Abstract:
The present invention provides a tensioning device, in particular for a timing chain of an internal combustion engine, having a housing, a tensioning piston guided in a piston bore of the housing, and a locking means for limiting the retraction motion of the tensioning piston into the piston bore of the housing. The locking means comprises a locking ring arranged on the tensioning piston. Furthermore, the locking means comprises a separate locking sleeve that can be reversibly radially expanded and is arranged at one tensioning end of the housing to introduce the locking ring into the locking means in the retraction direction of the tensioning piston and to retain the locking ring in the locking means in the tensioning direction of the tensioning piston.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International patent application PCT/EP2008/003229, filed on Apr. 22, 2008, which claims priority to foreign patent application DE 10 2007 039 438.3, filed on Aug. 21, 2007. 
     FIELD OF THE INVENTION 
     The present invention relates to a tensioning device, in particular for a timing chain of an internal combustion engine, having a housing, a tensioning piston guided in a piston bore of the housing, and locking means for limiting the retraction motion of the tensioning piston into the piston bore of the housing; the locking means comprises a locking ring arranged on the tensioning piston. 
     BACKGROUND OF THE INVENTION 
     Such tensioning devices are wide-spread and are in particular employed as chain tensioners in timing chain drives of internal combustion engines. Usually, they are arranged in the loose side of the chain drive, as during normal operation lower tension forces occur there on average, so that the pressure spring only has to be designed for these lower tension forces. These tensioning devices normally comprise a housing with a piston bore in which a tensioning piston is arranged which is pretensioned by means of the pressure spring. The hollow cylindrical tensioning piston forms a pressure chamber together with the piston bore of the housing, which chamber is filled with a hydraulic fluid for dampening the retraction motion of the tensioning piston. The pressure chamber is in communication with a hydraulic cycle, e.g. the engine oil cycle of an internal combustion engine, via a check valve to replace the hydraulic fluid escaping from the pressure chamber via a leakage gap formed by the piston and the tensioner housing, or via a corresponding vent hole. 
     Such tensioning devices often comprise locking means which prevent the retraction of the tensioning piston into the housing beyond a predetermined stop but permit further readjustment of the tensioner, e.g. in case of chain wear. For this, a front region of the tensioning piston which is not exposed to the pressure chamber is conventionally provided with a lateral toothing cooperating with a ratchet element. The pretensioned ratchet element permits the tensioning piston to slip through in the tensioning direction of the pressure spring and simultaneously prevents excessive retraction of the tensioning piston into the piston bore of the housing in case of relatively hard and strong vibration impacts. Such a tensioning device is known from DE 10014700 A1, where the piston pretensioned by the spring comprises two ratchet sections essentially arranged oppositely and in which the serrated profiling of the separate ratchet element engages. 
     Moreover, there are also tensioning devices with locking means which take care that a latching effect occurs and the tensioning piston can no longer be retracted in the piston bore when the pressure in the hydraulic cycle is not high enough, e.g. when the engine is being switched off and started. Such a tensioning device is known from EP 657662 A2. This tensioning device comprises a tensioning piston having several catch grooves at its outside circumference into which a spring-loaded catch piston engages. The front face of the catch piston is beveled and the pressure of the hydraulic medium can act on it so that it becomes disengaged. By the locking position of the catch piston, a certain degree of pretension is maintained even while the engine is standing still, despite a possible leakage of the hydraulic medium from the pressure chamber. With each new start up of the tensioning device, a certain tension therefore exists independently of the pressure in the hydraulic cycle. As soon as in the hydraulic cycle and thus also in the pressure chamber of the tensioning device sufficient pressure has built up, the hydraulic pressure also disengages the latching means and the tensioning piston operates in a conventional manner. 
