Rolling bearing cage for a dental technology bearing and method for manufacturing such a rolling bearing cage

Abstract

Rolling bearing cage (3) for a dental technology bearing with a speed characteristic nx dm ≥ 500,000 mm / min, where n is the speed in revolutions per minute and dm is the mean bearing diameter in millimeters, wherein the rolling bearing cage (3) comprises or consists of polyimide with embedded graphite; characterized by the fact that the graphite is spherical natural graphite; the graphite is present in a grain size distribution of 50% < 20 µm; the graphite has a grain size of no more than 50 µm; and the proportion of graphite in the rolling bearing cage (3) is 5 to 20 wt.%.

Description

[0001] The present invention relates to a rolling bearing cage for a dental technology bearing, that is, a cage for a rolling bearing in a medical device that operates at very high speeds, for example in a dental turbine or other dental hand instrument, and to a method for manufacturing a corresponding rolling bearing cage, in detail according to the preamble of the independent claims.

[0002] In dental hand instruments, for example a dental turbine, as specifically related to the present invention, high-speed rolling bearings are used which have a speed rating of at least 500,000 mm / min. Such a speed rating is obtained from the product of the maximum rotational speed per minute and the mean bearing diameter in millimeters, wherein the mean bearing diameter is generally defined by the so-called sphere center circle, that is, the diameter of the circle on which the centers of the rolling elements, or in the case of spheres, the sphere centers, lie.

[0003] High-speed bearings like these require measures to reduce cage wear, i.e., wear on the bearing cage that holds the rolling elements. Traditionally, such bearing cages are lubricated with oil, which can be added during the cage manufacturing process, for example, by impregnating the cage material. However, these bearings typically experience significant oil loss, necessitating relubrication. In practice, this oil loss also means that the oil can leak out during dental treatment, thus being unintentionally introduced into the patient's system. It is therefore understandable that the amount and type of oil used is limited.

[0004] DE 10 2006 030 836 B4 discloses numerous different measures for reducing cage wear in a rolling bearing used in dental technology. For example, fabric-reinforced phenolic resin, which can be impregnated with oil, is proposed as a material for the cage. Such fabric reinforcements are generally made of a fine cotton fabric, which is usually only thermally stable up to about 120°C. This is insufficient if sterilization at higher temperatures is required.

[0005] Another proposed measure involves the use of high-performance plastics, such as polyetheretherketone (PEEK), polyetherketone (PEK), polyamide-imide (PAI), and polyimide (PI), to which lubricating additives and fibers are added. These polyimide materials are produced using sintering technology, whereby the lubricating additives are typically incorporated into the material without forming a chemical bond.

[0006] Furthermore, polyamide and perfluoroalkyl substances and mixtures of these compounds with other polymer substances are described, as well as PAI with chemically coupled fluoropolymer, fluorocopolymer and fluorooligomer, for example polytetrafluoroethylene (PTFE).

[0007] According to DE 10 2006 030 836 B4, it is further proposed to add additional additives, fillers, and auxiliary materials to PAI with a chemically coupled fluoro copolymer or fluoropolymer. These additives are preferably distributed homogeneously within the compound material for the rolling bearing cage or applied only to its surface. For example, carbon fibers and / or graphite can be added to the compound material for the rolling bearing cage.

[0008] A disadvantage of such sliding additives is that they can reduce the mechanical strength of the rolling bearing cage, which can lead to fractures under mechanical stress during operation. The proportion of sliding additives is therefore limited to a relatively small amount, or, as described in DE 10 2006 030 836 B4, special materials, in this case PAI with chemically coupled fluoropolymer or fluorocopolymer, must be used, with the polymer content being 5 to 60 wt.%. However, in practice, the incorporation of graphite into a PAI material leads to reduced fracture toughness despite the aforementioned measures. Furthermore, the service life of the rolling bearing cage is limited by its exposure to high temperatures in saturated steam during steam sterilization.

[0009] US 6,113,278 A describes sintered polyimide as a material for a generic rolling bearing cage. Here, the lubricating properties are improved by impregnation with lubricating oil.

[0010] DE 198 48 051 B4 discloses a rolling bearing for devices rotating at high speed made of a compression-molded PEI with subsequent sintering at 400° C.

[0011] DE 103 51 346 A1 proposes epoxy resin with fibers for the cage of a corresponding ball bearing.

