The pinion from a very tall 2.76:1 ratio (left) has much more tooth contact than the 5.38
The axle manufacturer rates an axle several ways, including Gross Axle Weight Rating (GAWR, or GAW) and Output Torque (OT). GAW is the amount of weight the axle is rated to carry. OT is divided into two areas-Maximum Continuous Output Torque (COT) and Maximum Short Duration Output Torque (MOT). Both are measured in pound-feet. COT is simply the amount of torque the axle can handle hour after hour, day after day without failure. MOT rates the momentary torque spike the axle can handle, such as from a spinning tire suddenly gaining grip.
Eighty percent of the time, the axleshafts are the weakest link in any axle assembly. Their strength is generally a major part of the OT rating offered by the manufacturer because they are at the end of the torque multiplication. Lower gears produce the most axle load from torque multiplication, and tall gears the least. OT is generally calculated according to worst-case loads, i.e., the lowest ratio offered by the axle manufacturer (generally about 4.56:1). Yes, really low aftermarket ratios may push axle loads outside of the original manufacturer's parameters and you should factor this into your calculations. Bear in mind that as ratios get lower, the pinion-tooth count decreases, tooth engagement is less and tooth size gets smaller. It's difficult to calculate exactly where the "magic number" dividing line is for failure. It's far more likely if you are running a lot of gear reduction (torque multiplication) in front of the pinion from a deep First gear, splitter or low-geared T-case, or even if you have a very torquey engine. This scenario may also overstress the pinion shaft and you can calculate pinion-shaft strength the same way as with axles below. If your existing axle is close to the ragged edge of strength, you plan difficult four-wheeling, have a very heavy rig and need ratios lower than 4.56:1, the safest cure is to go up a ring-gear size so the pinion shaft and teeth will be more robust.
Differentials and carriers have weak links that are difficult to calculate. Remember that all the torque, including from the reduction of the ring-and-pinion gears, will be transferred through the carrier, pinion (spider) gears, pinion shaft and side gears. Some open diffs are notoriously strong, especially four-pinion carriers, and some are notoriously weak. There are some improved open carriers available for certain axles, but most people upgrade by installing aftermarket lockers or limited-slips that include new carriers, which are vastly stronger than the OE units.
Weak carriers may provide an even weaker link when used with plug-in lockers (i.e., Lock Right, E-Z Locker, Aussie Locker). If the carrier is a strong one, the plug-in locker is a great, inexpensive, effective alternative. If it's weak, it will fail even faster. Bottom line, if you have a notoriously weak carrier, opt for a locker or limited-slip that includes a new carrier, or upgrade the open carrier if that alternative exists.
Hubs, bearings and wheel studs also count in the strength equation. It's not hard to figure that big tires will put greater stress on wheel bearings and hubs. Fortunately, these parts are generally safe if your axle assembly is matched in strength according to your tire size. Often overlooked are wheel studs. No five- or six-lug 4x4 that works hard should have studs smaller than 1/2 an inch in diameter. These studs are the last direct torque connection between the axle and the wheel. It's generally easy to upgrade a hub from 7/16 to 1/2 inch. Going to 5/8-inch studs is more difficult. Small bolt patterns, such as the 5-on-4.5-inch used on many Jeeps, Chevy S10s, Rangers and so on are best upgraded to 5-on-5.5-inch or six-lug patterns if tire diameter will be greater than 35 inches.
The rigors of hard trail work and the extra leverage of big tires can stress a housing past the breakage point. Housing strength is a combination of the strength of the iron center housing and the diameter, wall thickness and material of the tubes. Given equal wall thickness, the larger-diameter tube is stronger and given equal diameter, a thicker wall tube is stronger. Cold Drawn or Drawn Over Mandrel (DOM) tube is stronger than the common HREW (Hot Rolled Electric Welded) tube, and seamless tubing is the strongest of all. Center housings are commonly of either grey cast iron or nodular iron, with nodular being stronger by 50 to 100 percent (depending on grade used). Since the late '80s, almost all housings are of nodular iron of one grade or another. Again, it's difficult to calculate precisely what's needed, but we can generalize. Based on observation, lighter rigs like Jeeps that are worked hard need at least 2.75- to 3-inch tubes, 0.250-inch wall, or better, in the rear, and 2.75-inch 0.313 or 0.380-inch wall up front. That's equivalent to many 1/2-ton trucks. Half-tons in the hard-worked category should have 2.75-inch, 0.50 wall or 3-inch 0.313 or 0.380 wall tubes up front (or better), and 3-inch 0.50 wall in back. Many solid-axle 3/4-tons have smallish axles up front, even though they may have 0.50 wall tubes, and should really be upgraded to 1-ton Dana 60s with 3.125 tubes with 0.380 or 0.50-inch-wall tubes.