Abstract:
A tensioning device is provided for an endless drive device, having a housing, a spring-loadable tensioning piston in a piston bore of the housing, a transport locking device, and a device to adjust the working range of the tensioning piston with a retainer element on the housing having at least one spring arm and a retainer profile arranged in the forward section of the tensioning piston in the tensioning direction. The transport locking device comprises a transport groove in the tensioning piston, the spring arm of the retainer element being engageable into the transport groove, and an unlocking element, wherein the unlocking element has at least one ramp section potentially in functional contact with the spring arm. The ramp section exerts an unlocking force on the spring arm over a travel path of the unlocking element relative to the tensioning piston, forcing said spring arm out of the transport groove.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to foreign German patent application No. DE 102012001074.5, filed on Jan. 20, 2012, the disclosure of which is incorporated by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention refers to a tensioning device for an endless drive device, such as a chain or belt, having a housing, a tensioning piston guided in a sliding manner against a spring in a piston bore of the housing, an adjustment device to adjust the working range of the tensioning piston, and a transport locking device, whereby the adjustment device comprises a retainer element arranged on the housing having at least one spring arm and a retainer profile arranged in the forward section of the tensioning piston in the tensioning direction. 
       BACKGROUND 
       [0003]    Such a tensioning device is known from DE 10 2009 035923 A1, for instance. The tensioning piston is held in its retracted transport state by means of a transport locking device. In this transport state, a compression spring that is compressed under tension is located in a pressure reservoir and pushes against the tensioning piston. The transport locking device comprises a straight retainer pin, which engages into corresponding eyes on the housing of the chain tensioner and comes to rest on the tensioning piston with one shoulder, therefore preventing the tensioning piston from being deployed further. This design is a flange tensioner, which in this transport state is brought into its predetermined position on the engine block by means of corresponding fastening devices. After the fastening to the engine block, the retainer pin is pulled and the tensioning piston is deployed due to the spring force of the compression spring arranged in the pressure reservoir. A type of retainer clamp is seated in the front section of the housing, where said retainer clamp engages into a corresponding retainer profile on the exterior surface of the tensioning piston with its two opposing spring arms. The retainer clamp is designed in such manner that it fulfills several functions. The spring force pushes the tensioning piston out far enough against the push contact surface of a pivoting tensioning rail that a chain, which is in contact with this tensioning rail, is correspondingly tensioned. This is typically a timing chain of a timing chain drive for an internal combustion engine. The spring arms of the retainer clamp engage into one of the retainer grooves of the retainer profile, causing the retainer clamp now to come to rest on the tensioning piston and be able to move together with said tensioning piston. Because of correspondingly shaped pocket sections, the retainer clamp can move up and down relative to the housing over a defined working range. The housing has suitable protrusions that engage into these pocket sections of the retainer clamp, serving as upper or lower stops for the retainer clamp. The retainer clamp and the retainer profile are configured in such a way that while the tensioning piston can be deployed beyond the working range to accommodate wear related chain lengthening so that the retainer clamp can be brought into engagement with another retainer groove, it is no longer possible for the tensioning piston to be retracted beyond the working range. This results in an automatic adjustment of the working range depending on the wear state (lengthening) of the chain. 
         [0004]    This known chain tensioner has the disadvantage that it can only be configured as a flange tensioner due to the transport locking device. 
         [0005]    A tensioning device configured as a screw-in chain tensioner is described by WO 2009/024196 A1, for instance. This device is equipped with a reversibly, radially expandable locking sleeve that is attached on the front end of the housing. An arresting ring is arranged at the front end of the tensioning piston that is initially located outside of the locking sleeve. Activating the tensioning device exerts pressure on the tensioning piston, so that the arresting ring is inserted into the radially expandable locking sleeve, causing said locking sleeve to expand radially. The arresting ring&#39;s travel is afterward restricted to a predefined region within the locking sleeve. The arresting ring can then again travel from retainer groove to retainer groove on a retainer profile of the tensioning piston as a function of the chain lengthening; however this travel is only in one direction, with the ring locking in the other direction. This design has the disadvantage that it has no transport locking device and that the tensioning piston must be completely retracted before its first use, so that the arresting ring enters the locking sleeve. To do so, the tensioning piston must be retracted against the force of the compression spring, either during assembly or in the installed state. Due to the constrained space in the engine compartment, this activation of the adjustment function is frequently difficult. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore the object of the present invention to provide a tensioning device of the previously named type, which provides in an advantageous manner a transport locking device and also an adjustment device for screwed-in or inserted housings. 
