Abstract:
A chain or belt tensioner comprises a plunger displaceable in a housing between retracted and extended positions. A stop ring between the plunger and housing frictionally contacts a smooth surface of the housing. A recess in the plunger has a tapered face for engagement with a bevelled surface of the stop ring. Inward movement of the plunger causes the tapered face to engage the stop ring and to force it to deform outwardly to a wedging position in which the stop ring is wedged between the tapered face and the surface thus preventing further inward movement of the plunger. The recess has a second face that engages the stop ring when the plunger moves out of the housing so as to slide it over the surface. The invention provides for a stop mechanism that enables retraction of the plunger to be prevented at an infinite number of positions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 119 to Great Britain Patent Application No. 0025246.0 filed Oct. 13, 2000. 
   STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable. 
   REFERENCE TO A MICROFICHE APPENDIX 
   Not Applicable. 
   BACKGROUND 
   1. Field of Invention 
   The present invention relates to a tensioner for a chain or belt and more particularly, but not exclusively, to a tensioner for imparting tension to a chain or belt used in a timing drive of an internal combustion engine of a vehicle. 
   2. Description of Related Art 
   Internal combustion engines of motor vehicles often include a timing belt or chain drive that passes over sprockets on the crankshaft and camshaft and is used to ensure that the camshaft is driven synchronously with the crankshaft. The tension in such a chain or belt varies considerably as a result of the expansion and contraction of engine components with temperature, torsional vibrations imparted from the crankshaft and camshaft, the engine speed and chain elongation as a result of chain wear or temperature variations in chain components. It is important to impart to and maintain tension in the chain or belt so as to reduce noise and the likelihood of the chain or belt jumping from the teeth of the sprockets. 
   Tensioners for chains or belts generally comprise a housing that defines an open-ended cylinder in which a plunger is slidably movable in a longitudinal direction and is biased outwardly of the cylinder by a coil spring so as to impart tension to the chain or belt. A variable volume fluid pressure chamber is defined between a hollow in the plunger and the cylinder walls. A check valve permits hydraulic fluid to pass from a source such as an oil pump into the pressure chamber but prevents passage of fluid in the reverse direction. The fluid in the pressurised chamber also serves to bias the plunger out of the cylinder towards the chain or belt. 
   The combined effect of the hydraulic fluid pressure and the coil spring biasing forces moves the plunger out of the housing and into contact with the chain or belt so as to take up the slack. An opposite resisting force is imparted from the chain to the plunger as a result of the tension induced in the chain by the drive. If the chain or belt is subjected to an increase in tension the resulting force applied to the plunger attempts to move it into the cylinder. However, such movement is prevented by the check valve which prevents the escape of hydraulic fluid out of the chamber. The incompressible nature of the hydraulic fluid prevents instant return movement of the plunger although a small annular clearance between the plunger and the cylinder wall may permit some fluid escape and limited slow retraction of the plunger. 
   When the internal combustion engine is idling or at rest there is little or no hydraulic fluid pressure and with insufficient fluid pressure in the chamber it is easier for the plunger to retract into the cylinder and for vibrations and noises to be transmitted from the chain. In order to address this problem it is common to provide a locking ratchet mechanism to prevent excessive retraction of the plunger even when the fluid pressure is low. 
   Known ratchet mechanisms for chain or belt tensioners comprise a rack defined on the plunger that is engageable by a pawl on the cylinder as described in U.S. Pat. Nos. 4,822,320 and 5,073,150. In an alternative embodiment a cylinder is provided with a helical toothed channel that is engageable by a pin on the plunger. The disadvantage of such ratchet mechanisms is that they are susceptible to poor performance and even jamming as a result of slight axial misalignment of the plunger in the chamber caused by the locking force is being applied on side only. Moreover, the components require careful machining that is relatively expensive to perform. 
