Abstract:
A fastening tool applies parts, in particular plastic parts, to objects. The plastic parts are supplied in the form of a belt in which the plastic parts are linked together by at least one flexible web. The fastening tool transports the belt by a feed mechanism such that one plastic part at a time is delivered to a fastening position. The feed mechanism has a fluid drive that is coupled by a translation-rotation converter to a transport gear rotatably mounted on the fastening tool and designed to positively or nonpositively engage the belt in order to transport it. The translation-rotation converter has an overrunning clutch whose driving gear is kinematically coupled to the fluid drive and whose driven gear is coupled to the transport gear in a rotationally fixed manner.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a fastening tool for applying plastic parts to objects wherein the plastic parts are supplied in the form of a belt in which the plastic parts are linked together by means of at least one flexible web, wherein the fastening tool is designed to transport the belt by means of a feed mechanism such that one plastic part at a time is delivered to a fastening position, and wherein the feed mechanism has a fluid drive that is coupled by a translation-rotation converter to a transport gear that is rotatably mounted on the fastening tool and is designed to positively or nonpositively engage the belt in order to transport it. 
   Such a fastening tool is known from DE 196 43 656 A1. 
   In many areas of prior art, it is necessary to attach parts to objects, for example nails in wooden objects, screws in wooden objects, plastic parts on metal objects such as metal sheets, etc. 
   The attachment of plastic parts, frequently called clips, to prepared or unprepared metal sheets, for example to welding studs, is an economically important field. In the automotive field, a variety of such plastic parts are attached to the vehicle body to perform holding functions, for example to support instrument panels, hold interior trim, carpeting, brake lines, electrical wiring, etc. 
   Since it is frequently necessary to attach a number of identical plastic parts in various positions in rapid sequence, the appropriate feeding of the plastic parts and the proper positioning of the plastic parts in the correct orientation prior to the actual fastening process is especially important. 
   It is an industry standard to provide the plastic parts as bulk material and then to separate them (using a vibrator, for example). Subsequently, one plastic part at a time is brought into fastening position by means of an appropriate feeding device. 
   Alternatively, there is known from EP 0,506,307 B1 a belt wherein the plastic parts are linked one after the other. Here, the plastic parts are molded onto one or two flexible webs during their molding process (generally by injection molding), where the flexible webs are either joined to other flexible webs or are fed into the injection molding machine as a continuous web. The belt is rolled up and transported to the production site in the rolled-up state. 
   This method of supplying plastic parts is offered by the company Tucker GmbH under the name “Plastifast.” 
   A fastening tool for attaching plastic parts supplied in this form is known from the aforementioned DE 196 43 656 A1. 
   In this prior art, the fastening tool has a housing with a handgrip. Supported on the housing is a magazine with two longitudinal webs into which the belt with the plastic parts is inserted. From there, the plastic parts are delivered to a head wherein the plastic parts are deflected in curved guides. Also located on the head is a feed mechanism to transport the belt. 
   The feed mechanism has a transport gear that positively engages the belt. The transport gear is coupled to a pneumatic drive by a translation-rotation converter. The translation-rotation converter has a number of laterally projecting pins at the transport gear. The pitch of these pins corresponds to the pitch of the teeth on the outer circumference of the transport gear. 
   The pneumatic drive has a carrier element that moves translationally back and forth and that “catches” one pin during each backward motion and “pushes against” one pin in front of it during each forward motion in order to turn the transport gear. 
   A complicated ratchet mechanism makes it possible for the carrier element to move backwards without moving the transport gear backwards. 
   The feed mechanism disclosed in this document can be improved from both functional and design standpoints. In order to ensure that a plastic part arrives at a fastening position every time even when problems arise in the transport mechanism, an external handwheel is provided by means of which the transport gear can be turned by hand. 
   Known from EP 0,131,851 A1 is a device for inserting and/or removing screws. In this device, screws are automatically fed, wherein the screws are inserted into a strip-shaped magazine and are pushed into a fastening position by spring pressure. 
   A similar mechanism is also known from U.S. Pat. No. 2,943,652. 
   A “machine-gun-style” lateral feeding of strips of nails to a fastening tool is known from DE 3,606,901 A1. 
