Hydraulic chain tensioner with molded plastic body

A hydraulic tensioner is designed for low cost and ease of manufacturing. A housing has a bore and a sleeve member received within the bore. A piston is slidably received within the sleeve member, forming a high pressure fluid chamber with the sleeve member. The housing can be constructed of inexpensive materials such as aluminum or plastic. The piston and sleeve member are constructed of drawn metal. The housing may be formed by injection molding. The sleeve member and other components of the tensioner may be assembled within the tensioner by inserting them into the mold during the injection molding process.

Reference is made to application Ser. No. 08/760,267, filed Dec. 5, 1996, 
now abandoned entitled, "Hydraulic Tensioner With Plastic Body And 
Piston," the subject matter of which relates to the present invention and 
which is incorporated herein by reference. 
Reference is also made to copending application Ser. No. 08/947,594, filed 
Oct. 9, 1997, entitled, "Hydraulic Chain Tensioner With Deep Drawn Cup," 
filed concurrently with this application, which is incorporated herein by 
reference. 
BACKGROUND OF THE INVENTION 
A tensioning device, such as a hydraulic tensioner, is used as a control 
device for a power transmission chain, or similar power transmission 
device, as the chain travels between a plurality of sprockets. In this 
device, the chain transmits power from a driving shaft to a driven shaft, 
so that part of the chain is slack and part of the chain is tight. 
Generally, it is important to impart and maintain a certain degree of 
tension in the chain to prevent noise, slippage, or the unmeshing of teeth 
in the case of a toothed chain. 
Prevention of such slippage is particularly important in the case of a 
chain driven camshaft in an internal combustion engine because jumping of 
teeth will throw off the camshaft timing, possibly causing damage or 
rendering the engine inoperative. However, in the harsh environment of an 
internal combustion engine, various factors can cause fluctuations in the 
chain tension. 
For instance, wide variations in temperature and thermal expansion 
coefficients among the various parts of the engine can cause the chain 
tension to vary between excessively high or low levels. During prolonged 
use, wear to the components of the power transmission system can cause a 
decrease in chain tension. In addition, camshaft and crankshaft induced 
torsional vibrations cause considerable variations in chain tensions. 
Reverse rotation of an engine, occurring for example in stopping or in 
failed attempts at starting, can also cause fluctuations in chain tension. 
For these reasons, a mechanism is desired to remove excessive tensioning 
forces on the tight side of the chain and to ensure the necessary tension 
on the slack side of the chain. 
Hydraulic tensioners are a common method of maintaining proper chain 
tension. In general, these mechanisms employ a lever arm that pushes 
against the chain on the slack side of the power transmission system. This 
lever arm must push toward the chain, tightening the chain, when the chain 
is slack, and must be very rigid when the chain tightens. 
To accomplish this result, a hydraulic tensioner typically comprises a rod 
or cylinder as a piston, which is biased in the direction of the chain by 
a tensioner spring. The piston is housed within a cylindrical housing, 
having an interior space which is open at the end facing the chain and 
closed at the other end. The interior space of the housing contains a 
pressure chamber in connection with a reservoir or exterior source of 
hydraulic fluid. The pressure chamber is typically formed between the 
housing and the piston, and it expands or contracts when the piston moves 
within the housing. 
Typically, valves are employed to regulate the flow of fluid into and out 
of pressure chamber. For instance, an inlet check valve typically includes 
a ball-check valve that opens to permit fluid flow in to the pressure 
chamber when the pressure inside the chamber has decreased as a result of 
outward movement of the piston. When the pressure in the pressure chamber 
is high, the inlet check valve closes, preventing fluid from exiting the 
pressure chamber, which in turn prevents the piston chamber from 
contracting, which in turn prevents the piston from retracting, achieving 
a so-called "no-return" function. 
