[001 ] This invention relates to a thrust washer for internal combustion engines provided with a metal base comprising at least one slip surface having a uniform surface profile without any points of inflection or high points.

Description of the state of the art

[002] Internal combustion engines are energy conversion mechanisms used by the great majority of motor vehicles and basically comprise two main parts - one or more cylinder heads and the engine block. Below the cylinder head(s) are located the combustion chambers and the cylinders and the crankshaft assembly is located in the engine block.

[003] The crankshaft is a fundamental component in the functioning of combustion engines, and is responsible for converting the energy generated by combustion of the air/fuel mixture into torque. The crankshaft, which is mounted in the block by supporting bearings, is usually of cast carbon steel and has connecting rods coupled to the crank pins and their geometry is reminiscent of a number of cranks placed side by side.

[004] Among the components which help to mount the crankshaft in the engine block are bushes and thrust washers, the thrust washers being responsible for withstanding the axial loads generated by the gearing or gearshift system.

[005] Thrust washers are generally located in one or more bearing positions, and basically comprise two half-moons or two half-washers positioned at the top and bottom of the bearing. These washers act in such a way as to cancel out possible axial forces acting on the crankshaft, so that when it is exposed to an axial load the force generated does not act directly on the engine block because the washer provides a supporting surface for the load, preventing wear on the block. [006] As may be seen in Figure 1 , a thrust washer is provided with a metal base, substantially of a semi-circumferential shape, and has two surfaces. The contact surface is associated with the engine block, and there is no relative movement between the washer and the block, while the slip surface remains in contact with the crankshaft, giving rise to relative movement between the washer and the crankshaft. Depending upon the applied load, the relative movement places a high load on the contact between the slip surface and the crankshaft, the slip surface being responsible for supporting the axial load produced. In addition to this, thrust washers also have an internal diameter, associated with the shaft, and channels for oil.

[007] The oil channels are fundamental to the proper functioning of thrust washers and ensure that surfaces are lubricated, preventing accelerated/excessive wear on the bearings. These channels are filled with oil so that the oil is distributed over the surface through the pull generated by the rotational movement.

[008] Both Otto and Diesel engines are subject to axial loads. Furthermore, these loads are even greater mainly in engines provided with automatic or automated gear changes. These gears help to reduce fuel consumption in motor vehicles, but they increase the applied loads, with the result that the thrust washers currently in use cannot withstand the forces and become worn or suffer other deficiencies.

[009] In order for the washer to support the loads generated an oil film has to be formed between the slip surface of the washer and the crankshaft. This oil film must maintain a thickness, and it is created through a phenomenon known as hydrodynamic support. This hydrodynamic support depends on two factors - the shape of the surface and the speed.

[0010] A washer is said to be hydrodynamically supported when there is no metal-to-metal contact, that is when there is a distance between the surfaces, this distance being the thickness of the oil film. For hydrodynamic support to occur the thickness of the oil film must be three times greater than the surface roughness.

[001 1 ] In order to achieve sufficient hydrodynamic support a specific rotation speed, and the best surface possible for that condition, have to be guaranteed. As the rotation speed varies in relation to the speed of the vehicle, the surface can be modified in such a way as to achieve sufficient support to support high axial loads and forces.

[0012] In the state of the art there are flat washers, convex washers and ramp-and-pad washers, which basically comprise at least one ramp section, at least one flat horizontal section and at least one channel.

[0013] Washers having a flat slip surface, as the name suggests, do not have any geometry on the slip surface, and only include oil channels. These washers have low resistance to wear and are only suitable for engines with low loads.

[0014] Convex washers are manufactured by a stamping process and comprise a curvilinear slip surface. They are cheaper, but they have low resistance to wear because of the great variation in the geometry of the slip surface, and because of the lack of flat sections. It should be noted that a flat section is essential in order to maintain hydrodynamic support.

[0015] Furthermore ramp-and-pad washers are produced by a machining process and comprise at least one ramp with a constant inclination, at least one horizontal flat surface and at least one oil channel. These washers ensure a high level of hydrodynamic support because the machining process guarantees that the washer is dimensionally stable, and the flat horizontal section ensures a high level of hydrodynamic support, improving the performance of the washers.

[0016] Although ramp-and-pad washers have greater resistance to wear and better performance, the connection between the ramp section and the horizontal section represents a sudden change in geometry. Mathematically speaking, this point of connection gives rise to an inflection point or vertex which interrupts the distribution of oil on the slip surface, reducing hydrodynannic support at that point. This inflection point or vertex results in a change in pressure behaviour, giving rise to instability.

[0017] One way of reducing this shortcoming is to round off the said vertex linking the ramp section to the flat horizontal section by means of a small radius. This configuration results in less instability in the washer, but it does not ensure excellent performance.

[0018] In view of the above, there has not yet been developed any thrust washer which has a surface which guarantees uniform hydrodynamic support which is not affected by any transition effect, imparting high wear resistance to the washer and consequently excellent performance.

Objects of the invention

[0019] The object of this invention is to provide a thrust washer, a washer comprising a base provided with one slip surface, a slip surface comprising at least one ramp section, at least one flat horizontal section, at least one oil channel, and a connection between the ramp section and the horizontal section which is without points of inflection or vertices.

[0020] Another object of this invention is to provide a thrust washer which ensures uniform hydrodynamic support without any transition between the surfaces.

[0021 ] A further object of this invention is to provide a thrust washer having high resistance to wear, and as a consequence excellent performance, especially in engines with high loads.

