The present invention relates to a ball screw which converts rotation into linear motion and, more particularly, to improvements in a ball screw used for heavy-load applications.
According to a conventional ball screw, the initial contact angle and the maximum contact angle of ball-screw threads to be formed on a screw shaft or a ball nut are respectively set to about 45 degree and about 65 degree in consideration of balanced workability, operability, and load-carrying capacity.
For example, in the case of a tube-circulated type ball screw as shown in FIG. 1, if a ball screw has ball circulation paths for a plurality of circuits, the attachment positions for the circulation tube 50 of each circulation paths are concentrated into one plane in order to reduce the number of machining processes. More specifically, a plurality of circulation tubes 50a, 50b, and 50c are usually designed so as to be axially arranged into a line and also arranged in the same phase with each other in the circumferential direction as shown in FIG. 1. Note that FIG. 1 shows the ball screw having three circulation circuits.
In a case where a ball screw is designed, a slightly large safety factor is set, so that a ball screw having a sufficient shaft diameter to satisfy working conditions is usually selected. At this time, if the shaft diameter of the ball screw cannot be increased, usually the number of circuits formed in a ball nut 51 or the number of balls is increased, or the lead of the thread of the ball screw is increased to thereby increase the diameter of a ball.
In the meanwhile, in the prior art, the selection of a suitable ball screw is made in consideration of load conditions on the basis of an assumption that load is uniformly exerted on all-balls rotating in the space between a screw shaft 52 and the ball nut 51. Namely, in the prior art, the suitable ball screw is selected by calculating a contact pressure on the basis of an averaged load which is obtained by dividing the load by the number of effective balls constrained to the ball nut 51, and comparing the thus-calculated contact pressure with a database concerning the function and life of the ball screw obtained as a result of tests.
In practice, however, the load is not uniformly exerted on all the so-called effective balls rotating in the space between the screw shaft 52 and the ball nut 51. Axial dispersion arise in the distribution of loads on the inside of the ball nut 51 owing to the elastic deformation of the screw shaft 52 and the ball nut 51 caused when they are subjected to the load. For example, similar to a case where heavy-weight material is machined on a work table supported by a ball screw, in a case where axial load Fa is exerted on the ball nut 51 of the ball screw and another axial load Fa' is exerted on the screw shaft 52 in the directions as shown in FIG. 2, the axial distribution of elastic displacements of the ball nut 51 and the screw shaft 52 in respective ball-screw threads 51a, 52a are represented by arrows shown in FIG. 2. It is understood that stress concentrates on the points of contact between the balls and the opposite ends of the ball nut 51 in accordance with the amount of elastic displacement of the screw shaft 52 and the ball nut 51.
The balls rotating in the respective circulation paths are sequentially scooped on and fed to a circulation tube 50 from the ball-screw threads 51a, 51b and are circulated. The balls are in contact with only the threads of the screw shaft 52 which are opposite to the ball nut 51. For this reason, dispersions also arise in the distribution of loads in the circumferential direction, thereby imposing heavy loads on a part of the effective balls. FIG. 3 shows the circulation of the balls of each circuit within the ball nut 51. As shown in FIG. 4, as a result of feeding the balls 1 to the circulation tube 50 from the ball-screw threads, the number of balls existing in the circumferential range of the ball nut 51 which has angle .beta. is comparatively reduced when the ball nut 51 is viewed in the direction of the axis. Eventually, the load exerted on the balls existing in the range having angle .beta. is comparatively increased.
As described above, as a result of the such concentrations of stress on the contact points between the balls and the opposite end portions of the ball nut 51 and the such increase in the load exerted on the balls which exist in the specific range of the ball circulation path, there are problems that the surface of the ball-screw threads 51a, 52a of the ball nut 51 and the screw shaft 52 is flaked at early time or abnormally abraded if the designed safety factor of the conventional ball screw is set on a small value.
For heavy load applications, if the safety factor is set to a large value, the size of the ball screw becomes large and fails to meet desired specifications or is costly.