Many tensioners also employ a pressure relief mechanism which allows fluid 
to exit the pressure chamber when the pressure in the chamber is high, 
thus allowing the piston to retract in response to rapid increases in 
chain tension. In some tensioners, the pressure relief mechanism is a 
spring biased check valve, which opens when the pressure in the pressure 
chamber becomes high. Some tensioners may employ a valve which performs 
both the inlet check function as well as the pressure relief function. 
Other mechanisms employ a restricted path through which fluid may exit the 
fluid chamber, such that the volume of flow exiting the fluid chamber is 
minimal unless the pressure in the fluid chamber is great. For instance, a 
restricted path may be provided through the clearance between the piston 
and bore, through a vent tube in the protruding end of the piston, or 
through a vent member between the fluid chamber and the fluid reservoir. 
A number of challenges exist in the design of hydraulic tensioners. One 
general design problem is the high cost and difficulty of manufacture and 
assembly. Traditionally, hydraulic tensioners have been constructed of 
cast iron housing bodies. The cast metal components provide the required 
close fit between the housing and the piston, and provide for strength and 
durability of the tensioner. However, this type of construction is 
expensive and difficult to manufacture. A need exists for a lower cost 
hydraulic tensioner which is easier to manufacture and assemble. 
One example of a tensioner design directed to reduced cost is described in 
Ojima et al., U.S. Pat. No. 5,037,357. Ojima et al. disclose a spring 
loaded tensioner including a body having a bearing surface, a first spring 
seated against the bearing surface and biasing a piston in a protruding 
direction. A second spring functions as a damper allowing the piston to 
retract in response to increasing tension in the belt or chain. The body 
may be made of sheet metal, allowing for low cost manufacturing. The 
disadvantages of this design include the reliance on springs to provide 
the "no return" and pressure relief functions. As a result, this design 
does not provide the advantages in performance provided by a hydraulic 
tensioner. 
Another tensioner known in the art employs a metal insert positioned within 
the bore of the housing body. The metal insert has cylindrical body and a 
solid bottom seated in the end of the bore. The fluid chamber is formed 
between the metal insert and the piston. Because the fluid chamber is 
formed with the metal insert, rather than the bore of the housing, the 
housing may be made of a less expensive material such as plastic. However, 
the metal insert may be difficult and expensive to manufacture and 
assemble. In particular, a cup-shaped insert may be difficult and 
expensive to manufacture. Moreover, this tensioner lacks a spring to bias 
the piston outward, and instead the tensioner relies on oil pressure in 
the fluid chamber to bias the piston outward. 
Another problem in the design of tensioners is excessive retraction of the 
piston during engine start up. Such retraction may produce undesirable 
noise in the system, or may allow the chain to slip or skip a tooth. One 
cause of such retraction is the leakage of oil from either the fluid 
chamber or the oil reservoir while the engine is off. For instance, fluid 
may leak from the fluid chamber through a clearance between the piston and 
the bore. Fluid may also leak from the oil reservoir, particularly in the 
case of an oil reservoir positioned in a cast iron housing. Such leakage 
may be accompanied by the introduction of air into the fluid chamber. 
Because air is more compressible than fluid, the presence of air in the 
fluid chamber allows significantly greater piston retraction, and reduced 
tensioner performance. 
Excessive piston retraction during engine start up may also be caused by 
force applied by the chain while the engine is off. For instance, if the 
vehicle is left on a hill, the rotational force on the wheels may cause 
increased chain tension in the engine. The resulting increase in chain 
tension may cause leak down of the piston, and poor tensioner performance 
during engine start-up. Thus, a need exists for a tensioner design which 
avoids excessive piston retraction during engine start-up. 
