The present invention relates generally to the automotive chain drive art and, more particularly, to a mechanical blade-type chain tensioner apparatus useful in confined spaces for applying a tensioning force to a chain traveling there past. Prior blade-type chain tensioning devices include a chain engaging blade or shoe member, typically molded from a resinous plastic material, having a metal spring installed therein to provide the shoe sub-assembly with the necessary rigidity and damping characteristics while taking advantage of the flexibility, low friction, and good wear properties of the plastic shoe.
FIG. 1 shows an exemplary known tensioner apparatus T′ comprising a bracket K typically defined from a metal stamping and a tensioner blade assembly BAS′ operably secured to the bracket. The bracket K is fixedly secured to an associated engine block EB (FIG. 2) as part of a chain drive system that is provided to phase or “time” the rotational position of one or more camshaft sprockets CMS with respect to the rotational position of the crankshaft sprocket CKS. A chain 15 such as a roller/bush chain or inverted tooth chain is engaged with the crankshaft sprocket CKS and the camshaft sprocket(s) CMS and phases/times the camshaft sprocket(s) to the crankshaft sprocket. The crankshaft sprocket CKS rotates in a direction DIR, and the chain 15 includes a taut strand portion 16 and a slack strand portion 17.
In the illustrated embodiment, the known tensioner T′ comprises an optional fixed chain guide portion FG. The fixed chain guide portion comprises a fixed guide flange XF that projects transversely from the main wall MW of the bracket K and that is engaged with and supports a fixed chain guide G defined from a polymeric (plastic) material. The fixed chain guide G includes a guide face GF that slidably engages and supports the taut strand 16 of the chain as shown in FIG. 2.
The tensioner T′ further comprises a tensioner portion TP. To define the tensioner portion TP, the bracket K comprises a pin P that is welded or otherwise securely affixed to the main wall MW and that projects perpendicularly outward there from. The bracket K further comprises a support flange TF that projects outwardly from the main wall MW. An end of the support flange TF forms or defines a ramp R, and an outer wall OW extends transversely from an outer end of the ramp R and extends parallel to the main wall MW such that a channel CH is defined between the main wall MW, the outer wall OW, and the ramp R.
A known tensioner blade assembly BAS′ includes a polymeric or “plastic” shoe B′ and a metal spring S releasably connected to the shoe B′. A first or pivot end B1′ of the blade assembly BAS′ includes a boss or barrel BL that includes pivot bore PB that is slidably received onto the pivot pin P. An opposite second or free end B2′ of the blade assembly BAS′ is located in the channel CH supported on the ramp R. The bracket K thus maintains the blade assembly BAS′ in its proper position with respect to the plane of the chain path while permitting sliding reciprocal translational motion of the second, free end B2′ on the ramp R as indicated by the arrow “TRANS” along with the related rotational movement of the blade assembly BAS′ at the pivot end B1′ as indicated by the arrow labeled “ROTATE” in response to changes in the tension and position of the slack strand 17 of the chain 15 and corresponding oscillatory movement of the slack strand 17 as indicated by the arrow “AMPL.” FIG. 2A is a partial view of the tensioner T′ that shows this operative movement of the blade assembly BAS′ using solid lines for a first position of the blade assembly BAS′ and phantom lines for a second position of the blade assembly BAS′. The pivot pin P can be replaced by a shoulder bolt or any other suitable fastener that allows rotation of the pivot end B1′ of the shoe B′ relative to the bracket K.
FIGS. 3-5 illustrates the known blade assembly BAS′ by itself, separated from the bracket K. The shoe B′ is a one-piece molded polymeric construction and the spring S is a one-piece metal stamping leaf-spring structure or the like. The shoe B′ includes a rear face RF that lies adjacent and/or slidably abuts the main wall of the bracket K when the blade assembly BAS′ is operatively installed on the bracket K. The shoe B′ also includes an opposite front face FF that is spaced from the bracket main wall MW and that faces away from the bracket main wall MW when the blade assembly BAS′ is operatively installed on the bracket T.
