Plastic nut for a ball screw spindle and method for manufacturing one

Abstract

Plastic nut (20) for a ball screw spindle (1) which defines a threaded bore (24) which has a spindle thread groove (22) and is formed by injection molding of fiber-reinforced plastic which contains matrix plastic (M) and reinforcing fibers (F) mixed therein, wherein: the spindle thread groove (22) is ground until the fibers (F) are exposed on a surface of the spindle thread groove (22).

Description

Technical field

[0001] The present invention relates to a plastic nut for a ball screw spindle and a method for manufacturing such a nut, and in particular to a plastic nut for a ball screw spindle manufactured by injection molding of fiber-reinforced plastic formed by mixing a fiber-based reinforcement material in matrix plastic, and to a method for manufacturing such a nut. Background of the invention

[0002] It is known to use a plastic nut for use in a feed spindle mechanism, which is produced by injection molding of fiber-reinforced plastic formed by mixing hair crystals (whiskers) into a matrix plastic consisting of PPS (polyphenylene sulfide resin). See, for example, patent document 1. Such a nut for use in a feed spindle mechanism can be produced either by milling the screw thread into it or by injection molding the screw thread into it. State of the art document(s) Patent document(s)

[0003] Patent document 1: JP 2001 - 116 102 A

[0004] Further publication: “Menges, G.; Geisbüsch, P.: The glass fiber orientation and its influence on the mechanical properties of thermoplastic injection-molded parts – An estimation method*). In: Colloid & Polymer Science, 260, 1982, pp. 73–81. – ISSN 1435-1536 (web) Summary of the invention; Problem to be solved by the invention

[0005] A feed spindle nut made of fiber-reinforced plastic is more durable than one made of ordinary plastic. However, when milling a screw thread, the presence of reinforcing fibers in the material causes a standard cutting tool to wear out relatively quickly. Therefore, a more expensive cutting tool, such as a diamond-coated tool, is required, resulting in higher manufacturing costs. Furthermore, since fiber-reinforced plastic has a higher coefficient of thermal expansion than steel, the milling speed must be controlled to prevent deformation from heat generated during the cutting process, which further contributes to the increased manufacturing costs.

[0006] A feed spindle nut made of fiber-reinforced plastic with a screw thread formed during an injection molding process can be manufactured more efficiently than a feed spindle nut with a screw thread formed by milling, and can therefore be produced at lower production costs. However, a thread produced by an injection molding process tends to wear relatively quickly when used in a feed spindle mechanism, and positional inaccuracies can arise at a relatively early stage of use due to the reduction in the thread's dimensional accuracy. Therefore, a feed spindle nut manufactured with a thread formed by an injection molding process may not be able to achieve the high dimensional accuracy that might be required.

[0007] The inventor of the present invention has carried out an extensive experimental study on the cause of early wear of the nut made of fiber-reinforced plastic with a screw thread formed by injection molding and has discovered a certain fact.

[0008] When fiber-reinforced plastic is injection molded, an outer skin layer, a few micrometers thick and free of fibers, forms on the outermost (surface) layer of the threaded groove. This outer skin layer consists only of the matrix plastic and has lower abrasion resistance than a material containing fibers. Therefore, it was concluded that the presence of this outer skin layer on the outermost layer of the threaded groove is a major cause of premature wear in nuts made of fiber-reinforced plastic.

[0009] When fiber-reinforced plastic is injection molded, an alignment layer is created inward of the outer skin layer. This layer contains a large number of fibers aligned along the plane of the outer skin layer. When the outer skin layer is worn away, the alignment layer is exposed on the surface of the threaded groove. Because a large proportion of the fibers in the alignment layer exposed to the threaded groove surface lie along the surface of the threaded groove, the fibers are not completely buried in the matrix plastic and are therefore relatively weakly bonded to the matrix plastic for the fibers.It was observed that the alignment layer exhibits greater abrasion resistance than the outer skin layer; however, the presence of the alignment layer causes the fibers to detach over time, and the resulting wear in the matrix plastic leads to a reduction in the dimensional accuracy of the threaded groove. Based on this finding, it was concluded that the presence of an alignment layer from the start of use is undesirable when particularly high dimensional accuracy is required. Furthermore, in ball screw applications, it was observed that the fiber fragments that detach from the matrix plastic can impede the rolling of the balls in the threaded groove, thus potentially impairing the function of the ball screw.

