In our last column we introduced the concept of a preventative maintenance program, and in this column we will discuss the basics of how to get started.
The key to a total preventative maintenance program (TPM) is that the entire work force and all levels of management must be committed to the program. Having a lack of commitment by either plant personnel or management is one of the main reasons that TPM programs fail.
One suggestion for getting a program started is to focus on a key area or areas that will allow the initial stages to be implemented by personnel who believe in the program. Success in one area would hopefully be noticed by others, and the program would grow to eventually include the entire plant.
Remember that TPM programs are founded on five basic concepts: 1) improving overall equipment effectiveness, 2) basic equipment care, 3) root cause and countermeasures, 4) planned maintenance system, and 5) predictive maintenance.
We will start applying these concepts at a critical system component or maybe one of the main bottlenecks of production. A critical system component would be one in which failure will ultimately cause production to stop. Examples in a sawmill might include the hog, air supply, or waste systems. A bottleneck is the one piece of equipment that limits your overall production capacity. It should be your head rig but it might be an edger, gang, or some other component of production that is the limit. You might already have another piece of equipment in mind, one that already has significant downtime or well known maintenance issues.
Before we get too far into implementing TPM, we need to determine how well your equipment is currently operating and how much improvement there is over time. After all, why implement TPM unless there will ultimately be some financial reward?
Measuring Overall Equipment Effectiveness
Measuring equipment effectiveness is termed measuring the overall equipment effectiveness (OEE). Taking the time to measure OEE of your equipment can often lead to some unwelcome surprises. We’re all very familiar with tracking loss of production with downtime, but OEE also takes into account set-up time and adjustments, idling and minor stoppages (maybe waiting for material), reduced operating speeds, process defects and reduced yield. OEE categorizes production losses by availability, performance and quality. Calculating OEE is vital to the implementation of your TPM program (notice that it is step 1).
Let’s walk through how to calculate the OEE for an edger in a hardwood sawmill. We visited sawmill X and during a normal production day and measured productive time, breakdowns, set-ups/adjustments and minor stoppages for the edger. For an eight hour shift the edger is expected to be available for eight work hours (a roamer is used to operate during breaks and lunch).
During the eight hour work period, the edger was not producing for a total of 48 minutes due to lumber handling problems and feedworks issues. This total was calculated over the eight hour shift and was made up of events that rarely took up more than 2 minutes. The edger was also not producing a total of 140 minutes due to a lack of material to process through the edger (breakdowns or problems with production equipment upstream). Again, this total was made up of smaller, less noticeable events.
The calculation for the OEE for this edger, based on the data we collected, is presented in Table 1. The machine cycle time is determined by the total number of boards the machine is designed to handle — 20 boards per minute for this example. The formulas used for each calculation are presented in the table.
The edger at this mill had a calculated OEE of 46.6%, meaning that we effectively use just under half the edger’s potential!
Most people are quite surprised when they first measure equipment OEE. Low OEE is typical because without any equipment maintenance system in place, most equipment is used ineffectively.
World class OEE measures are typically greater than 85%. Reaching an OEE of 100% is difficult if not impossible in most situations. However, by measuring OEE and striving for a goal of 100%, it allows us to become world class when it comes to equipment capability, reliability, and performance.
OEE, as measured in this example, gives us a standard way to analyze a piece of equipment and discover how its effectiveness can be improved. By knowing which components of OEE are lowest (OEE = Availability X Performance X Quality), we can target an area for improvement using cost-benefit decision making. Let’s take a closer look at each aspect of OEE.
Availability (60.8%)
If the equipment is operating only 60.8% of the time it should be operating, there is room for improvement.
Based on this information, it is time to take a closer look at the root cause of these operating losses. What were the underlying reasons the equipment was unavailable for 48 minutes? In this example there were problems with the feed works moving the boards through the edger.
The biggest loss of production time was the machine sitting idle due to production problems further up stream. Seemingly minor stoppages due to waiting for the resaw might seem small at the time but are often the main reason equipment availability is low. A few seconds here and there can add up to a large chunk of downtime over the course of a day — in this example 140 minutes!
Performance (80%)
The equipment’s performance efficiency is 80% of its designed or ideal capacity. Further investigation is needed to determine why the machine is not used to its capacity. Has the feed works speed been changed, are boards not available at the machine’s capacity, or do other problems exist? Based on the calculated performance, it is time to sit down with maintenance and the operator to determine the performance issue. More root-cause analysis is required.
Quality (95.7%)
Quality doesn’t look too low for this machine center. However, for an edger, we would expect closer to 99% quality. After all, most edging quality issues should be related to operator decision errors rather than equipment errors. Looking into the problems of this machine, it was determined that many boards were over-edged because the laser lines were not being properly aligned with the saws. This is an issue that could be eliminated with more routine maintenance or even better with checks used by the operator.
Bottom line
This mill was considering updating to a new edger to get higher output per day. However, after going through the exercise to calculate its OEE, it was determined that the machine had plenty of capacity, and that the capacity problems were elsewhere in the system.
Interestingly, we were using this calculation to look closely at maintenance issues for the machine, yet we determined that the larger issue at hand is the throughput or capacity.
This is just another example of how TPM can benefit the bottom line. We suggest that you find a critical piece of equipment at your operation and calculate its OEE as a way to start making changes.
Next time we will take another step towards TPM and discuss how to improve the OEE of equipment and reduce downtime.
(Brian Bond is a Virginia Tech associate professor and extension specialist in the department of wood science and forest products; Earl Kline is a Virginia Tech professor in the department of wood science and forest products.)