     A generic tensioning device is known from DE 3636918 A1. The tensioning piston that can be extended from the piston bore of a fixed adjuster housing acts on the flexible drive means in the tensioning direction. At the first startup of the tensioning device, the locking ring arranged on the piston snaps into a first catch position of the adjuster housing. If the pressure of the hydraulic medium in the pressure chamber is no longer sufficient to stop the piston during a retraction motion, e.g. during startup or in case of strong vibration impacts, the locking ring hits a stop edge of the piston, whereby the retraction motion of the piston into the piston bore is limited. If in case of wear of the drive means the initial position of the locking ring is no longer sufficient to define a suited working range of the tensioning piston, the locking ring is automatically moved to the next catch point of the adjuster housing by a second stop edge. 
     Such tensioning devices with ratchet elements or locking rings to limit the retraction motion of the tensioning piston into the housing are wide-spread and have proved themselves in use. The corresponding constructions of the locking means have a relatively elaborate design, where a partially complex manufacture and the observation of narrow tolerances are required for secure function. 
     Furthermore, these locking means usually consist of a series of components that have to be assembled in a complex manner and locked in a transport and/or assembly position. 
     SUMMARY OF THE INVENTION 
     In view of the disadvantages of the tensioning devices known from prior art, it is therefore an object of the present invention to simplify the construction of a tensioning device with locking means of the type mentioned in the beginning. 
     According to the invention, this object is achieved by the locking means comprising a retainer which is arranged at a tensioning end of the housing and can be reversibly radially expanded to introduce the locking ring into the locking means in the retraction direction of the tensioning piston and to hold the locking ring in the locking means in the tensioning direction of the tensioning piston. In contrast to the complex constructions and large structural volumes of conventional locking means for tensioning devices, in the embodiment according to the invention only a few simple components are required to restrict the active travel of the tensioning piston and to lock the retraction motion of the tensioning piston into the piston bore in case of strong vibrations or during start up (prevention of start chatter). The tensioning device according to the invention further realizes locking means with a very small structural volume that can be integrated in the existing dimensions of the housing and tensioning piston without any additional requirements being placed on the manufacture of the components. For example, apart from the use of standard components, simple inexpensive elements can be used. After the locking ring arranged on the tensioning piston has been received in the locking means by the first introduction of the tensioning piston into the piston bore of the housing, the ring of the locking means is retained in the locking means and possibly shifted on the piston during an extension motion of the tensioning piston. Thus, the locking means defines the travel of the tensioning piston. 
     One preferred embodiment provides for the retainer to be embodied as a separate sleeve fixed to the housing. A separate locking sleeve permits a simple manufacture of the tensioning device wherein the locking means is fabricated independently of the housing and subsequently mounted separately. In the process, the separate sleeve can be fixed to the housing by means of permanent as well as detachable connections, e.g. by welding, soldering or gluing, but also by screwing, clamping or catching. 
     The sleeve can favorably comprise an internally bent projection at the housing&#39;s side which engages in a groove at the housing to fix the sleeve to the housing. Such a catch-like connection permits a quick and inexpensive and nevertheless secure mounting of the sleeve to the housing. The sleeve favorably comprises one or several surrounding projections which can also be embodied as bent over wings/end regions of the sleeve itself. 
     For a simple readjustment of the locking ring on the tensioning piston, the sleeve can comprise an internally bent projection on the tensioning side which retains the locking ring in the tensioning direction or extending direction of the tensioning piston in the locking means. Due to the identical or similar embodiment of the internally bent projection on the housing&#39;s side and the tensioning side of the sleeve, in particular in the form of internally bent wings/end regions of the sleeve, manufacture can be facilitated and it is possible to mount the sleeve at alternate sides. 
     One advantageous embodiment of the invention provides for the sleeve to be formed by a meandering belt. Such a meandering belt is for example obtained by incising or slotting the sleeve alternately from both front sides, where the slots extend over more than one half, preferably more than 70% of the length of the sleeve. Forming the sleeve of a meandering belt permits the manufacture of the sleeve of a stable material having high strength, for example plastics or steel, in particular spring steel, while nevertheless radially flexible properties are allowed for, although the retainer or the retainer cage is still embodied rigidly axially to the axis of the tensioning piston. The meandering belt is shaped to a sleeve (in abutment, overlapping, ends connected or not, etc.). It is also possible to mold the sleeve as molded part (in this case, the belt and sleeve are produced quasi in one single operation). 
