31-01-2013, 02:51 PM
SPUR GEARS AND BEVEL GEARS
SPUR GEARS.doc (Size: 1.15 MB / Downloads: 63)
Introduction
Spur gears are simply toothed wheels used for transmitting power and motion from one shaft to another. The design of gears is highly standardised by the AGMA (American Gear Manufacturers’ Association). Like bearings, gears represent a product of excellence in engineering design.
The advantages of gear transmissions are: (1) transmit large power, (2) high efficiency, (3) stable speed ratio, (4) long life, and (5) reliable. The disadvantages are: (1) high requirements in manufacturing and assembling, (2) expensive, and (3) unsuitable for long distance transmission.
The design of spur gears requires the study and objective understanding of the following subjects:
-- Kinematics and geometrical shapes
-- Analysis of forces produced in gears and in gear trains
-- Design of gears against breakage
-- Design of gears against deterioration and wear
Meshing Geometry
When using spur gears the designer must use a small gear (pinion gear) that has at least a certain number of teeth. Below that number, all the load normally shared by several teeth will fall on just one tooth, which can exceed the yield strength of the single tooth. Since gears are used to change shaft rotational speed, we must also consider gear ratio..
Backlash
The more accurately the tooth profile is cut, and the centre-to-centre distance is maintained, the smaller the backlash. If the input shaft is fixed, backlash is the distance a tooth on the gear moves when contact is shifted from the forward tooth to the trailing tooth. Coarse gears have large backlash as shown in the table 14.3 on page 633 of the text.
Allowable Stresses on Gear Teeth
Bending Stresses
The allowable bending stress on a tooth could limit the power transfer for thinner teeth at low contact ratios. The simple graph on page 638 of the text deals with the bending stress at the root of a gear tooth. The additional tables here give more detailed information.
Usually gears fail because the tooth surface is deformed and wear takes place due to the small sliding action of the contact point. Thus the contact stress may also be the critical design stress. Values are shown in the graph on page 639. Additional information is given in the figures reproduced here.
Contact Stresses
If contact stresses are too high the surfaces of the gear teeth will be gouged. Lubrication films break down and metal-to-metal contact takes place. The teeth grind away the profile due to sliding. If the gear is surface hardened, surface fatigue may occur, resulting in surface flaking. These flakes can then become a grinding powder that rapidly destroys the machine.