Ball screw
Encyclopedia
A ball screw is a mechanical linear actuator
that translates rotational motion to linear motion with little friction
. A threaded shaft provides a helical raceway for ball bearing
s which act as a precision screw. As well as being able to apply or withstand high thrust loads, they can do so with minimum internal friction. They are made to close tolerances and are therefore suitable for use in situations in which high precision is necessary. The ball assembly acts as the nut while the threaded shaft is the screw.
In contrast to conventional leadscrew
s, ballscrews tend to be rather bulky, due to the need to have a mechanism to re-circulate the balls.
and in automobile power steering
to translate rotary motion from an electric motor to axial motion of the steering rack. They are also used in machine tool
s, robot
s and precision assembly equipment. High precision ball screws are used in stepper
s for semiconductor manufacturing.
to completely or partially enclose the working surfaces. Another solution is to use a positive pressure of filtered air when they are used in a semi-sealed or open enclosure.
While reducing friction, ball screws can operate with some preload, effectively eliminating backlash (slop) between input (rotation) and output (linear motion). This feature is essential when they are used in computer-controlled motion-control systems, e.g. CNC machine tools and high precision motion applications (e.g. wire bonding
).
s, as the stiffness of a servo motor
is required to keep the cutter from grabbing the work and self feeding, that is, where the cutter and workpiece exceed the optimum feedrate and effectively jam or crash together, ruining the cutter and workpiece. Cost is also a major factor as Acme
screws are cheaper to manufacture.
compared to alternatives. A typical ball screw may be 90 percent efficient, versus 50 percent efficiency of an Acme lead screw of equal size. The higher cost of ball screws may thus be offset by lower power requirements for the same net performance.
Instrument quality screw shafts are typically precise to 250 nanometers per centimeter. They are produced on precision milling machines with optical distance measuring equipment and special tooling. Similar machines are used to produce optical lenses and mirrors. Instrument screw shafts are generally made of Invar
, to prevent temperature from changing tolerances too much.
An external ballnut employs a stamped tube which picks up balls from raceway with use of small pick up finger. Balls travel inside of tube and are then replaced back in thread raceway.
An internal button ballnut employs a machined
or cast
button style return which allows balls to exit raceway track and move one thread and reenter raceway.
An endcap return ballnut employs a cap on the end of ball nut. The cap is machined to pick up balls out of the end of nut and direct them down holes which are bored
transversely down the ballnut. The compliment cap on the other side of nut directs balls back into raceway.
Where
is torque applied to screw or nut, 
is linear force applied, 
is ball screw lead, and 
is ball screw efficiency.
Linear actuator
A linear actuator is an actuator that creates linear motion .Mechanical and hydraulic actuation are the most common methods of achieving the linear motion...
that translates rotational motion to linear motion with little friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
. A threaded shaft provides a helical raceway for ball bearing
Ball bearing
A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit...
s which act as a precision screw. As well as being able to apply or withstand high thrust loads, they can do so with minimum internal friction. They are made to close tolerances and are therefore suitable for use in situations in which high precision is necessary. The ball assembly acts as the nut while the threaded shaft is the screw.
In contrast to conventional leadscrew
Leadscrew
A leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...
s, ballscrews tend to be rather bulky, due to the need to have a mechanism to re-circulate the balls.
Applications
Ball screws are used in aircraft and missiles to move control surfaces, especially for electric fly by wireFly by Wire
Fly by Wire: The Geese, the Glide, the Miracle on the Hudson is a book written in 2009 by William Langewiesche about US Airways Flight 1549 with emphasis on the role played by the advanced fly-by-wire flight control system of the aircraft....
and in automobile power steering
Power steering
Power steering helps drivers steer vehicles by augmenting steering effort of the steering wheel.Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver needs to provide only modest effort regardless of conditions. Power steering helps considerably when a...
to translate rotary motion from an electric motor to axial motion of the steering rack. They are also used in machine tool
Machine tool
A machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
s, robot
Robot
A robot is a mechanical or virtual intelligent agent that can perform tasks automatically or with guidance, typically by remote control. In practice a robot is usually an electro-mechanical machine that is guided by computer and electronic programming. Robots can be autonomous, semi-autonomous or...
s and precision assembly equipment. High precision ball screws are used in stepper
Stepper
A stepper is a device used in the manufacture of integrated circuits that is similar in operation to a slide projector or a photographic enlarger. Steppers are an essential part of the complex process, called photolithography, that creates millions of microscopic circuit elements on the surface of...
s for semiconductor manufacturing.
History
Historically, the first precise screwshafts were produced by starting with a low precision screwshaft, and then lapping the shaft with several spring-loaded nut laps. By rearranging and inverting the nut laps, the lengthwise errors of the nuts and shaft were averaged. Then, the very repeatable shaft's pitch is measured against a distance standard. A similar process is sometimes used today to produce reference standard screw shafts, or master manufacturing screw shafts.Description and operation
To maintain their inherent accuracy and ensure long life, great care is needed to avoid contamination with dirt and abrasive particles. This may be achieved by using rubber or leather bellowsBellows
A bellows is a device for delivering pressurized air in a controlled quantity to a controlled location.Basically, a bellows is a deformable container which has an outlet nozzle. When the volume of the bellows is decreased, the air escapes through the outlet...
to completely or partially enclose the working surfaces. Another solution is to use a positive pressure of filtered air when they are used in a semi-sealed or open enclosure.