   The tensioner described in U.S. Pat. No. 4,792,322 has a ratchet locking mechanism in which the locking force is applied uniformly around the plunger. It relies on the use of a radially resilient ring that moves between annular notches defined at axially spaced intervals in the interior wall of the cylinder. The ring serves to prevent excessive movement of the plunger into the cylinder by becoming trapped between an edge of the notch on the cylinder wall and a shoulder defined on the plunger. As the plunger extends out of the cylinder the ring is forced to move with it by contact with a stop face on the plunger whereupon it is moved into the next notch in the cylinder. The arrangement prevents retraction of the plunger but only to a predetermined number of discrete positions governed by the number of notches and their spacings. Like the other aforementioned ratchet designs, the locking of the plunger against retraction at discrete positions does not always ensure the appropriate tension is imparted to the chain. 
   U.S. Pat. No. 5,704,860 describes a chain tensioner having a radially resilient stop ring that co-operates with grooves in the wall of the plunger and housing. The ring is clamped between two contact surfaces on the housing and piston to lock the piston in the housing bore despite the urging force of a compression spring. If the piston is pushed into the housing the stop ring snaps into a groove so as to allow limited movement of the plunger in the housing. However, it does not move entirely clear of the plunger and further extension of the plunger encourages the stop ring to move along the housing. 
   EP 0260565 describes a chain tensioner in which a locking ring is radially resilient so that it is biased on to the plunger. It is moveable into a predetermined position where it is squashed between contact surfaces on the piston and housing and prevents extension of the plunger. However, it does not move clear of the plunger. 
   It is an object of the present invention to obviate or mitigate the aforesaid disadvantages. 
   BRIEF SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention there is provided a chain or belt tensioner comprising a plunger displaceable in a bore of a housing between retracted and extended positions, the plunger projecting from an open end of the bore in said extended position and biased towards the extended position by a biasing member, a radially resilient stop member disposed in the bore between the plunger and housing and engaged in frictional contact with a first surface of the plunger or the housing, and a recess in the plunger or housing and defined on a second surface opposite the first surface, the recess having a tapered face for engagement with a surface of the stop member, wherein the tapered face tapers radially outwardly in the direction of towards the open end of said bore so that movement of the plunger relative to the housing towards the retracted position causes the tapered face to engage said surface of the stop member and to force it to deform in a radial direction to a wedging position in which the stop member is wedged between the tapered face and the first surface so as to prevent further inward movement of the plunger, the recess having a second surface that is engageable with the stop member when the plunger is moved out of the housing towards the extended position so as to slide the member over said first surface, wherein the first surface is of substantially constant diameter. 
   The present invention provides for a mechanism by which the inward movement of the plunger can be stopped at an infinite number of plunger positions relative to the housing. This is achieved by use of the radially resilient stop member in combination with the tapered surface of the recess. The arrangement permits the stop member to be wedged between the housing and the plunger at any axial position along the housing, the member, when wedged, being in increased frictional contact with the first surface so as to prevent significant retraction of the plunger into the housing. The first surface may be defined by the bore in which case the recess is defined in the plunger and the stop member is deformed radially outwardly to prevent further inward movement of the plunger. Alternatively the first surface may be defined on the plunger in which case the recess is in the housing and the member is deformed radially inwardly. The elimination of the plurality of annular grooves or ratchet teeth designs removes the step-wise nature of the previous ratchet mechanisms. 
   The outside diameter of the ring in a relaxed condition is preferably greater than the inside diameter of the bore. When the first surface is defined by the bore the stop member is thus disposed in the recess in the plunger in a radially contracted condition. 
   Ideally said surface of the stop member is ramped so as to co-operate with the tapered face of the recess. 
   The stop member may be in the form of a ring that may have a radial cut or slot so as to permit radial deformation. 
   The tensioner may additionally comprise a groove provided in said housing at one end of said bore, the groove having a tapered edge directed radially inwards in the direction towards the extended position for housing the stop member during initial assembly of the tensioner. The housing bore has an open end and a closed end, the plunger having a first end that is inserted into the bore towards the closed end during assembly and an annular surface between its first end and the stop member recess. 
