At first glance, the axle shaft seems to be one of the simpler pieces in the axle assembly. There are differing opinions of what makes a great axle shaft. And, of course, there are always tradeoffs. I will review the major areas of the axle shafts and point out some things to keep a watchful eye on.
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Axle Shaft Function
The term “axle shaft” can some-times be confusing. It is also often overlooked as a simple component in the entire axle system. For a typical beam axle, this term refers to the shafts that connect the differential side gears to the wheels. A spline couples the axle to the differential side gear, and a bolted flange attaches the axle to the wheel and brake drum or rotor. The axle shaft is not to be confused with the complete axle assembly.
First off, keep in mind that from the engine, through the transmission, and all the way to the hypoid gear set, you are progressively multi-plying engine torque. The highest torque in the vehicle is the wheel torque, and the axle shafts are what transfer that torque to the wheels. Let’s look at some quick numbers to help illustrate how high this torque can be.
Let’s assume that we have a vehicle with an engine capable of producing 300 ft-lbs of torque, a transmission first-gear ratio of 2.45:1 (such as the Chrysler A727 Torque flite) with a torque converter stall ratio of 1.8:1, and a rear-axle ratio of 3.55:1.
Multiplying these together we get:
300 x 2.45 x 1.8 x 3.55 = 4,696 ft-lbs of torque at the ring gear
Assuming each axle shaft receives 50 percent of the total torque, we end up with about 2,350 ft-lbs of torque per axle shaft. That is significantly higher than the 300 ft-lbs of engine torque that we started with. If we have a spool in the rear axle, we can send all of the torque to one wheel.
However, some say that the tire slips relative to the ground before that happens. This is called skid torque, and there is truth to the statement. If the potential torque to the wheel is greater than the torque that the tire-to-road interface can support, the tire will spin relative to the road. Wheel spin is very common in many performance applications. If the tires are changed to slicks, their higher coefficient of friction increases the skid torque. Most folks typically plan for an impact factor of 1.2 to 1.5 times the skid torque value as the maximum torque to the shafts.
Axle shafts can bend from vehicle loads or side impact. For example, sliding sideways and hitting a curb can bend the axle flange; alternatively, an axle may fail in torsion just from too much torque.
Some vehicles have independent carrier axle assemblies and halfshafts between the differential side gears and the wheels (refer to Chapter 2). The halfshafts serve the same function as the axle shafts, but only carry torque. They do not support vehicle weight like a semi-float beam axle shaft.
Since the majority of muscle cars have semi-float axle housings, I will focus on that style axle shaft. Keep in mind that most of the following details can be applied to full-float and three-quarter-float axle housings as well (see Chapter 2).
Axle Shaft Geometry and Nomenclature
The semi-float axle shaft has two primary functions: the first is to transfer torque to the wheel, and the second is to support the weight of the vehicle on the rear wheels. Therefore, the shaft is resisting the twist of the driving torque and the bending from the vehicle load. Because the axle needs to be strong enough to transfer the torque and support the bending loads, it is very important to get the correct axle shafts for your application.
Original equipment axle shafts are fine for stock or mild cars. Any serious performance upgrades should include matched axle shafts in order to avoid any catastrophic failures. Since most factory semi-float axle assemblies use a C-washer-style axle shaft retention strategy, it is imperative to make sure that the axle shaft does not fail. If the shaft breaks, there is no longer anything holding the axle shaft in the axle housing, except maybe the brake hardware if you have disc brakes. How many times have you seen a wheel and tire on the side of the highway with the axle shaft still attached?
There is a secondary function that the semi-float axle shafts must also provide. The shaft must resist side-to-side wheel thrust loads, which are normally encountered during turn maneuvers. During the maneuver, as the driver turns the steering wheel the front tires articulate about the steer axis into the turn while the rear tires must be dragged through the turn. The rear wheels want to continue heading forward in a straight path. The act of dragging the rears tires through the turn pushes the outside axle shaft inboard with the inside shaft pulled out-board. For example, imagine taking a right-hand turn. The left rear-axle shaft pushes inward against the differential pin or thrust block, depend-ing on the type of differential, while the right rear-axle shaft pulls on the C-washer to differential side gear interface. Combining wider, better gripping tires such as drag racing slicks with tightly preloaded differential results in increased side-to-side thrust force.
