AUTOMATIC BELT TENSIONER

Abstract: A belt tensioner has a shaft (25) having an oil reservoir (29) in a lower portion thereof and having an external thread (25c) and rotatably mounted in a body (21), a torsion spring (27) provided between the body and the shaft so as to urge the shaft in a rotational direction, and a cylindrical rod having an internal thread (28c) engaged with the external thread of the shaft. A lower end of the rod (28) being projected from the body so as to be operatively connected to a tension pulley for a belt. The torsion spring and both the threads are arranged to project rod by the rotation of the shaft urged by the torsion spring. An oil passage (30) is provided in the shaft so as to communicate the oil reservoir with the bore above the internal thread of the rod.

Description: The present invention relates to an automatic tensioner for tensing a belt such as a timing belt for an automotive engine and more particularly to a spring loaded screw type tensioner. Japanese Patent Laid Open Nos. 56-83647 and 57-­ 40149 disclose screw type tensioners applied to chain driven camshaft mechanisms. The tensioner has a shaft applied with torque by a torsion spring and an actuating rod having a thread engaged with a thread of the shaft. The shaft is urged by the spring in a rotational direction to project the actuating rod. When the tension of the chain decreases as a result of stretching or expansion of the chain, the rod is caused to project further by the rotation of the shaft to push a chain guide, thereby automatically tensioning the chain. In the screw type tensioner, since a large force is continuously exerted on the thread, a square threaded screw device having a high rigidity is employed. In the square threaded screw device, the quantity of particles produced by abrasion of the threads increases, due to high friction between the threads. Therefore, it is preferable to use oil having a low viscosity rather than grease for lubrication in order to flush out the particles. Accordingly, although the conventional tensioner can be applied to a chain drive device mounted inside an engine body in which engine oil splashes to lubricate the device, it cannot be applied to a timing belt drive device arranged outside the engine body. The present invention seeks to provide an automatic belt tensioner wherein a reliably sealed lubricating system is provided so as to sufficiently flush out particles resulting from the abrasion of screw threads. Accordingly the present invention provides an automatic belt tensioner comprising a housing; a plunger mounted in a helical mounting in the housing with one end projecting so that rotation of the helical mounting causes the plunger to move axially in the housing; and a torsion spring connected between the housing and the helical mounting so as to urge the plunger outwardly of the housing for engagement, in use, with the belt; characterised by the provision of an oil reservoir for supplying oil to the threads of the helical mounting. A preferred form of the invention provides an automatic belt tensioner comprising a substantially vertically disposed cylindrical housing having an axial chamber, a shaft mounted on the housing and having an oil reservoir in a lower portion thereof and having an external thread, a torsion spring provided between the body and the shaft so as to urge the shaft in a rotational direction, and a cylindrical plunger having an axial bore and an internal thread provided at an intermediate portion with respect to the axial length of the bore and engaged with the external thread of the shaft. A lower end of the plunger projects from the body so as to be operatively connected to a tension pulley for a belt. The plunger is guided by guide means so as to be axially moved. Circulating means is provided for circulating oil between the oil reservoir in the shaft and the bore below the internal thread of the plunger. The torsion spring and both the threads are arranged to project the plunger by the rotation of the shaft urged by the torsion spring. In accordance with one aspect of the invention, the circulating means comprises a passage provided in the shaft so as to provide communication between the oil reservoir and the bore above the internal thread of the rod, and an air passage provided in the shaft to connect the oil reservoir with the atmosphere. One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is an elevational view of a timing belt drive device for an engine to which the present invention is applied; Figure 2 is a partial sectional view of a part of Figure 1; Figure 3 is a fragmentary sectional view of an automatic tensioner of the present invention; and Figure 4 is a sectional view taken along a line IV-IV of Figure 3. Referring to Figures 1 and 2, a crankshaft pulley 1 is connected to a camshaft pulley 2 through a toothed timing belt 5. An oil pump pulley 3 and an idler pulley 4 engages with the belt at the tight side of the run of the belt 5. A tension pulley 6 bears on the belt 5 at the slack side. The tension pulley 6 is rotatably supported on a central portion of an arm 7 through a bearing 9 mounted on a shaft 8. One end of the arm 7 is rotatably secured to the engine body by a shaft 10. An automatic tensioner 20 is provided adjacent to the other end of the arm 7. A tension spring 11 is provided to urge the arm 7 to assist the automatic tensioner 20 so as to decrease the vibration of the belt. Referring to Figures 3 and 4 the automatic tensioner 20 has a cylindrical body 21 having an axial chamber 21d. A bolt 22 is screwed in the body to cover an opening at the top of the body 21. A shaft 25 is axially disposed in the chamber 21d of the body 21. The upper end of the shaft 25 is rotatably mounted in a recess 21b formed in the body 21. The shaft 25 has a relatively large diameter upper portion and a smaller diameter lower portion, so that an annular shoulder 25a is formed between them. The smaller diameter portion engages in an aperture of corresponding diameter in the end 26a of a hollow tubular member 26. Disposed around the upper portion of the shaft 25 is a torsion spring 27, one end of which is engaged with a bore 25b formed on the top of the shaft and the other end to a groove 21c formed in the inner wall of the body 21, so as to apply torque to the shaft 25. An external square thread 25c is formed on the smaller diameter lower portion of the shaft 25. An oil reservoir 29 containing a low viscosity lubricating oil A is formed within the lower portion of shaft 25. A cylindrical plunger 28 has a bore 28c and an internal thread 28a at an intermediate position along the axial length of the bore. The plunger 28 is mounted in the hollow member 26, engaging the thread 28a with the thread 25c of the shaft 25. The rod 28 projects downwardly out of the body 21. A head portion 28b is formed at the bottom end for abutting against the arm 7. A radial oil passage 30 is formed in the wall of the shaft 25 at a portion above the engaging portion of the threads 25c and 28a so as to connect the oil reservoir 29 with a space formed between the shaft 25 and the plunger 28. A stem seal 31 is mounted between the top end of the rod 28 and the shaft 25 so as to prevent the oil from leaking out of the plunger 28. An oil seal 33 is arranged at an intermediate position with respect to the axial length of the rod 28 between the stopper 26 and the rod 28, and is secured by back-up rings 32. Thus, the oil is prevented from leaking out of the stopper. An oil seal 34 and an O-­ ring 35 are provided in the space between the body 21 and the member 26 thereby preventing the leakage of oil from the body 21. A radial air passage 36 is formed in the wall of the shaft 25 at a position above the level of the oil A. A circular bearing 23 is formed with an elongated guide opening 23a at the centre and having upwardly projecting engaging legs 23b at the periphery. The bearing 23 is attached to the bottom end of the body 21 and secured thereto by a snap ring 24. Each engaging leg 23b engages with a corresponding groove 21a formed on a lower periphery of the body 21. The lower portion of plunger 28 has two opposed flats corresponding to the shape of the opening 23a of the bearing 23, so that the plunger 28 is located in position by the bearing in such a way that it can be axially moved without rotating. The bearing 23 supports the member 26 and hence the shaft 25. The spring 27 and threads 25c and 28a are so arranged that when the shaft 25 is rotated by spring 27, the rod 28 projects as shown by a chain line in Figure 3. In operation, the rod 28 is urged by spring 27 to project, thereby rocking the arm 7 about the shaft 10 so that the tension pulley 6 is urged against the timing belt 5. At a position where the component of rotary force at the thread 28 a is generated by the reaction of the belt 5, balances with the torque of the torsion spring 27, the tension pulley 6 is held to provide a proper belt tension. When the crankshaft 1 is rotated, the timing belt 5 guided by the idler pulley 4 and the tension pulley 6 drives the camshaft pulley 2 and the oil pump pulley 3 in synchronism with the crankshaft 1. When load on the slack side of the belt 5 is decreased, for example, at the start of the engine, rod 28 is temporarily projected. During the normal operation of the engine, the rod 28 is projected and retracted in accordance with expansion of the belt at warm-up and with contraction of the belt at cold starting. Accordingly, the tension of the belt is automatically kept at a predetermined value by the tension pulley 6. When the rod 28 is projected increasing the volume of the space in the rod, oil A in the oil reservoir 29 flows through the oil passage 30 into the space between plunger 28 and shaft 25. At the same time, air is inducted into the oil reservoir 29 through the air passage 36. On the other hand, when the rod 28 is retracted decreasing the size of the space, the oil A in the space returns to the oil reservoir 29 through the passage 30 while the air in the oil reservoir 29 is discharged through the air passage 36. Since the level of the oil A is always shown above the threads 28a and 25c, the threads are thoroughly lubricated. Accordingly, the particles caused by abrasion of the threads collect in the bottom of the space in the plunger 28. The oil in the rod 28 is prevented from leaking by the stem seal 31. Even if excessive oil is discharged out of the rod 28, oil seals 33 and 34 prevent it from flowing out of the body 21. Accordingly the preferred form of the present invention provides an automatic tensioner which can be applied to a timing belt since lubricating oil is trapped inside the tensioner by reliable sealing means. Further, since the tensioner has its own oil reservoir it can be used independently in many other applications. While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made within the scope of the appended claims.

