WO2005120770A1

WO2005120770A1 - Automatic tool exchange type high speed spindle assembly with bearing protection structure

Info

Publication number: WO2005120770A1
Application number: PCT/KR2004/001355
Authority: WO
Inventors: Young-Jin Hyun
Original assignee: Yiwon Engineering Inc.
Priority date: 2004-06-07
Filing date: 2004-06-07
Publication date: 2005-12-22

Abstract

The spindle assembly of this invention includes a draw bar installed in a shaft in such a manner that the draw bar moves toward a tool while compressing a disc spring when pushing force of an unclamping unit is applied to the draw bar during an automatic tool exchange process, at least one pair of bearings installed between the shaft and a bearing case, an outer wheel fixing unit fixed to the bearing case in order to restrict an outer wheel of a front bearing, and a stopper unit provided between an inner wheel fixing unit and the outer wheel fixing unit so as to restrict the movement of the inner wheel if the inner wheel moves more than an allowed distance. The outer wheel fixing unit is coupled to the bearing case while forming a gap capable of compensating for a thickness tolerance of the front bearing therebetween.

Description

AUTOMATIC TOOL EXCHANGE TYPE HIGH SPEED SPINDLE ASSEMBLY WITH BEARING PROTECTION STRUCTURE

Technical Field The present invention relates to a spindle assembly of a machine tool, and more particularly to an automatic tool exchange type high speed spindle assembly having a bearing protection structure for preventing a bearing from being damaged when a shaft is pushed in a forward direction from a spindle of a machine tool having an automatic tool exchange function so as to exchange tools.