     A stop can be preferably provided at the tensioning end of the housing, wherein the stop locks the retraction motion of the tensioning piston when it contacts the locking ring. This simple construction permits a secure limitation of the retraction motion without requiring any additional components for this. 
     To securely position the locking ring on the piston, the outside wall of the tensioning piston can be provided with grooves, the locking ring being received in these grooves. The grooves prevent the locking ring from sliding on the tensioning piston in the locked position. Alternatively, the locking ring can also become jammed on the piston, so that no grooves on the piston are required in such an embodiment. Suitably, the grooves surround the outer wall of the piston and are separated from each other by an also surrounding web. The flutes surrounding the circumference of the piston permit the locking ring to be securely retained on the tensioning piston, as compared to grooves that are only arranged in some sections of the circumference, although the surrounding flutes have a lower depth than the grooves arranged in some sections. The webs between the grooves, which preferably have a wedge-like design, prevent a movement of the locking ring on the tensioning piston during the refraction motion and nevertheless permit the locking ring to easily slip over into the next groove, in particular if the beveled side of the wedge-shaped webs is arranged in the direction of the extending motion, i.e. they permit the readjustment of the tensioning device. In the process, the locking ring must be at least partially radially expandable to permit it to slip over the web to the next groove where it then elastically contracts again to realize the locking effect. 
     One advantageous embodiment provides for the tensioning device to be embodied as screw-in adjuster. Thanks to the small structural volume of the locking means, the embodiment of the tensioning device according to the invention is particularly suited for screw-in adjusters. Moreover, the locking means can also be employed in all conventional flange-type tensioning devices. Favorably, the outside diameter of the sleeve is here smaller than the core diameter of the screw-in thread. The adjustment of the diameter of the sleeve to the diameter of the thread permits to screw in the preassembled adjuster into a corresponding tapped bore to receive the tensioning device without any further operations in the final assembly. For preassembling the tensioning device at the manufacturer&#39;s, the tensioning piston is received in the piston bore and secured by calking in the housing, where applicable a locking sleeve is shifted over the piston and fixed to the housing, and the locking ring is shifted onto the tensioning piston into its first groove, i.e. it is not yet introduced into the locking means. In this transport position, the tensioning device is pushed into a corresponding opening and fixed or, in case of a screw-in adjuster, screwed in, at the final assembly without any further preparation. In the process, the tensioning piston moves into the piston bore of the housing until the locking ring is introduced into the locking means. The final assembly thus permits to make the locking means operate without any further unlocking. 
