“Quality levels” have been established by the American Gear Manufacturer’s Association (AGMA) to provide a common basis upon which to compare parts made by different methods and from different suppliers. The current standard on gear inspection (AGMA 2000) contains information on selecting quality levels, calculating tolerances, and measuring gear elements. When you put an AGMA quality level on a drawing you are specifying that inspections be done in accordance with the provisions of AGMA 2000. The standard outlines many different ways of certifying that a quality level has been met, and the manufacturer is free to select from those methods unless you require specific inspections on your drawing.
Tooth thickness measurements, for example, can be made with five different methods. If you plan to use a particular method for receiving inspection, that is what should be noted on the drawing. The gear manufacturer may then use whatever method he wants for in-process checks, but the final inspection report will contain measurements that are directly comparable to your results.
Most gear purchasers are not equipped to perform detailed gear inspections, and must rely on the supplier to certify the “quality” of the delivered goods. The gear designer has the option of requiring either “composite” or “individual element” (such as lead, profile, spacing, or runout) checks on the drawing, and may request copies of the “charts” (the graphical output of the gear inspection machine) from these checks. Making charts is very time consuming and most gear suppliers pass the cost of this labor on to the purchaser in the form of higher part prices or separate inspection fees. If you require charts be sure to indicate the sampling plan desired. 100% inspections are seldom cost effective. Table 6.1 is an example of a typical sampling plan.
|Lot Size||1% aql||4% aql|
|2 to 8||100%||1 of 2|
|9 to 15||100%||1 of 4|
|16 to 25||100%||1 of 7|
|26 to 50||1 of 2||1 of 7|
|51 to 90||1 of 4||1 of 12|
|91 to 150||1 of 7||1 of 18|
|151 to 280||1 of 7||1 of 21|
|281 to 500||1 of 7||1 of 40|
|501 to 1200||1 of 9||1 of 70|
|1201 to 3200||1 of 10||1 of 140|
|3201 to 10,000||1 of 16||1 of 400|
Based on Level II-A of Mil-STD-105D.
Modified for in-process inspection.
Modern inspection equipment can determine part characteristics with far greater accuracy than manufacturing can make the parts. Some types of gears, such as bevels and worms, have not been subjected to “individual element” checks because of equipment limitations and have always been accepted based upon “composite” methods. Just because it is possible to inspect the individual elements of spur and helical gears does not mean it is practical. It is not uncommon for a gear pair accepted by such methods to have “additive errors” that result in unacceptable performance. Conversely, a pair of “rejected” gears may have “complementary errors” and provide very acceptable performance.
“Elemental” checks provide valuable information for in-process control of manufacturing, but may not be the “best” criteria for final acceptance. The “composite” method of checking a gear against its mate or a master gear on a “tight mesh” center distance considers lead, profile, spacing, and runout errors. Both “flanks” of a tooth space are checked at the same time, however, when in actual operation only one is in mesh at a time. “Single flank”, also known as “transmitted error” testing is far more realistic but is not widely available. The author feels that a combination of composite checks and occasional contact-pattern checks (see Figure 6.1) under simulated “assembled” conditions is the most practical way to qualify a gearset and avoid field problems.
“Quality” costs when it comes to gears. Before you put a quality requirement on a drawing, make sure you really need it. Attempting to solve your “gear” problems by increasing quality requirements can be expensive and ineffective. The “right” quality level for you is the one that works in your application. It you find that one aspect of your gears (such as lead, profile, spacing, or runout) needs to be controlled more closely than the others, it is your option to tighten the tolerance on just that element. It is not uncommon, for example, to order a Quality 8 gear with Quality 10 spacing as long as it is noted on the drawing. The reference books listed at the end of this manual have extensive discussions on various aspects of gear quality.