INTRODUCTION

Superior Axle & Gear is well known as a manufacturer of top-quality aftermarket axle-shafts and gear-sets. Recently, we had an opportunity to tour the Superior facility in Los Angeles with Mike Denton Jr., who talked with us about what goes into designing an axle-shaft and then walked us through the steps involved in the manufacturing process. That’s convenient since axle-shafts are the subject of this installment in our ‘Ultimate Toyota-Based Rear Axle’ build-up.

All Toyota 8” and Non-TRD Tacoma/T-100/Tundra axles use 1.3” (major) diameter, 45 30-spline axle-shafts. We are working with these stock dimensions to ensure that our axle-shafts remain compatible with stock aftermarket carriers/lockers. It’s entirely possible to broach-out the side-gears in a Toyota ARB locker to accept a larger diameter axle, but Mike pointed out that a pair of his 30-spline axles would probably exceed the ultimate strength of something in the third member (meaning that the ring & pinion, carrier or drive flanges would fail before the axle-shafts).

Superior custom-made a set of 4340 full-floating axle-shafts for our Front Range Off-Road axle-housing and full-floating conversion kit. Because our axles use splines on both ends (at the differential and the drive-flange), we had a very straightforward profile (changes in axle diameter). Each axle has one seal-surface at the wheel-hub, which features smooth radii to reduce stress risers. Some people are making a big deal about rolled splines, but the truth is that properly hobbed splines (especially if cut before heat-treating) will offer comparable strength in most real-world applications. Very few custom axles actually feature rolled splines because of the tooling, additional setup time and expense associated with this step. It’s really only practical for production runs.

The ultimate strength of an axle is a result of proper design, material selection and the manufacturing process. There have been some fantastic technical articles related to axle technology and material selection published recently. We would encourage readers to check out the following articles for more information about materials and axleshaft design/strength.

‘Axles 101’, Four Wheeler Magazine (January ’05, Author: Jim Allen)

SUPERIOR SHOP TOUR

The countertops of the Superior offices are covered with all types of product samples.
Examining the design limitations of stock components is the first step towards improving (strengthening) any axle-shaft.

Broken parts make better parts. How’s that? Broken parts reveal flaws that can be addressed with improvements in design, materials and the manufacturing process. Mike explained to us how the nature of this failure indicates that there might not have been adequate spline-engagement because the break occurred halfway down the splined area of the shaft and the splines are straight on the broken axle-stub. Cracks running lengthwise down the shaft from several of the splines also indicate that the spline-root was an area of concentrated stress.

These samples parts are being analyzed to verify that proper penetration and Rockwell hardness has been achieved during the heat-treating process. Heat-treating is just as important as material selection or an axle’s profile design and must be performed by someone that is familiar with the material and desired characteristics. All of Superior’s heat-treating is performed in-house.

Here you can see the various lengths of raw billets they keep on hand. Superior processes over 300,000# of steel to make more than 10,000 axle-shafts each month. In addition to manufacturing their own line of performance products and private-label aftermarket products, Foote Axle (Superior’s parent company) also produces axle-shafts for OEM automotive and commercial/industrial applications.

The appropriate material is first cut to length in one of these huge hydraulic shears. Then the cut material is organized into manageable batches and processed according to the instructions/build-sheet that accompanies every batch of axles.

Because of California’s ‘Energy Crisis’, the workday at Superior begins before 3am in order to avoid the peak electricity rate hours. These guys handle themselves casually as they work with the heated axles, but they move with amazing efficiency to ensure that the material stays hot and workable. The bar-stock for semi-floating axle-shafts is cut extra-long so that one end can be forged into the axle-flange.

The bar-stock is forged into a semi-floating axle blank at red-hot temperatures and under extreme pressure.

The wheel flange is re-heated in the furnace between forgings. Careful control over the heating and cooling of the axles during manufacturing is a major part of their resulting strength.

The final forging of the wheel flanges takes place at near room temperature. This is impressive to watch, but it made our heads ring. The hammering motion of this machine is so powerful that it breaks at least one of those huge wooden beams each month.

Completed forgings are then CNC machined to final tolerances, except for bearing & seal surfaces, which are machined after heat-treating.

The tooling is incredible. These cutters are capable of removing up to 1/8” of 4340 in a single pass! Superior invests in the equipment and processes (like CNC lathes/mills & heat-treating equipment) that allow them to manufacture premium quality components in the US instead of manufacturing inexpensive parts overseas.

Superior also manufactures most of the tooling to keep their heavy equipment running in-house. Parts for many of their machines are so specialized that they can’t afford the downtime that it would take to order replacement parts from a manufacturer or tool-smith.

Axle flanges are drilled for wheel-studs in this specialized CNC mill.

Straightening the axles to eliminate run-out is impressive because it’s still all done free-hand. Francisco “Poncho”Garcia has been working at Superior straightening axles for many years. Each axle is rotated by hand, moving up and down the length of the shaft to measure the run-out and make any necessary adjustments.

These images illustrate the variety of dies used to roll axle splines. Different dies are used according to the pressure angle of the splines and diametric pitch (the number of splines relative to the axle diameter).

This is the spline-rolling machine that is used to literally roll the axle over the dies. Rolled splines are so strong because no material is removed (the splines are essentially cold-forged). Unfortunately, we didn’t get an opportunity to check out the hobbing process.

These images show how alloy axles are through-hardened with an electro-magnetic charge, which heats an entire axle evenly in a matter of seconds, before it is quenched. The result is an axle that is heat-treated evenly all the way through the part, as opposed to plain carbon-steel axles which can only be induction-hardened. Induction-hardening only penetrates the surface of an axle, which is great for a ring & pinion set but is not ideal for heat-treating axles.

Axles go through a final round of machining after heat-treating to ensure that the bearing and seal surfaces maintain accurate tolerances. Axle dimensions are then checked against the build-sheet again to ensure all the parts are 100% accurate.

This batch of axles is ready to be packaged and shipped. Many of these axles are being delivered to other aftermarket manufacturers/vendors to sell as part of custom-built axle assemblies. Next month we’ll take a look at the FROR axle-housing and full-floating conversion components.

ARB USA
http://www.arbusa.com

WFO Concepts
http://www.wfoconcepts.com

Superior Gear & Axle
http://www.superiorgear.com

Front Range Off-Road Fabrication
http://www.frontrangeoffroadfab.com