Ford Axle Ring and Pinion Assembly How to Guide

Many parts compose a rear axle and its most important function is to provide torque multiplication and speed reduction. This is accomplished with a right-angle drive gear arrangement called a hypoid gear set. The next most important aspect of the axle is to split torque to the wheels through a differential.

This chapter explores two of the most common manufacturing methods of hypoid gears: face-hobbed (or two-cut) and face-milled (or fivecut). It’s important to have an understanding of these two types because the pattern moves differently and at different rates depending on the type of manufacturing. More simply stated, if you try to shim the position of a face-milled gear using the face-hobbed process, you won’t achieve the correct position and gear contact pattern. The same is true for shimming a face-hobbed gear with a face-milled approach. Therefore, you need to accurately select the right process for the right gear.

Many people have selected the wrong process, even some who have been building axles for years. Making this mistake can be very frustrating because the assembly is suddenly noisy.

Face hobbing is a continuous indexing process in which the gear tooth surfaces are machined while both the cutter and the gear are rotating. The two machining steps are ring gear roughing and finoshing and pinion roughing and finishing. One machine is required per step.

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This is a typical ring and pinion from the 8.8-inch axle along with bearing, collapsible spacer, and pinion. The ring-and-pinion gear determines the gear ratio of the rear axle assembly, and therefore the gear ratio directly impacts vehicle acceleration and fuel economy. In addition, the face-hobbed and face-milled gears require different setup procedures, which are covered in this chapter.

This computer-generated ring gear tooth illustrates gear patterns better than actual hardware. The tooth profile on the left is the correct shape. The one on the right has been stretched to make it easier to see pattern changes. The outside diameter of the ring gear is referred to as the heel of the gear. The inside diameter is referred to as the toe of the gear. The convex (or bowed) surface of the gear tooth is the drive side. The concave (or curved in) side is the coast side.

The blue lines divide the gear face between the heel and toe portions. The red lines divide the gear between root and crown portions. The root is the valley between the gear teeth; the crown is the top surface of the tooth, which is sometimes called the top land or flank. Ideally, you want the pattern centered on the tooth face between the root and the crown, and also between the heel and the toe. In other words, you want the pattern to be at the intersection of the blue and red lines in this illustration. For high-performance applications, I favor the pattern being toward the toe portion of the tooth, so the entire gear-mounting system deflects (the pattern does not run off the tooth). This tighter pattern does have a slight noise disadvantage, which is generally acceptable for high performance applications.

This illustration shows the ideal bench pattern contact. You cannot always achieve this because of several factors, including housing machining tolerances, gear manufacturing, etc. You should always strive to have the pattern centered between the root and top surfaces of the gear. So you want the pattern to sit on the red line even if the pattern has to be closer to the heel or toe. The main factor for centering the pattern on the red line is the pinion mounting distance. If you are setting up a used gear set, focus more on the coast side as this surface has less wear on it and is a better indicator of gear position. Remove the used gear set to take contact pattern and backlash measurements. If the gear is quiet and works well in the vehicle and is only being changed to get a different ratio, for example, it is beneficial to have these measurements to act as a guide for re assembly into a different housing later.

 

Face milling is a single indexing process in which one gear tooth slot is machined at a time. In effect, the part is stationary while the cutter rotates. Two machines and two processes are needed to produce the gear: gear roughing both drive and coast sides and gear finishing both drive and coast sides.

The pinion requires three machines and three operations: roughing both drive and coast surfaces, finishing the drive side, and finishing the coast side.

Both types of gears are lapped together as a set and their ring and pinion must stay together. The ring/ pinion set and the differential bearing caps must stay matched in the axle.

All new Ford 8.8-inch original equipment and Ford Racing gears are face hobbed. Face-hobbed gears offer huge benefits from manufacturing and product strength standpoints, which is why the newer Ford 8.8-inch gears have switched to this process.

All Ford 9-inch gears (original equipment and aftermarket) and 8.8-inch aftermarket gears are face milled. (See my other CarTech book, High-Performance Differentials, Axles and Drivelines, for more details on both types.)

There are two reasons that the 9-inch gears are never face hobbed. The first is that the cutter path for face hobbing does not clear the straddle mount pinion on the 9-inch-style pinion head. (However, there are engineering solutions to resolve this.) Second, aftermarket gear producers have already installed the necessary manufacturing equipment for face milling. The machine cost to install face-hobbing equipment can quickly reach $2 million, and the typical aftermarket company cannot charge or sell enough gears to realistically recoup this initial capital expense.