[0010] In view of the results of the investigations carried out by the inventor, a primary objective of the present invention is to provide a nut for a ball screw spindle, which is made of fiber-reinforced plastic and has a screw thread produced by injection molding, which can reduce early wear of the nut and is able to maintain high dimensional accuracy over a long period of time. Means of solving the problem

[0011] The present invention provides a plastic nut for a ball screw spindle, which defines a threaded bore (24) having a screw thread groove (22) and is formed by injection molding of fiber-reinforced plastic containing matrix plastic (M) and reinforcing fibers F mixed therein, wherein: the screw thread groove (22) is ground until the fibers (F) are exposed on a surface of the screw thread groove (22).

[0012] This arrangement prevents premature wear, as the fibers (F) on the surface of the thread groove (22) are exposed from the start, and high dimensional accuracy can be maintained over an extended period of time.

[0013] Preferably, the plastic nut for a ball screw spindle of the present invention can further comprise an alignment layer (II) containing fibers (F) extending along a surface of the threaded groove (22), wherein the threaded groove (22) is ground until the alignment layer (II) is exposed.

[0014] This arrangement prevents premature wear, as the threaded groove is ground until the alignment layer (II) containing fibers (F) is reached, and high dimensional accuracy can be maintained over an extended period of time.

[0015] Preferably, the plastic nut for a ball screw spindle of the present invention can further comprise a core layer (III) containing fibers (F) that are not aligned, wherein the thread groove (22) is ground until the core layer (III) is exposed.

[0016] With this arrangement, early wear can be avoided even further, as the thread groove is ground until the core layer (III) containing non-aligned fibers (F) is reached, and high dimensional accuracy can be maintained over an extended period of time.

[0017] The present invention further provides a method for manufacturing a plastic nut for a ball screw spindle, which contains matrix plastic (M) and reinforcing fibers (F) mixed therein, comprising: a first step in which a nut (20) is injection molded which defines a threaded bore (24) having a pre-finishing screw thread groove which is configured and dimensioned using a metal casting kit according to pre-finishing specifications; and a second step in which the screw thread groove (22) is finished according to final specifications by grinding the pre-finishing screw thread groove until the fibers (F) are exposed to a surface of the screw thread groove.

[0018] This arrangement can prevent premature wear, as the pre-finishing screw thread groove is ground until the fibers (F) on the surface of the screw thread groove (22) are exposed, and high dimensional accuracy can be maintained over an extended period of time.

[0019] Preferably, in this method for manufacturing a plastic nut for a ball screw, immediately after the first step the nut is configured to have an outer skin layer (I) which defines an outermost layer and is free of fibers (F), an alignment layer (II) which is located further inward than the outer skin layer (I) and contains fibers (F) which are aligned along a surface of the outer skin layer, and a core layer (III) which is located further inward than the alignment layer (II) and contains unaligned fibers (F); and wherein the second step comprises a grinding step to remove at least the outer skin layer (I).

[0020] With this arrangement, the fibers (F) lie freely on the surface of the threaded groove (22) from the beginning of use, since the outer skin layer (I) is removed in the second step, so that early wear can be avoided and high dimensional accuracy can be maintained over a longer period of time.

[0021] Preferably, in this process for manufacturing a plastic nut for a ball screw spindle, the second step includes a grinding step to remove the alignment layer (II).

[0022] With this arrangement, the reduction in abrasion resistance due to the loosening of fibers (F) in the alignment layer (II) can be avoided, since the alignment layer (II), which contains a large number of aligned fibers, is essentially not present from the start of use.

[0023] Preferably, the second step in this method for manufacturing a plastic nut for a ball screw spindle comprises a grinding step which removes a quantity (G) of material suitable to expose the core layer (III) on the surface of the thread groove.