Claims: 1. An automatic belt tensioner comprising a housing (20); a plunger mounted in a helical mounting (25; 25c;, 28a) in the housing with one end (28b) projecting so that rotation of the helical mounting causes the plunger to move axially in the housing; and a torsion spring (27) connected between the housing and the helical mounting so as to urge the plunger outwardly of the housing for engagement, in use, with the belt; characterised by the provision of an oil reservoir (29) for supplying oil to the threads of the helical mounting. 2. An automatic tensioner according to claim 1 further characterised by circulating means (30; 36) for circulating the oil past the threads (25c; 28a) between the reservoir and a secondary chamber, as the mounting is rotated. 3. An automatic belt tensioner comprising: a substantially vertically disposed cylindrical housing having an axial chamber; a shaft having an oil reservoir in a lower portion thereof and having an external thread and rotatably mounted in the housing; a torsion spring provided between the housing and the shaft so as to urge the shaft in a rotational direction; a cylindrical plunger having an axial bore and an internal thread arranged at an intermediate portion with respect to the axial length of the bore and engaged with the external thread of the shaft, a lower end of the plunger projecting from the housing so as to be operatively engageable to a tension pulley for a belt; means for locating the plunger so as to permit axial movement; means for circulating oil between the oil reservoir in the shaft and the bore below the internal thread of the plunger; sealing means for preventing the oil from leaking out of the plunger; the torsion spring and the threads being so arranged as to urge the plunger towards an outward position. 4. An automatic belt tensioner according to claim 2 wherein the circulating means comprises a passage provided in the shaft so as to communicate the oil reservoir with the bore above the internal thread of the rod, and an air passage provided in the shaft to communicate the oil reservoir with the atmosphere.

Inventors: OZAWA, MASAHIRO, SAWAFUJI, SHUJI, YOSHIKAWA, HIROSHI
Applicants: FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1987
Main classification: F16H7
IPC codes: F16H7