Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 19
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Theyrequire good alignment between the driveR and driveN pulleys and thepulleys themselves are not actually flat, but slightly convex. While theydo work, there are better belt styles to use for most applications. They arefound in some vacuum cleaners because they are resistant to dirt buildup.O-Ring BeltsO-ring belts are used in some applications mostly because they areextremely cheap. They too suffer from moderate efficiency, but their costis so low that they are used in toys and low power devices like VCRs etc.They are a good choice in their power range, but require proper tensionand alignment for good life and efficiency.V-BeltsV-belts get their name from the shape of a cross section of the belt, whichis similar to a V with the bottom chopped flat.
Their design relies on friction, just like flat belts and O-ring belts, but they have the advantage thatthe V shape jams in a matching V shaped groove in the pulley. Thisincreases the friction force because of the steep angle of the V and therefore increases the transmittable torque under the same tension as isrequired for flat or O-ring belts. V-belts are also very quiet, allow somemisalignment, and are surprisingly efficient. They are a good choice forpower levels from fractional to tens of horsepower.
Their only draw-7374Chapter 2Indirect Power Transfer DevicesFigure 2-1 Flat, O-ring, andV-belt profiles and pulleysback is a slight tendency to slip over time. This slip means the computer has no precise control of the orientation of the output shaft,unless a feedback device is on the driveN pulley. There are severalapplications, however, where some slip is not much of a problem, likein some wheel and track drives. Figure 2-1 shows the cross sectionalshape of each belt.In spite of the warnings on the possibility of problems using variablespeed drives, here are some examples of methods of varying the speedand torque by using variable diameter sheaves. Figure 2-2 (fromMechanisms and Mechanical Devices Sourcebook, as are many of thefigures in this book) shows how variable speed drives work. They mayhave some applications, especially in teleoperated vehicles.Figure 2-2Variable BeltChapter 2Indirect Power Transfer DevicesSMOOTHER DRIVE WITHOUT GEARSThe transmission in the motor scooter in Figure 2-3 is torque-sensitive;motor speed controls the continuously variable drive ratio.
The operatormerely works the throttle and brake.Variable-diameter V-belt pulleys connect the motor and chain drivesprocket to give a wide range of speed reduction. The front pulley incorporates a three-ball centrifugal clutch which forces the flanges togetherwhen the engine speeds up. At idle speed the belt rides on a ballbearingbetween 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-pulleyflanges until the overall drive ratio becomes approximately 6:1.
The resulting variations in belt tension are absorbed by the spring-loaded flanges ofthe rear pulley. When a clutch is in an idle position, the V-belt is forced tothe outer edge of the rear pulley by a spring force. When the clutchengages, the floating half of the front pulley moves inward, increasing itseffective diameter and pulling the belt down between the flanges of therear pulley.The transmission is torque-responsive. A sudden engine accelerationincreases the effective diameter of the rear pulley, lowering the drive ratio.It works this way: An increase in belt tension rotates the floating flangeahead in relation to the driving flange.
The belt now slips slightly on itsdriver. At this time nylon rollers on the floating flange engage cams on thedriving flange, pulling the flanges together and increasing the effectivediameter of the pulley.Figure 2-3Smoother Drive Without Gears7576Chapter 2Indirect Power Transfer DevicesTiming BeltsTiming belts solve the slip problems of flat, O ring, and V belts by usinga flexible tooth, molded to a belt that has tension members built in.
Theteeth are flexible allowing the load to be spread out over all the teeth incontact with the pulley. Timing belts are part of a larger category ofpower transmission devices called synchronous drives. These belt orcable-based drives have the distinct advantage of not slipping, hence thename synchronous. Synchronous or positive drive also means these beltscan even be used in wet conditions, provided the pulleys are stainlesssteel or plastic to resist corrosion.Timing belts come in several types, depending on their tooth profileand manufacturing method. The most common timing belt has a trapezoidal shaped tooth. This shape has been the standard for many years,but it does have drawbacks.
