A SCREW AND NUT DEVICE EQUIPPED WITH A ROTATION BRAKE

Abstract: In a device for bringing about rotational braking in a screw and nut device incorporating a nut, which is rotatable and displaceable relativ to a threaded bar (2), and a helical spring (3) enclosing the bar, which spring at braking is clamped against the bar, for effecting a big braking moment and a small wear in the thread of the bar, there is arranged a helical element (11) with a loose fit in the thread groove of the bar, which element extends radially outside the thread flank tip (9). The helical spring is furthermore arranged with radial pre-tension on the thread flank tip between the winding turns of the helical element.

Description: The invention refers to a device of the type defined in the preamble of claim 1. Screw and nut devices with low internal friction, e.g. so called ball nut mechanisms, wherein a row of balls are arranged in the thread grooves between nut and screw, have the property of not being irreversible, which in certain cases may be an inconvenience. E.g. in a lifting device, where the load is lifted by such a screw and nut device, this property may cause the load to lower itself when no external driving torque acts upon the device. This may be a safety risk and different braking members, which prevent unintentional movement are earlier known. A helical spring may e.g. enclose the screw and be clamped against it for braking purpose, when the device is loaded in a direction opposite to the driving direction and no driving torque is applied (SE-A-8505997-0). In this known device the braking spring is located in the thread groove of the screw and it exerts a friction force in these during braking. This implies certain drawbacks. Then wear will occur in the thread, which will reduce the service life of the screw. The friction force furthermore will occur at the smallest distance from the rotation axis, when the contact takes place in the bottom of the thread, which gives the smallest possible breaking moment for a given frictional force. The purpose of the present invention is to provide a device of the type defined in the preamble, in which the biggest possible braking moment is obtained when exerting the smallest possible wear in the thread groove. This is obtained according to the invention in that the device has the features defined in claim 1. Such a device can be manufactured in a simple manner from nonexpensive parts and by simple operations, and the device is economic and reliable. Any lubricant layers in the thread groove are not destroyed by the braking device, which can slide against a surface upon the screw where the friction is big and is not reduced by presence of lubricant. The invention hereinafter will be further described with reference to the accompanying drawing, in which Fig. 1 and Fig. 2 show a longitudinal section and a cross section through a part of a device according to an embodiment of the invention. The device incorporates an element 1, which is preferably designed as a part of a cylinder and which is connected to or forms part of a not shown nut, which, in a manner known per se, is rotatable and displaceable on a threaded bar 2. The bar is enclosed by a helical spring 3 having an end portion 4 extending radially outwards from the bar 2 and projects into a space 5 between a surface 6 on the element 1 connected to the nut and another surface 7 on an element 8, which connects to a driving device, not further shown, by means of which the nut can be rotated thus that is moves along the bar 2. The driving device rotates during operation the element 8 thus that the surface 7 is pressed against the end portion 4 of the helical spring, and this is then pressed against the surface 6 thus that the element 1 and the nut are brought to rotate and move along the bar 2. The helical spring 3 thereby rotates freely about the bar 2, as the forces acting upon the helical spring tend to increase the circumference of the spring and thereby to produce a play between the spring and the bar. If the driving device does not exert any torque and element 1 instead tends to rotate due to the nut being axially loaded and is therefore exerted to a torque via the thread contact, then surface 6 is pressed against the end portion 4 of the helical spring, whereby the forces upon the helical spring become such that the spring tends to reduce its circumference and to press harder against the bar 2, thus that the spring is braked by friction on the bar and prevents relative rotation between the bar and the nut. The device thus is irreversible. A double acting function, i.e. the same function as described above but acting in both directions, is obtained if also the other end of the spring is provided with a portion projecting into a space, which is located diametrically to the first end portion between surfaces connected to elements 1 and 8, which are then preferably made as two half cylinders, such as shown in the drawing. According to the invention the helical spring 3 is arranged on the flank tip 9 of the thread of the bar 2. This means that the very thread groove is left unengaged by the spring, whereby wear caused by friction is avoided. The flank tip furthermore can be made thus that it gives a desired friction coefficient together with the spring without dependency of the fact that the thread groove must have such a surface that it is appropriate as a race track for rolling bodies or as a sliding surface against a nut thread. A further advantage of the helical spring being arranged on the flank tip is that this arrangement gives the biggest possible distance between the rotational axis 10 and the friction surface, which again gives the biggest possible braking torque when the rotation is braked. The helical spring is arranged with a radial pre­ tension on the thread flank tip in order to ascertain that an initial braking force always shall act in the contact spring -­ flank tip. Thereby it is ascertained that the force from the surface 6 acting upon the end portion 4 of the spring, when the nut tends to rotate due to the fact that it carries an external axial load, will become so big that the spring is contracted and is clamped against the bar 2. When the device is driven by a torque from the driving device via the element 8 and the surface 7, then the spring strives to expand, whereby the pre-tension is reduced thus that the braking force can be neglected in comparison with other occuring forces. In order to prevent the helical spring from being displaced thus that it is located in the thread groove on the bar 2 a helical element 11 according to the invention is arranged with a loose fit in the thread groove of the bar and extends radially outside the thread flank tip 9. The helical spring 3 thereby extends between the winding turns of the helical element 11, i.e. the spring 3 and the element 11 are concentric and can be considered as screwed into each other. The element 11 can be prevented from being screwed out of its position by at least one of its ends being provided with a radially projecting portion 12, which projects into the above-­ mentioned space 5 like the portion 4 of the spring 3. Due the fact that the element 11 has a loose fit in the thread groove it will never become clamped against this, but it only acts as a positioning member for the spring 3 and a spacer for its winding turns. The element 11 is preferably a helical spring shaped as a wire having a section corresponding to the shape of the thread groove in the bar 2. In a ball nut mechanism this form is circular, whereby the wire can have a circular section. The thread flank tip 9 is preferably cylindric, whereby the helical spring 3 has an internal surface with the corresponding cylindric shape. Such a shape is simple to produce and to machine. The bar may e.g. be ground in a centreless grinding machine and the helical spring may be made from a wire having a rectangular section. Other embodiments of the invention than the one described are possible within the scope of the claims. The shape of the thread groove and of the thread flank tip can be varied as well as the section of the spring 3 and of the element 11. Screw and nut can cooperate by means of rolling bodies or by direct thread flank contact.

Claims: 1. A device for bringing about rotational braking in a screw and nut device incorporating a nut, which is rotatable and displaceable relativ to a threaded bar (2), and a helical spring (3) enclosing the bar, which spring, when the device is axially loaded without being subjected to an external driving torque, is clamped against the bar and brakes the relative rotation between the nut and the bar,characterized thereby, that a helical element (11) is arranged with a loose fit in the thread groove of the bar and extends radially outside the thread flank tip (9), and that the helical spring (3) is arranged with radial pre-tension of the thread flank tip (9) between the winding turns of the helical element (11). 2. A device as claimed in claim 1,characterized thereby, that the helical element (11) is a second helical spring, which is concentric to and screwed into the first-mentioned helical spring (3). 3. A device as claimed in claim 1 or 2,characterized thereby, that the thread flank tip (9) is cylindric and that the inner surface of the first-mentioned helical spring (3) has a correspondingly cylindric shape. 4. A device as claimed in claim 3,characterized thereby, that the helical spring (3) is produced from a wire having a rectangular section.

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