GB2230826A - Bearing - Google Patents

Description

SLEDVE BEARING This invention relates to a sleeve bearing intencea for use in four-stroke internal combustion engines, particularly as a connecting rod bearing, As known by those skilled in the art, there is provided a clearance holding a film of lubricanting oil betseen the inner peripheral surface of the bearing and the crankpin.

According to the current technology concepts an adequate choice of the assembly clearance between the shaft and the bearing is made based on load and lubrication considerations, generally wit the aid of computerized methods. n such.

studies, minimum and maximum amounts for these clearences are defined by key parameters such as tho bearing operating temperature, minimum oil film thickness and maximum oil film pressure, Where the beating-to-shaft clearance is reduced the oil film is subjected to a high shearing stress causing the sleeve bearings to overheat, besides facilitating a contact between the shaft an > ' the bearing due to an excessively thin oil film. Under such conditions, the bearing material is subject to failure by overheating and excessive wear.

inversely, large clearances cause an excessive hydrodynamic pressure in conjunction with a reduction in the oil film thichness, thereby exposing the bearing material to fatigue failure and excessive wear, The numerical techniques currently omptoyed afford a prediction of acceptable limits for assembly clearances based on lubrificating oil characteristics and the bearing geometry and materials, The high-load four-stroke internal combustion engines designed accorirg to current trends have subjected bearing, particularly the connecting rod bearings, to an excessive load and wear which become prohibitive and eventually cause fatigue and overheating problems.These engines, in which hig operating speeds are combined with a low mass interest of every component, subject the connecting rods to high inertia loads thereby causinq distortions on their big end hole during operation.

Due to the connecting rod configuration and the manner bv which they transmit forces, the resulting distortions are such that during the exhaust stroke and, to a lesser degree, during the compression stroke the rod big end tends to have its diameter increased toward the centerline of the big end and small end holes, and reduced in a direction orthogonal to the said centerline.On the other hand, during the cxpansion stroke the big hole tends to have its diameter increased, or evon unchanged, in a direction orthogonal to the connecting rod big end and small end centerline, The amounts of these diametral variations are dependent on the inertia loads acting or. the connecting rods as well as the structural resistance 0 the rods which is related with both the quantity and the mass distribution in its design.

The reductiOn in clearence in the direction of closing of the connecting rod hole causes disturbances for the creation of a stable Lubricating oil film and a consecuential increase of the operating temperature. Under such conditions, a boaring-to-journal contact may take piace close to the point below the parting lJlane with a subsequent failure of the boaring material causod by excessive wear and overheating.

Owing to the instability or turbulence developed in the oil film this closing can further lead the bearing sliding surface to erosion damages caused by capitation in te oil film.

In the aforesaid cIrcumstances closing of the hole under inertIa load one must increase tho clearance in te direction ct closing thereby enabling a stable lubricating oil film, thus preventing heat to the generated above levels determined beforehand. This measure, an increase of the diametral clearance by the use of a pair of conventional halfshells, although solving problems arising out of the closing of the rod big end hole subjected to inertia loads, can on the other hand give rise to problems on the bearing placed in the rod half during tbe expansion stroke.An excessive increase in the operating clearance may create a highly detriments increase in the hydrodynamic pressure developed on the lubricating oil fIlm, in conjunction with an equally detrimental increase in the the mum oil film thinness. These two conditions may load to an recessive wear or fatigue failure of the bearing material.

With a view to overcoming the disadvantages resulting from the use of conventional bearings for applications where lubrification problems arising out of the said deformation are present, the present invention contemplates the use of connecting rod bearing halfshclls having geometric features intended to provide the halfshells with better lubrification conditions, thereby imparting a higher resistance to fatigue an wear.

It is, therefore, an object of the present invention to trovlde a connecting rod bearing whouse relief portions exhibit a reduced thickness as compared with the thickness if the central portion of the bearing, thereby defining an eccentricity.