The problems of leakage of fluid from the fluid chamber and undesired 
piston retraction also affect the means available for minimizing the cost 
to manufacture and assemble the tensioner. A poor fit between the piston 
and the bore of the housing allows greater leakage of fluid from the fluid 
chamber. Further, it is difficult and expensive to maintain the required 
close manufacturing tolerances of the bore and piston to avoid excessive 
leakage. Typically, the tensioner body is formed of cast iron or steel, 
with the bore machined out for the piston and check valve assembly. The 
disadvantages of this system include the expense of the casting system, 
which may require specialized manufacturing machines. In addition, the 
dimensional accuracy of the boring machine is limited. 
Accordingly, it is an object of the present invention to provide a 
hydraulic tensioner which can be inexpensively manufactured and assembled. 
It is another object of the present invention to provide a hydraulic 
tensioner having improved response to fluctuations in chain tension. It is 
a further object of this invention to provide a hydraulic tensioner having 
improved performance at engine start-up. It is yet another object of this 
invention to provide a hydraulic tensioner which is less susceptible to 
leakage from its fluid chamber. 
It is another object of this invention to provide a method of producing a 
less expensive and more effective hydraulic tensioner. 
SUMMARY OF THE INVENTION 
The present invention is directed to a hydraulic tensioner having a sleeve 
received in a bore of a housing body, such that the fluid chamber is 
formed between the sleeve and the piston. The housing body may be made of 
plastic. In addition, the housing body may be formed by injection molding, 
thereby allowing the sleeve to be molded into the housing body, providing 
for a leak-resistant fluid chamber. 
According to one aspect of the present invention, there is provided a 
tensioner for a wrapped power transmission device, e.g. a chain linking at 
least two rotating members such as a pair of sprockets. The tensioner has 
a plastic body having a bore. A sleeve is positioned within the bore. A 
piston is slidably received within the sleeve, and is biased toward the 
chain by a piston spring. The piston and sleeve form a fluid chamber 
within the bore. The fluid chamber is in connection with a source of 
fluid, and a check valve may be provided within fluid chamber, to regulate 
fluid flow in to and out of the fluid chamber. 
In some embodiments, the tensioner is manufactured by injection molding. In 
this process, generally, a plastic material is melted and then injected 
into the cavity of a mold. Once the melted plastic is in the mold, it 
cools to a shape that reflects the cavity. A variation of this process, 
known as insert molding, involves inserting other components into the mold 
prior to the injection of the melted plastic. Upon insertion, the melted 
plastic flows around and surrounds the inserted component. When the 
plastic cools and hardens, the inserted component is firmly embedded in 
the plastic body. 
Construction of the tensioner through injection molding and insert molding 
provides a number of advantages over conventional methods. For instance, 
the bonding and interlocking of components assembled through insert 
molding is superior to the bonding and interlocking of components which 
are joined through conventional methods, such as press fitting. In 
addition, a seal can be formed between components which is difficult or 
impossible to achieve through other methods of construction. Insert 
molding is also easier and less expensive to accomplish than conventional 
methods. 
In one embodiment of the present invention, the plastic housing body may be 
formed through injection molding. In addition, the sleeve may be insert 
molded into the housing body during the injection molding process. The 
sleeve may be positioned within the mold such that the injected melted 
plastic flows around the sleeve, forming a leak-resistant seal with the 
sleeve. In other embodiments, grooves may be provided on the exterior 
surface of the sleeve, to facilitate and strengthen the joining of the 
sleeve and the housing body. Injection molding and insert molding are 
manufacturing processes known generally in the art, and are described, for 
instance, in U.S. Pat. No. 5,215,341 and U.S. Pat. No. 4,269,387, both of 
which are incorporated by reference into the present application. 
In other embodiments of the present invention, other components of the 
tensioner may also be insert molded during the injection molding process. 
For instance, a seal ring may be insert molded into the end of the bore, 
to provide a seat for the check valve. In another embodiment, a support 
member may be insert molded into a position against the sleeve, such that 
a portion of the support member is embedded in the housing body. When the 
melted plastic cools and hardens, the support member, by its positioning 
against the sleeve, helps to hold the sleeve in place with respect to the 
housing body. In addition, the support member supports the load applied by 
the check valve on the housing. 