         [0007]    This object is attained according to the invention in that the transport locking device comprises a transport groove in the tensioning piston, the spring arm of the retainer element, where said spring arm can be engaged in the transport groove, and an unlocking element, and the unlocking element has at least one ramp section that can be brought into functional contact with the spring arm, where said ramp section exerts an unlocking force on the spring arm over a specified travel distance of the unlocking element relative to the tensioning piston, forcing the spring arm from the transport groove. This configuration precisely differentiates between the retainer profile, which provides for the adjustment of the working range in interaction with the spring arm, and a transport groove into which the spring arm engages without the tensioning piston being able to deploy. A suitably configured unlocking element then forces the spring arm from the transport groove, permitting the tensioning piston to deploy. Preferably, the unlocking element executes a guided path of motion, resulting in a defined interaction between the ramp section and the spring arm. Ideally, the unlocking element is connected with a moving element of the tensioning device, or configured as a unit with the tensioning device. 
         [0008]    According to a favorable configuration, the unlocking element is equipped with an entrainment device that can be brought into functional contact with the tensioning piston to ensure that the at least one spring arm can be securely engaged with the retainer profile after the unlocking process, where said entrainment device is configured and positioned in such a manner that the entrainment device is brought into functional contact with the deploying tensioning piston after the spring arm is disengaged from the transport groove, the tensioning piston carries the unlocking element along, and the ramp section is therefore brought out of functional contact with the spring arm of the retainer element and engages into the retainer profile. Due to the spring force, the deploying tensioning piston thus again forces the ramp section out of engagement with the spring arm, so that the spring arm again moves toward the tensioning piston and can engage into the retainer profile. This process is non-critical because the spring arm by itself cannot prevent the deployment of the tensioning piston in the interaction between the spring arm and the retainer profile, but this is instead achieved by the pressing onto the tensioning rail, which is in contact with the chain. 
         [0009]    For reasons of safety and improved provision of function, it is advantageous according to one version for the retainer element to have two opposing spring arms that function as a type of clamp, and the unlocking element to have a chevron type ramp section that can be inserted between the two spring arms, causing the spring arms to be spread apart and to be disengaged from the transport groove. The tensioning piston is accordingly encompassed by the unlocking element in a clamp-like manner. When the spring arms are in contact with the tensioning piston it is not mandatory that these continue to exert a force. However, a corresponding spring force is required to move the spring arms to the outside, away from the tensioning piston, where said spring force is required for the unlocking or adjustment processes. The ramp sections accordingly point away from each other in such a manner that they can be inserted between the two spring arms and force these away from each other, causing the retainer element to be spread apart. This process causes the two spring arms to glide out of the transport groove. 
         [0010]    A particularly simple configuration forms the retainer element from a bent round wire clamp, the spring arms of which are attached by means of stop tabs that encompass the housing at least in sections, where said stop tabs can be brought into functional contact with stop protrusions on the housing in order to adjust the working range. These stop tabs, which are preferably formed as a single part from the round wire, therefore constitute the stop for the continued upward travel of the retainer element. During the adjustment process the tensioning piston is forced further from the housing, whereas the retainer element contacts the stop protrusions with the stop tabs and is prevented from traveling further. Due to the retainer profile, this in turn results in a spreading apart of the at least one spring arm, whereby said spring arm can engage into another location of the retainer profile. 
         [0011]    Ideally, another version provides that the spring arms, which are engaged with the retainer profile, protrude radially in such a manner that the spring arms can be brought into contact with stop surfaces on the housing on the retracting end of the working range when the tensioning piston is retracted in order to prevent a further retraction of the tensioning piston. The interaction between the stop surfaces and the radially protruding spring arms causes a blockage of the retraction travel of the tensioning piston. This can require a corresponding shape of the stop surfaces (e.g. in the shape of a cone, etc.), causing the at least one spring arm to be forced more strongly in the direction of the tensioning piston. The retainer profile can also be configured in such a manner that a gliding out of the at least one spring arm from the retainer profile is not possible when contacting the stop surface. 
