(based on involute double helical teeth with parallel input and output shafts)

Gear teeth must have an even load across the entire face width to minimize stress on the teeth. The contact between gear teeth is line contact; therefore, the alignment between the rotating elements (pinion and gear) is critical. Tooth alignment is controlled by the accuracy of the rotating elements, the housing, and the bearings assembly. Tooth contact should be checked on all new installations, after any disassembly of the gear unit, and after any major housing-to-foundation change. It can also be checked as a part of routine ntenance or when a problem related to alignment is suspected. Contact should be checked on the job foundation to be sure that the unit will operate properly.
Tooth contact can be checked two ways:

• Soft Blue – Apply soft machinist’s bluing or transfer bluing to three or four teeth on the pinion in two places, 180º apart and that gear is rolled by hand through mesh with its mating gear. The transfer of the blue from one gear to the other gear is read as the contact. The soft blue method is usually performed first. Because the unit is not running, this check does not give true contact; however, it does give a good indication of what contact will be. If it indicates inadequate contact, maintenance personnel might opt not to start the unit until contact is corrected. If the unit has been disassembled, then a soft blue check before the housing cover is installed might save another dismantling to correct contact. This is especially important if a new set of rotating elements or bearings is installed. The contact should be checked at three places around the gear, approximately 120º apart. However, the blue must be reapplied and smoothed on the pinion after each meshing.
• Hard Blue – the hard blue application is for running contact check. The gear teeth is painted with hard or layout blue, then run the gear unit, and observe the pattern of “wear off” of the bluing. The blue is applied to an area three- or four-teeth wide at four places on the gear and at two places on the pinion. The unit is run (usually at full speed). Running conditions can vary from no load to full load. It is best to run the unit at very light load (up to 20%) for two hours or so, and then shut it down and check the contact. With higher loads, the unit should run a shorter time before checking contact. The objective is to run the unit just long enough to wear the blue off the areas of higher contact stress. High loads can mask poor contact and give a false reading.
Contact checking can usually be accomplished through the inspection cover port. Occasionally, soft blue checking is done with the housing cover removed, such as during the reassembly process.
Interpretation of Tooth Contact
The following is information to be used only for guidance. Exactly what contact should be acceptable should be based on the manufacturer’s recommendations and experience. In most cases, the gearbox manufacturer should be consulted on how to correct poor contact. Assuming properly manufactured parts, minor corrections can be made to the tooth contact by shimming the gear housing. Note that tip or root relief modifications are designed to improve load distribution when a unit is operating under load, but they can make contact appear quite bad under no load, as in a soft blue check.
Generally, with a soft blue check, one is looking for some blue to transfer, usually in a line that covers at least 80% of the face width, a centralized 60% coverage, or acceptable contact patterns consistent with those illustrated in Figure . Do not be alarmed by a lack of blue covering the flank of the tooth; flank contact should normally not extend entirely to the tip of the tooth.
Figure below illustrates various examples of tooth contact patterns. Keep in mind that a soft blue contact will not produce such dark impressions – look for the same pattern in a “sketchy” impression. The hard blue operational/running tooth contact check can be done from no load to full load, and the results will vary with the load condition. If the unit is run at no load, the test will usually appear similar to a soft blue check. More blue will wear off the pinion than the gear due to the higher number of cycles the pinion encounters. As the load increases, blue will wear off more of the tooth flank. Look for evidence of even load across as much of the gear tooth, both flank and face width, as possible.