Teeth, gear

Although the Rockwell test is intended to be used on flat parallel-sided specimens, its use can be extended to rounded surfaces by using a curvature correction factor. Compound surfaces such as gear teeth can be tested but the results must be corrected for curvature. 

Elastohydrodynamic Lubrication (EHL). Lubrication needs in many machines ate minimized by carrying the load on concentrated contacts in ball and toUet beatings, gear teeth, cams, and some friction drives. With the load concentrated on a small elastically deformed area, these EHL contacts ate commonly characterized by a very thin separating hydrodynamic oil film which supports local stresses that tax the fatigue strength of the strongest steels. 

Teeth have an involute profile, a curve that is generated by the rolhng of a straight line over a circle, as shown in Fig. 29-83. Although many cui ves could be used for gear teeth, the involute is the preferred one because it allows the correc t engagement, even when the distance between the centers of the gears is not accurately held. 

Slides Fatigue fracture surfaces components failed by fatigue, e.g. gear teeth, half-... 

Rpm X number of gear teeth (always present) Gear mesh frequency Record signal for reference Distress noisy and shows increase m GML and harmonics One-half GML—even number of teeth with machining error... 

AGMA 211.02, Surface Durability (Pitting) of Helical and Herringbone Gear Teeth, Washington, D.C., 1969. 

A method of assuring nearly perfeet tooth eontaet is to mateh-lap the gear teeth after nitriding. Lapping eliminates the break-in period, whieh otherwise takes from 70 to 120 hours. It is during the break-in period, that tooth surfaee distress usually oeeurs. 

For maximum reliability, it is reeommended that nitrided gear teeth be speeified. Experienee indieates that the extra eost of mateh lapping is justified. 

To promote even wear, a hunting tooth design is desirable. In this design, a pinion tooth does not contact a given gear tooth more than once until it has meshed with all the other gear teeth. 

Hardening, gear teeth, 331 Hardness, gear coupling teeth, 336... 

External gear None Power input and heat generated increases with increasing viscosity May be suitable for handling viscosities up to 25 000 centristokes without modification. For high viscosities (a) Clearances may be increased (b) Speed reduced (c) Number of gear teeth reduced... 

Gear backlash is the play between teeth measured at the pitch circle. It is the distance between the involutes of the mating gear teeth, as illustrated in Figure 39.17. 

Input speed x Number of input gear teeth Number of intermediate gear teeth... 

All gear sets create a frequency component referred to as gear mesh. The fundamental gearmesh frequency is equal to the number of gear teeth times the running speed of the shaft. In addition, all gear sets create a series of sidebands or modulations that are visible on both sides of the primary gear mesh frequency. 

If the shafts are too far apart, the teeth mesh above the pitch line, which increases the clearance between teeth and amplifies the energy of the actual gear-mesh frequency and all of its sidebands. In addition, the load-bearing characteristics of the gear teeth are greatly reduced. Since the force is focused on the tip of each tooth where there is less cross-section, the stress in each tooth is greatly increased. The potential for tooth failure increases in direct proportion to the amount of excess clearance between the shafts. 

The lubrication requirements for single-screw type compressors are not severe, but in oil-flooded rotary units, the oxidizing conditions are extremely severe because fine droplets of oil are mixed intimately with hot compressed air. In some screw-type air compressors, the rotors are gear driven and do not make contact. In others, one rotor drives the other. The heaviest contact loads occur where power is transmitted from the female to the male rotor here the lubricant encounters physical conditions similar to those between mating gear teeth. This arduous combination of circumstances places a great demand on the chemical stability, and lubricating power, of the oil. 

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