Abstract:
A hydraulic chain tensioner for an internal combustion engine includes a support body forming a cylindrical housing which receives a piston slidable in the housing and which carries a pressure pad acted upon by the piston and arranged to come into contact with the chain. A high-pressure chamber is defined by the housing and the piston. A source of pressurized hydraulic liquid supplies the tensioner with lubricating liquid and completes the filling of the high-pressure chamber. The high-pressure chamber is connected to the ambient surroundings by gas-purging orifice

Description:
FIELD OF THE INVENTION 
     The present invention relates to hydraulic chain tensioners for internal combustion engines. 
     The invention is particularly concerned with a hydraulic chain tensioner comprising: a support body forming a cylindrical housing receiving a piston slidable in the housing and carrying a pressure pad acted upon by the piston and arranged to come into contact with the chain, a high-pressure chamber defined by the housing and by the piston and arranged for communication with a pressurized hydraulic liquid thereby to supply the tensioner with lubricating liquid and to complete the filling of the high-pressure chamber. 
     DESCRIPTION OF THE PRIOR ART 
     Hydraulic tensioners of this type are already known for chains or other transmission elements referred to as endless linkages such as those described for example in FR-A-2 610 685. These tensioners are connected to a source of pressurized hydraulic liquid such as the oil pump of the heat engine. The tensioner exerts pressure via its piston onto the chain. When the chain becomes slack, the piston exits its housing so that the pressure of the liquid in the housing falls and when this pressure is less than the pressure of the hydraulic liquid, the valve opens and hydraulic liquid enters the so-called high-pressure chamber of the tensioner. The tensioner opposes the vibrating movements of the chain since its chamber contains a liquid which is incompressible in principle. 
     However, sometimes the hydraulic liquid, i.e. the oil of the lubrication circuit, contains dissolved gasses or gasses in the form of an emulsion. These gasses can separate progressively from the liquid and form a cushion of air in the high-pressure chamber of the tensioner. The behaviour of the tensioner is then changed in relationship to the volume of compressed gas. Moreover, and in particular after a prolonged stoppage of the engine, it may be the case that the tensioner does not immediately carry out its function after the engine has been started up, for as long as the pressure of the hydraulic liquid supplied by the liquid source fails to reach the appropriate level. 
     SUMMARY OF THE INVENTION 
     The present invention proposes to develop an improved hydraulic tensioner for an endless chain or linkage of the type first referred to above. 
     To this end the invention relates to a hydraulic tensioner of the type initially defined above having the high-pressure chamber connected to the ambient surroundings via a gas-purging orifice. 
     According to one advantageous preferred feature, a compression spring acts between the support body and the pad pressing on the endless chain or linkage. This spring makes it possible to absorb vibrations of the chain when the engine is first started up, when the high-pressure chamber has not yet been charged with pressurized liquid. 
     This spring may be helicoidal compression spring surrounding the piston and bearing against an edge of the housing receiving the piston. 
     The spring can alternatively be a helicoidal compression spring housed in the high-pressure chamber between the base of said chamber and the adjacent end of the piston. 
     In both cases it is advantageous to provide the device with a locking member permitting the pad to be held in a retracted piston for mounting operations and arranged to be released only when the tensioner has been installed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in more detail hereinafter with reference to the accompanying drawings. In the drawings: 
     FIG. 1 is a cross-sectional view of a hydraulic tensioner according to the prior art, in the retracted position, 
     FIG. 2 is a cross-sectional view of the tensioner of FIG. 1, in the deployed position, 
     FIG. 3 is a cross-sectional view of a hydraulic tensioner according to the invention, in the retracted position, 
     FIG. 4 is a view of the tensioner of FIG. 3, in the deployed position. 
     FIG. 5 is a cross-sectional view of a different embodiment of a tensioner according to the invention, in the retracted position, 
     FIG. 6 is a cross-sectional view of the tensioner of FIG. 5, in the deployed position. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to FIGS. 1 and 2, a known hydraulic tensioner for an endless chain or linkage comprises a support  1  serving to mount the tensioner. This support  1  comprises a duct  11  for the passage of hydraulic liquid, connected to a source  2  of hydraulic liquid such as a pump supplying lubrication oil. 
     The support  1  receives a casing  3  forming the housing for a piston  4  comprising a cylindrical sleeve  41  sliding in the casing  3  and a base  42  defining a high-pressure chamber  5  with the base  32  of the casing  3 . The base  42  is provided with a ball valve  43  with a calibrated spring  44  defining the opening pressure of the ball valve  43 . The direction of passage is indicated by the arrow A. The cylindrical sleeve  41  of the piston  4  is provided, over a certain length, with a cut-away or free part  411  into which opens a through orifice  412  communicating with the cavity  45  of the piston. This cavity  45  is connected to the high-pressure chamber  5  via the valve  43 . 
     The casing  3  comprises, in its peripheral surface, a port  311  connecting the duct  11  with the cut-away zone  411  of the piston  4  to permit entry of hydraulic liquid no matter what the displacement position of the piston  4  with respect to the casing  3 . The base  42  of the piston comprises a gasket  46  providing a seal. 
     Thus, in this known tensioner, the high-pressure chamber is closed in a sealed manner by the casing  3  and the base  42  of the piston. 
     The piston  4  bears on the strand  8  of the chain by way of a pad  6  pushed by the piston  4 . The pad  6  is formed by a support strip  61  carrying a slide element  62 . The pad  6  has a double orifice  63 ,  64  extending through it, which issues into the cavity  45  in order to permit passage of the oil serving to lubricate the strand  8  of the chain. The support  1  comprises a guide surface  13  serving to guide the movement of the pad  6 . 
