Abstract:
A hydraulic tensioning device for a power transmission system having a belt or a chain includes a cylinder and piston assembly of which a movable element, generally the piston, acts on the belt or chain so as to ensure that it is tensioned and thus ensures taking up of any slack in the chain or belt. The piston includes a plurality of piston elements slidable along a central axis of the cylinder.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to hydraulic tensioners used as a control device for a power transmission chain drive in automotive timing applications and more particularly to a hydraulic tensioner having a long maximum useful stroke, while having a normal cylinder-piston assembly length in the retracted state. 
     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 a chain travels between a plurality of sprockets. As a chain transmits power from a driving sprocket to a driven sprocket, one portion or strand of the chain between the sprockets will be tight while the other portion of the chain will be slack. In order to impart and maintain a certain degree of tension in the slack portion of the chain, a hydraulic tensioner provides a piston that presses against a tensioner arm or other chain guiding mechanism. 
     Prevention of excess slack in the chain is particularly important in the case of a chain driven camshaft in an internal combustion engine because a chain without sufficient tension can skip a tooth or otherwise 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 can cause a decrease in chain tension. In addition, camshaft and crankshaft induced torsional vibrations cause considerable variations in chain tension. 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 such as a hydraulic tensioner is desired to ensure the necessary tension on the slack side of the chain. 
     Typically, a hydraulic tensioner includes a piston in the form of a hollow cylinder. The piston slides within a bore in the housing and is biased outward from the housing in the direction of the tensioner arm and chain by a piston spring. The interior of the piston forms a high pressure fluid chamber with the bore or opening in the housing. The high pressure chamber is connected through a one way check valve to a low pressure chamber or reservoir, which provides or is connected to an exterior source of hydraulic fluid. 
     Upon start-up, the force of the spring on the piston causes the piston to move further outward as the chain begins to move. Outward movement of the piston creates a low pressure condition in the high pressure fluid chamber, or pressure differential across the inlet check valve. Accordingly, the inlet check valve opens and permits the flow of fluid from the reservoir, or low pressure chamber, into the high pressure chamber. When the high pressure chamber is sufficiently filled with fluid, the force on the chain that moves the piston inward will be balanced by the outward force from the spring and the resistance force of the fluid in the chamber. The force of the chain against the fluid in the chamber also causes the check valve to close, which prevents further addition of fluid to the chamber. 
     U.S. Pat. No. 4,826,470 to Breon et al. discloses a cylinder/piston assembly tensioning device, in which the cylinder forms a chamber for pressurized fluid. A static plunger is interposed between the cylinder and the piston, and a spring is interposed between a flange of the static plunger and the piston. The usable stroke of the piston is limited to the extension of the piston skirt, which must never completely leave the cylinder. This patent also describes a piston formed by two parts that create between them a path for the fluid to exit. The maximum useful stroke of the piston is nevertheless limited by the axial length of the skirt integral with the piston head. 
     U.S. Pat. No. 4,963,121 to Himura et al. also discloses a tensioning device for a belt or chain drive mechanism that includes a hollow piston element slidable inside a stationary cylinder. A fluid chamber is defined between two reciprocally sliding tubular elements of which one is fixed to the cylinder and the other moves under the action of a fluid and of a spring, integrally with the piston. The maximum useful outward stroke of the piston is defined by the length of the skirt integral with the piston head. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, the tensioner includes a cylinder, which may be in the form of a bore in the tensioner housing, and a piston movable with respect to the cylinder. The piston includes two or more reciprocally sliding elements. A first distal element includes a piston head and a skirt. A second proximal element, or plurality of proximal elements, includes a skirt. A spring and pressurized fluid exert a bias between the end of the cylinder and the piston head on the distal element of the piston. Facing walls of the skirts of the piston elements and the cylinder have reciprocal engaging means to limit the movement of each with respect to the other. 
     The tensioner of the present invention allows the piston to extend from the cylinder for a longer stroke than previous tensioners. Yet, the axial measurement of the tensioner or axial length of the tensioner when retracted is equal to previous tensioning devices. 