[0012] EP 2 886 583 A1 discloses a polyimide powder with high thermo-oxidative resistance and a molded body produced from such a powder. The molded body can be produced by direct forming or hot-compression molding and can be used, for example, as a plastic molded body for grippers in the glass industry, or as bearings or roller bushings, seals and sealing rings, guides, valve seats, shut-off valves, brake pads, valves in turbochargers or compressors, bearing components such as cages or balls, piston rings, etc. The molded body is produced using a raw powder comprising a polyimide powder whose polyimide polymer is composed of at least two components, with the further addition of additives to form a compound.All additives that can withstand the hot compression molding or direct forming process without damage are suitable, for example, polytetrafluoroethylene, graphite, molybdenum disulfide, boron nitride, metal oxides, etc. The amount of additive is virtually unlimited and can range from 0.1 wt.% to 90 wt.%. The ideal particle size can be determined using a laser diffraction particle size analyzer and is in the range of 15 to 80 µm. An ideal processing temperature range of 300°C to 500°C is specified.

[0013] US 2014 / 0 226 928 A1 discloses a rolling bearing cage with the features summarized in the preamble of claim 1. The rolling bearing cage can be made of a polymer material, wherein polymer materials can be mixed with additives, including graphite. The polymer material itself can, for example, comprise polyimide.

[0014] DE 196 06 249 A1 discloses a crown cage for a ball bearing and a dental handpiece. The crown cage is manufactured by injection molding from thermoplastic resin, for example polyimide resin with 10 to 20 wt% graphite.

[0015] A product data sheet for natural graphite GNP from Suter Kunststoffe AG ​​(version 2.0 / 2016-04-14) describes natural graphite and its possible applications.

[0016] Due to increased sterilization requirements in the dental field and increased demands on service life, there is a need for a roller bearing cage that is improved compared to the aforementioned designs and that advantageously has high mechanical strength and at the same time high temperature resistance and suitability for regular sterilization even at temperatures of 120°.

[0017] The present invention is therefore based on the objective of providing a rolling bearing cage for a dental technology bearing with a speed characteristic nx dm ≥ 500,000 mm / min, where n is the speed per minute and dm is the mean bearing diameter in millimeters, which has a particularly long service life even with frequent sterilizations under high temperatures.

[0018] The problem according to the invention is solved by a rolling bearing cage having the features of claim 1 and by a method for manufacturing such a rolling bearing cage comprising the steps of claim 11. Advantageous and particularly expedient embodiments of the invention are specified in the dependent claims.

[0019] A rolling bearing cage according to the invention for a dental technology bearing, for example for a dental turbine, with a speed characteristic of nx dm ≥ 500,000 mm / min, where n is the speed and dm is the mean bearing diameter, comprises polyimide with embedded graphite or consists of polyimide with embedded graphite. "Consists of" means that the rolling bearing cage is made exclusively of polyimide and graphite or at least no other component is provided in a quantity exceeding 1% by weight, wherein the sum of all other components is limited to a maximum of 5% by weight.

[0020] According to the invention, the graphite incorporated into the polyimide is spherical natural graphite.

[0021] In particular, the shape of graphite is described as spherical if two ellipsoids are determined for a corresponding piece of graphite, namely a smallest triaxial ellipsoid and a largest triaxial ellipsoid.

[0022] The smallest triaxial ellipsoid intersects the surface of the graphite part at points of its smallest diameter. The largest ellipsoid intersects the surface of the graphite part at points of its largest diameter. The graphite part is considered spherical, in particular, if the semi-axes of the smallest ellipsoid differ in length by no more than 20%, and the semi-axes of the largest ellipsoid differ in length by no more than 20%, provided that the centers of the ellipsoids are no more than 25% apart in the length of the largest semi-axes of the largest ellipsoid, and the lengths of the corresponding semi-axes of the smallest and largest ellipsoids differ by no more than 25% of the longer of the respective longer semi-axes. If the largest (outer) ellipsoid is more than 50% larger than the smallest (inner) ellipsoid, the shape is considered plate-like.

[0023] The graphite is preferably in a highly pure form, with a purity of at least 95% according to DIN 51903.

[0024] According to the invention, the graphite is present in a grain size distribution of 50% < 20 µm. To determine this, the percentage of grains by weight can be plotted against the classified equivalent diameter on the abscissa. 50% of the grains therefore have an equivalent diameter of less than 20 µm. The equivalent diameter can be determined, in particular, by sieving with a mesh size of 20 µm.

