BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a hand-held drive-in tool for driving fastening elements in a constructional component or a workpiece and including a drive-in ram displaceably supported in a guide for driving fastening elements in the constructional component or the workpiece, a drive for driving the drive-in ram and having a driving spring member for displacing the drive-in ram, a device for preloading the driving spring member, a locking device having a locking position in which the locking device retains the driving spring member in its preloaded position and including locking means. 
   2. Description of the Prior Art 
   In a drive-in tool described above, the driving spring member is formed as a mechanical spring. 
   The advantage of drive-in tools of the type described above consists in using a low-cost mechanical driving spring member, which permits to economically manufacture this type of drive-in tools. Because a preloading process can last only from ten to several hundred milliseconds, in particular when very strong driving springs are used, it is advantageous when the preloading process is already completely carried out before actuation of the actuation switch of the drive-in tool. It is further necessary that the driving spring is fixed in its preloaded position before actuation of the drive-in process by a locking device, directly or indirectly, e.g., via another element such as the drive-in ram. 
   A drive-in tool of the type described above is disclosed in U.S. Pat. No. 3,847,322. In the disclosed drive-in tool, a drive-in ram is preloaded against a driving spring member by a motor-driven preloading mechanism. A locking device retains the drive-in ram and the driving spring member in the preloaded position. To this end, the locking device has locking means that lockingly engages a locking surface on the drive-in ram. The locking device is released by an actuation switch, whereby the locking device is lifted off its locking position by a motor-driven mechanism and is displaced in a release position. In the release position of the locking device, the drive-in ram is displaced in the setting direction by the biasing force of the driving spring member for driving a fastening element in a workpiece. 
   The drawback of the known drive-in tool consists in that the sliding friction between the locking means and the locking surface is relatively high, so that lifting of the locking means off is rather, slow-going. Furthermore, loss of the material due to the sliding friction under a surface pressure is rather high. 
   Accordingly, an object of the present invention is a drive-in tool in which the above-discussed drawbacks a known drive-in tool is eliminated. 
   SUMMARY OF THE INVENTION 
   This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing a drive-in tool in which the drive-in ram is provided with a locking stop, and the locking device includes an annular member pivotally arranged on the locking device and rollable off the locking stop. 
   With the rollable-off annular member, the frictional resistance is noticeably reduced upon release of the annular member that serves as a locking member, because instead of the sliding friction, a much smaller rolling friction takes place as the annular member rolls off the locking stop. The release of the locking member is easy-going and requires, therefore, much less energy. Further, the wear of the cooperating parts is noticeably reduced. 
   Advantageously, the locking device includes a support pivotable about a pivot axis and the annular member is supported on a rotational axis provided on the support. The rotational axis of the annular member extends parallel to the pivot axis of the support. 
   Thereby, all frictional losses except the rolling friction losses are prevented. 
   Advantageously, the annular member is formed as a roller bearing, which minimizes the rolling friction. The roller bearings are cost-effective as standard parts are used. Thereby, no high additional costs are involved. 
   It is further advantageous when a contact of the annular member with the locking stop is spaced, in a direction opposite a direction of a locking pivotal movement of the support from a point of the locking stop closest to the pivot axis of the support. 
   The novel features of the present invention, which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiment, when read with reference to the accompanying drawings. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings show: 
       FIG. 1  a longitudinal cross-sectional view of a drive-in tool according to the present invention; 
       FIG. 2  a view similar to that of  FIG. 1 , with the drive-in tool in an actuated condition; and 
       FIG. 3  a detail of the drive-in tool showing the portion III in  FIG. 1 . 
   


   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A drive-in tool  10  according to the present invention, which is shown in  FIG. 1 , includes a housing  11  and a drive generally designated with a reference numeral  30  and arranged in the housing  11 . The drive  30  drives a drive-in ram  13  displaceable in a guide  12  likewise located in the housing  11 . The drive-in ram  13  has a drive-in section  14  for driving a fastening element  60  and a head section  15 . 
