IRREVERSIBLE SCREW AND NUT DRIVING DEVICE

Abstract: In a screw and nut driving device with low internal friction, e.g. a ball nut mechanism, irreversibility is effected in a simple and efficient manner when the nut (2) is rotatably driven and enclosed by an annular space in which a helical spring (7) is arranged in such a manner that its ends (8, 9) are acted upon by the driving force from the external driving source, thus that the spring can rotate freely in the space. When the external driving source is disconnected and the nut, due to axial load is subjected to a torque, the ends of the spring are acted upon thus that the spring is disformed to friction locking against a non-rotating surface, whereby the nut is prevented from rotation.

Description: The invention refers to a device of the type defined in the preamble of claim 1. Screw and nut driving devices, which incorporate ball or roller nuts, i.e. where balls or rollers recirculate in grooves constituted by the nut and screw threads, have so small friction losses that they are not irreversible. Small friction losses are of course desirable from energy aspects, but non-irreversible devices are inappropriate from safety aspects in certain connections e.g. in lifting devices. Different blocking and braking arrangements have earlier been proposed for solving this problem. They generally have been arranged in connection to the driving motor and they have locked or braked the output shaft of the motor, often in dependency of the energy supply to the motor, which is generally arranged for driving the screw, thus that the nut without rotation is displaced along the screw, when this rotates. Rotation of the screw is however nonappropriate at rapid approach, as a long and slender screw ought to rotate at a lower speed than its critical speed. Locking and braking means between the motor and the screw increase the dimension of the device. The purpose of the present invention is to provide a screw and nut driving device of the type defined in the preamble, with aid of which the approach can be carried out at a high speed and irreversibility can be brought about with simple means in direct connection to the nut. This is obtained according to the invention in that the device is made with the features defined in claim 1. Such a device can be made very compact, and a conventional driving system can be used for rotating the nut. Irreversibility in both rotation directions can easily be obtained with simple and inexpensive means. The invention hereinafter will be further described with reference to the accompanying drawing, in which Figure 1 shows a longitudinal section and Figures 2 and 3 show cross-sections along lines II-II and III-III in Figure 1 of a device according to an embodiment of the invention. The device incorporates a screw in the form of a threaded rod 1, and a nut 2 enclosing the screw. The nut and the screw cooperate via a number of balls, which recirculate between the screw and the nut in grooves constituted by the screw and nut threads and a recirculation portion 3 arranged on the outside of the nut. The nut is arranged in a not further shown element, e.g. a load carrier in a lifting device, which element is displaceable along the screw and is driven by the relative rotation of the nut and screw. The nut is supported in the element in two ball bearings 4,5, the outer race rings of which are arranged in seats in the element and the inner race rings of which enclose sleeves in which the nut is arranged. A sleeve 6 is connected to a not shown driving member, e.g. an electric motor arranged upon the element, which is displaceable along the screw, with which driving member the sleeve and thereby the nut can be rotated about the non-rotating screw in the non-rotating element, which at rotation of the nut is displaced along the screw. A helical spring 7 is arranged in an annular space between the nut and the element. The ends 8,9 of the spring are so located that they, when the nut is driven by an external driving source via the sleeve 6, are acted upon by the driving force thus that the spring can rotate freely in the space and that, when the nut due to axial load on the element is subjected to a torque, without being driven by the external driving source are acted upon by the nut thus that the spring is disformed to frictional locking against a surface connected to the element and delimiting the space for the spring, whereby the nut is prevented from rotation relative to the element. In the embodiment shown this is obtained in that the sleeve 6 is rotatably supported upon a further sleeve 10 and that the sleeves have one mainly semicircular portion 11,12 each, which portions project into the space housing the helical spring, whereby each one of the helical spring ends are bent into one slot 13,14 between the different ends on the portions 11,12, thus that the ends of the spring have a certain play in the circumferential direction of the slots. The sleeve 10 is rigidly fitted to the nut 2, and when the sleeve 6 is rotated by an external driving source, the portion 11 will be pressed against one 8,9 of the spring ends, thus that the spring tends to reduce its circumference, whereby it can rotate freely in the space. The spring end in is then pressed against an end surface on the portion 12, thus that the sleeve 10 and thereby the nut 2 are brought to rotate. If, however the sleeve 6 is not acted upon by any external driving source but is disconnected, and the nut 2 instead exerts a turning moment upon the sleeve 10, e.g. by being loaded in the longitudinal direction of the screw 1 by a load from a load carrier, the portion 12 will be pressed against one of the spring ends 8,9 thus that the spring tends to increase its circumference, whereby it contacts a surface delimiting the space and being connected to the element. Thereby the device will be frictionally locked, whereby the nut is prevented from rotating relative to the screw 1. The device is thus irreversible although it incorporates a ball nut mechanism having low friction against the screw. The surface against which the spring 7 engages at looking forms preferably a cylindrical wall in a bore in a sleeve 15, which is rotatably arranged in a seat in the element, which is displaceable along the screw 1. The rotation of the sleeve 15 is locked by an excess load catch in the form of a spring loaded ball 16, which is pushed into one of several recesses 17 about the circumference of the sleeve 15. At a certain torque the ball is pressed out of the recess and the sleeve can be rotated, whereby it is ascertained that the device is not subjected to too large loads. The moment carrying ability must of course fullfil all safety requirements. In order to maintain the connection between the nut and the screw even if the recirculating balls should fall out of the track, a safety nut 18 is arranged in connection to the nut 2. The safety nut is provided with thread, which cooperates directly with the thread of the screw 1 and is in normal operation non-loaded. Due to the location of the spring 7 around the nut 2 it is achieved a compact and space saving arrangement. The rather big diameter of the spring means that the friction force between spring and sleeve 15 gives a big braking moment.

Claims: 1. A screw and nut driving device incorporating a screw (1) and a nut (2) enclosing the screw and cooperating therewith, said nut being arranged in an element displaceable along the screw and driven by the relative rotation between nut and screw,characterized therein, that the nut is supported in the element and is rotatably driven, whereby the device incorporates a helical spring (7) provided in an annular space between the nut and the element, the ends (8,9) of which spring being acted upon by the driving power, when the nut is driven by an external driving source, thus that the spring can rotate freely in the space and when the nut due to axial load is subjected to a torque by the element without being driven by an external driving source being acted upon by the nut, thus that the spring is disformed to friction locking against a surface associated with the element and delimiting the space, whereby the nut is prevented from rotating relative to the element. 2. A driving device as claimed in claim 1,characterized therein, that said surface cooperating with the helical spring (7) is arranged upon a member (15) rotatably arranged in the element, which is displaceable along the screw (1), and that a rotation catch (16,17) which is released at a predetermined torque is arranged between the member and the element.

Inventors: NILSSON, SVEN WALTER
Applicants: SKF NOVA AB
Application year: 1987
Main classification: F16H25
IPC codes: F16H25, F16D51