   In addition, a fastening tool for inserting screws is known from U.S. Pat. No. 5,144,870. The screws are fed from a belt into a guide. The screws are fed by a spring-loaded drive oriented in the longitudinal direction which has a gear rack which an elastically-preloaded tooth engages. 
   All of the above-mentioned solutions for fastening tools are either very costly or not very reliable in their designs. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In view of this situation, an object of the present invention is to specify a fastening tool which has a feed mechanism that is simple in design and reliable. 
   This object is attained with the aforementioned fastening tool for attaching parts to objects in that the translation-rotation converter has an overrunning clutch whose driving gear is kinematically coupled to the fluid drive and whose driven gear is coupled to the transport gear in a rotationally fixed manner. 
   Overrunning clutches are devices wherein the connection between two shafts is released when the driven shaft rotates faster than the driving shaft. Thus, torque can only be transmitted in one direction of rotation. 
   Such overrunning clutches are commercially available as prefabricated components. 
   In the scope of the present invention, the overrunning clutch makes it possible for a translationally acting fluid drive to convert stroke motions of a certain extent into rotational motions of the transport gear. In contrast, the return stroke is not transmitted to the transport gear. 
   Overall, this results in a simple and robust feed mechanism for the fastening tool. 
   The object is fully attained in this manner. 
   The fluid drive preferably has a piston/cylinder device with a variable stroke. 
   In the present context, “variable stroke” means that the piston/cylinder device need not necessarily perform its full stroke during every actuation process. For example, the stroke depends on the opposing force present. 
   In this way, it is possible to variably achieve a specified stroke through a sum of multiple full strokes and one partial stroke. 
   In this way, the design complexity as a whole is further reduced. 
   It is particularly advantageous when the fluid drive is a pneumatic drive. 
   Compressed air is a working medium that is commonly present in the production facilities in question here. Of course, it is also possible to implement the fluid drive hydraulically as an alternative. 
   In accordance with an embodiment that is preferable as a whole, the driven gear is arranged coaxially inside the driving gear. 
   This has the consequence that the fluid drive can exert a larger torque on the driving gear as a result of its larger radius. It is also advantageous that the driven gear can have a larger axial extent, for example in order to support multiple individual transport gears in a rotationally fixed manner. 
   Alternatively, it is also possible, however, for the driving gear to be located coaxially inside the driven gear. 
   In this case, the driven gear can also be the transport gear. 
   Moreover, it is advantageous for the transport gear to be removably coupled to the driven gear of the overrunning clutch. 
   In this way, it is possible to easily refit the fastening tool for different kinds of parts, specifically in that a transport gear adapted to the spacing-and/or shape of the parts on the particular belt is used. 
   It is also advantageous when the transport gear is embodied as a ring gear that can be pushed onto the driven gear of the overrunning clutch. 
   In this way, the transport gears can be manufactured in a relatively simple way. The rotationally fixed connection can be solely frictional (press-fit). However, a keyed connection or a similar coupling that is positive-acting in the rotational direction is preferred. 
   In a particularly preferred embodiment, the driving gear of the overrunning clutch is coupled to the fluid drive by means of a crank mechanism. 
   Crank mechanisms can be designed so as to be relatively simple and robust. 
   In this regard, it is especially preferred for the driving gear to have a laterally projecting pin that engages a longitudinal recess in a connecting block that is coupled to the fluid drive. 
   It is a matter of course that the longitudinal recess is oriented transverse, and preferably perpendicular, to the stroke direction of the fluid drive. In this way, the longitudinal recess can convert stroke motions into rotational motions of the driving gear and accommodate the resulting motion of the pin transverse to the stroke direction. 
   On the whole, it is additionally advantageous for the fastening position to be defined by a stop. 
   In accordance with an especially preferred embodiment, during a transport process the fluid drive subjects the translation-rotation converter to a drive force by means of which the next plastic part to be fastened is transported to a stop that defines the fastening position. 
   During operation, the fluid drive generally exerts a force (which can be variable on account of the differing torques at the translation-rotation converter) on the transport gear and consequently on the plastic parts to be transported. As a result, it is not necessary to adjust the feed mechanism&#39;s extent of motion to the particular spacing of the plastic parts on the belt. The part in the forwardmost position is transported into the fastening position in that the fluid drive is actuated until the part is in the fastening position. Depending on the spacing of the plastic parts it may be necessary to perform multiple individual strokes during this process. 
