Build Your Own Combat Robot phần 4 ppsx

■ Power transmission Every device and component that transmits power from the motor to the wheels including the speed reducer.. The purpose of the power transmission is to transmit the r

Power Transmission:

Getting Power to Your Wheels

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chine along, but these sorts of propellants are not allowed in most competitionsand would prove to be quite ineffective anyway Moving parts that actually touchthe floor are the preferred method of providing controlled movement to your ro-bot, with wheels being the most chosen method

The following are some definitions used in this book:

■ Speed reduction Transforming high RPM and low torque power

into low RPM, high torque power

■ Speed reducer The device that does the speed reduction.

■ Gear reduction Speed reduction using gears.

■ Power transmission Every device and component that transmits power

from the motor to the wheels (including the speed reducer)

■ Transmission A speed reducer with more than one reduction ratio Note

that a transmission is only one component in the “power transmission.”The two terms are not interchangeable

The purpose of the power transmission is to transmit the rotational energyfrom the motors to the wheels of the robot, and many different ways can be em-

ployed to do this The simplest way is to use a direct drive method With this

method, the wheel’s hub, or axle, is directly connected to the motor—either rectly on the output shaft of the motor or the output shaft of a gearmotor

di-A gearmotor is a single unit with a gearbox and a motor combined into one

con-venient package The gearbox is used to decrease the rotational speed of the motor

to a more usable output shaft speed Many electric motors’ rotational speeds rangebetween 3000 to 20000 RPM This speed is too fast for directly driving a robot’swheels—unless you want your robot to move at warp speed The gearbox also in-creases the actual torque of the electric motor to a much higher value on the outputshaft The higher torque will give your robot more pushing power

Although many robot builders use the gearmotor approach, some have usednon-gearmotors to power the wheels directly For example, the middle and heavy-weight entries from team Whyachi used direct-drive Magmotors in their robots

Early in the robot design process, you usually decide that you want your robot

to move at a certain speed and have the ability to push a certain amount of weight.These specifications can help you select an appropriately sized motor

Ideally, you will be able to find a prepackaged gearmotor that will meet yourspecifications If you cannot find the perfect gearmotor, you will have to settle forwhatever you can find and live with a different robot speed and strength—or youcan build your own speed reducer

The type of power transmission you’ll need for your robot is a simple duction setup, not the type of power transmission commonly found in automobiles

speed-re-or motspeed-re-orcycles In some cases, you may want to increase the speed of a gearmotspeed-re-or; but in most cases, you will be reducing the speed of the motor This type of power

transmission usually consists of a set of chains and sprockets, timing belts,V-belts, gears, or even a secondary gear box

The power transmission is also often used to transmit the power of the motor totwo or more robot wheels In most cases, two separate axles are driven at the sametime through chains and sprockets, timing belts, and V-belts

True Story: Grant Imahara and Deadblow

“The most spectacular failure I had was in Las Vegas, during season 2.0,” says

Grant Imahara, the renowned builder behind Deadblow “I was waiting to fight

a robot named Kegger built by a team called ‘Poor College Kids.’ It was probably

going to be a prettyeasymatch, but BattleBots teaches you not to be overconfident,

because anything can happen.”

Indeed, Grant has seen just about everything He was there at the birth of

the sport, since Marc Thorpe, an Industrial Light and Magic co-worker, created

Robot Wars in 1995 and gave Grant tickets to attend “I was captivated, and

knew that I had to build a robot of my own.”

Deadblow was the result of that obsession; and at this particular event, Grant

found himself charging the onboard air tanks—essential to power the weapons—

in preparation for competition “I was filling my two onboard air tanks from an

external SCUBA tank, which was a pretty standard thing for me to do I had

done it a million times But this time I heard a loud ‘pop,’ followed by a rush

of high pressure air coming out of the robot.”

Grant describes how the nearby mass of people backed away uneasily at the

ominous sound of rushing air “That pop meant that I had ruptured one of my

air lines and the weapon— Deadblow’s onlyweapon—couldn’t work without air I

knew that if the robot couldn’t fight at its designated time, I would have to forfeit

my match.”

