SEAL MEMBER FOR BALL SCREW DEVICE

Abstract

A seal member for a ball screw device, which is fixed to an axial end surface of a nut member of the ball screw device and is configured to seal a gap between the nut member and the screw shaft, the seal member including: a plurality of base plates which each have a through hole having an inner diameter larger than an outer diameter of the screw shaft; a spacer plate, which is sandwiched between two base plates adjacent to each other, and has a seal accommodation hole having an inner diameter larger than an inner diameter of a through hole of the base plates; and a seal plate, which has a shaft-fitting hole corresponding to a sectional shape of the screw shaft in a direction perpendicular to an axial direction, and is arranged in the seal accommodation hole of the spacer plates.

Claims

1. A seal member for a ball screw device, which is fixed to an axial end surface of a nut member of the ball screw device, and is configured to seal a gap between the nut member and the screw shaft, the seal member comprising: a plurality of base plates each having a through hole having an inner diameter larger than an outer diameter of the screw shaft; a spacer plate, which is sandwiched between two base plates adjacent to each other, and has a seal accommodation hole having an inner diameter larger than an inner diameter of a through hole of the base plates; and a seal plate, which has a shaft-fitting hole corresponding to a sectional shape of the screw shaft in a direction perpendicular to an axial direction, and is arranged in the seal accommodation hole of the spacer plates, wherein a space defined between the two base plates adjacent to each other is set larger than an axial thickness of the seal plate.

2. The seal member for a ball screw device according to claim 1, further comprising a connection member configured to pass through the base plates and the spacer plate, to thereby fix the base plates and the spacer plate to an axial end surface of the nut member, wherein the seal plate is arranged between the two base plates and in a seal accommodation hole of the spacer plate.

3-6. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a front view for illustrating one example of a ball screw device to which a seal member according to the present invention is applicable.

[0013] FIG. 2 is an exploded perspective view for illustrating a seal member according to a first embodiment of the present invention.

[0014] FIG. 3 is a main-part enlarged sectional view for illustrating the seal member when a thin film seal is used as the seal plate.

[0015] FIG. 4 is a main-part enlarged sectional view for illustrating the seal member when a surface-contact seal is used as the seal plate.

[0016] FIG. 5 is an explanatory view for illustrating a contact state between the surface-contact seal and a screw shaft.

[0017] FIG. 6 is a sectional view for illustrating a contact state between a projection of the surface-contact seal and a rolling groove of the screw shaft.

[0018] FIG. 7 is an explanatory view for illustrating an upper limit value of an axial thickness of the surface-contact seal.

[0019] FIG. 8 is an explanatory view for illustrating a calculation formula for deriving the upper limit value of the axial thickness of the surface-contact seal.

[0020] FIG. 9 is an explanatory view for illustrating a groove width of the rolling groove formed in the screw shaft.

[0021] FIG. 10 is an exploded perspective view for illustrating a seal member according to a second embodiment of the present invention.

MODE FOR CARRYING OUT THE INVENTION

[0022] Now, with reference to the accompanying drawings, detailed description is made of a seal member for a ball screw device according to the present invention.

[0023] FIG. 1 is an illustration of one example of the ball screw device to which the seal member according to the present invention is mounted.

[0024] This ball screw device 1 includes a screw shaft 2 and a cylindrical nut member 3. The screw shaft 2 has, on its outer peripheral surface, a rolling groove 20 for balls formed in a spiral shape with a predetermined lead. The nut member 3 is threadedly engaged with a circumference of the screw shaft 2 through intermediation of a plurality of balls. For example, through rotation of the screw shaft 2 relative to the nut member 3, the nut member 3 moves in an axial direction of the screw shaft 2. Alternatively, through rotation of the nut member 3 relative to the screw shaft 2, the screw shaft 2 moves in an axial direction of the nut member 3. The screw shaft 2 may be so-called multiple thread screw having a plurality of rolling grooves 20.

