Patent Publication Number: US-2012024249-A1

Title: Hydraulic backlash compensating element

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority of DE 2010 033 091.4 filed Aug. 2, 2010, which is incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates to a hydraulic backlash compensating element for an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     A hydraulic backlash compensating element of the abovementioned type is known from the general prior art. The hydraulic backlash compensating element has an oil-filled piston/cylinder unit, a check valve and a spring. Hydraulic backlash compensating elements are encountered in valves, support elements or tensioning elements for tensioning a traction means in a traction drive of an internal combustion engine, for example. 
     Inside a housing there is a piston with an integrated check valve. The two elements can be moved relative to one another, form a defined leakage gap at the contact surface and are pushed apart by an internal spring. While the piston enters the housing under the action of an external force, a high pressure arises in the oil-filled pressure chamber formed by the housing and the piston when the check valve is closed. A small quantity of oil is discharged to the surroundings through the very narrow leakage gap. When the external force acting on the piston weakens, the internal spring pushes the hydraulic element apart. This gives rise to a differential pressure which opens the check valve, and the extra quantity of oil required for the compensating process flows in. The check valve has a valve element which interacts with a valve seat. In general, a valve spring is provided in the check valve, pressing on the valve element in the direction of the valve seat in order in this way to close an inlet duct. In order to obtain effective damping in the hydraulic backlash compensating element, it is important that the check valve should open and close precisely. The opening and closing times and the speed of response of the check valve depend, inter alia, on the mass of the valve element. With increasingly dynamic processes in the internal combustion engine, various malfunctions occur in conventional check valves having steel balls as valve elements, e.g. noise, bouncing of the valve element, inadequate closure and wear on the valve element and the valve seat. 
     OBJECT OF THE INVENTION 
     It is therefore the underlying object of the invention to specify a hydraulic backlash compensating element which has a check valve that avoids the above-mentioned disadvantages. 
     SUMMARY OF THE INVENTION 
     According to the invention, which relates to a hydraulic backlash compensating element for an internal combustion engine, which comprises a housing that has an opening and a pressure-actuated piston which is mounted in the opening of the housing that has a check valve which can allow oil to flow into the housing and has a valve seat and a valve element arranged movably within therein, the object is achieved by virtue of the fact that the valve element of the check valve is a hollow ball. The particular advantage here is that a hollow ball is lighter than a ball made of solid material. Owing to the lower mass, there is an improvement in the dynamic response of the valve element, leading to improvements in the response times of the check valve of the hydraulic backlash compensating element. Another advantage is that the force with which the valve element strikes against the valve seat decreases with the weight of the valve element. As a result, the wear on the valve seat and on the valve element is reduced. 
     As a specific embodiment of the invention, it is proposed that the hollow ball be produced from a sintered inorganic material. In general, the valve elements used in hydraulic backlash compensating elements are solid steel balls having a diameter of 2-3 mm. To enable a robust hollow ball of such a size to be produced, a special manufacturing process is required. This process has been developed only recently by IFAM (Fraunhofer—Institut für Fertigungstechnik und Angewandte Materialforschung) and hollomet GmbH, see also WO 2001/54846 A2. Expanded polystyrene balls are used as the starting material. In a fluidized bed process, an air stream blows the balls upwards and keeps them suspended while a suspension of metal powder and binder is sprayed onto them from above. Once the metal layer is thick enough, a heat treatment is carried out: in a first step, all the organic constituents, the polystyrene and the binder vaporize. The residual materials are gaseous and escape through the pores in the metal layer. What remains is a fragile metal ball. This is sintered at just below the melting temperature. During this process, the metal powder granules combine and the shell becomes hard and impermeable. The ball is now sufficiently robust to be ground in a grinding machine. The wall thickness can be set to thicknesses of between a few tenths of a millimeter and one millimeter. A hollow ball produced in the manner described has a low inertia, on the one hand, and, given the appropriate wall thickness, also has the required robustness, on the other. A hollow ball produced by this process is therefore eminently suitable as a valve element in a check valve subjected to dynamic loading in a hydraulic backlash compensating element. 
     According to a preferred development of the invention, it is proposed that the hollow ball have a filling. A partially filled hollow ball makes it possible to optimize the noise behavior of the valve since the noises associated with the impact of the valve element in the valve seat are attenuated. The rebounding of the valve element during closure onto the valve seat is likewise reduced. As a result, there is an improvement in the closing behavior and hence also in the response time of the valve. The filling can be composed of various materials. Gases, gels or colloids can be used as a filling, for example, depending on the application. Solids, sand, powder or even liquids are likewise conceivable, and the liquids in turn may differ in their viscosity. 
