Linear motion drives are mechanical transmission systems which are used to convert rotary motion into linear motion. The conventional thread forms like vee or square are not suitable in CNC because of their high wear and less efficiency. Therefore CNC machines generally employ ball screw for driving their workpiece carriages. These drives provide backlash free operation with low friction-wear characteristics. These are efficient and accurate in comparison with that of nut-and-screw drives. Most widely used linear motion drives are ball screws.

A linear actuator is an actuator that produces motion in a straight line. Linear actuators are extensively required in machine tools and industrial machinery. Hydraulic or pneumatic cylinders inherently produce linear motion. Many other mechanisms are used to generate linear motion from a rotating motor.

1.  Mechanical actuators

These actuators convert rotary motion into linear motion. Conversion is made by using various types of mechanisms such as:

• Screw: This is a simple machine known as screw. By rotating the screw shaft, the actuator's nut moves in a line.
  
  
• Wheel and axle: Hoist, winch, rack and pinion, chain drive, belt drive, rigid chain and rigid belt actuators operate on the principle of the wheel and axle. A rotating wheel moves a cable, rack, chain or belt to produce linear motion.
  
  
• Cam: discussed in last lecture.

Hydraulic actuators utilize pressurized fluid to produce a linear motion where as pneumatic systems employ compressed air for the same purpose. We will be discussing about these systems in Modules 4 and 5.

2.  Piezoelectric actuators

These actuators work on the principle of Piezoelectricity which states that application of a voltage to a crystal material such as Quartz causes it to expand. However, very high voltages produce only tiny expansions. As a result, though the piezoelectric actuators achieve extremely fine positioning resolution, but also have a very short range of motion. In addition, piezoelectric materials exhibit hysteresis which makes it difficult to control their expansion in a repeatable manner.

3.  Electro-mechanical actuators

Electro-mechanical actuators are similar to mechanical actuators except that the control knob or handle is replaced with an electric motor. Rotary motion of the motor is converted to linear displacement. In this type of actuators, an electric motor is mechanically connected to rotate a lead screw. A lead screw has a continuous helical thread machined on its circumference running along the length (similar to the thread on a bolt). Threaded onto the lead screw is a lead nut or ball nut with corresponding helical threads. The nut is prevented from rotating with the lead screw (typically the nut interlocks with a non-rotating part of the actuator body). Therefore, when the lead screw is rotated, the nut will be driven along the threads. The direction of motion of the nut depends on the direction of rotation of the lead screw. By connecting linkages to the nut, the motion can be converted to usable linear displacement.

There are many types of motors that can be used in a linear actuator system. These include dc brush, dc brushless, stepper, or in some cases, even induction motors. Electromechanical linear actuators find applications in robotics, optical and laser equipments, or X-Y tables with fine resolution in microns.

4.  Linear motors

The working principle of a linear motor is similar to that of a rotary electric motor. It has a rotor and the stator circular magnetic field components are laid down in a straight line. Since the motor moves in a linear fashion, no lead screw is needed to convert rotary motion into linear. Linear motors can be used in outdoor or dirty environments; however the electromagnetic drive should be waterproofed and sealed against moisture and corrosion.