Paul E. Sandin - Robot Mechanisms and Mechanical Devices Illustrated (779750), страница 23
Текст из файла (страница 23)
The two gearshave slightly different numbers ofteeth.Teeth make the difference. Motion between the two gears depends on aslight difference in their number of teeth (usually one or two teeth). Butdrives with gears that have up to a difference of 10 teeth have been devised.On each revolution of the wave former, there is a relative motionbetween the two gears that equals the difference in their numbers ofteeth.
The reduction ratio equals the number of teeth in the output geardivided by the difference in their numbers of teeth.Two-stage (Figure 2-26) and four-stage (Figure 2-27) gear reducersare made by combining flexible and solid gears with multiple rows ofteeth and driving the flexible gears with a common wave former.Hermetic sealing is accomplished by making the flexible gear serve asa full seal and by taking output rotation from the solid gear.Figure 2-26 A two-stage speed reducer is driven by a common-wave former operating against an integral flexible gearfor both stages.Figure 2-27 A four-stage speed reducer can, theoretically,attain reductions of millions to one.
The train is both compact and simple.102Chapter 2Indirect Power Transfer DevicesHIGH-SPEED GEARHEADS IMPROVE SMALLSERVO PERFORMANCEThe factory-made precision gearheads now available for installation inthe latest smaller-sized servosystems can improve their performancewhile eliminating the external gears, belts, and pulleys commonly usedin earlier larger servosystems. The gearheads can be coupled to thesmaller, higher-speed servomotors, resulting in simpler systems withlower power consumption and operating costs.Gearheads, now being made in both in-line and right-angle configurations, can be mounted directly to the drive motor shafts.
They canconvert high-speed, low-torque rotary motion to a low-speed, hightorque output. The latest models are smaller and more accurate thantheir predecessors, and they have been designed to be compatible withthe smaller, more precise servomotors being offered today.Gearheads have often been selected for driving long trains of mechanisms in machines that perform such tasks as feeding wire, wood, ormetal for further processing. However, the use of an in-line gearheadadds to the space occupied by these machines, and this can be a problemwhere factory floor space is restricted. One way to avoid this problem isto choose a right-angle gearhead (Figure 2-28).
It can be mounted vertically beneath the host machine or even horizontally on the machine bed.Horizontal mounting can save space because the gearheads and motorscan be positioned behind the machine, away from the operator.Bevel gears are commonly used in right-angle drives because they canprovide precise motion. Conically shaped bevel gears with straight- orspiral-cut teeth allow mating shafts to intersect at 90º angles. Straight-cutbevel gears typically have contact ratios of about 1.4, but the simultaneous mating of straight teeth along their entire lengths causes more vibration and noise than the mating of spiral-bevel gear teeth. By contrast, spiral-bevel gear teeth engage and disengage gradually and precisely withcontact ratios of 2.0 to 3.0, making little noise.
The higher contact ratiosof spiral-bevel gears permit them to drive loads that are 20 to 30% greaterthan those possible with straight bevel gears. Moreover, the spiral-bevelteeth mesh with a rolling action that increases their precision and alsoreduces friction. As a result, operating efficiencies can exceed 90%.Simplify the MountingThe smaller servomotors now available force gearheads to operate athigher speeds, making vibrations more likely. Inadvertent misalignmentbetween servomotors and gearboxes, which often occurs during installation, is a common source of vibration. The mounting of conventionalChapter 2Indirect Power Transfer DevicesFigure 2-28 This right-angle gearhead is designed for high-performance servo applications. It includes helical planetary output gears, a rigid sun gear, spiral bevel gears, and abalanced input pinion.
Courtesy of Bayside Controls Inc.motors with gearboxes requires several precise connections. The outputshaft of the motor must be attached to the pinion gear that slips into a setof planetary gears in the end of the gearbox, and an adapter plate mustjoint the motor to the gearbox. Unfortunately, each of these connectionscan introduce slight alignment errors that accumulate to cause overallmotor/gearbox misalignment.103104Chapter 2Indirect Power Transfer DevicesThe pinion is the key to smooth operation because it must be alignedexactly with the motor shaft and gearbox. Until recently it has been standardpractice to mount pinions in the field when the motors were connected to thegearboxes.