   The first and second end faces of the recess are preferably spaced apart by a distance greater than the axial length of the stop member so as to permit travel of the recess over the member. 
   The leading edge of the stop member is preferably rounded at its radially outer extremity so as to assist in its removal from the housing. 
   A locking member may be provided between the plunger and housing, the member being radially resilient and having an outside diameter greater than the inside diameter of the bore, the housing and plunger having respective locking grooves for receipt of the locking member, wherein the plunger is moveable from a locked position in which the locking member is trapped between the respective locking grooves so as to prevent extension of the plunger and an unlocked position in which the locking member is received in a relaxed state in the locking groove of the housing such that it does not project into the bore of the housing and the plunger is able to move relative to the housing and locking member. 
   According to a second aspect of the present invention there is provided a chain or belt comprising a plunger displaceable in a bore of a housing between retracted and extended positions and biased towards the extended position by a biasing member, the housing having a first recess and the plunger having a second recess, the first recess having a locking edge, and a radially resilient locking member that is slidable with the plunger relative to the housing from a first position in which it is trapped between said first and second recesses and abuts the locking edge so as to prevent movement of the plunger out of the housing and a second position in which it is received in the first recess such that it is clear of the plunger and allows it to move relative to the housing in either direction between extended and retracted positions. 
   According to a third aspect of the present invention there is provided a chain or belt tensioner comprising a plunger for displacement in a bore of a housing between retracted and extended positions, the plunger projecting from an open end of the bore in said extended position and biased towards the extended position by a biasing member, the plunger having a leading end that is inserted into the bore of the housing during assembly of the tensioner, the leading end of the plunger or the end of the housing at the entry to the bore having a tapered surface that tapers inwardly in the direction of entry of the plunger into the bore, a radially resilient locking member disposed in a recess in the housing or the plunger, a first locking surface defined by the recess and a second locking surface defined on an opposing surface of the plunger or housing, wherein during insertion of the plunger into the housing bore the tapered surface engages the locking member and deforms it in a radial direction such that it rides over it and moves longitudinally relative to said opposing surface and engages said second locking surface whereupon subsequent attempted extension of the plunger out of the housing is limited to a position where the locking member is trapped between said first and second locking edges. 
   According to a fourth aspect of the present invention there is provided a method for arming a chain or belt tensioner, the tensioner comprising a plunger for displacement in a bore of a housing between retracted and extended positions, the plunger projecting from an open end of the bore in said extended position and biased towards the extended position by a biasing member, the plunger having a leading end that is inserted into the bore of the housing during assembly of the tensioner, the leading end of the plunger or the end of the housing at the entry to the bore having a tapered surface that tapers inwardly in the direction of entry of the plunger into the bore, a radially resilient locking member disposed in a recess in the housing or the plunger, a first locking surface defined by the recess and a second locking surface defined on an opposing surface of the plunger or housing, the method comprising the step of inserting the plunger into the housing bore such that the tapered surface engages the locking member and deforms it in a radial direction such that it rides over it and moves longitudinally relative to said opposing surface and engages said second locking surface whereupon subsequent attempted extension of the plunger out of the housing is limited to a position where the locking member is trapped between said first and second locking edges. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
       FIG. 1  is a partial longitudinal view of a tensioner of the present invention shown in part cross-section in an initial stage of assembly, the structure shown being symmetrical about the centre line; 
       FIGS. 2 and 3  are views corresponding to that of FIG.  1  and show subsequent stages in the assembly of the tensioner; 
       FIGS. 4  to  6  are views corresponding to that of FIG.  1  and show the release of a plunger of the tensioner; 
       FIG. 7  is a view corresponding to previous views and shows the tensioner in a locked configuration; 
       FIGS. 8 and 9  are partial views in part cross-section showing dismantling of the tensioner; 
       FIGS. 10  to  12  are views corresponding to those of  FIGS. 2  to  4  showing a modified embodiment of the tensioner of the present invention; 
       FIG. 13  is a partial sectioned view of an alternative tensioner embodiment; and 
       FIGS. 14  to  16  are views corresponding to those of  FIGS. 3 ,  4  and  7  showing an alternative embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, the exemplary chain tensioner has a body  1  that is designed to be inserted into a bore defined in an internal combustion engine cylinder block (not shown). To this end the body  1  has, at one end, a head  2  with a plurality of flats for engagement with an appropriate fastening tool and an adjacent threaded portion  3  by which the tensioner is screw-engaged in the engine bore. 
