# Sizing coil loops.

Sizing coil loops

It is a common misunderstanding that increasing the distance between coil feed and straightener (a longer loop in the horizontal dimension) will provide a larger amount of slack material in the loop. This is not only untrue, but in fact the result may be less slack in the loop.

Slack is gained only by increasing the loop in the vertical direction. As Figure 2 shows, the amount of slack material available in the loop is the difference between, one, the horizontal distance between feed and straightener, and two, the total length of material in the loop.

You can see from this example that placing the straightener farther from the feed results in substantially less slack material in the loop (3 ft versus 1 ft). The amount of available slack can be further increased by raising the feed-line height, or by installing a loop pit.

Pulls longer loop

In addition to providing less material storage, placing the straightener farther from the feed will also cause it to work harder. The feed will be forced to pull a longer loop (25 ft versus 12 ft) on each cycle. What's more, the reduced amount of slack will impose additional wear on straightener and reel. This is caused by the increased frequency of starting and stopping.

Mild steel can be forced into a radius 360 times its thickness without reintroducing a set into the material. Knowing this, you can calculate the best loop size for any gauge of mild-steel coil stock.

Tougher materials normally can be put into a tighter radius, while softer materials may require a larger radius. In any case, the 360-times-thickness measure is a good starting point.

Guide to length

A useful guide to calculating loop length is given in Figure 3 and its table. Let's look at an example:

The material we will be running is 0.062"-thick mild steel. For this material, our chart recommends a loop length if 96". Remember, this is the horizontal distance from where the material radius begins upon leaving the straightener, and where it ends upon entering the feed. The 96" value is not the total length of material in the loop.

Let's assume our pass line height is 42". At this height, the chart shows we will have 38" of slack material in our loop. (In most cases, the pass line height at the straightener will be different from that at the feed. An average of the two gives a usable figure.)

Now, let's assume the feed length of the job we're running will be 18". A good rule of thumb is that slack accumulation should be at least three times the feed length. In this case, we don't have this much slack (38" actual versus 54" required).

The solution is a loop pit, as shown in Figures 1 and 3. If the original pass line height is beyond 720 times the material thickness (360 x 2 x material thickness), every inch of loop depth will add 2" to slack accumulation.

In our example, an additional 8" of depth would add 16" to the accumulation. This would provide us with the three feed-lengths of slack we require.

PHOTO : 1. In this press installation, a loop pit--at right in the photo--is used to gain the required amount of slack in the loop.

PHOTO : 2. Placing the straightener farther from the feed actually results in less slack material in the loop.

PHOTO : 3. Guide for calculating correct loop length.