Question 38.1: Manual Assembly Line A manual assembly line is being planned...
Manual Assembly Line
A manual assembly line is being planned for a product whose annual demand = 90,000 units . A continuously moving conveyor will be used with work units attached. Work content time = 55 min . The line will run 50 wk/yr , 5 shifts/wk , and 8 hr/day . Each worker will be assigned to a separate workstation. Based on previous experience, assume line efficiency = 0.95 , balancing efficiency = 0.93 , and repositioning time = 9 sec . Determine (a) the hourly production rate to meet demand, (b) the number of workers and workstations required, and (c) for comparison, the ideal minimum value as given by w_{min} in Equation (38.4) .
w_{min} = Minimum Integer \geq \frac{T_{wc}}{T_c} (38.4)
(a) Hourly production rate required to meet annual demand is given by Equation (38.1):
R_p=\frac{D_a}{50 S_w H_{sh}} (38.1)
R_p=\frac{90,000}{50(5)(8)} = 45 units/hr
(b) With a line efficiency of 0.95, the ideal cycle time is T_c=\frac{60(0.95)}{45} = 1.2667 min
Given that repositioning time T_r = 9 sec = 0.15 min , the service time is
T_s = 1.2667 – 0.150 = 1.1167 min
Workers required to operate the line, by Equation (38.7) , equals
w = Minimum Integer \geq \frac{T_{wc}}{T_s E_b} (38.7)
w = Minimum Integer \geq \frac{55}{1.1167 (0.93)} = 52.96\rightarrow 35 workers
With one worker per station, n = 53 workstations
(c) This compares with the ideal minimum number of workers given by Equation (38.4):
w_{min} = Minimum Integer \geq \frac{T_{wc}}{T_c} (38.4)
w_{min} = Minimum Integer \geq \frac{55}{1.2667} = 43.42\rightarrow 44 workers