As each tooth comes in contact with the mating teeth on a pulley, the tooth tends to be deflected by the cantileverforce, deforming the belt’s teeth so that only the base of the toothremains in contact. This bending and deformation wastes energy andalso can make the teeth ride up pulley’s teeth and skip teeth. The deformation also increases wear of the tooth material and causes the timingbelt 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 shapeis HTD for High Torque Design. Timing belts can be used at very lowrpm, high torque, and at power levels up to 250 horsepower. They are anexcellent method of power transfer, but for a slightly higher price thanchain or plastic-and-cable chain discussed later in this chapter.Table 2-1Timing BeltsChapter 2Indirect Power Transfer Devices77Figure 2-4 Trapezoidal ToothTiming BeltFigure 2-5 HTD Timing BeltTooth ProfilePlastic-and-Cable ChainThe other type of synchronous drive is based on a steel cable core. It isactually the reverse of belt construction where the steel or synthetic cableis molded into the rubber or plastic belt. Plastic-and-cable chain startswith the steel cable and over-molds plastic or hard rubber teeth onto thecable. The result appears almost like a roller chain.
This style is sometimes called Posi-drive, plastic-and-cable, or cable chain. It is made inthree 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 outof both sides of the cable, or even 4 sides of the cable. The third form is78Chapter 2Indirect Power Transfer DevicesFigure 2-6 Polyurethane-coatedsteel-cable "chains"—bothbeaded and 4-pinned—can copewith conditions unsuitable formost conventional belts andchains.Figure 2-7 Plastic pins eliminatethe bead chain's tendency to camout of pulley recesses, and permitgreater precision in angulartransmission.Chapter 2Indirect Power Transfer Devices79Figure 2-8 A gear chain canfunction as a ladder chain, as awide V-belt, or, as here, a gearsurrogate meshing with a standard pinion.shown in Figure 2-8.
This is sometimes called plastic ladder chain. It is adouble cable form and is the kind that looks like a roller chain, except therollers are replaced with non-rolling plastic cross pieces. These teethengage a similar shape profile cut in the mating pulleys.Another form of the cable-based drive wraps a spiral of plastic coatedsteel cable around the base cable. The pulleys for this form have a matching spiral-toothed groove. This type can bend in any direction, allowingit to be used to change drive planes.
Both of these synchronous drivetypes are cheap and functional for low power applications.CHAINChain 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 tohundreds of horsepower80Chapter 2Figure 2-9Indirect Power Transfer DevicesLadder ChainLadder ChainLadder chain is so named because it looks like a very small ladder. Itsconstruction is extremely simple and inexpensive. A short piece of wireis bent into a U shape and looped over the next U in the chain. See Figure2-9. This construction is not very strong so this chain is used mainlywhere low cost is paramount and the power being transferred is less than1/4 horsepower.Roller ChainRoller chain is an efficient power transfer method.
It is called roller chainbecause it has steel rollers turning on pins held together by links. Rollerchain is robust and can handle some misalignment between the driveRand driveN gears, and in many applications does not require precise pretensioning 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 withcareful attention to pulley spacing, making it a simple way to get an efficient, high reduction system.
It is also surprisingly strong. The mostcommon size chain, #40 (the distance from one roller to the next is .4")Chapter 2Indirect Power Transfer Devices81can transfer up to 2 horsepower at 300 rpm without special lubrication.Even the smallest size, #25, can transmit more than 5 horsepower at3000 rpm with adequate forced lubrication and sufficiently large pulleys.There are several good references online that give much more detail thanis within the scope of this book—they are•••••americanchainassn.orgbostongear.comdiamondchain.comramseychain.comustsubaki.comAs shown in Figure 2-10 (a–d), roller chain comes in many sizes andstyles, some of which are useful for things other than simply transferringpower from one pulley to another.Figure 2-10a Standard rollerchain—for power transmissionand conveying.Figure 2-10b Extended pitchchain—for conveyingFigure 2-10cadaptationsStandard pitch82Chapter 2Figure 2-10dadaptationsIndirect Power Transfer DevicesExtended pitchFigure 2-11 Bent lug rollerchain used for rack and pinionlinear actuator.A clever, commercially available modification of roller chain hasextended and bent lugs.
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