   A bolt guide  17 , which is arranged coaxially with the guide  12 , adjoins the guide  12  at the end of the guide  12  facing in the drive-in direction  27 . A magazine  61 , in which fastening elements  60  are stored, projects sidewise from the bolt guide  17 . 
   The drive  30  includes a driving spring member  31  which is indirectly supported against the housing  11  at a support location  36  at one of its ends and which engages the head section  15  of the drive-in ram  13  with another of its end. The driving spring member can be formed, e.g., as a composite spring or as a steel spring and is formed as e.g., a helical spring. 
   In its loaded condition  22 , which is shown in  FIG. 1 , the drive-in ram  13  is resiliently preloaded against the driving spring member  31  and has its head section  15  inserted in a cylindrical guide space  37  defined by the driving spring member  31  and the support location  36  for the driving spring member  31 . The possibility to displace the head section  15  in the guide space  37  within the means defining the support location and, in particular, within the driving spring member  31  permits to obtain advantageously a compact construction. 
   In the loaded position  22 , the drive-in ram  13  is retained by a locking device generally designated with a reference numeral  50 . The locking device  50  has a locking member in form of annular member  59  that is rotatably supported on a support  51 . In a locking position  54  (see  FIG. 1 ), the annular member  59  engages with a locking surface  59  a locking stop  53  formed on a projection  58  of the drive-in ram  13 , retaining the drive-in ram  13  against action of the biasing force of the driving spring member  31 . The locking surface  59  circumferentially surrounds the annular member  49  that is formed, e.g., as a roller bearing. With the use of a roller bearing as the annular member  49 , the rolling friction can be reduced to a minimum. The support  51  is formed as a pivot arm supported on a shaft  48  of a servo motor  52  which displaces the locking device  50  in its release position  55  shown in  FIG. 2 , as it would be described further below. The shaft  48  defines a pivot axis A around which the support  51  pivots. The rotational axis D of the annular member  49  extends parallel to the pivot axis A of the support  51  in order to prevent any other friction losses except the occurring roller friction. As shown in  FIG. 3 , the contact K of the locking surface  59  or of the annular member  49  with the locking stop  53  is spaced from a point P of the locking stop  53  closest to the pivot axis A, with respect to the closing direction S (the arrow) of the support  51 , whereby the support  51 , together with the annular member  49 , forms self-closing locking means. 
   The servo motor  52  is connected by a first electrical conductor  56  with the tool control unit  23  that controls the operation of the servo motor  52 . 
   The drive-in tool  10  has a handle  20  on which an actuation switch  19  for actuating a drive-in process with the drive-in tool is arranged. Further, a receptacle  18  for receiving a network-dependent power source  21  is provided in the handle  20 . The power source  21  supplies the drive-in tool  10  with the electrical energy. In the disclosed embodiment, the power source  21  has at least one accumulator. The power source  21  is connected with the control unit  23  and the actuation switch  19  by respective supply conductors  24 . A switching conductor  57  connects the control unit  23  with the actuation switch  19 . 
   On the muzzle  62  of the drive-in tool  10 , there is provided a switch  29  connected with the control unit  23  by a switching conductor  28 . The switch  29  communicates an electrical signal to the control unit  23  as soon as the drive-in tool  10  is pressed again the workpiece U, as shown in  FIG. 2 . Thereby, the switch  29  insures that the control unit  23  initiates a setting process with the drive-in tool  10  only then when the drive-in tool  10  is properly pressed against the workpiece U. 
   The drive-in tool  10  further has a tensioning or preloading device, which is generally designated, with a reference numeral  70 . The preloading device  70  has a motor  71  for driving a drive roller  72 . A second control conductor  74  electrically connects the motor  71  with the control unit  23  that actuates the motor  71  when, e.g., the drive-in ram  13  is located in its end position in the drive-in direction  27  or when the drive-in tool  10  is lifted off the workpiece U. The motor  71  has an output member  75  such as a driven gear which is connectable with the drive roller  72 . The drive roller  72  is supported rotatably on a longitudinally adjustable control arm  78  of an adjustment element  76  formed as a solenoid. The adjustment element  76  is connected with the control unit  23  by an adjustment conductor  77 . During an operation, the drive roller  72  is connected with the output member  75  that rotates the drive roller  72  in the direction of arrow  73  shown with dash lines. 
