Duplex Spring

A duplex spring puts two torsion springs of different inside diameters on the same torsion shaft. One spring fits inside the other. The inner spring has a smaller inside diameter that clears the shaft. The outer spring is larger and clears the inner spring. Both springs wind in the same direction. They work in parallel and add their torque outputs together.

The result is roughly twice the lifting force of a single spring of the same length. No extra shaft space is needed. This is why duplex springs are used on heavy doors or when the shaft is already as long as the header allows.

When duplex springs are used:

• Heavy doors: A solid wood or thick insulated steel door can weigh 300-500 pounds or more. A single spring with that torque rating would be very long or need a very thick wire. Splitting the load between two nested springs is the practical solution.
• Limited shaft space: Sometimes there is no room to add a second spring beside the first. A duplex setup fits both springs in the space of one.
• Replacement matching: An existing duplex system needs a duplex replacement to keep the spring geometry correct.

For example, a 400-pound solid wood carriage-house door on a 14-foot shaft might use a 2-inch outside spring and a 1-3/4-inch inside spring, each 30 inches long. Together they produce the combined IPPT (inch-pounds per turn) needed to lift that door.

Unlike a side-by-side two-spring system, a duplex system uses one set of winding cones and stationary cones for both springs. The cones grip the outer spring. The inner spring transfers force through shared shaft contact and cone clamping. Winding a duplex spring uses the same bar-and-count method as a single spring.