   The tensioner body  1  is generally cylindrical and has a blind bore  4  that is open at one end to receive a slidable hollow plunger  5 . The exposed end  5   a  of the plunger  5  is closed and engageable with a movable chain guide or shoe (not shown). The other end of the plunger  5  has a chamfered outer edge  6  the purpose of which will become apparent later. 
   The interior of the bore  4  and the plunger  5  define a variable volume pressure chamber  7  that is filled with hydraulic oil. The chamber  7  is supplied with oil from an oil reservoir (not shown) via a passage  8  and a ball check valve  9 . When the pressure of the oil supplied through the passage  8  exceeds that in the chamber  7  the ball is lifted from its check valve seat and permits passage of oil into the chamber  7 . Conversely, when the oil pressure in the chamber  7  exceeds that of the supply the ball returns to its seat and the check valve  9  is closed so as to prevent escape of oil from the chamber  7 . 
   The plunger  5  has a small central opening  10  at its closed end that serves as a vent for air and excess fluid in the pressure chamber  7  as is well known. The presence of any trapped air in the oil in the chamber  7  is undesirable and is vented to atmosphere via the vent opening  10 . Access to the vent opening  10  is restricted by a venting plug  11  of known design so as to limit the escape of air and fluid. 
   A compression spring  12  is disposed coaxially between the closed end of the plunger  5  and the base of the housing bore  4  (for clarity only part of spring is shown in the figures) and serves to bias the plunger  5  outwards of the housing body  1  towards the chain so as to impart tension thereto. 
   The plunger  5  has two spaced annular grooves  13 ,  14  defined on its outer surface. A first groove  13  is defined near the closed end of the plunger  5  and is designed to receive an annular locking ring  15 . A second groove  14  is disposed towards the opposite end of the plunger  5  and, in use, receives an annular stop ring  16 . 
   The first groove  13  has a first end face  17  nearest the closed end of the plunger  5  and which extends in a substantially radial plane and a second end face  18  that is tapered radially inwardly in the direction towards the first face  17 . The associated locking ring  15  has a radial slot to render it radially expansible or contractible and resilient such that it recovers its original diameter if forced to contract or expand. The ring  15  is of triangular or trapezoidal cross-section with a bevelled edge  19  facing the tapered end face  18  of the groove  13 . The ring  15  may be manufactured from, for example, a material such as aluminum, steel or durable plastics. 
   The second annular groove  14  is longer in the axial direction of the tensioner than the first groove  13  and has a radially extending first end face  20  furthest from the closed end of the plunger  5  and a second end face  21  that is tapered radially outwardly in the direction towards the first groove  13 . The associated stop ring  16  is of similar configuration to the locking ring  15  but is disposed in the tensioner in a reverse orientation i.e. with a bevelled edge  16   a  facing the tapered end face  21  of the groove  13  and has a rounded edge  22  at the radial extremity that faces towards the open end of the bore  4 . 
   The interior surface of the body  1  that defines the bore  4  has two spaced annular grooves  23 ,  24  to correspond to those of the plunger  5 . A locking groove  23  has a main portion  25  with a profile complementary to the locking ring  15  and is designed to receive the ring  15  such that it does not project radially therefrom. It has a leading shoulder  26  that is only marginally larger in diameter that the bore  4  and terminates in a ramped locking edge  27 . A stop ring groove  24  has a radially extending end face  28  and a tapered end face  29  extending radially inwards in the direction towards the open end of the bore  4 . The surface of the body  1  between the two grooves  23 ,  24  is relatively smooth and is free of notches or grooves. 
   To assemble or reset the tensioner the stop ring  16  is first placed in the stop ring groove  24  of the body  1  and the locking ring  15  is placed around the first groove  13  in the plunger  5 . The inside diameter of the stop ring  16  is slightly less than that of the stop ring groove  24  such that it projects radially inwardly therefrom and the locking ring  15  is slightly larger in diameter than the first groove  13  on the plunger such that it projects radially outwards therefrom. The plunger  5  is pushed into the chamber  7  so that its chamfered edge  6  comes into abutment with the bevelled edge  16   a  of the stop ring  16  (FIG.  1 ). By virtue of the co-operation of the abutting edges  6 ,  16   a  further insertion of the plunger  5  forces the stop ring  16  to expand radially outwards into the stop ring groove  24  where it is held by the outer surface of the plunger  5  ( FIG. 2 ) until it comes into alignment with the second groove  14  on the plunger  5  whereupon it relaxes radially into that groove  14  (FIG.  3 ). At the same time the locking ring  15  is forced into the bore  4  by deforming it radially inwardly so that it passes into the leading shoulder  26  of the locking ring groove  23  in the body  1  and is contracted into the first groove  13  in the plunger  5 . At this point the plunger  5  is locked against movement out of the body  1  by the locking edge  27  and the tensioner may be inserted into the engine cylinder block with little risk of the plunger  5  falling out of the housing body  1  (FIG.  3 ). 
   Once the tensioner is in placed in the engine, the plunger  5  is released by pushing it further into the body  1  until the locking ring  15  is brought into register with the main portion  25  of the locking ring groove  23  in the body  1  whereupon it relaxes radially and expands into the main portion  25  (FIG.  4 ). The plunger  5  is then free to extend out of the body  1  under the action of the biasing spring  12  and, when the engine is running, under the pressure of the hydraulic oil in the pressure chamber  7 . The movement of the plunger  5  in this direction brings the first end face  20  of the second groove  14  in the plunger  5  into abutment with the stop ring  16  such that it is carried with the plunger  5  (FIG.  5 ). Continued extension of the plunger  5  brings the leading edge  22  of the stop ring  16  into abutment with the tapered second end face  29  of the stop ring groove  24  (see  FIG. 5 ) and the ring  16  is forced to contract so that it is sandwiched radially between the smooth portion of the bore  4  and the second groove  14  in the plunger  5 . In this position the stop ring  16  is in frictional contact with the housing but is free to slide over the smooth surface of the bore  4  in the direction of the open end so that it is carried by the plunger  5  during extension (FIG.  6 ). However, should the plunger  5  be forced to retract owing to a sudden increase in tension in the chain the second groove  14  of the plunger moves over the stop ring  16  (which is in frictional contact with the bore  4 ) thereby allowing a small inward movement of the plunger  5  until the tapered end face  21  of the groove  14  comes into abutment with the bevelled edge  16   a  of the stop ring  16 . The tapered end face  21  acts as a cam surface to force the stop ring  16  radially outwards so that it is wedged against the smooth portion of the bore  2  and further inward movement of the plunger  5  is prevented by the increased frictional contact between the ring  16  and body  1  (FIG.  7 ). 
   When there is a sudden increase in chain tension and therefore load on the plunger the limited retraction of the plunger is restricted by virtue of the oil pressure in the chamber  7  and is only permitted by virtue of a limited flow of hydraulic oil via the vent  10  and plug  11 . The retracting motion of the plunger  5  is thus damped by the oil. In the instance where the there is insufficient oil pressure in the chamber  7  such as during engine start-up or idling the stop ring  16  serves to prevent excessive retraction of the plunger  5 . 
