GB2225080A - Diaphragm sealed tensioner for toothed belts - Google Patents

Abstract

A toothed belt tensioner comprises a piston 24 slidable within a cylinder 20 and having a piston rod 21 sealed by a diaphragm so that closed oil chambers 31, 32 are provided on both sides of the piston 24. The diaphragm is secured both to the inner surface of the cylinder 20 and to the piston rod 21, and is shaped so that it has no convolutions and no elements directed toward the inner surface of the cylinder. The absence of such convolutions and elements prevents sliding of the diaphragm against the inner surface of the cylinder upon movement of the piston, thereby preventing the premature wear of the diaphragm. A one-way check valve 40 is provided between the oil chambers 31, 32. Belt tensioning force is provided by a spring 23 in chamber 32. <IMAGE>

Description

Description of Invention DIAPHRAGM SEALED TENSIONER FOR TOOTHED BELTS Field of the Invention This invention relates to a tensioner for toothed belts, and more particularly to a tensioner having a novel oil diaphragm which is designed in such a way as to avoid wear resulting from sliding of diaphragm elements against fixed parts of the tensioner.

Background of the Invention It is often necessary to impart tension to a toothed belt in order to prevent the unmeshing of its teeth. For example, where the belt is used for driving the camshaft of an engine, excessive changes in belt tension may occur during wide variations in the ambient temperature. This is mainly due to the different coefficients of extension between the belt and the body of the engine. Vibration, noise, and unmeshing of the teeth may occur as a result of the variations in belt tension.

U. S. patent 4,708,696, dated November 2A, 1987, describe a typical tensioner which includes a piston rod which extends from a piston slidably fitting, with a clearance, in a cylinder. The cylinder is separated by the piston into front and rear chambers which are filled with oil. A passage and a one-way checks valve in the piston communicates between the chambers allowing for quick retraction when the belt tightens and slow return when it relaxes.

Since the oil in the front chamber is sealed by ring-like seals held between the inner surface of the cylinder and the outer surface of the piston rod, leakage of oil as well as infiltration of air therethrough is unavoidable. Furthermore, an oil reservoir is required in order to accommodate the difference in volumes caused by the presence of the piston rod.

One solution of this problem, as shown in United States patent 4,798,563 dated January 17, 1989, is to seal the front chamber by means of a diaphragm, the ends of which are secured to the inner surface of the cylinder and the outer surface of the piston rod, respectively. However, the diaphragm used in patent 4,798,563 is a bellows type diaphragm having convolutions composed of elements alternately directed away from and toward the inner surface of the cylinder. With this bellows design, there is the possibility that the crest portions of the diaphragm will slide against the inner surface of the cylinder when reduced tension of the belt allows the piston to protrude. The collision and sliding are more likely to occur in the sections of the diaphragm which are closer to the end of the diaphragm fixed to the inner surface of the cylinder.Collision and sliding subject the crest portions of the diaphragm to wear. In the worst case, the diaphragm will break at those worn portions, permitting oil to leak, and thereby causing the tensioner to fail.

Additionally, the mold used in making bellowstype diaphragms must be separable along a longitudinal plane. Consequently, parting lines will appear on the external surface of the diaphragm. If a parting line is allowed to come into contact with the inner surface of the cylinder, premature wear of the cylinder and of the diaphragm will result.

Summarv of the Invention The tensioner according to the present invention is disposed so that the distal end of the piston rod urges a roller against a slackening side of a toothed belt. The piston rod moves back and forth as the tension of the belt varies. When the piston retracts as a result of an increase in tension, the oil in the rear cylinder chamber flows into the front cylinder chamber through the clearance between the piston and the cylinder. The diaphragm sealing the front chamber deforms to accommodate the increase in oil volume in the front chamber. The viscosity of the oil flowing through the clearance causes the piston to move slbwly in the chamber. However, when the tension of the belt is reduced, the piston rod rapidly protrudes due to the urging force of the spring.A one-way check valve enables the oil in the front chamber to flow into the rear chamber, thus restoring the tension of the belt.

The principal object of the invention is to solve the aforementioned problems of collision, wear and leakage that exist with current tensioners. The present invention is able to solve these problems by providing a tensioner in which the diaphragm does not have a portion which is directed toward the inner surface of the cylinder from the end secured to the cylinder. This prevents the diaphragm from making contact with the inner surface of the cylinder.

More specifically, the diaphragm is shaped so that, for any curve defined by the intersection of an axial plane with the diaphragm, the radial distance r, measure from the piston to any point on the curve decreases monotonically with increasing distance s measured along the curve from the inner surface of the cylinder to said point. In other words, at substantially any point on the curve dr Z 0.

ds Preferably the diaphragm is continuously directed away from the cylinder from the end secured to the cylinder toward the end secured to the piston rod.

It is also an object of the invention to provide a simple and effective diaphragm structure in a tensioner which is inexpensive and reliable.

Still other objects of the invention will be apparent from the following detailed description when read in conjunction with the drawings.

Brief DescriPtion of the Drawings FIG. 1 is a longitudinal section of a preferred tensioner in accordance with the invention; FIG. 2 is a left-hand side elevation of FIG. 1; and FIGS. 3 to 5 are fragmentary longitudlnal sections of the tensioners according to other er.bod,mer.ts of this invention having alternative diaphragm structures.

Detailed Description of the Invention In FIG. 1, piston 24 is slidable in cylinder 20, there being a slight clearance between the outer surface of the piston and the inner surface of the cylinder. A piston rod 21, having flat portions 22 at its distal end, is press-fit into piston 24.

A ring 27, provided in the cylinder and engageable by piston 24, functions as a stop when the piston rod 21 reaches its most extended position.

