Patent Application
20090111628
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 of the kind that is used, for example, in a timing drive of an internal combustion engine of a vehicle.
Internal combustion engines of motor vehicles often include a timing belt or chain 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, vibration and harshness 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 hollow plunger is slidably movable in a longitudinal direction and is biased axially out 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 the plunger interior 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.
In some tensioners of this kind the hydraulic fluid is diverted from the engine supply to the check valve of the tensioner from a pressurised source via a radial passage in a wall of the tensioner housing.
One known tensioner of this kind is described in our European patent No. 1215415. The tensioner has a main body of steel that is screw-fitted into a bore defined in the engine block so as to ensure that it is correctly positioned with respect to the chain or belt, and a head that remains exposed for engagement with an insertion tool. An external thread is defined on the body adjacent to the head for connection into a complementary thread of the supporting bore. The variable volume fluid pressure chamber extends rearwardly into the area surrounded by the exterior thread so that the check valve is disposed in the tensioner head. The oil is supplied to the check valve from an oil pump or the like via an oil passage defined by a small bore that is machined through the wall of the tensioner housing from a position adjacent to the start of the external thread. In order to take account of the misalignment between the oil supply outlet and the inlet of the oil passage there is a groove defined across the thread. That way the tensioner of the present invention can be used in existing engines without the need for modification to alter the position of the egression of fluid from the reservoir.
The production of a tensioner of this kind involves the manufacturing step of drilling the oil passage through the wall of the steel tensioner housing and a groove across the external thread. This is a relatively expensive operation and it is an object of the present invention to obviate or mitigate this and other disadvantages.
A further object of the present invention is to provide for a chain or belt tensioner that is of relatively low cost to manufacture without its performance or quality being compromised.
According to a first aspect of the present invention there is provided a chain or belt tensioner comprising a housing for receipt of a reciprocal plunger that acts directly or indirectly on a chain in order to tension it, the housing having a first and second parts, the first part defining a first bore that extends between a first closed end and an open second end, the second part being substantially tubular and defining a second bore for receipt of the plunger, a first end that is received in said first bore such that there is an overlapping portion of the first and second parts, and has a second open end; an inlet in the housing for the supply of hydraulic fluid, a check valve disposed in said housing between the inlet and the second bore for controlling the delivery of a hydraulic fluid into the second bore, a fluid passage extending from the inlet, into the open second end of the first part of the housing to the check valve, at least part of the passage being defined along the length of the overlapping portion of the first and second parts of the housing.
The arrangement of the present invention enables the first bore to extend further into the housing and therefore the check valve can be positioned further into housing so that there is more room to accommodate longer lengths of plunger in comparison to previous designs. The invention allows the check valve to be offset from the inlet by providing for a fluid passage that extends between the housing parts.
The provision of two parts of the housing allows for the provision of the passage between the parts rather than having to drill a hole through the housing in order to provide communication between the check valve and the fluid inlet.
The passage may be defined by one or more grooves on the outside surface of the second part of the housing. The groove may extend from the inlet at a position outside of the first part of the housing and then between the housing parts. The passage may be of helical form and may interconnect an annular chamber, defined by clearance between first and second parts of the housing, and the fluid inlet. The first end of the second part of the housing may have an external diameter less than that of the bore in the first part so as to provide a clearance that defines the annular chamber.
The check valve may be disposed in the second part of housing at, or adjacent to, the first end thereof.
A fluid filter may be disposed between the passage and the check valve.
The first part of the housing may be made of a different material to the second part of the housing. The second part of the housing may be manufactured from steel and the first part may be manufactured from a material having a lower hardness value.
The first part of the housing may comprise a formation by which it can be engaged with a tool for insertion into an engine block.
The first part of the housing may have an annular wall with a thread thereon for screw engagement with a complementary thread in the engine block.
At least part of the passage may be axially collateral and/or concentric with the thread.
The first and second parts of the housing may engage in a friction fit.
A reciprocal plunger is ideally provided in the second bore and is preferably biased out of the housing in order to make direct or indirect contact with the chain to be tensioned. The plunger may have a bore which may combine with the second bore in the housing to form a variable volume pressure chamber. A biasing member such as, for example, a compression spring, may be provided in the bore of said plunger and biases the plunger to extend out of the second bore in the housing.