[0025] Moreover, a seal member 5 configured to seal a gap defined between the nut member 3 and the screw shaft 2 is mounted to an end surface of the nut member 3 in the axial direction. In FIG. 1, the seal member 5 is depicted only on one end surface of the nut member 3. However, the seal member 5 is usually mounted also to another end surface of the nut member 3.

[0026] FIG. 2 is an illustration of a seal member according to a first embodiment of the present invention, and is an exploded perspective view for illustrating a state in which the seal member 5 is removed from the nut member 3. The nut member 3 is formed into a cylindrical shape with a through hole for allowing the screw shaft 2 to be inserted therethrough, and a spiral load track 30 opposed to the rolling groove 20 formed in the screw shaft 2 is provided on an inner peripheral surface of the through hole. Balls 4 are arrayed on the load track 30, and roll on the load track 30 while bearing a load between the nut member 3 and the screw shaft 2.

[0027] The nut member 3 has an endless circulation path for the balls 4. In the nut member 3 illustrated in FIG. 2, a return path for the balls 4 is provided along the axial direction of the nut member 3, and a circulation support member 31 is mounted to an axial end surface of the nut member 3. The circulation support member 31 guides the balls 4 between the load track 30 and the return path, and constructs an endless circulation path for the balls with several turns of the spiral load track set as one circuit. As a method for constructing the endless circulation path for the balls in the nut member, there may be adopted any suitable structure such as so-called deflector type with one turn of the load track set as one circuit or so-called return pipe type in which a pipe member connecting several turns of the load track is mounted to the nut member.

[0028] The seal member 5 includes a seal plate 50, two base plates 51, and spacer plates 52. The seal plate 50 is held in slide contact with the screw shaft 2. The two base plates 51 are located on both sides of the seal plate 50. The spacer plates 52 are sandwiched between the two base plates 51.

[0029] The base plate 51 is formed into a ring shape which has a through hole having an inner diameter slightly larger than an outer diameter of the screw shaft 2. A plurality of mounting holes 51a which allow fixing screws 53 being connection members to pass therethrough are formed at equal intervals in a circumferential direction. The fixing screws 53 pass through the base plates 51 to be fastened to the nut member 3.

[0030] The spacer plate 52 is formed into a ring shape which has a seal accommodation hole 52b having an inner diameter larger than an inner diameter of the through hole of the base plate 51, and cutout portions 52a configured to allow the fixing screws 53 to pass therethrough are formed at equal intervals in the circumferential direction. When the two base plates 51 are to be fastened to the nut member 3, the spacer plates 52 are sandwiched between the base plates 51 to be integrated with the base plates. Moreover, when the fixing screws 53 are to be fastened, through fitting of the cutout portions 52a to the fixing screws 53, the spacer plates 52 are correctly positioned between the two base plates 51. In FIG. 2, two spacer plates 52 are arranged between two base plates 51. However, the number of spacer plates 52 to be sandwiched between the two base plates 51 and the plate thickness of each spacer plate 52 can be suitably determined in accordance with a material or a plate thickness of the seal plate 50.

[0031] The seal plate 50 is formed into a ring shape which has a shaft-fitting hole 50a corresponding to a sectional shape perpendicular to the axial direction of the screw shaft 2. Therefore, when the screw shaft 2 is inserted through the shaft-fitting hole 50a, an inner peripheral edge of the seal plate 50, that is, an inner peripheral surface of the shaft-fitting hole 50a is fitted to a periphery of the screw shaft 2. An outer diameter of the seal plate 50 is set larger than an inner diameter of the through hole of the base plate 51 and smaller than an inner diameter of the seal accommodation hole 52b of the spacer plate 52. The outer diameter of the seal plate 50 is smaller than the inner diameter of the seal accommodation hole 52b of the spacer plate 52. Thus, the fixing screws 53 passing through the base plates 51 and the spacer plates 52 are located on a radially outer side with respect to the seal plate 50, and the fixing screws 53 do not pass through the seal plate 50.

