LECTURE №10

 

DRIVES AND MECHANISMS. LINEAR MOTION DRIVES

 

10.1. MECHANICAL ACTUATORS

 

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.

 

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 lecture 4 and 5.

 

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.

 

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.

 

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.

 

10.2. BALL-SCREW BASED LINEAR DRIVES

 

Ball screw is also called as ball bearing screw or recirculating ball-screw. It consists of a screw spindle, a nut, balls and integrated ball return mechanism a shown in Figure 10.2.1.

 

 

Fig. 10.2.1. Ball-screw configuration

 

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 10.2.1., 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.

 

Characteristics of ball screws

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.

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.

Thread Form

The thread form used in these screws can either be gothic arc type (Fig. 10.2.2 a) or circular arc type (Fig. 10.2.2 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. 10.2.2. 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.

 

Preloading

 

 

Fig. 10.2.3. 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.

 

Effects of preload

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.

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.

Disadvantages of ball screws:

Ø  tend to vibrate;

Ø  require periodic overhauling to maintain their efficiency;

Ø  inclusion of dirt or foreign particles reduces the life of the screws;

Ø  not as stiff as other power screws, thus deflection and critical speed can cause difficulties;

Ø  they are not self-locking screws hence cannot be used in holding devices such as vices;

Ø  require high levels of lubrication.

Applications of ball screws:

o    ball screws are employed in cutting machines, such as machining center and nc lathe where accurate positioning of the table is desired;

o    used in the equipments such as lithographic equipment or inspection apparatus where precise positioning is vital;

o    high precision ball screws are used in steppers for semiconductor manufacturing industries for precision assembly of micro parts;

o    used in robotics application where precision positioning is needed;

o    used in medical examination equipments since they are highly accurate and provide smooth motion.

 

10.3. INDEXING MECHANISMS

 

Mechanism is a system of rigid elements arranged and connected to transmit motion in a predetermined fashion. Indexing mechanisms generally converts a rotating or oscillatory motion to a series of step movements of the output link or shaft. In machine tools the cutting tool has to be indexed in the tool turret after each operation. Also in production machines the product has to be indexed from station to station and need to be stopped if any operation is being performed in the station. Such motions can be accomplished by indexing mechanisms. Indexing mechanisms are also useful for machine tool feeds. There are several methods used to index but important types are ratchet and pawl, rack and pinion, Geneva mechanism and cam drive.

 

Ratchet and pawl mechanism

 

 

Fig. 10.3.1. Ratchet and pawl mechanism

 

A ratchet is a device that allows linear or rotary motion in only one direction. Figure 10.3.1 shows a schematic of the same. It is used in rotary machines to index air operated indexing tables. Ratchets consist of a gearwheel and a pivoting spring loaded pawl that engages the teeth. The teeth or the pawl, are at an angle so that when the teeth are moving in one direction the pawl slides in between the teeth. The spring forces the pawl back into the depression between the next teeth. The ratchet and pawl are not mechanically interlocked hence easy to set up. The table may over travel if the table is heavy when they are disengaged. Maintenance of this system is easy.

 

Rack and pinion mechanism

 

 

Fig. 10.3.2. Rack and pinion mechanism

A rack and pinion gear arrangement usually converts rotary motion from a pinion to linear motion of a rack. But in indexing mechanism the reverse case holds true. The device uses a piston to drive the rack, which causes the pinion gear and attached indexing table to rotate (Fig. 10.3.2). A clutch is used to provide rotation in the desired direction. This mechanism is simple but is not considered suitable for high-speed operation.

 

Geneva mechanism

The Geneva drive is also commonly called a Maltese cross mechanism. The Geneva mechanism translates a continuous rotation into an intermittent rotary motion. The rotating drive wheel has a pin that reaches into a slot of the driven wheel. The drive wheel also has a raised circular blocking disc that locks the driven wheel in position between steps (Fig. 10.3.3). There are three basic types of Geneva motion mechanisms namely external, internal and spherical. The spherical Geneva mechanism is very rarely used. In the simplest form, the driven wheel has four slots and hence for each rotation of the drive wheel it advances by one step of 90°.

 

Fig. 10.3.3. Geneva mechanism

 

If the driven wheel has n slots, it advances by 360°/n per full rotation of the drive wheel. In an internal Geneva drive the axis of the drive wheel of the internal drive is supported on only one side (Fig. 10.3.4). The angle by which the drive wheel has to rotate to effect one step rotation of the driven wheel is always smaller than 180° in an external Geneva drive and is always greater than 180° in an internal one. The external form is the more common, as it can be built smaller and can withstand higher mechanical stresses.