Background Art Generally, when an automatic tool exchange unit is used in a spindle assembly of a machine tool, a tool holder is fixed to a taper seat of a spindle by means of elastic force of a disc spring of a draw bar. In contrast, in order to release the tool holder from the spindle, it is necessary to push the draw bar toward a tool by applying predetermined force greater than the elastic force of the disc spring to the draw bar. Such a mechanical action is carried out by means of an unclamping unit using a pneumatic actuator or a hydraulic actuator. Pushing force applied to the draw bar is transferred to the shaft through the disc spring of the draw bar, so that the pushing force is applied to a bearing. Accordingly, balls and a raceway of the bearing are subject to overload, so permanent deformation may occur in the surfaces of the balls and the raceway of the bearing. A conventional spindle has a low rotational speed, so it does not produce a problem if the balls and the raceway of the bearing are slightly damaged. However, currently used machine tools are operated at high speeds, so the spindle provided in the machine tool may rotate at a high speed in a range of about 30000 to 50000 rpm. Accordingly, the dynamic characteristic of the spindle rotating at a high speed may be significantly deteriorated even if slight deformation occurs in the balls and the raceway of the bearing used for the high-speed spindle while significantly shortening the life span of the bearing. In most cases, automatic tool exchange units are used without considering such a problem, so it is necessary to frequently exchange an expensive high-performance bearing within a short period of time. As shown in FIG. 1, the bearing may be damaged when the tool is exchanged because pushing force of an unclamping unit 20 applied to a draw bar 12 is transferred to a shaft 10 through a disc spring 16 so that the shaft 10 pushes an inner wheel 44 of the bearing 40. At this time, balls 42 of the bearing 40 interposed between the inner wheel 44 and an outer wheel 46 of the bearing 40 are subject to overload, so that permanent deformation may occur in surfaces of the balls 42 and a raceway of the bearing. Therefore, if a stopper structure is provided in front of the bearing in such a manner that the inner wheel 44 of the bearing 40 is not over-pushed more than a predetermined distance, the balls 42 interposed between the inner wheel 44 and the outer wheel 46 of the bearing 40 can be prevented from being overloaded. As shown in FIG. 2, according to a currently used spindle structure, the inner wheel 44 and the outer wheel 46 of the bearing 40 are fixedly coupled with a lock nut 36, respectively. In addition, an anti-vibration cover 32 is fixedly coupled to a left side of a bearing case 30. For instance, if a predetermined gap SI is formed between the anti-vibration cover 32 and the lock nut 36 for fixing the inner wheel 44, when the shaft 10 moves in the left direction more than a predetermined distance, the lock nut 36 positioned adjacent to the inner wheel 44 of the bearing 40 makes contact with the anti-vibration cover 32, thereby stopping the shaft 10. Such a stopper structure can protect the bearing 40 because the shaft 10 moves along the moving direction of the draw bar 12 when the draw bar 12 is pushed in the left direction and the movement of the shaft 10 is stopped when the lock nut 36 makes contact with the anti-vibration cover 32 even though the draw bar 12 moves further. At this time, the balls 42 of the bearing 40 are maintained in an idle state while being spaced from the inner wheel 44 or the outer wheel 46, and the pushing force applied to the draw bar 12 is only transferred to the anti- vibration cover 32, which is an outer wheel fixing unit, through the shaft 10 without being applied to the bearing 40. The above stopper structure can be embodied because the spindle structure has a slight margin space capable of allowing the inner wheel 44 and the outer wheel 46 of the bearing 40 to move in an axial direction with respect to the balls 42. Accordingly, as shown in FIG. 3a, an assembling work for the bearing 40 may be carried out by applying pre-pressure to the bearing 40 in such a manner that the balls 42 make contact with the raceway of the inner wheel 44 and the outer wheel 46 with a predetermined contact angle. At this time, if the inner wheel 44 of the bearing 40 moves in the left direction while fixing the outer wheel 46 as shown in FIG. 3b, the balls 42 of the bearing 40 are maintained in an idle position without being restricted by the raceway of the inner wheel 44 and the outer wheel 46. In addition, if the inner wheel 44 of the bearing 40 further moves in the left direction as shown in FIG. 3c, the balls 42 make contact with the other portion of the raceway. In this state, if the inner wheel 44 of the bearing 40 is further pushed in the left direction caused by external force applied to the inner wheel 44 of the bearing 40, permanent deformation may occur between the balls 42 and the raceway of the bearing 40, causing damage to the bearing 40. As mentioned above, the bearing structure has a margin for allowing the balls 42 to be placed in the idle position with respect to the raceway within a relative displacement between the inner wheel 44 and the outer wheel 46. In addition, the above-mentioned stopper structure prevents the inner wheel 44 of the bearing 40 from being moved more than a predetermined distance, that is, more than the margin distance of the bearing, when the shaft 10 moves in the left direction. Thus, the gap SI between the anti-vibration cover 32 and the lock nut 36 of the inner wheel 44 must be formed within the predetermined margin distance of the bearing. However, the above-mentioned stopper structure has a problem in that the balls 42 may be maintained in the idle position with respect to the inner wheel 44 or the outer wheel 46 of the bearing 40 within an idle position allowing a distance of about 0.1 to 0.3mm when the inner wheel 44 moves with the shaft 10. That is, when the shaft 10 moves toward the tool within the distance range of about 0.1 to 0.3mm, the balls 42 can be maintained in the idle position with respect to the inner wheel 44 or the outer wheel 46 of the bearing 40. However, if the shaft 10 moves further, the balls 42 are again restricted by means of the inner and outer wheels 44 and 46 of the bearing 40. In this state, if the inner wheel 44 is further moved by means of excessive force applied to the inner wheel 44, the bearing 40 may be permanently damaged. For this reason, the stopper structure must be installed in a predetermined position for allowing the inner wheel 44 to move within an allowed distance. However, the stopper structure installed as mentioned above may cause a problem in that the stopper structure must be frequently exchanged whenever the bearing is exchanged. This is because the axial tolerance of the bearing, that is, the thickness tolerance of the bearing is in a range of about 0.1 to 0.4 mm, which means that the thickness of the bearing may vary depending on sorts of the bearings. If the stopper structure is installed in relation to a normal-sized bearing having a margin distance of 0.1mm, a user must select the same bearing when exchanging the normal-sized bearing. If the user uses another bearing having a thickness larger than the thickness of the normal- sized bearing by 0,1mm, the lock nut of the inner wheel fixed to the shaft makes contact with the stopper structure so that the spindle cannot rotate. In addition, a spindle bearing of a machine tool generally includes at least two bearings, which are coupled to each other in series. Accordingly, thickness variation of the spindle bearing may become enlarged due to the thickness tolerance thereof. That is, due to a thickness difference of the bearing, the stopper structure must be installed in a predetermined position by measuring the thickness of the bearing whenever the bearing is exchanged.