P ower Transmission Basics

As stated, the purpose of the power transmission is to reduce the speed of the tor to some usable speed for the robot and to transmit the power to the wheels.The speed of a robot is a function of the rotational speed of the wheels and the di-

mo-ameter of the wheels Equation 1 shows this relationship, where v is the velocity of the robot, D is the diameter of the driven wheels, and N is the rotational speed of

the wheel So, to determine the required rotational speed of the wheel, Equation 1

is solved for N, which is shown in Equation 2.

If your robot has 10-inch-diameter wheels and the rotational speed of the robot

is 300 RPM, the speed of the robot will be 9,425 inches per minute, or about 8.9 milesper hour (MPH) If you want your robot to move 20 MPH, this same wheel willhave to spin at 673 RPM This is one fast robot

After you have an idea of the wheel speed you want, you need to determine howmuch of a speed reduction in the power transmission you will need to convert themotor speed to the wheel speed This is done by using a combination of differentsprocket diameters, pulley diameters, or gear diameters The speed ratios of a geartrain are just a ratio of the gear diameters

Figure 6-1 shows a sketch of the same type of speed reduction The leftmostsketch shows two gears in mesh, and the sketch on the right shows a belt/chaingear reduction One thing to note here is that with the gear reduction, the direction

of the driven gear is opposite of that of the driving gear With the belt/chain tem, the directions of both pulleys/sprockets are the same

sys-Grant Imahara and Deadblow (continued)

Fortunately, the Washburn family was nearby in the contestant stands ShaneWashburn, Grant explains, was a co-worker at ILM and he had fought against Grant

with his bot Red Scorpion in previous years Moreover, Shane’s father, Ray, was

a welder and hydraulics expert, and his brother, Jon, was an emergency medicaltechnician “They heard the air line rupture and were immediately at my side While

I was desperately trying to turn off the SCUBA tank, the Washburns and my crewwere taking the screws out of the top cover We wheeled my robot out of the way

and the BattleBots people and Team Poor College Kids graciously allowed another

match to go before us This bought a little time, but not much Ray ran all the wayback to the pits and grabbed all the air fittings from my toolbox We fixed it there

on the spot in about five minutes—I couldn’t have done it without their help.”

Despite the catastrophic failure, Grant adds that he went on to beat Kegger with

just a single onboard air tank But there’s a lesson in the story: “Always inspect all ofyour equipment for wear and damage, even if you don’t think you had any.”

6.1 6.2

Equation 3 shows how the speed of the output gear relates to the speed of theinput gear.

In Equation 3, D 1 and N 1are the diameter and rotational speed of the driving

gear, and D 2 and N 2are the diameter and rotational speed of the driven (output) gear

When D 1 is greater than D 2, the output gear will spin faster than the driving gear;

when D 1 is less than D 2, the output gear will spin slower (gear reduction) than thedriving gear When driving two shafts together, such as a front and rear axle beingdriven with only one motor, the gear/sprocket diameters between the two axlesmust be the same or the wheels will spin at different speeds

If you have a 3000 RPM motor and you want a wheel speed of 300 RPM, youwill have to reduce the speed of the motor by a factor of 10 By looking at equa-

tion 3, you can see that the output gear, D 2, will have to be 10 times bigger than

the input gear, D 1 This is a pretty big gear reduction with only two gears If youwere using a 1.5-inch-diameter gear on the motor shaft, you would have to use a15-inch-diameter gear on the wheel If the wheel is only 10 inches in diameter, thegear’s diameter will cause the gear to strike the ground, since it is larger than thewheel When this type of situation occurs, three or more gears/pulleys/sprocketsmust be used together

Figure 6-2 shows a more complex speed reduction

Though the configuration shown in Figure 6-2 seems complicated, it can besimplified by looking at it as two separate two-gear systems In this example, thespeed of gear number 2 is the same as what is shown in Equation 3 The speed of

gear number 4, N 4, is first shown in Equation 4 that follows It has the same exactform as what is seen in Equation 3 Since gears numbers 2 and 3 are physically at-tached to the same shaft, they will both spin at the same speed, which is shown inEquation 5 Because of this, you can substitute Equation 3 into Equation 4 to determine the final speed of the output shaft Equation 6 shows the speed reduction for

