McGraw-Hill - The Robot Builder''''s Bonanza Part 8 docx

Read more about servo motors in Chapter 20, “Working with ServoMotors.”Calculating the Speed of Robot TravelThe speed of the drive motors is one of two elements that determines the trave

motor, of course) Read more about servo motors in Chapter 20, “Working with ServoMotors.”

Calculating the Speed of Robot TravelThe speed of the drive motors is one of two elements that determines the travel speed ofyour robot The other is the diameter of the wheels For most applications, the speed of thedrive motors should be under 130 rpm (under load) With wheels of average size, the resul-tant travel speed will be approximately four feet per second That’s actually pretty fast Abetter travel speed is one to two feet per second (approximately 65 rpm), which requiressmaller diameter wheels, a slower motor, or both

How do you calculate the travel speed of your robot? Follow these steps:

1. Divide the rpm speed of the motor by 60 The result is the revolutions of the motor persecond (rps) A 100-rpm motor runs at 1.66 rps

2. Multiply the diameter of the drive wheel by pi, or approximately 3.14 This yields the

cir-cumference of the wheel A 7-inch wheel has a circir-cumference of about 21.98 inches

3. Multiply the speed of the motor (in rps) by the circumference of the wheel The result

is the number of linear inches covered by the wheel in one second

With a 100-rpm motor and 7-inch wheel, the robot will travel at a top speed of 35.168inches per second, or just under three feet That’s about two miles per hour! You can read-ily see that you can slow down a robot by decreasing the size of the wheel By reducingthe wheel to 5 inches instead of 8, the same 100-rpm motor will propel the robot at about

25 inches per second By reducing the motor speed to, say, 75 rpm, the travel speed fallseven more, to 19.625 inches per second Now that’s more reasonable

CALCULATING THE SPEED OF ROBOT TRAVEL 231

Steeringwheel

Drivewheels

FIGURE 16.12 In tricycle ing, one drive motor powers the robot; a single wheel in front steers the robot Be wary of short wheelbases as this can introduce tipping when the robot turns.

steer-Bear in mind that the actual travel speed once the robot is all put together may be lowerthan this The heavier the robot, the larger the load on the motors, so the slower they will turn.

Round Robots or Square?

Robots can’t locomote where they can’t fit Obviously, a robot that’s too large to fit throughdoorways and halls will have a hard time of it In addition, the overall shape of a robot willalso dictate how maneuverable it is, especially indoors If you want to navigate your robot

in tight areas, you should consider its basic shape: round or square

■ A round robot is generally able to pass through smaller openings, no matter what its entation when going through the opening (see Fig 16.14) To make a round robot, youmust either buy or make a rounded base or frame Whether you’re working with metal,steel, or wood, a round base or frame is not as easy to construct as a square one

ori-■ A square robot must orient itself so that it passes through openings straight ahead ratherthan at an angle Square-shaped robot bases and frames are easier to construct thanround ones

While you’re deciding whether to build a round- or square-shaped robot, consider that

a circle of a given diameter has less surface area than a square of the same width Forexample, a 10-inch circle has a surface area of about 78 square inches Moreover, becausethe surface of the base is circular, less of it will be useful for your robot (unless your print-

ed circuit boards are also circular) Conversely, a 10-inch-by-10-inch square robot has asurface area of 100 inches Such a robot could be reduced to about 8.5 inches square, and

it would have about the same surface area as a 10-inch round robot, and its surface areawould be generally more usable

Steering anddrive wheels

FIGURE 16.13 An omnidirectional robot uses the

same wheels for drive and steering.

ROUND ROBOTS OR SQUARE? 233

Path of robot

Path of robot

Path of robot

Square robot of same dimensions as circular robot won't fit through opening

Square robot of slightly smaller dimensions as circular robot fits through opening 10"

8.5"

10"

FIGURE 16.14 A round robot versus a square robot All

things being equal, a round robot is better able to navigate through small openings.

However, rounded robots also have less usable surface area, so a square-shaped robot can be made smaller and still support the same onboard “real estate.”

From Here

To learn more about… Read

Selecting wood, plastic, or metal to Chapter 8–10construct your robot

Choosing a battery for your robot Chapter 15, “All about Batteries and Robot Power

Motors are the muscles of robots Attach a motor to a set of wheels and your robotcan scoot around the floor Attach a motor to a lever, and the shoulder joint for yourrobot can move up and down Attach a motor to a roller, and the head of your robotcan turn back and forth, scanning its environment There are many kinds of motors;however, only a select few are truly suitable for homebrew robotics In this chapter,we’ll examine the various types of motors and how they are used.

