Transfer Case

Transfer Case

In a 4WD vehicle, as mentioned earlier, the transfer case delivers power to both the front and rear assemblies. Two drive shafts normally operate from the transfer case, one to each drive axle.

The transfer case itself is constructed similar to a standard transmission. It uses shift forks to select the operating mode, plus spines, gears, shims, bearings, and other components found in manual and automatic transmissions. The outer case of the unit is made of cast iron or aluminium. It is filled with lubricant (oil) that cuts friction on all moving parts. Seals hold the lubricant in the case and prevent leakage around shafts and yokes. Shims set up the proper clearance between the internal components and the case.

Driveline Windup

It must be kept in mind that vehicles with two driving axles have different gear ratios between the front and rear driving axles, resulting in a pull-push action. The result of having the two axle ratios is a phenomenon called driveline windup. Driveline windup can be explained by associating the driveline to a torsion bar. The driveline twists up when both driving axles are rotating at different speeds, pushing and pulling the vehicle on hard, dry pavement. Also remember that neither the front nor rear axle has any compensating factor for speed and gear ratio differences between the front and rear driving axles.

Driveline windup can cause handling problems, particularly when rounding turns on dry pavement. This is because the front axle wheels must travel farther than the rear axle wheels when rounding a curve. On wet or slippery roads, the front and rear wheels slide enough to prevent damage to the driveline components. However, this may not be the case on dry surfaces. This is why many older 4WD systems that do not include components to dissipate driveline windup can only be safely driven on wet or slippery surfaces.

Interaxle Differentials The most common method of dissipating driveline windup is to include a third or transaxle differential in the transfer case gearing.

The front and rear drivelines are connected to the interaxle differential inside the transfer case. Just as a drive axle differential allows for different left and right drive axle shaft speeds, the interaxle differential allows for different front and rear driveline shaft speeds. The driveline windup, developed as a result of different front and rear axle gear ratios, is dissipated by the interaxle differential.

While the interaxle differential solves the problem of driveline windup during turns, it also lowers performance in poor traction conditions. This is because the interaxle differential will tend to deliver more power to the wheels with the least traction. The result is increased slippage, the exact opposite of what is desired.

To counteract this problem, some interaxle differentials are designed much like a limited slip differentials are designed much like a limited slip differential. They use a multiple-disc clutch pack to maintain a predetermined amount of torque transfer before the differential action begins to take effect. Other systems, use a cone braking system rather than a clutch pack. However, the end result is the same. Power is supplied to both axles regardless of the traction encountered.

Most systems also give the driver the option of locking the interaxle differential in certain operating modes. This eliminates the differential action altogether. However, the interaxle differential should only be locked while driving in slippery conditions.

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