Materials and Manufacturing Methods
Original equipment axle shafts are typically made from high-grade steel, generally SAE 1039, 1055, 1050, or 1541. They are forged and then heat treated with induction coils. The induction heat-treating process quickly draws the shaft through an electromagnetic coil. As a consequence it is heated and quenched in a controlled fashion to harden the surface of the shaft while the center core remains relatively soft, maintaining its ductility. Original equipment axle shaft hardness depth is typically between 0.100 and 0.180 inch. The shafts are not usually hardened all the way through in order to maintain the ductility and toughness of the softer core.
Surface hardening provides sufficient wear resistance for bearing surfaces and seal journals for long life. As a shaft becomes harder, it typically becomes more brittle and less resistant to shock loads. This is one reason it is important to have the correct axle shaft for your application. In some applications, you may want a through-hardened shaft. In typical street-use applications, you want to have ductile shafts. Purpose-built drag cars require through-hardened shafts and special gear sets (see Chapter 6).
As the horsepower of the vehicle increases and subsequent wheel torque is generated, stronger materials like SAE 1550 or 4340, or even a more exotic 300M, are needed for extreme-duty axle shafts. The SAE 4300 series material is referred to as a high-strength low-alloy material. Strengthening materials like chromium and molybdenum are added to the steel to create chromoly (also called cromo, or CRMO) alloy grades.
Next, let’s focus on the shaft itself and the important attachment features. On the inboard end of the shaft are the C-washer (or C-clip) groove and the mating spline to the differential side gear.
The C-washer groove at the end of the axle shaft is a critical feature, as that section of the shaft is responsible for retaining the axle shaft in the axle assembly and resisting pull-out loads. The end of the axle also contacts the differential carrier pin and at times shows signs of wear over the life of the vehicle.
The serrated area on the end of the shaft is called the spline. It allows the shaft to be installed and removed for service. Splines are a long discussion with many folks: number of teeth, pressure angle, length of engagement, involute versus straight-cut profile, and so on.
Number of Teeth and Pressure Angle
First, we must examine the spline tooth count and why it is important. In most circles, people generalize tooth count as “the more the better.” Shaft diameter is related to the spline diametral pitch and tooth count. For applications only subjected to torque (as opposed to a propshaft spline which slides under load), a 45-degree pressure angle is the most common. Most pinion interface splines and axle shaft splines have a 45-degree pressure angle. The main reason for this is financial, since a spline rolling rack with a 45-degree pressure angle can yield approximately 50 percent longer tool life when compared to a 30-degree pressure-angle rack.
There are exceptions, such as the Mopar 83⁄4-inch axles from 1957 to 1964 and some Buick, Oldsmobile, and Pontiac axles from a similar vintage, which utilize a 30-degree pressure angle. The rolling process presses the spline shape into the axle shaft prior to heat-treating. Extreme pressure is exerted on the shaft during the spline-rolling process, dis-placing the material to form the tooth profile. This movement of material, which is similar to the gear forging process, produces a finer grain structure not achievable with a cut spline. Cut splines create stress risers in the shaft, making it approximately 30 percent weaker.
Most axle splines have a spline pitch of 24/48. The top number, 24, is the diametral pitch. The bottom number is stub pitch, and is always twice the diametral pitch. This gives us the proportions of the specific tooth geometry for the spline. Diametral pitch is the number of spline teeth per inch of pitch diameter. The diametral pitch determines the thickness of each spline tooth, and in combination with the number of teeth, also determines the pitch diameter.
Never mix and match different pressure-angle or pitch spline shafts with side gears. They may assemble but will not have the proper tooth surface engagement and are a guar-anteed recipe for failure.
What does all this mean? If the pressure angle and spline pitch remain constant, then as the tooth count increases, so does the actual spline diameter. As the spline diameter increases, the torque-carrying capacity also increases. Therefore, the more teeth on the spline, the stronger that interface will be. As the diameter of the shaft increases, the maximum stress or load that the shaft can withstand increases by the diameter, cubed. Therefore, just a one- or two-tooth increase in the spline count improves the strength significantly.
For curious readers, the equations can be approximated as follows for an external-fillet root-side-fit spline with a pressure angle of 45 degrees:
Major Diameter =(Number of teeth + 1) ÷ Diametral Pitch
Minor Diameter =(Number of teeth–1) ÷ Diametral Pitch
Just substitute the number of teeth and then solve for diameters in inches. The minor diameter equation is an approximation, as there are many factors that go into calculating the exact value.
