Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 20
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These lugs can be bolted directly to pads andused for tracks on tracked vehicles, simplifying this sometimes complicated part of a high-mobility robot. Care must be taken to keep the padsas thin as possible, or to space them out to every other bent lug becausedebris can jam between the pads and cause problems. This is why trackson excavators and military tanks are specially designed with the pivotpoint as close to the ground as possible.
Other than that small issue, however, this chain can be and has been used as the backbone for tracks.Rack and Pinion Chain DriveBent lug roller chain can also be used as a low cost rack and pinion driveto get linear motion from rotary motion. Though crude, this systemworks well if noise and a slightly non-smooth linear motion can be tolerated. Figure 2-11 shows a basic layout for this concept.Timing or Silent ChainSilent chain gets its name from the fact that it is very quiet, even at highspeeds and loads.
It is also more efficient than roller chain because theclever shape of its inverted teeth provide smooth transfer of power fromChapter 2Indirect Power Transfer Devices83Figure 2-12 Silent chain toothprofilethe chain to the pulley. It is intolerant of grit, is somewhat more expensive, and requires more precision in alignment between the driveR anddriveN pulleys than a roller chain.It is a very good choice for transmitting high horsepower at thousandsof rpm from an electric motor or an internal combustion engine to thetransmission of large vehicles. It was used in the Oldsmobile Toranadoautomobile in the late 1970s, where it transmitted several hundred horsepower from the engine to the transmission.
It is not made in small sizesbecause of the special shape of its teeth (Figure 2-12) and is designedmainly for power ranges from tens to hundreds of horsepower.With proper design and simple maintenance, a silent chain drive willlast for thousands of hours. If high efficiency and high power arerequired with operation in a clean environment, and the higher price canbe afforded, silent chain is the best choice of any power transfer device inthis book.FRICTION DRIVESPower can be transferred by friction alone.
This technique is usuallyreserved for special cases, where its short life is acceptable. Its claim tofame is its high efficiency and ability to vary speed. The usual layoutfor a variable speed friction drive is a hardened steel wheel mounted onthe input shaft, which is pushed very hard against a steel disk mountedon the output shaft. Efficiencies can be high, but the high forcesrequired to carry the torque through only friction wear out the mating84Chapter 2Indirect Power Transfer Devicessurfaces at a high rate.
This drive has been used with some success inwalk-behind lawn mowers, but its life in that application is usuallyonly a couple seasons. Figure 2-13 shows one of several versions of afriction drive.CONE DRIVE NEEDS NO GEARS OR PULLEYSA variable-speed-transmission cone drive operates without gears orpulleys. The drive unit has its own limited slip differential and clutch.As the drawing shows, two cones made of brake lining material aremounted on a shaft directly connected to the engine.
These drive twolarger steel conical disks mounted on the output shaft. The outer disksare mounted on pivoting frames that can be moved by a simple controlrod.To center the frames and to provide some resistance when the outerdisks are moved, two torsion bars attached to the main frame connectand support the disk-support frames. By altering the position of theframes relative to the driving cones, the direction of rotation and speedcan be varied.The unit was invented by Marion H. Davis of Indiana.Figure 2-13 Cone drive operates without lubrication.Chapter 2Indirect Power Transfer DevicesGEARSGears are the most common form of power transmission for several reasons.
They can be scaled to transmit power from small battery poweredwatch motors (or even microscopic), up to the power from thousandhorsepower gas turbine engines. Properly mounted and lubricated, theytransmit power efficiently, smoothly, and quietly. They can transmitpower between shafts that are parallel, intersecting, or even skew. Forall their pluses, there are a few important things to remember aboutgears. To be efficient and quiet, they require high precision, both in theshape of the teeth and the distance between one gear and its matinggear.
They do not tolerate dirt and must be enclosed in a sealed case thatkeeps the teeth clean and contains the required lubricating oil or grease.In general, gears are an excellent choice for the majority of power transmission applications.Gears come in many forms and standard sizes, both inch and metric.They vary in diameter, tooth size, face width (the width of the gear), andtooth shape. Any two gears with the same tooth size can be usedtogether, allowing very large ratios in a single stage.
Large ratiosbetween a single pair of gears cause problems with tooth wear and areusually obtained by using cluster gears to reduce the gearbox’s overallsize. Figure 2-14 shows an example of a cluster gear. Cluster gearsreduce the size of a gearbox by adding an interim stage of gears.
Theyare ubiquitous in practically every gearbox with a gear ratio of more than5:1, with the exception of planetary and worm gearboxes.Gears are available as spur, internal, helical, double helical (herringbone), bevel, spiral bevel, miter, face, hypoid, rack, straight worm, double enveloping worm, and harmonic. Each type has its own pros andcons, including differences in efficiency, allowable ratios, mating shaftangles, noise, and cost.
Figure 2-15 shows the basic tooth profile of aspur gear.Gears are versatile mechanical components capable of performingmany different kinds of power transmission or motion control. Examplesof these are••••••Changing rotational speed.Changing rotational direction.Changing the angular orientation of rotational motion.Multiplication or division of torque or magnitude of rotation.Converting rotational to linear motion and its reverse.Offsetting or changing the location of rotating motion.8586Chapter 2Indirect Power Transfer DevicesFigure 2-14 Cluster gearFigure 2-15 Gear ToothTerminologyChapter 2Indirect Power Transfer DevicesGear Tooth Geometry: This is determined primarily by pitch, depth,and pressure angle.Gear Terminologyaddendum: The radial distance between the top land and the pitch circle.addendum circle: The circle defining the outer diameter of the gear.circular pitch: The distance along the pitch circle from a point on onetooth to a corresponding point on an adjacent tooth.
It is also the sum ofthe tooth thickness and the space width, measured in inches or millimeters.clearance: The radial distance between the bottom land and the clearance circle.contact ratio: The ratio of the number of teeth in contact to the numberof those not in contact.dedendum circle: The theoretical circle through the bottom lands of agear.dedendum: The radial distance between the pitch circle and the dedendum circle.depth: A number standardized in terms of pitch. Full-depth teeth have aworking depth of 2/P. If the teeth have equal addenda (as in standardinterchangeable gears), the addendum is 1/P. Full-depth gear teeth have alarger contact ratio than stub teeth, and their working depth is about 20%more than that of stub gear teeth.
Gears with a small number of teethmight require undercutting to prevent one interfering with another during engagement.diametral pitch (P): The ratio of the number of teeth to the pitch diameter. A measure of the coarseness of a gear, it is the index of tooth sizewhen U.S. units are used, expressed as teeth per inch.pitch: A standard pitch is typically a whole number when measured as adiametral pitch (P).
Coarse-pitch gears have teeth larger than a diametral pitch of 20 (typically 0.5 to 19.99). Fine-pitch gears usually haveteeth of diametral pitch greater than 20. The usual maximum fineness is120 diametral pitch, but involute-tooth gears can be made with diametralpitches as fine as 200, and cycloidal tooth gears can be made with diametral pitches to 350.pitch circle: A theoretical circle upon which all calculations are based.8788Chapter 2Indirect Power Transfer Devicespitch diameter: The diameter of the pitch circle, the imaginary circlethat rolls without slipping with the pitch circle of the mating gear, measured in inches or millimeters.pressure angle: The angle between the tooth profile and a line perpendicular to the pitch circle, usually at the point where the pitch circle andthe tooth profile intersect.
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