Rolling bearing ring by metal injection molding process

A rolling bearing providing a first ring and a second ring in relative rotation one each other, and at least one row of rolling elements being arranged between the said rings. At least one the rings is made by metal injection molding process including the successive steps of mixing a metal powder with a thermoplastic binder, forming a part by injection of the mixed powder in a closed die, debinding such a formed part in a furnace, sintering to densify the part, and quenching to set a ring hardness, to improve wear resistance and fatigue life.

This application claims priority to German patent application no. 102018208947.7 filed on Jun. 6, 2018, the contents of which is fully incorporated herein by reference.

TECHNOLOGICAL FIELD

The present invention relates to a rolling bearing, for use in any application with rotating parts, for example automotive vehicles, thermal and electrical motors, torque transmission devices.

BACKGROUND

A rolling bearing comprises a first ring and a second ring in relative rotation one each other, at least one row of rolling elements being arranged between the said rings.

The rings may be of solid type. A solid type is to be understood as a ring obtained by machining, grinding, from a metal tube shock, bar stock, rough forgings and/or roller blanks, to provide the ring final shape and dimensions. However, the manufacturing and material costs are intended to be decreased. The solid rings are of relatively important weight and, in many applications, it is desirable to reduce the rolling bearing weight. It is also desirable to use a manufacturing process less expensive and enabling complex shapes to the rings while keeping the load support function. Such solid-type rings may equip ball bearings of deep groove and types, roller bearings of toroidal, cylindrical, spherical, needle roller types, for example.

Alternatively, the rings may be stamped and cut from a metal blank sheet. A wide variety of ring shapes may be formed according to this manufacturing process, but limits the ability for supporting higher loads. Such rings may equip rolling bearings for clutch bearing units, steering units, suspension thrusts, for example.

SUMMARY

The aim of the invention is to overcome these drawbacks by proposing a rolling bearing having rings made by a process that permits shapes of high complexity.

To this end, the invention relates to a rolling bearing comprises a first ring and a second ring in relative rotation one each other, at least one row of rolling elements being arranged between the said rings.

According to the invention, at least one the rings is made by metal injection molding process including the successive steps of mixing a metal powder with a thermoplastic binder, forming a part by injection of the mixed powder in a closed die, debinding such a formed part in a furnace, sintering to densify the part, and quenching to set at least one of the rings hardness, to improve wear resistance and fatigue life.

Thanks to the invention, a ring for a rolling bearing of complex shape can me manufactured for use in a rolling bearing. The metal injection molding process prevents the use of additional secondary machining, and also prevents waste. The ring surfaces are fine without additional refining process step.

The ring density can be defined as high as necessary during the sintering step, and is suitable for use in a variety of rolling bearing applications.

According to further aspects of the invention which are advantageous but not compulsory, such a rolling bearing may incorporate one or several of the following features:

Rolling elements are rollers or needles.

Rolling elements are balls.

The metal powder is a steel alloy.

The metal powder is a steel alloy comprising chromium, molybdenum and high carbon through hardening steel. Advantageously, the metal powder is the steel alloy 100Cr6.

The metal powder is a steel alloy comprising nickel, chromium and molybdenum hardening steel. Advantageously, the metal powder is the steel alloy 21NiCrMo2.

The metal powder is an alloy of steel comprising nickel, carbon and molybdenum hardening steel. Advantageously, the metal powder is the steel alloy AISI 4620.

The limit of elasticity of at least one of the rings is comprised between 1000 MPa and 1500 MPa, and advantageously substantially equal to 1200 MPa.

The ultimate tensile strength of at least one of the rings is comprised between 1200 MPa and 1700 MPa, and advantageously substantially equal to 1500 MPa.

The ring is of surface roughness comprised between 250 and 830 Hv. The ring surface roughness is advantageously superior to 650 Hv on the rear abutment surface.

The process further comprises finishing operations of ring surfaces.

DETAILED DESCRIPTION

TheFIG. 1discloses a rotating bearing assembly1. For example, assembly1may be used in an internal combustion engine of a motor vehicle, or in an electric motor.

Assembly1comprises a rotating shaft2of longitudinal rotating axis X2, a stationary housing3, for example a stationary engine block, and a rolling bearing4to support in rotation said rotating shaft2with respect to said stationary housing3.

The rolling bearing4is of deep groove ball bearing type and is of longitudinal rotating axis X4. Axis X4and X2are common.