         [0012]    According to a preferred embodiment, the tensioning device comprises a tensioning rail, whereby the unlocking element is arranged on the tensioning rail. The retainer element therefore interacts with an unlocking element on the tensioning rail. An unlocking accordingly can only occur when these two elements interact with each other. This generally occurs during the assembly process, in that the retainer element moves toward the tensioning rail, or the tensioning rail is pivoted in the direction of the retainer element. 
         [0013]    In an advantageous arrangement, the entrainment device can be formed by the push contact surface of the tensioning rail, which comes into functional contact with a front face of the tensioning piston. First, the unlocking element therefore comes into contact with the retainer element and disengages the at least one spring arm, causing the tensioning piston to propel from the housing due to the spring force. After a generally short travel, the front face of the tensioning piston then impacts the push contact surface of the tensioning rail, and subsequently causes a pivoting motion of the tensioning rail until the chain is adequately tensioned. Because this causes the tensioning piston to also be deployed further and the tensioning rail to be pivoted back, the unlocking element is also disengaged from the retainer element, causing the spring arm to spring back and to engage with the retainer profile. 
         [0014]    A further configuration provides that the unlocking element is arranged such that it can slide in the tensioning direction in the front section of the tensioning piston and opposite to the tensioning direction relative to the tensioning piston. The unlocking element is therefore guided by the tensioning piston itself and comes to rest on said tensioning piston, and is correspondingly arranged such that it can slide for the unlocking process. Because of this measure, no special designs must be arranged on other components, such as the tensioning rail. All elements necessary for the unlocking and adjustment functions are therefore combined into a single sub-assembly. 
         [0015]    Preferably, the unlocking element can be ring-shaped in design and be mated on the front terminating section of the tensioning piston and be secured by means of at least one retainer tab that engages on the tensioning piston. Due to the retainer tab, certain positions of the unlocking element can be specified along the terminating section of the tensioning piston. Position specifications for the unlocking element can accordingly be made before and also after the unlocking process. 
         [0016]    In connection with this, it is advantageous if the at least one retainer tab engages into the transport groove in a transport state, and the unlocking element protrudes past the terminating section of the tensioning piston in this transport state. This causes the unlocking element to first come into contact with the tensioning rail before the tensioning piston contacts the tensioning rail. Pushing the unlocking element onto the tensioning rail can therefore be employed for unlocking purposes. 
         [0017]    Moreover, the unlocking element can have at least one spreader arm that protrudes in a direction parallel to the axis opposite to the tensioning direction of the tensioning piston, where the at least one ramp section and/or the at least one retainer tab is arranged on said spreader arm. When the unlocking element slides in the axial direction, this spreader arm can then force the at least one spring arm away from the tensioning piston, so that said tensioning piston comes out from the unlocking groove. At the same time, or alternatively, the spreader arm can also have the at least one retainer tab, so that the starting position of the unlocking element and/or also the terminating position is secured on the tensioning piston after the unlocking process. 
         [0018]    One version correspondingly provides that the at least one retainer tab is configured and positioned in such a manner that after an unlocking and relative sliding of the tensioning piston, the at least one retainer tab engages into the retainer profile, securing the unlocking element in this unlocked state. 
         [0019]    Preferably, the overall height of the unlocking element and the position of the at least one retainer tab can be selected in such a manner that the tensioning piston protrudes from the unlocking element in the unlocked state. This ensures that in normal operation of the tensioning device, the unlocking element is secured in a position on the tensioning piston that renders it essentially without function, and where said unlocking element preferably has no contact with the tensioning rail. 
         [0020]    According to one version, the entrainment device can ideally be formed by a stop shoulder arranged in the ring-shaped unlocking element, where said stop shoulder comes into contact with a stop on the tensioning piston, so that the unlocking element can move together with the deploying tensioning piston and can be disengaged from the retainer element. 
         [0021]    According to one configuration, the retainer profile is formed by ring grooves arranged adjacent to each other below the transport groove, whereby the retainer tab in the unlocked state engages into the ring groove that is physically closest to the transport groove. This makes further measures on the tensioning piston unnecessary to secure the unlocking element in the unlocked state on the tensioning piston, but instead permits the use of already existing structures. 