While reducing friction, ball screws can operate with some preload, effectively eliminating backlash (slop) between input (rotation) and output (linear motion). This feature is essential when they are used in computer-controlled motion-control systems, e.g. CNC machine tools and high precision motion applications (e.g. wire bonding
Wire bonding
Wire bonding is the primary method of making interconnections between an integrated circuit and a printed circuit board during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one PCB to another...
).
Disadvantages
Depending upon their lead angle, ball screws can be back-driven due to their low internal friction (i.e. the screw shaft can be driven linearly to rotate the ball nut). They are usually undesirable for hand-fed machine toolMachine tool
A machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
s, as the stiffness of a servo motor
Servo motor
thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...
is required to keep the cutter from grabbing the work and self feeding, that is, where the cutter and workpiece exceed the optimum feedrate and effectively jam or crash together, ruining the cutter and workpiece. Cost is also a major factor as Acme
Acme thread form
Trapezoidal thread forms are screw thread profiles with trapezoidal outlines. They are the most common forms used for leadscrews . They offer high strength and ease of manufacture. They are typically found where large loads or high accuracy are required, as in a vise or the leadscrew of a lathe...
screws are cheaper to manufacture.
Advantages
Low friction in ball screws yields high mechanical efficiencyMechanical efficiency
Mechanical efficiency measures the effectiveness of a machine in transforming the energy and power that is input to the device into an output force and movement...
compared to alternatives. A typical ball screw may be 90 percent efficient, versus 50 percent efficiency of an Acme lead screw of equal size. The higher cost of ball screws may thus be offset by lower power requirements for the same net performance.
Manufacture
Ball screw shafts may be fabricated by rolling, yielding a less precise, but inexpensive and mechanically efficient product. Rolled ball screws have a positional precision of several thousandths of an inch per foot.Accuracy
High-precision screw shafts are typically precise to one thousandth of an inch per foot or better. They have historically been machined to gross shape, case hardened and then ground. The three step process is needed because high temperature machining distorts the work-piece. Hard whirling is a recent (2008) precision machining technique that minimizes heating of the work, and can produce precision screws from case-hardened bar stock.Instrument quality screw shafts are typically precise to 250 nanometers per centimeter. They are produced on precision milling machines with optical distance measuring equipment and special tooling. Similar machines are used to produce optical lenses and mirrors. Instrument screw shafts are generally made of Invar
Invar
Invar, also known generically as FeNi36 , is a nickel steel alloy notable for its uniquely low coefficient of thermal expansion . The name, Invar, comes from the word invariable, referring to its lack of expansion or contraction with temperature changes.It was invented in 1896 by Swiss scientist...
, to prevent temperature from changing tolerances too much.
Ball return systems
The bearing balls travel inside the screw and nut thread. If the ball nut did not have a return mechanism the balls would fall out of the end of the ball nut when they reached the end of the nut. For this reason several different recirculation methods have been developed.An external ballnut employs a stamped tube which picks up balls from raceway with use of small pick up finger. Balls travel inside of tube and are then replaced back in thread raceway.
An internal button ballnut employs a machined
Machining
Conventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...
or cast
Casting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
button style return which allows balls to exit raceway track and move one thread and reenter raceway.
An endcap return ballnut employs a cap on the end of ball nut. The cap is machined to pick up balls out of the end of nut and direct them down holes which are bored
Boring
Boring often refers to anything that causes boredom.Boring may also refer to:Making holes*Boring , the drilling of holes or tunnels in the earth**Tunnel boring machine machine used in boring tunnels or shafts...
transversely down the ballnut. The compliment cap on the other side of nut directs balls back into raceway.
Equations
Where
is torque applied to screw or nut, is linear force applied, is ball screw lead, and is ball screw efficiency.See also
- Ball splineBall splineBall splines are a special type of linear motion bearing that are used to provide nearly frictionless linear motion while allowing the member to transmit torque simultaneously. There are grooves ground along the length of the shaft for the recirculating ground balls to run inside...
- JackscrewJackscrewA jackscrew is a type of jack which is operated by turning a leadscrew. In the form of a screw jack it is commonly used to lift heavy weights such as the foundations of houses, or large vehicles.-Advantages:...
- Roller screwRoller screwA roller screw, also known as a planetary roller screw or satellite roller screw, is a low-friction precision mechanical device for converting rotational motion to linear motion, or vice versa. Planetary roller screws are most commonly used as the actuator mechanism in electro-mechanical linear...
- Linear motion bearingLinear motion bearingA linear-motion bearing or linear slide is a bearing designed to provide free motion in one dimension. There are many different types of linear motion bearings and this family of products is generally broken down into two sub-categories: rolling-element and plane.Motorized linear slides such as...
- Recirculating ballRecirculating ballRecirculating ball, also known as recirculating ball and nut or worm and sector, is a steering mechanism commonly found in older automobiles, and some trucks...