Manufacturing
Here is an overview of rear axleshaft manufacturing.
Hobbed splines (top) are cut into the shaft. Rolled splines (bottom) don’t remove material
Splines:
• Hobbed: This process carves involute (curved) splines into the shaft. Differential side gear splines are involute, so torque is spread more evenly when the splines align. Some smaller-volume businesses straight-cut their splines. This concentrates the stress on only part of the tooth instead of spreading the load more evenly. Straight cutting can also penetrate below the shaft’s case-hardening.
• Rolled: A machine with expensive dies forms involute splines under high pressure. No material is removed.
Major Spline Diameter: The maximum distance across the splines (as determined by a caliper).
Minor Spline Diameter: The diameter at the base of the splines, which may or may not be the shaft’s minimum diameter.
[1.50 inches 1.31 inches 1.16 inches 1.10 inches 1.03 inches] Here is a sample of many Dan
Pitch Diameter: The diameter at the splines’ midway points.
Minimum Diameter: The skinniest part of the shaft, which is sometimes the minor spline diameter but can also be a necked-down area somewhere else on the shaft.
Diametrical Pitch: A mathematical calculation devised to keep the splines’ centerlines constant at any diameter. The current standard is 24-pitch. This means that the midpoints of the splines on a 24-spline shaft are 1 inch in diameters, the midpoints for a 35-spline shaft are 11⁄2 inches in diameter, and so on.
Pressure Angle: A spline’s tooth angle. It varies among manufacturers and is not interchangeable: For example, 30-degree Dana inner splines aren’t compatible with 45-degree Ford or Toyota differentials, even if the spline count is the same. Wider-angle splines produce slightly stronger shafts because the minor spline diameter is greater.
Generally, more splines means a larger-diameter, stronger shaft (the 14-bolt 30-spline is an exception). Just as fine-thread fasteners spread the load over a greater surface compared to coarse threads, smaller teeth/more splines are stronger than “coarse” splines, which are cut deeper into the shaft (minimizing the minor diameter). According to Dynatrac, performance aftermarket shafts for big-tired 4x4s start at 35-spline.
This cross-section shows induction hardening. The dark outer area’s hardness has a rating
Hardening/Finishing Treatments
Hardening treatments increase shaft strength by altering the metal’s crystal-line structure. Heat creates carbon atoms, which are trapped thanks to prompt quenching with oil or water. The Rockwell scale, expressed in HRC, is the most common hardness measuring system for axleshafts.
Through Hardening: The entire piece of metal is theoretically hardened uniformly from case to core.
Induction Hardening: The method most commonly used for axleshafts. An electromagnetic field, with a current that can be varied depending on the material’s diameter and the desired depth of the hardened layer, heats the metal. Quickly thereafter the shaft is cooled.
Nitriding: This process uses a high-nitrogen solution such as ammonia to harden the surface while the metal is heated in an enclosure.
Other metal treatments and finishing processes further increase a shaft’s durability. Some of the possibilities are as follows.
Polishing: Micropolishing increases strength by minimizing surface irregularities and stress risers. 4340 chromoly has a fairly tight grain and does not benefit as much from polishing as the carbon-group steels do. Bearing/seal areas are normally ground or polished to minimize scoring or maximize sealing of these contact areas.
Cryogenic Freezing: Nitrogen gas is used to cool the part to about 300 degrees F below zero. This tightens the metal’s grain and improves fatigue-resistance more than overall strength. It is one of the more expensive treatments.
Black Oxide: This coating is primarily for resisting corrosion.
Gun Drilling: This process bores out the shaft’s core. Done properly, it saves weight with minimal strength loss. Gun drilling and cryogenic freezing aren’t in high demand in the value-conscious recreational 4x4 market. These options are investigated mainly by racers looking for every available edge.
Annealing: This process actually softens metal through heating and slow cooling.
Shot Peening: Small-particle blasting for surface stress relief is aerospace-spec for some parts. Foote Axle & Forge feels that proper manufacturing, including accurate induction hardening, precludes the need for shot peening, which is difficult to precisely regulate.
The photos and captions here show how Foote Axle & Forge builds shafts from raw round bars. Many well known axle “manufacturers” are actually finishers, buying blanks from companies such as Foote and then finish-machining them as necessary for desired length, spline spec, and wheel bolt pattern. Finishers sometimes offer optional services like gun drilling and cryo freezing.