collar  20 . 
     Referring to  FIG. 6 , a sectional view of the housing  40 , the piston  42 , the ball cage  46 , and the collet  44  taken along the line  6 - 6  of  FIG. 2  is shown. The collet  44 , the ball cage  46 , and the piston  42  are all co-linear about the longitudinal axis  38  which extends out of the page. Each of the fingers  120  of the collet  44  extends radially from the longitudinal axis  38 , which extends out of the page in the Figure, and the bore  126  for the torsion bar  30  to the ball channel  132  of the collet  44 . Each of the ball channels  132  of the collet  44  is aligned with the ball channel  110  in the piston  42 . A ball bearing  48  is interposed between the ball channel  132  and  110  of the collet  44  and the piston  42 , respectively. 
     Still referring to  FIG. 6 , the ball cage  46  is interposed between the collet  44  and the piston  42 . The ball cage  46  is used for ease of the installation of components of the pneumatic collet  20  (i.e., the collet  44  and the ball bearing  48 ) into the housing  40  during assembly of the pneumatic collet  20 . In the embodiment shown, the ball cage  46  does not affect the workings of the pneumatic collet  20  during operation. 
     The housing  40  has a pair of feet  140  for mounting the pneumatic collet  20 . 
     The ball bearings  48  and the ball channels  110  and  132  of the piston  42  and the collet  44 , respectively, are sized so that the ball bearing engages a substantial portion of each of the channels. The contact is generally in a line contact wherein the line contact of each of the ball bearings  48  is in a plane that is perpendicular to the longitudinal axis  38 . 
     Referring to  FIG. 7 , a cross-sectional view of the pneumatic collet  20  is shown. The piston  42  is located in the chamber  88  with the annular ring  94  of the piston located in the annular chamber  112 . The annular chamber  112  has a clamping port  142  that opens on the piston face wall  74  of the housing  40  and an unclamping port  144  that opens on the piston face wall  84  of the sleeve  50 . The ports  142  and  144  are connected to pneumatic air, shop air, through a plurality of lines  32  and a controller  34 , as seen in  FIG. 1 . 
     During operation of the test stand  22 , the inputting of air in the clamping port  142  in the housing  40  exerts pressure against the annular piston head  98  of the annular ring  94  of the piston. This pressure causes the piston to move towards the bottom of the page in  FIG. 7 . Likewise the inputting of air in the unclamping port  144  in the sleeve  50  exerts pressure against the other annular piston head  98  of the annular ring  94  of the piston  42 . This pressure causes the piston  42  to move upward on the page in  FIG. 7 . In both cases, the port  142  or  144  that is not receiving air, is open to allow air in the annular chamber  112  on that side of the annular ring  94  of the piston  42  to exit the chamber  112  as the piston  42  moves in that direction. It is recognized that a vacuum could be placed on that side of the chamber  112  to assist in movement of the piston  42 . 
     Referring to  FIG. 8 , an enlarged view of the relationship of the ball channels  132  and  110  of the collet  44  and the piston  42 , respectively is shown. The ball channels  132  and  110  of both the collet  44  and the piston  42  are set at an angle relative to the longitudinal axis  38 . The ball channels  132  on the collet  44  are set at an angle α such that the saddle  150 , the lowest point, of the ball channel  132  is further from the longitudinal axis  38  at the distal ends  152  of the fingers  120  than at the base  116  of the collet  44 . The ball channels  110  on the piston  42  are set at an angle β such that the saddle  154  of the ball channel  110  is closer to the longitudinal axis  38  at the end nearer the base  64  of the housing  40  than at the end in proximity to the cover  58 . In some arrangements, the angles α and β are between 1 and 2 degrees. In one arrangement, the angles α and β are identical. When the angles α and β are identical, the line of the saddles  150  and  154  of the aligned ball channels of the collet  44  and the piston  42  are parallel and at an angle α relative to the longitudinal axis  38 . 
     As the piston  42  is moved to the left in  FIG. 8 , the ball bearings  48  are forced to the left and radially inward because of the shape and alignment of the ball channels  110 . As the ball bearings  48  are moved to the left, the distal end  152  of the fingers  120  of the collet  44  are forced (deflected) radially inward by the ball bearings  48 . The diameter of the smaller diameter portion  130  of the bore  126  is narrowed as the fingers  120  flex (deflect) inward and the fingers  120  of the collet  44  grab the torsion bar  30 . The torsion bar  30  is therefore not allowed to rotate at this location. The piston  42 , ball bearings  48 , and the fingers  120  are shown in phantom in the clamped position. 
     When air is input into the unclamping port  144  of the pneumatic collet  20  and allowed to vent from the clamping port  142 , as best seen in  FIG. 7 , the piston  42  is forced to the right in  FIG. 8 . As the piston  42  moves to the right the resilience of the fingers  120  of the collet  44  and the alignment of the ball channels  110  and  132  force the ball bearings  48  towards the right. This allows the bore  126  located in the collet  44  to return to its normal size wherein the torsion bar  30  extends through the pneumatic collet  20  but is limited in its movement by the pneumatic collet  20 . This is considered the un-clamped position. 
