How Cost of Work Completed Affects Machine Utilization

By Mike Vorster, Contributing Editor | September 28, 2010
Mike Vorster, Equipment Executive

Mike Vorster

David H. Burrows Professor of Construction Engineering and Management at Virginia Tech. See for full archives of “Equipment Executive.”
How Machines Interact with Work

Production is an interplay between the machine that is used to perform the work and the work itself. This illustration shows how machine and work fit together. By understanding these dynamics, managers canidentify the factors that influence production.

We have written many Equipment Executive articles on construction equipment fleet-management strategies, including dozens that focus on how best to lower the owning and operating cost per hour. Coupled with cost management, however, is the realization that construction equipment-using entities survive, grow and flourish by using that equipment efficiently.

Construction machines exist to do work, so fleet managers must keep an eye on cost per unit of work in addition to owning and operating cost. The cost per unit of work can be calculated using the following principal production equation:

Principal production equation

Minimizing the numerator — cost per hour — will reduce the cost per unit of work completed. But it is just as effective, and just as important, to maximize the denominator — production per hour.

Production is an interplay between the machine that is used to perform the work and the work itself. Our illustration shows how machine and work fit together, and we can identify and understand the many factors that influence production.

The characteristics of the construction machine are defined by the engine that provides the power, the transmission that converts this power into available force, the traction system that moves the machine and generates the usable forces, and the implement that actually does the work.

The characteristics of the work can be divided into three broad categories. First, the grade, rolling and working resistance that the machine experiences as it loosens, lifts or moves its load; second, the distance traveled in a cycle; and third, the properties of the material that define the machine's ability to break, move, lift or load the material in question.

The available force, the usable force, and the skill of the operator determine how much force can be applied to overcome the resistance experienced by the machine and hence the speed with which the machine is able to perform its work. Speed coupled with distance determines the cycle time and the number of cycles the machine can complete in each working hour.

The size and shape of the truck body, excavator bucket, or dozer blade (the implements doing the work) join with the properties of the material to determine the volume that can be moved per cycle. Cycles per hour and volume per cycle determine the following second main production equation:

Production equation

Our knowledge of equipment costs and a relentless focus on fuel, repair cost, purchase price, and utilization put us in a position to lower the cost per hour of the equipment used. Our illustration and equations help us to understand the factors that determine production and affect the unit cost of producing work.

From this exercise, we can hone in on six important concepts:

1. As fleet manager, you should provide equipment that performs to specification. Make sure that each machine delivers the power, force and capacity intended and that it has the ability to perform the required work.

2. Operators must have the skills and capabilities necessary to apply to the machine and use its capabilities at maximum safe speeds.

3. The job must be planned to ensure that operations, workflow and the jobsite function in such a way where the speed and capability of the equipment can be used to minimize cycle times.

4. The organization must manage the work, plan operations, and provide the infrastructure needed to make every minute of every hour count. Maximize cycles per hour as well as cycles per day.

5. Ensure that there is a match betweenthe bodies, buckets and blades that push, lift, load and transport the material. Ensure that every load is at capacity and that volumeper cycle is maximized.

6. Machines usually work in groups or spreads, so managers must balance production spreads to maximize the capacity of the constraining resource, reduce waiting time, and achieve the best unit cost for the spread overall.