    
    
     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. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 is a sectional view along a central axis of a cylinder-piston assembly of a tensioning device according to the invention, such as a plane substantially parallel to the drawing sheet and through the device shown in FIG. 5, in a retracted state. 
     FIG. 2 is a sectional view similar to FIG. 1 with the tensioning device shown in a partially extended state. 
     FIG. 3 is a sectional view similar to the previous figures, with the tensioning device shown in the position of maximum extension. 
     FIG. 4 illustrates a tensioning device with an oscillating shoe positioned on a chain drive of a timing system. 
     FIG. 5 illustrates a tensioning device with a translating shoe positioned on a chain drive of a timing system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A tensioning device illustrated in the figures is indicated generally by reference number  10 . 
     The tensioning device  10  includes a body or cylinder  20  with a hollow center or bore, and a piston or plunger  30  formed of a plurality of elements. The piston includes at least two elements  31  and  32 , respectively. The body or cylinder  20  has fixing tabs  21  for securing the tensioner to an engine body. The center of the cylinder, or bore, forms an internal cylindrical chamber  22 . The chamber  22  communicates with a supply of pressurized fluid, generally oil, through an opening  23 , and a check valve  24 , in a manner well known and described in the prior art. 
     The piston  30  includes a first element or distal element  31  and a second element or proximal element  32 . The first element includes a crown or piston head,  310 , and a cylindrical skirt  311 . The second, proximal element  32  includes a cylindrical skirt  321 . Between the bottom of the cylindrical chamber  22  of the cylinder and the head  310  of the distal element a piston spring or pressure spring  40  is interposed. The outer wall of the skirt  311  of the distal element of the piston has a seat  312  for a piston ring  313 , in an intermediate position along the length of the skirt  311 . The inner surface of the piston element  32  has a groove  322  near the distal end thereof, to allow widening or expansion of the piston ring  313 . Engagement of the piston ring  313  in the groove  322  prevents further extension of the distal element  31  of the piston with respect to the proximal element  32  of the piston. 
     The outer wall of the proximal element  32  of the piston has a seat  323  for a piston ring  324 . In the proximity of the end the inner face of the cylinder  20 , a corresponding groove  202  is able to receive the elastic ring  324  to stop further extension of the element  32  of the piston from the cylinder. 
     The annular grooves or housings  322 ,  202  preferably have the respective distal wall forming a steep step outwards (that is, toward the right in FIG. 1) and the proximal wall gently sloping inward, that is, toward the left in FIG.  1 . The proximal element  32  of the piston  30  has a step  325  in its proximal part facing toward the axis to form an abutment for distal element  31  in a retracted position. 
     In the retracted or resting position illustrated in FIG. 1, the two elements  31  and  32  of the piston  30  are in the retracted state, i.e., the element  31  is completely or almost completely received inside the cylinder  20 . This position can coincide with the initially set position of the tensioning device. The head  310  of the device may be brought into contact with a shoe, generally an oscillating shoe (FIG. 4) or a translating shoe (FIG. 5) which can be pushed against chain C, or the like, to tension it. When fluid is introduced through the opening  23  and the check valve  24  into the chamber  22 , the piston is biased out of the cylinder through the action of the fluid and the spring  40 . The bottom of the chamber  22  of the cylinder and the head  310  of the distal element  31  of the piston are pushed apart by the spring. This causes the element  31  to be extended (towards the right in FIG. 1) with respect to the second element  32 , and the element  32  to be extended with respect to the cylinder  20 . The position of the elements  31  and  32  with respect to each other and with respect to the cylinder  20  is determined by the balance of forces exerted by the pressure of the fluid and the spring, on one hand, and the tension of the chain on the other. 
     The extension of the piston elements is limited by engagement of the piston ring  312  in the groove  322 , and by engagement of the piston ring  323  in the groove  202 . The maximum extension position is illustrated in FIG.  3 . As can be seen from the figures, for tensioning devices having a same axial length L, (FIG.  1 ), in the retracted position, a conventional tensioning device could give a maximum useful stroke L 1  as shown in FIG. 2, while the new tensioning device permits a useful stroke L 2 , marked in FIG. 3, which is considerably greater than L 1 . 
     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.