Sandin P.E. Robot mechanisms and mechanical devices illustrated

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74 Chapter 2 Indirect Power Transfer Devices

back is a slight tendency to slip over time. This slip means the com-

puter has no precise control of the orientation of the output shaft,

unless a feedback device is on the driveN pulley. There are several

applications, however, where some slip is not much of a problem, like

in some wheel and track drives. Figure 2-1 shows the cross sectional

shape of each belt.

In spite of the warnings on the possibility of problems using variable

speed drives, here are some examples of methods of varying the speed

and torque by using variable diameter sheaves. Figure 2-2 (from

Mechanisms and Mechanical Devices Sourcebook, as are many of the

figures in this book) shows how variable speed drives work. They may

have some applications, especially in teleoperated vehicles.

Figure 2-1 Flat, O-ring, and

V-belt profiles and pulleys

Figure 2-2 Variable Belt

Chapter 2 Indirect Power Transfer Devices 75

SMOOTHER DRIVE WITHOUT GEARS

The transmission in the motor scooter in Figure 2-3 is torque-sensitive;

motor speed controls the continuously variable drive ratio. The operator

merely works the throttle and brake.

Variable-diameter V-belt pulleys connect the motor and chain drive

sprocket to give a wide range of speed reduction. The front pulley incor-

porates a three-ball centrifugal clutch which forces the flanges together

when the engine speeds up. At idle speed the belt rides on a ballbearing

between the retracted flanges of the pulley. During starting and warmup,

a lockout prevents the forward clutch from operating.

Upon initial engagement, the overall drive ratio is approximately 18:1.

As engine speed increases, the belt rides higher up on the forward-pulley

flanges until the overall drive ratio becomes approximately 6:1. The result-

ing variations in belt tension are absorbed by the spring-loaded flanges of

the rear pulley. When a clutch is in an idle position, the V-belt is forced to

the outer edge of the rear pulley by a spring force. When the clutch

engages, the floating half of the front pulley moves inward, increasing its

effective diameter and pulling the belt down between the flanges of the

rear pulley.

The transmission is torque-responsive. A sudden engine acceleration

increases the effective diameter of the rear pulley, lowering the drive ratio.

It works this way: An increase in belt tension rotates the floating flange

ahead in relation to the driving flange. The belt now slips slightly on its

driver. At this time nylon rollers on the floating flange engage cams on the

driving flange, pulling the flanges together and increasing the effective

diameter of the pulley.

Figure 2-3 Smoother Drive Without Gears

76 Chapter 2 Indirect Power Transfer Devices

Table 2-1 Timing Belts

Timing Belts

Timing belts solve the slip problems of flat, O ring, and V belts by using

a flexible tooth, molded to a belt that has tension members built in. The

teeth are flexible allowing the load to be spread out over all the teeth in

contact with the pulley. Timing belts are part of a larger category of

power transmission devices called synchronous drives. These belt or

cable-based drives have the distinct advantage of not slipping, hence the

name synchronous. Synchronous or positive drive also means these belts

can even be used in wet conditions, provided the pulleys are stainless

steel or plastic to resist corrosion.

Timing belts come in several types, depending on their tooth profile

and manufacturing method. The most common timing belt has a trape-

zoidal shaped tooth. This shape has been the standard for many years,

but it does have drawbacks. As each tooth comes in contact with the mat-

ing teeth on a pulley, the tooth tends to be deflected by the cantilever

force, deforming the belt’s teeth so that only the base of the tooth

remains in contact. This bending and deformation wastes energy and

also can make the teeth ride up pulley’s teeth and skip teeth. The defor-

mation also increases wear of the tooth material and causes the timing

belt drive to be somewhat noisy.

Several other shapes have been developed to improve on this design,

the best of which is the curved tooth profile. A trade name for this shape

is HTD for High Torque Design. Timing belts can be used at very low

rpm, high torque, and at power levels up to 250 horsepower. They are an

excellent method of power transfer, but for a slightly higher price than

chain or plastic-and-cable chain discussed later in this chapter.

Chapter 2 Indirect Power Transfer Devices 77

Plastic-and-Cable Chain

The other type of synchronous drive is based on a steel cable core. It is

actually the reverse of belt construction where the steel or synthetic cable

is molded into the rubber or plastic belt. Plastic-and-cable chain starts

with the steel cable and over-molds plastic or hard rubber teeth onto the

cable. The result appears almost like a roller chain. This style is some-

times called Posi-drive, plastic-and-cable, or cable chain. It is made in

three basic forms.

The simplest is molding beads onto the cable as shown in Figure 2-6.

Figure 2-7 shows a single cable form where the plastic teeth protrude out

of both sides of the cable, or even 4 sides of the cable. The third form is

Figure 2-5 HTD Timing Belt

Tooth Profile

Figure 2-4 Trapezoidal Tooth

Timing Belt

78 Chapter 2 Indirect Power Transfer Devices

Figure 2-7 Plastic pins eliminate

the bead chain's tendency to cam

out of pulley recesses, and permit

greater precision in angular

transmission.

Figure 2-6 Polyurethane-coated

steel-cable "chains"—both

beaded and 4-pinned—can cope

with conditions unsuitable for

most conventional belts and

chains.

