Ingeniero

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 arethe 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. Crossedhelicals 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. Special gears such as spiroid, helicon beveleid andface should be avoided as much as possible because of limited features, complex forms to produce and inspect, limited fabrication sources, and relative high cost. 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 thedesired 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 astandardized 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

Type Spur

Helical

Crossed - helical

Internal spur

Bevel

Worm mesh

Specials (face, Spiroid, Helicon,Beveloid)

TABLE 1.8 SUMMARY AND EVALUATION OF GEAR TYPES Precision Comments Features Applications Rating Regarding Precision excellent Parallel shafting Applicable to all Simplest tooth elements offering max High speeds types of trains and imum precision. First choice, recom and loads a wide range of mended for all gear meshes, except Highest efficiency velocity ratios, where very high speeds andloads or special features of other types, such as right - angle drive, cannot be avoided. good Parallel shafting Most applicable to Equivalent quality to spurs except for Very high speeds high speeds and complication of helix angle. and loads loads; also used Recommended for all high- speed Efficiency slightly wherever spurs and high-load meshes. Axial thrust less than spur mesh are used. componentmust be accommodated. poor Skewed shafting Relatively low To be avoided for precision meshes. Point contact velocity ratio; Point contact limits capacity and precision. High sliding low speeds and Suitable for right - angle drives if low speeds light loads only. A less expensive substitute for bevel light load. Any angle gears. Good lubrication essential because skew shafts. of point contact andhigh sliding action. fair Parallel shafts Internal drives Not recommended for precision meshes High speeds requiring high because of design, fabricabon, and High loads speeds and high inspection limitations. Should only be loads; offers low used when internal feature is necessary. sliding and high stress loading; good for high capacity, long tie. Used In planetary gears to produce large reduction...