   As soon as the plastic part strikes the stop, and thus is in the fastening position, a corresponding opposing force acts on the fluid drive, so that the latter stays in the last position it has assumed. With continuing actuation, the fluid drive then continues to press the plastic part against the stop, for example, so that exact positioning in the fastening position is always ensured. Alternatively, the overrunning clutch implements the position-holding in that the driven gear supports itself thereupon. 
   On the whole, it is further preferred if the driving gear has a predefined scope of rotational motion of less than 180°. 
   This is because it is important that the overrunning clutch property is used such that the return stroke of the fluid drive results in a reverse rotation of the driving gear. As a rule, the scope of rotational motion will be significantly less than 180°. The precise values, however, depend on the design parameters, the tribological properties of the materials used, etc. 
   It is a matter of course that the features described above and those that are described below can be used not just in the combinations described, but also individually or in other combinations, without exceeding the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Example embodiments of the invention are depicted in the drawings and are explained in detail in the descriptions that follow. Shown in: 
       FIG. 1  is a schematic perspective view of a fastening tool in accordance with prior art; 
       FIG. 2  is the head of a fastening tool in accordance with the present invention, in a partially sectional view; 
       FIG. 3  is a schematic side view of the feed mechanism of the head shown in  FIG. 2 ; and 
       FIG. 4  is a schematic partial axial sectional view of the feed mechanism from  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , a fastening tool in accordance with prior art is labeled  10  as a whole. 
   The general construction of the fastening tool corresponds to the fastening tool disclosed in the document DE 196 43 656 A1. 
   The fastening tool  10  serves to attach plastic parts  12  to an object  14 . 
   For this purpose, a plastic part  12  is transported to a forward-located fastening position  18  that is aligned with a longitudinal axis  16  of the fastening tool  10 . 
   In the present case, the plastic part  12  has a cylindrical projection that is inserted into a blind hole of an object  14 . 
   Alternatively, the projection of the plastic part  12  could also be tubular in design. In this case, there would for example be a projecting stud provided on the object  14 , welded on by a stud-welding process, for example. 
   The fastening tool  10  is not limited to the plastic part  12  shown. Rather, a number of different plastic parts  12  can be fastened to a wide variety of different objects  14  using the fastening tool  10 . 
   In every case, the plastic parts  12  are provided in the form of a belt  20 . The belt  20  has two parallel flexible webs  22 ,  24 . The plastic parts  12  are injection-molded onto the webs  22 ,  24  with a predefined spacing A. 
   Such a belt is known in principle from the aforementioned EP 0,506,307 B1. 
   The belt  20  is inserted into the fastening tool  10  from behind in a feeder device  26  that is oriented approximately parallel to the longitudinal axis  16 . The belt can, for example, be provided rolled up on appropriate carriers and is automatically unrolled therefrom by the fastening tool  10  as described below. 
   The fastening tool  10  has a housing  30 , from the bottom of which extends a hand grip  32 . 
   Supply lines  34  are connected to the bottom of the hand grip, in the present case a compressed air line and an electrical supply line. 
   A switch  36  for actuation by the index finger is provided on the hand grip. 
   Actuating the switch  36  causes the plastic part  12  that is located in the fastening position  18  to be separated from the webs  22 ,  24  and applied to the object  14 . Moreover, the following plastic part  12 ′ is subsequently transported to the fastening position  18 . 
   The housing  30  also has a magazine holder  38  on which an elongated magazine  40  is supported. The magazine  40  has two parallel longitudinal profiles  42 ,  44  that are oriented approximately parallel to the longitudinal axis  16 . The belt  20  is passed between the longitudinal profiles  42 ,  44 . The magazine  40  has a length such that a predefined number of plastic parts  12 , in the present case approximately six, are lined up one behind the other in the magazine  40 . The belt  20  is held in a stabilized manner in the axial direction in the magazine  40 . Moreover, the magazine  40  is designed so as to swivel about the longitudinal axis  16 , as is schematically indicated under  46 . 
   In addition, the housing  30  has a head  48  that adjoins the front of the magazine  40 . When the magazine  40  swivels, the head  48  is swiveled with it. 