AC or DC?

Direct current—DC—dominates the field of robotics, either mobile or stationary

DC is used as the main power source for operating the onboard electronics, foropening and closing solenoids, and, yes, for running motors Few robots usemotors designed to operate from AC, even those automatons used in factories.Such robots convert the AC power to DC, then distribute the DC to various sub-systems of the machine

DC motors may be the motors of choice, but that doesn’t mean you should usejust any DC motor in your robot designs When looking for suitable motors, be surethe ones you buy are reversible Few robotic applications call for just unidirectional(one-direction) motors You must be able to operate the motor in one direction, stop

it, and change its direction DC motors are inherently bidirectional, but some designlimitations may prevent reversibility

The most important factor is the commutator brushes If the brushes are slanted, themotor probably can’t be reversed In addition, the internal wiring of some DC motors pre-vents them from going in any but one direction Spotting the unusual wiring scheme byjust looking at the exterior or the motor is difficult, at best, even for a seasoned motor user.The best and easiest test is to try the motor with a suitable battery or DC power supply.Apply the power leads from the motor to the terminals of the battery or supply Note thedirection of rotation of the motor shaft Now, reverse the power leads from the motor Themotor shaft should rotate in reverse.

Continuous or Stepping

DC motors can be either continuous or stepping Here is the difference: with a continuous motor, like the ones in Fig 17.1, the application of power causes the shaft to rotate con-

tinually The shaft stops only when the power is removed or if the motor is stalled because

it can no longer drive the load attached to it

With stepping motors, shown in Fig 17.2, the application of power causes the shaft to

rotate a few degrees, then stop Continuous rotation of the shaft requires that the power bepulsed to the motor As with continuous DC motors, there are subtypes of stepping motors.Permanent magnet steppers are the ones you’re likely to encounter, and they are also theeasiest to use

The design differences between continuous and stepping DC motors need to beaddressed in detail Chapter 18, “Working with DC Motors,” focuses entirely on continu-

FIGURE 17.1 An assortment of DC motors.

ous motors Chapter 19, “Working with Stepper Motors,” focuses entirely on the steppingvariety Although these two chapters focus on the main drive motors of your robot, you canapply the information to motors used for other purposes as well.

Servo Motors

A special “subset” of continuous motors is the servo motor, which in typical cases bines a continuous DC motor with a “feedback loop” to ensure the accurate positioning ofthe motor A common form of servo motor is the kind used in model and hobby radio-con-trolled (R/C) cars and planes

com-R/C servos are in plentiful supply, and their cost is reasonable (about $10–12 for basicunits) Though R/C servos are continuous DC motors at heart, we will devote a separatechapter just to them See Chapter 20, “Working with Servo Motors,” for more information

on using R/C servo motors not only to drive your robot creations across the floor but tooperate robot legs, arms, hands, heads, and just about any other appendage

Other Motor TypesThere are many other types of motors, some of which may be useful in your hobby robot,some of which will not DC, stepper, and servo motors are the most common, but you mayalso see references to some of the following:

OTHER MOTOR TYPES 237

FIGURE 17.2 An assortment of stepper motors.

■ Brushless DC This is a kind of DC motor that has no brushes It is controlled

elec-tronically Brushless DC motors are commonly used in fans inside computers and formotors in VCRs and videodisc players

■ Switched reluctance This is a DC motor without permanent magnets.

■ Synchronous Also known as brushless AC, this motor operates synchronously with the

phase of the power supply current These motors function much like stepper motors,which will be discussed in Chapter 19

■ Synchro These motors are considered distinct from the synchronous variety, described

above Synchro motors are commonly designed to be used in pairs, where a “master”motor electrically controls a “slave” motor Rotation of the master causes an equalamount of rotation in the slave

■ AC induction This is the ordinary AC motor used in fans, kitchen mixers, and many

other applications

■ Sel-Syn This is a brand name, often used to refer to synchronous AC motors.