[0024] According to this arrangement, early wear can be avoided even further, since the core layer (III) on the surface of the thread groove (22) is exposed from the beginning of use, and high dimensional accuracy can be maintained over a longer period of time. Effect of the invention

[0025] According to the plastic nut for a ball screw spindle and the method for manufacturing it according to the present invention, premature wear can be avoided because the screw thread groove is ground until the fibers in the matrix plastic on the surface of the screw thread groove are exposed, and high dimensional accuracy can be maintained over an extended period of time. Brief description of the drawings Fig. Figure 1 is a perspective view showing a nut for a ball screw spindle according to the present invention; Fig. Figure 2 is a sectional view of the ball screw spindle; Fig. Figure 3A is a diagram showing the internal structure of the thread groove area of ​​the plastic nut for a ball screw spindle before the grinding process in the illustrated embodiment, and Fig. Figure 3B is a diagram showing the internal structure of the thread groove area of ​​the plastic nut for a ball screw spindle after the grinding process in the illustrated embodiment; Fig. Figure 4A is a microscopic photograph of the surface of the thread groove area of ​​the plastic nut for a ball screw spindle before the grinding step in the illustrated embodiment. Fig. Figure 4B is a microscopic photograph of the surface of the thread groove area of ​​the plastic nut for a ball screw spindle in the illustrated embodiment, after the outer skin layer has been removed, and Fig. 4C is a microscopic photograph of the surface of the thread groove area of ​​the plastic nut for a ball screw spindle after the alignment layer is removed, in the embodiment shown; Fig. Figure 5 is a sectional view of a ball bearing; and Fig. Figure 6 is a sectional view of a sliding bearing bushing. Description of the preferred embodiment(s)

[0026] An embodiment of the ball screw spindle in which the plastic nut is used is described below with reference to drawings 1 to 4.

[0027] As in Fig. 1 and Fig. As shown in Figure 2, a ball screw 1 has a spindle shaft 10 with a male spindle shaft groove 12 (hereinafter referred to as the male thread groove 12) formed on its outer cylindrical surface, a nut (plastic nut for a ball screw) 20 defining a threaded bore 24 which has a female screw thread groove 22 (hereinafter referred to as the female thread groove 22) formed on its inner cylindrical surface, and a pair of end deflectors 30A and 30B attached to the two longitudinal ends of the nut 20. The spindle shaft 10 is made of rolled stainless steel, the nut 20 is made of injection-molded fiber-reinforced plastic, and the end deflectors 30A and 30B are made of injection-molded high-polymer plastic such as POM (polyacetal resin) and PPS (polyphenylene sulfide resin).

[0028] Each of the end deflectors 30A, 30B is fitted into a recess 28 formed in the corresponding longitudinal end of the nut 20 and is securely fastened therein by means of an end deflector fastening element 40. Each end deflector fastening element 40 consists of a coil of spring steel wire with slightly less than one turn and is engaged jointly by the male threaded groove 12, the female threaded groove 22 and an engagement groove 34 formed in the corresponding end deflector 30A, 30B in smooth continuation with the female threaded groove 22, so that the end deflector 30 is held firmly on the nut 20.

[0029] The spindle shaft 10 is guided longitudinally through the threaded bore 24, and a ball track 14 is defined between the spindle shaft 10 and the threaded bore 24. The male threaded groove 12 and the female threaded groove 22 are each provided with a Gothic arch configuration, combining a pair of identical arches in a symmetrical pattern in the illustrated embodiment. Several balls (steel balls) 16, typically made of stainless steel, are loaded into the ball track 14 and a pair of ball return tracks 26 and 32, formed in the nut 20 and the end deflectors 30 respectively, for free rolling motion.

[0030] The ball return track 26 formed in the nut 20 extends longitudinally parallel to the threaded bore 24 on a radial side of the threaded bore 24 and communicates at each longitudinal end with the ball return track 32, which is formed in a hook shape in the corresponding end deflector 30. As shown in Fig. As can be seen in Figure 2, when, for example, the balls 16 move from left to right in the ball return track 32, one of the end deflectors 30A picks up the balls 16 from the ball roller track 14 into the corresponding ball return track 32, while the other end deflector 30B returns the balls 16 from the corresponding ball return track 32 to the ball roller track 14. As a result, when the spindle shaft 10 is screwed into the nut 20 via the balls 16, the rotation of the threaded shaft 10 is converted into the linear longitudinal movement of the nut 20, while the balls 16 circulate in the nut 20.

[0031] The material and manufacturing process of the nut 20 and the surface finish of the female threaded groove 22 are discussed below.