     The present invention furthermore relates to a method for activating a return interlock or locking means for a preassembled screw-in tensioner with a housing and a tensioning piston guided in a piston bore of the housing, wherein a first part of the return interlock is arranged at the housing and a second part of the return interlock is arranged at the tensioning piston separately from the first part. The method comprises screwing in the preassembled screw-in tensioner into a corresponding final assembly opening, usually a tapped bore in the engine block of an internal combustion engine, touching the tensioning element to be tensioned by the screw-in tensioner, introducing the tensioning piston into the piston bore of the housing by further screwing in the screw-in tensioner into the final assembly opening, and interlocking the first and the second parts of the return interlock and activating the return interlock. In contrast to conventional tensioning devices with a locking means or return interlock operatively assembled during preassembly, the method according to the invention permits to avoid the usually necessary latching of the locking means after preassembly and the corresponding unlatching after final assembly. In the method according to the invention, the first and the second parts of the return interlock are arranged separately in the axial direction after preassembly of the screw-in tensioner, where the axial distance of the first and the second parts is preferably smaller than the screw-in path of the tensioner in the corresponding final assembly opening. In an advantageous embodiment of the return interlock of which the working range can be adjusted, the first part of the return interlock is a reversibly radially expandable locking sleeve arranged at one tensioning end of the housing, and the second part is a locking ring arranged on the tensioning piston which, during the retraction motion of the tensioning piston into the piston bore during final assembly, first radially expands the locking sleeve and is then received in the locking sleeve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Below, the construction and function of the tensioning device according to the invention will be illustrated more in detail with reference to the enclosed drawings and one embodiment. In the drawings: 
         FIG. 1  shows a cross-section through a housing and a tensioning piston for a tensioning device according to the invention in a not assembled state, 
         FIG. 2  shows a cross-section through the housing and the tensioning piston of  FIG. 1  in an assembled state, 
         FIG. 3   a  shows a cross-section through a locking sleeve for a tensioning device according to the invention, 
         FIG. 3   b  shows a perspective view of the locking sleeve of  FIG. 3   a,    
         FIG. 4   a  shows the tensioning device of  FIG. 2  and the locking sleeve of  FIG. 3  in an assembly arrangement, 
         FIG. 4   b  shows the tensioning device of  FIG. 2  and the locking sleeve of  FIG. 3  in an assembled state, 
         FIG. 5   a  shows the tensioning device of  FIG. 4   b  and a locking ring for a tensioning device according to the invention in an assembly arrangement, 
         FIG. 5   b  shows the tensioning device of  FIG. 4   b  and the locking ring in an assembled state, 
         FIG. 6   a  shows the tensioning device of  FIG. 5   b  in the transport state in a final assembly arrangement, 
         FIG. 6   b  shows the tensioning device according to the invention in the finally assembled state, 
         FIG. 6   c  shows an enlarged representation of detail VI of  FIG. 6   b.    
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a hollow cylindrical housing  2  and an also hollow cylindrical tensioning piston  3  for a tensioning device  1  according to the invention for the timing chain drive of an internal combustion engine. A piston bore  4  is provided in the hollow cylindrical part of the housing  2  in which the tensioning piston  3  can be guided moving in the longitudinal direction with respect to axis A. At the bottom of the piston bore  4 , a seat  5  for receiving the check valve and a chamber  6  are provided. The chamber  6  is connected to a non-depicted hydraulic system via a bore  7 , preferably to the oil cycle of an internal combustion engine. The housing  2  furthermore comprises a threaded section  8  on its outside circumference as well as a flange  9  in the base area of the housing. With the threaded section  8 , the tensioning device  1  can be mounted in the engine block of an internal combustion engine, where the flange  9  limits the screwing in of the tensioning device  1 . Alternatively, the housing  2  or the tensioning device  1 , respectively, can also be designed as flange housing or flange tensioning device. At the tensioning end of the housing  2 , i.e. at the open front face of the hollow cylindrical housing  2 , a groove  10  is provided at the outside circumference in addition to the otherwise conventional embodiment of the housing. 
     The tensioning piston  3  is also essentially designed like a conventional tensioning piston, and at the closed front face of the hollow cylindrical piston, i.e. at the tensioning side of the tensioning piston  3 , it comprises a vent hole  11 . In addition, several grooves or flutes  12  surrounding the outside circumference are provided in the front region of the tensioning piston  3 , i.e. on the tensioning side, which are each separated by a wedge-shaped web  13 . 
