GB589596A - Improvements in or relating to screw-and-nut actuating mechanism particularly adapted for use in machine tool transmissions - Google Patents

Abstract

589,596. Screw-and-nut gearing. STEVENS, A. H. (Cincinnati Milling Machine Co.). May 5, 1943, No. 7122. [Class 80 (iii)] To reduce friction in precision screw-and-nut mechanism, for a milling or other machine tool, a nut 16, Fig. 2 (and/or screw), has one or more closed-ended helical grooves 23, 24 on each of its opposed helical faces, which are supplied with oil under pressure through independent ducts 34, 35, 29 and 33, 36, 31 respectively, which oil passes out through the clearance space between the opposed working faces of the screw 11 and nut 16, through wide clearances 39, 40 at the crests and cores of the threads to escape at the ends of the nut to a supply reservoir from which it is pumped at constant flow-rate to each supply duct. A pressure-gradient, determined by the hydraulic flow-resistance, exists in the working clearancespace and this, together with the pressure in the appropriate groove, supports the axial load on the screw. Variations in load cause variation in clearance, which in turn varies the flow-resistance and thus the pressure in the grooves, increase in pressure at one face being accompanied by reduction at the opposite face, so that, under load, the screw-thread automatically takes up an intermediate position in the working clearance in which the opposing pressure and the load balance, with the screw-thread out of metallic contact with the faces of the nut-thread. Initial adjustment of axial clearance is preferably provided by making the nut in two relatively rotatable halves with the split in a radial plane, the halves being axially spaced either by a replaceable ground shim, Fig. 4 (not shown), or, as shown in Fig. 1, the nut-halves 45, 46 are driven through a helical gear-pair 61, 62 with gears of opposite' helixangle so that axial adjustment of the gear-pair, by screwed end-collars 73, turns the nut-halves 45, 46 relatively to each other. In this case there are oil-supply grooves only in the one helical working-face of each nut-half, feed being through annular spaces 94, 95 and ducts 90, 91 respectively, and return from the ends of the nut-halves being through ducts 39<1>, 40<1>, 41<1>. The independent supply of pressure-oil to the grooves 23, 24 in the opposed helical faces of the nut may be by two separate pumps or, as shown in Fig. 1, a pressure-conduit 78, leading from a single pump, feeds the groovesupply ducts 90, 91 through an automatic flowproportioning device comprising a pressureresponsive drift-block 86, which, by moving towards or away from the entries of orifices 88, 89, leading to the thread-supply ducts 90, 91, under changes in pressure in the ducts, is capable of increasing or reducing the flow-resistance in the supply to one duct and simultaneously reducing or increasing the resistance to the other duct. In operation, an increase in axial load on the screw 11 will cause reduction in the clearance on the loaded thread-face and increase in clearance on the opposite thread-face. The consequent pressure rise in that duct 90, 91, leading to the thread-clearance of greater resistance, causes the block 86 to move away from the orifice 88, 89 of that duct, thus reducing its orifice flow-resistance and simultaneously increasing that of the other duct, so that the overall flow-resistance of the path through the clearance at each of the thread-faces, and thus the rate of flow through each path, are kept constant. The part 10 traversed by the screw is a support for a cutting or other tool in which the screw 11 is non-rotatably mounted and the screw is prevented from sagging by end supports 19.