Similarly, the injection molding process may be used to form the means by 
which the tensioner can be installed in the engine or other application. 
For instance, metal inserts may be insert molded into the housing body. 
These inserts may be sleeves or threaded bores, through which the 
tensioner may be attached to the engine by bolts. 
In another embodiment of the present invention, a fluid reservoir is 
incorporated into the housing body. By constructing the tensioner by 
injection molding and insert molding, a fluid reservoir may be 
incorporated in the housing body which is less susceptible to leakage than 
the fluid reservoir of a multiple-component metal tensioner. A seal may be 
employed to ensure that the reservoir is leak proof. In addition, 
incorporation of a fluid reservoir into the body of a plastic tensioner 
may be accomplished more easily and inexpensively as compared to 
incorporation of a fluid reservoir in a metal tensioner. 
In another embodiment of the present invention, a clip rack is employed to 
prevent undesired retraction of the piston. The clip rack is attached to 
the sleeve or housing body and engages grooves on the outer surface of the 
piston. The grooves are configured to allow extension of the piston, but 
to resist retraction of the piston. In this way, excessive retraction of 
the piston, particularly during engine start-up, is prevented. 
For a better understanding of these and other aspects and objects of the 
invention, reference should be made to the following detailed description 
taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning now to the drawings, FIG. 1 illustrates one embodiment of the 
present invention. A power transmission device 10 comprises a chain 12 
operating between two sprockets 14, 16. A lever arm 18 mounted on a pivot 
20 presses against the chain to maintain tension. A hydraulic tensioner 
100 has a piston 130, which applies force to the lever arm 18. 
FIG. 2 shows a cross sectional view of the embodiment of the present 
invention shown in FIG. 1. The tensioner 100 includes a housing 102 having 
a bore 104 and an inner surface 106 on this bore. A sleeve 105 is received 
within the bore 104 of the housing body 102. Preferably, the bore and 
sleeve are substantially cylindrical in shape. The piston 130, which is 
also preferably cylindrical, is slidably received in the sleeve 105, 
forming a fluid chamber 180 with the sleeve 105. The piston 130 is biased 
out of the housing 102 by a piston spring 170, such that the piston tip 
131 pushes against lever arm 18, as shown in FIG. 1. An elastic seal 
member, such as an O-ring (not shown), may facilitate a seal between the 
piston 130 and the sleeve 105. 
Preferably, the sleeve 105 is metal, and is formed by turning, such as on a 
lathe or screw machine. This method allows for greater dimensional 
accuracy than conventional methods. The sleeve is also easier and less 
costly to manufacture as compared to a cup-shaped insert having a solid 
bottom. The sleeve may also be constructed through other methods known in 
the art such as drawing or casting. 
The housing 102 may be constructed of any materials known in the art, such 
as steel or aluminum, but is preferably constructed of plastic, and is 
preferably formed through an injection molding process. An example of a 
suitable material for the housing 102 is polyphenylene sulfide (PPS) with 
glass and mineral fillers. 
The sleeve 105 is securely fixed within the bore 104. In one embodiment, 
the sleeve has an outer surface 108 having a plurality of projections or 
recesses 110. These projections or recesses 110 on the sleeve outer 
surface 108 mesh with corresponding projections or recesses 112 on the 
bore inner surface 106. Alternatively, projections or recesses (not shown) 
may be provided on a portion of the sleeve inner circumferential surface 
109 which is in contact with the housing body 102. Other means of securely 
fixing the sleeve 105 within the bore 104, including methods known in the 
art employing friction and/or mechanical interlocking, such as threads, 
keyways, or splines, may also be used within the scope of the present 
invention. 
In another embodiment of the present invention, shown schematically in FIG. 