Note that AC motors aren’t always operated at 50/60 Hz, which is common for hold current Motors for 400-Hz operation, for example, are common in surplus stores andare used for both aircraft and industrial applications

house-Motor SpecificationsMotors come with extensive specifications The meaning and purpose of some of the specifi-cations are obvious; others aren’t Let’s take a look at the primary specifications of motors—voltage, current draw, speed, and torque—and see how they relate to your robot designs

OPERATING VOLTAGE

All motors are rated by their operating voltage With small DC “hobby” motors, the rating

is actually a range, usually 1.5 to 6 volts Some high-quality DC motors are designed for

a specific voltage, such as 12 or 24 volts The kinds of motors of most interest to robotbuilders are the low-voltage variety—those that operate at 1.5 to 12 volts

Most motors can be operated satisfactorily at voltages higher or lower than those ified A 12-volt motor is likely to run at 8 volts, but it may not be as powerful as it could

spec-be, and it will run slower (an exception to this is stepper motors; see Chapter 19, “Workingwith Stepper Motors,” for details) You’ll find that most motors will refuse to run, or willnot run well, at voltages under 50 percent of the specified rating

Similarly, a 12-volt motor is likely to run at 16 volts As you may expect, the speed ofthe shaft rotation increases, and the motor will exhibit greater power I do not recommendthat you run a motor continuously at more than 30 or 40 percent its rated voltage, however.The windings may overheat, which may cause permanent damage Motors designed forhigh-speed operation may turn faster than their ball-bearing construction allows

If you don’t know the voltage rating of a motor, you can take a guess at it by trying ious voltages and seeing which one provides the greatest power with the least amount ofheat dissipated through the windings (and felt on the outside of the case) You can also lis-ten to the motor It should not seem as if it is straining under the stress of high speeds

var-CURRENT DRAW

Current draw is the amount of current, in milliamps or amps, that the motor requires from

the power supply Current draw is more important when the specification describes motorloading, that is, when the motor is turning something or doing some work The current draw

of a free-running (no-load) motor can be quite low But have that same motor spin a wheel,which in turn moves a robot across the floor, and the current draw jumps 300, 500, even

1000 percent

With most permanent magnet motors (the most popular kind), current draw increases withload You can see this visually in Fig 17.3 The more the motor has to work to turn the shaft,the more current is required The load used by the manufacturer when testing the motor isn’tstandardized, so in your application the current draw may be more or less than that specified

A point is reached when the motor does all the work it can do, and no more current willflow through it The shaft stops rotating; the motor has “stalled.” Some motors, but notmany, are rated (by the manufacturer) by the amount of current they draw when stalled.This is considered the worse-case condition The motor will never draw more than this cur-rent unless it is shorted out, so if the system is designed to handle the stall current it canhandle anything Motors rated by their stall current will be labeled as such Motorsdesigned for the military, available through surplus stores, are typically rated by their stall current When providing motors for your robots, you should always know the approx-imate current draw under load Most volt-ohm meters can test current Some special-pur-pose amp meters are made just for the job

Be aware that some volt-ohm meters can’t handle the kind of current pulled through amotor Most digital meters (discussed more completely in Chapter 3, “Tools and Supplies”)can’t deal with more than 200 to 400 milliamps of current Even small hobby motors candraw in excess of this Be sure your meter can accommodate current up to 5 or 10 amps

If your meter cannot register this high without popping fuses or burning up, insert a1- to 10-ohm power resistor (10 to 20 watts) between one of the motor terminals and thepositive supply rail, as shown in Fig 17.4 With the meter set on DC voltage, measurethe voltage developed across the resistor

A bit of Ohm’s law, I  E/R (I is current, E is voltage, R is resistance) reveals the

cur-rent draw through the motor For example, if the resistance is 10 ohms and the voltage is2.86 volts, the current draw is 286 mA You can watch the voltage go up (and therefore thecurrent too) by loading the shaft of the motor

MOTOR SPECIFICATIONS 239

6543210

Current(amps)

Load (lb-ft)

0 1 2 3 4 5 6 7 8 9

Increasing load

FIGURE 17.3 The current draw of a motor increases in

proportion to the load on the motor shaft.

