A support nut

Abstract: A support nut in machine elements of the screw-spindle­ /nut comprising a supporting device which extends along the machine element for the purpose of preventing the occurrence of natural vibrations therein. The support nut (6) includes a ball race (7) which is arranged coaxially with the spindle. The ball race is provided with mutually parallel grooves (8), which extend trans­ versely to the axis (9) of the ball race and the spind­ le, and balls (10) which run in the grooves (8) and in the screw threads of the spindle. A ball-retainer (11) is arranged coaxially between the ball race and the spindle and is freely rotatable in relation to the ball race (7), and a nut-section (12) is arranged coaxially around the ball race (7). A locking element (22, 22′) is arranged between the nut-section (12) and the ball race (7), such as to enable the ball race to rotate relative to the nut-section (12) subsequent to exceed­ ing a predetermined torque between the ball race (7) and the nut-section (12).

Description: The present invention relates to a screw spindle-nut assembly in accordance with the preamble of Claim 1. When shafts, axles and, in the present case, screw spindles rotate, natural vibrations are induced in the spindle when the spindle reaches its critical speed of rotation, this critical speed being contingent on the diameter of the spindle in relation to its length and on the speed at which the spindle rotates. The smaller the diameter/length ratio, the lower the speed at which the spindle may be permitted to rotate if the spindle is to be prevented from reaching its critical speed. Consequently, the rotational speed of longer spindles must be kept low, and the speed at which the nut moves will therefore be correspondingly slow. In order to enable the speed of screw-spindles to be increased without inducing natural vibrations therein, it is usual at the present time to use support sleeves or collars, the smooth bores of which embrace the spindle and which support against the supporting device of the arrangement, the supporting device normally being a cylinder which surrounds the spindle. These support sleeves are moved with the aid of pull-rods, in a manner such that when the nut is located in one terminal position of the machine element, one support sleeve will be located between the nut and a mutually adjacent end of the cylinder and the other support sleeve will be located roughly centrally in the cylinder. The two support sleeves are connected together by means of the pull-rods. Thus, when the nut moves in the cylinder as the spindle rotates, the nut will ultimately engage the centre support sleeve so that the sleeve and nut move together during continued movement of the nut. Consequently, this support sleeve, with the pull-rods, will draw with it the end-located support sleeve which will therewith move out of the cylinder at the speed of the nut, towards the centre of said cylinder where the support sleeve remains stationary when the nut and the support sleeve coacting therewith reach the other terminal position of the cylinder. Although this known type of support means is relatively simple, it creates a large amount of noise when the arrangement is in operation and causes wear on the crests of the screw-spindle. The object of the present invention is to eliminate these drawbacks associated with known techniques, such as to prevent the occurrence of natural vibrations or oscillations in the type of machine element intended here. This object is achieved with the invention having the characterizing features set forth in the following claims. The invention will now be described in more detail with reference to an exemplifying embodiment thereof illustrated in the accompanying drawings, in which Figure 1 illustrates schematically a machine element of the kind intended here provided with support nuts in accordance with the invention; Figure 2 is a longitudinal sectional view of one half of an inventive support nut; Figure 3 is a part cross-sectional view of the support nut shown in Figure 2; and Figure 4 illustrates an alternative embodiment of the invention. In the Figures of the drawing the reference numeral 1 identifies a schematically illustrated screw-spindle which has a pitch appropriate for the purpose concerned and a part-circular thread-profile (not shown). The spindle 1 is disposed coaxially in a cylinder 2 and the ends of the spindle are journalled in the end walls of the cylinder. The spindle extends through the left-hand end-wall of the Figure 1 embodiment, and is connected in some appropriate manner to drive means operative to rotate the spindle 1. A slot 3 extends along the length of the cylinder 2. The nut driven by the spindle 1 is referenced 4 and is provided with a dogging or drive element 5 which extends through the slot 3. The drive element is intended to transmit the movement of the nut 4 in a manner to satisfy the purpose for which the arrangement is intended. An inventive support nut 6 is mounted on both sides of the nut 4, as hereinafter described in more detail. As will be seen more clearly from Figure 2, the support nut 6 has a ball race 7 which is arranged coaxially around the spindle 1 and is provided with tracks or grooves 8 which extend parallel with one another and at right angles to the