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Axles Schmaxles

A DISCUSSION OF CONTEMPORARY SHAFT TECHNOLOGY

Drive axles are subjected to immense torque loads. If this isn’t evident, multiply the engine torque by the transmission ration and the ring-and-pinion ratio (torque x low gear ratio x rearend ratio),and you see that numbers get prodigious way fast. And what’s on the receiving end of this force? The drive axles of a 1,300hp drag car must collectively withstand upwards of 12,000 lb-ft of torque every time the drive drops the hammer. That’s why drag racing’s rule-makers mandated aftermarket axles more than 30 years ago. So while the performance requirements of street vehicles typically don’t require beefy racing axles, driveline swaps, power adders, and/or narrowing the existing axle housing most assuredly cry for sturdier replacements.
For some insight into axle technology-past, present, and future-we called upon the man responsible for the industry’s first warranted alloy racing axle and for establishing the first contingency award programs for axles with the major drag racing sanctioning bodies more than 20 years ago. Recognized as a leading exponent of state-of-the-art manufacturing technology, Mark Williams Enterprises (MW) began building race cars in the early ‘60s. Back then, the primary method of making custom-length axles for narrowed rearends was founded on junkyard cores. He’d chop off the shaft and bore a large hold in the center of the flange, weld in a new shaft of the proper length, and machine the splines in a mill. (Henry’s Machine in Bellflower, California, was probably the best-known source for these two-piece units.) Sometimes, the stock shaft was thick enough and could simply be shortened and resplined, but the cutting tools of the day weren’s up to machining the heat-treated stock, so it would typically require annealing the shaft to a softer state, then re-splining it.
Speed equipment pioneer Ed Donovan developed the first one-piece forge axles, but their $300 price tag (about $3,000 in today’s bucks) fairly limited their use to Top Fuel.
Williams saw the need for a popularly priced one-piece forged axle for other realms and set about developing his Hi-Torque line. The first production pieces went into Judy Lilly’s “68 SS/AA Plymouth Barracuda. As for current axles, Williams allows three basic categories: carbon steel OE, induction-hardened aftermarket, and through-hardened alloy steel. Further, a number of physical characteristics must by considered, including type of material, heat treatment, spline count and shape, shaft profile, bearing size, flange design, and mass (important in racing only).
The most important (and the most-often overlooked aspect of axle strength concerns the splined end. Viewed as a cross-section of the end, the outer tips of the splines define the major diameter. The bottom of the grooves between the spline defines the minor diameter, and it is this dimension that determines the strength of the shaft. The pressure angle (or the basic included angle of the spline) is next in importance. Some are at 30 degrees (60-degree included angle), while the optimum for racing applications is 45 degrees (90-degree included angle) because the spline is shorter-allowing for a larger minor diameter of a given od axle.
According to the old SAE handbook, all modern axles are known as 24-pitch. If the shaft had a 1-inch circular pitch diameter (the mid-point between the major and minor diameters), it would exhibit exactly 24 splines (or teeth). The distance between the centerline of adjacent splines remains constant, so as the diameter of the shaft increases, so does spline count. For examle, a 35-spline axle has a major diameter of 1,500 inches; a 40-spline axle is 1.708 inches in diameter. Another key aspect of the spline is its shape. All OE axles, differentials, and so on, have involute splines, which means that the faces of the splines are slightly curved to provide optimum contact and even pressure distribution during engagement. The only way to achieve this shape is through hobbing (or rolling) the spline. Axles that have been resplined or manufactured using a flycutting procedure, however, have straight-cut splines. Matching a flat axle spline with an involute spline differential concentrates pressure on both the internal and external item, creating excessive levels of stress. In terms of reliability, an involute spline beats a straight-cut version hands-down.

 

If you’re a racer, the final consideration is mass. Most competitors who seek ultimate performance (as opposed to category and bracket racers primarily interested in consistency) desire to reduce unsprung rotational weight. For such applications, MW offers its Super Ligh Line that features pocket-milled flanges, a tuliped end, and undercut bearing diameters. They’re rifle-drilled 7/8-inch. Compared to conventional designs, they’re about 12 pounds lighter per pair.Recently, MW introduced its MasterLine series, a budget priced ($295 the pair) performance axle (designed for the street and bracket racing markets) that is made to exact length and features hobbed, involute splines that match a stock differential, posi-traction, or aftermarket spools. They maintainthree popular bolt patterns, 5-on-4 ½, 4 ¾-, and 5-inch bolt circles for ½ inch studs (5/8-inch upgrade is available).
What does Williams’ recommend? Any car worthy of a 9.99 or better elapsed time should be equipped with 35-spline axles at least. Heavier cars, or those with an anticipated load of 9,000 lb-ft should have nothing less than 40 splines. For bracket and street use, avoid OE 28-spline axles; in fact, even 31-spline axles are borderline. A late-model Camaro with 7.62 inch ring gear and pencil-thin 26-spline axles are at the absolute bottom of the reliability scale (but street tires will fry before the parts will break). Further, says Williams, pairing a spool with 28- or 31-spline axles is a surefire driveline grenade waiting to pop.
There is no substitute for substance. Material strength and heat treatment notwithstanding, size is the most important consideration. Williams also cautions the use of straight-cut splines-especially in conjunction with posi-traction and spools that accept involute (curved) splines. Consider the pressure angle; you can’t mix 30-degree and 45-degree components together. Even if they happen to engage, the contact points will be focused on a very concentrated area. Williams recommends that you have a clear understanding of the physical properties of all components and of the load potential involved when you choose an axle to fit you needs. Competent technical assistance from any manufacturer is as close as your telephone, fax or computer.


