Calculation of machine hour rate

There are more than 150 machines in our company.For calculationg Machine Hour Rate (for cost capturing using activity rate) we take the annual depreciation as machine cost.After the bookvalue becomes zero, what should be the machine cost? We operate these machines in 1,2 and 3 shifts.This depends on the production requirements.Can I change the rate of depreciation depending on the shifts for which the machines are operated?  Pl. help.