   Provided on the head  48  are two curved profiles  50 ,  52  which constitute a continuation of the longitudinal profiles  42 ,  44  and deflect the belt  20  by approximately 90°. 
   A transverse stop  54  that defines the fastening position  18  is located at the end of the curved profiles  50 ,  52 . 
   Moreover, located below the head  48  is a collecting means  56  for the webs  20 ,  22 , which are fed out downward after the plastic parts  12  are separated at the fastening position  18 . 
     FIG. 2  shows a perspective view of the head  48  in a partially sectional representation. 
   It can be seen that a ram  60  aligned with the longitudinal axis  16  is provided in the head  48 . When the fastening tool  10  is actuated, the ram  60  is moved forward to separate the plastic part  12  that is located in the fastening position  18  from the webs  20 ,  22  and attach it to the object  14 . 
   Moreover, a feed mechanism  62  is provided in order to further transport the belt  20  after each fastening operation so that the next plastic part  12 ′ arrives at the fastening position  18 . 
   The construction of the fastening tool  10  described thus far corresponds to the construction of the fastening tool that is known from the aforementioned DE 196 43 656 A1. 
   The present fastening tool  10  differs therefrom in particular through the feed mechanism  62 . 
   The feed mechanism  62  has a fluid drive in the form of a piston/cylinder device  64  aligned approximately parallel to the longitudinal axis  16 . The piston/cylinder device  64  is connected to the pneumatic line  34 , which is not shown in the figures. 
   A piston  66  of the piston/cylinder device  64  extends forward, parallel to the longitudinal axis  16 , and is rigidly connected to a connecting block  68 . 
   The connecting block  68  has a connecting hole  70  perpendicular to the piston axis. 
   An overrunning clutch device  72  is provided next to the connecting block  68 . 
   The overrunning clutch device  72  has an external driving gear  74  and an internal driven gear  76 , with an overrunning hub  78  disposed between them. The structure of such overrunning hubs is known per se, so the internal structure is not described in detail here. 
   Provided on the driving gear  74  is a laterally projecting pin  80  that engages in the elongated hole  70 . 
   During forward stroke motions of the piston/cylinder device  64  (out of the plane of the picture in  FIG. 2 ), the pin  80  is carried along by the connecting block  68  so that the driving gear  74  turns. The overrunning hub  78  is arranged such that the driven gear  76  is carried along in this rotation. 
   On the return stroke of the piston/cylinder device  64 , the connecting block  68  is moved back and again takes the pin  80  with it. During this process, the driving gear  74  is rotated back to the starting position. The driven gear  76  remains in its previous position. 
   Hence, the connecting block  68 , the elongated hole  70 , the overrunning clutch device  72  and the pin  80  constitute a translation-rotation converter. 
   In the present case, the driven gear  76  is designed as a shaft that runs transverse to the longitudinal axis  16  over nearly the entire width of the head  48 . 
   Two transport gears  84 ,  86  of a transport gear arrangement  82  are attached to the driven gear  76  in a rotationally fixed manner. 
   The transport gears  84 ,  86  are each arranged in the vicinity of the curved profiles  50  or  52 , respectively, and engage the belt  20  symmetrically about a center line which is not shown. In other words, the plastic parts  12  are transported between the two transport gears  84 ,  86 . 
   The outer circumferences of the transport gears  84 ,  86  are provided with teeth  88  in order to be able to positively engage the belt  20 . 
   The function of the feed mechanism  62  and of the associated overrunning clutch device  72  is described below on the basis of schematic  FIGS. 3 and 4 . 
     FIG. 3  shows the direction of rotation  90  of the driving gear  74  during a forward stroke of the piston/cylinder device  64 . The direction of rotation of the driving gear  74  during a return stroke is labeled  92 . The only direction of rotation of the driven gear  76  is labeled  94 . The directions of rotation  90  and  94  are identical. The only direction of rotation of the transport gear  84  is labeled  95 . 
