i

The i-th preventive maintenance cycle

c_o

Unit downtime costs

N

The optimal number of preventative maintenance

C_pm

Total cost of preventive maintenance

t_i

Time interval between i-1th and i-th preventive maintenance cycle

C_r

Cost of equipment replacement

R_min

Reliability threshold

C_d

Maintenance downtime costs

h_i(i)

The failure rate function of the equipment during the i-th preventative maintenance cycle

α_i

The scale parameters of the weibull distribution

t_p

Time of single prevention maintenance

β_i

The shape parameters of weibull distribution

t_f

Time of single unplanned maintenance

a_i

Service age decreasing factor

c_P

Costs of single preventive maintenance

b_i

Failure rate increasing factor

c_f

Costs of single unplanned maintenance

γ_i

Improvement factor