Rolling bearing4comprises an inner ring8with an inner cylindrical bore9. According to one embodiment, the inner ring8is rotating, bore9being mounted on an outer cylindrical surface7of a rotary shaft2. Advantageously, inner ring8is press fitted onto shaft end but inner ring8can be securely fixed to said shaft end by any other suitable fixing means. Inner ring8is fastened in rotation with rotating shaft2. Inner ring8further comprises an outer surface provided with a concave portion10forming an inner raceway for a plurality of rolling elements11.

Rolling bearing4comprises an outer ring12with an outer cylindrical surface13mounted in a cylindrical bore14of stationary housing3. Advantageously, outer ring12is press fitted into housing3but inner ring12can be securely fixed to said housing3by any other suitable fixing means. Outer ring12is stationary. Outer ring12further comprises an inner bore provided with a concave portion15forming an outer raceway for a plurality of rolling elements11.

Alternatively, the inner ring8may be a stationary ring, the outer ring12being a rotating ring.

Rolling elements11, here balls, are radially arranged between raceways10,15of inner and outer rings8,12, respectively.

Rolling elements11are circumferentially maintained by a cage16. Cage16comprises an annular heel17designed to be arranged axially on one side of the rolling elements11and radially between the outer surface of inner ring and the inner bore of outer ring. Cage16further comprises a plurality of projected portions18that axially extend from said annular heel17. Projected portions18are formed integral with heel17and between them delimit pockets19in which the rolling elements11are housed. Projected portions18form separation walls between two circumferentially adjacent pockets.

Advantageously, each of the projected portions18can be axially extended on its free end by a claw20so as to snap rolling elements11in a pocket19. Alternatively, projected portions do not comprise such claws, or only a limited number of projected portions of cage comprise claws.

In the illustrated embodiment, the rolling bearing4further comprises sealing means21provided on one axial side of the rolling bearing4towards the shaft end. According to an alternate embodiment, the rolling bearing4may comprise sealing means on both axial sides.

Sealing means21comprise a stiffening insert22and a sealing gasket23.

Stiffening insert22is annular and extend radially. Sealing gasket23is fixed to said stiffening insert22. Advantageously, stiffening insert22is made of metal, and sealing gasket is made of polymer material.

Sealing gasket23comprises an annular anchorage portion24on a first radial end. Anchorage portion24is fitted into an annular groove25provided in the inner bore of outer ring12. Annular groove25is provided on one axial side of the outer raceway15for the rolling elements11. According to the invention, groove25is on the axial side oriented towards the shaft end16.

Sealing gasket further comprises an annular sealing lip26on a second radial end, opposite radially to said anchorage portion24. Sealing lip26is in sliding contact with a portion of the outer surface of inner ring8on the axial side of inner raceway10oriented towards the shaft end16. In the present embodiment, sealing lip26contacts an annular groove provided on the outer surface of inner ring8. Alternatively, sealing lip26may contact an outer cylindrical surface of inner ring8.

According to the invention, the inner rings8and the outer ring12of rolling bearing4are each made by metal injection molding process that includes the successive following steps.

A metal powder with a thermoplastic binder are mixed together. Mixing coats the metal powder particles with the binder, breaks up agglomerates and permits to obtain homogeneity of distribution of metal power particles and binder. Mixing is performed in any suitable mixing means, such as single or twin screw extruder, plunger extruder, double planetary mixer, twin cam mixer, for example. Advantageously, the metal powder is a steel alloy. According to a first embodiment, the metal powder is a steel alloy comprising chromium, molybdenum and high carbon through hardening steel. Advantageously, the metal powder is the steel alloy 100Cr6. According to a second embodiment, the metal powder is a steel alloy comprising nickel, chromium and molybdenum hardening steel. Advantageously, the metal powder is the steel alloy 21NiCrMo2. According to another embodiment, the metal powder is an alloy of steel comprising nickel, carbon and molybdenum hardening steel. Advantageously, the metal powder is the steel alloy AISI 4620.

A ring part is formed by injection of the mixed powder in a closed die. The mixed powder is progressively filled in a closed die, said die defining a hollow shape of the inner ring8. The same applies for outer ring12. The mixed powder fills the entire cavity without any void. The mixture of metal powder and binder being homogeneous, the metal powder and the binder are homogeneously distributed within said closed die. The mixed powder is filled in at pressure and temperature conditions suitable for the mixed powder composition. Furthermore, the temperature must be sufficient to the thermoplastic binder be aggregated with the metal power particles. The binder holds the metal particles together, but is unable to support stress, thermal gradients that could be applied to a rolling bearing4in some applications.