[0022] Yet a further object of this invention is to provide a thrust washer which has a dimensionally stable surface on the slip surface and an oil film thickness which is greater than three times the surface roughness of the slip surface.

Brief description of the invention

[0023] The objects of this invention are accomplished by a thrust washer for internal combustion engines provided with a metal base comprising a contact surface associated with the engine block and a slip surface, the slip surface comprising a surface profile incorporating at least one ramp section, at least one flat horizontal section, and at least one oil channel, the ramp section and the flat horizontal section being connected through at least one asymptotic curve, the ramp section tending towards the flat horizontal surface asymptotically.

[0024] The objects of this invention are also accomplished through an internal combustion engine comprising at least one thrust washer as described above.

Brief description of the drawings

[0025] This invention will be described more particularly below on the basis of an embodiment illustrated in the drawings. The drawings show:

[0026] Figure 1 - a perspective view of a thrust washer together with all its constituent parts;

[0027] Figure 2 - a front view of the thrust washer according to this invention;

[0028] Figure 3 - a detailed view of a cross-section B-B in Figure 2;

[0029] Figure 4 - a comparative graph of the hydrodynamic pressure generated by thrust washers according to the state of the art and according to this invention;

[0030] Figure 5 - a comparative graph of the hydrodynamic pressure generated by thrust washers according to the state of the art and according to this invention.

Detailed description of the drawings

[0031 ] This invention relates to a thrust washer 1 for internal combustion engines provided with a metal base comprising at least one slip surface 3 having a uniform surface profile without points of inflection or vertices.

[0032] As already mentioned in the state of the art, thrust washers 1 are components which help mount the crankshaft in the block of an internal combustion engine, and are responsible for withstanding the axial loads generated by the gearing or gear change system.

[0033] As may be seen in Figure 1 , thrust washer 1 is provided with a metal base, substantially of semi-circumferential shape, and comprises two surfaces. Contact surface 2 is associated with the engine block, and there is no relative movement between washer 1 and the block, while slip surface 3 remains in contact with the crankshaft, generating a relative movement between washer 1 and the crankshaft. Depending upon the applied load, the relative movement places a high load on the contact between slip surface 3 and the crankshaft, while slip surface 3 is responsible for supporting the axial load generated. In addition to this, thrust washer 1 also comprises a diameter for association with the bearing, and at least one oil channel 6.

[0034] In order for washer 1 to support the loads generated an oil film must be formed between slip surface 3 of washer 1 and the crankshaft. This oil film must maintain a thickness and is produced by means of a phenomenon known as hydrodynamic support. This hydrodynamic support depends on two factors - the shape of the surface and the rotation speed of the crankshaft.

[0035] Washer 1 is said to maintain hydrodynamic support when there is no metal-to-metal contact, that is there is a distance between the surfaces, this distance being the thickness of the oil film.

[0036] In order to have sufficient hydrodynamic support, a specific rotation speed has to be ensured, and the best surface possible for this condition also has to be guaranteed. As the rotation speed varies according to the speed of the vehicle, the surface of the slip surface has to be altered so as to achieve sufficient support to support high loads and axial forces.

[0037] In this respect this invention provides a preferred configuration for thrust washer 1 in Figures 2 and 3, comprising a surface profile of slip surface 3 incorporating at least one ramp section 4, at least one flat horizontal section 5 and at least one oil channel 6, ramp section 4 and flat horizontal section 5 being connected by means of an asymptotic curve.

[0038] Ramp section 4 starts with the inclination of the curve and tends asymptotically towards flat horizontal section 5. This means that ramp section 4 and flat horizontal section 5 are connected through a constant curve which is smoothly concordant, eliminating possible points of inflection or vertices in the connection.

[0039] As explained previously in the state of the art, the presence of points of inflection or vertices in the connection between ramp section 4 and flat horizontal section 5 results in a change in the behaviour of the oil pressure, giving rise to instability in the hydrodynamic support of washer 1 .

[0040] Thus this invention provides a thrust washer 1 which ensures more uniform hydrodynamic support, cancelling out the transitional effects produced by points of inflection or vertices.

[0041 ] This more uniform hydrodynamic support means that there is no metal-to-metal contact between slip surface 3 and the crankshaft, ensuring a distance between the surfaces or an oil film thickness at least three times the surface roughness.

[0042] Furthermore washer 1 according to this invention is less influenced by the dimensional tolerance of the surface. The tolerance in the angle of the straight ramp represented in the state of the art affects the result of hydrodynamic support and load support. Change in the tolerance in washer 1 according to this invention has less effect on these properties.

[0043] Viewing Figure 4, the influence of the surface profile of slip surface 3 in relation to the angular position of that surface 3 in connection with the oil pressure maintained on that surface can be seen. In addition to this Figure 5 shows the profile of slip surface 3 produced in accordance with the angular position of surface 3. [0044] State of the art 1 shows a surface profile for a convex washer, while state of the art 2 shows the surface profile for a ramp-and-pad washer provided with a straight ramp.

[0045] Examining Figures 4 and 5, it will be concluded that the surface profile of slip surface 3 provided by this invention gives rise to a smaller oil pressure, but with greater hydrodynamic support and a thicker oil film.

[0046] In view of the above, thrust washer 1 according to this invention offers an increase in the maximum supported load, increased wear resistance and, as a consequence, excellent performance.

[0047] Although a preferred embodiment has been described, it should be understood that the scope of this invention covers other possible variations, being limited only by the content of the appended claims, including possible equivalents therein.