According to the present invention there is provided a bearing halfshell for internal combustion engine connecting rods, the bearing halfshell having a central plane of symmetry, a pair of diametrically opposed ends located in a bearing parting plane and an inner slide surface, wherein the inner slide surface has a substantially constant radius of curvature from the central plane of symmetry up to two regions of said inner surface located one on each side of the central plane of symmetry and in a radial plane which defines an angle with the bearing parting plane, the inner surface being defined, between each one of said regions and a respective end located on one same side of the central plane of symmetry, by a surface which gradually decreases away from the central plane of symmetry in a more accentuated manner than that corresponding to the radius of curvature of the inner surface, thereby defining an eccentricity on the respective end portion of the inner surface.

A bearing halfshell according to the present invention is shown in a cross sectional view on the appended drawing.

As dcpicted in the drawing bearing halfshell 10 has a central plane of symmetry A, a pair of diametrally opposed ends 11 located on the parting plane B and an inner sliding surface 12.

According to the preset invention, the inner sliding surface 12 of the halfshell has a substar.~ially constant radIus of curvature from the central plane of symmetry A up to two portions of the inner surface 12 located one on each side of the central plane of symmetry A and on a respective radial plane P which defines wit the bearing parting plane B an acute angle & preferably from about - 50 to 450 Fror. each of said regions and up to the respective end 11 of the halfshell 10, the inner surface 12 is defined by a surface 13 which gradually decreases away from the central plane of symmetry A in a more accentuated manner than that one obtained with the radius of curvature of the inner surface 12, in order to provide an eccentricity # in the respective end portion of t'le inner surface 12, the amount of the eccentricity e varying from 0,01 to 0,25mm.

Besides providing a relief of the hydrodynamic pressures developed on the portion subjected to the highest loads on the oil film between the bearing and the shaft, the relief portions with a reduced thickness function as an oil pocket which ensures a stable oil film free of any turbulence or any other factor of distress. This in turn allows the bearing to operate at lower temperatures, which increases the lubricating oil viscosity and in consequence the minimum oil film thickness without increasing the peak oil film pressure, On the other hand, a number of the known bearing half shells has their central portion provided with a hole for the passage of oil intended to lubricate the cylinder iiner and the piston pin and/or cool the piston.For this reason, a substantial amount of this lubricating oil is deviated from tat portion of the bearing subjected, to the higher loads, thereby causing a shortage of oil which can load to an excessive wear, fatigue and an early failure of the bearing.

This problem too is solved by the oil pocket created by the bearing design of the invention, in that said portion desing will be always provided with an oil film having a thickness respire: for a proper operating of the bearing.

One additiona advantage provided by the bearing half shell object of the present invention lies in the fact the halfshell cn be installed on connecting rods where the location of the halfshell in the bearing cap is not precise, without jeopardizing its lubricating condition,

Claims (4)

1. CLAI,MS: 1. A bearing halfshell for internal canbustion engine connecting rods, the bearing halfshell having a central plane of symmetry, a pair of diametrically opposed ends located in a bearing parting plane and an inner slide surface, wherein the inner slide surfacefhas a substantially constant radius of curvature from the central plane of symmetry up to two regions of said inner surface located one on each side of the central plane of symmetry and in a radial plane which defines an angle with the bearing parting plane, the inner surface being defined, between each one of said regions and a respective end located on one same side of the central plane of symmetry, by a surface which gradually decreases away fran the central plane of symmetry in a more accentuated manner than that corresponding to the radius of curvature of the inner surface, thereby defining an eccentricity on the respective end portion of the inner surface.
2. 2. A bearing halfshell as claimed in claim 1, wherein each radial plane defines with the bearing parting plane an angle from 50 to 450.
3. 3. A bearing halfshell as claimed in claim 1, wherein the eccentricities, range from 0.01 to 0.25 mm.
4. 4. A bearing halfshell substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawing.