Abstract:
A clip-type mechanism for tensioning slack in a chain. A spring-biased piston is positioned in a bore in a housing. A ratchet rack is positioned on the piston, along with a clip member. A recess in the housing bore has an annular member with a hardened surface to reduce wear.

Description:
TECHNICAL FIELD 
     The present invention relates to a tensioner for producing tension in a chain, and more specifically to a clip-type tensioner having a ratchet mechanism. 
     BACKGROUND OF THE INVENTION 
     Clip-type tensioners, as they are known, have been used in the past as chain tensioners. A clip-type tensioner consists of a housing, a piston which is slidably supported in a hole of the housing, a spring for urging the piston so as to protrude from the hole, and a ratchet mechanism. The ratchet mechanism has a number of rack teeth on the outer periphery of the piston, a clip which engages with the rack teeth, and a recess on the inner peripheral surface of the housing. One example is shown in FIG. 1 of Japanese Unexamined Patent Application Publication 2005-98420. The recess on the inner peripheral surface of the housing has a front end face which comes into contact with the clip when the piston advances, allowing the piston to advance, and a rear end face (seat face) which comes into contact with the clip when the piston retracts, thereby holding the clip between the rack teeth and restricting retraction of the piston. 
     When slack is produced in the chain during operation, the piston advances toward the chain under the elastic repelling force of the spring and protrudes outside the housing The clip which has engaged with the rack teeth on the outer periphery of the piston comes into contact with the front end face of the recess at this point, which means that the piston is allowed to advance. On the other hand, when tension in the chain increases, the excessive pressing force from the chain causes the piston to retract, pushing it into the housing The clip which has engaged with the rack teeth on the outer periphery of the piston comes into contact with the rear end face of the recess at this point, which means that retraction of the piston is restricted. 
     However, with the conventional clip-type tensioner described above, when the piston has been pushed into the housing, the clip collides with the rear end face of the recess, causing the clip to exert an excessive pressing force on the rear end face of the recess. Consequently, the rear end face of the recess is worn with repeated usage. 
     SUMMARY OF THE INVENTION 
     The present invention has been devised in view of the situation outlined above, and the issue addressed by the present invention lies in providing a clip-type tensioner with which wear can be reduced to lengthen the lifespan. 
     The clip tensioner according to a preferred embodiment of the invention is provided with: a housing in which a hole is formed which is open at one end; a piston which is slidably supported in the hole; and a spring which urges the piston so as to protrude from the hole. The ratchet mechanism consists of: a number of rack teeth which are formed on the outer periphery of the piston; a clip having a circular portion whereof the diameter can be enlarged, which elastically engages with the rack teeth in a diametric direction; and a recess provided in the housing and accommodating the clip. The recess has a first face which comes into contact with the clip when the piston advances, allowing the piston to advance, and a seat face which comes into contact with the clip when the piston retracts, thereby holding the clip between the rack teeth and restricting retraction of the piston. An annular member is provided at the second face opposite the first face inside the recess, and the annular member has an oblique face which constitutes the seat face of the recess, the oblique face having a higher surface hardness than the second face of the recess. 
     When slack is produced in the chain during operation, the piston advances under the elastic repelling force of the spring and protrudes outside the housing The clip which has engaged with the rack teeth on the outer periphery of the piston comes into contact with the first face of the recess in the housing at this point, which means that the piston is allowed to advance. On the other hand, when tension in the chain increases during operation, the excessive pressing force from the chain causes the piston to retract, pushing it into the housing The clip which has engaged with the rack teeth on the outer periphery of the piston comes into contact with the seat face of the recess in the housing at this point, which means that retraction of the piston is restricted. 
     Preferably the seat face is constituted by the oblique face of the annular member which is provided at the second face inside the recess, and the oblique face has a higher surface hardness than the second face of the recess, and therefore wear of the seat face can be reduced, which makes it possible to improve the lifespan of the tensioner. It is unnecessary to use special materials for the housing or to subject the housing to special heat treatment in order to improve the wear resistance of the seat face of the recess, which makes it possible to inexpensively reduce the wear on the seat face. 
     The annular member is preferably made of a wear resistant material. The annular member can be made of steel, such as carbon steel or stainless steel, for example, or a sintered alloy. The annular member can also be press-fit into the recess. 
     The housing is preferably made of aluminum, titanium or magnesium, or an alloy or sintered alloy of aluminum, titanium or magnesium. Aluminum, titanium and magnesium are generally soft but lightweight, and therefore the seat face of the recess can be endowed with high hardness by providing the annular member in the recess of the housing, while the tensioner as a whole can be made lighter. The housing may also be made of an alloy (including a sintered alloy) of aluminum, titanium or magnesium. 
     With the clip-type tensioner according to a preferred embodiment of the present invention, when the excessive pressing force from the chain during operation causes the piston to retract and to be pushed inside the housing, the clip which has engaged with the rack teeth on the outer periphery of the piston comes into contact with the seat face of the recess in the housing so that retraction of the piston is restricted, but in this case the seat face consists of the oblique face of the annular member which is provided at the second face of the recess, and the surface hardness of the oblique face is greater than the surface hardness of the second face of the recess, and therefore wear of the seat face can be reduced, and the lifespan of the tensioner can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view in longitudinal section of the clip-type tensioner according to an exemplary embodiment of the present invention; 
         FIG. 2  shows an enlargement of some of the rack teeth of the piston and illustrates the operational effects of this exemplary embodiment; and 
         FIG. 3  is a view in longitudinal section of the annular member 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 to 3  illustrate a clip-type tensioner according to an exemplary embodiment of the present invention, where  FIG. 1  is a view in longitudinal section of the clip-type tensioner according to this exemplary embodiment,  FIG. 2  shows an enlargement of some of the rack teeth of the piston and illustrates the operational effects of this exemplary embodiment, and  FIG. 3  is a view in longitudinal section of the annular member. 
