EP1907595A1

EP1907595A1 - Heat-treating of rolling elements for bearings, and furnace for implementing such treatment

Info

Publication number: EP1907595A1
Application number: EP05778666A
Authority: EP
Inventors: Herminio Maorenzic
Original assignee: Euroball SpA
Current assignee: NN Europe SpA
Priority date: 2005-07-20
Filing date: 2005-07-20
Publication date: 2008-04-09
Also published as: WO2007010567A1; US20080308978A1

Abstract

Heat-treating of rolling elements for bearings; according to the treatment, the rolling elements are guided from an entrance to an exit by gravity or mechanically and are heated during the movement towards the exit by induction heating. The speed of the balls can be controlled by making their guiding path sinuous and/or by rotation thereof.

Description

- i -

HEAT-TREATING OF ROLLING ELEMENTS FOR BEARINGS, AND FURNACE FOR IMPLEMENTING SUCH TREATMENT

TECHNICAL FIELD

5 The present invention refers to heat-treating of rolling elements for bearings, particularly to a treatment for hardening said rolling elements, to which the following specification specifically refers but without impairing its generality.

D

BACKGROUND ART

For hardening balls and rollers for bearings it is known to use furnaces provided with a conveyor, for example a belt or screw conveyor, that feeds either continuously or in groups

5 the balls/rollers along a direction in line within a chamber, the walls of which are heated by electric resistances at the required temperature. The furnaces of the known type above described are designed for a relatively high output, so as to heat, each, the rolling elements coming from several molding

3 lines, associated to the production of respective series of rolling elements different one from the other, for example different in diameter.

Between the furnace and each line an associated storage unit ) or buffer is provided, that accumulates a certain quantity of rolling elements produced waiting to enter into the furnace according to a required schedule: in particular, their entrance is allowed after the conveyor has been operated empty for a certain time, the so called "type change" period, allowing some space from one group of rolling elements and the following one.

The heating is followed by quenching, normally in oil, and then by tempering.

The known furnaces described above are scarcelly satisfying, as they are realtively bulky, involve high maintenance costs and require relatively long starting up times (approximately one day) to reach the required temperature, due to the large size of the heated chamber, necessary to satisfy the production cycle of all the upstream production lines, with consequently high energy costs without actual production. Such costs without actual production occur also during the "type change" period (approximately half an hour) described above.

Moreover there exists the possibility (1 over 10000 approx.) that a ball/roller gets trapped in a portion of the conveyor inside the furnace, and is then casually released during treatment of a successive series having a different diameter, thus creating therein an anomaly that is difficult to detect. The ball/roller released may cause failure in the machinery downstream the furnace, should it have a larger diameter than those with which it has casually mingled, while going through all the following treatments and giving rise to complaints by end customers who receive a non homogeneous lot, should it have a smaller diameter. Moreover, the oil for the quenching, although widely used, is undesirable as it involves fire risks and requires recycling of vapours during hardening, washing of the rolling elements at the exit from the hardening bath, and a recycling process for the oil itself at the end of use.

DISCLOSURE OF INVENTION

The aim of the present invention is to provide a heat-treating of rolling elements for bearings, that allows to solve in a simple and economic way the above problems.

According to the present invention a heat-treating of rolling elements for bearings is provided, as defined in claim 1. According to the present invention, a furnace for heat- treating of rolling elements as defined in claim 16, and a production plant of rolling elements as defined in claim 35, are implemented.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention a preferred embodiment is described, for descriptive and not limitative purposes, with reference to the enclosed drawings, wherein : figure 1 is a partial and schematic view of a production plant of rolling elements for bearings; - figure 2 illustrates, schematically and partly in section, a hardening and tempering group forming part of the plant of figure 1 for implementing a heat-treating of rolling elements for bearings according to the present invention; figure 3 illustrates, schematically, in an enlarged scale and in section, a furnace of the hardening and tempering group in figure 2; figure 4 illustrates, in perspective, in a further enlarged scale and with parts cut off for clarity purposes, a detail of figure 3; and figures 5 e 6 are schematic views of respective furnace alternatives of figure 3.