The rotational speed of a motor is given in revolutions per minute (rpm) Most continuous

DC motors have a normal operating speed of 4000 to 7000 rpm However, some purpose motors, such as those used in tape recorders and computer disk drives, operate asslow as 2000 to 3000 rpm For just about all robotic applications, these speeds are muchtoo high You must reduce the speed to no more than 150 rpm (even less for motors dri-ving arms and grippers) by using a gear train You can obtain some reduction by using elec-tronic control, as described in Part 5 of this book, “Computers and Electronic Control.”However, such control is designed to make fine-tuned speed adjustments, not reduce therotation of the motor from 5000 rpm to 50 rpm See the later sections of this chapter formore details on gear trains and how they are used

special-Note that the speed of stepping motors is not rated in rpm but in steps (or pulses) persecond The speed of a stepper motor is a function of the number of steps that are required

to make one full revolution plus the number of steps applied to the motor each second As

a comparison, the majority of light- and medium-duty stepper motors operate at theequivalent of 100 to 140 rpm See Chapter 19, “Working with Stepper Motors,” for moreinformation

FIGURE 17.4 How to test the current draw of a motor by measuring the

voltage developed across an in-line resistor The actual value of the resistor can vary, but it should be under about

20 ohms Be sure the resistor is a high-wattage type.

slows down, straining under the workload Reduce the torque even more, and the load mayprove too demanding for the motor The motor will stall to a grinding halt, and in doing soeat up current (and put out a lot of heat).

Torque is perhaps the most confusing design aspect of motors This is not because there

is anything inherently difficult about it but because motor manufacturers have yet to settle

on a standard means of measurement Motors made for industry are rated one way, motorsfor the military another

At its most basic level, torque is measured by attaching a lever to the end of the motorshaft and a weight or gauge on the end of that lever, as depicted in Fig 17.5 The lever can

be any number of lengths: one centimeter, one inch, or one foot Remember this because

it plays an important role in torque measurement The weight can either be a hunk of lead

or, more commonly, a spring-loaded scale (as shown in the figure) Turn the motor on and

it turns the lever The amount of weight it lifts is the torque of the motor There is more tomotor testing than this, of course, but it’ll do for the moment

Now for the ratings game Remember the length of the lever? That length is used in thetorque specification If the lever is one inch long, and the weight successfully lifted is twoounces, then the motor is said to have a torque of two ounce-inches, or oz-in (Some peo-ple reverse the “ounce” and “inches” and come up with “inch-ounces.”)

The unit of length for the lever usually depends on the unit of measurement given for theweight When the weight is in grams, the lever is in centimeters (gm-cm) When the weight

is in ounces, as already seen, the lever used is in inches (oz-in) Finally, when the weight is

in pounds, the lever used is commonly in feet (lb-ft) Like the ounce-inch measurement,gram-centimeter and pound-foot specifications can be reversed—“centimeter-gram” or

“foot-pound.” Note that these easy-to-follow conventions aren’t always used Some motorsmay be rated by a mixture of the standards—ounces and feet or pounds and inches

MOTOR SPECIFICATIONS 241

Length

Scale Upward pull

Motor

Lever

to motor

FIGURE 17.5 The torque of a motor is measured by

attach-ing a weight or scale to the end of a lever and mounting the lever of the motor shaft.

STALL OR RUNNING TORQUE

Most motors are rated by their running torque, or the force they exert as long as the

shaft continues to rotate For robotic applications, it’s the most important ratingbecause it determines how large the load can be and still guarantee that the motorturns How running torque tests are conducted varies from one motor manufacturer toanother, so results can differ The tests are impractical to duplicate in the home shop,unless you have an elaborate slip-clutch test stand, precision scale, and sundry othertest jigs

If the motor(s) you are looking at don’t have running torque ratings, you must mate their relative strength This can be done by mounting them on a makeshift wood ormetal platform, attaching wheels to them, and having them scoot around the floor If themotor supports the platform, start piling on weights If the motor continues to operatewith, say, 40 or 50 pounds of junk on the platform, you’ve got an excellent motor for dri-ving your robot

esti-Some motors you may test aren’t designed for hauling heavy loads, but they may besuitable for operating arms, grippers, and other mechanical components You can test therelative strength of these motors by securing them in a vise, then attaching a large pair ofVise-Grips or other lockable pliers to them Use your own hand as a test jig, or rig one upwith fishing weights Determine the rotational power of the motor by applying juice to themotor and seeing how many weights it can successfully handle

Such crude tests make more sense if you have a “standard” by which to judge ers If you’ve designed a robotic arm before, for example, and are making another one,test the motors that you successfully used in your prototype If subsequent motors fail

oth-to match or exceed the test results of the standard, you know they are unsuitable for the test