6.3

FIGURE 6-1

Simple speed

reduction schematic

the gear reduction shown in Figure 6-2 The D 1 / D 2is the first gear reduction ratio,

and D 3 / D4is the second gear reduction ratio

In the previous example, you looked at a speed reduction of 10 With the ble-speed reduction system, you have a lot of options for choosing gear diameters.The product of the first and second stages in the speed reducer must be 10 For ex-ample, you can choose the first gear reduction to be 4 and the second gear reduction

dou-to be 2.5 In this case, you can use the same 1.5-inch-diameter gear on the modou-torshaft, and then the second gear should be 6 inches in diameter This is smaller thanthe 10-inch-diameter wheels used in this example The third gear could be a2-inch-diameter gear, which would mean that the last gear should be 2.5 timeslarger or 5 inches in diameter These gear sizes are much more manageable thantrying to do this entire gear reduction in one step

As a general rule, the greater the gear reduction, the more gears you will have touse to achieve the gear reduction In the real world, you may not find the exactgear and sprocket diameters you want This may be because the actual sizes do notexist For example, if you are using sprockets instead of gears, it is rare to be able

to find a sprocket that has a diameter 10 times greater than the driving sprocket.You will usually have to choose components that are close to the values you want.Thus, the speed reduction will be a little lower or higher than what you want

Torque

The output torque is also a function of the gear ratios, but the torque and gear ratioshave an inverse relationship When the speed is reduced, the torque on the outputshaft is increased Conversely, when the speed is increased, the output torque is re-duced Equation 7 shows the torque relationships from Figure 6-1 The direction inwhich the torque is being applied is identical to the rotational directions

T 1 and D 1 are the torque and the diameter of gear 1, and T 2 and D 2are the torque

and diameter of gear 2 If D 2 is greater than D 1, the output torque is increased.From Figure 6-2, the output torque is shown in equation 8

In the previous example, where we were looking for a 10-to-1 speed reduction,this will increase the output torque by a factor of 10

During the robot design process, the power transmission must be considered atthe same time while you’re selecting the motors The number of gears, sprockets,and pulleys and their sizes can have a significant impact on the overall structuraldesign of the robot To simplify the overall power transmission design, you shouldchoose a motor that has the lowest RPM so that the number of components in thepower transmission (or speed reducer) can be minimized

of a gear, the torque is equal to the force being applied to the gear teeth multiplied

by the radius of the gear Equation 9 shows this relationship, where T is the torque, F is the applied force, and r is the distance from the center of rotation and

6.8 6.7

where the force is being exerted Equation 10 shows how the force is related to theapplied torque.

Using this relationship, you might think that your 500 in.-lb torque motors andyour 10-inch-diameter wheeled robot would have a pushing force of 100 pounds(100 pounds = 500 in.-ibs / 5-inch radius) But this isn’t the case Wheel frictionbecomes part of the equation Without friction, powered wheels will never move avehicle, and turning the vehicle would be virtually impossible In most mechanicaldevices, friction is undesirable; but for wheels, friction is good For combat ro-bots, the more friction you can get the better your robot can push The frictionalforce to move an object across a horizontal floor is equal to the product of the co-efficient of friction between the floor and the object’s surface and the weight of theobject Equation 11 shows you how it works:

where F fis the frictional force,µ is the coefficient of friction, and F wis the weight ofthe object

Figure 6-3 shows a schematic of the various forces acting on a wheel F wis theweight force acting on this wheel For a really rough approximation, this valuecould be estimated by dividing the robot’s total weight by the number of itswheels This applies only a rough estimate to the weight of a wheel, and it is trueonly if the robot’s center of gravity is at the geometrical center between the wheels.Computer-aided design (CAD) software can help provide the actual values for thewheels, or they can be directly measured by putting a scale under each wheel

So how much can a robot push? The maximum pushing force will be equal to

the sum of all the frictional forces, F f, for all of the wheels When the reactionforces of an immovable object, such as a wall or a bigger robot, exceeds the totalfrictional forces, your robot will stop moving—and, in this case, your robot couldactually be pushed backward! By combining Equations 9 and 11, the torque re-quired to produce the maximum pushing force will be as shown in Equation 12

For a robot with all identical wheels and motors that can deliver all the torque it

could need, the total maximum pushing force, F max, will become the product of theweight of the robot and the coefficient of friction Equation 13 shows this