[0025] The graphite has a grain size of no more than 50 µm. This prevents a significant loss of fracture toughness.

[0026] The invention provides for a graphite content in the rolling bearing cage of 5 to 20 wt.%, based on the total weight of the rolling bearing cage. However, the graphite content in the rolling bearing cage is particularly preferably more than 12 wt.%, especially between 12.5 and 20 wt.%, for example between 14 and 20 wt.%.

[0027] Particularly good results regarding mechanical strength and service life can be achieved when a semi-finished product for the rolling bearing cage, especially a round bar, is manufactured using hot compression molding (HCM). In this hot compression molding process, the material, which is in powder form, is inserted into the mold under simultaneous application of defined pressure and temperature. Alternatively, a cage or ring can be manufactured from the components polyimide and graphite using direct forming (DF) under defined pressure, followed by sintering.

[0028] The graphite is advantageously distributed evenly throughout the polyimide, that is, throughout the entire rolling bearing cage. For this purpose, polyimide powder and graphite powder can first be mixed uniformly before being subjected to temperature and pressure.

[0029] The HCM process for polyimide is particularly cost-effective when graphite has already been incorporated.

[0030] The rolling bearing cage according to the invention, or the material from which the rolling bearing cage is manufactured, advantageously has an elongation at break of 3.5 to 5.5% according to EN ISO 527 and / or a bending elongation of 3.0 to 6.0% according to EN ISO 178.

[0031] By using polyimide (PI), especially a comparatively more hydrolysis-stable polyimide instead of polyamide-imide (PAI), significantly greater mechanical strength, particularly maximum tensile strength and breaking strength, can be achieved, especially with an increasing number of sterilization cycles. The aforementioned proportion of 5 to 20 wt% graphite represents an optimum balance between mechanical strength, particularly breaking strength, and lubrication properties.

[0032] The service life of the rolling bearing cage can also be significantly increased by using PI instead of PAI according to the invention. Service life tests have shown an increase of more than 20%.

[0033] A method according to the invention for manufacturing a rolling bearing cage provides a polyimide powder and a graphite powder, wherein the powders – the polyimide powder and the graphite powder – are homogeneously mixed together so that the graphite powder is uniformly distributed in the polyimide powder. Alternatively, the graphite can be incorporated into the powder by polymerization. Subsequently, a semi-finished product for manufacturing rolling bearing cages is produced by shaping and heating the powder mixture, in particular by the hot pressing process (HCM) with simultaneous defined pressure application and heating.

[0034] During temperature (heating) and pressure application, the powder or powder mixture is preferably subjected to a pressure of 100 bar to 1000 bar and simultaneously to a temperature above the glass transition temperature of polyimide. These conditions are preferably maintained until sintering is complete.

[0035] The powder is advantageously mixed in a weight ratio of 5 to 20 wt% graphite powder and 95 to 80 wt% polyimide powder. A graphite powder content of more than 12 wt% is particularly beneficial.

[0036] The maximum grain size of the graphite in the graphite powder can be achieved, for example, by sieving with a sieve with a mesh size of 20 µm to 100 µm, in particular 40 µm to 80 µm.

[0037] The invention will below be described by way of example using an embodiment and the figures.

[0038] They show: Fig. 1 an axial section through a rolling bearing with spherical rolling elements and a rolling bearing cage according to the invention; Fig. 2 an axial section through a dental turbine with a rolling bearing cage according to the invention; Fig. 3 A schematic representation to illustrate the method according to the invention.

[0039] The one in Fig. Figure 1 of the depicted rolling bearing – ball bearing – has an outer ring 1 and an inner ring 2 radially opposite it on the inside. Rolling elements 6, here in the form of balls, are positioned between the outer ring 1 and the inner ring 2, and are held in their position relative to each other by the rolling bearing cage 3.

[0040] The rolling bearing is closed off by a cover plate 4, which is held in the outer ring 1 by means of a snap ring 5.

[0041] According to the invention, the rolling bearing cage 3 consists of polyimide with embedded graphite. The embedded graphite reduces wear between the rolling bearing cage 3 and the rolling elements 6, as well as, optionally, wear during relative rotation between the rolling bearing cage 3 and the outer ring 1 and / or the inner ring 2.

