Gear Rating Suite 2.2
Frequently Asked Questions


Q: During setup, a message box says, “A file being copied is older than the file currently on your system. It is recommended that you keep your existing file.” What should I do?
A: Click on “Yes” to keep the existing file. The files included on the setup disk are intentionally not the latest available files. This was done to ensure that the included files are compatible with older systems. 

Q: Why are the values from this program not exactes matches with the values in 2101-C95?
A: This program is based on 2001-C95 that is dimensioned in U.S. customary units. Due to rounding conversions between values and constants in 2001 and 2101, there is not an exact numeric equality between the two documents. As such, the user will find minor differences in values that are not significant to the accuracy of the standard.

Q: I noticed that several of the ISO 6336 standards have more than one file. For example, there’s "ISO_6336-2-1996," "ISO_6336-2+Cor.1-1998" and "ISO_6336-2+Cor.2-1999." Which of these applies to the rating program?
A: For ISO standards, what AGMA calls an "errata" is called a "Technical Corrigendum." Each of the Corrigendum documents contains only the changes to the standard. So to apply the ISO standard correctly, you need to review both the original standard (i.e. “ISO_6336-2-1996”) and then see if the preceding Corrigendum modifies the portion of the standard you are concerned with. In this example, there have been two different Corrigenda, one in 1998 and another in 1999. The AGMA standards modify the original document and indicate this on the cover page. For example, if you look at the AGMA 2000-A88 document, you will see it includes a 1992 errata.

Q: Is there an easy way to view more than one document on the CD-ROM at the same time? It would be helpful to be able to see both a document and its Corrigendum at the same time.
A: Yes! You can access the viewer window while the first document is still open by clicking on the "View ISO/AGMA Standards" item on the Taskbar, then selecting the next standard to open. You can repeat the process as often as desired. To then switch between documents, you can either select the document you want from the "Window" pull-down menu. Another way is by selecting "Cascade" or "Tile" from this same "Window" pull-down menu. Now you can easily click from window to window.

Q: When you use a tool with one pressure angle to produce a gear with a different standard pressure angle (short pitch hob), how do you rate the gears? 
A: There are two types of "short pitch" hobbing situations:
Case 1: Correct base pitch. In this case, the normal base pitch (& cutting axial pitch) is the same on the hob and when grinding. To use the GRS for this case, use the hob's geometry (pressure angle & normal base pitch) when entering the gear set geometry into the program. The actual gear and pinion tooth thickness should be used, so the x factor for rating will not be the same as on the drawings. The trochoid will be correct, because it's generated by the hob. And the involute will also be correct since the base pitch is the same.

If different tools (with different normal diametral pitches or modules) are used for the two parts, you will need to make two runs. In the first run, use the pinion tool for both parts and ignore the rating given for the gear. In the second run, use the gear (wheel) tool for both parts and ignore the rating given for the pinion. The set rating is then the combination of the pinion rating from the first run and the gear rating from the second run.

Case 2: Slightly different base pitch, followed by grinding to get the correct base pitch. GRS will only approximate the rating for this case. This is typically used only when you are trying to replicate an existing design and don't have the exact hob that was originally used. In this case, as a practical matter, the difference in normal base pitch must be very small.

For example, if you wanted to make a 5 MOD-17°NPA gear (0.5914" NBP) but wanted to use a 5NDP-20°NPA hob (0.5904" NBP), you may be able to grind the part to the correct base pitch even though the hob used is slightly different. But there is no simple way to rate the gear in this situation, because there is no single geometry that describes both the trochoid and involute of the finished gear. You must remember that the "J" factor is influenced by both trochoid and involute; whereas the "I" factor is only influenced by involute.

The closest way to approximate this would be to input a pitch that would cut at the same generating pitch diameter as the actual hob used. Then, adjust the pressure angle so the resulting normal base pitch is the same as the finished gear. You also would have to be careful to input a cutter addendum that causes the root diameter calculated by the GRS to match the actual one on the finished gear. The axial pitch must also be the same. Any rating produced by this (case 2) method is beyond the scope of the rating standards, so you will need to verify the design through your own experience.

It should be noted that most designers don't know enough detailed tooling geometry to do this calculation. It is usually ignored in gear rating to a standard, because the changes in rating calculated are probably not "real" within the manufacturing tolerances of tool, part and metallurgy of small lot series produced gears. The exceptions are in specialized applications like aircraft and automobile. Instead of relying on standards, the users typically employ their own calculation methods, confirmed by full-size testing of the completed transmissions.

Q: Why is "short pitch" hobbing used? 
A: "Short pitch" hobbing may be used to improve the trochoid geometry so the Start of Involute (SOI) diameter is brought closer to the Root diameter. It is also used to improve hob life. For high-pressure angles, the hob becomes pointed. Since most cutting action and hob wear occurs at the tip of the hob, pointed hobs are not economical. A longer wearing hob (with bigger tip radius) can be made by reducing the hob pressure angle and shortening the circular pitch. Maag shapers making coarse pitch pregrind gear teeth often use a rack cutter of lower pressure angle and the correct base pitch to finish the trochoid and protuberance area of the gear and a standard cutter to form the involute, which is also a form of short pitching. The purpose of this is to produce the desired protuberance shape for grinding wheel clearance with a larger cutter tip radius, while controlling the amount of grind stock removal.

Q: How come the "ECHO OF INPUT" of the AGMA 2001 calculation lists 0.00000 for pinion torque when I enter torque or power?
A: On the top of the AGMA Factors window, you selected "Power Rating, Calculate from Safety Factor". Since you requested that the program provide a power rating based on your safety factor, torque is not needed as an input, and is not sent to the rating program. If you were to change this selection to "Safety Factor, Calculate from Power," then the torque would appear in the input list.

Q: If I select four contacts per revolution (as in a sun gear meshing with four planets), does the program divide the input torque by four, assuming equal load sharing?
A: The selection of four contacts per flank per revolution (as in a sun gear meshing with four planets) does not divide the input torque by four, assuming equal load sharing. The full input torque is applied to the mesh. The number of contacts per revolution is used to increase (as a simple multiplier) the number of cycles used in the calculation of life cycle factor. In the case of multiple meshes of unequal power, the rating of the highest power mesh will be slightly conservative, since this power will be assumed to apply to all meshes. The number of contacts per revolution can be used not only for a planetary set, but also for a bull gear driving multiple pinions, as with integrally geared compressors.