[0032] The matrix plastic for the fiber-reinforced plastic from which the nut 20 is manufactured can consist of a super-construction plastic such as PPS (polyphenylene sulfide resin). The fibers that can be used for the reinforcing fibers can be short fibers with a diameter of 0.1 µm to 5.0 µm and a length of 1 µm to 100 µm. The fibers can be glass fibers, carbon or graphite fibers, polyamide fibers, boron fibers, aramid fibers, silicon carbide whiskers, silicon nitride whiskers, zinc oxide whiskers, titanium boride whiskers, metal fibers, silicon fibers, and titanium oxide fibers.

[0033] The manufacturing process of the nut 20 can include a first step in which the entire nut 20 is injection molded in a mold using fiber-reinforced plastic containing matrix plastic and embedded fibers as the injection molding material. This allows the nut 20 to be provided with a pre-finishing thread groove configured and dimensioned according to specified pre-finishing parameters, which can later be finished as the final female thread groove 22. The manufacturing process can further include a second step in which the pre-finishing thread groove is finished according to the specifications of the final female thread groove 22 using a screw grinding machine until the fibers of the fiber-reinforced plastic are exposed on the surface of the thread groove after the injection molding (curing) process.

[0034] The pre-finishing thread groove produced by the injection molding process is provided with the same chamfer and thread pitch as the final female thread groove 22 and the same configuration as the final female thread groove 22 or the configuration of the Gothic arch, but is one parting quantity less deep than the final female thread groove 22 G (see Fig. 3A), where the amount of material removed, G, is to be ground off in the second step. In other words, the pre-finishing thread groove has a smaller cross-sectional area than the final female thread groove 22, corresponding to the amount of material removed, G, of material to be ground off.

[0035] Since the final female thread groove 22 and the pre-finishing thread groove have the same configuration, the amount of material G to be ground away is uniform across the entire thread groove. Therefore, the entire surface of the thread groove is ground in the first grinding pass, and the amount of material removed is considerable. This allows the grinding operation to be performed efficiently. The pre-finishing thread groove can be cast with a high degree of precision, which should be sufficient to allow the thread groove to be used as is, as a normal thread groove with an acceptably high degree of precision in terms of configuration and dimensions. Consequently, the grinding operation in the finishing process can be performed with a minimal amount of material removal in a short time.

[0036] Fig. Figure 3A schematically shows the internal structure of the thread groove area of ​​the nut 20 (nut material) immediately before the first step of the grinding work and Fig. Figure 3B schematically shows the internal structure of the thread groove area of ​​nut 20 after completion of the grinding operation. In drawings 3A and 3B, the letter M denotes matrix plastic, the letter F denotes fibers, and the letter B denotes voids. Fig. 3A designates the reference numeral 23 as the pre-finishing thread groove.

[0037] After the injection molding process and before grinding, the fibers F, as in Fig. As can be seen in Figure 3A, the outermost layer of the mother 20 is not present. More precisely, the mother 20 has an outer skin layer I, consisting exclusively of matrix plastic M; an alignment layer II, located further inward than the outer skin layer I and containing fibers that are largely aligned along the surface of the outer skin layer I; and a core layer III, located further inward than the alignment layer II and containing fibers that are largely unaligned. In reality, there may not be clear boundaries between the different layers, and the outer skin layer I may gradually transition into the alignment layer II, and the alignment layer II may gradually transition into the core layer III.

[0038] In the second step, the female pre-finishing thread groove 23 (which is to be completed as the female thread groove 22) is ground so that at least the outer skin layer I is removed until the fibers F in the matrix plastic M on the surface of the female thread groove 22 are exposed. Preferably, the alignment layer II is removed by a specific amount G until the core layer III on the surface of the female thread groove 22 is exposed by this grinding process. This grinding process causes the female thread groove 22 to assume the prescribed final depth. Since the outer skin layer II is approximately 1 µm to 2 µm thick and the alignment layer II is approximately 50 µm to 100 µm thick, the amount G removed can be 0.2 mm or less.

[0039] Preferably, the removal quantity G is chosen as a minimum quantity required to expose the core layer III on the surface of the female thread groove 22, so that the voids B do not have to be exposed on the surface of the female thread groove 22 and the time required for the grinding operation can be minimized.