A ratchet mechanism may be provided for preventing undesirable retraction of the plunger into the housing. The ratchet mechanism may include a cylinder on which teeth of the ratchet are formed. A washer may be provided between the cylinder ratchet mechanism and an internal wall of the housing.
The second part of the housing may define a third bore at said first end, the check valve being disposed in said third bore.
According to a further aspect of the present invention there is provided a tensioner as defined above and in combination with a support member in which the tensioner is received. The support member may be part of the engine block of an internal combustion engine.
According to another aspect of the present invention there is provided a method for assembling a chain or belt tensioner, the tensioner comprising a housing for receipt of a reciprocal plunger that acts directly or indirectly on a chain in order to tension it, the housing having a first and second parts, the first part defining a first bore that extends between a first closed end and an open second end, the second part being substantially tubular and defining a second bore for receipt of the plunger, the method comprising the steps of heating at least the first part of the housing so that it expands and the first bore is enlarged, inserting a first end of the second part in said first bore such that there is an overlapping portion of the first and second parts, and allowing the assembled parts to cool such that the first part contracts on to the overlapping portion of the second part.
A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:
Referring now to the drawings, the exemplary chain tensioner has a generally cylindrical housing constructed from two connected parts 10, 12. A first part, hereinafter referred to as the plug 10, receives one end of a second part, hereinafter referred to as the insert body 12, in a close-fitting relationship. The plug 10 is manufactured from aluminium whereas the body 12, which needs to be harder wearing, is made from steel. It is to be appreciated that any alternative suitable materials may be used. Whereas the insert body 12 needs to be manufactured from a material that has high wear and fatigue resistance the plug will generally be made from a material with a lower hardness value so that it can be moulded or easily machined. The plug is also ideally light and has anti-corrosive properties.
The plug 10 is of cylindrical form with an annular wall 13 that defines a bore 14 of substantially constant diameter and an end wall 15 that closes the bore at one end. The outer surface of the annular wall 13 has a screw thread formation 16 towards its open end by which the plug 10 may be threadedly engaged in a bore in an engine block 17 (see
The housing insert body 12 is generally cylindrical having open ends with a first bore 19 that extends from a first end 20 and a second larger diameter bore 21 that extends from the second end 22 and meets with the first bore 19 at a radial step 23. The outer surface of the body varies in diameter: starting at the first end of the body 20 that is received in the bore 14 of the plug 10 and moving from left to right in the orientation shown in
The second diameter is designed to be to be more or less equal to, or very slightly less than, the diameter of the plug bore 14 so that when the body 12 is inserted into the plug 10 the outside surface of the intermediate portion 26 and the inside surface of the plug bore 14 engage in a friction, press or interference fit. With the body 12 fully inserted into the plug 10, a tapered annular chamber 29 is defined by the radial clearance between the plug bore 14 and the first portion 24 and taper 25 of the housing body 12. Rotational and/or axial stop features may be provided to ensure that the two parts engage correctly. It can be seen that when assembled in this way, the helical groove 27 starts at a location adjacent to the annular wall 13 of the plug 10 and extends (from right to left in
The first bore 19 in the body 12 is configured to receive a ball check valve 30 and a fluid filter 31 as shown. The second bore 21 is designed to receive a slidable plunger 32. It can be seen that the transition step 23 between the first and second bores 19, 21 is at an axial position roughly coincident with the end of the taper 25 on the outside surface of the body 12.
As can be seen from
The tensioner is shown screw-fitted into a bore 36 in part of the engine block 17 in
The plunger 32 has a plurality of passages 40 that extend radially from the blind bore 34 at a position immediately adjacent the solid portion 32b. These allow oil to bleed from the plunger bore 34 into the second bore 21 of the body 12, via the small annular clearance between the plunger 32 and the housing body 12, when there is excess oil pressure. The passages 40 and the annular clearance thus serve as a throttle during relative movement of the plunger 32 in the housing body 12. The hydraulic oil acts on both sides of the plunger 32 with equal pressure and serves to damp movement of the plunger.
