Question #1 – Table 7, footnote 4 reads:
4) The microstructure requirements apply only to those portions of the gear material where the teeth will be located to a depth equal to that of 1.2 times the tooth depth.
The wording is confusing here; how can teeth be located at 1.2 times its own depth? If the intent is to check microstructure at 1.2 times the tooth depth, it should just state that: “….apply only to those portions of the gear material at 1.2 times the tooth depth.”
Response to Question #1
The wording in the standard should be interpreted such that the microstructure requirements are applied only to those portions of the gear material which lie within a hollow cylinder that is defined by:
- An outside diameter that equals the tip diameter of the gear teeth
- An inside diameter that equals tip diameter of the gear minus 2.4 x (whole depth of the teeth)
- The two flat planes that establish the total face width of the gear
Question #2 – Tables 7 & 9, Decarburization test
Does this test only apply to the tooth flank surface where tooth contact is expected. In other words, is decarb allowed in other non-contact areas of the tooth, such as top land and toe end?
Response to Question #2
Table 9 states “No partial decarb apparent at 400X, except in unground roots”. The standard is silent (in both the tables and footnotes) on defining other specific surface areas to which the partial decarb criteria should be applied. It is typical that the heat treatment processes will cause minor carbon depletion on all surfaces. This minor carbon depletion has almost no deleterious effect on the performance of the material. On surfaces that are machined after carburizing, this partial decarb layer is normally removed.
Therefore, it is appropriate to interpret that the “No partial decarb apparent at 400X” criteria should be applied to all surfaces that receive a final finishing operation after carburizing and be excluded for other surfaces.
Question #3 – Tables 7-10, Cleanliness
All tables have footnotes which state:
The grade cleanliness requirements apply only to those portions of the gear material where the teeth will be located to a distance below the finished tip diameter of at least two times the tooth depth. On external gears, this portion of the gear blank normally will be less than 25 percent of the radius.
Yet in the Table columns there are references to AMS 2301 andASTM A534 or A866. Both AMS 2301 and ASTM A534/A866 are typically used for HEAT qualification, though AMS 2301 also covers product qualification. The problem is that for:
AMS2301: after sample conversion per AMS 2301 section 3.1.3, it would be difficult to nearly impossible where on the AMS2301 specimen corresponds to the portions of gear described in Tables 7-10’s footnote (above). We cannot work to both AMS 2301 and the locations in the footnote at the same time.
ASTM A534/A866 only cover Heat Qualification. Test sample is forged to a 4”x4” square and the micro-cleanliness specimens taken from the 1/4T location of the test bars. Again, they don’t necessarily represent the locations in AGMA’s footnote.
If AGMA 2001’s intention is to use material capable (or certified) ofAMS 2301 or ASTM A534/A866, the footnote should be dropped because the location described in the footnote isn’t really applicable to the testing locations of the AMS or ASTM specs.
Response to Question #3
The intent of the HGRC during the development of the 2001-B88 was to define suitable acceptance criteria to ensure that the gear blank material in the region of the teeth would be able to successfully withstand the loads applied to the teeth. For the cleanliness criteria, the wording of the standard should be interpreted such that the cleanliness requirements are applied only to those portions of the gear material which lie within a hollow cylinder that is defined by:
- An outside diameter that equals the tip diameter of the gear teeth
- An inside diameter that equals tip diameter of the gear minus 2.4 x (whole depth of the teeth)
- The two flat planes that establish the total face width of the gear
During the development of AGMA 2001-B88, the Helical Gear Rating Committee attempted to reference currently available published standards to specify all of the acceptance criteria for the metallurgical factors that were evaluated. Thus the measurement process and acceptance criteria of AMS 2301 or ASTM 866 should be applied, but the specimen location, selection and preparation description are not applicable.