All About Alloys & Splines, Floaters & Flanges
Nobody likes to get the shaft in the proverbial sense. But getting the right shaft for your axle can keep you from being stuck in the yuck with no torque to the tires. This article hones in on the materials and manufacturing of OE and aftermarket axleshafts. We’re limiting it to rear shafts; front shafts might be covered in an upcoming issue.
An axleshaft’s overall strength is determined by what Randy’s Ring & Pinion calls DMD: diameter, materials, design. We’ll start with materials.
Most readers know that steel is a generic name for various iron alloys, or mixture. Iron (Fe) is steel’s base metal, with carbon (C) and such other elements as manganese (Mn), phosphorous (P), sulfur (S), silicon (Si), nickel (Ni), chromium (Cr), molybdenum (Mo) and vanadium (V) sometimes added to affect hardness, machinability, and heat-treatability.
Axleshafts typically use medium-carbon steel. Carbon increases strength but makes the steel less ductile or flexible. Aftermarket alloy shafts often have a higher carbon content than OE shafts and also introduce other metals such as chromium, molybdenum, and nickel into the mix. These metals increase strength and improve case hardening.
Here’s an overview of the AISI (Ameri-can Iron & Steel Institute) material grades most often used for automotive axleshafts. (The Society of Automotive Engineers tends to use AISI’s designations; MIL-Spec, ASTM, ASM, and international designations can vary.) The raw stock sometimes includes abbreviations that refer to the manufacturing process: H is hot-rolled, CD is cold-drawn, A is annealed, and Q is quenched, for example.
The first two numbers refer to the alloy and the last two the average carbon content. The 1xxx numbers are primarily the Carbon Group, the 4xxx has many Nickel-Chromium-Molybdenum Group (chromoly) members, and the three-number designations are considered aircraft-grade steels.
1340: This high-manganese grade was the OE material years ago. Many early Dana/Spicer axleshafts used 1340. Modern higher-performance applications need stronger material.
1040: OE axleshafts are typically made from induction-hardened 1040 because it strikes a compromise between strength and ductility. 1040 is also easier to machine than harder alloys.
1050: Thanks to its higher carbon content, 1050 is about 38 percent stronger than 1040. It is used in certain OE applications and also for some aftermarket OE-replacement shafts.
1541: This high-alloy grade is popular with aftermarket manufacturers.
1541H: An even better aftermarket shaft material, this grade adds silicon to 1541 to increase strength and heat-treatability. 1541H can be 50 percent stronger than OE 1040 and about 12 percent stouter than 1050.
4140: 41xx designates the chromoly group. Chromium offers three benefits: improved hardness, better elasticity during quenching, and greater corrosion resistance. Molybdenum and nickel further increase hardness. This steel is also commonly used for U-joints, spindles, and camshafts.
4340: Also in the chromoly family, 4340 is about twice as strong as OE 1040. It is also ductile enough to absorb the shock of abrupt acceleration, taking some load off of the differential. 4340 is popular for performance-aftermarket front shafts because it is strong and more affordable than some of the more exotic alloys.
300M: Also known as 4340M, it is similar to 4340, only with vanadium added plus additional silicon and slightly more carbon and manganese. It is mainly used in aircraft applications where high strength and ductility are required for such components as landing gears. 300M is also normally through-hardened and is about 150 percent stronger than OE 1040. It is expensive, harder to machine than other shaft materials, and manufactured in much lower quantities than the other steel grades.
Hy-Tuf: (ASM-6418, SAE 4625M4, MIL S-71083, aka Maxi-Drive) is a chromoly well known in drag racing circles. Strange Engineering and others sell Hy-Tuf shafts that have a Rockwell (HRC) hardness rating of 46-48 throughout. (Hy-Tuf is generally through-hardened.) The recipe is high in silicon and manganese. Hy-Tuf is more affordable and available than 300M.
8620: Also in the Nickel-Chromium-Molybdenum Group, this low-alloy (lower carbon) grade isn’t viable for drive axles—tensile strength is about half of the 1040 material typically used for OE shafts. 8620 is commonly used for components that need a hard surface to combat wear and a ductile core, such as ring-and-pinion gears.
Each steel grade has an acceptable range of carbon and other elements. (Strength ratings in the accompanying chart are approximate; a shaft’s actual strength is determined by the specific recipe used by the steel mill, by the hardening process, and by size.) The cost of the finished product often reflects the quality of the materials. Raw steel that is consistently on the upper end of spec tends to cost more than shipments that vary from load to load or are on the lower end of the acceptable range. Harder alloys are more difficult to machine, which also adds to the cost.