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