   In order to dismantle the tensioner for replacement, servicing or repair etc. the plunger  5  is pulled outwards so that the stop ring  16  travels over the bore  4  and the locking ring  15  in the locking groove  23  of the housing. It will be noted from  FIG. 8  that the radial thickness of the locking ring  15  is closely matched to the depth of the main portion  25  of the locking ring groove  23  so that the stop ring  16  can travel over the locking ring  15  without difficulty until its rounded edge  22  comes into abutment with the locking edge  27  of the leading shoulder  26 . Further pulling of the plunger  5  forces the rounded edge  22  of the stop ring  15  to ride over the ramped locking edge  27  of the shoulder thereby enabling the plunger  5  to be fully removed from the housing body  1  (FIG.  9 ). 
   It is to be understood that the locking ring may be omitted from the design or replaced by other locking means. 
     FIGS. 10  to  12  illustrate a modified tensioner in which the stop ring  116  operates as before but additionally performs the function of the locking ring. 
   The structure of the tensioner body  101  and the plunger  105  differs only in that the grooves  13  and  23  are eliminated and in that there is provided a shallow step  140  immediately adjacent the second groove  114  in the plunger  105  at the end opposite the tapered end face  121 . The step  140  defines a stepped radially extending edge  141 . The stop ring  116  is initially placed in the stop ring groove  124  and is loaded on to the plunger  105  by pushing the plunger inwardly of the body so that it rides over the chamfered end surface  106 .  FIG. 10  shows the stop ring  116  received in the stop ring groove  124  in the body  101 . As the stop ring  116  abuts the end face  128  of the stop ring groove  124  further insertion of the plunger brings it into register with the step  140  (see FIG.  11 ). The tensioner is now armed such that if an attempt is made to remove the plunger  105  from the body  101  the leading edge of the stop ring  116  is trapped brought into contact with the tapered end face  129  of the stop ring groove  124  and the ring  116  trapped between it and the step edge  141  so that removal of the plunger is prevented as before (FIG.  12 ). This armed configuration is released as before by pushing the plunger into the tensioner body so that the stop ring  116  abuts the radially extending end face  128  of the stop ring groove  124  (as in  FIG. 11 ) and is then forced to move into the groove  114  in the plunger  105  whereupon the tensioner is ready for use. 
   It is to be understood that the tapered end faces  21 ,  121 ,  29 ,  129  of the grooves need not be planar as shown in  FIGS. 1  to  12  but may be of any suitable smooth profile that reduces (or increases) in diameter. An example of an arcuate taper is shown at  221  in FIG.  13 . Similarly the complementary bevelled surface on the stop or locking rings may also be other than planar. 
   It will be appreciated that numerous modifications to the above described design may be made without departing from the scope of the invention as defined in the appended claims. For example, whilst the above described embodiment of the tensioner is used for tensioning a chain the same design may be used with minor modification to tension a belt. Moreover, the positions of the stop ring and associated groove may be reversed such that, in use, the stop ring is seated in a groove defined in the housing. In such an embodiment movement of the plunger out of the housing is permitted by virtue of it having a smooth surface that is able to slide over the stop ring (which has an inside diameter smaller than the outside diameter of the plunger) but retraction of the plunger into the housing is limited by the interaction of the bevelled surface of the ring with the tapered surface of the groove which forces the ring to contract radially on to the plunger in a wedging action. An example is shown in  FIGS. 14  to  16 . In  FIG. 14  the tensioner is armed in that the stop ring  316  locks the plunger  305  relative to the body  301  between a stepped edge  341  on the body and a tapered end face  350  of a groove  351  in the plunger  305 . The plunger is released as shown in  FIG. 15  by pushing it inwardly of the body as indicated by the arrow. In  FIG. 16  the plunger is shown in use and prevented from further retraction into the body by the wedging action of the stop ring. Again initial arming of the tensioner is achieved by inserting the plunger  305  into the housing  301  with the stop ring  316  in the groove  351 . The ring  316  rides over the chamfered surface  306  defined on the housing at the bore entry and is moved to the stepped edge  341 . 
   The foregoing disclosure and description of the preferred embodiment are illustrative and explanatory thereof, and various changes in the components, circuit elements, circuit configurations, and signal connections, as well as in the details of the illustrated circuitry and construction and method of operation may be made without departing from the spirit and scope of the invention.