Click here to see a comparison of axles.

Material and heat treatment are the next areas of concern. OE carbon steel axles (typically SAE 1055 or 1541) are induction-hardened only up to the bearing surface, leaving the flange much softer than the rest of the shaft. That’s so people can run into curgs and not snap the flange off, or so the story goes. Induction hardening (shaft passes through and electromagnetic coil that excites a powerful current within the shaft, thereby heating it) penetrates about 0.150-inch, so the axle core remains relativley soft. Typically, the shaft surface hardness is 55-58 Rockwell and very stiff (almost brittle). Great for curb-bangers, but certainly not capable of handling the shock loads associated with massive torque.
On the flip side, MW’s Hi-Torque axles are made of high-strength chromium-molybdenum-nichel alloy and are subsequently heat-treated via a lengthy process called Austempering. In this phase, the forging is submerged in a special solution at 1,550 degrees F in a vertical furnace for more than an hour. This treatment provides an ultimate tensile strength of 225,000-253,000 psi along with exceptional ductility (the ability to change shape or form without breaking). The shaft surface hardness is about 50 Rockwell-far less brittle than an induction-hardened carbon steel axle. When these forgings are subjected to thousands of lb-ft, they twist and rebound like a torsion bar instead of snapping like a wishbone.
The profile of the axleshaft strongly influences ductility. MW’s Hi-Torque unit tapers from the axle bearing shoulder (1.774 inches) down to the minor diameter of the spline. This “triangulation” gives the axle more resistance to bowing (more powerful cars can actualy create a toe-in situation and impede performance). Moreover, about one-third of the axle remains in the minor diameter to allow torsion bar-like twisting where and when it should happen and prevents the axle from permanently moving out of shape. The shoulder of the axle bearing is another area to examine. Standard 12-Bolt Chevy axles use a 1.400-inch id bearing, while the one for the small Ford is 1.378. Most mid-range performance axles typically exhibit a 1.562-inch id bearing; racing axles, like the MW Hi-Torque, employ a 1.774-inch bearing. The larger the bearing, the greater the surface area to carry weight and dissipate the load.
Another area to consider is flange design and method of wheel attachment. The flange on OE axles is typically 3/8-inch thick and receives press-in studs with a serrated shank. Aftermarket axles generally have thicker flanges and are drilled and tapped for ½-inch diameter cap screws. Racing axles are prepped with 5/8-inch holes and have 11/16-inch shoulder drive studs that pilot the wheels.

STRENGTH COMPARISON OF COMMON O.E. AUTOMOTIVE & RACING SPLINES
# Teeth Pressure Angle Common Application Major Diameter Minor Diameter % Change in Diameter % Change in Strength
30 45 deg Basis For Comparison: GM 12 Bolt 1.2917 1.2083 0.0% 0.0%
             
28 30 deg GM Buick & Pontiac '64-'70 Axle 1.1960 1.127 -7.9% -21.9%
30 30 deg 8-3/4" Mopar '57-'64 1.2793 1.1960 -1.0% -3.0%
31 30 deg Olds/Pontiac '57-'64 1.3210 1.2377 2.4% 7.5%
35 30 deg Dana 60 Strange & Lenco spools 1.4876 1.4043 16.2% 57.0%
             
26 45 deg GM 10 bolt 7-1/2" '82 & later 1.1250 1.0417 -13.8% -35.9%
28 45 deg GM 10 bolt 8.5" & 8.2" '65-'81 & Ford 9" & 8.8" 1.2083 1.1250 -6.9% -19.3%
31 45 deg 9" Ford 1.3333 1.2500 3.5% 10.7%
33 45 deg Strange 9" Ford spools 1.4167 1.3333 10.3% 34.4%
35 45 deg Mark Williams 35 Spline spools 1.5000 1.4167 17.2% 61.2%
40 45 deg Mark Williams 9" Dana 60 Spools 1.7083 1.6250 34.5% 143.2%