A compression spring 41 is disposed coaxially in the blind bore 34 of the plunger 32, a first end of the spring 41 bearing against a shoulder 42 defined on the nozzle 39 and a second end of the spring bearing against the bottom of the blind bore 34 in the plunger. The spring 41 serves to bias the plunger 32 outwards of the housing body 12 towards the chain so as to impart tension thereto.
The second end 22 of the housing body 12 is closed by an annular seal 43 disposed between the solid part 32b of the plunger and an annular recess 44 defined in the inside surface of the second bore 21 in the body 12.
Interposed between the plunger 32 and the compression spring 41 there is provided a ratchet locking mechanism 45 of conventional structure and which is designed to prevent excessive retraction of the plunger 32 against the spring force even when the oil pressure is low. The mechanism is embodied by a cylinder 46 with a helically toothed channel 47 in which a pin 48 on the plunger 32 is engaged in stepwise fashion. A hardened steel washer 49 is provided between ratchet cylinder and the bottom of the second bore such that it abuts the stepped transition 23 between the first and second bores. This prevents friction between the ratchet cylinder 46 and the bottom of the second bore 21 and significantly reduces the possibility of particles becoming entrained in the oil in the pressure chamber 35.
At engine start-up or during rapid acceleration, when the oil pressure is low, the spring 41 biases the plunger 32 out of the housing body 12 and the ratchet mechanism 45 prevents excessive retraction should the chain apply a force against the plunger. Thereafter, the oil pressure increases and maintains the biasing force applied to extend the plunger 32. The oil acts on a greater surface area inside the plunger bore 34 and therefore the force that tends to move the plunger out of the housing bore is greater than that acting in the opposite direction.
The tensioner of the present invention can be used in existing engines without the need for modification to alter the position of the existing annular recess 38 in the engine block. The helical groove 27 between the two parts of the tensioner can take any convenient form provided it carries oil from the supply to the annular chamber 29. It is the provision of the groove 27 that permits the check valve 30 to be located at a deeper position in the tensioner housing than in conventional designs. The compact arrangement thereby enables the tensioner to be shorter in length or for the plunger 32 and housing bore 19, 21 to be longer and thereby allow a longer stroke.
It is to be understood that instead of the groove 27 being provided on the outside surface of the housing body it may be defined on the inside surface of the housing plug 10 provided there is adequate communication with the annular recess 38 in the engine block 17.
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, any number of helical grooves 27 may be provided to allow passage of the oil between the two parts of the housing from the inlet to the annular chamber. The number and the shape of the grooves can be varied and are selected to provide the most appropriate oil feed rate to the tensioner for any particular application. The groove width or length may be varied to adjust the flow resistance of the oil or conversely the flow rate may purposely be increased by enlarging the cross-sectional area of the groove. Furthermore, the plug may be made from any suitable cost-saving material, such as plastics, since it does not need to have the same hard-wearing characteristics as the insert body. Moreover, the connection between the plug and insert body can take any suitable form. Examples of different connections are described in the alternative embodiments of
In the embodiment of
In an alternative embodiment shown in
In the embodiment of
In
In the embodiment of
Further examples of alternative connections between the plug and housing body include a formation on the external surface of the housing body that is designed to engage with the inside surface of the plug in a friction fit or which engages with and deforms the material of the plug or an adhesive bonding between the two components. It will be appreciated that in all embodiments the connection between the housing and plug does not interfere with the passage of oil to the check valve.
Another alternative way of connecting the plug and the housing is to use heat. The aluminium plug (at least) can be heated by thermal induction such that it expands and the steel insert body can then be inserted into the bore in the plug. When the assembled components are left to cool the plug contracts to its original size such that the inside diameter of its bore engages on the outside diameter of the insert body in an interference fit. In order to release the components they can both be heated as the materials have different thermal expansion coefficients such that the aluminium expands at a greater rate than the steel to facilitate release.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0607775.4 | Apr 2006 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP07/03726 | 4/20/2007 | WO | 00 | 10/17/2008 |