axis 9 of the ball race and the spindle. The tracks 8 have a part-circular profile, corresponding to the thread profile of the spindle 1, although this is not necessary per se. In the case of the illustrated embodiment, the ball race has five tracks 8. Balls 10 (of which only two are shown in Figure 2) are active between the spindle 1 and the ball race 7. A ball retainer or cage 11 is arranged between the ball race and the spindle 1 and is freely rotatable in relation to the ball race 7 and is provided with recesses for accommodating respective balls 10. Arranged coaxially around the spindle 1 and the ball race 7 is a nut-section, generally referenced 12. For manufacturing reasons of a technical nature, the nut-section of the illustrated embodiment comprises two parts 13 and 14. In the illustrated embodiment, the ball retainer 11 is journalled at the nut-part 13 by means of journalling balls 15. The ball race 7 is pivotally journalled to the nut-part 13 by means of journalling balls 16 independently of the ball retainer 11. For manufacturing reasons of a technical nature, a mounting ring 17 is screwed into the right-hand end of the support nut of Figure 1. The mounting ring 17 has tracks for respective journalling balls 15 and 16. The mounting ring 17 is locked in position with the aid of a locking screw 18. The outer, sleeve-like part 14 of the nut-section 12 is attached to the nut-part 13 in some appropriate manner, for instance by shrink-fitting. The support nut is also provided with two guide members 19 in the form, for instances, of screws having prism-shaped heads. These guide members are intended to run in guide channels or grooves in the cylinder 2, so as to enable the nut 6 to move axially along the cylinder without rotating or twisting. The guide channels are not shown in Figure 1. The ball race 7 is provided with a smoothed-down part 20 on the outside thereof, approximately in the centre of the ball race, as will be seen from Figures 2 and 3. The nut-part 13 is provided with a hole 21 axially centrally of the smoothed-down part 20. The hole 21 accommodates a ball 22, which abuts the smoothed-down part 20. On the opposite side of the smoothed-down part 20, the ball 22 rests against a flexible cushion, for instance a rubber pad 23, which functions to urge the ball 22 against the smoothed-down part 20 with a given biassing force. The reference numeral 24 identifies rings of antifriction material, for instance Teflon®. These rings 24 slide against the inner surface of the cylinder 2 and guide the nut for movement in the cylinder. The manner of operation of the arrangement will be described briefly with reference to Figure 1. The nut 4 is located in the left-hand terminal position, with a support nut 6 located between the end surface of the nut 4 and the nearest cylinder end-wall. A further support nut 6 is located approximately in the centre of the spindle 1. When the spindle 1 is rotated by a motor (not shown) the nut 4 will move along the spindle, in the illustrated case to the right, at a speed corresponding to the screw-pitch of the spindle multiplied by the speed of spindle rotation. Because the balls of respective support nuts 6, said balls rolling in the grooves defined by the screw-thread, are accommodated in the freely pivotal ball-retainer 11 in relation to the support nut, the support nut will move (to the right) at half the speed of the nut 4, rotation of the support nut being prevented by the guide members 19. Consequently, departing from the position of the nut 4 and the support nuts 6 illustrated in Figure 1, when the spindle 1 is rotated, the nut 4 will move at the aforesaid speed, whereas the support nuts 6 will move at half this speed and the right-hand support nut will reach the right-hand end-wall of the cylinder at the same time as the nut 4 reaches said support nut. The left-hand support nut 6 will have moved forwards to a position approximately midway of the spindle during this time period. When the spindle is rotated in the opposite direction, the aforedescribed sequence of movements will be repeated, but with the nut 4 and the support nuts 6 moving to the left in Figure 1. Because the support nuts 6 are not forcibly guided in any way and because, consequently, the support nut or support nuts is/are able to move at a faster speed than the aforesaid speed, as occurs when a support nut slips, the support nut (for instance the right-hand support nut 6) will reach the end-wall of the cylinder before the nut 4 reaches its terminal position. Because the support nut 6 is now prevented from moving axially to the right (according to the illustrated embodiment), continued rotation of the spindle 1 will cause the thread-flanks of the spindle to dog the balls 10 accommodated in the ball keeper 11 and force the ball race 7 to rotate against the actual action of the ball 22 and the rubber pad 23. Thus, when the support nut 6 is stationary, the spindle 1 can be allowed to rotate until the nut 4 has reached its intended terminal position. The support nut 6 is, in this way, automatically brought to the correct position on the screw spindle 1. When the screw spindle is subsequently rotated in the other direction, in order to move the nut 4 to the left in the illustrated embodiment, the smoothed-down part 20 of the support nut 6 will