Hypoid gear set technology has been in use for well over 100 years. One example is the ring-and-pinion gears in our axles. The assembly basically serves two purposes: First, it translates the transmission rotation 90 degrees to drive the wheels. Second, it slows the rotation speed out of the transmission to a more useable wheel axle speed, while increasing available torque.
Setting up a ring-and-pinion requires some bit of knowledge as to how the two gears interact and the physical forces involved. When a gear set is machined and delivered from the factory, it is provided with setup specifications to allow precise mating of the paired ring-and-pinion gears. Proper installation requires specialty tools and attention to detail in order to yield an assembly that is strong, long lasting, and quiet.
The pinion gear spins on a set of tapered roller bearings in the axle housing or in the third member of an axle. The mating ring gear is bolted to the differential carrier and spins on a set of tapered roller bearings as well.
Two dimensions are vital when setting up a gear assembly for an axle. These are the pinion checking distance (CD) and backlash. CD (or pinion depth) determines how deep the pinion rides in the differential or third-member housing and its relationship to the center axis of the ring gear. Backlash (BL) determines how closely the pinion sits to the ring gear and how tight they mesh with each other.
You want the BL to be neither too loose (poor mesh and driveline slop) nor too tight (excessive heat and gear wear). Because the pinion gear is beveled, these two dimensions are intertwined, but the pinion depth is set first, followed by backlash.
In addition to these two dimensions, there are the pinion and carrier bearing preloads. New tapered roller bearings are installed under slight pressure to allow for wear dimensions during bearing break-in, while still keeping the assembly free from exhibiting any play after the bearings set in.
Setting up gears takes some knowledge, tools, and technique to assemble a reliable and quiet-running set. When built properly, a gear set should give you trouble-free service for many miles and allow the axles to spin your tires through the rough stuff.
Step By Step
This is a fully assembled removable third member. The ring gear is bolted to the differential and the pinion gear is inside this assembly with its shaft exiting the housing to mate with the driveshaft yoke. Always replace a ring-and-pinion as a set; they are manufactured and lapped to each other as a matching set.
Here, you can see the pinion depth, or checking distance, with respect
to the ring gear horizontal centerline. The gear manufacturer provides this
dimension, or similar, and usually etches it on the face of the pinion gear for each gear set.
The pinion gear typically engages the ring gear below its vertical centerline
as shown in this side view. This is referred to as a low-pinion gear set. If the pinion gear engages the ring gear above its centerline, it is referred to as
a high-pinion gear set.
Shims are typically placed behind the rear pinion bearing before the bearing is pressed on the gear. These shims determine the pinion checking distance. As an alternative, some housings place shims behind the rear bearing race to set the pinion distance.
A high-pinion axle (or reverse-spiral axle) with the pinion above the centerline is stronger than a low-pinion axle when used in a front-axle application. These types use reverse-cut gear sets that are not interchangeable with standard-rotation gear sets. The term “reverse-rotation” is often mistakenly used to reference a “reverse-cut axle.” However, this is a misnomer, as the axle and its shafts do not actually move in a reverse direction.
checking distance determines where the pinion will ride in the gear width of the ring gear. With the pinion installed in the axle housing or third member, and the pinion depth set, the differential
carrier with the
mounted ring gear is installed and the backlash adjustment begins.
Third members, such as this one from a Toyota truck, typically use adjuster nuts to set the position of the differential carrier and the carrier bearing preload. On axles where the differential carrier rides in bearing saddles in the axle housing, the differential may be set with adjuster nuts or with shims between the carrier bearings and the axle housing.
The backlash is adjusted by moving the differential carrier in the axle housing or the third member using the adjuster nuts or shims next to the carrier bearings. The ring gear can be moved closer to the pinion (reduced backlash) or farther from the pinion (increased backlash). The depth at which the pinion mates into the ring gear can determine gear strength and wear performance.
Once the correct backlash is set, check the mesh pattern using gear marking compound. Marking compound is a greasy substance used to check the mesh pattern. A small amount of the compound is applied in several locations around the ring gear. Then the pinion is rotated, causing the compound to be tracked around the ring gear. You can then observe the mesh pattern in the compound on the ring gear teeth.
If the pattern shows the mesh point too close to the heel or toe of the ring gear, the pinion checking distance must be readjusted. This requires removing the rear pinion bearing or race to change the shim stack and starting the setup process again.
When installing the pinion, there are two ways to set the bearing preload. One is to use a crush sleeve (left). With this method a non-reusable crush tube is collapsed between the bearings when the assembly is tightened. The second method uses a solid cast-iron spacer (right). Obviously, the solid spacer does not collapse, so shims are used with the solid spacer to accurately set the bearing preload. Shims are added and subtracted to get the proper preload. There are advantages and disadvantages to both methods. The solid spacer takes more effort to install, but is considered more stout. Should the pinion flange take a very hard hit on a rock or other immovable object, it is possible for a crush sleeve to be impacted and crushed slightly more. When this happens, it leaves some freeplay in the bearings, and the assembly loosens slightly.
In general, you want the pinion to mesh near the center of the face of the ring gear tooth. That is, it should be about centered heel to toe, and centered top to root. Note also there is a drive and coast side to the ring gear teeth. The drive side is the more critical pattern of the two. Examples of possible gear mesh patterns can be found with the gear installation instructions that come with a new gear set.
During assembly, the pinion bearing preload is set with the crush sleeve or pinion spacer shims. It’s checked with a sensitive torque wrench to ensure proper tension.