Introduction to Ball Screws
The main function of a ball screw system is the efficient transfer of rotational motion to linear motion, sometimes to extremely high degrees of precision, accuracy, and repeatability. Ball screws are typically differentiated by the manner in which the bearing threads were fabricated, either by a bulk deformation process called rolling, or by a precision grinding process. For a more detailed discussion about the differences between these two types ball screw systems as general principles useful for selecting between the two types, see the previous blog entry Comparing Rolled and Ground Ball Screws.
However, sometimes far more can be gleaned from specific, concrete examples of common machine applications that utilize rolled or ground ball screws. These types of illustrations provide the engineer with a few productive, real life illustrations that will serve as general operating principles to aide in the selection process between rolled and ground ball screws for new applications.
Precision X/Y Positioning Tables
Dimensional inspection tables come in a wide variety of designs, shapes, and styles and are widely used in manufacturing, inspection, and other related research applications. In general, they typically employ a precision ground ball screw system in order to achieve the level of precision required to position the tables for a given application. The ground ball screws at the heart of both large and small positioning tables are driven by servomotors. These servomotor/ground ball screw systems are able to attain extremely high levels of precision (1 microinch in certain cases) with excellent repeatability.
Motorized XY Stage
Steppers Used in Lithography Equipment
Ball screws are also present in step photolithography machines that fabricate cutting edge microscopic integrated circuits which have helped to drive huge advances in raw computing power. At a high level, photolithography is a microfabrication process that harnesses reactions caused by light exposure to thin film substrates present on silicon wafers. This process effectively etches geometric patterns onto the wafers, which forms the foundation of the integrated circuit. In order to accomplish this, a component called a stepper controls the position and location of the light exposure on the wafer and therefore must be able to operate on a microscale at extremely high degrees of precision.
Within this stepper, in a similar fashion as the X/Y positioning tables, the ball screw actuator is paired with a servomotor and a harmonic gear reducer. Only high precision ground ball screws are capable of achieving the high precision and repeatability that is required to precisely control the light exposure on the wafers to the high levels of repeatability that is required for the process. These precision ground ball screws operate smoothly, quietly, and efficiently and are the best choice for a machine application that requires a similar level of precision as the lithographic machine application.
Ball Screw Actuator
Automotive Power Steering
Finally, the most commonly encountered ball screw in everyday life is the typical automobile’s power steering system. To contrast the previous two examples, the typical automobile power steering system does not require nearly the same degree of precision and positional repeatability that the X/Y table requires. The rotary motion that the driver applies to the steering wheel rotates the ball screw, translating to the corresponding ball nut which acts as a piston within the pressurized fluid system at the heart of the of the automobile’s power steering. The ball nut engages the Pitman arm via mating teeth on its underside. This Pitman arm serves as the primary linkage from the power steering box to the center link that actually moves the wheels. From this illustration, it’s not too difficult to see how the motion of the ball nut moves the Pitman arm, which controls the motion of the center linkage and the motion of the automobile’s front wheels.
The ball nut engages the Pitman arm via mating teeth on its underside.
For an application like automotive power steering, rolled ball screws are feasible and far more economical due to the fact that the main function of the ball screw is simply to convert rotatory motion into linear motion in the power steering piston. Because of this, a rolled ball screw nut is more than adequate to achieve the motion translation and lower degree of positioning required for this application.
Conclusion
Hopefully these examples help to provide context for the types of applications that typically use ground and rolled ball screws. With a firm grasp of the types of applications that may require a precision ground ball screw, the engineer should be far better equipped to select and size the proper ball screw required in order to satisfy a wide range of mechanical demands.
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