[0040] The outer skin layer I and the alignment layer II are thus removed, and the core layer II is ground down, but only to the extent that the voids B, which may exist in the core layer III, are not exposed on the surface of the female thread groove 22. This is important for the female thread groove 22 so that it can achieve a high degree of abrasion resistance.

[0041] By performing the grinding work until the core layer III is reached beyond the alignment layer II, longitudinal end sections are removed from those fibers that are located in the part of the material of the nut 20 which is to be removed (corresponding to the removal quantity G) and that are aligned in the depth direction (or from the fibers F that are located next to the alignment layer II), so that the end surfaces of such fibers are exposed in alignment on the surface of the female thread groove 22.

[0042] Drawings 4A to 4C are microscopic photographs showing the threaded spindle area of ​​nut 20. Fig. Figure 4A shows the surface of the threaded groove before grinding. Fig. Figure 4B shows the surface of the threaded groove after the outer skin layer has been removed, and Fig. Figure 4C shows the surface of the threaded groove after the alignment layer has been removed.

[0043] In Fig. 4A No fibers are observed on the surface of the thread groove. Fig. 4B shows that the alignment layer II is exposed on the surface of the threaded groove, and the many white lines visible on the threaded spindle surface indicate the presence of fibers F1 extending along the surface of the threaded groove. Fig. 4C the core layer III is exposed on the surface of the thread groove and the presence of fibers F1 extending along the surface of the thread groove is barely visible, but the ground end surfaces of fibers F2 exposed on the surface of the thread groove are visible in large numbers.

[0044] If the outer skin layer I, which consists exclusively of the matrix plastic, is absent from the start of use due to the execution of the second step on the surface of the female thread groove, premature wear of the female thread groove can be avoided. If the alignment layer II, which contains a large number of aligned fibers F1, is essentially absent from the start of use on the surface of the thread groove, the reduction in abrasion resistance due to the dissolution of fibers F1 in the alignment layer II can be avoided, and the abrasion resistance of the female thread groove 22 can be improved.If the longitudinal end sections of the fibers F2 present in the core layer III are removed by grinding and the end surfaces of these end sections are exposed in large numbers, aligned with the surface of the female thread groove, such fibers can further act as solid piles driven into the matrix plastic M, thus further increasing the abrasion resistance of the female thread groove due to the high abrasion resistance of the fibers F2. Due to the reduction or absence of fiber detachment, interference with the rolling motion of the balls 16 in the ball track 14 caused by detached fibers can be avoided.

[0045] By grinding and removing the outer skin layer I, or both the outer skin layer I and the alignment layer II, by grinding the surface of the threaded groove until the core layer III is reached, or by grinding and removing the longitudinal end sections of the fibers F2, which are aligned longitudinally in the core layer III, until the end surfaces of these end sections are flush with the surface of the female threaded groove, the abrasion resistance of the female threaded groove 22 can be improved to a correspondingly high degree. Any one of these four grinding stages can be selected depending on the required abrasion resistance of the female threaded groove 22.

[0046] The present invention has been described with reference to a preferred embodiment, but is not limited by this embodiment and can be modified in various ways without departing from the spirit of the present invention.

[0047] For example, the circulation of the spheres can be achieved not only by using end deflectors, but also by using tubes, other types of deflectors, and end caps. The configuration of the spindle thread groove is not limited to the configuration of the Gothic arch, but can also be circular, elliptical, or V-shaped.

[0048] The components included in the embodiment mentioned above are not essential in their entirety for the realization of the present invention, but can be partially replaced and omitted without departing from the spirit of the present invention.

[0049] The present invention is not limited to a plastic nut for a ball screw spindle, but can also be applied to other fiber-reinforced plastic parts that have a rolling or sliding surface, such as ball screw spindle shafts, sliding screw spindle shafts, linear guides with rolling balls, sliders with guide grooves for rolling balls, outer and inner raceways of roller and ball bearings and bushings for sliding bearings.

[0050] Fig. Figure 5 shows a ball bearing 50. The ball bearing 50 comprises an outer race 54 with a ball rolling groove 52 defined by an arcuate lower surface on its inner circumference, an inner race 58 with a ball rolling groove 56 defined by an arcuate lower surface on its outer circumference, and several steel or ceramic balls 60 positioned in rolling engagement between the ball rolling groove 52 of the outer race 54 and the ball rolling groove 56 of the inner race 58 in a mutually concentric arrangement.