BEST MODE FOR CARRYING OUT THE INVENTION

In figure 1, by 1 is indicated, as a whole, a plant (partially and schematically illustrated) for the production of rolling elements for bearings, that is balls, rollers and needles. Plant 1 includes a plurality of moulding lines 3a-3d, from where respective series of steel rolling elements to be treated exit, indicated respectively by 4a-4d. Each series includes rolling elements which are all the same but different from those of other series, for example in the diameter.

Directly downstream the exit of each line 3a-3d, an associated hardening and tempering assembly 5a-5d is provided, that carries out a hardening treatment followed by tempering and that is dedicated to the specific type and/or diameter of the rolling elements 4a-4d to be treated.

With reference to figure 2, each assembly 5 includes a furnace 6, that receives the rolling elements 4 by gravity through a vertical conduit 7 and carries out an austenitizing heating of steel .

When exiting from the furnace 6, the heated rolling elements

(indicated by 8) fall by gravity through a vertical conduit 9 into a tank 10 containing a quenching fluid, particularly water. The elements 4 remain in the bath for a determined time while they are transported upwards outside the tank 10 by a screw conveyor 11. At the exit of the conveyor 11, the quenched rolling elements (indicated by 12) fall by gravity through a vertical conduit 13 into another furnace 14, that carries out the tempering and, at the exit, drops in a container 15 the rolling elements hardened and tempered (indicated by 16). The furnaces 6,14 are carried by respective structures 17,18 secured to the tank 10 and to the container 15 respectively, and have an inclination A adjustable with respect to a horizontal direction B, and are substantially the same one another, and therefore only furnace 6 will be described shown in detail in figure 3, with reference to the case where the steel rolling elements 4 are defined by balls.

The furnace 6 includes two heads 21,22 opposite and coupled to conduits 7 e 9, respectively and defining an entrance chamber j 23 and an exit chamber 24. The chamber 23 is closed at the top by a rotating door 25, described in detail here in below, while the chamber 24 is closed at the bottom by the surface 26 of the water contained in the conduit 9 and in the tank 10 (figure 2) .

The furnace 6 includes a guide device 27 for feeding the balls 4 from the chamber 23 to the chamber 24 in a preset time, during which the balls 4 are heated at a preset temperature by induction, that is by the variation of a magnetic field, the flux lines of which pass in between the chambers 23,24: such variation generates electric currents induced in the balls 4 that are being fed, which therefore heat up due to Joule effect. The magnetic field is generated by a device 28, defined particularly by a coil or inductor having a straightforward axis 29 and fed with high frequency alternated voltage. The furnace 6 is assembled with the chamber 23 higher up than the chamber 24, so that the inclination A of the axis 29 is about 30°.

The device 27 includes a fixed cylindrical liner 30 coaxial with the coil 28 and ending with two sections 31,32 opposed, that protrude into the heads 21,22 and delimit part of the chambers 23, 24. The liner 30 includes a intermediate section 33 enclosed by the coil 28 and housing a cylindrical coaxial body 34, rotating about the axis 29 and being part of the device 27.

With reference to figures 3 and 4, the external cylindrical surface of the body 34 has a plurality of grooves 35 equally spaced about the axis 29 and defining, together with the internal liner 30, respective channels, the transversal section of which is similar by excess to the diameter of the balls 4 for guiding the balls 4 from the chamber 23 to the chamber 24 along respective rows. The entrance of the grooves 35 has a widening defined by a slope 3β directed in a rotation 5 direction C of the body 34 (figure 4), for faciliting the automatic fitting into the grooves 35 of the balls 4 that are collected by gravity on the section 31. The grooves 35 include an intermediate shaped section 37, that is completely housed in the coil 28, has a length that is greater than the axial O length of the coil 28, thereby making the path of the balls 4 in between the chambers 23 and 24 longer, and has such a shape that it makes the descent of the balls 4 slower. Particularly, the section 37 of each groove 35 is sinuous, has an average straight course parallel to the axis 29 and includes

L5 alternating cavities 40,41 having concavities opposite to one another and connected one another by substantially circumferential branches 43 (figure 4) .