         [0022]    Moreover, the invention refers to a chain drive, in particular a timing or auxiliary device drive of an internal combustion engine, having a drive chain sprocket, at least one driven chain sprocket, a drive chain that couples the drive chain sprocket and the at least one driven chain sprocket to each other, and a tensioning device in accordance with one of the claims  1  to  14  that tensions the drive chain. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The following provides a more detailed, drawing-based discussion of embodiments of the present invention. Shown are: 
           [0024]      FIG. 1  a perspective schematic representation of a timing chain drive in accordance with a first embodiment, 
           [0025]      FIG. 2  a schematic side view of the tensioning device from  FIG. 1  in the installation state before the unlocking, 
           [0026]      FIG. 3  a perspective representation of the housing of the chain tensioner from  FIG. 1 , 
           [0027]      FIG. 4  a magnified perspective representation of the tensioning piston of the chain tensioner from  FIG. 1 , 
           [0028]      FIG. 5  a magnified perspective representation of the retainer element from  FIG. 1 , 
           [0029]      FIG. 6  the tensioning device from  FIG. 1  at the beginning of the unlocking process, 
           [0030]      FIG. 7  a perspective representation of a section of the tensioning device during the unlocking process, 
           [0031]      FIG. 8  a side view of the tensioning device from  FIG. 1  after the unlocking process, 
           [0032]      FIG. 9  a second embodiment of a chain tensioner according to the invention in a front view, 
           [0033]      FIG. 10  the chain tensioner from  FIG. 9  cross-sectioned along the line X-X, 
           [0034]      FIG. 11  the housing of the chain tensioner from  FIG. 9  in a perspective representation, 
           [0035]      FIG. 12  the housing from  FIG. 11  in a full cross-section, 
           [0036]      FIG. 13  the piston of the chain tensioner from  FIG. 9  in a full cross-section, 
           [0037]      FIG. 14  the retainer element from  FIG. 9  in a magnified front view, 
           [0038]      FIG. 15  the retainer element from  FIG. 14  in a back view, 
           [0039]      FIG. 16  the unlocking element from  FIG. 9  in a magnified full cross-section, 
           [0040]      FIG. 17  the unlocking element from  FIG. 16  in a half cross-section rotated by 90°, 
           [0041]      FIG. 18  the upper section of the chain tensioner before the unlocking in a magnified front view, 
           [0042]      FIG. 19  a magnified perspective representation of the state as in  FIG. 18 , 
           [0043]      FIG. 20  the upper section of the chain tensioner from  FIG. 9  during the unlocking process in a magnified front view, 
           [0044]      FIG. 21  a perspective representation of the state as in  FIG. 20 , 
           [0045]      FIG. 22  the upper section of the chain tensioner from  FIG. 9  after the unlocking in a magnified front view, and 
           [0046]      FIG. 23  the upper section of the chain tensioner in the state as in  FIG. 22  in a magnified perspective representation (partially cross-sectioned). 
       
    
    
       [0047]      FIG. 1 to 8  will now be employed to provide a more detailed discussion of a first embodiment of a tensioning device according to the invention as follows. 
       DETAILED DESCRIPTION 
       [0048]      FIG. 1  is a representation of the significant components making up a timing chain drive  1 . This is comprised of a chain tensioner  2 , a pivot bolt  3  around which a pivoting tensioning rail  4  is arranged, a guide rail  5 , a timing chain  6  and—only shown schematically—the camshaft chain sprockets  7  and  8  and the crankshaft chain sprocket  9 . The chain drive  1  couples the crankshaft (not shown) with the camshafts (not shown). The guide rail  5  is arranged in the driving side of the timing chain drive  1 , whereas the tensioning rail  4  is pressed against the slack span of the timing chain drivel. 
         [0049]    The chain tensioner  2  is an insert chain tensioner, the housing  10  of which is inserted into a corresponding receiver  11  on the engine block, which is not shown in detail. For this purpose, housing  10  has a fastening flange  12  with attachment bores  13  and a ring-shaped fully circumferential flange  14  that is arranged at a distance to these, the exterior surface of which has a ring groove  15  to accept an O-ring. The housing  10  furthermore has a tube-shaped section  17  extending vertically away from fastening flange  12  attached by means of a tab section  16 , where this tube-shaped section  17  supports the flange  14  already mentioned above along its outer circumference. The tube-shaped section  17  is closed toward the fastening flange  12 , thus forming a cylindrical receiver bore  18  with a closed base. The tube-shaped section  17  has a supply bore  19  between fastening flange  12  and flange  14 , where said supply bore  19  creates a connection to the engine hydraulic systems. Housing  10  is inserted into receiver  11  and attached by means of attachment screws  20 , which are inserted into fastening bores  13  and screwed into the receiver. For this purpose, a flange gasket is employed between fastening flange  12  and receiver  11 , and an O-ring is inserted into ring groove  15 , where neither the flange gasket nor the O-ring are shown in detail. This creates a sealed region within receiver  11  between fastening flange  12  and flange  14 . A feed bore of the oil hydraulic system terminates in this region, so that oil can flow into the interior of receiver bore  18  by means of supply bore  19 . 
