This invention relates to an apparatus for converting linear motion into rotary motion, and vice versa. More particularly, it relates to a helical ball gearing for translating axial or linear motion into rotary motion and vice versa, for use in mechanical components and machinery.
As is known in the art, a helical ball gearing for converting axial motion to rotary motion of a machine component, e.g. a rotary pillar or support for driving swivelling vehicle doors, has a non-rotatable piston rod, which is axially displaceable, for example with the help of a pneumatic drive, and which is locked against rotation, e.g. by keyways. When the piston rod is linearly displaced in the axial direction, a rotor performs a corresponding rotary motion, because of balls disposed in helical grooves of the rotor and piston rod. In existing devices of this kind, the balls are supported with the help of a ball cage, which is basically a sleeve having bores corresponding to and accommodating the balls.
For reliable operation of this type of ball gearing, the bores provided in the ball cage for guiding the balls must have exactly the same lead and pitch as the helical grooves of the rotor. Even minor deviations cause distortions impairing the smooth rolling of the balls in the helical grooves.
Furthermore, the ball cage may slightly change its position. Changes in position can be attributed to an irregular non-positive lock in the gearing and the effect of the force of gravity acting on the balls or the ball cage. As a result, before the rotor reaches its end position, the ball cage may run up against an obstacle. In this case the helical ball drive would then be more of a sliding helical drive, as the balls could no longer spin freely.
The problems arising from a change in position of the ball cage are increased when the ball gearing is operating in a vertical position. In this case, due to the force of gravity, the balls and the ball cage are subjected to strong impulses, e.g., jarring forces, in the direction of movement of the piston rod, e.g., during starting and stopping. Therefore, the proper and reliable operation of the known ball gearing device previously described cannot be assured.
Special problems are encountered with swivelling doors, e.g. on vehicles, which are driven by rotary pillars or supports. The rotary motion of the rotary pillars or supports is often produced via a ball gearing device, particularly when such rotary pillars or supports must operate for long periods. It has been found that the force required for the opening and closing movement can be transmitted into the rotor without the swivelling door showing any tendency to jam. However, if the door has to be opened by hand, in which case the resulting force acts on the rotor, a substantial amount of force has to be applied to open the door. The force required to open the door, in this case, will exceed the capability of some persons, e.g. passengers, thereby preventing them from opening the door.
Accordingly, it is an object of the present invention to provide a helical ball gearing for converting linear motion into rotary motion, having consistent and smooth operation, even after long periods of operation.
It is also an object of the present invention to provide a helical ball gearing having an improved means for guiding the balls, thereby allowing the helical ball gearing to operate reliably in any orientation.