Disclosure of the Invention Therefore, the present invention has been made in view of the above-mentioned problems occurring in the prior art, and it is a first object of the present invention to provide a high speed spindle assembly capable of effectively preventing a bearing from being damaged when a tool is automatically exchanged by preventing a shaft from being moved more that an allowed distance when a draw bar and the shaft are moved toward the tool by means of pushing force of an unclamping unit through fixing a stopper unit to a bearing case within the allowed distance. A second object of the present invention is to provide a spindle assembly of a machine tool having a superior automatic tool exchange function, in which it is not necessary to exchange the stopper unit according to a thickness tolerance of a bearing whenever the bearing is exchanged. To accomplish the above objects, the present invention provides a spindle assembly comprising a draw bar installed in a shaft in such a manner that the draw bar moves toward a tool while compressing a disc spring when pushing force of an unclamping unit is applied to the draw bar during an automatic tool exchange process; at least one pair of bearings installed between the shaft and a bearing case; an outer wheel fixing unit fixed to the bearing case in order to restrict an outer wheel of a front bearing; and a stopper unit attached to the outer wheel fixing unit adjacent to the bearing and coaxially with a rotational center of a spindle, wherein the stopper unit is a part of an anti-vibration cover, a first gap is formed between an inner wheel fixing unit of the front bearing adjacent to the shaft and the stopper unit in order to allow an inner wheel of the front bearing to move within an allowed distance, and a second gap is formed between the outer wheel fixing unit and the bearing case in order to compensate for a thickness tolerance of the front bearing. According to the present invention having the above construction, when the draw bar and the shaft are moved toward the tool by means of pushing force of the unclamping unit during the automatic tool exchange process, the inner wheel of the front bearing moving in the forward direction by the shaft makes contact with the stopper unit attached to the outer wheel fixing unit within the predetermined allowed distance, so the inner wheel cannot move further, thereby preventing the bearing from being damaged. In addition, the size of the gap for compensating for the thickness tolerance of the bearing is flexibly adjusted according to the thickness tolerance of the bearing, so that it is not necessary to frequently exchange the stopper unit whenever the bearing is exchanged.

Brief Description of the Drawings The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: FIG. 1 is a sectional view showing a conventional high speed spindle assembly; FIG. 2 is a sectional view showing a front bearing structure of a conventional high speed spindle assembly; FIG. 3 is a sectional view showing a bearing structure ; FIG. 4 is an enlarged sectional view of a front bearing section of a high speed spindle assembly according to a first embodiment of the present invention; FIG. 5 is an enlarged sectional view of a front bearing section of a high speed spindle assembly according to a second embodiment of the present invention; and FIG. 6 is an enlarged sectional view of a front bearing section of a high speed spindle assembly according to a third embodiment of the present invention.

Best Mode for Carrying Out the Invention Hereinafter, a preferred embodiment of the present invention will be described with reference to accompanying drawings. FIG. 4 shows a front bearing section of a high speed spindle assembly according to a first embodiment of the present invention. Referring to FIG. 4, an outer wheel 46 of a front bearing 40 is installed such that the outer wheel 46 of the front bearing 40 is restricted by means of an outer wheel fixing unit 32b fixed to a bearing case 30. A stopper unit 32a is attached to the outer wheel fixing unit 32b coaxially with a rotational center of the spindle. An inner wheel 44 of the front bearing 40 adjacent to a shaft 10 is fixed by means of a lock nut 36. A gap SI is formed between the lock nut 36 and the stopper unit 32a in order to allow the inner wheel 44 to move within an allowed distance . In other words, since the inner wheel 44 of the front bearing 40 is fixed to the shaft 10 by means of the lock nut 36, when the shaft 10 moves in the forward direction for exchanging a tool, the inner wheel 44 of the front bearing 40 also moves together with the shaft 10. At this time, the movement of the inner wheel 44 of the front bearing 40 may be restricted within a range of the gap SI. That is, in order to prevent the inner wheel 44 of the front bearing 40 from being moved out of the allowed distance, the stopper unit 32a is installed in front of the lock nut 36. The stopper unit 32a fixed to a bearing case 30 by means of a bolt 34 is a part of an anti-vibration cover 32 and the outer wheel fixing unit 32b. In addition, the outer wheel fixing unit 32b is installed in the bearing case 30 while forming a gap Sll for compensating for a thickness tolerance of the bearing therebetween. Since the thickness tolerance may vary depending on sorts of bearings, if the bearing is exchanged with a bearing having a thickness tolerance larger than the allowed moving distance of the bearing, an inner wheel fixing unit makes contact with the stopper unit 32a, thereby preventing a spindle from being rotated. Accordingly, the gap Sll is formed between the outer wheel fixing unit 32b and the bearing case 30 in order to ensure the rotation of the spindle. If the bearing is exchanged with a bearing having a smaller thickness tolerance, the size of the gap Sll becomes reduced and the size of the gap SI formed between the stopper unit 32a and the inner wheel fixing unit is constantly maintained. In contrast, if the bearing is exchanged with a bearing having a greater thickness tolerance, the size of the gap Sll becomes enlarged and the size of the gap SI is constantly maintained. FIG. 5 shows a second embodiment of the present invention. Referring to FIG. 5, a first collar 54 is additionally inserted between the inner wheel 44 of the front bearing 40 and the lock nut 36. In addition, a second collar 52 is installed between the outer wheel 46 of the front bearing 40 and the outer wheel fixing unit 32b. The second collar 52 is positioned in front of the first collar 54 and spaced from the first collar 54 by an allowed moving distance of 0.1mm. In other words, the first collar 54 is installed between the inner wheel 44 of the front bearing 40 and the lock nut 36 so as to support the inner wheel 44, the second collar 52 is installed between the outer wheel 46 of the front bearing 40 and the outer wheel fixing unit 32b in order to support the outer wheel 46, and a gap S2 is formed between the first and second collars 54 and 52 in order to permit the inner wheel 44 to move within the allowed distance of 0.1mm, thereby forming a predetermined stopper unit . When the draw bar 12 is pushed in the left direction, the second collar 52 prevents the first collar 54 from being moved together with the shaft 10 more than the allowed distance of 0.1mm, which is a size of the gap S2, so that the movement of the shaft 10 is stopped even though the draw bar 12 moves further. Accordingly, the bearings 40 and 40a can be effectively protected. In addition, the gap Sll is formed between the outer wheel fixing unit 32b and the bearing case 30 so as to allow the stopper unit to constantly maintain the allowed distance between the stopper unit and the bearing 40 regardless of the thickness of the bearing 40. FIG. 6 shows a third embodiment of the present invention. According to the third embodiment of the present invention, a stopper unit is positioned rearward of the bearings 40 and 40a, differently from ^"the first and second embodiments of the present invention. That is, the stopper unit 30 is attached to a rear portion of the bearing case 30, a locking section 10a having a size larger than an inner diameter of the stopper unit 30a is formed in the shaft 10 coupled with the inner wheel 44 of the bearings 40 and 40a, while forming a gap between the locking section 10a and a rear portion of the stopper unit 30a. Similar to the stopper structures of the first and second embodiments, the stopper structure of the third embodiment also prevents the shaft 10 and the inner wheel 44 of the bearing from being moved in the forward direction more that the allowed distance. The allowed distance is set to 0.1mm for a general bearing for the high speed spindle. Meanwhile, a gap Sll is formed between the outer wheel fixing unit 32b and the bearing case 30 in order to compensate for the thickness tolerance of the bearing when exchanging the bearing. If the bearing has a thin thickness, the size of the gap Sll becomes reduced. In addition, if the bearing has a thick thickness, the size of the gap Sll becomes enlarged, thereby constantly maintaining an interval between the anti-vibration cover 32 and the lock nut 36.