     In  FIG. 2 , the housing  2  and the tensioning piston  3  are shown in the assembled state. Here, the tensioning piston  3  is received in the piston bore  4  of the housing  2  so as to move in the longitudinal direction, where the housing is calked at the open front face such that the tensioning piston  3  can no longer come out of the piston bore  4  after assembly. The housing  2  and the tensioning piston  3  together form a pressure chamber  14 . The pressure chamber  14  is connected to the hydraulic system via a check valve  15  which is received in the seat  5  at the bottom of the piston bore  4 , the chamber  6  filled with a hydraulic medium and the bore  7 . The tensioning piston  3  is pretensioned with a helical pressure spring  16  in the axial direction. The pressure spring  16  is only schematically shown in  FIG. 2 . A packing element  17  is furthermore arranged in the hollow cylindrical tensioning piston  3  and reduces the volume of the pressure chamber  14  and with its head  18  seals the vent hole  11  on the front side in the tensioning piston  3  with respect to the pressure chamber  4  filled with the hydraulic medium. For this, the head  18  of the packing element  17  is pressed against the front end of the hollow cylindrical tensioning piston  3  by the pressure spring  16  which supports itself against the check valve  15 . In front of the flange  9  of the housing  2 , two sealing rings  19 ,  20  are provided in the tensioning direction which seal the tensioning device  1  in the finally assembled state in the engine block of an internal combustion engine. 
     In  FIG. 3   a , a locking sleeve  21  is shown in a cross-section. The locking sleeve  21  preferably consists of steel, in particular spring steel. The front ends of the sleeve  21  are bent inwards on both sides, where the bent over sections of the sleeve project like wings with respect to the internal wall of the sleeve. The end of the wings  22  has the largest distance with respect to the internal wall of the sleeve  21 . The locking sleeve  21  is furthermore alternately slotted from both front sides, where the slots  23  extend over 75% of the length of the locking sleeve with internally bent wings  22 . By the slots being arranged in an alternating manner, the locking sleeve  21  is formed by a surrounding meandering belt which permits a radial expansion of the locking sleeve  21 , but simultaneously also ensures the reversible return motion. 
       FIG. 3   b  shows a perspective representation of the locking sleeve  21 . As can be seen in  FIGS. 3   a  and  3   b , the locking sleeve  21  is embodied as symmetric part, despite the internally bent projecting wings  22  and the slots  23 , and can therefore be used from both sides in the same manner. 
       FIG. 4   a  shows the tensioning device  1  with the housing  2  and the tensioning piston  3  guided in the piston bore  4  of the housing  2  as well as the locking sleeve  21  in the assembly position in front of the tensioning end of the tensioning piston  3 . During the assembly of the locking sleeve  21 , the same is pushed over the head of the tensioning piston  3  on the tensioning side in the direction of arrow. In the process, the sleeve  21  is already slightly expanded to slide over the head of the tensioning piston  3 . Alternatively, the head on the tensioning side can also be dimensioned such that it exactly fits through the opening of the locking sleeve  21  formed by the wings  22 . The locking sleeve  21  is pushed over the tensioning piston  3  until it reaches the housing  2 . When it is further pushed over the tensioning end of the housing  2 , the locking sleeve  21  radially expands, and when it is further pushed over the outside wall of the housing, the wings  22  projecting inwards engage in the surrounding groove  10  and the locking sleeve  21  contracts again radially until it assumes its original state, as shown in  FIG. 4   b . The engagement of the locking sleeve  21  in the groove  10  with the internally projecting wings  22  permits a very simple and effective mounting of the locking sleeve at the housing  2 . 
     The tensioning device  1  of  FIG. 5   a  with the locking sleeve  21  shifted onto it and caught corresponds to the tensioning device of  FIG. 4   b . Furthermore, a locking ring  24  is arranged in front of the tensioning end of the tensioning piston  3  for further preassembly. During assembly, the locking ring  24  is shifted onto the head of the tensioning piston  3  in the direction of arrow and over the head to the first surrounding flute  12  of the tensioning piston  3 . For this, the locking ring has an elastic design, so that it correspondingly deforms radially to reach the first flute  12  over the head of the tensioning piston  3 . Alternatively, the locking ring  24  can also be interrupted to permit the required radial deformation. Other constructive embodiments of the head of the tensioning piston  3  on the tensioning side and of the locking ring  24  which permit to shift the locking ring  24  over the head of the tensioning piston  3  to the first flute  12  are also possible. 