     As shown in  FIG. 1 , a clip-type tensioner  1  has: a housing  2  made of aluminum, for example, in which is formed a hole  2   a  which is open at one end; a hollow piston  3  which is slidably supported in the hole  2   a  of the housing  2 ; and a spring  4  for urging the piston  3  so as to protrude from the hole  2   a  (only part of this spring is depicted). Moreover, the material from which the housing  2  is made is not limited to aluminum—it may be titanium or magnesium, or an alloy (sintered alloy) thereof. 
     An oil passage  25  for supplying the oil from a hydraulic pressure supply source (not depicted) to a chamber  30  which is formed by the space inside the hole  2   a  in the housing  2  and the piston  3  is formed at the bottom of the housing  2 . Furthermore, screw holes  125 ,  126  into which mounting screws for mounting the tensioner  1  are inserted are provided on the housing  2 . 
     A number of rack teeth  31  are formed on the outer periphery of the piston  3 . A clip  6  having a circular portion whereof the diameter can be enlarged, which elastically engages with the rack teeth  31  in a diametric direction is fitted on the rack teeth  31 . A recess  20  is formed on the inner peripheral surface at the upper part of the housing  2 . The clip  6  is accommodated inside the recess  20 . 
     As shown in  FIG. 2 , the recess  20  has a first face  21  which is formed at the upper end, and a second face  22  which is formed at the lower end and lies opposite the first face  21 . An annular member  5  is provided at the second face  22 . As shown in  FIG. 3 , the annular member  5  has a flat bottom face  51 , an oblique face  52  shaped like an inverted cone which is formed above the flat face, and inner and outer cylindrical peripheral faces  53 ,  54 . 
     The outer peripheral face  54  of the annular member  5  is press-fitted at a side wall  23  of the recess  20  in the housing  2 , and the bottom face  51  of the annular member  5  is in contact with the second face  22  of the recess  20 . The oblique face  52  of the annular member  5  constitutes the seat face  52  with which the clip  6  can make contact inside the recess  20 . The seat face  52  has a higher surface hardness than the second face  22  of the recess  20 . 
     The annular member  5  is preferably made of a wear resistant material. To be more specific, the annular member  5  is made of a steel material, such as carbon steel or stainless steel, for example, or a sintered alloy. 
     The rack teeth  31  have a bottom part of maximum depth which is formed at a position toward the front end of the piston, and a gently inclined face which is a continuation of the bottom part and is formed at a position toward the rear end of the piston. The clip  6  is seated in the bottom part of the rack teeth  31 . 
     When the piston  3  advances with the clip  6  fitted in the rack teeth  31  of the piston  3 , the first face  21  of the recess  20  is in contact with the clip  6 , allowing the piston  3  to advance; when the piston  3  retracts with the clip  6  fitted in the rack teeth  31  of the piston  3 , the seat face  52  is in contact with the clip  6  which is held between the rack teeth  31  so that retraction of the piston  3  is restricted. The ratchet mechanism consists of the rack teeth  31 , the first face  21 , the seat face  52 , and the clip  6 . 
     The operational effects of this exemplary embodiment will be described next. 
     First of all, when slack is produced in the chain (not depicted) during operation, the piston  3  advances under the elastic repelling force of the spring  4  and protrudes from the hole  2   a  in the housing  2 , while the clip  6  which has engaged with the rack teeth  31  on the outer periphery of the piston moves together with the piston  3 . At this point, if there is a large amount of movement of the piston  3 , the clip  6  comes into contact with the first face  21  of the recess  20  in the housing  2  and only the piston  3  advances from this state, removing slack from the chain. 
     Next, when the tension in the chain increases during operation, the excessive pressing force from the chain causes the piston  3  to retract, pushing it into the hole  2   a  in the housing  2 , and the clip  6  which has engaged with the rack teeth  31  on the outer periphery of the piston moves together with the piston  3 . Then, when the clip  6  comes into contact with the seat face  52  of the recess  20  in the housing  2 , the clip  6  is held between the rack teeth  31  and the seat face  52  (see  FIG. 2 ), whereby retraction of the piston  3  is restricted. 
     While the invention described in connection with various embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention covers all alternatives, modifications, and equivalents as may be included within the spirit and scope of the appended claims. 
     In this case, the seat face  52  is constituted by the oblique face  52  of the annular member  5  which is provided at the second face  22  inside the recess  20 , and the oblique face  52  has a higher surface hardness than the second face  22  of the recess  20 , and therefore wear of the seat face  52  can be reduced, which makes it possible to improve the lifespan of the tensioner. Furthermore, in this case, it is unnecessary to make the whole of the housing  2  out of special materials or to subject the housing  2  to special heat treatment in order to improve the wear resistance of the seat face  52  of the recess  20 , which makes it possible to inexpensively reduce the wear on the seat face. 
     Furthermore, when the housing is made from aluminum, titanium or magnesium, these materials are generally soft but lightweight, and therefore the seat face  52  of the recess  20  can be endowed with high hardness by providing the annular member  5  in the recess  20 , while the tensioner as a whole can be made lighter.