Industrial Applicability As can be seen from the foregoing, according to the present invention having the above construction, in order to prevent the balls and the raceway of the bearing from being damaged when the draw bar and the shaft are moved toward the tool by means of pushing force of the unclamping unit during the automatic tool exchange process in the high speed spindle, the movement of the inner wheel of the bearing is restricted within an allowed distance by means of the stopper unit installed at a front or a rear portion of the bearing case. In addition, the gap is formed between the bearing case and the outer wheel fixing unit so as to constantly maintain an interval between the anti-vibration cover (or stopper unit) and the lock nut regardless of the thickness tolerance of the bearing when the bearing is exchanged. Accordingly, it is not necessary to frequently exchange the stopper unit according to the thickness tolerance of the bearing whenever the bearing is exchanged.

Claims

1. A spindle assembly comprising: a draw bar installed in a shaft in such a manner that the draw bar moves toward a tool while compressing a disc spring when pushing force of an unclamping unit is applied to the draw bar during an automatic tool exchange process; at least one pair of bearings installed between the shaft and a bearing case; an outer wheel fixing unit fixed to the bearing case in order to restrict an outer wheel of a front bearing; and a stopper unit attached to the outer wheel fixing unit adjacent to the bearing and coaxially with a rotational center of a spindle, wherein the stopper unit is a part of an anti-vibration cover, a first gap is formed between an inner wheel fixing unit of the front bearing adjacent to the shaft and the stopper unit in order to allow an inner wheel of the front bearing to move within an allowed distance, and a second gap is formed between the outer wheel fixing unit and the bearing case in order to compensate for a thickness tolerance of the front bearing.

2. The spindle assembly as claimed in claim 1, wherein a first collar is installed between the inner wheel of the front bearing and a lock nut so as to support the inner wheel, a second collar is installed between the outer wheel of the front bearing and the outer wheel fixing unit in order to support the outer wheel, and a third gap is formed between the first and second collars in order to permit the inner wheel to move within the allowed distance.

3. The spindle assembly as claimed in claim 1, wherein the stopper unit is installed in a rear portion of the bearing case such that the stopper unit is adjacent to the outer wheel at a rear portion of the front bearing, and a locking section is formed in the shaft coupled with the inner wheel of the front bearing, while forming a gap between the locking section and a rear portion of the stopper unit for allowing an inner wheel of the front bearing to move within the allowed distance.