Ford Differentials. Joseph Palazzolo

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       The Ford Explorer uses an independent-style carrier made out of cast aluminum. The ring gear speed sensor is shown in the upper right-hand portion of the photograph. This sensor is for the anti-lock brake system.

      The 8.5- and 8.8-inch gears have even found themselves in the independent-carrier-style axles. The Ford Mustang and Thunderbird used this style for a few model years. There were also 8-inch ring gear independent carriers in production.

Front Axle Applications

      While the majority of this book focuses on rear-wheel-drive applications, there are also four-wheel-drive applications of the Ford 8.8-inch and 9-inch axles. When used in these applications the axles are very similar to a rear axle except they have a different wheel end arrangement that allows for the steering knuckles to be mounted.

      When these axles are used to drive the front wheels, the hypoid gear set is basically flipped over and special reverse-cut tooth geometry is utilized for the 8.8-inch version. Reverse cut just means that the spiral hand is the opposite of traditional ring gears for a rear axle application; it is the mirror image of a standard rear axle gear (see Chapter 3).

      This means that the standard 8.8-inch rear axle gears cannot be used for a front axle application and vice versa. During the manufacturing process, the gear blank forgings and the gear design are the same; just the spiral angle is machined to the opposite hand.

       Reverse-Rotation Gear Set

      The most common 8.8-inch front axle application is the F-150/ Expedition platform. Four-wheel-drive vehicles use a unique reverse-rotation-style hypoid gear set, and the pinion is set above center on the front axle. With the pinion in that location, a special provision is required to supply adequate lubrication to the pinion bearings.

      The production axle is an independent-style axle arrangement with halfshafts to the drive the wheels. The stub shafts that come out of the aluminum axle housing are a 28-tooth spline shaft. These 28-tooth side gears in the differential are unique as they have a snap-ring groove machined in them to retain the stub shafts and do not rely on C-washer retention as does the rear axle version. The main reason for the pinion to be above center is to reduce the angles on the front propshaft universal joints.

      This special 8.8-inch, reverse-cut-style gear set is also used on the Currie high-pinion, 9-inch axles. They have unique cast gear cases that accommodate the smaller 8.8-inch offset but still retain the 9-inch-style differential and bearing support structure for the ring gear. The pinion is not the traditional cartridge style as on the common 9-inch; it actually operates similar to the 8.8-inch-style gear set.

       Straddle-Mount Pinion Bearing

      Another difference between the 8.8-inch arrangement and the traditional 9-inch is that the 8.8-inch gears do not have the trunnion on the pinion head, so the high-pinion 9-inch axles do not have the straddle-mounted pinion bearing arrangement. Therefore, the high-pinion, 9-inch axle used for off-road and lifted truck applications is more of a hybrid combination of the 9-inch structure with 8.8-inch gears. This axle assembly usually drives 35-inch tires and transmits up to 400 hp. The hybrid third member can also be used in the rear axle application, again limiting power and tire size. The high-pinion third members have some unique features added to the gear case in order to catch and direct oil to the bearings.

       From left to right are: 10-inch, 9-inch, and GM G-Body 7.5-inch 10-bolt.

      It is possible to use a traditional 9-inch rear axle third member and just flip it over to drive the front wheels. But in this configuration, the hypoid gears are typically driving on the coast side of the gears, which are not as strong, with a typical reduction in load-carrying capacity of about 20 to 30 percent along with poor oil flow.

       9-inch Gear Arrangement

      Hi9 has developed an actual 9-inch gear arrangement, also called TrueHi9. Again, the pinion located above center helps with driveshaft angles and ground clearance, but it requires unique oil porting and baffling in order to catch and feed oil to the pinion bearings. This arrangement uses a unique 9-inch-style gear set, produced by Richmond Gear, that has the reverse-cut method to drive on the drive side of the gear tooth faces.

      Hi9 also offers the MegaHi9. It uses gears that are made out of the stronger SAE9310 material (as compared to SAE 8620), along with a 35-tooth splined pinion arrangement. The material change makes the gears approximately 15 percent stronger.

      The TrueHi9 design has a distinctive thrust button feature to help combat ring gear separating forces along with a unique reinforced gear case in the pinion pocket bearing area. This thrust button is just a hardened, threaded, adjustable support that is very close to the ring gear back face. When the ring gear deflects from high-torque loads, the thrust button surface actually contacts a specially machined surface on the back face of the ring gear and resists that deflection.

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