The minor diameter is the value used to analyze shear stress in the splined region of the axle. The graph shows spline tooth count and shear stress with a baseline 30-tooth spline, which is a good baseline to compare the stock Chrysler 8.75-inch and GM 12-bolt. The Ford 8.8-inch can be either a 28- or 31-tooth spline axle shaft, depending on the vehicle application.
As I stated earlier, when the minor diameter increases, the overall torque-carrying capacity of the spline also increases. Since the pitch is the same for the splines, we can rely on just tooth count.
Length of Engagement
The axle shaft’s external spline engages with the side-gear internal-mating spline upon assembly. The typical side-gear spline length is approximately 1 inch for most passenger car axles. Some aftermarket axle shaft manufacturers produce axle shafts that have a spline length of a few inches or more. This allows them to cut the end of the shaft to length, machine-in the C-washer groove, and ship the part. This reduces their inventory and costs. This method usually leads to problems later on with the axle shaft. The spline to shaft transition zone is very important, as is the shaft length and stiffness. You really do not want unnecessary spline cut into the shaft.
Remember that spline strength is a function of the diameter raised to the third power. Well, the ability of the shaft to resist twisting is a function of the diameter raised to the fourth power. You actually want the shaft to be able handle some twisting to help absorb shock and impact loads experienced from pot holes, wheel hop, and even wide-open-throttle launches. The ability of the shaft to twist and return to its original shape is based on the diameter, as mentioned above, and the overall length of the shaft being twisted. Some aftermarket axle shafts even have a gradual taper from the spline minor diameter to the wheel-bearing diameter on the opposite end. This twisting can actually protect the differential gears and hypoid gear set from heavy shock loads.
The ideal condition is to have the shaft start at the minor diameter of the spline. As the spline chart shows, we calculated strength with that diameter; the smaller diameter allows for a smoother transition of load to the shaft as compared to an abrupt step.
The ideal design for maximum strength is where the spline minor diameter transitions down to a smaller shaft diameter. This reduces the stress riser at the smaller or minor diameter of the spline. This is not always possible and by no means implies that other geometries won’t work for your application. This is simply the preferred case.
Just one last comment on shaft length: Most axle shafts are the same length for the left and right sides of the vehicles. There are some left- and right-side axle shafts that are not always the exact same length. Some muscle cars have the engine and transmission slightly offset to the passenger side and the engine and transmission are tipped downward.(I will talk more about powertrain orientation in Chapter 8.) Since the powertrain is offset, the differential pin is not typically centered in the vehicle, and therefore different-length shafts are required on each side.
The asymmetric axle length also implies that the axles have unequal stiffness in torsion. The longer shaft, usually on the driver’s side, twists slightly more, which can lead to a couple of common failures. Since the longer shaft can handle more twist angle before permanent damage, the short shaft typically fails. In that situation, there are some vehicles that perpetually break the shorter shaft when the longer shaft remains intact. However, if the load conditions are such that the long shaft is repeatedly twisted more than the shorter shaft, it may fail from fatigue before the short shaft. Again, an equal load shared by same-length shafts would be ideal.
The GM 10- and 12-bolt axles do not have this issue as both shafts are the same length. On older vehicles, you may recognize the following: The 1983 to 1992 Ford Rangers and Bronco IIs have unequal-length axle shafts. The one on the passenger’s side is about 15⁄8 inches longer than the one on the driver’s side. How-ever, the driver’s-side axle shaft is the same length as on the 1979 to 1993 Mustangs. So in the scrap yards, there are many Rangers and Bronco IIs with just the driver’s-side axle shaft missing. It’s easy to convert your Mustang to a five-lug bolt pat-tern. You just need to acquire two of these shafts.
To better understand this, let’s look at the mathematical equation that governs the angle of shaft twist for a given shaft geometry.
α =584 x T x L ÷ D4 x G
α = angle of twist in degrees
T = torque applied measured in inch-pounds
L = shaft length in inches
D = shaft outside diameter and
G = modulus of elasticity in torsion of the shaft (for steel this is 11,500,000 psi)
If we want to compare the angle of twist of the left versus the right shaft, we would end up with:
αright ÷ αleft
Assuming that both shafts are steel, the value for G is the same and drops out of the equation. Now we are comparing torque, length, and diameter.
If we desire equal torque from side to side and we must use different shaft lengths; the only other variable left to tweak is diameter. Therefore, if we want to balance the torque from side to side, we can achieve that with unequal-length axle shafts that have different diameters.