         [0050]    Two opposing stop protrusions  21  are located at a distance to flange  14  along the front end of housing  10 . The front face of housing  10  has two protruding chamfered sections  22  that have a beveled stop edge  23 . The chamfered sections  22  and stop edge  23  form sections of a circular shape that correspond to the shape of the tube-shaped section  17 . For instance, if stop edge  23  were fully circumferential, it would form an internal taper. 
         [0051]    The tensioning piston  24  shown in  FIG. 4  is inserted into receiver bore  18  in a longitudinally movable manner. The front section protrudes from housing  10  with front face  25 . The tensioning piston  24  is configured as a hollow piston in the known manner. The interior of receiver bore  18  also contains a compression spring, which is not shown, and a mushroom shaped packing, which is forced by the compression spring against the base of a rear-facing bore opening in tensioning piston  24 . These designs are already known, which is why they are not described in greater detail here. Furthermore, a check valve is arranged in the interior of housing  10  downstream from supply bore  19 , where oil is able to flow through said check valve into, but not out of, housing  10 . The tensioning piston  24  further has a vent bore  26  in front face  25 , where said vent bore  26  is connected to the pressure reservoir (not shown) via choke grooves, where said pressure reservoir is formed between housing  10  and tensioning piston  24 . For this purpose the rear section  27  of tensioning piston  24  is guided into receiver bore  18  with a sufficiently tight fit to merely create a leakage gap that is tuned for damping purposes. The tensioning piston  24  has a retainer profile  28  in the front section. The retainer profile  28  consists of adjacent, similarly configured, ring-shaped circumferential retainer grooves  29 . A ring-shaped circumferential transport groove  30  is also located between retainer profile  28  and front face  25 , where said transport groove  30  is deeper than retainer grooves  29  and has straight side flanks. 
         [0052]    The retainer element  31  shown in  FIG. 5  is also needed to create a transport locking device and an adjustment device. This retainer element  31  is designated as a retainer clamp bent from a round wire. Spring steel is preferably used for this purpose. The retainer element  31  has a first and second spring arm  32  and  33 . Each of the two spring arms  32 ,  33  has an arched retainer section  34  at its center, where the arch shape of said arched retainer section  34  is adjusted to the radius of retainer grooves  29  and transport groove  30 . The otherwise U-shaped spring arms  32  and  33  are connected to each other by means of a bar  35  on one side, whereas the open ends  36 . 1  and  36 . 2  on the opposite side are adjacent to each other and approximately imitate a bar. 
         [0053]    The transport locking device furthermore comprises an unlocking element  37  arranged on tensioning rail  4 , where said unlocking element  37  has the shape of a projecting tab and is equipped with ramp sections  38 . 1  and  38 . 2  on each of its opposing sides. The width of unlocking element  37  is selected such that its ramp sections  38 . 1  and  38 . 2 , which form a chevron, can be inserted between the two spring arms  32  and  33  of retainer element  31  arranged on the tensioning piston  24  in order to spread apart said spring arms  32  and  33 . The positioning on the backside of tensioning rail  4  is performed such that this interaction between unlocking element  37  and retainer element  31  can occur when tensioning rail  4  pivots around pivot bolt  3  and chain tensioner  2  is located in receiver  11 . The tensioning rail  4  furthermore has a push contact surface  39  on its rear side that is intended to come into contact with front face  25  of tensioning piston  24 . 
         [0054]    The following provides a more detailed explanation of the operation and function of the embodiment shown. 