Another torque specification, stall torque, is sometimes provided by the manufacturer

instead of or in addition to running torque (this is especially true of stepping motors) Stalltorque is the force exerted by the motor when the shaft is clamped tight There is an indi-rect relationship between stall torque and running torque, and although it varies frommotor to motor you can use the stall torque rating when you select candidate motors foryour robot designs

Gears and Gear ReductionWe’ve already discussed the fact that the normal running speed of motors is far too fast formost robotics applications Locomotion systems need motors with running speeds of 75 to

150 rpm Any faster than this, and the robot will skim across the floor and bash into wallsand people Arms, gripper mechanisms, and most other mechanical subsystems need evenslower motors The motor for positioning the shoulder joint of an arm needs to have aspeed of less than 20 rpm; 5 to 8 rpm is even better

There are two general ways to decrease motor speed significantly: build a biggermotor (impractical) or add gear reduction Gear reduction is used in your car, on yourbicycle, in the washing machine and dryer, and in countless other motor-operatedmechanisms

GEARS 101

Gears perform two important duties First, they can make the number of revolutionsapplied to one gear greater or lesser than the number of revolutions of another gear that isconnected to it They also increase or decrease power, depending on how the gears are ori-ented Gears can also serve to simply transfer force from one place to another

Gears are actually round levers, and it may help to explain how gears function by firstexamining the basic mechanical lever Place a lever on a fulcrum so the majority of thelever is on one side Push up on the long side, and the short side moves in proportion.Although you may move the lever several feet, the short side is moved only a few inches.Also note that the force available on the short end is proportionately larger than the forceapplied on the long end You use this wonderful fact of physics when you dig a rock out ofthe ground with your shovel or jack up your car to replace a tire

Now back to gears Attach a small gear to a large gear, as shown in Fig 17.6 The smallgear is directly driven by a motor For each revolution of the small gear, the large gear turnsone half a revolution Expressed another way, if the motor and small gear turn at 1000 rpm,the large gear turns at 500 rpm The gear ratio is said to be 2:1

Note that another important thing happens, just as it did with the lever and fulcrum

Decreasing the speed of the motor also increases its torque The power output is

approxi-mately twice the input Some power is lost in the reduction process due to the friction ofthe gears If the drive and driven gears are the same size, the rotation speed is neitherincreased nor decreased, and the torque is not affected (apart from small frictional losses).You can use same-size gears in robotics design to transfer motive power from one shaft toanother, such as driving a set of wheels at the same speed and in the same direction

ESTABLISHING GEAR REDUCTION

Gears are an old invention, going back to ancient Greece Today’s gears are more refined,and they are available in all sorts of styles and materials However, they are still based on

GEARS AND GEAR REDUCTION 243

15 teeth

30 teeth

Gear ratio to

FIGURE 17.6 A representation of a 2:1

gear reduction ratio.

the old Greek design in which the teeth from the two mating gears mesh with each other.The teeth provide an active physical connection between the two gears, and the force istransferred from one gear to another.

Gears with the same size teeth are usually characterized not by their physical size but

by the number of teeth around their circumference In the example in Fig 17.6, the smallgear contains 15 teeth, the large gear 30 teeth And, you can string together a number ofgears one after the other, all with varying numbers of teeth (see Fig 17.7) Attach atachometer to the hub of each gear, and you can measure its speed You’ll discover the fol-lowing two facts:

■ The speed always decreases when going from a small to a large gear

■ The speed always increases when going from a large to a small gear

There are plenty of times when you need to reduce the speed of a motor from 5000 rpm

to 50 rpm That kind of speed reduction requires a reduction ratio of 100:1 To accomplishthat with just two gears you would need, as an example, a drive gear that has 10 teeth and

a driven gear that has 1000 teeth That 1000-tooth gear would be quite large, bigger thanthe drive motor itself

You can reduce the speed of a motor in steps by using the arrangement shown in Fig 17.8.Here, the driver gear turns a larger “hub” gear, which in turn has a smaller gear permanent-

ly attached to its shaft The small hub gear turns the driven gear to produce the final outputspeed, in this case 50 rpm You can repeat this process over and over again until the output speed is but a tiny fraction of the input speed This is the arrangement most oftenused in motor gear reduction systems

