An Interactive Program

Abstract

We describe the development of a program to balance 1- or 2-sided

assembly lines for a manufacturer of utility vehicles. The program is

highly interactive, runs on a personal computer, and is in use now. We

also discuss some theoretical properties of 2-sided lines. Finally, we include

the data for a real assembly line.

Key words: assembly lines, line balancing, heuristics, software

Supported in part by the National Science Foundation (DDM-9101581), and by the Office

of Naval Research (N00014-89-J-1571). Address: School of Industrial and Systems Engineering,

Georgia Institute of Technology, Atlanta, GA 30332

1

1 Introduction

A manufacturer of small utility vehicles rebalances its several very flexible assembly

lines as often as once each week to better match production rate to

market demand that is both seasonal and "lumpy". Each line produces a single

model of vehicle. The assembly at each station is done by a worker who is either

a permanent employee of the manufacturer or else a temporary employee hired

from a pool of contract labor. The company wants to minimize the number of

stations (workers) required to produce at any given rate. Adjustments to the

number of workers are made by hiring the appropriate number of temporaries.

This flexibility is important to the manufacturer: their main product is a golf

cart, so that demand is highly seasonal.

Each line has 2 sides, left and right, with stations in pairs directly opposite

each other so that each vehicle is worked on by two people simultaneously. Some

tasks can be assigned only to one side of the line (e.g. "mount the left wheel");

some can be assigned to either side of the line (e.g. "install the hood ornament");

and some tasks must be assigned to both sides of the line simultaneously, so

that the pair of workers on opposite sides can collaborate (e.g. "install the rear

seat"). Each of the several products consists of 80-300 tasks. (The data for one

of the products appears in an appendix.)

The manufacturer requested software to enable an engineer to rebalance a

line quickly. It was especially important that the program be fast, convenient,

and interactive. This was important not only for reconfiguring the lines, but

also for testing the effects of design changes on the manufacturing process. For

example, an engineer might redesign a fender, thereby changing some of the

tasks associated with installing it; then using the program to rebalance the line

with the hypothetical changes, he could see how the line might change.

There is a rich literature on assembly line balancing. However, most of it

concerns algorithms to balance an idealized 1-sided line. Our paper complements

this literature by describing the inevitable compromises made in translating

abstract models to a commercial product in use on the shop floor. In addition

2

Product Editor

Task Editor

Line Editor

Station Editor

Line-balancing engine

..............................................................................................................................

..........................

...................................................................................................................................................................................................................................................................................................................................................................................... .......................... ..................................................................................................................................................................................................................................................................................................................................................................................... ...........................

..........................

..............................................................................................................................

..........................

..............................................................................................................................

..........................

..........................

Figure 1: General structure of the program.

we raise some new issues in balancing a 2-sided line. Finally, we include the

data for a real assembly line, which to our knowledge is the first to appear in

the open literature in almost 20 years.

2 General structure of the program

The program consists of two main functional units, the Product/Task Editor and

the Line/Station Editor. The Product Editor manages the data of a product,