10.0 GEAR TYPE EVALUATION
The choice of gear type is dependent upon a number
of considerations involving physical space and shaft arrangement, load, gear ratio, and
desired precision or quality level. A general guide is to choose the simplest gear type
that can accomplish the objectives.
Spur gears are the first choice if they can do the job, as they
are the easiest to make. That means they are the least expensive and, if required, can be
made to the highest precision. Helical gears are slightly more complicated than
straight spurs, but are the choice if loads and speeds are demanding. Helicals are
superior to spurs in load capacity. Also, they offer avoidance of undercutting in small
tooth number pinions; and helicals can be designed to neatly span non-standard center
distances. Crossed helicals are an acceptable skew shaft drive only if the loads are
small. Worm gearing and bevels offer right angle drives for skew and intersecting shafts
respectively. Each offers special features and advantages if needed.
Internal gears can fill a real need nicely, but they should only
be used when the application requires their unique feature.
Table 1.8 summarizes comments and evaluations of
the various gear types.
11.0 CRITERIA OF GEAR QUALITY
In addition to the sizing of gear parameters, it is necessary to ensure that their specifications and manufacture result in the desired gear quality, This includes not only tolerances, but an understanding of what compromises gear quality.
11.1 Basic Gear Formats
Specification of a gear requires a drawing that shows details of the gear body, the mounting design, face width, any special features, and the fundamental and essential gear data. This gear data can be efficiently and consistently specified on the gear drawing in a standardized block format. The format varies in accordance with gear type. A typical data block for standard fine-pitch spur gears is given in Figure 1.47. Formats for coarse pitch gears, helical gears and other gear types are given in detail in the appendix of Ref. 5.
T68 Next Page