7, the sleeve 105 is positioned in the housing 102 by insert molding. The 
sleeve 105 is constructed as described above. The sleeve 105 is then 
positioned in a mold 700 suitable for forming the desired housing. Melted 
plastic is then injected into the mold 700. The melted plastic then 
surrounds the sleeve 105 within the mold, and cools to form the housing 
body 102. The sleeve 105 is thereby embedded within the housing 102. The 
joining of the sleeve 105 and the housing 102 may include, for instance, 
mechanical interlocking, chemical bonding, thermal bonding and/or adhesion 
between the sleeve 105 and the housing 102. The sleeve 105 and housing 102 
may be formed of materials well known in the art to facilitate such 
bonding or adhesion. 
In another embodiment of the present invention, as shown in FIGS. 1 and 2, 
a support element 114 is provided to secure the sleeve 105 within the 
housing body 102. Prior to injection of the melted plastic into the mold 
cavity, the support element 114 is placed in the sleeve 105 such that a 
portion of the support element is positioned against the sleeve 105. After 
the melted plastic cools to form the housing body 102, the support element 
114 is firmly embedded in the housing body, and the sleeve 105 is securely 
fixed with respect to the housing body 102 by way of its contact with the 
support element 114. Moreover, the support element 114 is also positioned 
to support the load applied by the check valve 202. 
The positioning of the support element 114 may be such to prevent either 
outward, rotational, and/or lateral movement of the sleeve 105 with 
respect to the bore 104 of the housing body 102. For instance, the support 
element 114 may be a ring with a portion of the ring positioned within a 
groove 116 in the sleeve 105. In FIG. 2, the support member 114 is 
received on the inner circumferential surface 109 of the sleeve 105, below 
the fluid chamber 180. In other embodiments, the support member 114 could 
be received in other portions of the sleeve 105, such as on the outer 
circumferential surface 110 of the sleeve 105. Furthermore, the support 
element 114 is not limited to a ring and groove configuration. For 
instance, pins, keyways, or splines (not shown) could be employed to 
provide interlocking and/or bonding between the sleeve 105 and housing 
body 102 within the scope of the present invention. 
In the embodiment shown in FIGS. 1 and 2, the fluid chamber 180 is 
connected by a channel 103 to the external source of fluid (not shown). In 
the preferred embodiment, valves which regulate fluid flow in to and out 
of the high pressure fluid chamber 180 are assembled within the fluid 
chamber. In one embodiment, an inlet check valve is employed. This valve 
allows fluid to flow from the external source of fluid into the fluid 
chamber 180, but does not permit fluid to flow in the reverse direction. 
In another embodiment (not shown), a pressure relief valve is also 
employed. This valve allows fluid to exit the fluid chamber, but only if 
the pressure in the fluid chamber increases to a certain predetermined 
value. In yet another embodiment, an integral inlet check and pressure 
relief valve is employed. This valve performs both an inlet check function 
as well as a pressure relief function. Check valves suitable for use with 
the present invention are well known in the art. 
As shown in FIG. 1, a spring biased check valve 202 is preferably employed. 
This valve includes a valve member 206 biased by a valve spring 208 which 
is braced against a valve support member 210. The valve member, which is 
preferably a ball, is biased toward a seal member 204 which serves as a 
valve seat. This seal member provides a seal between the fluid chamber 180 
and the source of fluid when the check valve is closed. In one embodiment 
of the present invention, the seal member is assembled within the 
tensioner by insert molding, as described above. Preferably, the seal 
member is formed of nylon 6/6, but the seal member may be formed of any 
material which is suitably pliable to form a seal between the valve member 
206 and the housing 102. 
As shown in FIGS. 2, 3, 4, 5, and 6, the housing 102 may include means for 
fastening the tensioner within its operating environment, such as the 
engine of an automobile. Preferably, a plurality of openings 312 are 
provided in the housing 102, through which the housing 102 may be attached 
to the engine by fastening means well known in the art, such as bolts or 
screws (not shown). Metal inserts 310 may be received in the openings 312 
to provide a stronger and more durable construction. These metal inserts, 
which are preferably steel, may be held in the openings 312 by 
conventional means such as pins or bolts 314. Preferably, the metal 
inserts 310 are assembled in the housing 102 by insert molding, as 
described above. 