 

 

Fig. 10.3.4. Internal Geneva mechanism

 

Because the driven wheel always under full control of the driver, impact is a problem. It can be reduced by designing the pin in such a way that the pin picks up the driven member as slowly as possible. Both the Geneva mechanisms can be used for light and heavy duty applications. Generally, they are used in assembly machines.

Intermittent linear motion from rotary motion can also be obtained using Geneva mechanism (Fig. 10.3.5). This type of movement is basically required in packaging, assembly operations, stamping, embossing operations in manufacturing automation.

 

 

Fig. 10.3.5. Linear intermittent motion using Geneva mechanism

 

10.4. MOTION PICTURE PROJECTORS

 

Geneva drive mechanism is used in conventional-mechanical type movie projectors. Figure 10.4.1 shows the schematic of movie projector with Geneva mechanism. The film does not run continuously through the projector. It is requited that the film should advance frame by frame and stands still in front of the lens for fraction of a second. Modern film projectors use an electronically controlled indexing mechanism which allows the fast-forwarding of the film.

 

 

Fig. 10.4.1. Motion picture projector with Geneva mechanism

 

Machining centers are used to carry out multiple operations like drilling, milling, boring etc. in one set up on multiple faces of the workpiece. These operations require a number of different tools. Tool changing operation is time consuming which reduces the machine utilization. Hence the tools should be automatically changed to reduce the idle time. This can be achieved by using automatic tool changer (ATC) facility. It helps the workpiece to be machined in one setup which increases the machine utilization and productivity. Large numbers of tools can be stored in tool magazines. Tool magazines are specified by their storage capacity, tool change procedure and shape. The storage capacity ranges from 12 to 200. Some of the magazines are discuseed as follows.

 

Tool turret

It is the simplest form of tool magazine. Figure 10.4.2 the schematic of a turret with a capacity to hold twelve tools. It consists of a tool storage without any tool changer.

 

Fig. 10.4.2. Tool turret

The turret is indexed in the required position for desired machining operation. Advantage of the turret is that the tool can easily be identified, but the time consumed for tool change is more unless the tool is in the adjecent slot.

Tool magazines are generally employed in CNC drilling and milling machines. Compared to tool turrets the tool magazines can hold more number of tools therefore proper management of tools is essential. Duplication of the tools is possible and a new tool of same type may be selected when a particular tool is worn off. The power required to move the tools in a tool magazine is more in comparison with that required in tool turrets. The following are some of the tool magazines used in automation:

o    disc or drum type;

o    chain type;

o    disk or drum type.

 

Disc type magazine

The disc type tool magazine rotates to get the desired tool in position with the tool change arm (Fig. 10.4.3). Larger the diameter of the disc/drum more the number of tools it can hold. It has pockets where tool can be inserted. In case of drum type magazine which can store large amount of tools, the pockets are on the surface along the length. It carries about 12 to 50 tools. If the number of tools are less the disc is mounted on top of the spindle to minimize the travel of tool between the spindle and the disc. If the tools are more then, the disc is wall mounted or mounted on the machining center column. If the disc is column mounted then, it needs an additional linear motion to move it to the loading station for tool change.

 

 

Fig. 10.4.3. Drum magazine

 

The tool change activity requires the following motions:

a)       the spindle stops at the correct orientation for the tool change arm to pick the tool from the spindle;

b)      tool change arm moves to the spindle;

c)       tool change arm picks the tool from the spindle;

d)      tool change arm indexes to reach the tool magazine;

e)       tool magazine indexes so that the tool from the spindle can be placed;

f)        the tool is placed in the tool magazine;

g)       the tool magazine indexes to bring the required tool to the tool change position;

h)      tool change arm picks the tool from the tool magazine;

i)         tool change arm indexes to reach the spindle;

j)         new tool is placed in the spindle;

k)      tool change arm moves back to its parking position.

 

Advantages of automatic tool changer

Advantages of automatic tool changer:

·          increase in operator safety by changing tools automatically;

·          changes the tools in seconds for maintenance and repair;

·          increases flexibility;

·          heavy and large multi-tools can easily be handled;

·          decreases total production time;

The parts riding on the rotating table are positioned at each station for their processing or assembly operation. This type of equipment is called as an indexing machine or dial index machine. These are generally used to carry out assembly operations of small sized products such as watches, jewelery, electronic circuits, small molds/dies, consumer appliances etc.