     Details of an exemplary test operation will now be provided. Referring back to  FIG. 1 , the drive mechanism  24  to be tested is brought into the shop. A torsion bar  30  is secured to the shaft  28  of the drive mechanism  24  for the purpose of testing. In some arrangements, a plurality of pneumatic collets  20  are positioned such that the torsion bar extends through the bore  126 , as seen in  FIGS. 3 and 8 . The pneumatic collets  20  are secured in the desired position that is the proper length along the torsion bar  30 ; aligned and held in place with feet  140 . Each collet  20  is connected to shop air through a series of lines  32  and the controller  34 . 
     Still referring to  FIG. 1 , the drive mechanism  24  is run through a series of tests. The pneumatic collets  20  are each in the un-clamped position, that is with the piston  42  to the right in  FIG. 3 . The fingers  120  of the collet  44  are not engaging the torsion bar  30 . 
     When the test calls for a specific pneumatic collet  20  to grab the torsion bar  30 , the controller  34  allows air to be fed into the clamping port  142  of the specific pneumatic collet  20  and allows air to be vented out of the un-clamping port  144 . As described above, the piston  42  is moved from the unclamped position to the clamped position. The movement of the piston  42  causes the ball bearings  48  to move both towards the distal end  152  of the fingers  120  and radially inward as shown in phantom in  FIG. 8 . This movement results in the fingers  120  narrowing the size of the smaller diameter portion  130  of the bore  126  sufficiently to have the fingers  120  grab the torsion bar  30  with sufficient force to limit rotation of the torsion bar  30  at this location. The pneumatic collet  20  imparts a high radial load with a relatively small input of work. This high radial load is necessary because of the small torsion bar surface and low friction. 
     When the test calls for the specific pneumatic collet  20  to release the torsion bar, the controller  34  allows air to be fed into the un-clamping port  144  of the specific pneumatic collet  20  and allows air to be vented out of the clamping port  142 . As described above, the piston  42  is moved from the clamped position to the un-clamped position. The movement of the piston  42  in conjunction with resilience of the fingers  120  of the collet  44  causes the ball bearings  48  to move both away from the distal end  152  of the fingers  120  and radially outward as shown in solid line in  FIG. 8 . This movement results in the fingers  120  returning to their normal position and the size of the smaller diameter portion  130  of the bore  126  being sufficiently large so that the fingers  120  do not interfere with movement of the torsion bar  30 . Air can be sent to another pneumatic collet  20  to move it to the clamped position while this pneumatic collet  20  is moved to the unclamped position. The unclamping is easy to perform with little load. This is in contrast with standard metal-to-metal devices which require a large impact load. 
     In an embodiment contemplated for a torsion bar having a diameter smaller than 0.2 inches, the pneumatic collet  20  is connected to pneumatic pressure or shop air in the range of 50 to 80 psi (pounds per square inch). The movement of the piston  42  from the first position to the second position is approximately 0.100 inches. The ball bearings  48  are made of stainless steel having a hardness of 60 Rc (Rockwell C hardness). The piston and collet are made of hardened steel having a hardness in the range of 50 Rc to 60 Rc. The tolerance of the channels and the ball bearings are in the range of +/−0.0004 inches. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
     For example, the pneumatic collet  20  has been described in relation to a test stand  22  for the testing of a drive mechanism  24 . In the embodiment shown, the collet  44  is retained by the housing to eliminate rotation. Likewise, the piston  42  cannot rotate because of the ball bearings  48  in the channels  110  and the aligned channels  132  in the collet  44 . While the torsion bar  30  is free to rotate a complete 360 degrees when unclamped, the torsion bar  30  is limited in rotation when clamped by the fingers  120  of the collet  44 . It is recognized that the pneumatic collet  20  could be adapted for use with a lathe or drill where stock or tools are clamped for rotational motion. In this embodiment, the collet would be allowed to rotate relative to the housing. 
     The pneumatic collet  20  can be used with any geometric shaped torsion bar  30  including torsion bars having a cross sectional area that is circular, square, or hexagonal. 
     In the embodiment shown, the collet  44  has six fingers  120  and six ball bearings  48 . It is recognized that fewer or more fingers and ball bearings can be used. The number of fingers and ball bearings is chosen to provide sufficient clamping around the collet fingers and have sufficient size ball bearings to transfer the load from the piston to the collet. Therefore, if a larger torsion bar  30  is used and therefore a larger bore  126 , an increase in the number of fingers  120  and ball bearings  48  may be desired. Also, increase in the size of fingers  120  and ball bearings  48  may be desired. 
     While hydraulics have shortcomings as discussed above, it is recognized that it may be desirable to use hydraulics in place of pneumatics in certain situations. Likewise, it is recognized that a manual lever can be coupled to the piston to move the piston between its positions.
A pneumatic collet assembly has a housing, a piston, a collet, and ball bearings. The piston is disposed within the housing. A bore within the piston receives the collet. The collet has a base and fingers. The fingers of the collet define a center channel or bore configured to receive a torsion bar. The central channel extends along a longitudinal axis. The ball bearings are disposed between the piston and the fingers of the collet. The piston is configured to move between a first position along the longitudinal axis and a second position along the longitudinal axis in response to pressure changes within the housing. The piston increases force against the ball bearings to urge the fingers of the collet against the torsion bar to clamp the torsion bar.