If the motor torque can produce a force greater than the frictional force, thewheels will spin If the maximum torque of the motors cannot produce forcesgreater than the frictional forces, your robot’s motors will stall when you run upagainst another robot or a wall In Chapter 4, you learned that stalling a motor isnot a good idea, so it is a better idea to have the wheels spin rather than beingstalled Equation 14 shows the stall torque relationship for each wheel This infor-mation can be used to help you determine the speed reduction in the power trans-mission and help you pick the right-sized motors Equation 13 is a ratherinteresting equation This maximum force is the maximum force your robot canexert, or it is the force another robot needs to exert on your robot to push itaround This force is a function of two things: weight of the robot and the coeffi-cient of friction between the robot’s wheels and the ground So, this tells you thatincreasing your robot’s weight can give you a competitive advantage

One of the difficult tasks in determining the pushing force is determining thecoefficient of friction The coefficient of friction between rubber and dry metalsurfaces can range from 0.5 to 3.0 In your high school science classes, you probablylearned that the coefficient of friction cannot be greater than 1.0 This is true forhard, solid objects; but with soft rubber materials, other physics are involved It isnot uncommon to find soft, gummy rubber that has coefficients of friction greaterthe 1.0, and some materials have a coefficient of friction as high as 3.0 For allpractical purposes, the coefficient of friction for common rubber tires and steelsurfaces is between 0.5 and 1.0

The other factor that affects the coefficient of friction is how much dirt is on thesurface A dirty surface will reduce the overall coefficient of friction This is whyoff-road tires have knobby treads to help improve the friction, or traction

As a worse-case situation, assume that the coefficient of friction is equal to 1and size all your components so you will not stall the motors in these conditions.This will give most robots a small safety margin If you want to be more conserva-tive, use a coefficient of friction greater than 1

6.14 6.12

6.13

Location of the Locomotion Components

Most combat robots are fairly simple, internally They consist of a power source, aset of batteries; motors for the wheels; a radio-controlled (R/C) system receiver;controllers to take an R/C signal from the receiver and send power to the motors;and a weapon system’s actuators, if they’re required in your design Other compo-nents appear in various robot designs, such as microcontrollers to process incom-ing data to pulse width modulation signals, DC to DC converters, fans to coolcontrollers, and so on, but these are generally smaller and can be placed intight-fitting places

The location of the main drive motors is the most critical concern in the placement

of large robot subsystems Usually, these motors are quite large The other largesubsystems, such as batteries and controllers, can be located “wherever possible.”Motors have to be close to the wheels, and their position and orientation is critical.Quite often they are mounted in the lowest part of the robot The motors must bepositioned accurately, especially if a series of gears are used to transmit the power

to the wheels, and the chains or gears need to be aligned in the same plane as thewheel system

Mounting the Motors

Mounting of the motors in any application is important, but combat robots ent another magnitude of problems for their motors The motors are trying towrench themselves out of their mounts from extreme torque conditions At thesame time, their mounts are being shaken so intensely that the mounting screwscan be sheared in half So you must design your robot to handle such extremes.Quite often, a DC motor you might find in a surplus catalog has several threadedholes in the front face where the output shaft is located Using these mounting holesfor screwing the motor to a plate is okay for the types of applications for which themotor was originally designed, but using these holes may not suit an extreme situa-tion in combat robots To determine whether these holes are suitable, you may need

pres-to subject the mopres-tor / mounting brackets pres-to a shock test The large inertial mass ofthe motor may just shear off the screws as you slam the assembly into your garagefloor Unfortunately, you might have to use an “easy-out” to remove the remainingportion of the screws Use your judgment here

You’re in a far better situation if your motors have a flange mount around thefront face of the motor If you need more strength, you can drill out the threadedholes and make larger holes for through-hole, high-strength bolts A flanged basemount can be found on some older motors Flange-based motors offer a higherstrength method of mounting compared with the threaded face hole method.Another method to use for mounting motors is to secure the face with the exist-ing mounting holes to a motor bracket you’ve fabricated, and then secure the backpart of the motor with several high-strength clamps and a machined block inwhich to rest the motor Use high-strength hose clamps that have a machinedscrew—not the “pot metal” types found in some hose clamps This back clampingwill prevent the heavy motor from moving See Figure 6-4