USING MOTORS WITH GEAR REDUCTION

It’s always easiest to use DC motors that already have a gear reduction box built onto them,such as the motor in Fig 17.9 R/C servo motors already incorporate gear reduction, andstepper motors may not require it This fact saves you from having to find a gear reducerthat fits the motor and application and attach it yourself When selecting gear motors,you’ll be most interested in the output speed of the gearbox, not the actual running speed

20 teeth

30 teeth 1,200 rpm

FIGURE 17.7 Gears driven by the 20-tooth gear on the left rotate at different

speeds, depending on their diameter.

of the motor Note as well that the running and stall torque of the motor will be greatlyincreased Make sure that the torque specification on the motor is for the output of thegearbox, not the motor itself.

With most gear reduction systems, the output shaft is opposite the input shaft (but ally off center) With other boxes, the output and input are on the same side of the box.When the shafts are at 90 degrees from one another, the reduction box is said to be a “right-angle drive.” If you have the option of choosing, select the kind of gear reduction that bestsuits the design of your robot I have found that the “shafts on opposite sides” is the all-around best choice Right-angle drives also come in handy, but they usually carry highprice tags

usu-When using motors without built-in gear reduction, you’ll need to add reduction boxes,such as the model shown in Fig 17.10, or make your own Although it is possible to doboth of these yourself, there are many pitfalls:

■ Shaft diameters of motors and ready-made gearboxes may differ, so you must be surethat the motor and gearbox mate

■ Separate gear reduction boxes are hard to find Most must be cannibalized from salvagemotors Old AC motors are one source of surplus boxes

■ When designing your own gear reduction box, you must take care to ensure that all thegears have the same hub size and that meshing gears exactly match each other

■ Machining the gearbox requires precision, since even a small error can cause the gears

to mesh improperly

ANATOMY OF A GEAR

Gears consist of teeth, but these teeth can come in any number of styles, sizes, and tations Spur gears are the most common type The teeth surround the outside edge of thegear, as shown in Fig 17.11 Spur gears are used when the drive and driven shafts are par-allel Bevel gears have teeth on the surface of the circle rather than the edge They are used

orien-GEARS AND GEAR REDUCTION 245

60-tooth gear

12-tooth pinion fixed on 60-tooth gear 12-tooth

driver (1,000 rpm)

48-tooth driven gear (50 rpm)

FIGURE 17.8 True gear reduction is achieved by

ganging gears on the same shaft.

FIGURE 17.9 A motor with an enclosed gearbox These are ideal for robotics use.

FIGURE 17.10 A gear reduction box, originally removed from an open-frame

AC motor On this unit, the input and output shafts are on the same side.

to transmit power to perpendicular shafts Miter gears serve a similar function but aredesigned so that no reduction takes place Spur, bevel, and miter gears are reversible That

is, unless the gear ratio is very large, you can drive the gears from either end of the gearsystem, thus increasing or decreasing the input speed

Worm gears transmit power perpendicularly, like bevel and miter gears, but their design

is unique The worm (or lead screw) resembles a threaded rod The rod provides the power

As it turns, the threads engage a modified spur gear (the modification takes into ation the cylindrical shape of the worm)

consider-Worm gear systems are specifically designed for large-scale reduction The gearing isnot usually reversible; you can’t drive the worm by turning the spur gear This is an impor-tant point because it gives worm gear systems a kind of automatic locking capability Workgears are particularly well suited for arm mechanisms in which you want the joints toremain where they are With a traditional gear system, the arm may droop or sink back due

to gravity once the power from the drive motor is removed

Rack gears are like spur gears unrolled into a flat rod They are primarily intended totransmit rotational motion to linear motion Racks have a kind of self-locking characteris-tic as well, but it’s not as strong as that found in worm gears

The size of gear teeth is expressed as pitch, which is roughly calculated by counting the

number of teeth on the gear and dividing it by the diameter of the gear For example, a gearthat measures two inches and has 48 teeth has a tooth pitch of about 24 Common pitchesare 12 (large), 24, 32, and 48 Some gears have extra-fine 64-pitch teeth, but these are

GEARS AND GEAR REDUCTION 247

FIGURE 17.11 Spur gears These particular gears are made of nylon and have

aluminum hubs It’s better to use metal hubs in which the gear is secured to the shaft with a setscrew.