Since the relationship is a function of diameter raised to the fourth power, small changes in diameter yield large changes in the deflection angle. Superior Axle makes equal-stiffness axle shafts that compensate for that situation. Some question how important it is to balance the torque and twist. It may seem minor, but the fatigue from twisting is balanced in each shaft. In the end, if you do not have a fatigue life issue in the vehicle, then this application of technology is trivial.
Involute vs. Straight-Cut Profile
The axle shaft spline feature must mate with the side gear spline on the differential or spool inside of the axle assembly. The side-gear spline-tooth profile is an involute tooth form profile. Basically, the tooth surface geometry follows a curved shape, and it is difficult to see with the naked eye because the involute profile is a slight curve. This slight curvature provides an optimized contact surface and better load distribution across the face of the spline. The involute profile has a concave shape that is formed from either the hobbing or rolling process. Both of these manufacturing meth-ods require some specific tooling.
Unfortunately, a small number of people modify existing shafts by resplining them when they are shortened. Occasionally a low-cost vendor cuts corners and foregoes the correct tooling to produce the spline geometry, instead cutting straight splines into the axle.
When a straight-cut spline is mated with an involute spline on the side gear, the splines make contact only on the tallest point of the involute profile, which creates very high stresses and excessive wear on the spline surface. Essentially it’s a failure waiting to happen. So you can see why it is imperative to match the involute spline profile of the side gear with the correct profile on the axle shaft. It is best to avoid straight-cut splines.
The portion of the axle shaft that rides on the wheel bearings needs to be carefully examined, especially if you are going to re-use your shafts. Since the axle shafts acts as the inner race for this bearing, it is quite common to see this surface degrade after many miles of usage. I have seen many shafts with a light gray look in this area, as opposed to the typical mirror-like finish that you want. Also, look closely for any nicks or grooves. Minor imperfections can be buffed out with emery cloth but major nicks warrant a new axle shaft.
Flange, Lug Studs and Nuts
Some axle shafts are actually two pieces. Some Chrysler shafts made in the early to mid 1960s have the flange end fit into a tapered shaft.
The typical factory axle flange is about 5/16 inch thick, while aftermarket axles are much thicker in this area at around 7/16 inch. Also, the after-market shafts have a large, generous radius to blend the flange area to the shaft diameter. This gradual radius helps to reduce stress risers at this interface.
Most new axle shafts do not come with studs installed. If they are the press-in-place style, then you can press them in place or just have the axle flange span across your bench vise and hammer the studs in, which usually requires a heavy hammer to install. The other option is to use a press to get the studs driven into place.
Some builders take a shortcut and try to use the lug nuts to install new studs. This is not recommended. The load required to pull the stud in place can damage the new stud threads and the nut. Also, if the studs won’t stay in place or rotate in the axle shaft hub, do yourself a favor and install new ones.
Most aftermarket axle shafts can accommodate push-in-style and thread-in-style studs. Usually the thread-in style is larger in diameter (perhaps 1/2-inch) when compared to the factory 7/16-inch thread. This can be an important detail if you are going to buy new wheels soon. You may want 1/2-inch studs in your axles, and choose to buy the wheels at the same time.
Another important area for overall axle shaft strength is the shaft-to-flange transition. This area can be overlooked but careful comparison of factory shafts and most aftermarkets shafts reveal a more gradual transition with the aftermarket shafts.
The NHRA rules state that the wheel studs and lug nuts must not be made of aluminum or titanium, so you need to use high-quality steel parts for this high-performance application. The function of retaining the wheels to the axle shafts is not the place to try to save weight. The rules also state that you must have at least the stud diameter amount of thread engagement on the lug nut. So, if you have 1/2-inch-diameter studs, the lug nuts need to be at least 1/2 inch of thread for engagement.
Some aftermarket studs are quite long in order to accommodate wheel spacers and thick wheels. Make certain to get the correct studs for your intended use. Do not con-fuse how many threads stick out past the lug nut as engagement. The rules do not state that the studs need to stick out past the lug nut. It is a good idea to have a few threads past the lug nut but there’s no reason for it to be excessive.
So even though the axle shafts are often overlooked, they are just as important as any other component within the complete axle. As with most of the axle components, there is some confusion about what method is best to determine which shafts to use. The answer is that it depends on your application. As I have discussed, there is a fair amount of research that goes into axle shafts, studs, and bearings. Don’t let these parts be the weak link in your axle system.
Written by Joe Palazzolo and Posted with Permission of CarTechBooks