be rotated away from the ball 22 and therewith prevent further rotation of the ball race 7. Although only one smoothed-down part 20 has been shown in Figure 3, it will be understood that a plurality of such smoothed-down parts can be disposed around the ball race 7. In this case, subsequent to the aforedescribed self-adjustment, the ball 22 will always be located in the proximity of or in abutment with one such smoothed-down part, thereby bringing the support nut 6 to a drive position more quickly. It is also conceivable to use more balls 22. Figure 4 illustrates another embodiment of the braking element. In the case of this embodiment, the braking element comprises a body 22′, for instance a ring which extends around the ball race 7 and which may be attached to said ball race 7 and which is in frictional engagement with the nut-part 13. Thus, the body 22′ functions to establish a firm connection between the ball race 7 and the nut-section 12, in a manner similar to that described with reference to the ball 22 and the smoothed-down part 20, up to a given, predetermined torque between the ball race 7 and the nut section 12 at which the frictional holding force between these members is exceeded and the ball race 7 rotates in relation to the nut-section 12. The body 22′ may comprise a plurality of segments disposed around the ball race 7 and may, of course, be attached to the nut-part 13 instead of to the ball race 7. In order to ensure that the support nut will move smoothly and to eliminate certain tolerance errors in the screw spindle, the ball race 7 is allowed to "tilt" to some extent in relation to the nut-section 12. This is achieved by giving the tracks in the nut-part 13 in which the journal balls 16 run a part-profile generated by the sector of a circle having a radius r whose centre point o lies on the axis 9 and in a plane which bisects the ball race 7 perpendicularly into two equal parts. It will be understood that various members of the inventive arrangement can be replaced with other, technically equivalent members. For instance, the pad 23 can be replaced with a pressure spring and, furthermore, the aforesaid supporting device may have the form of rails or the like.

Claims: A screw spindle-nut assembly with the screw spindle (1) rotatable supported in a supporting device (2) and with the nut (4) nonrotatable driven by the screw spindle (1) and provided with a drive element (5), the assembly being provided with a vibration damping support nut (6); characterized in that the support nut (6) includes a ball race which is arranged coaxially with the screw spindle (1) and which is provided with mutually parallel grooves (8) which extend transversely to the axis (9) of the ball race and the spindle, balls (10) which run in the grooves (8) and the screw heads of the spindle (1), a ball-retainer (11) which is freely rotatable in relation to the ball race (7) and arranged coaxially between said ball race and the spindle (1), a nut-section (12) coaxially arranged around the ball race (7), and means (19) making the support nut (6) able to move axially along the screw spindle (1) without rotating in relation to the supporting device (2); and in that a locking element (22, 22') is arranged between the nut-section (12) and the ball race (7) such as to enable the ball race to rotate relative to the nut-section (12) when a predetermined torque between the ball race (7) and the nut-section (12) is exceeded. A screw spindle-nut assembly according to Claim 1, characterized in that the locking element comprises one or more smoothed-down parts (20) on and around the peripheral outer surface of the ball race (7), and further comprises a ball (22) which is disposed in the nut-section (12) and which is urged resiliently (23) into engagement with the smoothed-down part (20). A screw spindle-nut assembly according to Claim 1, characterized in that the locking element comprises one or more bodies (22') which act frictionally between the ball race (7) and the nut-section (12). A screw spindle-nut assembly according to any one of the preceding claims, characterized in that the ball-retainer (11) is journalled (15) in the nut-section (12). A screw spindle-nut assembly according to any one of the preceding claims, characterized in that the ball race (7) is journalled in the nut-section (12) by means of angle-contact ball-bearings (16). A screw spindle-nut assembly according to Claim 5, characterized in that respective ball-bearing tracks in the nut-section (12) have a profile generated by a radius having a centre (0) on the axis (9) of the ball race (7) and in a plane which extends at right angles to said axis and which is located centrally between the angle-contact ball-bearings (16). A screw spindle-nut assembly according to any one of the preceding claims, characterized in that the support device is a cylinder (2) provided with a groove in which guide members (19) mounted on the support nut (6) are intended to run when the nut moves. A screw spindle-nut assembly according to Claim 2, characterized in that the nut-section has provided therein a flexible pad (23) which urges the ball (22) against the ball race (7).

Inventors: GRANBOM BO, GRANBOM, BO
Applicants: GRANBOM BO, GRANBOM, BO
Application year: 1990
Main classification: F16C29
IPC codes: B23Q5, F16H25