   When the drive-in tool  10  is actuated by a main switch, not shown, the control unit  23  firstly ascertains that the drive-in ram  13  is located in its preloaded position  22  shown in  FIG. 1 . If this is not the case, then the adjustment element  76  displaces the drive roller  72  into engagement with the output member  75  driven by the motor  71 . Simultaneously, the drive roller  72  engages the drive-in ram  13  which is displaceable by the drive roller  72  rotatable in the direction of arrow  73 , in the direction of the drive  30 . This preloads the driving spring member  31  of the drive  30 . When the drive-in ram  13  and the driving spring member  31  reach their preloaded or initial position  22 , the annular member  49  engages with its locking surface  59  the locking stop  53  of the drive-in ram  13 , retaining the drive-in ram in the preloaded position  22 . To this end, the support  51 , together with the annular member  49 , can be spring-biased in the direction of the drive-in ram  13  or into a position in which it engages a stop  47 . 
   Upon engagement of the annular member  49  with the locking stop  53 , the motor  71  can be turned off by the control unit  23  and the adjustment element  76 , which is also controlled by the control unit  23 , displaces the drive roller  72  from the position in which it engages the output element  75  and the drive-in ram  13 , into its disengaged position. 
   When the drive-in tool  10  is pressed against a workpiece U, as shown in  FIG. 2 , firstly, the switch  29  sets the control unit  23  into a setting-ready condition. Upon actuation of the actuation switch  19  by a user, the control unit  23  displaces the locking device  50  in its release position  55  in which the support  51 , together with the annular member  49 , is lifted off the drive-in ram  13 . At that, the annular member  49  rolls with its locking surface  59  along the locking stop  53  on the projection  58  downwardly with a very small rolling resistance. 
   The locking surface  59  must not be continuous but also can be discontinuous, e.g., be formed as a structural or profiled surface. 
   Upon lifting of the locking device  50  off, the driving spring member  31  of the drive  30  displaces the drive-in ram  13  in the drive-in direction  27 , whereby the fastening element  60  is driven in the workpiece U. 
   For returning the drive-in ram  13  and for preloading the driving spring member  31 , at the end of the drive-in process, the preloading device  70  is actuated by the control unit  23 , when the drive-in tool  10  is lifted again off the workpiece U. To this end, the switch  29  communicates an appropriate signal to the control unit  23 . The preloading device  70  displaces the drive-in ram  13  in the above described manner against the driving spring member  31  of the drive  30 , preloading the driving spring member  31 . The drive-in ram  13  displaces the driving spring member  31  until the support  51 , together with the annular member  49  is displaced again into its locking position  54  on the locking stop  53  on the drive-in ram  13 . 
   Though the present invention was shown and described with references to the preferred embodiment, such is merely illustrative of the present invention and is not to be construed as a limitation thereof and various modifications of the present invention will be apparent to those skilled in the art. It is therefore not intended that the present invention be limited to the disclosed embodiment or details thereof, and the present invention includes all variations and/or alternative embodiments within the spirit and scope of the present invention as defined by the appended claims.
A hand-held drive-in tool for driving fastening elements in includes a drive-in ram ( 13) displaceably supported in the guide ( 12), a spring member ( 31) for displacing the drive-in ram ( 13), a device ( 70) for preloading the driving spring member ( 31) and a locking device ( 50) having a locking position ( 54) in which the locking device ( 50) retains the driving spring member ( 31) in its preloaded position ( 22) and includes an annular member ( 49) pivotally arranged on the locking device ( 50) and rollable off the locking stop ( 53) provided on the drive-in ram ( 13).