     According to another embodiment of the invention, it is envisaged that the wall of the hollow ball be composed of layers of different materials and/or that the wall be composed of a porous material. With surface layers of different hardness, it is possible to reduce both the wear on the valve element and the wear on the valve seat. A wall of porous material, on the other hand, can optimize the rebound behavior. Another variant envisages combining a layer of porous material with a hardened layer. It is likewise conceivable to use a harder substrate material, which is provided with a rubber coating in order to protect the valve seat and the valve element from wear. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the invention are depicted in the figures, which are described in detail below, although the invention is not restricted to these illustrative embodiments. In the drawing: 
         FIG. 1  shows a hydraulic backlash compensating element embodied as a hydraulic belt tensioner, 
         FIG. 2  shows a hydraulic backlash compensating element embodied as a hydraulic chain tensioner, 
         FIG. 3  shows a hydraulic backlash compensating element embodied as a support element, 
         FIG. 4  shows a check valve in cross section, 
         FIG. 5  shows a valve element as a hollow ball, 
         FIG. 6  shows a hollow ball with filling material, and 
         FIG. 7  shows a hollow ball, the wall of which is composed of layers of different materials. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Various embodiments of hydraulic backlash compensating elements are depicted in  FIGS. 1 to 3 . Since the construction of all three embodiments is virtually identical, the description of the figures will be given by way of example for  FIG. 2 , which shows a hydraulic backlash compensating element  1  designed as a hydraulic chain tensioner. Inside a housing  2  there is a piston  3  with an integrated check valve  4 . The piston  3  is arranged within the housing  2  in a manner which allows longitudinal movement, and the two elements are pushed apart by an internal spring  6 . A defined leakage gap  10  is formed between the outer surface of the piston  3  and the inner surface of the housing  2 . While the piston  3  enters the housing  2  under the action of an external force, e.g. that of a traction means or a cam (not shown here), a high pressure arises in a pressure chamber  12  enclosed by the housing and the piston when the check valve  4  is closed. A small quantity of oil passes into the engine compartment through the very narrow leakage gap  10 . When the external force acting on the piston  3  weakens, the internal spring  6  pushes the backlash compensating element  1  apart. This gives rise to a differential pressure which opens the check valve  4 . The extra quantity of oil required for the compensating process can flow in via an oil feed opening  13  arranged in the housing  2 . 
     The check valve  4  has a valve element  7  which interacts with a valve seat  5 . In general, a valve spring  11  is provided in the check valve  4 . This pushes the valve element  7  in the direction of the valve seat  5  and thus closes the oil feed opening  13  as the differential pressure falls. In order to obtain effective damping in the hydraulic backlash compensating element  1 , it is important that the check valve  4  should open and close precisely. The opening and closing times and the speed of response of the check valve  4  depend on the mass of the valve element  4 . The lighter the valve element  7 , the more rapid is the response of the check valve  4 . For this reason, the valve element  7  is embodied as a hollow ball in order to reduce the mass. 
       FIG. 4  shows a detail view of the check valve  4  in the closed state when the valve spring  11  is pressing the valve element  7  into the valve seat  5 .  FIG. 5  shows the valve element  7  as a hollow ball in cross section. The wall thickness  9  of the hollow ball can vary, depending on requirements.  FIGS. 6 and 7  show further possible embodiments of the valve element  7  for achieving optimum closing and opening behavior of the check valve  4  of the hydraulic backlash compensating element  1 .  FIG. 6  shows a valve element  7  as a filled hollow ball. The filling  8  makes it possible to optimize the noise behavior of the check valve  4  since the noises associated with the impact of the valve element  7  in the valve seat  5  are attenuated. The rebounding of the valve element  7  during closure onto the valve scat  5  is likewise reduced. As a result, there is an improvement in the closing behavior and hence also in the response time of the check valve  4 .  FIG. 7  shows a valve element  7  embodied as a hollow ball, the wall  9  of which is composed of layers of different materials. With surface layers of different hardness, it is possible to reduce both the wear on the valve element  7  and the wear on the valve seat  5 . 
     LIST OF REFERENCE SIGNS 
     
         
           1  Hydraulic Backlash Compensating Element 
           2  Housing 
           3  Piston 
           4  Check Valve 
           5  Valve Seat 
           6  Spring 
           7  Valve Element 
           8  Filling 
           9  Wall 
           10  Leakage Gap 
           11  Valve Spring 
           12  Pressure Chamber 
           13  Oil Feed Opening