Total Productive Maintenance

Part 2: Eliminating Six Big Losses to Improve Equipment Effectiveness

by Roger A.P. Fielding, BENCHMARKS

The Cost of Downtime

The interest on money borrowed to finance the purchase of extrusion presses and their ancillary equipment is paid 24 hours a day, 365 days each year. The effectiveness of maintenance activities is measured by the availability of the presses. The benchmark is to have the presses available for operating all the time. Since the output from the press defines the throughput of the extrusion plant, the cost per operating hour of the presses can be expressed as the total plant operating expense divided by the time the press is producing extrusions. This is the true cost of unplanned downtime (Goldratt, 1986).

The Challenge of Reliability

Modern aluminum extrusion systems are made up of many components operating in unison. And, whereas earlier presses operated with separate, discrete sub-systems of furnaces, runout, handling, stretcher and saws — thereby allowing for some operation in the case of breakdown or adjustment — the overall reliability of the modern system is no better than the reliability of the weakest link. The problem is well documented (Buffa, 1972; Benson, 1951).

In recent years, extrusion press systems have become more reliable through the use of programmable logic controllers and sophisticated software diagnostic programs. Improvements to the engineering of components, such as belts, couplings, valves and microswitches, have increased the mean time between failures (MTBF) of these components, and the development of non-contact position sensing devices has removed some components from the system. Understanding the mechanisms of wear has resulted in increasingly stringent requirements for the control of cleanliness and temperature of hydraulic fluids. The combination of these developments has improved inherent reliability and has permitted the introduction and development of automated extrusion press systems.

The downtime of an extrusion press system of log or billet racks, furnace, hot shear, press and runout, pullers, cooling table, stretcher, batching table and feed conveyors, saw table (with its feed conveyors), saw and stacker will be dependent on the reliability of each of the working component parts. And, whereas a "reliability" of 99% might have been acceptable when presses extruded onto a fixed runout table with little or no additional handling, it is totally unacceptable today. The reliability of the automated press system is the product of the reliability of each of the component parts. If each of the sub-systems listed above were only 99% reliable, the system would be "down" 12% of the time!

In planning to improve reliability, the design and maintenance engineers must consider the inherent reliability of each part, and the probable mean time between failure of each part; the number of working parts, and the time taken to fix a problem. A problem that only takes a few seconds to correct is not much of a problem if it occurs infrequently. But, if it recurs with every billet pushed—as with a discard which fails to separate from the shear or spacers which do not feed consistently to the stacker—it will require the undivided attention of an operator at some time during each working cycle. Again, the reliability of the system is a function of the reliability of the sub-systems.

Press Maintenance

The press system must be maintained to meet its performance capability. The billet furnace must deliver billet at the specified temperatures. The press must be correctly aligned. Pressures, speeds and clearances must be maintained to prevent the creation of mushrooms and flashes, and to ensure that the butt shear cuts cleanly and the fixed dummy block operates properly. The equipment must be available for operation at all times.

Making equipment available for use requires an understanding of why it breaks down. Understanding why machinery doesn’t work comes from taking appropriate measurements and an intimate knowledge of machine design.

Press Performance

The productive capability of the press and its ancillary equipment was defined when it was installed. Subsequent upgrades may have increased its capability, but the initial specification defines the minimal acceptable performance. The press must be capable of producing the required quantity and quality of extrusions in minimum time with minimum losses.

The press system mustn’t be the cause of slow working or lost time. The billet heater must supply the required quantity of billet, at the required time and at the correct temperatures. The dead cycle must be that specified by the machine builder. The pullers and cut-off saws or shears must operate within the dead cycle, and the transfer arms must clear the run-out for the next extrusion. The stretcher must be equipped to keep the cooling tables clear. And, the saw and stackers must keep up with the press.

The press system mustn’t be the cause of scrap. Handling mustn’t damage the extrusions. Most modern handling systems use pullers operating over protected non-marking runout tables, using transfer arms with rectilinear motions and felt belting to carry extrusions through the cooling section, stretcher and batching section to the saw table. However, there are still many handling systems comprising sloping lift-offs, walking beams with a wide variety of coverings, and with no mechanical means of transferring the extrusions into and out of the stretcher. These are often sources of product damage.

Safety

The press system mustn’t be the source of accidents. Presses, stretchers, saws and stacking systems must be protected to ensure safe operation. They must conform to standards of noise suppression. Hydraulic pumps, tanks, pipe couplings and hoses must be guarded against the dangers of fire. Potential sources of sparks and open flame must be guarded.

The other four steps to the successful introduction of Total Productive Maintenance will be discussed in subsequent articles.

Total Productive Maintenance - Part 3