A ball screw is a linear mechanical actuator that translates rotational motion into linear motion with minimal friction. A threaded shaft provides a helical raceway for ball bearings that act as precision screws. As well as being able to apply or withstand high thrust loads, they can do so with minimum internal friction. They are made to cover tolerances and are therefore suitable for use in situations where high precision is required. The ball assembly acts as a nut while the threaded shaft is a screw. Unlike conventional leadscrews, ballscrews tend to be rather large, because of the need to have mechanisms to circulate the ball.
Another form of linear actuator based on a rotating rod is a nonwoven ball, a.k.a. "rolling ring drive". In this design, three (or more) roller bearings are arranged symmetrically on the housing surrounding the smooth actuator rod or shaft (without screw). The bearing is set at an angle to the rod, and this angle determines the direction and rate of linear motion per rod revolution. The advantage of this design over conventional ballscrew or leadscrew is the practical removal of backlash and loading caused by preload nuts.
Video Ball screw
Apps
Ball screws are used in aircraft and missiles to move the control surfaces, especially for electric fly by wire, and in car power steering to translate rotary motion from the electric motor to axial movement of the steering rack. They are also used in machine tools, robots, and precision assembly equipment. High-precision ball screws are used in steppers for semiconductor manufacturing. They are also incorporated into the actuator mechanisms of computer-controlled self-pleasure devices.
Maps Ball screw
History
The ball screw was created by Rudolph G. Boehm of Sugar Land, Texas. He was awarded in 1929 US Patent 1,704,031 entitled "Antifriction nut".
Initial precision screwshaft is produced by starting with a low precision screwshaft, and then pounding the shaft with multiple spring trims. By rearranging and reversing the nut lap, the elongated error of the nut and axis is averaged. Then, the pitch of the recurrent pitch is measured by the standard distance. A similar process is sometimes used today to produce standard reference screw axes, or master-making screw shafts.
The ball screw is used to extend the Deployable Tower Assembly (DTA) structure on the James Webb Space Telescope
Description and operation
To maintain its accuracy and ensure long service life, care should be taken to avoid contamination with dirt and abrasive particles. This can be achieved by using rubber or leather bellows to fully or partially attach the work surface. Another solution is to use positive pressure from filtered air when used in semi-closed or open enclosures.
While reducing friction, ball screws can operate with multiple preload, effectively eliminating backlash (slop) between input (rotation) and output (Linear motion). This feature is important when used in computer controlled motion control systems, for example, CNC machine tools and high precision motion applications (eg, wire bonds).
Disadvantages
Depending on the angle of their lead, the ball screw can be moved back because of low internal friction (ie, the screw shaft can be pushed linearly to rotate the ball nut). They are usually undesirable for hand-feeding machine tools, because of the stiffness of servo motors needed to keep the cutter from reaching the job and feeding itself, ie, where the cutter and workpiece exceed the optimum feed rate and effectively jam or crash together , damaging cutter and workpiece. Cost is also a major factor because Acme screws are cheaper to produce.
Benefits
Low friction on the ball screws results in high mechanical efficiency compared to alternatives. Typical ball screws may be 90 percent efficient, compared to 20 to 25 percent Acme tin screw efficiency of the same size. The lack of shear friction between the nuts and screws is suitable to extend the life of screw assemblies (especially in non-backlash systems), reducing downtime for component maintenance and replacement, while also decreasing demand for lubrication. These, combined with their overall performance benefits and reduced power requirements, can offset the initial cost of using a ball screw.
Spherical screws can also reduce or eliminate common reactions to lead screw and nut combinations. The ball may have been loaded so there is no "wobble" between the ball screw and the ball nut. This is highly desirable in applications where the load on the screw varies rapidly, such as machining equipment.
Producing
Screw shaft balls can be made by rolling, resulting in less precise, but inexpensive and mechanically efficient products. Screws of rolled balls have a precision position of several thousandth of an inch per foot.
Accuracy
A high-precision screw shaft is usually appropriate for one-thousandth of an inch per foot (830 nanometers per centimeter) or better. They have historically been worked out for rough shapes, cases hardened and then milled. A three-step process is required because high-temperature machining distorts the workpiece. Hard whirling is the latest precision machining technique (2008) that minimizes heating work, and can produce precision screws from case-hardened bar rods.
The screw screw quality of the instrument is usually appropriate for 250 nanometers per centimeter. They are manufactured on precision milling machines with optical distance measuring devices and special tools. A similar machine is used to produce optical lenses and mirrors. The instrument screw shaft is generally made of Invar, to prevent the temperature from changing the tolerance too much.
System returns
The circulating ball moves in the form of threaded screw and nut, and the ball is recirculated through various types of return mechanism. If the ball of peanuts does not have a return mechanism the ball will fall from the tip of the nut bulb when they reach the tip of the nut. For this reason several different recirculation methods have been developed.
External ballnuts use a patterned tube that picks up the ball from the raceway by using a small thumb. Balls travel inside the tube and then replaced back on the yarn racing track.
Ballnut the internal button using a button style or a button that returns the ball that allows the ball to exit the raceway path and move one thread then re-enter the race track.
A ballnut produces a back ending cap at the tip of a nut ball. Cover is done to get the ball from the tip of the nut and direct it to the boring pit transversely below the ballnut. A complementary cover on the other side of the bean directs the ball back to the raceway.
Profile thread
To get the proper ball rolling action, like on a standard ball bearing, it is necessary that, when loaded in one direction, the ball makes contact at one point with the nut, and one point with the screw. In practice, most ball screws are designed to be easily loaded, so there is at least a small charge on the ball at four points, two contacts with the nut and two contacts with the screw. This is done by using a screw profile that has slightly larger radius than a sphere, small difference of radius (eg a simple V thread with a flat face does not fit) so that the elastic deformation around the contact point allows a small, non contact area -nol must be obtained, like other rolling element pads. For this purpose, the thread is usually done as a "gothic arch" profile. If a simple semi-circular thread profile is used, the contact will only be at two points, on the outside and inside, which will not withstand axial load.
Preloading
To eliminate backlash and obtain optimum rigidity and wear characteristics for a particular application, the amount of controlled preload is usually applied. This is done in some cases by machining the components in such a way that the balls are "tight" fit when assembled, but this gives poor control over the preload, and can not be adjusted to allow for wear. It is more common to design the ball nut effectively two separate mechanically coupled nuts, adjusting well with one nut rotation to the other, thus creating relative axial displacement, or by holding both bolts together axially and rotating one against the etc., so the set of balls is axially shifted to make preload.
Equation
Di mana adalah torsi yang diterapkan ke sekrup atau mur, adalah gaya linier yang diterapkan, adalah sekrup bola, dan adalah efisiensi sekrup bola.
Standar Sekrup Bola
National and international standards are used to standardize the definition, environmental requirements, and test methods used for ball screws. The selection of standards to be used is an agreement between the supplier and the user and has some significance in the screw design. In the United States, ASME has developed the B5.48-1977 Standard titled "Screw Ball".
See also
- Spline ball
- Jackscrew
- Leadscrew
- Screw rotator
- Linear motion pad
- The recycle ball
- Linear Actuators
References
Source of the article : Wikipedia
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