by Jim Kerr
Click image to enlarge.
Eaton has been in the axle business for decades. The latest design inaxles was recently introduced on the head-turning Hummer H2, and it is a driver selectable electric locking rear axle assembly. While the Hummer H2 may be the first to use this unit, I predict we will see wide spread use of it on many trucks and SUVs.
To activate the locking axle, the driver simply pushes a button on the dashboard. In the Hummer H2, the transfer case module is used to control the system. If the vehicle is in low range, the vehicle speed is less than 4.8 kph, and wheel speed difference is less than 50 rpm, the control computer sends electrical power to the rear axle housing. Of course, this programming is specific to the H2. The system could be manually controlled, but then there would be no safety systems to prevent engagement that could damage the axle.
One more programming feature is included on the H2. If the vehicle goes faster than 32 kph, the computer unlocks the axle. Why? Because an open differential is more stable at higher speeds. For example, I was recently asked why Jaguar did not install a limited slip differential in the supercharged S-Type R sedan. With 400 horsepower, this addition would seem natural, but instead Jaguar controls wheel spin with traction control that applies brakes and decreases engine power.
If a limited slip or locked rear axle was used, there is enough horsepower to spin both back wheels, especially on slippery surfaces. As soon as both wheels spin, there is no traction and the slightest deviation can cause the vehicle to loose control. This may not be a problem at very low vehicle speeds, but at higher speeds, the vehicle can spin dangerously.
With an open (unlocked) differential, only one wheel spins, so the stationary one still has traction and keeps the vehicle stable. Using traction control transfers power to the stationary wheel, but also decreases power so it is safer.
Looking at the Eaton locking axle construction, it is so simple that I wonder why it hadn’t been thought of before. When the computer sends electricity to the rear axle, it energises an electromagnetic coil on one side of the rotating differential. The electromagnet sits between two bearings that allow it to remain stationary while the differential turns.
Once energised, the electromagnet attracts and stops a washer-like steel plate. This plate has ramps and balls that roll up the ramps, placing side thrust on another plate. As this second plate is moved sideways, pins on the back side are forced into engagement with holes in the gear inside the differential that is connected to the axle. Because no differential action can take place, the axle is locked to the rotating housing and both axles turn at the same rate. Turn the magnet off and springs push the locking pins and plate outward, unlocking the axle.
Locking axles may provide excellent traction, but make it difficult to turn corners. Try rolling a tube in a circle around one point and you will quickly see why. The inside end has to only rotate a little, while the outer end has to rotate many times. The same happens on a vehicle when cornering. One tire, or perhaps both, will have to slide.
Front locking axles are not generally used because they impede steering. This would be dangerous. The only vehicle I am aware of with a locking front axle is the Mercedes-Benz G500. This military based SUV uses a locking transfer case, rear differential, and front differential. With all three locked, traction is superb on the roughest terrain, but turning is a real challenge. The G500 locking front axle is intended for use in the most severe conditions, and only to get through it.
Locking differentials offer the best of limited slip and open differentials. For off-road use, they provide ultimate traction, while pavement handling is improved in the open mode. This simple addition makes vehicles much more versatile.