     At the end of preassembly, as shown in  FIG. 5   b , the tensioning device  1  is provided with a locking sleeve  21  shifted onto the housing  2  and a locking ring  24  shifted onto the tensioning piston  3 . 
       FIG. 6   a  shows the transport position and installation state of the tensioning device  1  with the locking sleeve  21  shifted onto the housing and the locking ring  24  positioned on the tensioning piston  3  corresponding to  FIG. 5   b . For or during final assembly, the tensioning piston  3  is, together with the locking ring  24 , pushed into the piston bore  4  of the housing  2  in direction of arrow against the pretension of the pressure spring  16 . As soon as the locking ring  24  reaches the locking sleeve  21 , the locking sleeve  21  expands at the tensioning side until the locking ring  24  gets behind the internally projecting wings  22  of the locking sleeve  21  when the tensioning piston  3  is further shifted inside. Then, the locking sleeve  21  contracts again radially at the tensioning end, so that the locking ring  24  is located between the front side of the housing  2  at its tensioning end and the wings  22  of the locking sleeve  21 , see  FIG. 6   b.    
       FIG. 6   c  shows in detail the arrangement of the locking ring  24  between the wings  22  on the tensioning side of the locking sleeve and the front stop  25  at the tensioning end of the housing  2 . The locking ring  24  can no longer travel beyond the wings  22  of the locking sleeve  21  when the piston  3  is extended. Equally, when the piston  3  is retracted into the housing  2 , the further retraction motion of the tensioning piston into the piston bore is prevented when the locking ring  24  contacts the stop  25 . 
     When the tensioning device  1  according to the invention is designed as a screw-in adjuster, the tensioning device  1  can be screwed into a corresponding opening at the engine block in the transport position which is shown in  FIG. 6   a  without any preparation or subsequent unlatching. While it is being screwed in, the tensioning piston  3  is pressed into the piston bore  4  of the housing  2  until the locking ring  24  is received in the locking sleeve  21 . 
     After the installation of the tensioning device  1  according to the invention, the tensioning piston  3  is, when it is put into operation for the first time, pushed forwards until the tensioning piston  3  generates sufficient pretension at a tensioning element (not shown) lying against the head of the tensioning piston  3 , which is due to the pretension of the pressure spring  16  as well as due to the pressure of the hydraulic medium in the pressure chamber  14 . If in the process the tensioning piston  3  exits from the piston bore  4  far enough for the locking ring  24  to hit the wings  22  of the locking sleeve, the locking ring  24  is retained in its position by the wings  22  of the locking sleeve  21 , while the tensioning piston  3  further extends, so that the locking ring  24  slides over the wedge-shaped web  13  into the next flute  12 . Here, the web  13  is beveled in the slipping direction of the locking ring  24  to permit the radial expansion of the locking ring and the slipping over into the next flute  12 . Depending on the position of the locking ring  24  on one of the flutes  12  on the tensioning piston  3 , the travel of the tensioning piston between the wings  22  is defined on the tensioning side of the locking sleeve  21  and the stop surface  25  at the front side of the housing  2 . 
     If the tensioning piston  3  retracts due to heavy vibration impacts or due to a low hydraulic pressure when the internal combustion engine is started to such an extent that the locking ring  24  contacts the stop  25  at the housing  2 , the retraction motion is locked as the locking ring  24  cannot radially expand at the straight side of the web  13 . 
     In case of wear of the drive means tensioned by the tensioning device  1  according to the invention, the locking ring  24  contacts again the wings  22  on the tensioning side of the locking sleeve  21 . If now the force applied by the pressure spring  16  and the hydraulic pressure in the pressure chamber  14  is sufficient to press the tensioning piston  3  further into the tensioning direction, the locking ring  24  radially expands again and slips over the next web  13  into the following flute  12 . By this automatic readjustment of the tensioning device  1 , the operating position of the tensioning piston  3  is changed without the travel being changed.