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.

5.  Ballscrew based linear drives

Fig.4.4.1 Ballscrew configuration

Ball screw is also called as ball bearing screw or recirculating ballscrew. It consists of a screw spindle, a nut, balls and integrated ball return mechanism a shown in Figure 4.4.1. The flanged nut is attached to the moving part of CNC machine tool. As the screw rotates, the nut translates the moving part along the guide ways. However, since the groove in the ball screw is helical, its steel balls roll along the helical groove, and, then, they may go out of the ball nut unless they are arrested at a certain spot. Thus, it is necessary to change their path after they have reached a certain spot by guiding them, one after another, back to their “starting point” (formation of a recirculation path). The recirculation parts play that role. When the screw shaft is rotating, as shown in Figure 4.4.1, a steel ball at point (A) travels 3 turns of screw groove, rolling along the grooves of the screw shaft and the ball nut, and eventually reaches point (B). Then, the ball is forced to change its pathway at the tip of the tube, passing back through the tube, until it finally returns to point (A). Whenever the nut strokes on the screw shaft, the balls repeat the same recirculation inside the return tube.

When debris or foreign matter enter the inside of the nut, it could affect smoothness in operation or cause premature wearing, either of which could adversely affect the ball screw's functions. To prevent such things from occurring, seals are provided to keep contaminants out. There are various types of seals viz. plastic seal or brush type of seal used in ball-screw drives.

5.1  Characteristics of ball screws:

5.1.1 High mechanical efficiency

In ball screws, about 90% or more of the force used to rotate the screw shaft can be converted to the force to move the ball nut. Since friction loss is extremely low, the amount of force used to rotate the screw shaft is as low as one third of that needed for the acme thread lead screw.

5.1.2 Low in wear

Because of rolling contact, wear is less than that of sliding contact. Thus, the accuracy is high. Ball screws move smoothly enough under very slow speed. They run smoothly even under a load.

5.1.3  Thread Form

The thread form used in these screws can either be gothic arc type (fig. 4.4.2.a) or circular arc type (fig. 4.4.2.b). The friction in this kind of arrangement is of rolling type. This reduces its wear as comparison with conventional sliding friction screws drives.

Fig. 4.4.2 Thread forms (a) Gothic arc (b) Circular arc

Recirculating ball screws are of two types. In one arrangement the balls are returned using an external tube. In the other arrangement the balls are returned to the start of the thread in the nut through a channel inside the nut.

5.3  Preloading

Fig. 4.4.3 Double nut preloading system

In order to obtain bidirectional motion of the carriage without any positional error, the backlash between the nut and screw should be minimum. Zero backlash can be obtained by fitting two nuts with preloading (tension or compression) or by applying a load which exceeds the maximum operating load. Figure 4.4.3 shows double nut preloading system. A shim plate (spacer) is inserted between two nuts for preloading. Preload is to create elastic deformations (deflections) in steel balls and ball grooves in the nut and the screw shaft in advance by providing an axial load. As a result the balls in one of the nuts contact the one side of the thread and balls in the other nut contact the opposite side.

5.3.1 Effects of preload

  • Zero backlash: It eliminates axial play between a screw shaft and a ball nut.
  • It minimizes elastic deformation caused by external force, thus the rigidity enhances.

In case mounting errors, misalignment between the screw shaft and the nut may occur this further generates distortion forces. This could lead to the problems such as,

  • Shortened service life
  • Adverse effect on smooth operation
  • Reduced positioning accuracy
  • Generation of noise or vibration
  • Breakage of screw shaft

5.4 Advantages of ball screws

  • Highly efficient and reliable.
  • Less starting torque.
  • Lower co efficient of friction compared to sliding type screws and run at cooler temperatures
  • Power transmission efficiency is very high and is of the order of 95 %.
  • Could be easily preloaded to eliminate backlash.
  • The friction force is virtually independent of the travel velocity and the friction at rest is very small; consequently, the stick-slip phenomenon is practically absent, ensuring uniformity of motion.
  • Has longer thread life hence need to be replaced less frequently.
  • Ball screws are well -suited to high through output, high speed applications or those with continuous or long cycle times.
  • Smooth movement over full range of travel.

5.5 Disadvantages of ball screws

  • Tend to vibrate.
  • Require periodic overhauling to maintain their efficiency.
  • Inclusion of dirt or foreign particles reduces the life of the screws.
  • Not as stiff as other power screws, thus deflection and critical speed can cause difficulties.
  • They are not self-locking screws hence cannot be used in holding devices such as vices.
  • Require high levels of lubrication.

5.6 Applications of ball screws:

  • Ball screws are employed in cutting machines, such as machining center and NC lathe where accurate positioning of the table is desired
  • Used in the equipments such as lithographic equipment or inspection apparatus where precise positioning is vital
  • High precision ball screws are used in steppers for semiconductor manufacturing industries for precision assembly of micro parts.
  • Used in robotics application where precision positioning is needed.
  • Used in medical examination equipments since they are highly accurate and provide smooth motion.


Last modified: Wednesday, 26 September 2018, 1:30 PM