   The teeth  88  are spaced apart in the circumferential direction by an angle α A  which corresponds to the spacing A of the plastic parts  12  on the belt  20 . The teeth  88  can be radially slotted in order to accommodate the webs  20  and  22 , but this is not shown in the diagrams. The longitudinal extent of the elongated hole  70  is labeled h. The longitudinal extent h of the elongated hole  70  determines the extent of motion of the driving gear  74 . Starting from the center position shown in  FIG. 3 , the driving gear can be moved in the direction of rotation  90  until the pin  80  strikes the lower end of the elongated hole  70 , indicated by  80 ′. In the opposite direction of rotation  92 , the corresponding position of the pin  80  is labeled  80 ″. 
   Thus, a maximum angle of rotation α max  of the driving gear  74  of less than 180° results. It is a matter of course that the angle α max  must be significantly smaller than 180° in order to avoid self-locking, at least when the friction between the pin  80  and the internal circumference of the elongated hole  70  is not negligible. In  FIG. 3 , the angle α max  is approximately 130°. However, significantly smaller and larger angles can be chosen as a function of tribological boundary conditions. 
   The longitudinal extent h of the elongated hole  70  is always smaller than the radius r that separates the pin  80  from the center point of the overrunning clutch device  72 . This ensures that the driving gear  74  does not execute a full rotation, but instead is only turned back and forth. 
   The stroke distance of the piston/cylinder device  64  defined by the distance between the positions  80 ′ and  80 ″ is shown as H. 
   It can also be seen from  FIG. 4  that the driven gear  76 , which is only partially visible, can be provided with a keyway  96  to make it possible to easily connect the transport gears  84 ,  86  to the driven gear  76  in a rotationally fixed manner. 
   During operation, the transport of a subsequent plastic part  12 ′ into the fastening position  18  is performed as follows. 
   The piston/cylinder device  64  is subjected to compressed air. Consequently, the driving gear  74  is rotated in the direction of rotation  90 . If the plastic part  12 ′ arrives at the fastening position  18  before the pin  80  is in the end position  80 ′, the transport process is completed. However, the piston/cylinder device  64  continues to be subjected to compressed air in order to hold the plastic part  12 ′ in the fastening position  18 . Alternatively, the supply of compressed air to the piston/cylinder device  64  is stopped as soon as the plastic part  12 ′ is in the fastening position  18 . 
   If the plastic part  12 ′ has not yet reached the fastening position  18  when the pin  80  is in the end position  80 ′, a return stroke of the connecting block  68  takes place. As a result of the overrunning hub  76 , the transport gears  84 ,  86  remain in the position they have reached. As soon as the connecting block has brought the pin  80  into the position  80 ″, the stroke direction is reversed again. The driving gear  74  then carries the driven gear and consequently the transport gears  84 ,  86  with it once more. This process is continued until the plastic part  12 ′ is in the fastening position  18 . 
   Of course, the number of repetitions is a function of the stroke H and of the spacing A of the plastic parts  12  on the belt  20 . 
   If plastic parts other than the plastic parts  12  shown are to be fastened using the fastening tool  10 , the transport gears  84 ,  86  are exchanged for transport gears that are matched to the other plastic parts. It is a matter of course that for the other transport gears  84 ,  86  the shape of the teeth  88  as well as their angular spacing α A  can be different from those of the transport gears  84 ,  86  shown. However, the effective outer diameter of the transport gears  84 ,  86  is the same for all different types of plastic parts. Adaptation in this regard is not necessary, since the transport mechanism  62  shown can achieve the spacing A corresponding to one transport step using either one stroke H, a partial stroke, or a number of strokes H plus a partial stroke if required. 
   If only one partial stroke is necessary, it is possible to either design the piston/cylinder device with a variable stroke such that it again initiates a return stroke starting at the end position of the partial stroke, or alternatively, it is also possible to keep the stroke constant and to continue it from a given position that corresponds to that of a plastic part in the attachment position  18 . 
   Instead of an arrangement of the driving gear coaxially outside the driven gear, it is also possible to arrange the driving gear coaxially inside of the driven gear. 
   Moreover, in the present case it is advantageous with regard to the assembled depth in the direction of the width of the fastening tool  10  for the transport gears  84 ,  86  to be embodied as ring gears. However, it is also possible to design them as solid gears and to provide other means for rotationally fixed coupling with the driven gear  76 . 
   Instead of the crank mechanism with the connecting block  68  and the pin  80 , other crank mechanisms or other types of translation-rotation converters are also possible.