Note that I do not cover ring gear spacers or different “series” of differential cases because they do not pertain to Ford 8.8-inch or 9-inch axles. General Motors and other axle manufacturers use a different ring gear mounting distance based on ratio and, therefore, sometimes require unique differential cases when changing ratios. In contrast, the Ford axle differential case can work with nearly any ratio, which simplifies determining a suitable case for a rebuild. When the ratio starts to get numerically high, above 4.56:1, there may be some unique things required to install the gear set. This is not typical for car tires but can be common for off-road trucks or trucks with large-diameter tires.

 

Face-Hobbed Gears

These include OE and replacement 8.8-inch gears.

 

This computer model of the ring gear tooth face illustrates a face-hobbed tooth form. Note that the tooth profile has a uniform depth across the entire face.

You want to have the pattern centered, and this diagram illustrates certain shift patterns. Changes in backlash (B/L) for a face-hobbed gear set are shown. Notice that the black arrows represent backlash increasing or decreasing. As backlash increases the pattern shifts toward the top of the tooth surface. The direction and slope of the arrows shows that the pattern moves slower up the tooth on the drive side as compared to the coast side. Also, the pattern moves faster from heel to toe with backlash changes on the drive side. This makes it easiest to concentrate on the drive side of the pattern, as the coast-side pattern moves slower with backlash changes.

This diagram illustrates how the pattern shifts with changes in the pinion mounting distance (PMD). The PMD is changed with the pinion head shim for the 8.8-inch axle (the 9-inch axle uses the pinion cartridge shim). As you increase the PMD, which is accomplished with a thinner shim, the pattern shifts closer to the top face on the drive and coast surfaces. At the same time, the pattern shifts toward the toe on the drive surface of the tooth and toward the heel on the coast surface.

On Ford OE and replacement gears, face-hobbed ring gears do not have the tapered back face machined. Here, you can see the area between the differential case mounting flange and the ring gear teeth. Close inspection reveals that the beveled surface is rough and has been left as an as-forged surface. This is an obvious sign of the type of process that was used to produce the gear—face hobbing. (On a factory face-milled gear, this surface is machined.)

In this diagram, the pattern is toward the heel and root on the drive surface and toward the toe and root on the coast surface. Decreasing the pinion shim thickness allows you to move the pattern toward the top of the tooth.

In this example, the pattern is really close to correct as far as root to top of the tooth surface; it is just a little high. You just need to center it between heel and toe on each surface. Decreasing the backlash corrects this pattern.

Here, the pattern is toward the toe on the drive surface and toward the heel on the coast surface. The pattern is also a little low, or closer to the root on both surfaces. Increasing the backlash corrects this pattern.

 

Face-Milled Gears

These include all 9-inch, aftermarket 8.8-inch, and early OE 8.8- inch gears.

 

The tapered-tooth depth of a face-milled profile can be measured, but it is also visually apparent. The tooth depth varies from just over .500 inch to under .300 inch from one end to the other. This is relevant so you understand the correct method to shim the gear position and subsequent bench contact pattern.

This is a computer model of the face-milled tooth profile. Based on the manufacturing process, the tooth profi le has a varying tooth depth from the inside of the gear to the outside.

 

 

 

 

 

 

 

Break–In Procedure

The original equipment gears that came with your car or truck have a special phosphate coating on them. This coating offers additional protection to the gear tooth faces during the break-in mileage of the axle. Most aftermarket gears do not have this coating and therefore a break-in procedure is required.

Even with this coating, I highly recommend a break-in procedure for any rebuild and new gears. Keep in mind that the higher offset of the 9-inch axle makes it prone to more heat generation than the 8.8-inch axle. New gears and bearings tend to generate more heat until they are broken-in. Always use the gear manufacturer’s recommendation for break-in procedure and lubrication.

The first 100 miles is the most critical. Use the axle at street speeds of 30 to 45 mph and stay below 60 mph for the first trip, which is usually less than 15 miles. Then allow the axle to cool for at least 30 minutes. Repeat this process for the first 100 miles on the new gears. Never subject the axle to full throttle or aggressive throttle accelerations in the first 500 miles because this exposes the components to excessive heat and can cause premature failure. Also never go to the race track within the first 500 miles because the axle is not prepared to withstand extreme loads.