The body 34 and the liner 30 are made of a material that is i0 amagnetic, so as not to guide the flux lines of the magnetic field generated by the coil 28, has a hardness exceeding 8, and preferibly at least equal to 9, in Mohs Scale, and is resistant to deformation in a range of temperatures that exceeds heating temperature (approx. 850°) of the balls 4 with :5 which it comes into contact. Particularly, the liner 30 is made of a ceramic material including allumina in a percentage exceeding 99%, and preferably at least equal to 99,7%, the body 34 instead is formed from a solid ceramic material, that has relatively low hardness in order to be mechanically workable (for example a hardness equal to 3 in Mohs Scale) , and is treated by suitable heating steps and coated with a suitable paint for its hardening before use. For example, the body 34 is made of an allumina based material known by its trade name RESCOR 960 (registered trademark) .

According to what is illustrated in figure 3, the body 34 is fitted in a fixed position on a coaxial shaft 45, that is driven in rotation in the direction C by a ratio-motor 46 (schematically illustrated) located outside the head 21 and controlled for rotating the body 34. The rotation velocity of the body 34 is adjustable according to the time range during which the balls 4 have to stay in the variable magnetic field and, therefore, have to be heated.

The shaft 45 extends axially through the chambers 23,24, the head 21 and the ratio-motor 46, is axially hollow and defines, in one of its ends, and entry 47 for a nitrogen flow, that axially crosses the body 34 in an sealed way with respect to the grooves 35 and ends up through an exit 48 in the chamber 24. The closure at the water surface 26 generates a counter pressure in the chamber 24, whereby nitrogen flows in the grooves 35 in a flow direction that is opposite to that of the balls 4 and generates an oxigen free protective atmosphere, thus avoiding steel decarburation.

Nitrogen is fed by a device 49, schematically illustrated and not described in detail, and is heated before entering into the chamber 24, to avoid defects in the material due to localized hardening of the balls 4 in the furnace 6. Particularly, considering that the shaft 45_. is made of metal, the magnetic field heats by induction the section (not illustrated) of shaft 45 inside the body 34, and this internal section then heats the nitrogen.

When the furnace 6 is not crossed by the balls, a control unit 55 acts on the coil 28 power supply for keeping the remaining temperature of the internal section of shaft 45 at about 300°.

In such a way, also the temperature of the body 34 remains around that range, thus avoiding thermal shocks on the material of body 34 in contact with the balls 4 that are then heated.

The door 25 is normally closed to avoid the exit of nitrogen from the furnace 6 and opens automatically when the weight of the balls 4 dropped in the conduit 7 exceed a threshold value: for example, the door 25 is closed by an elastic means 51 that exerts a calibrated and antagonistic bias as compared to the weight of the balls 4 abbuting on the door 25. In alternative, the door 25 is opened/closed by an actuator.

The opening of the door 25 is controlled by a retaining device 52, schematically illustrated and not described in detail, so that it occurs exclusively when the nitrogen pressure in the chamber 23 (detected by a sensor 53) exceeds a threshold value, indicative of the fact that the nitrogen protective atmosphere has already formed inside the furnace.

With reference to figure 4, the balls 4 entered at the opening of the door 25, fall into section 31 and enter in the grooves 35, that are located near the bottom generatrix D of the liner 30, automatically by gravity and by the rotation effect of the slopes 36, forming, in each groove 35, an associated row that is discontinuous. During rotation, the balls 4, when they are near the lateral generatrix E rotated by 90° with respect to the generatrix D in the direction C, stop in the cavities 40, without going up the branches 43 in spite of the inclination A; when they are near a top generatrix F (rotated by another 90°), they fall by gravity towards exit 24 into the relevant grooves 35; when they are near a lateral generatrix G rotated by another 90°, they stop in the cavities 41; finally, when they are again in the bottom position (generatrix D) , they fall by gravity to the exit 24, and so on.

Again with reference to figure 3, during feeding in, the change in the magnetic field, due to feeding frequency of the coil 28, generates the induced electric currents that heat the balls 4. In the chamber 24, the surface temperature of the heated balls 8 is remotely detected, particularly by means of an infrared laser tracking sensor 54 carried by the head 22. The power and/or the frequency of the magnetic field are then adjusted by unit 55 in response to the detected temperature, so as to control in closed-loop the heat-treating and to reach - li ¬

the requested temperature.