         [0055]    In the initial state, as is shown for instance in  FIG. 2  and  FIG. 6 , the chain tensioner  2  is in its transport state. In this transport state, the tensioning piston  24  is retracted very far, so that retainer element  31  arranged on the outside of housing  10  engages into transport groove  30  using its retainer sections  34 , supporting itself on the opposite side on the bottom sides of stop protrusions  21  using bar  35  and also the open ends  36 . 1  and  36 . 2 . Due to the straight side surfaces of transport groove  30 , the compression spring (not shown) preloaded in the interior of chain tensioner  2  is unable to force out tensioning piston  24  against the resistance of retainer element  31 , but instead, the already mentioned securing of the transport state is achieved. 
         [0056]    When tensioning rail  4  now pivots in a clockwise direction according to  FIG. 2 , unlocking element  37  is inserted between the two spring arms  32  and  33 , specifically on the side where the two open ends  36 . 1  and  36 . 2  lie on top of each other, so that ramp sections  38 . 1  and  38 . 2  press against the respective spring arm  32  and  33  (see  FIG. 7 ). This causes spring arms  32 ,  33  to move away from each other, spreading apart retainer element  31 . This in turn causes retainer sections  34  to slide out from transport groove  30 . Unlocking occurs as soon as the force of the compression spring acting in the interior is large enough, and tensioning piston  24  disengages and propels from housing  10  and in between spring arms  32  and  33 . As soon as front face  25  of tensioning piston  24  comes into contact with push contact area  39 , the tensioning piston  24  propelling to the outside also forces tensioning rail  4  back in the opposite direction, that is to say in a counter-clockwise direction. In this way, the tensioning piston  24  causes unlocking element  37  to be carried along as well and thus to be moved out of the area between the two spring arms  32  and  33 , also causing ramp sections  38 . 1  and  38 . 2  to no longer be in functional contact with spring arms  32  and  33 . This ends the spreading apart of retainer element  31 , and as soon as an equilibrium has been established between the compression spring in the interior of chain tensioner  2  and the force of timing chain  6  acting against tensioning rail  4 , retainer sections  34  engage into the retainer groove  29  of retainer profile  28  located at the same height. This state is represented in  FIG. 8 . In this case the retainer mechanism engages into the third retainer groove  29  of retainer profile  28  for illustration purposes. The push contact area  39  therefore essentially functions as an entrainment device for unlocking element  37 , causing it to disengage from retainer element  31 . 
         [0057]    This design also provides an adjustment device in addition to the transport locking device. The retainer element  31  is shaped in such a manner that a working range is defined between the contact of retainer sections  34  and stop edges  23  when tensioning piston  24  retracts, and the contact of bar  35  and open ends  36 . 1  and  36 . 2  along the bottom side of stop protrusions  21  when tensioning piston  24  deploys. The travel distance that tensioning piston  24  can travel within the working range is slightly larger than the center-to-center distance between two retainer grooves  29 . Within this range, retainer element  31  moves together with tensioning piston  24  and has no further effect. But as soon as an adjustment is needed because tensioning piston  24  deploys further, e.g. due to wear (lengthening) of timing chain  6 , bar  35  as well as open ends  36 . 1  and  36 . 2  contact the bottom side of stop protrusions  21 . But the hydraulic pressure in the interior of chain tensioner  2  is so large that tensioning piston  24  is forced further to the outside. Due to the round shape of retainer grooves  29 , retainer element  31  can spread apart, so that spring arms  32  and  33  are forced to the outside, allowing retainer sections  34  to engage into the next adjacent retainer groove  29  after a corresponding travel distance of tensioning piston  24 , causing an adjustment to the working range. In addition to this further deployment of tensioning piston  24 , which retainer element  31  does not oppose, retainer element  31  also performs the arresting function in the opposite direction. When large vibration movements occur in timing chain drive  1 , such as occur during the starting process of the internal combustion engine, the retraction motion of tensioning piston  24  is limited by retainer element  31 . In particular during the startup process and due to the leakage gap seal between housing  10  and tensioning piston  24 , sufficient hydraulic pressure has not yet developed in the interior of the pressure reservoir at engine startup, therefore resulting in insufficient damping. In this state, when tensioning piston  24  retracts against the spring force, retainer element  31  comes into contact with chamfered sections  22  on housing  10  after a corresponding retraction motion. This specifically applies for retainer sections  34 , which come into contact with the beveled stop edges  23 . Due to the beveled configuration of stop edges  23 , retainer sections  34  are forced in the direction of tensioning piston  24 , preventing retainer sections  34  from sliding out of the corresponding retainer groove  29 . This prevents tensioning piston  24  from retracting beyond the working range. The chain tensioner  2  is therefore equipped with an adjustment device that adjusts to the wear state of the chain drive. 
