1、兰州交通大学毕业设计（论文） 1 Gears and gear drive Gears are the most durable and rugged of all mechanical drives. They can transmit high power at efficiencies up to 98% and with long service lives. For this reason, gears rather than belts or chains are found in automotive transmissions and most heavy-duty machi
2、ne drives. On the other hand, gears are more expensive than other drives, especially if they are machined and not made from power metal or plastic. Gear cost increases sharply with demands for high precision and accuracy. So it is important to establish tolerance requirements appropriate for the app
3、lication. Gears that transmit heavy loads or than operate at high speeds are not particularly expensive, but gears that must do both are costly. Silent gears also are expensive. Instrument and computer gears tend to be costly because speed or displacement ratios must be exact. At the other extreme,
4、gears operating at low speed in exposed locations are normally termed no critical and are made to minimum quality standards. For tooth forms, size, and quality, industrial practice is to follow standards set up by the American Gear Manufactures Association (AGMA). Tooth form Standards published by A
5、GMA establish gear proportions and tooth profiles. Tooth geometry is determined primarily by pitch, depth, and pressure angle. Pitch： Standards pitches are usually whole numbers when measured as diametral pitch P. Coarse-pitch gearing has teeth larger than 20 diametral pitch usually 0.5 to 19.99. Fi
6、ne-pitch gearing usually has teeth of diametral pitch 20 to 200. Depth: Standardized in terms of pitch. Standard full-depth have working depth of 2/p. If the teeth have equal addenda(as in standard interchangeable gears) the addendum is 1/p. Stub teeth have a working depth usually 20% less than full
7、-depth teeth. Full-depth teeth have a larger contract ratio than stub teeth. Gears with small numbers of teeth may have undercut so than they do not interfere with one another during engagement. Undercutting reduce active profile and weakens the tooth. Mating gears with long and short addendum have
8、larger load-carrying capacity than standard gears. The addendum of the smaller gear (pinion) is increased while that of larger 兰州交通大学毕业设计（论文） 2 gear is decreased, leaving the whole depth the same. This form is know as recess-action gearing. Pressure Angle: Standard angles are 020 and 025 . Earlier s
9、tandards include a 14- 02/1 pressure angle that is still used. Pressure angle affects the force that tends to separate mating gears. High pressure angle decreases the contact ratio (ratio of the number of teeth in contact) but provides a tooth of higher capacity and allows gears to have fewer teeth
10、without undercutting. Backlash: Shortest distances between the non-contacting surfaces of adjacent teeth . Gears are commonly specified according to AGMA Class Number, which is a code denoting important quality characteristics. Quality number denote tooth-element tolerances. The higher the number, t
11、he closer the tolerance. Number 8 to 16 apply to fine-pitch gearing. Gears are heat-treated by case-hardening, through-hardening, nitriding, or precipitation hardening. In general, harder gears are stronger and last longer than soft ones. Thus, hardening is a device that cuts the weight and size of
12、gears. Some processes, such as flame-hardening, improve service life but do not necessarily improve strength. Design checklist The larger in a pair is called the gear, the smaller is called the pinion. Gear Ratio: The number of teeth in the gear divide by the number of teeth in the pinion. Also, rat
13、io of the speed of the pinion to the speed of the gear. In reduction gears, the ratio of input to output speeds. Gear Efficiency: Ratio of output power to input power. (includes consideration of power losses in the gears, in bearings, and from windage and churning of lubricant.) Speed: In a given ge
14、ar normally limited to some specific pitchline velocity. Speed capabilities can be increased by improving accuracy of the gear teeth and by improving balance of the rotating parts. Power: Load and speed capacity is determined by gear dimensions and by type of gear. Helical and helical-type gears hav
15、e the greatest capacity (to approximately 30,000 hp). Spiral bevel gear are normally limited to 5,000 hp, and worm gears are usually limited to about 750 hp. Special requirements Matched-Set Gearing: In applications requiring extremely high accuracy, it may be 兰州交通大学毕业设计（论文） 3 necessary to match pin
16、ion and gear profiles and leads so that mismatch does not exceed the tolerance on profile or lead for the intended application. Tooth Spacing: Some gears require high accuracy in the circular of teeth. Thus, specification of pitch may be required in addition to an accuracy class specification. Backl
17、ash: The AMGA standards recommend backlash ranges to provide proper running clearances for mating gears. An overly tight mesh may produce overload. However, zero backlash is required in some applications. Quiet Gears: To make gears as quit as possible, specify the finest pitch allowable for load con
18、ditions. (In some instances, however, pitch is coarsened to change mesh frequency to produce a more pleasant, lower-pitch sound.) Use a low pressure angle. Use a modified profile to include root and tip relief. Allow enough backlash. Use high quality numbers. Specify a surface finish of 20 in. or be
19、tter. Balance the gear set. Use a nonintegral ratio so that the same teeth do not repeatedly engage if both gear and pinion are hardened steel. (If the gear is made of a soft material, an integral ratio allows the gear to cold-work and conform to the pinion, thereby promoting quiet operation.) Make
20、sure critical are at least 20% apart from operating speeding or speed multiples and from frequency of tooth mesh. Multiple mesh gear Multiple mesh refers to move than one pair of gear operating in a train. Can be on parallel or nonparallel axes and on intersection or nonintersecting shafts. They per
21、mit higer speed ratios than are feasible with a single pair of gears . Series trains:Overall ratio is input shaft speed divided by output speed ,also the product of individual ratios at each mesh ,except in planetary gears .Ratio is most easily found by dividing the product of numbers of teeth of dr
22、iven gears by the product of numbers of teeth of driving gears. Speed increasers (with step-up rather than step-down ratios) may require special care in manufacturing and design. They often involve high speeds and may creste problems in gear dynamics. Also, frictional and drag forces are magnified w
23、hich, in extreme cases , may lead to operational problems. Epicyclic Gearing:Normally, a gear axis remains fixed and only the gears rotates. But in an epicyclic gear train, various gears axes rotate about one anther to provide specialized output motions. With suitable clutchse and brakes, an epicyclic train serves as the planetary
