In such screw spindle drives that are known (for example, from German Auslegeschrift No. 1,625,526 or German Offenlegungsschrift No. 2,031,390), the pitch of the thread grooves of the screw spindle and nut is greater in each case than the diameter of the balls. This has the disadvantage that in the case of pitches which are to be smaller than a certain ball diameter, balls with a smaller diameter must also be employed in each case. Balls with a smaller diameter, however, have a lower load-bearing capacity because of their higher contact pressure.
Many screw spindle drives have a so-called ball return means by which the balls are carried back again across one or more thread grooves into the preceding thread grooves. Such ball return means, however, increase the external diameter of the nut. In the case of screw spindle drives which are intended only for a limited mutual axial movement, a ball return means can be dispensed with. However, precautions must then be taken so that the balls in a ring thereof do not move apart. Precautionary arrangements of this kind are, for example, a ball cage or retainer. If such a ball cage is absent, a stop projecting into the thread grooves of the nut must be provided at both ends of the latter the stop preventing the balls leaving the thread grooves of the nut. If, however, the balls are not held together by a ball cage, one or more balls come into abutment against the stop prematurely and can no longer roll in the thread grooves on further rotation of the nut or spindle. The easy mutual movement of the screw spindle and nut is thereby hampered and increased wear moreover occurs in consequence of sliding friction. Ball cages have the disadvantage of occasioning additional manufacturing costs, having a braking action in consequence of the friction of the balls on the ball cage and, moreover, being also subjected to wear because of this friction.
In the case of screw spindle drives having ball return means, the nut has an insert piece having a return passage open towards the spindle and by which the balls are returned to the preceding thread groove at any given time. In such case, the nut has a relatively large axial length and also a relatively large diameter, because this diameter is determined essentially by the height of the return passage. In the known screw spindle drives, the height of the return passage corresponds approximately to one and a half times the ball diameter, since the depth of the thread grooves in the spindle is only a little smaller than the radius of the balls. So that the balls can be lifted at any given time out of a thread groove and over the rib present between the two thread grooves, the return passage must have the height mentioned. The large pitch of the thread grooves furthermore has the disadvantage that the return passage, in which the balls are returned by one pitch into the preceding thread groove within a comparatively small circumferential angle, must be inclined to a comparatively great extent in the opposite direction to the thread grooves. Since no transmission of force in the axial direction can take place between the nut and screw spindle in the region of the return passage, it is endeavoured to accommodate this return passage in a circumferential angle which is as small as possible. The marked pitch of the return passage has the disadvantage that a marked deflection of the balls takes place in this passage, which has associated with it a correspondingly heavy wear and also the development of noise. Particularly because of their large overall axial length, a plurality of nuts cannot be arranged in series in the known screw spindle drives, since this would result in too great a total length. Such an arrangement of a plurality of nuts in series may be desirable, however, in order to achieve freedom from play by mutual pre-stressing of the nuts and also to multiply the load-bearing capacity of the nuts.
The problem underlying the invention is to avoid the above disadvantages and provide a screw spindle drive which in conjunction with a large ball diameter has a small pitch and consequently a large load-bearing capacity and the nut of which has a smaller over-all size and which is cheap to produce.
According to the invention, this is achieved in that the pitch of the thread grooves of the screw spindle and nut is smaller than the diameter of the balls.
For a certain ball diameter, the novel screw spindle drive may have any desired pitch which is smaller than the ball diameter.
As a result of this construction, optimum utilization of space for the surfaces rolling on one another is achieved. Neither on the spindle nor on the nut are any parts not serving as rolling surfaces present between the individual thread grooves, so that an optimum over-all length can be obtained for the nut. In screw spindle drives having a ball return means the over-all radial dimension of the nut also becomes altogether smaller. In fact, in the screw spindle drive according to the invention, the depth of the thread grooves is substantially smaller than the radius of the balls. Consequently, the depth of the return passage needs to be only slightly larger than the ball diameter, which again ultimately results advantageously in a small external diameter of the nut. The balls, which are relatively large in diameter in comparison with the pitch, have a lower contact pressure than smaller balls and consequently give the drive a higher load-bearing capacity with a lower degree of wear than would be present in a screw spindle drive of known type with the same pitch, but with smaller balls. Since the thread grooves in the new screw spindle drive are less deep than in known screw spindle drives, less metal-removal work by cutting is necessary for producing them, so that the new drive is also cheaper to manufacture. In this connection, the larger ball diameter likewise has an advantageous effect, because the thread grooves, the radius of which corresponds to that of the balls, also have a larger radius and are consequently simpler to grind than thread grooves of smaller radius such as would be necessary for balls having smaller diameters. In particular because of the smaller axial length of the nut, as many nuts as desired can be arranged in series by the unit part assembly technique, the total length of this arrangement being kept within permissible limits. Basically, the unit part assembly technique makes possible the mass production of separate nuts of the same kind in large numbers. One or more nuts are arranged in series according to the desired load-bearing capacity of the screw spindle drive. If it is desired to achieve freedom from play, it is sufficient if this unit, by which the desired load-bearing capacity in one direction is achieved, is opposed by only a single nut for pre-stressing. In known drives, on the other hand, the axial length of the prestressing nut also always corresponds to the axial length of the actual load-bearing nut. Furthermore, the new screw spindle drive also operates with particularly low wear and noise. In fact, if the pitch is smaller than the thread diameter, a smaller pitch is also obtained in the return passage and, consequently, a smaller deflection of the balls than in previous drives.
In a screw spindle drive having a ball return means, there is provided in the nut an insert piece having a return passage open towards the spindle and by which the balls are returned at any given time to the preceding thread groove. In such screw spindle drives, the axial length of the nut advantageously corresponds approximately to twice the ball diameter. In this way, a small over-all length of the nut is obtained.
In order to reduce this over-all axial length still further, it is advantageous if the axial length of the nut is only slightly greater than the sum of one pitch of the thread grooves and one ball diameter. The slightly greater length of the nut compared with the said sum is due to the special design of the insert piece with the return passage.
According to the invention, screw spindle drives without a ball return means are so designed that the balls are in mutual contact and are arranged at any given time in the form of a closed ring of balls, the last ball in one thread groove being in contact with the first ball in the adjacent thread groove.
In addition to the above-mentioned advantages, further advantages are also obtained. As a result of the elimination of ball return means, the over-all radial dimension of the nut also becomes altogether smaller. Owing to the fact that the last ball in one thread groove is in contact with the first ball in the adjacent thread groove, the balls cannot move apart. A ball cage can therefore be dispensed with and the balls can be arranged lying closely against one another, which likewise contributes towards an increase in the load-bearing capacity in consequence of the increase in the number of balls. Moreover, there is no danger of individual balls running prematurely against the end stops, so that the ease of action of the drive is ensured at all times, with the avoidance of unnecessary wear.
In a double or multi-thread screw spindle drive, the problem underlying the invention is solved in that in the case of a double or multi-thread screw spindle drive without ball return means the lead of the thread grooves of the screw spindle and nut is smaller than twice, or a multiple of, the diameter of the balls and the balls of two adjacent rows interengage alternately in contact with each other. In this way, substantially the same advantages are obtained in double or multi-thread screw spindle drives as in a single-thread screw spindle drive. As a result of the interengagement of the balls of two adjacent thread grooves, moving apart of the balls is prevented.
The invention is described in detail hereinafter with reference to several embodiments shown in the drawings.