[0032] Thus, when the spacer plates 52 and the seal plate 50 are sandwiched between the two base plates 51, and the fixing screws 53 are fastened to the nut member 3, the seal plate 50 is located inside the seal accommodation holes 52b of the spacer plates 52, and movement of the seal plate 50 in the axial direction of the screw shaft 2 is restricted by the base plates 51. Accordingly, the seal plate 50 is irremovably held inside the seal accommodation holes 52b. On this occasion, a space corresponding to the thickness of the spacer plates 52 is defined between the two base plates 51. The seal plate 50 is accommodated in this space, and is held between the two base plates 51.

[0033] A material of the seal plate 50 may suitably be selected in accordance with a use environment of the ball screw device 1 to which the seal member 5 is mounted. For example, there may be used a material formed of a synthetic resin film or a material formed of a fiber sheet having rubber or synthetic resin impregnated therein.

[0034] FIG. 3 is a sectional view for illustrating one example of a state in which the seal plate 50 is mounted to the seal member 5.

[0035] In the example illustrated in FIG. 3, a thin film seal 50A formed of a synthetic resin film is used as the seal plate 50, and one spacer plate 52 is sandwiched between the two base plates 51. The thin film seal 50A is used mainly for the purpose of preventing leakage of the lubricant from the inside of the nut member 3 to the outside, but also serves to remove a foreign object adhering to the screw shaft 2. The inner peripheral surface of the shaft-fitting hole 50a in the thin film seal 50A is opposed to the outer peripheral surface of the screw shaft 2 through a minute gap which is slightly larger than an oil film thickness. When the thin film seal 50A is excessively brought into slide contact with the screw shaft 2, the thin film seal 50A is bent, and hence there is a risk of causing an increase in gap between the inner peripheral surface of the shaft-fitting hole 50a and the screw shaft 2. Moreover, there is also a risk of causing expansion of the screw shaft 2 due to generation of a friction heat. Therefore, when the thin film seal 50A is to be mounted to the nut member, it is important to prevent the thin film seal 50A from being excessively pressed against the screw shaft 2 and prevent the thin film seal from being bent.

[0036] Thus, when such thin film seal 50A is to be used, a gap t.sub.1 defined between the two base plates 51 by the spacer plates 52 is set larger than an axial thickness of the thin film seal 50A. With this, the thin film seal 50A is held between the two base plates 51 through a slight gap without being sandwiched, and is freely displaceable in a radial direction of the screw shaft 2. As a result, the thin film seal 50A is always held at an optimum position with respect to the screw shaft 2, and hence the inner peripheral edge of the thin film seal 50A, that is, the inner peripheral surface of the shaft-fitting hole 50a can be prevented from being excessively pressed against the screw shaft 2. Moreover, the two base plates 51 are provided on both sides of the thin film seal 50A through a minute gap. Therefore, even when the inner peripheral edge of the thin film seal 50A is in contact with the screw shaft, bending of the thin film seal 50A can be prevented. With this, the function of the thin film seal 50A described above can be maximally exerted.

[0037] For clear illustration of the presence of the gap defined between the thin film seal 50A and the base plates 51, the gap is illustrated with emphasis in FIG. 3. However, an actual size of the gap is smaller than a plate thickness of the thin film seal 50A.

[0038] FIG. 4 is a sectional view for illustrating another example of a state in which the seal plate 50 is mounted to the seal member 5.

[0039] In the example illustrated in FIG. 4, a surface-contact seal 50B is used as the seal plate 50. The surface-contact seal 50B is evenly held in contact with the screw shaft 2 at low contact pressure similarly to the thin film seal 50A, but secures the performance of removing foreign objects with a contact area set larger than that of the thin film seal 50A. Therefore, an axial thickness of the surface-contact seal 50B is larger than that of the thin film seal 50A, and two spacer plates 52 are sandwiched between the two base plates 51.