         [0058]      FIG. 9 to 23  are employed to explain a second embodiment of the present invention as follows. When pertinent, the same reference numbers are used for the same and functionally equivalent components and reference is made in this regard to the above description. The following is strictly intended to make reference to the significant differences to the aforementioned exemplary embodiment. The chain tensioner  2  shown in  FIGS. 9 and 10  can be employed in a functionally equivalent manner with the other elements of the timing chain drive  1  shown in  FIG. 1 . Only the unlocking element  37  arranged on tensioning rail  4  is omitted and is replaced by a different design described below. 
         [0059]    The chain tensioner  2  shown in  FIGS. 9 and 10  has a housing  10  (see also  FIGS. 11 and 12 ), which is configured as a screw-in housing. For this purpose, housing  10  is screwed into a bore (not shown) in the engine block. This engine block bore has a feed bore to provide a connection to the engine oil hydraulic system. This feed bore communicates with a section that is located between threaded section  40  and hex head  41  of housing  10 . This section contains supply bore  19  extending radially to the outside, which is in contact with receiver bore  18  via a central connection bore  42 . A receiver pocket  43  is located at the base of receiver bore  18 , which serves to seat a check valve that is not shown. A gasket is inserted between hex head  41  and the engine block, so that a reliable fluid connection is provided between the engine oil hydraulic system and chain tensioner  2 . The shape of tensioning piston  24  essentially corresponds to the tensioning piston of the aforementioned embodiment. The tensioning piston  24  is only extended slightly above transport groove  30 , and has a cylindrical extension  44  with a slightly smaller diameter. The interior structure and configuration as a hollow piston is also essentially equivalent in shape to the aforementioned embodiment. For reasons of simplification, the check valve as well as the compression spring and the packing have been omitted from  FIG. 10 . For the largest portion, or all of their lengths, these are located within the hollow space  45  inside tensioning piston  24 . 
         [0060]    The stop protrusions  21  are configured slightly larger in this embodiment and shaped together with chamfered sections  22 , so that these respectively form a ring section. 
         [0061]    The retainer element  31 , which is arranged on the outside of housing  10  and is engaged with tensioning piston  24 , is in turn bent from a round wire and is shaped slightly differently from the first embodiment because of the fact that stop protrusions  21  and chamfered sections  22  are located on top of each other. The spring arms  32  and  33  and retainer sections  34  are essentially identical. In place of bar  35 , spring arms  32  and  33  are connected to each other on one side by means of stop tabs  45 . 1  and  45 . 2 , which partially encompass housing  10  and have a common tab  46 . Stop tabs  47  are arranged on the opposite side on the applicable ends of spring arms  32  and  33 , where said stop tabs  47  each partially encompass housing  10 . The upper lip of stop tabs  45 . 1  and  45 . 2  and of stop arms  47  are at the same height. 
         [0062]    In the transport state shown in  FIGS. 9 and 18 , retainer sections  34  of spring arms  32  and  33  are engaged into transport groove  30  of tensioning piston  24  and the upper lips of stop tabs  45 . 1  and  45 . 2  and stop arms  47  are located on the bottom side of stop protrusions  21 . This secures tensioning piston  24  in the transport state. 
         [0063]    Unlocking is performed by an unlocking element  48  in the shape of a ring-shaped cap located on extension  44  of tensioning piston  24 . The ring-shaped unlocking element  48  has two diametrically arranged spreader arms  49  that are parallel to the axis and located on the bottom side. Each spreader arm  49  has a ramp section  50 . 1  and  50 . 2 , respectively on its outward pointing sides. This creates a chevron that can be inserted into the area between the two spring arms  32  and  33  and spread these apart. Inward protruding retainer tabs  51  are located on the lower end of spreader arms  49 , where the shape of said retainer tabs  51  is selected in a manner permitting these to engage into transport groove  30  and to also engage into retainer groove  29  located below transport groove  30 . The unlocking element  48  is designed to be movable by precisely this amount along extension  44  relative to tensioning piston  24 . The entrainment device is formed by a stop shoulder  52  in the opening of the ring-shaped unlocking element  48  and a stop flange  53  correspondingly located above transport groove  30 . Longitudinal grooves  54 , which are associated with spreader arm  49 , are located in unlocking element  48 , where said longitudinal grooves  54  facilitate the injection molding process and also provide for improved flexibility of spreader arms  49 . 
