US 20080287233 A1
A tightener (1) cooperating with a belt in the presence of oil, and having a pin (2) having a first axis (A); a base plate (15) integral with and substantially perpendicular to the pin (2); a cam arm (3) connected in rotary manner to the pin (2) and movable with respect to the base plate (15); a contact member (5) carried by the cam arm (3); and an elastic member (4) connected to the cam arm (3) to activate the contact member (5) against the belt. The tightener (1) also has a friction member (16) made of oil-resistant material and axially supporting the cam arm (3) on the base plate (15); and axial elastic means (24) for loading the friction member (16) against the base plate (15).
1. A tightener for a belt drive operating in the presence of oil, comprising a pin having a first axis; a base plate integral with and substantially perpendicular to said pin; a cam arm connected in rotary manner to said pin and movable with respect to said base plate; a contact member carried by said arm; and an elastic member connected to said arm to activate said contact member against said belt; a friction member axially supporting said arm on said base plate; and axial elastic means for loading said friction member against said base plate; characterized in that said friction member is made of an oil-resistant polymer material and is radially interposed between said elastic member and said contact member.
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The present invention relates to a tightener for a belt drive operating in the presence of oil.
Known belt drives, such as for driving an internal combustion engine camshaft, are normally used in dry applications, in which a tightener is used to maintain correct tension of the belt.
Cam tighteners for dry applications are known comprising a fixed member with a supporting pin; a spring-loaded cam arm; an idle pulley fitted to the cam arm and cooperating with a timing belt; and a bushing of friction material interposed radially between the fixed pin and the cam arm to damp vibration of the belt.
For applications in the presence of oil, chains are normally used, which cooperate with a shoe to maintain correct tension.
Belt applications in the presence of oil have recently been developed, in which known tighteners have proved ineffective in maintaining correct tension.
It is an object of the present invention to provide a tightener for a belt drive operating in the presence of oil, designed to eliminate the aforementioned drawback.
According to the present invention, there is provided a tightener as claimed in Claim 1.
A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:
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More specifically, fixed pin 2 has a cylindrical mounting surface 6 having an axis A; a through hole 7 having an axis B eccentric with respect to axis A; an end shoulder 8; and a contoured head 9 at the opposite end of cylindrical surface 6 to end shoulder 8. Through hole 7 extends through contoured head 9, which is symmetrical with respect to a plane through axes A and B.
Cam arm 3 fits radially onto mounting surface 6 by means of a bushing 10 made of antifriction material, e.g. teflon, and has an annular housing 11, and a cylindrical outer surface 12 having an axis C eccentric with respect to axes A and B. More specifically, cylindrical outer surface 12 supports hollow pulley 5 by means of a bearing 13, and housing 11 is open towards end shoulder 8, houses the whole of tensioning spring 4, and has a slot for connection to an end portion 14 of tensioning spring 4.
Cam arm 3 is supported axially by an annular base plate 15 connected perpendicularly to fixed pin 2, and by a friction ring 16 interposed between cam arm 3 and base plate 15 and made of material capable of maintaining friction even in the presence of oil, such as BERAL 1122 marketed by Federal-Mogul Friction Products Gmbh.
More specifically, the friction material used comprises an oil-resistant binder comprising caoutchouc; and a mixture of additives for obtaining friction in the presence of oil and comprising rock wool, magnesium oxide, graphite, and clay. The physical characteristics of the material used are: specific weight (measured at 20° C.) 2.04*10̂−3 kg/cm̂3; thermal conductivity 1.00 W/m*° C.; approximate dry friction coefficient 0.47; and approximate oil-bath friction coefficient 0.10.
More specifically, cam arm 3 has an integral, radially peripheral collar 17 facing base plate 15 and defining a seat to make friction ring 16 radially integral with cam arm 3.
Base plate 15 comprises an inner edge 18 which rests axially on end shoulder 8; and a projection 19 projecting towards cam arm 3 and cooperating both with arm 3, to define two limit positions, and with an end portion (not shown) opposite end portion 14, to tension spring 4.
On the opposite side to base plate 15, cam arm 3 and contoured head 9 of fixed pin 2 define an annular seat 21 coaxial with axis A and for housing a supporting cup made of plastic, a Belleville washer 24, and a circular cover plate 25.
More specifically, cup 23 comprises, integrally, a flat portion 26 perpendicular to axis A; and a peripheral edge 27 parallel to axis A. Peripheral edge 27 houses Bellville washer 24 radially, and, together with cover plate 25, defines an annular gap 28 to allow oil in to lubricate Belleville washer 24 and mounting surface 6. Belleville washer 24 is pressed axially against flat portion 26 by cover plate 25. More specifically, cover plate 25 is connected angularly rigidly to contoured head 9 by a matching shape fit, and is fixed axially by caulking.
Tightener 1 is fitted to a wall of an internal combustion engine by means of a screw 29 housed inside through hole 7. More specifically, contoured head 9 is bounded by a supporting surface 30 projecting axially with respect to cover plate 25, and end shoulder 8 is bounded by a supporting surface 31 projecting axially with respect to base plate 15, so that, when tightener 1 is fixed to the engine, the head 32 of screw 29 cooperates with supporting surface 30, and supporting surface 31 directly contacts the wall of the engine to keep base plate 15 detached. The pull exerted by the screw is therefore transmitted to fixed pin 2, and the axial load of friction ring 16, which is much less than the pull exerted by the screw, is controlled accurately by sizing Belleville washer 24. Belleville washer 24, in fact, is used in the maximum-compression condition, in which it exerts substantially constant force alongside minor variations in the axial dimension of cam arm 3, caused, for example, by in-service wear of friction ring 16.
The advantages of the tightener according to the present invention are as follows.
Using an axially-loaded, oil-resistant friction ring 16 provides for achieving satisfactory damping in the presence of oil. Moreover, tightener 1 comprises numerous component parts in common with a dry-operating tightener, thus enabling mass production cost benefits. More specifically, by replacing friction ring 16 with a PTFE ring, tightener 1 may also be used dry.
Moreover, using an oil-resistant friction material, cost is further reduced by eliminating in-process cleaning of metal parts, i.e. fixed pin 2 and cam arm 3.
The fact that the whole of tensioning spring 4 is housed axially inside housing 11 of cam arm 3 provides for reducing length to adapt to applications featuring belts operating in the presence of oil, and which generally tend to replace chains which permit particularly short lengths.
Clearly, changes may be made to the tightener as described and illustrated herein without, however, departing from the scope of the present invention as defined in the accompanying Claims.
For example, friction ring 16 may be housed inside a groove defined by cam arm 3, or may be glued.
Hollow pulley 5 and bearing 13 may be replaced by a shoe having a ring member fitted to cylindrical outer surface 12 and integral with cam arm 3.
To reduce the number of component parts of tightener 1, supporting cup 23 may be eliminated, and Belleville washer 24 may contact cam arm 3 directly, provided gap 28 is sized to allow sufficient oil through to lubricate the surfaces in relative motion.
The friction material used comprises a caoutchouc-based binder if a flexible friction ring 16 is required. If a rigid friction ring is required, caoutchouc is not used.