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Intermittent loading， such as that from gear couplings mounted on ther-mally expanding shafting， is a different problem. This problem is accom-modated in a double-helical unit by judicious selection of helix angle and coupling size so that axial coupling forces resulting from the transmission of torque are less than the thrust force produced by each helix of the gear. This design assures that the coupling will slip to relieve any axial loading and that the power balance on the two helices will be maintained.
High-speed gear couplings are selected in which the pitch diameter is substantially smaller than that of the pinion. Axial forces produced will behigh， and the combination should be examined very closely as a potential trouble source.

.ouble-helical speed gear is the first choice in calculating the accuracy of loading and smoothness of operation. Predictable performance renders unnecessary any deviations from easily defined and measured geometry. These gear sets will be more efficient and have unmatched reliability if properly applied. Reasonable intelligence must be exercised in coupling selection. This discretion will assure vibration and noise levels that are often indistinguishable from that of the connected machinery.

In single-helical gearing， all forces externally generated must be added tothe thrust produced by the gear itself， and the total is used on each shaft to select the high-speed shaft thrust bearing. An error in thrust or bearing capacity estimation will result in frequent failures of the thrust bearing or the associated shafting. Single-helical gearing， due to asymmetrical loadingfrom the helix， has two sources of design difficulty which do not exist in double-helical gear sets. The effective center of tooth pressure oscillates backand forth across theface， putting substantial alternating loads on the shaft bearings. This oscillation results in peakbearing loads substantially greaterthan those calculated， which can lead to early bearing failure. In addition， the helix-induced thrust force causes the gearing to try to skew in thehousing， unbalancing the bearing loading and forcing the gearing to run out of parallel. Crowning of single-helical gearing is used to counter the effects of shaft misalignment.
Factors Affecting Gear Design
A transverse section through a gear and pinion mesh is shown in Figure 14-1 with some of the major points in gear and pinion interaction. Figure 14-2 shows the terminology used to describe helical gear. The majorfactors affecting gear performance are: (1) pressureangle， (2) helix angle，
(3) tooth hardness， (4) scoring， (5) gear accuracy， (6) bearingtypes， (7) servicefactor， (8) gear housing， and (9) lubrication.

.ressure Angle

 

The decision regarding the pressure angle is one that the designer has tomake early in the design stage. Conventionally， pressure angles have ranged between 14.50 and 250. Changes in the pressure angle affect both the contactratio and the length of line of action. As the pressure angle increases， the contact ratio and the length of line of action decreases as seen in Figures 14-3 and 14-4. The contact ratio is an indication of the number of teeth incontact. As a generalrule， the higher the contact ratio， the less noise the gears will generate.
The strength of the tooth is an important factor in the selection of the pressure angle. Figure 14-5 shows the variation of gear tooth geometry andpressure angle. The higher the pressureangle， the higher the tooth strength.

Figure 14-3. Variation of transverse contact ratio with pressure angle and helixangle. (Courtesy of LufkinIndustries， Inc.)

The noise that the gears generate decreases with a contact ratio increase.Thus， the selection of pressure angles involves many factors. Normal angles in use today are between 17.50 and 22.50. Higher pressure angles increase thebearing loadings， but this increase is not a determining factor when selecting pressure angles.
　
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