Torsion Springs - By Jim Hauter

By Jim Hauter*

Picture this: You’re on a service call 35 miles from your office, servicing a broken torsion spring. But you don’t have an exact match on your truck. You need to get the door working and do not want to make a return service call to this out-of-the-way job.

The broken spring is a .243 x 1.75 x 31. In your truck, you have 2-inch ID springs in .234 and .262 wire. What will work?

The first thing to remember when doing any spring conversion is to use the same size or larger size wire on the converted spring. Converting a spring with a smaller wire may result in an over-wound spring that will not operate the door properly.

The first step is to determine the IPPT (inch pounds per turn) of the broken spring. This can be accomplished through the use of a Torsion Spring Rate Book. As show in Figure 1, the IPPT of the broken spring is 42.6. The next step is to match the IPPT of the stock spring (.262 x 2-inch ID). Remember, if the spring is the same wire size as the original spring, the cycle life will remain the same. If you choose a spring with a larger wire size, the cycle life will increase.

As shown in Figure 2, by matching the IPPT of the original spring, a .262 x 2-inch x 39 1/4-inch will work as a replacement for the broken spring. Also note, the replacement spring is rated for more turns than the original, resulting in increased cycle life. When increasing in wire size, it’s important to remember your replacement spring may be longer than the original.

Step by Step

If you don’t have a rate book handy, you can still do a conversion - as long as the wire size of the new spring remains unchanged. Here are some easy steps to follow:

Step 1: Measure the inside diameter, wire size and length of the original spring. Remember, your replacement spring must be the same wire size as the original spring.

Step 2: Divide the mean diameter of the original spring by the mean diameter of the replacement spring. (Mean diameter = inside diameter plus the wire size.)

Step 3: Multiply the result by the original spring’s length to determine the length of your replacement spring. For example, if you need to replace a broken .234 x 1 3/4” x 29 1/2” and you have a .234 wire in a 2-inch inside diameter, the equation would look like this:

1.75 + .2343 = 1.9843 (mean diameter of old spring)
2.00 + .2343 = 2.2343 (mean diameter of new spring)
1.9843 / 2.2343 = .888 .888 x 29 1/2 = approximately 26 1/4

* Therefore, you need to cut a .2343 x 2-inch x 26 1/4 inch .

Formula

Mean Diameter (old) / Mean Diameter (new) x Length of Old = Replacement Spring Length

This article reprinted from Professional Door Dealer