[0051] The outer race 54 and the inner race 58 are made of injection-molded fiber-reinforced plastic. The matrix plastic for the fiber-reinforced plastic, which forms the outer race 54 and the inner race 58, can consist of a super-engineering plastic such as PPS (polyphenylene sulfide resin). The fibers that can be used for the reinforcement can be short fibers with a diameter of 0.1 µm to 5.0 µm and a length of 1 µm to 100 µm. The fibers can be glass fibers, carbon or graphite fibers, polyamide fibers, boron fibers, aramid fibers, silicon carbide whiskers, silicon nitride whiskers, zinc oxide whiskers, titanium boride whiskers, metal fibers, silicon fibers, and titanium oxide fibers.

[0052] The manufacturing process of the outer running ring 54 and the inner running ring 58 can include a first step in which the outer running ring 54 and the inner running ring 58 are injection molded in casting assemblies using fiber-reinforced plastic containing matrix plastic and fibers mixed therein as the plastic material for the injection molding process, so that the outer running ring 54 and the inner running ring 58 can be provided with pre-coating ball rolling grooves configured and dimensioned according to pre-coating specifications, which can later be completed as final ball rolling grooves 52 and 56, respectively.The manufacturing process may further include a second step in which the pre-finishing ball rolling grooves are completed according to the specifications of the final ball rolling grooves 52 and 56 using a grinding machine until the fibers of the fiber-reinforced plastic are exposed on the surfaces of the ball rolling grooves, after the injection molding and possibly curing process.

[0053] After the injection molding process and before grinding, the fibers F in the outer race 54 and the inner race 58 are, similar to the plastic nut 20 discussed above, not present in the outermost layer. More precisely, the outer race 54 and the inner race 58 each comprise an outer skin layer consisting entirely of matrix plastic, an alignment layer located further inward than the outer skin layer and containing fibers that are largely aligned along the surface of the outer skin layer, and a core layer located further inward than the alignment layer and containing fibers that are largely unaligned.In the outer running ring 54 and also in the inner running ring 58, there are in reality not necessarily clear boundaries between the different layers, and the outer skin layer can gradually transition into the alignment layer, and the alignment layer can gradually transition into the core layer.

[0054] In the second step, the pre-finishing ball grooves (which are to be completed as the final ball grooves 52 and 56) are ground so that at least the outer skin layer is removed until the fibers in the matrix plastic on the surface of the ball grooves 52 and 56 are exposed. Preferably, the alignment layer is removed by a specific amount until the core layer on the surface of each ball groove 52, 56 is exposed by this grinding process. This grinding process causes the ball grooves 52 and 56 to reach their prescribed final depth. Preferably, the amount of material removed is chosen as the minimum required to expose the core layer on the surface of the ball grooves 52 and 56, so that the voids are not necessarily exposed on the surface of the ball grooves 52 and 56 and the time required for the grinding operation can be minimized.

[0055] The outer skin layer and the alignment layer are thus removed, and the core layer is ground down, but only to the extent that any voids that may be present in the core layer are not exposed on the surface of the ball grooves 52 and 56. This is important so that the ball grooves 52 and 56 achieve a high degree of abrasion resistance.

[0056] By performing the grinding work until the core layer is reached beyond the alignment layer, longitudinal end regions are removed from the fibers that are located in the part of the material to be removed and are aligned in the depth direction (or from the fibers that are located next to the alignment layer), so that the end surfaces of such fibers are exposed in alignment on the surface of the ball roller grooves 52 and 56.

[0057] If the outer skin layer, consisting solely of the matrix plastic, is absent from the surfaces of the ball rolling grooves 52 and 56 from the start of use due to the execution of the second step, premature wear of the ball rolling grooves 52 and 56 can be avoided. If the alignment layer, containing a large quantity of aligned fibers, is absent from the surface of the ball rolling grooves 52 and 56 from the start of use, the reduction in abrasion resistance due to fiber dissolution in the alignment layer can be avoided, and the abrasion resistance of the ball rolling grooves 52 and 56 can be improved.If the longitudinal end regions of the fibers present in the core layer are removed by grinding and the end surfaces of these end regions are exposed in large numbers in alignment on the surface of the ball rolling grooves 52 and 56, such fibers also act as solid piles driven into the matrix plastic, so that, due to the high abrasion resistance of the fibers, the abrasion resistance of the ball rolling grooves 52 and 56 can be increased even further.

