i think yu is asking for the MTBF? most manufacturers have a guide for that.
ere Hippoo - from the Ashcroft site.... (for the Rangie)
RRC, Workshop Manual, Section 41 Transfer Gearbox, Overhaul, Dismantle, Inspection & Overhaul.
You can either do this test on the bench or in the car with the handbrake on and one front wheel lifted.
Viscous unit - rolling resistance Bench check NOTE: Testing should be carried out in an ambient of 20 ?C.
1) Secure the output shaft ...
2) Apply a clockwise torque of 27 Nm to the (front propshaft) output flange nut. If no resistance to turn is felt, unit requires replacing.
3) If resistance to turn is felt, apply a clockwise torque of 20Nm to the (front propshaft) output flange nut for 1 minute, this should result in a rotation of approximately 25? - 30?. If no rotation or a greater force is required, unit requires replacing.
27 Nm = 20 lbf-ft 20 Nm = 15 lbf-ft
thats what we need for the freebie