The pivot end B1′ of the shoe B′ and a free end B2′ of the shoe B′ define respective first and second spring-receiving slots L1′,L2′ for receiving and retaining opposite ends S1,S2 of the spring S. The shoe B′ includes a central body B3 that extends between and interconnects the pivot and free ends B1′,B2′. An upper or outer surface OS of the central body B3 provides a chain contact surface for being slidably engaged by an associated chain being tensioned. The central body B3 includes a lower or inner surface IS that is defined by the underside of the central body B3 that is opposite the outer surface OS. The inner surface IS is contacted by an arched central portion S3 of the spring S.
The first end S1 of the spring S is retained in the slot L1′ between a first outer wall W1 and the main wall MW of the bracket K. The first outer wall W1 abuts or lies closely adjacent an outer spring edge E1 and an inner spring edge E2 lies closely adjacent and/or in contact with the main wall MW of the bracket K. The second end S2 of the spring S is retained in the slot L2′ between a second outer wall W2 and the bracket main wall MW. The second outer wall W2 abuts or lies closely adjacent the outer spring edge E1 and the inner spring edge E2 abuts and/or lies closely adjacent the bracket main wall MW. Accordingly, as can be seen in FIG. 4, the inner spring edge E2 is located adjacent and flush with the rear face RF of the shoe B′ which causes the inner spring edge E2 to contact the main wall MW of the bracket K during use of the tensioner T′. Repetitive high-speed sliding contact between the inner spring edge E2 and the bracket main wall MW causes polishing of the main wall MW—and in cases where the chain drive dynamics are excessive—erosion of the main wall is known to occur, particularly near the spring ends, and this may induce relative movement of the spring S relative to the plastic shoe B′ due to friction between the spring inner edge E2 and the bracket main wall MW, which leads to abrading or cutting damage of the plastic shoe B′ in the areas where it is contacted by the spring S.
Several prior art patents show such blade-type mechanical chain tensioning devices that include a blade spring that can be manufactured from spring steel strip stock without any notches or cutouts to the spring and which are normally cut to length from the steel strip stock and then formed to provide a desired free height. For example, prior U.S. Pat. No. 5,286,234 to James D. Young discloses a chain tensioner wherein such a blade spring is received laterally into a slot of an elongated plastic shoe and the spring is laterally restrained on one side by the outside walls of the shoe at each end of the slot and on the opposite side by the tensioner bracket wall.
Another structure is described in U.S. Pat. No. 5,266,066 to David C. White, in which the installed blade spring is laterally restrained by a “fence”—effectively, a longitudinal rib—on the underside (concave side) of the shoe.
These prior art blade tensioners have disadvantages, however, for some chain drive applications having higher camshaft and/or crankshaft torsional vibrations that may result in increased blade flexing during engine operation and thereby requiring higher tensioning loads to limit the amplitude of the blade flexing at its central region. A particular drawback associated with the tensioner defined in U.S. Pat. No. 5,286,234 to Young is the potential for excessive erosion to the bracket wall caused by spring edge-to-wall contact with the more dynamically active chain drive systems. A particular limitation with the blade shoe as disclosed in U.S. Pat. No. 5,266,066 to White is that during engine operation the underside of the blade at its central region will be more highly stressed—particularly at the outermost fiber of the fence—than for a blade having a rectangular cross-section. High stresses may also occur in the fillet region between the fence and blade central body.
A further construction is shown in Ferenc et al. U.S. Pat. No. 5,711,732. This blade-type tensioner does not have the limitations of the above described prior art devices, but this is achieved by the design compromise of incorporating central notches at each end of the blade spring which have been found to abrade the plastic blade shoe over time as the blade flexes and the ends of the spring slide relative to the adjacent portions of the plastic shoe.