Full Floating Shafts

An easy example is to compare the rear axle of a (rear wheel drive) passenger car and the rear axle of a large truck.

The rear wheel drive axle shaft has a bearing at the hub(wheel) end which is secured by a pressfit collar, the other end of the axle is supported by the bearing in the differential (pumpkin to some). The load of the vehicle is supported by the axle is mostly on the outer bearing with a canterlever action to the inner differential bearing. This is known as a semifloating axle.

The truck axle is different in that the Hub is supported by a pair of bearings directly onto the axle housing. This bears all of the load. The axle which is bolted to the end of the hub sits in the differential to provide drive only. This is a full floating axle.

There is a a conversion from Harrop Engineering to convert Ford 9 inch rear ends to full floating which uses a shaft splined at both ends, one end fits into the differential and the other to a mating cap which is bolted to the hub.

The benefit of a full floating rear axle is that the load is only carried by the housing leaving the shaft to just deal with torsion of the drive.

In a semi floating rear axle there is a potential for the shaft to fail due to stress caused by supporting excess weight(overloading) and the torsion/shock load of the drive. A broken axle will lead to the wheel assembly parting company with the vehicle.

This is why 40/60 series Landcruisers with full floating rear axles are a sought after item.

An example of a full floating shaft is a wrist (gudgeon) pin. These pins are normally press fitted into either the piston or the connecting rod at the factory. When this type of assembly is used, no axial stops are required. In order to reduce friction, engine modifiers sometimes hone both the hole in the piston and the wrist pin to achieve a sliding fit for the pin in both parts. When this is done, the pin is "fully floated" and provisions must be made to keep it from scoring the cylinder walls. This is normally accomplished with circlips in the piston or teflon end plugs.