[0040] The surface-contact seal 50B can be produced with an elastic member such as rubber or elastomer by die forming with the screw shaft 2 as a core. For example, the surface-contact seal 50B is obtained in the following manner as disclosed in Japanese Patent Application Laid-open No. 2014-156888. Specifically, woven fabric having, for example, rubber or elastomer impregnated therein is wound around an outer peripheral surface of the screw shaft 2 as a core. After that, a skeleton layer made of synthetic resin is formed by die forming around the woven fabric while an appropriate pressure being applied, thereby being capable of forming a seal base member having a through hole matching with a shape of the outer peripheral surface of the screw shaft 2. Then, after the screw shaft 2 as a core is taken out from the seal base member, the seal base member is cut into a donut shape with a suitable thickness. Accordingly, the surface-contact seal 50B is obtained. In the surface-contact seal 50B produced in such a manner, the shape of the outer peripheral surface of the screw shaft 2 is transferred to the inner peripheral surface of the shaft-fitting hole 50a. Thus, a spiral projection matching with the rolling groove 20 of the screw shaft is formed in the inner peripheral surface of the shaft-fitting hole. When the surface-contact seal 50B is fitted to the screw shaft 2, the surface-contact seal 50B and the screw shaft 2 are evenly brought into contact with each other without a gap at low surface pressure.

[0041] In order to secure the even contact between the surface-contact seal 50B and the screw shaft 2, when the surface-contact seal 50B is to be fixed between the two base plates 51, the surface-contact seal 50B is first fitted to the screw shaft 2. After the surface-contact seal 50B is positioned by the screw shaft 2, the fixing screws 53 are fastened so that the surface-contact seal 50B is sandwiched between the two base plates 51.

[0042] Moreover, the surface-contact seal 50B formed of the elastic member such as rubber or elastomer, when being slightly squeezed in its thickness direction (axial direction), is capable of improving close contact between the inner peripheral surface of the shaft-fitting hole 50a and the screw shaft. Specifically, a gap t.sub.2 defined between the two base plates 51 by the spacer plates is set slightly smaller than an axial thickness of the surface-contact surface seal 50B so that the surface-contact seal 50B is fixed in a state of being squeezed between the two base plates 51 adjacent to each other. An inner peripheral surface of the shaft-fitting hole 50a of the squeezed surface-contact seal 50B slightly expands toward the radially outer surface of the screw shaft 2. As a result, the inner peripheral surface of the shaft-fitting hole 50a can reliably be brought into close contact with respect to the circumference of the screw shaft 2. FIG. 4 is an illustration of such case, from which the state in which the inner peripheral edge of the surface-contact seal 50B is pressed against the screw shaft 2 to be slightly expanded.

[0043] Next, an appropriate axial thickness of the surface-contact seal 50B is to be discussed.

[0044] As described above, with regard to the surface-contact seal 50B, a wide contact area is set while the surface-contact seal 50B is held in contact with respect to the screw shaft 2 at low contact surface pressure, thereby securing the performance of removing foreign objects adhering to the screw shaft 2. Therefore, in the view point of enhancing the performance of removing the foreign objects, when the axial thickness of the surface-contact seal 50B is set larger, the contact area with respect to the screw shaft 2 increases, and the foreign object removing performance of the surface-contact seal 50B is enhanced.

[0045] However, the surface-contact seal 50B produced with a material such as rubber or elastomer by die forming is liable to cause contraction after forming. When the axial thickness of the surface-contact seal 50B is large, the spiral projection formed on the inner peripheral surface of the shaft-fitting hole 50a of in the surface-contact seal 50B causes excessive interference with the rolling groove 20 of the screw shaft 2, which may cause formation of a gap between the inner peripheral surface of the shaft-fitting hole 50a and the screw shaft 2. When the contact pressure between the inner peripheral surface of the shaft-fitting hole 50a and the screw shaft 2 becomes stronger, generation of a friction heat may cause expansion of the screw shaft 2 or a rise in rotational torque required for rotational drive of the screw shaft. Moreover, when the gap is formed between the inner peripheral surface of the shaft-fitting hole 50a and the screw shaft 2, the sealability therebetween is impaired, and hence foreign objects may be more liable to enter the nut member 3 from the outside.

