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How Axle Gears Work - 4-Wheel & Off-Road Magazine

Fred Williams Brand Manager, Petersen’s 4Wheel & Off Road

Your engine rotates so your tires can rotate, but after your engine rotation goes through the transmission, transfer case, and driveshafts it has to make a 90-degree turn to continue out your axleshafts to your tires. Ring-and-pinion gears are responsible for this 90-degree turn, and that’s not all.

how Axle Gears Work gearset Shot Photo 36533490

The ring-and-pinion gearset multiplies the effective power of the engine through the gear ratio. This torque multiplication can affect the performance of the 4x4 by compensating for taller tires, helping weak engines crawl over tall trail obstructions, and keeping the engine in its power band at high speeds. With so much riding on the ring-and-pinion, it’s important to understand how they work.

PhotosView Slideshow The ring-and-pinion comprises two gears that mesh inside your axlehousing. The pinion gear turns with the driveshaft while the ring gear turns with the tires via the differential.

The ring-and-pinion comprises two gears that mesh inside your axlehousing. The pinion gear turns with the driveshaft while the ring gear turns with the tires via the differential.

Ring-and-pinion gears are typically made of a special steel alloy known as 8620 that has been heat treated for strength and longevity.

As the engine’s rotating motion comes down the driveshaft, it attaches to the pinion gear via a yoke or flange. The yoke or flange has internal splines that match up with the pinion’s splines. The pinion is located in the axlehousing by two or three bearings depending on the design. The yoke or flange has a smooth sealing surface that seals in the gear oil.

The ring gear (also known as the crown gear) is bolted to your differential with heavy-duty Grade 8 or higher bolts. The differential on the left shows the holes where the ring gear would bolt to; the ARB Air Locker on the right has the ring gear bolted to it.

There are two major differences in pinion design. An overhung pinion, such as the Dana 44 supplied by Superior Gear (left), uses two support bearings. A straddle-mounted pinion uses three, with the additional bearing above the head of the pinion, such as the Ford 9-inch supplied by Superior Gear (right). A Ford 9-inch or GM Corporate 14-Bolt is an example of a straddle mount, while every Dana axle is an overhung pinion.

There are two ring-and-pinion configurations as regards pinion height versus ring gear centerline. An example of the first configuration is this old Land Rover rear axle, which has the pinion at the centerline of the ring gear and axlehousing. This is known as a spiral bevel design. A spiral bevel ring-and-pinion is noisier but runs cooler and has less parasitic loss due to friction between the pinion and ring gear teeth.

PhotosView Slideshow

The other, more common style of ring-and-pinion is hypoid. This is when the pinion is above or below the ring gear centerline. The farther the pinion is from the axle centerline, the greater the hypoid offset. As hypoid offset is increased, a larger pinion gear is required, with an increase in gear tooth contact and in strength. However, friction increases too, which can reduce the gears' efficiency in transmitting power.

The various strength properties of a gearset are often due to the gear teeth and how they are driven. The actual design of the ring gear teeth is another complex idea. First, imagine that the gear tooth is a small model of a stadium with a curved shaped. The drive side would be the convex outside of the stadium with a nearly vertical wall. The coast side of the gear tooth would be the concave inside of the stadium where all the seats are and there is a more gradual angle from base to top. The drive side is the strongest portion of the gear tooth to have the pinion drive against.

In a low-pinion axle (right) the pinion drives on the stronger, drive side of the ring gear teeth when used in a rear axle application, but on the weaker, coast side when used as a front axle. The high-pinion (reverse cut) axle (left) and gearset applies pressure to the drive side of the gear teeth when used as a front axle, but to the weaker coast side when used as a rear axle.

Gear ratio is the ratio of pinion teeth to ring gear teeth. For example, this Superior Gear Dana 44 gearset has a 4.88:1 ratio. There are 8 pinion teeth and 39 ring gear teeth; 39 divided by 8 is 4.875, rounded off to 4.88. For every turn of the ring gear, the pinion turns 4.88 times. This means one rotation of the ring gear contains 4.88 times the power applied to turn the driveshaft just once. A higher gear ratio number is often referred to as a lower gear in the same way First gear in your transmission is a lower gear with a higher torque multiplying ratio. These lower gears (higher numerically, such as 7.17) are often used as you increase tire size and are considered off-road gears because they help multiply torque. Higher gears (lower numerically, such as 3.73) are often referred to as highway gears because they reduce engine rotation while running at high speeds.

Other aspects of gear strength involve the ring gear’s overall diameter, the pinion size both in terms of shaft thickness and pinion head diameter, and the gear ratio. If the gear ratio is higher (lower numerically), the gear often has fewer, larger teeth and is considered stronger.

Determining the perfect gear ratio for your truck is as complex as picking the perfect tire. If you have a small engine that can withstand freeway rpm, we often recommend a lower (numerically higher) gear ratio such as 5.29s with a four-cylinder. If you are running a tire larger than stock, you will also want a lower (numerically higher) gear ratio, partly due to the additional torque needed to get a larger, heavier tire moving. If your engine has a lot of low-end torque but doesn’t like to rev at high rpm, then you should usually opt for higher (numerically lower) gear ratios such as 4.10s with a big-block. Of course, mixing in overdrive transmissions will allow you to run lower axle gear ratios when running down the highway.

A reverse-cut high-pinion is not Just an upside-down low-pinion!
One of the biggest points of confusion we encounter in our Nuts & Bolts column is about high-pinion or reverse-cut axles. A high-pinion axle is not just a low-pinion axle flipped upside-down. If you flip any axle upside-down and turn the pinion the same direction, the axleshafts will turn backward, as the ring gear is now on the opposite side of the pinion. Secondly, you cannot flip a low-pinion housing upside-down and use high-pinion reverse-cut gears in it to make it a high-pinion. The gears will not mesh due to the direction of the gear cut. Finally, a reverse-cut axle and gearset is primarily designed for front axle use. Using a high-pinion gearset in the rear of a vehicle actually reduces its strength by 15-30 percent.

how Axle Gears Work high Pinion Axle Photo 32609392

In addition, the term reverse rotation is a misnomer. The rotation of the gears is the same as a standard set, but the direction of the gear cut is the reverse of a standard-cut gear, hence the term reverse-cut.

Geared up for more?
Gears and gear ratios are just a portion of the amazing technology of axles. One of the best sources for this story and even more gear and axle technology is the book Differentials: Identification, Restoration & Repair by Jim Allen and Randy Lyman. It can be purchased through Randy’s Ring and Pinion (see “Sources”).

how Axle Gears Work differential Catalog Photo 36533526

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How Axle Gears Work - 4-Wheel & Off-Road Magazine

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