[0058] By grinding and removing the outer skin layer, or both the outer skin layer and the alignment layer, by grinding the surface of the ball roller grooves 52 and 56 until the core layer is reached, or by grinding and removing the longitudinal end sections of the fibers aligned in the depth direction within the core layer until the end surfaces of these end sections are exposed and aligned on the surface of the ball roller grooves 52 and 56, the abrasion resistance of the ball roller grooves 52 and 56 can be improved to a correspondingly high degree. Any one of these four grinding stages can be selected depending on the abrasion resistance requirements of the ball roller grooves 52 and 56.

[0059] Fig.Figure 6 is a sectional view of a plain bearing bushing 70. The matrix plastic for the fiber-reinforced plastic from which the plain bearing bushing 70 is manufactured can consist of a super-engineering plastic such as PPS (polyphenylene sulfide resin). The fibers that can be used for the reinforcing fibers can consist of short fibers with a diameter of 0.1 µm to 5.0 µm and a length of 1 µm to 100 µm. The fibers can consist of glass fibers, carbon or graphite fibers, polyamide fibers, boron fibers, aramid fibers, silicon carbide whiskers, silicon nitride whiskers, zinc oxide whiskers, titanium boride whiskers, metal fibers, silicon fibers, and titanium oxide fibers.

[0060] The manufacturing process of the plain bearing bushing 70 can include a first step in which the entire plain bearing bushing 70 is injection molded in a molding kit using fiber-reinforced plastic containing matrix plastic and embedded fibers as the plastic material for the injection molding process. This allows the plain bearing bushing 70 to be provided with a pre-finishing bore configured and dimensioned according to specified pre-finishing parameters, which can later be finished as the final bearing bore 72. The manufacturing process can further include a second step in which the pre-finishing bearing bore is finished according to the specifications of the final bearing bore 72 using a grinding machine until the fibers of the fiber-reinforced plastic are exposed on the surface of the bearing bore 72 after the injection molding and / or curing process.

[0061] After the injection molding process and before grinding, the fibers in the outermost layer of the sliding bearing bushing 70 are absent, similar to the plastic nut 20 discussed above. More precisely, the sliding bearing bushing 70 has an outer skin layer consisting solely of matrix plastic, an alignment layer located further inward than the outer skin layer and containing fibers that are largely aligned along the surface of the outer skin layer, and a core layer located further inward than the alignment layer and containing fibers that are largely unaligned. In reality, there are not necessarily clear boundaries between the different layers in the sliding bearing bushing 70, and the outer skin layer can gradually transition into the alignment layer, and the alignment layer can gradually transition into the core layer.

[0062] In the second step, the pre-finishing bore (which is to be completed as the final bearing bore 72) is ground so that at least the outer skin layer is removed until the fibers in the matrix plastic on the surface (inner surface) of the bearing bore 72 are exposed. Preferably, a certain amount of material is removed from the alignment layer until the core layer on the surface of the bearing bore 72 is exposed by this grinding process. This grinding process causes the bearing bore 72 to assume the prescribed final inner diameter. Preferably, the amount of material removed is chosen as a minimum amount required to expose the core layer on the surface of the bearing bore 72, so that the voids are not necessarily exposed on the surface of the bearing bore 72 and the time required for the grinding operation can be minimized.

[0063] The outer skin layer and the alignment layer are thus removed, and the core layer is ground down, but only to the extent that any voids that may be present in the core layer are not exposed on the surface of the bearing bore 72. This is important for ensuring that the bearing bore 72 achieves a high degree of abrasion resistance.

[0064] By performing the grinding work until the core layer is reached beyond the alignment layer, longitudinal end sections are removed from those fibers that are located in the part of the material to be removed and are aligned in the longitudinal direction (or from the fibers that are located next to the alignment layer), so that the end surfaces of such fibers are exposed in alignment on the surface of the bearing bore 72.