[0046] FIG. 5 and FIG. 6 are explanatory views for illustrating the interference between the projection of the surface-contact seal 50B and the rolling groove 20 of the screw shaft 2. In FIG. 5, the outer peripheral surface of the screw shaft 2 is developed, and the surface-contact seal 50B overlaps thereon. The rolling groove 20 for the balls 4 is formed with a predetermined lead in the screw shaft 2, and the rolling groove 20 is inclined at a lead angle with respect to a plane orthogonal to the axial direction of the screw shaft 2. The lead corresponds to a length by which the rolling groove proceeds in the axial direction of the screw shaft when turning around the screw shaft by one turn. When the lead is represented by L, and a circumferential length of the screw shaft 2 is represented by Dn, the lead angle can be expressed by =tan.sup.1(L/Dn).

[0047] On inner peripheral surface of the surface-contact seal 50B formed by die forming using the screw shaft 2 as a core, there is formed the spiral projection 54 matching with the rolling groove 20. The projection 54 originally is formed so as to have the same lead and lead angle with the rolling groove 20. However, when the surface-contact seal 50B contracts after completion of forming, as indicated by two-dot chain lines in FIG. 5, the lead angle of the projection 54 is liable to become smaller than the lead angle of the rolling groove 20. Therefore, when the surface-contact seal 50B is mounted to the nut member 3 and fitted to the screw shaft 2, as illustrated in FIG. 5, there are formed regions X in which the projection 54 and the rolling groove 20 having different lead angles interfere with each other.

[0048] FIG. 6 is a sectional view for illustrating the screw shaft 2 and the surface-contact seal 50B taken along an A direction (see FIG. 5) inclined by the lead angle with respect to the axial direction O of the screw shaft 2, and is an illustration of a contact state between the screw shaft 2 and the surface-contact seal 50B. As illustrated in FIG. 6, when the interference regions X described above are generated in a plane extending along the A direction so as to be located on both sides of the thread ridge 21 of the screw shaft 2, a force Facts on the surface-contact seal 50B in a direction of pushing up the surface-contact seal 50B from the outer peripheral surface of the screw shaft 2. As a result, as illustrated in FIG. 6, the surface-contact seal 50B floats upward from the outer peripheral surface of the screw shaft 2, and hence a gap is liable to be formed between the projection 54 of the surface-contact seal 50B and the rolling groove 20 of the screw shaft 2.

[0049] In order to prevent such floating of the surface-contact seal 50B and enhance the close contact between the surface-contact seal 50B and the screw shaft 2, it is effective to set the axial thickness T of the surface-contact seal 50B to a predetermined value or less to prevent generation of the interference regions X on both sides of the thread ridge 21 of the screw shaft 2. FIG. 7 is a view corresponding to FIG. 5 described above, and is an illustration of a case in which the axial thickness T of the surface-contact seal 50B is smaller than that given in FIG. 5. As illustrated in FIG. 7, when the axial thickness T of the surface-contact seal 50B falls within a width of the thread ridge 21 of the screw shaft 2 indicated by a line segment Y, in the A direction inclined by the lead angle with respect to the axial direction O, the interference regions X are not generated on both sides of the thread ridge 21 of the screw shaft 2 at any position in the circumferential direction of the surface-contact seal 50B. Accordingly, generation of the force F of pushing up the surface-contact seal 50B from the outer peripheral surface of the screw shaft 2 can be suppressed. Here, the line segment Y in FIG. 7 is a line segment extending along the A direction and across the thread ridge 21 in a shortest distance.

[0050] That is, in a case in which the screw shaft 2 and the surface-contact seal 50B are matched with each other in respective axial directions and fitted to each other, when the axial thickness T of the surface-contact seal 50B falls within the range of the line segment Y extending across the thread ridge 21 in a shortest distance, a gap is less liable to be formed between the projection 54 of the surface-contact seal 50B and the rolling groove 20 of the screw shaft 2, thereby being capable of enhancing the close contact between the surface-contact seal 50B and the screw shaft 2.

