A New Standard Under Development: Specifying Powder Metallurgy Gears

The best PM manufacturing suppliers that specialize in making gears have a gear engineer on staff who works with the customer to make them successful.

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In the early nineties, I was working at a transmission manufacturing facility in Indiana that made lawn and garden transmissions. Their products used large numbers of powder metal and cut gears. On a typical day, 15,000 PM gears would be assembled into product. This company made transmissions full of PM gears for 30 years or more—millions of gears every year.

They were having all kinds of problems, so they recruited me to work for them as a gear engineer and power transmission designer. Having written extensive gear and bearing calculation programs in college, I had an intimate knowledge and love for the work, but I had never designed gears out of powder metal. I was familiar with standards for commercial and industrial gears, having worked for several years designing mill gear reducers, so I knew what to do. I immediately began searching for some industry standards on PM gearing. I was shocked to find out that there weren’t any.

Background
Back in the late 1980s, there was a stark disconnect between the PM manufacturers and those who purchase PM gear components. At that time, there were no standards relating to durability or strength of PM gearing. Knowledge of bending or pitting was simply not available outside of select companies that had rigorously tested and painstakingly developed their products. The only data available for mechanical properties was MPIF Standard 35 put out by the Metal Powder Industries Federation. It is an excellent standard, but one must never confuse PM mechanical properties with gear durability. Like plastics—to some extent—there are variables involved that are not found in wrought steel.

In 1993, under the leadership of our good friend Irving Laskin, the American Gear Manufacturing Association formed a committee for developing and writing powder metallurgy gear standards. The committee started out with several companies who were making PM gears and a few gear engineers who were designing the PM products into lawn and garden, various consumer goods, and automotive applications. Irving was a highly respected gearing consultant and mentor of mine for decades who often shared with me that several OEM’s manufacturing a plethora of geared devices were desperate for knowledge of medium-strength power gearing. These companies simply needed to leverage the incredible economy of volume that PM offers.

The first and most pressing issue the PM Gear committee took up in 1993 was to create a document that outlined the vital issues the buyer (customer) needed to know about purchasing PM gearing. At the time of the first document release, there were 25 active participating members on the PM committee and many more associate members. Five years later, and after much discussion and work, AGMA 6008-A98 was formally released. This document was deemed so imperative for the industry that it superseded any attempt by the committee to develop a specific gear ratings standard. It would be nearly another eight years before the spur gear standard was published. At this time in powder metal gearing history, there were still misunderstandings between PM manufacturers and customers regarding the differences between wrought steel gearing and PM products in design, materials, and process. Some of this is still true today, but to a much lesser extent. The best PM manufacturing suppliers that specialize in making gears have a gear engineer on staff who works with the customer to make them successful. These engineers also tend to be active participants on the PM committee.

The document “Specifications for Powder Metallurgy Gears,” released in 1998, is the only one of its kind, specifically devoted to gearing. It remains a significant source of published information on this subject.

Today, the AGMA PM Gearing Committee is making large efforts to significantly expand 6008-A98. When it is ready to go to the membership for ballot, it will be the most comprehensive authority on specifying PM gears that we know of. Here are some of the new expansion items the committee is considering:

• Similarities of some PM alloys to specific wrought materials
In some cases, there are PM alloys that exhibit very similar performance to steel gears.

• Avoiding dangers of grease lubrication with PM materials and how to manage and avoid loss of lubrication
PM materials can be oil impregnable. This is a great property, but it can be a double-edged sword. Specific knowledge is required to leverage the obtainable lubrication advantages in PM materials.

• Possibilities and limitations of multi-cavity tools
It is possible to make multiple parts from a single press. However, only specific geometry makes this practical.

• PM alloys and their specific applications and advantages in PM gear performance
PM materials cover a wide range of special alloys with unique performance with materials not available any other way.

• Process and treatments
Answering the question: Why would we consider one process or treatment over the other? Some treatments are unique to PM components. Since mold is not generating, specific optimization in root strength can be realized.

• Specific heat treatments/processes and their relationship in gear performance
Some PM treatments will give equal performance to wrought steel alloys.

• Sinter bonding: how, why, and the major advantages
Sinter bonding basically takes to net shape PM parts before sintering and assembles them while in their “green” un-sintered state. After sintering they become one ridged assembly. Multi-level net shape subassemblies or assemblies are possible.

• Green machining
There are applications where a significant advantage can be obtained by machining geometry that cannot be pressed in a non-sintered part.

• Green inspection with a significant emphasis on green roll testing (double flank inspection) and how it relates to the sintered part quality
This is a technique specific to gearing to evaluate tooling set up and general quality and correlate it back to a finished, hardened part.

• Tolerance: An index of typical length/diameter/geometry tolerances by process and treatment for guidance in the design process.

• Tooling: Understanding the tools that make PM gears (a most misunderstood area) for buyers.

• Tooling: Specific knowledge for specifying multi-level parts.

• Crush ribs and special tooth densifiers for strengthening critical areas.

• Understanding and predicting sectional density variations with multi-level parts.

• Molding integral one piece, multi-level net shape gear & shaft/gear and hub geometries.

• Special tooling and characteristics of PM helical gears
Helical gears are more difficult and require experience and understanding of design, tooling and process. Helical gears can be made very successfully and with excellent accuracy.

• Secondary operations
• Shot peening, peening in general (various types and how it works with PM)
• Steam treat: how, why and unique benefits
• Resin impregnation
• Roll densifying: surface strengthening increasing gear accuracy methods
• Understanding Oil impregnations (typical / special) lubricants
• Rust preventatives–recommendations, uses
• PM related honing, grinding and secondary machining of sintered gears

• Methods for increasing gear accuracy of pressed & molded gears

• New technologies and techniques that enhance performance in PM gears

• Statistical Process controls and Process Capability for high volume PM

• Chamfers, radii: things that should not be done in the part design.

AGMA Published Documents
(ANSI/AGMA 6008-A98) “Specifications for Powder Metallurgy Gears”
(AGMA 942-A12)  “Metallurgical Specifications for Powder Metallurgy, PM, Steel Gearing”
(AGMA 930-A05) “Calculated Bending Load Capacity of Powder Metallurgy (P/M) External Spur Gears”

Documents Currently Under Development
(AGMA 936-AXX) “Calculated Bending Load Capacity of Powder Metallurgy (P/M) External Helical Gears”
(ANSI/AGMA 6008-AXX) “Specifications for Powder Metallurgy Gears” Revised & Updated:
(AGMA 944-AXX) “Mechanisms of Powder Metal, PM, Gear Failures”

If you or your company designs with uses or manufactures PM gearing, the AGMA PM committee invites you to join the discussions and development of these key technical documents. Active AGMA members enjoy the wealth of knowledge, intelligence, expertise, and industry contacts that comes with participation in this outstanding process. For more information, please feel free to contact Amy Lane: Lane@AGMA.org.