During the break-in procedure, force speed differences across the clutch pack by driving in circles (clockwise and counterclockwise or figure 8s) to move lube through the clutch pack.

Change the axle oil after the first 500 miles. This removes any metal debris that was generated during break-in and if the oil was partially overheated, it is replaced. This may be a little too cautious, but the cost of oil is cheap insurance to make certain that you have years of hassle free performance.

 

Gear Ratio Selection and Tooth Combinations

As modern vehicles have become more refined and quiet, a certain level of gear design and attention to gear tooth mesh frequency is required. The pinion is usually the weaker gear in the gear train and the more teeth on the pinion the better. The OE gears typically avoid having pinions with fewer than nine teeth on them. It is possible to make smaller pinions work but they may require special grades of steel and heat-treat processes.

 

Non-Hunting Ratio

The Ford 9-inch axle came from the factory with ratios that are whole numbers, such as 3.0:1, which was achieved with a 39-tooth ring gear meshing with a 13-tooth pinion gear. This type of ratio is referred to as non-hunting because any given pinion tooth always contacts the same three-ring gear teeth per revolution.

An easy way to tell if you have a non-hunting ratio is to find out whether there is a whole number that can be multiplied by your ratio to come up with a whole-number result. For example, a 3.55:1 ratio cannot be multiplied by any whole number to get a whole number, so it is not non-hunting. But a 3.50:1 ratio can be multiplied by many whole numbers to get a whole number (2 for example, yielding another whole number, 7.0), so it is non-hunting.

The other way to tell is to write down the two multiplication pairs of each tooth-count and look for common factors. For instance 13 is a prime number, which has no whole number multiplication pairs, but 39 is not a prime number and has the factors 3 and 13. If there are no common factors, it is a non-hunting gear.

 

Following the break-in procedure and using the correct lubricant is crucial. This gear set was assembled and immediately driven to the track. The axle was filled with high-quality synthetic oil in the hope that it would allow the break-in process to be skipped. There was so much heat generated that the pinion teeth literally melted and tore off the shaft. Again, it is not recommended to use synthetic oil during break-in, especially on 9-inch gears.

 

Typically, a non-hunting ratio is timed, meaning it has timing marks on the ring and pinion teeth that need to be aligned during assembly. The main reason for this is to re-match the teeth to each other that were matched during the lapping process when the gears were produced. If the marks are not aligned correctly, there is usually a gear whine issue in the vehicle and the contact pattern may be less than ideal. If you have an OE whole-number ratio, look for these timing marks. Aftermarket gears typically do not have them.

 

This is a list of some common ratios and their gear and pinion tooth combinations. The 8.8-inch factory and service ratios are highlighted in yellow. The 9-inch ratios from the factory are in blue. The ratio 4.56:1 (in green) is common for both axle sizes. You can see how the 2.86, 3.00, 3.25, 3.50:1, etc. are prone to gear whine issues, simply based on tooth combination. So, depending on the need for a specific ratio, I avoid the tooth combinations that are prone to make noise, which are the ratios with a common factor. This is not to say that all nonhunting ratios are noisy but if the vehicle is sensitive to this type of frequency, these ratios cause a problem. Ironically, many aftermarket gears with a 3.70:1 ratio tend to make noise. Note that the list only includes ratios up to 6.0:1. There are even higher values available in the aftermarket.

 

Semi-Hunting Ratio

A semi-hunting gear ratio is a ratio that has common factors, but the number of revolutions for them to come in contact again is more than a single revolution. A 3.5:1 ratio is a good example because it has a tooth combination of 35 and 10. A common factor of 5 is the tooth combination, but because the pinion with 10 teeth is not a prime number, it requires more than one ring gear revolution to align with the same pinion tooth. In this case, two revolutions are required in order for the pinion and ring teeth to align as they started. This type of ratio should also have timing marks but do not always come this way.

 

Full-Hunting Ratio

A full hunting (or just hunting) tooth combination is desirable for its low noise along with ease of lapping. This is the reason that most modern ratios utilize prime numbers for the tooth combinations. In other words, the pinion teeth mesh with most of the ring gear prior to encountering the original tooth.

Written by Joe Palazzolo and Republished with Permission of CarTech Inc