Thermal Considerations for the Motor

One of the drawbacks of using a higher-than-recommended voltage on a DC motor

is the possibility of overheating Even though combat matches generally last only afew minutes, intense heat built up in a motor can destroy it This is not a powertransmission issue, but it certainly is a mounting consideration Some motors use afan at one end to draw in air for cooling, but the intermittent action of the motormay mean that the motor is cooking in its built-up heat while it is off You mustalso remember that the windings that heat up are in the armature, which is the ro-tating component that is isolated from the case, so heat sinks are not as effective asone might think If the armature heats up too much, it can begin to disintegrate,slinging wire pieces all over the inside of the motor If that happens, you’re in for abad day

How can you keep these motors cool? If you’ve run the motor on your benchwhile under load and you’ve noticed that the case gets extremely hot, you maywant to mount it in a machined aluminum block to absorb and conduct the excessheat away from the motor Some competitors have also used a small blower toforce air through the motor to augment the fan Have the fan run even when themotor is off to continue the cooling process as much as possible

FIGURE 6-4

Clamping method

to produce a secure

motor mount.

Methods of Power Transmission

In previous chapters, several methods of interconnecting the motors with thewheels have been discussed In direct-drive methods, the motor or gearmotor’soutput shaft is connected directly to the wheels (see Figure 6-5)

Indirect-drive methods include a chain, belt, and even a series of flexible plings The following sections will discuss various chain and belt drive systems.Numerous types of flexible shaft couplings are available, such as universal joints,shear couplings, spider couplings, grid couplings, offset couplings, chain cou-plings, gear and sleeve couplings, bellows couplings, and helical beam couplings.The main advantage of these shaft couplings is that they can connect two shaftsthat are slightly misaligned Figure 6-6 shows a Lovejoy flexible coupling A

cou-Lovejoy coupling is a spider coupling They come with three different parts: two

bodies and a spider The shaft bodies come with different bore diameters so thatdifferent shaft diameters can be coupled together The spider’s material is madeout of urethane, Hytrel, or rubber The selection of the spider material is based onthe applications the coupling is going to be used for

For high-powered robots, careful design of the components and mounting cations will be needed to minimize shaft misalignment

C hain Drive Systems

Rather than starting with some more exotic designs that use a flexible shaft oreven an articulated shaft fitted with swivel joints, let’s instead jump right to themethod that is used the most—a chain drive This type of interconnection betweenthe wheels and motors offers a lot of pluses If the proper chain is used, it has thecapacity to transfer a lot of power to the wheels It also has the ability to take up

“slop” in the system without requiring precise spacing between the motor andwheel/axle sprocket

Buying the Chain

What is the proper chain for your robot? You might be tempted to use a bicyclechain Hey, you can pedal hard, even stand on the pedals when going uphill, andstill not break the chain The quality of mass-marketed bicycle chains is not up toindustrial standards, however Invest a few bucks in some good roller chain It will

be money well spent and can save you from a few headaches in the long run

The proper term for this type of chain is single strand roller chain Generally,

the pitch on these types of chains ranges from 1/4 inch to 3/4 inch A 1/2 inch pitchmeans that the spacing of the sprocket’s teeth are 1/2 inch apart (or the chain’srollers are 1/2 inch apart) The industrial roller chain is specified with an ANSInumber, generally 25 to 80 See Table 6-1 for a list of some of the common chains

A typical ANSI #40 industrial roller chain, for example, will have a 1/2-inch pitchand a 5/16-inch roller width; it will have a maximum allowable load of 810 pounds;and the chain will break when the load gets up to 4,300 pounds The maximum al-lowable load is based on continuous operation Exceeding the maximum allowableload will shorten the life of the chain If you exceed the average tensile strength,the chain will break

Some builders have ganged up two sprockets on each end to double thestrength In actuality, the strength is not quite doubled due to slight differences in