[0051] FIG. 8 is an explanatory view for illustrating a relationship between a length of the line segment Y and the axial thickness T of the surface-contact seal 50B. A line segment P indicated by a one-dot chain line in FIG. 8 represents an axial pitch of the rolling groove 20 in the outer peripheral surface of the screw shaft. When one rolling groove is formed in the screw shaft, the pitch P is the same as the lead L. When two rolling grooves are formed in the screw shaft, the pitch P corresponds to L/2. Thus, a pitch P.sub.A of the rolling groove in the A direction is expressed by P cos . Thus, when the groove width of the rolling groove 20 is represented by W.sub.1, a width W.sub.2 of the thread ridge in the A direction is expressed by P cos W.sub.1. The width W.sub.2 corresponds to the length of the line segment Y illustrated in FIG. 7. The groove width W.sub.1 of the rolling groove 20 is, as illustrated in FIG. 9, an intersection between the outer peripheral surface of the screw shaft 2 and a spherical surface of the balls 4 rolling on the rolling groove 20, and is different from a width of the rolling groove 20 formed in the screw shaft 2.

[0052] When a length obtained by projecting the width W.sub.2 of the thread ridge in the A direction in the axial direction O of the screw shaft is represented by W.sub.0, W.sub.0=P cos.sup.2 W.sub.1 cos is given. When the axial thickness T of the surface-contact seal 50B is equal to or less than the width W.sub.0, as described above, the interference regions X are not generated on both sides of the thread ridge 21 of the screw shaft 2 at any position in the circumferential direction of the surface-contact seal 50B. Accordingly, the close contact between the surface-contact seal 50B and the outer peripheral surface of the screw shaft 2 can be enhanced. That is, when the axial thickness T of the surface-contact seal 50B is set so that the following conditions are given, the surface-contact seal 50B is evenly brought into contact with the outer peripheral surface of the screw shaft, and hence the sealing performance can reliably be given.

TP cos.sup.2 W.sub.1 cos

[0053] T: axial thickness of surface-contact seal 50B

[0054] P: pitch of rolling groove 20 in outer peripheral surface of screw shaft 2

[0055] : lead angle of rolling groove 20

[0056] W.sub.1: groove width of rolling groove 20

[0057] As described above, through changes in plate thickness or the number of the spacer plates, the seal member 5 to which the present invention is applied is adaptable to the plate thicknesses of various seal plates 50, and the seal plate 50 can be mounted to the nut member 3 in an optimum fixing method in accordance with a material of the seal plate 50. Thus, according to the seal member 5 of the present invention, the sealing member 5 can flexibly adapt to changes in material and plate thickness of the seal plate 50 in accordance with a use environment of the ball screw device 1. Moreover, even when the material or the plate thickness of the seal plate 50 is changed, standardization can be achieved without changes of the base plates 51 and the spacer plates 52, thereby being capable of reducing production cost or management cost for components.

[0058] FIG. 10 is an exploded perspective view for illustrating a seal member according to a second embodiment of the present invention. In the first embodiment described with reference to FIG. 2, one seal plate 50 is arranged between two base plates 51. However, in the second embodiment, three base plates 51 and two seal plates 50 are stacked alternately to forma so-called double-seal seal member.

[0059] The spacer plates 52 are each sandwiched between base plates 51 adjacent to each other similarly to the first embodiment described above, and the spacer plates 52 define an accommodation space for the seal plate 50. In FIG. 10, two spacer plates 52 are arranged between two base plates 51 adjacent to each other. However, the number of the spacer plates 52 can be suitably increased or decreased in accordance with a plate thickness of the seal plate 50.

[0060] The seal member according to the present invention, as illustrated in the second embodiment, can suitably be changed not only in material or plate thickness of the seal plate 50 but also in the number of the seal plate 50, thereby being capable of suitably achieving a performance in accordance with a use environment of the ball screw device 1.

[0061] In the embodiment described above, the fixing screws 53 being connection members passing through the base plates 51 and the spacer plates 52 are used to fasten the base plates 51 and the spacer plates 52 to the nut member 3. However, for example, rivets may be used as the connection members to integrate the base plates 51 and the spacer plates 52 with each other in advance to form the seal member 5, and the integrated seal member 5 may be mounted to the nut member by other means such as a stopper ring.