In one embodiment of the present invention, the tensioner incorporates a 
fluid reservoir within the body of the tensioner. As shown in FIGS. 3, 4, 
and 5, the housing body 102 may include a cavity 410 in connection with 
the fluid chamber 180. This cavity 410 may be filled with fluid to provide 
a fluid reservoir for the tensioner. In the embodiments shown in FIGS. 3, 
4, and 5, the cavity 410 is sealed on one side by a plate 412. The plate 
412 may be fastened to the piston body by conventional means, such as 
bolts or screws. As shown in FIG. 4, a seal may be provided between the 
plate and the housing body by an elastic member, such as an O-ring 414. 
The O-ring 414 may be insert molded onto the housing 102, as described 
above, or received in a groove 416 on the housing 102. 
Alternatively, as shown in FIG. 5, a seal may be provided between the plate 
412 and the housing 102 by means of a deformable ridge 418, such as a 
crush ridge, around the periphery of the cavity 410 on the housing 102. In 
another embodiment, a liquid seal material may be applied between the 
plate and the housing body to provide a seal therebetween. Other means of 
providing a seal between a plate and a body having a fluid filled cavity 
are well known in the art and may be employed within the scope of the 
present invention. 
As shown in FIGS. 2 and 3, one embodiment of the present invention employs 
a clip rack 500 to prevent the piston 130 from retracting too far when the 
engine is stopped. This clip rack 500 includes a clip 510 attached to the 
sleeve 105 or housing 102. The clip has a flange 512 which engages a 
plurality of grooves 514 on the outer surface of the piston. These grooves 
514 are configured to easily allow extension of the piston 130, but to 
prevent piston retraction unless a strong retracting force is applied to 
the piston 130. For instance, as shown in FIG. 2, each groove may have a 
sloped side 516, on the side closest to the piston tip 131, and a stepped 
side 518 on the side furthest from the piston tip 131. During extension, 
the flange 512 of the clip rack slides easily along the sloped side 516 of 
each ridge. During retraction, the flange 512 of the clip rack engages the 
stepped side 518 of each groove 514, thereby impeding retraction of the 
piston 131. 
In some embodiments of the present invention, an air vent is provided in 
the tip of the piston 130 to allow air to escape the fluid chamber. In one 
embodiment, shown in FIG. 3, a vent tube 610 is positioned in the tip of 
the piston 130. The vent tube 610 may be filled with a porous substance 
such that air may pass through the tube, but a fluid, due to its 
viscosity, is partially or fully prevented from passing through the tube. 
Thus, air may escape the fluid chamber, but fluid does not easily escape 
the fluid chamber. Preferably, the porous substance is powdered metal, but 
other substances well known in the art may also be employed within the 
scope of the present invention. 
In another embodiment, a vent disk 612 is positioned in the end of the 
fluid chamber 180 closest to the piston tip 131. This vent disk 612 
contains a tortuous path (not shown) having one end in connection with the 
fluid chamber and the other end in connection with the exterior of the 
tensioner. Vent disks having a tortuous path for the passage of air are 
well known in the art. 
Those skilled in the art to which the invention pertains may make 
modifications and other embodiments employing the principles of this 
invention without departing from its spirit or essential characteristics 
particularly upon considering the foregoing teachings. The described 
embodiments are to be considered in all respects only as illustrative and 
not restrictive and the scope of the invention is, therefore, indicated by 
the appended claims rather than by the foregoing description Consequently, 
while the invention has been described with reference to particular 
embodiments, modifications of structure, sequence, materials and the like 
would be apparent to those skilled in the art, yet still fall within the 
scope of the invention.