This procedure often caused the assembly to vibrate. Engineersrealized that the integration of gearheads into the servomotor packagewould solve this problem, but the drawback to the integrated unit is that failure of either component would require replacement of the whole unit.A more practical solution is to make the pinion part of the gearheadassembly because gearheads with built-in pinions are easier to mount toservomotors than gearheads with field-installed pinions. It is only necessary to insert the motor shaft into the collar that extends from the gearhead’s rear housing, tighten the clamp with a wrench, and bolt the motorto the gearhead.Pinions installed at the factory ensure smooth-running gearheadsbecause they are balanced before they are mounted. This procedure permits them to spin at high speed without wobbling.
As a result, the balanced pinions minimize friction and thus cause less wear, noise, andvibration than field-installed pinions.However, the factory-installed pinion requires a floating bearing tosupport the shaft with a pinion on one end. The Bayside Motion Groupof Bayside Controls Inc., Port Washington, New York, developed a selfaligning bearing for this purpose. Bayside gearheads with these pinionsare rated for input speeds up to 5000 rpm. A collar on the pinion shaft’sother end mounts to the motor shaft.
The bearing holds the pinion inplace until it is mounted. At that time a pair of bearings in the servomotor support the coupled shaft. The self-aligning feature of the floatingbearing lets the motor bearing support the shaft after installation.The pinion and floating bearing help to seal the unit during its operation. The pinion rests in a blind hole and seals the rear of the gearhead.This seal keeps out dirt while retaining the lubricants within the housing.Consequently, light grease and semifluid lubricants can replace heavygrease.Cost-Effective AdditionThe installation of gearheads can smooth the operation of servosystems aswell as reduce system costs.
The addition of a gearhead to the system doesnot necessarily add to overall operating costs because its purchase pricecan be offset by reductions in operating costs. Smaller servomotors inherently draw less current than larger ones, thus reducing operating costs, butthose power savings are greatest in applications calling for low speed andhigh torque because direct-drive servomotors must be considerably largerthan servomotors coupled to gearheads to perform the same work.Chapter 2Indirect Power Transfer DevicesSmall direct-drive servomotors assigned to high-speed/low-torqueapplications might be able to perform the work satisfactorily without agearhead.
In those instances servo/gearhead combinations might not beas cost-effective because power consumption will be comparable.Nevertheless, gearheads will still improve efficiency and, over time, evensmall decreases in power consumption due to the use of smaller-sizedservos will result in reduced operating costs.The decision to purchase a precision gearhead should be evaluated ona case-by-case basis. The first step is to determine speed and torquerequirements. Then keep in mind that although in high-speed/low-torqueapplications a direct-drive system might be satisfactory, low-speed/hightorque applications almost always require gearheads. Then a decisioncan be made after weighing the purchase price of the gearhead againstanticipated servosystem operating expenses in either operating mode toestimate savings.The planetary gearbox is one of the most efficient and compact gearbox designs.
Its internal coaxial layout reduces efficiency robbing sideloads on the gear’s shafts. Figure 2-29 and the following tables give theformulas required to calculate the input/output ratios. In spite of itshigher cost, a planetary gearbox is frequently the best choice for mediumratio power transfer. Because of its even greater precision requirementthan spur gears, it is usually better to buy an off-the-shelf gearbox than todesign your own.Figure 2-29 Simple Planetaries and Inversions105This page intentionally left blank.Chapter 3 Direct PowerTransfer DevicesCopyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.This page intentionally left blank.COUPLINGSAt some point in a mobile robot designer’s career there will be a need tocouple two shafts together.
Fortunately, there are many commerciallyavailable couplers to pick from, each with its own strengths and weaknesses. Couplers are available in two major styles: solid and flexible.Solid couplers must be strong enough to hold the shafts’ ends together asif they were one shaft. Flexible couplers allow for misalignment and areused where the two shafts are already running in their own bearings, butmight be slightly out of alignment. The only other complication is thatthe shafts may be different diameters, or have different end details likesplined, keyed, hex, square, or smooth.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