Chapter 2 Indirect Power Transfer Devices 79

shown in Figure 2-8. This is sometimes called plastic ladder chain. It is a

double cable form and is the kind that looks like a roller chain, except the

rollers are replaced with non-rolling plastic cross pieces. These teeth

engage a similar shape profile cut in the mating pulleys.

Another form of the cable-based drive wraps a spiral of plastic coated

steel cable around the base cable. The pulleys for this form have a match-

ing spiral-toothed groove. This type can bend in any direction, allowing

it to be used to change drive planes. Both of these synchronous drive

types are cheap and functional for low power applications.

CHAIN

Chain comes in three basic types.

• Ladder chain, generally used for power levels below 1/4 horsepower

• Roller chain, for fractional to hundreds of horsepower

• Timing chain, also called silent chain, for power levels in the tens to

hundreds of horsepower

Figure 2-8 A gear chain can

function as a ladder chain, as a

wide V-belt, or, as here, a gear

surrogate meshing with a stan-

dard pinion.

80 Chapter 2 Indirect Power Transfer Devices

Ladder Chain

Ladder chain is so named because it looks like a very small ladder. Its

construction is extremely simple and inexpensive. A short piece of wire

is bent into a U shape and looped over the next U in the chain. See Figure

2-9. This construction is not very strong so this chain is used mainly

where low cost is paramount and the power being transferred is less than

1/4 horsepower.

Roller Chain

Roller chain is an efficient power transfer method. It is called roller chain

because it has steel rollers turning on pins held together by links. Roller

chain is robust and can handle some misalignment between the driveR

and driveN gears, and in many applications does not require precise pre-

tensioning of the pulleys. It has two minor weaknesses.

1. It doesn’t tolerate sand or abrasive environments very well.

2. It can be noisy.

Roller chain can be used for single stage reductions of up to 6:1 with

careful attention to pulley spacing, making it a simple way to get an effi-

cient, high reduction system. It is also surprisingly strong. The most

common size chain, #40 (the distance from one roller to the next is .4")

Figure 2-9 Ladder Chain

Chapter 2 Indirect Power Transfer Devices 81

can transfer up to 2 horsepower at 300 rpm without special lubrication.

Even the smallest size, #25, can transmit more than 5 horsepower at

3000 rpm with adequate forced lubrication and sufficiently large pulleys.

There are several good references online that give much more detail than

is within the scope of this book—they are

• americanchainassn.org

• bostongear.com

• diamondchain.com

• ramseychain.com

• ustsubaki.com

As shown in Figure 2-10 (a–d), roller chain comes in many sizes and

styles, some of which are useful for things other than simply transferring

power from one pulley to another.

Figure 2-10a Standard roller

chain—for power transmission

and conveying.

Figure 2-10b Extended pitch

chain—for conveying

Figure 2-10c Standard pitch

adaptations

82 Chapter 2 Indirect Power Transfer Devices

A clever, commercially available modification of roller chain has

extended and bent lugs. These lugs can be bolted directly to pads and

used for tracks on tracked vehicles, simplifying this sometimes compli-

cated part of a high-mobility robot. Care must be taken to keep the pads

as thin as possible, or to space them out to every other bent lug because

debris can jam between the pads and cause problems. This is why tracks

on excavators and military tanks are specially designed with the pivot

point as close to the ground as possible. Other than that small issue, how-

ever, this chain can be and has been used as the backbone for tracks.

Rack and Pinion Chain Drive

Bent lug roller chain can also be used as a low cost rack and pinion drive

to get linear motion from rotary motion. Though crude, this system

works well if noise and a slightly non-smooth linear motion can be toler-

ated. Figure 2-11 shows a basic layout for this concept.

Timing or Silent Chain

Silent chain gets its name from the fact that it is very quiet, even at high

speeds and loads. It is also more efficient than roller chain because the

clever shape of its inverted teeth provide smooth transfer of power from

Figure 2-11 Bent lug roller

chain used for rack and pinion

linear actuator.

Figure 2-10d Extended pitch

adaptations

Chapter 2 Indirect Power Transfer Devices 83

the chain to the pulley. It is intolerant of grit, is somewhat more expen-

sive, and requires more precision in alignment between the driveR and

driveN pulleys than a roller chain.

It is a very good choice for transmitting high horsepower at thousands

of rpm from an electric motor or an internal combustion engine to the

transmission of large vehicles. It was used in the Oldsmobile Toranado

automobile in the late 1970s, where it transmitted several hundred horse-

power from the engine to the transmission. It is not made in small sizes

because of the special shape of its teeth (Figure 2-12) and is designed

mainly for power ranges from tens to hundreds of horsepower.

With proper design and simple maintenance, a silent chain drive will

last for thousands of hours. If high efficiency and high power are

required with operation in a clean environment, and the higher price can

be afforded, silent chain is the best choice of any power transfer device in

this book.

FRICTION DRIVES

Power can be transferred by friction alone. This technique is usually

reserved for special cases, where its short life is acceptable. Its claim to

fame is its high efficiency and ability to vary speed. The usual layout

for a variable speed friction drive is a hardened steel wheel mounted on

the input shaft, which is pushed very hard against a steel disk mounted

on the output shaft. Efficiencies can be high, but the high forces

required to carry the torque through only friction wear out the mating

Figure 2-12 Silent chain tooth

profile