Is Cut-To-Length Line speed the same as yield (output)?
Misconception: Line speed is a specification often used to help determine the production capability of a line. Line speed refers to the speed of the material during normal line operation. However, line speed is often misinterpreted as system output or yield which actually refers to the amount of material processed in a given minute.
Line speed and yield would be identical if you were evaluating a continuously operating processing line such as a Slitter. However, it is frequently misapplied to intermittently operating or variable speed Cut-To-Length Lines.
Clarification: In the case of a start/stop Cut-To-Length Line, line speed refers to the speed of the material while it is moving. However, because this type of system operates intermittently, the material is stopped momentarily while it is being sheared. As a result, yield differs from actual line speed.
A dual speed hump table Cut-To-Length Line operates with two speed modes, in one mode line speed is relatively high. However, while the strip is being sheared, the line is in a creep mode and the material is actually moving considerably slower.
Consequently the yield is an average between the two speeds and the duration of time spent in each mode.
The Feed System in a looping type line operates intermittently. An accumulation loop between the Feed and Leveler allows the material to be leveled continuously while the Feed stops the strip momentarily for shearing.
While feed speed may easily exceed 600 FPM, because the material is stopped for shearing, yield again varies from actual line speed.
To determine yield, you must again average the feed speed, dwell time, deceleration and acceleration rates.
Summary: Although line speed and yield can be identical, there is a distinct difference between the two.
The most meaningful statistic is true yield. The easiest way to determine yield on any Cut-To-Length Line is to simply count the number of parts produced in a given minute and multiply that number by the part length. This will be the yield for that particular part.
In addition to part length, it is also important to consider the circumstances under which this speed was achieved, i.e. what type of material was being processed, how wide, what gage, and the part tolerance being attained.
These variables will ultimately determine the true yield a system is capable of producing.
Conclusion: Although most misconceptions are based on assumptions and misinterpreted information, they are often perceived as being fact. However, by reviewing the surrounding information more closely, most can be explained.