According to the alternative embodiment schematically illustrated in figure 5, the furnace 6 includes a plurality of devices 27 housed in a single coil 28 in equally spaced positions around the axis 29. The bodies 34 are driven in rotation in the respective liners 30 about their own axes 56 parallel to axis 29, by just one motor (not illustrated) by means of a transmission 57, for example of the gear tipe.

According to the alternative embodiment schematically illustrated in figure 6 (particularly valid for the heat- treating of rollers) , the device 27 is replaced by a plurality of fixed channels 58 that are equally spaced around the axis 29 and guide the rows of rolling elements 4, preferably upwards or horizontally.

Upstream the channels 58, a loading automatic motorized device 59, is provided which fits a rotating element 4 at a time into each channel 58 and is associated to a one-way device 60 that prevents loosing the rolling elements 4 at the entrance into the channels 58. The device 59, during fitting in operation, develops a biasing function over the rolling elements previously fitted in, so as to convey them "by steps" towards the exit of the channels 58, with a speed or at a pace that is adjusted according to the length of the channels 58 and to the time required for the heating. As discussed above it is evident that the described heat- treating allows to have extremely compact furnaces. Particularly, a high ratio between productivity (expressed in kilos treated daily) and surface occupied by the furnace 5 (expressed in square meters) is obtained: this ratio is about 3 times as much as compared with known solutions that involve large resistance furnaces.

Moreover, the furnaces can be dedicated and made permanent one LO for each line 3a-3d, avoiding the risks of mixing together different types of rolling elements, and avoiding also the use of storage units or buffers at the exit of the lines 3a-3d.

Moreover, the magnetic field does not require starting up L5 time, and can be deactivated whenever there is no actual passing by of rolling elements, consequently saving energy as compared with resistance furnaces.

Moreover, the devices 27 and the channels 58 allow to treat .0 the balls 4 continuously and in line, to keep them together equally spaced about the axis 29, and to convey them along paths that are coaxial to axis 29, thus achieving a better induction effect as well as an easier construction of the furnace 6 for conveying the balls 4. 25

As compared with a free fall by gravity condition, the balls 4 are slowed down by the combined action of the rotation and of the cavities 40,41, so as to remain within the magnetic field for a period of time (for example 12 seconds) which, is enough to carry out the heat-treating completely, but limiting the axial length of the coil 28 and hence of the furnace 6.

Moreover, the quenching in water avoids fire risks and does not require complicated recycling and/or washing apparatuses for the rolling elements after hardening.

It is finally clear that to the heat-treating and to the furnace described changes and variations can be made within the scope of the present invention, as defined in the enclosed claims. For example, the magnetic field could have flow lines oriented in a different way as compared with those generated by the coil 28, but which can nevertheles still induce electric current in the rolling elements during feeding; and/or the treatment could be applied in different types of steel annealing; and/or the grooves 35 and the channels 58 could follow a path that is different from that shown; and/or the inclination A could be different from the one indicated, if necessary it could be at 90°; and/or the descent of the rolling elements 4 by gravity could be slowed down in a different way from that described, for example by adjustable friction.

Claims

1.- Heat-treating of rolling elements for bearings, including ^■ the steps of:

5 - feeding in said rolling elements from an entrance towards an exit, which is different from said entrance;

- heating the rolling elements while displacing them from said entrance towards said exit; characterized in that the heating is carried out by means of O electric currents induced in the rolling elements by changes in a magnetic field during feeding in.

2,- Treatment according to claim 1, characterized in that said magnetic field has an axis, and in that .said rolling .5 elements are fed in, globally, in an equally spaced way around said axis.

3.- Treatment according to claim 1 or 2, characterized in that the rolling elements are guided in a row. >0

4.- Treatment according to any of the previous claims, characterized in that the rolling elements are fed in downwards towards said exit by the effect of gravity.

!5 5.- Treatment according to claim 4, characterized in that the descent by gravity of said rolling elements is slowed down between said entrance and said exit.

6.- Treatment according to claim 5, characterized in that the slowing down is carried out moving a guide passage of said rolling elements.

5 7.- Treatment according to claim 6, characterized in that said magnetic field has an axis, and in that the slowing down is carried out by rotating said guide passage around a direction parallel to said axis.

O 8.- Treatment according to claim 6 o 7, characterized by- stopping the rolling elements in a section of said guide passage for some positions of said guide passage.