The thermoplastic binder is then removed from the ring material compound by debinding the formed part in a furnace. The debinding at high temperature is the preferred process instead of using solvent extraction processes. Indeed, the debinding by solvent extraction requires the immersion of ring in a fluid that dissolves the thermoplastic binder, and has the disadvantage for the present final application to leave an open-pore structure. On the contrary, the debinding in a furnace extracts the thermoplastic binder from the pores as a fluid, the thermoplastic binder passing to a liquid and/or vapor state under the effect of high temperature.

This debinding step is combined/followed by sintering the ring, the metal particles being heated and soldering to each other, also under the effect of high temperature. The homogeneity of the mixed powder during the mixing step and then the injecting step leads to a homogeneous sintered structure of ring material and at an acceptable density for the rolling bearing application, with controlled dimensions and properties. The exemplary embodiment of steel alloys 21NiCrMo2 and AISI 4620 permits to reach the desired ring characteristics.

The limit of elasticity of at least one of the rings is comprised between 1000 MPa and 1500 MPa, and advantageously substantially equal to 1200 MPa.

The ultimate tensile strength of at least one of the rings is comprised between 1200 MPa and 1700 MPa, and advantageously substantially equal to 1500 MPa.

The ring is then quenched to set at least one of the rings hardness, to improve wear resistance and fatigue life.

Applicants respectfully submit that no new matter is added to the application by the amendments to the specification.

Advantageously, the process may further comprise finishing operations on the ring surfaces, in particular on the raceways10,15.

The ring is of surface roughness comprised between 250 and 830 Hv. Advantageously, the surface roughness may be superior to 650 Hv to support high loads.

The present invention has been illustrated on the basis of a deep groove rolling bearing4for a rolling bearing assembly1which can for example be used in an internal combustion engine of a motor vehicle, or in an electric motor. It is also possible, without departing from the scope of the invention, to provide a rolling bearing with more than one row of rolling elements, with other types of rolling elements, and suitable for any other rolling applications.

A second example of rolling bearing30is illustrated in theFIG. 2. Rolling bearing30is a clutch thrust bearing for use in a clutch bearing unit in the automotive applications. Said rolling bearing30is suitable for use in any other application with rotating movement.

The rolling bearing30is annular and centered on a central rotating axis X30. The ball bearing30comprises a fixed inner ring31, a rotatable outer ring32, and one series of balls33located in a raceway chamber34defined between the rings31,32. The balls33are circumferentially equally spaced and held by a cage35, each of said balls33being arranged in a corresponding pocket36provided circumferentially to an annular heel37. In the illustrated embodiment, the heel37is axially arranged between a free edge of a toroidal portion38of the fixed inner ring31, and a radial portion39of the rotatable outer ring32. A clutch thrust bearing30provided with balls33as rolling elements between rings31,32is of reduced friction torque compared to other types of rolling bearings, provided with rollers or needles.

The rotatable outer ring32comprises a toroidal portion40of inner toroidal surface forming an outer raceway for the balls33.

Advantageously, the rotatable outer ring32further comprises a radial portion39that outwardly radially extends from an inner side of said toroidal portion40. Said radial portion39has an axial contact surface41suitable to actuate a coupling member (not represented), for example a diaphragm, to engage a clutch system of the vehicle.

Advantageously, the toroidal portion40of rotatable outer ring32is provided with a flange42. Flange42has a tubular outer portion covering the toroidal portion40, and a downwards radial projection that maintains the balls33in the rolling chamber34.

The fixed inner ring31comprises a toroidal portion38of outside toroidal surface forming an inner raceway for the balls33, and a radial portion43that outwardly radially extends from said toroidal portion38. Said radial portion43has an axial contact surface44in axial abutment against an axially movable member (not illustrated). The ball bearing4is set in axial movement by the transmission of the movement of axially movable member to the axial contact surface44of fixed inner ring31, and the transmitted to the clutch coupling member.

According to the invention, the inner ring31and the outer ring32of rolling bearing4are each made by metal injection molding process that includes the steps as previously described.

Moreover, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.