ANSI No Pitch,

chain-link spacing and subsequent uneven loading on one of the two chains, but

we won’t cover the dynamics and physics of this scenario This is still an able method of applying redundancy for safety When one of the chains fails, youstill have another to carry most of the load Double-strand roller chain is the bestway to increase load capacity, and the cost of this type of chain is only about twicethat of single-strand chain

accept-Most supply houses will supply the chain as a random-length loop or as longpieces of various lengths Cutting the chain may require that you punch or drill outthe rivet on one part of a link You can buy a set of chain maintenance tools forin-the-field chain repairs; these would include a roller chain breaking tool, which

is far easier to use than a hammer and a punch Also available are chain pin tracting tools and a unique roller chain puller that allows you to tighten the chain be-fore inserting a master link connector For maximum chain strength, a chain can

ex-be custom ordered from the manufacturer in the exact length you need If youchoose to go this route, you will not need a master link

The master link is a separately purchased connector link that allows you to

cre-ate a continuous loop of chain You should also buy several extra master link nectors to fasten the chain together at the length you’ll want This fastener consists

con-of a side piece con-of a link with two pins that fit in the roller parts con-of the two ends con-ofthe chain, and a figure-8 side piece to fit over the pins on the other side A clipsnaps over the slotted ends of the pins, locking the master link in place Figure 6-7shows a typical chain

Chain Sprockets

The sprockets used with roller chains look a little bit like gears, but they have morerounded teeth and are not meant to mesh with each other like a “standard” gear.For combat robots, you should buy only steel sprockets for their strength Thesesprockets are specified by an ANSI number (sprockets and chains must have thesame ANSI number, or they will not mesh together because the pitch lengths willnot be the same), the number of teeth on the sprocket, and the shaft bore size.Most sprockets you will find include a keyway to lock them to a shaft with a similar

FIGURE 6-7

A typical

ANSI #40 chain.

keyway Some of the smaller diameter sprockets may have one or two set screws inthe place of a keyway These will work adequately with a flattened area on theshaft for lower torque applications, such as for small hobby robots For combatrobots, use keyways on all sprockets, gears, and pulleys Doing so is a battle-proven method to secure components to shafts.

You might also want to apply one or more idler sprockets to take up slack in thechain Quite often you place your motor(s) and wheel(s) in set locations and thenapply the chain More than likely, you’ll find that the chain is too loose (or maybetoo tight) Having a bit of slack in the chain and using a sprocket idler on a smallspring-loaded lever arm will keep the chain at a specified tightness and will pre-vent the chain from flying outward with centrifugal force under high speeds

When implementing a sprocket and chain system, all of the sprockets musthave the same pitch as the chain to which they are connected When calculatingthe speed and torque ratios, you should use the number of teeth instead of usingthe actual diameter If you use the sprocket diameter, use the specified pitch diam-eter, not the outside diameter of the sprocket The pitch diameter is the actual di-ameter in which the chain will wrap around the sprocket

To locate the sprockets on the robot, you can determine the distance betweenthe sprockets in two ways The proper method would be to calculate the centerdistances and then design the robot to accommodate the dimensions Appendix Cshows the calculations for determining the center distances The other method,which is used by many beginners, is to place the two sprockets wherever you wantthem and then take a long length of chain and wrap it around both sprockets,holding the two ends in your hand Then you cut the chain at the appropriateplace, apply the master link, and possibly use an idler sprocket to take up theslack Figure 6-8 shows a sprocket

FIGURE 6-8

A typical 12-tooth

ANSI #40 sprocket.

B elt Drive Systems

In addition to chain drive systems, a belt drive system can be used to transmitpower from the motor to other devices such as wheels and weapons Many differ-ent types of belt drive systems are available, but the three most common are flatbelt, synchronous belt, and V-belt systems

Flat Belts

Flat belts are commonly used for applications that need high belt speeds, small ley diameters, and low amounts of noise Flat belts are in common use when onelarge motor drives several different pieces of machinery They cannot be used forapplications in which absolute synchronization between two pulleys is required.This is because these belts require friction to maintain motion, and slippage orcreepage can occur Flat belts must be kept under tension to transmit power fromone pulley to another Because of this, a belt tensioning device is required.One advantage of this type of system is that a flat belt could be wrapped directlybetween the motor shaft and larger diameter pulley attached directly to the robotwheel A similar application is commonly seen inside small electronic equipmentsuch as tape recorders and videocassette recorders, and you can find them turningthe rotary brushes in vacuum cleaners

pul-The drawback to these types of systems is that the two pulley surfaces must beperfectly parallel If they are not, the belts will run off the pulleys To prevent thisfrom happening, flanges need to be placed on the sides of the pulleys to constrainthe belts in place