[0065] If the outer skin layer, consisting solely of the matrix plastic, is absent from the surface of the bearing bore 72 due to the execution of the second step from the start of use, premature wear of the bearing bore 72 can be avoided. If the alignment layer, which contains a large number of aligned fibers, is absent from the surface of the bearing bore 72 from the start of use, the reduction in abrasion resistance due to fiber detachment in the alignment layer can be avoided, and the abrasion resistance of the bearing bore 72 can be improved.If the longitudinal end sections of the fibers present in the core layer are removed by grinding and the end surfaces of these end sections are exposed in large numbers in alignment on the surface of the bearing bore 72, such fibers further act as solid piles driven into the matrix plastic, so that due to the high abrasion resistance of the fibers the abrasion resistance of the bearing bore 72 can be increased even further.

[0066] By grinding and removing the outer skin layer, or both the outer skin layer and the alignment layer, by grinding the surface of the bearing bore 72 until the core layer is reached, or by grinding and removing the longitudinal end sections of the fibers that are aligned in the depth direction within the core layer until the end surfaces of these end sections are flush with the surface of the bearing bore 72, the wear resistance of the bearing bore 72 can be improved to a correspondingly high degree. Any one of these four grinding stages can be selected depending on the required wear resistance of the bearing bore 72.

[0067] Although the present invention has been described with reference to preferred embodiments, the person skilled in the art will recognize that various modifications are possible without leaving the scope of the present invention.

[0068] The components included in the embodiment mentioned above are not essential for the execution of the present invention, but can be partially replaced and omitted without deviating from the spirit of the present invention.

[0069] The content of the original Japanese patent application (JP 2013-24877 filed on February 12, 2013), the priority of which is claimed under the Paris Convention for the present application, is included in this application by reference. Reference symbol list 1 ball screw 10 Spindle shaft 12 male spindle thread grooves 14 Ball track 16 balls 20 Nuts (plastic nuts for a ball screw spindle) 22 female spindle thread groove 24 threaded holes 26 Ball return track 30 End deflector 32 Ball return track 50 ball bearings 52 ball groove 54 outer running ring 56 ball groove 58 inner running ring 70 Plain bearing bushing 72 bearing bore I Outer skin layer II alignment layer III core layer B Empty space F Fiber G removal quantity M Matrix plastic

Claims

[1] A plastic nut (20) for a ball screw (1), which defines a threaded bore (24) having a screw thread groove (22) and is formed by injection molding fiber-reinforced plastic containing matrix plastic (M) and reinforcing fibers (F) mixed therein, wherein: the screw thread groove (22) is ground until the fibers (F) are exposed on a surface of the screw thread groove (22). [2] A plastic nut (20) for a ball screw (1) according to claim 1, further comprising an alignment layer (II) containing fibers (F) extending along a surface of the screw thread groove (22), wherein the screw thread groove (22) is ground until the alignment layer (II) is exposed. [3] A plastic nut (20) for a ball screw (1) according to claim 1, further comprising a core layer (III) containing fibers (F) that are not aligned, wherein the screw thread groove (22) is ground until the core layer (III) is exposed. [4] Method for producing a plastic nut (20) for a ball screw (1) containing matrix plastic (M) and reinforcing fibers (F) mixed therein, comprising: a first step in which a nut (20) is injection-molded using a metal casting assembly (24), the nut defining a threaded bore having a pre-finishing spindle thread groove configured and dimensioned according to pre-finishing specifications; and a second step in which the spindle thread groove (22) is completed according to final specifications by grinding the pre-finishing spindle thread groove until the fibers (F) are exposed on a surface of the threaded spindle groove (22). [5] A method for manufacturing a plastic nut (20) for a ball screw (1) according to claim 4, wherein immediately after the first step, the nut comprises: an outer skin layer (I) defining an outermost layer and being fiber-free (F), an alignment layer (II) located further inward than the outer skin layer (I) and containing fibers (F) aligned along a surface of the outer skin layer, and a core layer (III) located further inward than the alignment layer (II) and containing fibers (F) that are not aligned; and wherein the second step comprises a grinding step for removing at least the outer skin layer (I). [6] A method for manufacturing a plastic nut for a ball screw according to claim 5, wherein the second step comprises a grinding step for removing the alignment layer (II).

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