         [0064]      FIG. 18 to 23  are now employed to explain the operation and function of this embodiment as follows. 
         [0065]      FIGS. 18 and 19  are representations of the transport state, in which retainer element  31  is engaged into transport groove  30  and unlocking element  48  also engages into transport groove  30  between the two spring arms  32  and  33  using retainer tabs  51 . This locates stop shoulder  52  at a distance to stop flange  53  that essentially corresponds to the center-to-center distance between transport groove  30  and the uppermost retainer groove  29 . 
         [0066]    In order to unlock the transport state, chain tensioner  2  is now screwed into the engine block by means of its screw-in housing  10 . Due to the unlocking element  48  protruding past front face  25 , unlocking element  48  first comes into contact with push contact surface  39  of tensioning rail  4 . Push contact surface  49  of tensioning rail  4  and the front face of unlocking element  48  are shaped accordingly. Due to the increasing pressure created by the screw-in process, tensioning rail  4  now exerts an increasingly greater force on unlocking element  48 , until the correspondingly shaped retainer tab  51  glides out of transport groove  30 . For this purpose, retainer tab  51  has a downward facing, flat ramp section that facilitates this gliding out process. Moreover, spreader arms  49  are equipped with sufficient flexibility, so that this may be attained with adequate force. As soon as the unlocking of retainer tabs  51  has been accomplished, unlocking element  48  slides relative to tensioning piston  24  along extension  44 . As can be seen in  FIGS. 20 and 21 , ramp sections  50 . 1  and  50 . 2  come into contact with spring arms  32  and  33  during this process and force these to the outside, so that retainer element  31  is spread apart until retainer sections  34  disengage from transport groove  30 , releasing tensioning piston  24 . Due to the spring force of the compression spring arranged in the interior of chain tensioner  2 , tensioning piston  24  is subsequently propelled upward and also performs a relative motion to unlocking element  48  due to mass inertia. This relative motion accomplishes a stop contact of stop flange  53  with stop shoulder  52  of unlocking element  48 , subsequently causing unlocking element  48  to be carried along by tensioning piston  24 , so that spreader arms  49  are again retracted from the area between spring arms  32  and  33 , and ramp sections  50 . 1  and  50 . 2  are no longer engaged with spring arms  32  and  33 . Retainer tabs  51  engage in the uppermost retainer groove  29  coterminous with the stop contact of stop flange  53  on stop shoulder  52 , so that unlocking element  48  is secured in this state, e.g. does not cause noise during operation. The relative motion between tensioning piston  24  and unlocking element  48  until the stop contact with stop shoulder  52  is selected such that tensioning piston  24  protrudes slightly from unlocking element  48  in the final state of the tensioning piston (as can be specifically seen in  FIG. 22 ), so that only front face  25  of tensioning piston  24  is in contact with the push contact area  39  of tensioning rail  4 . 
         [0067]    In addition to this automatic unlocking of tensioning piston  24  from the transport state, this embodiment also provides an adjustment device. In this embodiment, the working range is slightly larger and extends approximately for the heights of three adjacent retainer grooves  29 . As can be seen e.g. in  FIG. 22 , retainer element  41  is initially engaged with the uppermost retainer groove  29 . The upper lips of stop tabs  45 . 1  and  45 . 2  come into contact with the bottom side of stop protrusions  21 , thus limiting the upward travel or forward motion in tensioning direction S of tensioning piston  24 . Starting from this state, as soon as chain wear permits tensioning piston  24  to deploy further, the groove shape of retainer grooves  29  causes spring arms  32  and  33  to spread toward the outside, and, because consistent wear is assumed, to snap into the next lower retainer groove  29 . A blockage of a further retraction motion of tensioning piston  24  beyond the working range occurs in the opposite direction. For this purpose, chamfered sections  22  are in turn equipped with beveled stop edges  23 , onto which retainer sections  34  of spring arms  32  and  33  make a stop contact when tensioning piston  24  performs a retraction motion. Here as well, the beveled stop edges  23  cause retainer sections  34  to be forced in the direction of tensioning piston  24 , thus preventing a sliding out of the respective retainer groove  29  and therefore blocking a further retraction motion of tensioning piston  24 .