For combat robotic applications, these types of belts can be used for spinningweapon systems If the weapon gets stalled, the motor will slip under the belt,which helps to protect the motor from stalling and burning out These types ofbelts also offer little power transmission ability due to the small frictional area ateach pulley

Synchronous Belts

Synchronous belts are more commonly known as timing belts The name timing

belt is derived from their popular use in car engines, where they’re placed between

the cam and crankshaft and are used to synchronize the cams inside the engine.Timing belts are similar to flat belts in their operation The physical difference be-tween these two belts is that the timing belts have teeth on one or both sides of thebelt This allows timing belts to synchronize the speeds between all the pulleys thatare being driven by the belt Figure 6-9 shows a timing belt

Because the teeth on the belts are used to drive the pulleys, similar to the chaindrive systems, the belt tension requirements are much less for synchronous belts

than regular flat belts Timing belts can transmit significantly more torque than ular flat belts They provide a much more quiet operation than chain drive systems.They have no backlash (they don’t slop when changing directions), so they are ideal

reg-in precise positionreg-ing systems such as automated and robotic machreg-ine tools

For combat robots, synchronous belts can be used to convert a two-wheel-driverobot into a four-wheel-drive robot, and they can be used for speed reductions.The drawbacks to timing belts are that the costs for the belts and pulleys are fairlyhigh compared to belt systems and chain drive systems, and they require the pul-leys to be precisely aligned and in the same plane with each other

Table 6-2 shows a list of the traditional belt sizes Table 6-3 shows a list of performance belt sizes

For a similar belt pitch, the high-performance belts are significantly strongerthan the standard belts Consult belt manufacturers such as Gates Rubber Com-pany or Stock Drive Products to obtain actual belt specifications for your speedand torque requirements As with chain drive systems, different pitches have dif-ferent belt designations A timing belt’s ability to transmit torque is based on the

belt’s power rating (torque × speed) and the belt’s width factor The baseline

width factor for timing belts is 1 inch

To determine the load-carrying capability of a timing belt, you multiply thepower rating of the belt by the belt width factor and divide the result by the rota-tional speed of the smallest pulley diameter With timing belts, general relationshipscan be used to describe the load-carrying capabilities of the belts You can obtainthis information directly from the belt manufacturer, who should also provide a beltdesign datasheet that will explain how to compute these values directly

The pulley centerline distances are computed in a similar manner to how thecenterline distances are computed with chain drive systems The calculations areshown in Appendix C They require more work to implement because the centerdistances have to be determined after the selection of the timing belt is made Timingbelts are available only in fixed lengths

Belt Type Pitch, mm Pitch, in.

2 mm GT 2.0 0.079

3 mm GT 3.0 0.118

5 mm GT 5.0 0.197 3mm HTD 3.0 0.118

5 mm HTD 5.0 0.197

TABLE 6-3 High-Performance Belt Size Designations ■

The power transmitting capability of a V-belt is dependent on the belt tensionand the angle of wrap around the sheave The greater the belt tension, the greaterthe torque transmitting capability As with synchronous belts, V-belts are avail-able only in fixed lengths To determine which size of V-belt to use, you shouldconsult the belt specification datasheets from the belt manufacturer.

For combat robots, V-belts could be used for drive belts in the power sion and for speed reduction applications But the most common use for V-belts isfor driving weapons As with flat belts, using V-belts in this way will allow the belt

transmis-to slip if the weapon is stalled With V-belts, more transmis-torque can be transmitted fromthe motor to the weapons, thus making them more effective than regular flat belts.The belt slippage when the weapon has stalled may be desirable in this situationbecause the drive motors are protected from complete stall and possible burnout

The compact form of a power transmission is to use a gearbox between your tors and wheels Earlier, we talked about using gearmotors for robots Agearmotor consists of a gearbox mounted to an electric motor Inside the gearboxare gears, shafts, bearings, oil/grease, and a rigid case A gearbox consists of pre-cisely designed components Within a gearbox there are various configurations ofgears to obtain the speed reduction The common methods consists of spurs gears,planetary gears, helical gears, worm gears (shown in Figure 6-10), or some combi-nation of these gears