[0042] In the Fig. Figure 2 shows the head section of a dental turbine, with an arrangement of components corresponding to the head housing already disclosed in DE 102 59 003 A1. The head section comprises a housing 7 in which the rotor shaft 9 of the rotor 8 is mounted. The rotor 8 carries a corresponding tool 10.

[0043] The bearing arrangement of the rotor shaft 9 comprises rolling bearings 11 and 12 with rolling bearing cages 3 according to the invention.

[0044] In the Fig.Figure 3 schematically shows, in detail a to d, a method according to the invention for manufacturing a rolling bearing cage 3 according to the invention. According to detail a, polyimide powder 14 is homogeneously mixed with graphite powder 13 by means of a mixer 16. According to detail b, the powder mixture 18 is introduced into a lower mold half 15, which can be heated by one or more heating coils 19.

[0045] According to detail c, an upper mold half 17 is placed on the lower mold half 15 to form a mold cavity 21 containing the enclosed powder mixture 18. Pressure and temperature are then applied to the powder mixture 18 to carry out the HCM process.

[0046] According to detail d, the rolling bearing cage 3 is ejected from the mold cavity 21, for example by means of a ram 20, after the upper mold half 17 has been lifted from the lower mold half 15. Typically, the rolling bearing cage 3 is machined, for example by cutting.

[0047] By using a particularly suitable pl polymer according to the invention, with significantly increased hydrolysis resistance compared to known bearing materials while simultaneously meeting the high mechanical and thermal requirements, a dental technology bearing with the aforementioned high speed characteristic can be provided, which also has a very long service life even with frequent sterilizations at high temperatures. Reference symbol list 1 outer ring 2 inner ring 3 roller bearing cages 4 Cover plate 5 snap rings 6 rolling elements 7 cases 8 Rotor 9 Rotor shaft 10 tools 11 rolling bearings 12 rolling bearings 13 Graphite powder 14 polyimide powders 15 lower half of the mold 16 mixers 17 upper half of the mold 18 Powder Mixture 19 heating coil 20 pestles 21 Mold cavity

Claims

[1] Rolling bearing cage (3) for a dental technology bearing with a speed characteristic nx dm ≥ 500,000 mm / min, where n is the speed in revolutions per minute and dm is the mean bearing diameter in millimeters, wherein the rolling bearing cage (3) comprises or consists of polyimide with embedded graphite; characterized by , that the graphite is spherical natural graphite; the graphite is present in a grain size distribution of 50% < 20 µm; the graphite has a grain size of no more than 50 µm; and the proportion of graphite in the rolling bearing cage (3) is 5 to 20 wt.%. [2] Rolling bearing cage (3) according to claim 1, characterized by that the graphite is of high purity, with a purity of at least 95% according to DIN 51903. [3] Rolling bearing cage (3) according to one of claims 1 or 2, characterized by , that the proportion of graphite in the rolling bearing cage (3) is more than 12 wt.%. [4] Rolling bearing cage (3) according to any one of claims 1 to 3, characterized by , that the rolling bearing cage (3) is made from a semi-finished product produced by hot pressing. [5] Rolling bearing cage (3) according to any one of claims 1 to 4, characterized by , that the proportion of graphite in the rolling bearing cage (3) is at least 7 wt.% [6] Method for manufacturing a rolling bearing cage (3) according to any one of claims 1 to 5, comprising the following steps: 6.1 Providing a polyimide powder (14); 6.2 Providing a graphite powder (13); 6.3 homogeneous mixing of the polyimide powder (14) and the graphite powder (13); 6.4 Forming and heating the powder mixture (18) of polyimide powder (14) and graphite powder (13) to produce a semi-finished product; 6.5 Manufacturing the rolling bearing cage (3) from the semi-finished product. [7] Method according to claim 6, characterized bythat the forming and heating takes place using the direct forming process. [8] Method according to claim 6, characterized by that the forming and heating takes place using the hot pressing process, especially with simultaneous heating and pressure application. [9] Method according to one of claims 6 or 8, characterized by , that the powder mixture (18) is subjected to a pressure of 100 bar to 1000 bar and simultaneously to a temperature above the glass transition point of polyimide until sintering is completed. [10] Method according to any one of claims 6 to 9, characterized by , that the powders are mixed in a weight ratio of 5 to 20% graphite powder (13), in particular 7 to 20% graphite powder (13), and 95 to 80% polyimide powder (14).

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