     In the known example illustrated in FIGS. 1 and 2, the tensioner also serves as a support for the other strand  8 ′ of the chain by way of an extension  6 ′ of the main body of the support. 
     FIG. 2 shows the known tensioner in the operational position. The high-pressure chamber  5  is filled with liquid at a certain pressure and the pad  6  is pushed against the strand  8  of the chain at the pressure transmitted by the piston. As the hydraulic fluid in the high-pressure chamber  5  is incompressible the chain cannot vibrate. 
     It should be noted that the other strand  8 ′ is not tensioned by the tensioner since the tensioner is mounted in a fixed manner and it exerts its pressure on the strand  8 , the strand  8 ′ simply being supported against the corresponding lower part  6 ′ of the support  1 . 
     A first embodiment of a tensioner according to the invention will now be described with the aid of FIGS. 3 and 4. 
     According to FIG. 3, the invention relates to a hydraulic tensioner for an endless chain and linkage, represented by the strand  108  which lies apart from the tensioner when the tensioner is in the retracted position. The tensioner comprises a support  101  intended to mount it. This support comprises a duct  111  through which it receives hydraulic liquid from a source  102  of hydraulic liquid, for example, the lubrication oil pump. The support  101  comprises a casing  103  housing a piston  104  formed from a cylindrical sleeve  141  and from a base  142 . The base is provided with a ball valve  143  loaded by a calibrated spring  144  to permit passage of pressurized liquid in the direction indicated by the arrow A. 
     The periphery of the cylindrical sleeve  141  is provided, over a part of its length, with a cut-away zone  147  into which issues a through orifice  148  communicating with a cavity  145  of the piston. The valve  143  permits communication between the cavity  145  and a high-pressure chamber  105 . 
     The cut-away zone  147  extends over a length of the piston substantially corresponding to its length of travel. This zone  147  communicates with the duct  111  by a port  137  to permit entry of pressurized liquid from the source  102  through the duct  111 , the port  137 , the cut-away peripheral zone  147  and the orifice  148  as far as the cavity  145  in order to supply the high-pressure chamber  105  via the valve  143 . The base of the piston also includes a sealing gasket  146  and the top of the piston or at least the sleeve  141  bears against a pad  106  formed from a support strip  161  and a slide element  162 . These two elements are each traversed by an orifice  163 ,  164 . These orifices provide communication between the cavity  145  and the outer surface of the slide element  162  and permit passage of the lubrication liquid to lubricate the strand  108  when the pad  106  is pushed against the strand  108  (operational position illustrated in FIG.  4 ). 
     The pad  106  is guided in its translational movement by a guide surface  113  of the support  101 . 
     A compression spring  107  is mounted between the support  101  and the pad  106 . In the embodiment of FIG. 3 this helicoidal compression spring  107  bears against the upper edge  131  of the casing  103  and the lower surface of the strip  161  of the pad. A locking member  171  has a hook  172  to hold the pad  106  in the retracted position illustrated in FIG.  3 . This locking member  171  is carried by a fixed element of the installation. 
     The locking member  171  opposes the resilient force exerted by the compression spring  107 . 
     FIG. 4 shows the hydraulic tensioner according to the invention in the operational position. In this position the pad  106  is pushed against the strand  108  of the endless chain or linkage. This bearing relationship is initially produced by the spring  107  after the pad is released, i.e. after removal of the lock  171  (this lock is not shown in FIG.  4 ). 
     The base  132  of the casing  103  and the base  117  of the support  101  are traversed by an orifice  190 , which has a fixed cross-section, in the base  117  and which is calibrated in the casing  103  by a throttling zone  191 . This calibrated orifice  190 ,  191  is of a size sufficient to permit air trapped in the high-pressure chamber  105  to escape but the orifice is sufficiently small to retain the hydraulic liquid. This orifice  190 ,  191  permits purging of the high-pressure chamber  105  so that it contains only liquid and not a mixture of liquid and gas or a cushion of gas or air with the liquid. 
     As gas is released, if this is the case, from the highly pressurized hydraulic liquid contained in the chamber  105 , this gas escapes via the orifice  190  and its throttle zone  191 . This reduction in the volume of the high-pressure chamber  105  is compensated by the arrival of a new quantity of liquid through the valve  143 . 
     Even before the pressurized liquid arrives in the high-pressure chamber  105  of the tensioner, the spring  107  pushes the slide element  162  of the pad  106  against the strand  108 . The piston  104  does not necessarily occupy the position illustrated in FIG. 4 at this moment. It is only when the pressure of the hydraulic liquid supplied by the source  102  charges the high-pressure chamber  105  and pushes the piston  104  that it occupies the position illustrated in FIG.  4 . 
     FIGS. 5 and 6 show another embodiment of hydraulic tensioner according to the invention. This embodiment is similar to the first embodiment and the references used to designate the different elements are the same as those used above merely increased by  100 . 
     The description will be limited to the differences. 
     According to this second embodiment, the compression spring acting between the support  201  and the strand  208  of the chain is not mounted around the piston  204  but between the base  232  of the casing  203  and the base  242  of the piston  204 . In order to form an abutment surface for the spring  207  a washer  248  is provided which bears against the base  242  of the piston  204  and protects the gasket  246 . 
     The pad  206  is again held by a locking member  271  which acts against the action of the spring  207 . 
     The operation of this second embodiment of the hydraulic tensioner and in particular the evacuation of air contained in the high-pressure chamber  205 , which is of dimensions necessarily larger than those of the first embodiment since it houses the compression spring  207 , is similar to that described above. The feature of this second embodiment is that of housing the spring  207  in a protected manner. In this second embodiment, the pad  206  is still pressed, if only gently, against the strand  208  by the spring by way of the piston  204 , whereas in the first embodiment the spring does not act by way of the piston but directly by the spring acting between the support and the pad.