Optimizing The Gearing

The most important factor influencing the durability of a gear set, and hence the gear size, is the hardness of the gear teeth. It is often possible to reduce considerably—sometimes by as much as half—the overall dimensions of a gear set by changing from medium-hardened gears (about 300 Brinell) to full-hardened gears (about 55 to 60 Rockwell C).

other factors play a role in minimizing the dimensions of a gear set; for example, the ratio between face width and pitch diameter and the proper pressure angle and pitch of teeth. Thus, when it comes to deciding between a set of standard catalog gears and gears designed specifically to meet the requirements of the application, the question of cost versus optimizing comes to bear. In general, where there is only a limited number of units to be made, the catalog gears are much less expensive and also much more readily available. There are many applications that call for critical power, speed, or space requirements, however, and it may pay in these applications to select gears that are designed for that application.

Minimizing Gear Noise Specifying or designing a gear set to produce low noise and vibration levels frequently leads to choices that are the opposite of those for optimizing the gears for strength and size. Generally, a parallel-shaft gearing rather than right-angle gearing is preferred for quiet operation because of greater geometric control, inherent ability to maintain tight manufacturing tolerances, and minimum friction during tooth contact. Helical gears, in particular, can have more than one tooth in contact (helical overlap), and some experience has shown as much as a 12-dB reduction in noise using helical instead of spur gears. Double helical or herringbone gearing has the problem of manufacturing the two helices with precisely the same phase and accuracy. Helical gearing must have thrust bearings or collars on each element and so produce an overturning moment. The overturning moment is more pronounced with single stage high ratio units such as large diameter, narrow face-width gears, and small diameter pinions. These problems concerning double and single helical units are easily handled with proper design and manufacturing.

For quiet, smooth operation, the gears should be designed with some or all of the following properties:

1. Select the finest pitch allowable under load considerations.

2. Employ the lowest pressure angle: 14 1 and 20° are most commonly used.

3. Modify the involute profile to include tip and root relief with a crowned flank to ensure smooth sliding into and out of contact without knocking and to compensate for small misalignments.

4. Allow adequate backlash (clearance) for thermal and centrifugal expansion, but not so much as to prevent proper contact.

5. Specify the higher AGMA quality levels, which will reduce the total dynamic load. Generally, AGMA quality 12 or better is required for smooth, quiet operation.

6. Maintain surface finishes of at least 20 microinches Ra (surface roughness average value).

7. Maintain rotor alignments and runouts accurately.

8. Limit rotor unbalance per plane to less than

Umax = 3 W/N in US units, and Umax = 4760 W/N in SI units, where Umax = residual unbalance, oz • in (g • mm) W = static weight on the journal, lb (kg) N = maximum continuous speed, rpm

9. Provide a nonintegral ratio ("hunting tooth") to prevent a tooth on the pinion from periodically contacting the same teeth on the mating gear.

10. Have resonances of rotating system members (critical speeds) at least 30% away from operating speed, multiples of rotating speeds, and tooth-mesh frequencies.

11. Have resonances of gear cases and other supporting members 20% away from operating speeds, multiples, and tooth-mesh frequencies.

12. Specify the highest-viscosity lubricant consistent with design and application.

13. Select rolling element bearings to minimize noise generation. Generally, hydrodynamic sleeve bearings are quieter than antifriction types but are more difficult to apply.

14. Because housing design is another area where noise and vibration reductions can be obtained, select an acoustically absorbent material for the housing or design the housing with built-in isolation mounts to cut down any vibration attenuation.

15. For parallel shaft gearing, it is recommended that speed increasers be up meshed and speed reducers be down meshed to prevent rotor instability.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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