Slanted oil paths 30 are provided within piston 24 to provide communication between the front oil chamber 31 and the rear oil chamber 32.

A one-way check valve 40 is provided between the front and rear oil chambers 31 and 32. The check valve 40 comprises a retainer 39 holding a ball 38 which is urged toward the opening of a ball seat 36 by a relatively weak spring 37. The check valve allows oil to flow readily from the front chamber 31 into the rear chamber 32, but blocks flow of oil in the opposite direction thereby requiring the oil to flow through the clearance between the piston and the cylinder (or through another restricted path if one is provided). A compression spring 23 is disposed between the retainer 39 and an end surface of the cylinder 20. Also provided in the rear chamber of the cylinder is an oil dummy 41.

Spring 23 applies a tension force to a toothed belt (not shown) through piston 24 and piston rod 21.

When the belt stretches due to an increase in ambient temperature, the belt tension decreases and the piston rod 21 moves forward under the force of compression spring 23 to return the belt tension to its original value. The hydraulic pressure in the front chamber 31 increases due to the advancing movement of piston rod 21.

The check valve 40 opens and enables rapid advance of piston rod 21 to effect a quick recovery of the belt tension.

Front chamber 31 is defined by the cylinder 20, piston rod 21 and diaphragm 11. The two opposite ends of the diaphragm are secured to cylinder 20 and piston rod 21 respectively. Diaphragm 11, once fixed to cylinder 20 and piston rod 21, does not have any portions which are directed toward the inner surface of the cylinder 20.

Thus, the diaphragm will not contact the inner surface of the cylinder 20. Even if such contact were to occur, the shape of the diaphragm is such that the portion nearest the inner surface of the cylinder rolls smoothly along the cylinder surface instead of sliding.

In order for the diaphragm to have no portions or convolutions directed toward the inner surface of the cylinder, it should be shaped so that, for any curve defined by the intersection of an axial plane with the diaphragm, the radial distance r measured from the piston rod to any point on the curve decreases monotonically with increasing distance S measured along the curve from the inner surface of the cylinder to the point. In other words the derivative dr < O at ds substantially all points on the curve.

The two opposite ends of diaphragm 11 are secured to cylinder 20 and piston rod 21 by means of fittings 12 and 13 respectively so as to avoid sliding contact by the diaphragm with the inner surface of cylinder 20 and the outer surface of piston rod 21.

Reference numeral 14 denotes a cap which is press-fit into fitting 12. As shown in FIG. 2, the shape of the opening of the cap 14 conforms to the shape of the distal end of the piston rod 21. This prevents the piston rod from rotating and prevents twisting of the diaphragm.

Cylinder 20 can be filled with oil through opening 16, which is in communication with the front chamber, i.e. the chamber defined by the diaphragm, the cylinder, the piston and the piston rod. By providing the oil filling opening in the front chamber, which, unlike the rear chamber, is not subject to significant pressure rise, it is possible to provide an easily machined oil filling opening and plug in the form of a simple cylindrical bore and complementary plug. Oil introduced into the front chamber through opening 16 flows to the rear chamber through oil path 30 and check valve 40 when the piston moves in a direction to cause expansion of the rear chamber.

FIGS. 3 to 5 illustrate other embodiments of the present invention in which modified diaphragms lla, llb, and llc are shown. As illustrated, diaphragm 11 may have different configurations, so long as it does not have a portion directed toward the inner surface of the cylinder 20 from the end fixed to cylinder 20. As shown in FIGS. 3, 4 and 5, cap 14a may be separate from the fittings which convert the diaphragm to the cylinder.

The present invention, when constructed as above described, may be operated in the same manner as a conventional tensioner, but has the advantage that its diaphragm is prevented from sliding against the inner surface of the cylinder, thus avoiding premature wear of the diaphragm.

Claims (9)

CLAIMS:

1. A tensioner for toothed belts comprising a cylinder having an inner surface, a piston slidable therein with clearance therebetween, and a piston rod extending from the piston, the piston dividing the cylinder into front and rear chambers, each chamber being filled with oil; spring means biasing said piston rod in a protruding direction, means providing an oil path communicating between said front chamber and said rear chamber for allowing the flow of oil into the rear chamber; and a one-way check valve in said oil path; said front chamber being sealed by a diaphragm the ends of which are secured to said cylinder and the piston rod, respectively; characterized in that said diaphragm does not have a portion which is directed toward the inner surface of the cylinder from the end secured to the cylinder.

2. A tensioner according to claim 1 in which at least a portion of the diaphragm extending from the end thereof secured to the cylinder is shaped for smooth rolling contact with said inner surface of the cylinder.

3. A tensioner according to claim 1 having port means communicating with the front chamber for filling said chambers with oil.

4. A tensioner according to claim 1 having port means in the form of a cylindrical bore communicating with the front chamber for filling said chambers with oil, and complementary removable plug means for closing said port means.

5. A tensioner according to claim 1 in which the diaphragm is shaped so that, from the end of the diaphragm secured to the cylinder toward the end of the diaphragm secured to the piston rod, the diaphragm is continuously directed away from the cylinder.

6. A tensioner according to claim 1 in which the diaphragm is shaped in that, for any curve defined by the intersection of an axial plane with said diaphragm, the radial distance measured from the piston rod to any point on the curve decreases monotonically with increasing distance measured along the curve from the inner surface of the cylinder to said point.

7. A tensioner according to claim 1 in which the diaphragm is shaped so that, at substantially any point on any curve defined by the intersection of an axial plane with said diaphragm, the derivative dr LO ds where r is the radial distance from the piston rod to the curve and s is the distance measured along the curve from the inner surface of the cylinder.

8. A tensioner for toothed belts substantially as hereinbefore described with reference to the accompanying drawings.

9. Any novel feature or combination of features described herein.