9.- Treatment according to claim 3, characterized in that L5 the rolling elements are fed in towards said exit step by step one after the other.

10.- Treatment according to any of the previous claims, characterized in that the rolling elements are contacted by >0 nitrogen during feeding in between said entrance and said exit, and in that nitrogen is pre-heated before contacting the rolling elements.

11.- Treatment according to claim 10, characterized by 15 letting the rolling elements to be treated entry into a chamber defining said entrance when the nitrogen pressure in said chamber exceeds a threshold value.

12.- Treatment according to any of the previous claims, characterized by detecting the surface temperature of the rolling elements in said exit.

13.- Treatment according to claim 12, characterized in that said magnetic field is generated by an inductor, and in that the power and/or electric supply frequency of said inductor is changed as a function of the detected surface temperature.

14.- Hardening of rolling elements for bearings, including: a heat-treating carried out according to any of the previous claims, and a following quenching; characterized in that said quenching is carried out in water.

15,- Hardening and tempering of rolling elements for bearings, including a hardening according to claim 14, and a following tempering carried out according to any one of the claims from 1 to 13.

16.- Furnace for implementing heat-treating of rolling elements for bearings according to any of the claims from 1 to 13; the furnace including: conveying means for feeding in said rolling elements from an entrance to an exit, different from said entrance; - heating means for heating up the rolling elements during movement from said entrance to said exit; characterized in that said heating means include magnetic field generator means; and said conveying means include guide means for guiding said rolling elements into said magnetic field, so as to achieve an induction heating.

17.- Furnace according to claim 16, characterized in that said guide means are shaped and/ore sized so as to guide the rolling elements in a row.

18.- Furnace according to claim 16 o 17, characterized in that said magnetic field generator means include an inductor having an axis, and in that said guide means include guide passages equally spaced around said axis.

19.- Furnace according to claim 18, characterized in that said guide passages have a extension that is longer than the axial lenght of said inductor.

20.- Furnace according to claim 19, characterized in that said guide passages have been made in a casing made of amagnetic material.

21.- Furnace according to claim 20, characterized in that i said casing is formed from a ceramic material that is workable and that is subjected to heat treatment and painted to be hardened before it is installed in the furnace.

22.- Furnace according to any of the claims from 16 to 21, characterized in that said entrance is located higher up than said exit.

23.- Furnace according to claim 22, characterized in that said conveying means include slowing down means to slow down the descent by gravity of the rolling elements from said entrance to said exit.

24,- Furnace according to claim 23, characterized in that said slowing down means include means for moving said guide means .

25.- Furnace according to claim 24, characterized in that said guide means include at least a guide rotating passage, and in that said slowing down means include at least a section of said guide passage and rotating means of said guide passage.

26.- Furnace according to claim 25, characterized in that said guide passage includes, one after the other, cavities having concavities opposite one another.

27,- Furnace according to claim 25 o 26, characterized in i that said guide passage is defined, at least in part, by a groove on a lateral external surface of a cylindrical body.

28.- Furnace according to any of the claims from 16 to 27, characterized in that it includes channelling means for generating a nitrogen protective atmosphere aroung the rolling elements, and pre-heating means for said nitrogen.

29,- Furnace according to claim 28, characterized in that said channelling means include a section that extends along an axis of said magnetic field and is sealed from said rolling elements.

30.- Furnace according to claim 28 or 29, characterized in that said entrance is defined by a chamber provided with a mobile door that opens to let in the rolling elements; first retention means being provided to keep said door closed when nitrogen pressure in said chamber is below a threshold value.

31.- Furnace according to claim 30, characterized in that it includes second retention means for keeping said door closed when the weight of the rolling elements abutting on said door is below a thershold value.

32.- Furnace according to any of the claims from 16 to 31, characterized in that it includes sensor means to detect the surface temperature of the rolling elements treated in said exit.

33.- Furnace according to claim 32, characterized in that said sensor means include infrared laser tracking means.

34.- Furnace according to claim 32 o 33, characterized in that it includes adjustment means acting on said magnetic field generator means in response to the detected surface temperature to change the power and/or frequency of said magnetic field.

35.- Production plant of rolling elements for bearings including a plurality of moulding lines in parallel one another and, downstream each line, an associated dedicated furnace implemented according to any of the claims from 16 a 34.