Q. What is a reasonable figure for maintenance downtime?
A. I know you didn't intend it to be, but that's a loaded question. Depending upon your source, you could be told anything from 20 percent to less than 10 percent downtime should be expected.
That's kind of like telling you you'll get 32 miles to the gallon without knowing what type of vehicle you have. Maintenance downtime is defined as the amount of downtime that results from parts being worn and needing to be repaired, adjusted or replaced. The compatibility of the interfaces between the machine, the operator and the materials, along with the design of the machine, will determine what the downtime will be.
When you purchase a piece of equipment, you should require a "performance guarantee" that stipulates how efficiently you can expect the machine to run. The original equipment manufacturer (OEM) will need to agree to the terms before the purchase. The efficiency of the machine depends greatly upon the material run through it. If you are running a high-speed can line with material tolerances in the thousandth of an inch, you would expect the machine to run more efficiently than if you were processing naturally occurring (and variable) products such as vegetables. That being said, manufacturers design their equipment to run at about 97.5 to 99 percent efficient. A multi-day test is usually run to verify the machine is capable of running at this efficiency, paying close attention to whether the problems that occurred were material, operator or machine related.
If you have four significant pieces of equipment on your processing line and you know the efficiency of each, multiply each efficiency number together (.99 X .99 X .99 X .99 = .96 percent efficiency) and subtract the result from 1.0 (in this case, resulting in 4.0 percent downtime). You can see that if you have a complicated line with 15 pieces of equipment, your efficiency drops to 86 percent with downtime of 14 percent. Downtime numbers are as unique as the process you are trying to run. The most important facet of downtime is getting everyone affected in agreement of the reasonable expectation.