Constant Force Spring Explanation - Part 1

Posted by Catherine Tyger on Thu, Jul 19, 2012 @ 09:37 AM

In Springs, constant force

What differentiates the Constant Force Spring from conventional extension and compression (round wire) springs? As shown by the graph below, Constant Force Springs are not bound by Hooke’s Law and provide a constant force over the length of travel. Hooke’s Law basically states that the extension of a spring is directly proportional to the load applied to it. Therefore, the further an extension spring is extended, the greater the force.

The load of a Constant Force Spring does not follow Hooke’s Law as shown in the graph. We can categorize the springs Vulcan produces as Constant Force, Gradient Positive and Gradient Negative. This installment of the blog will focus on the standard Constant Force Spring.

Vulcan Chart resized 600

A Constant Force Spring is manufactured from flat strip steel. The amount of stress imparted into the spring determines the diameter, force and cycle life of the spring. The action of a Constant Force Spring is similar to extending a ribbon; the spring material extends and retracts. The only portion of the spring that is working at any given time is the part tangent to the coil. The force then is constant as long as the diameter is constant. On extremely long designs the?re is a mechanical advantage to the changing diameters and the force increases slightly as the spring is extended. However, this amount of increase is minimal compared to extension springs and can be negated in many cases if necessary.

The life cycle of a Constant Force Spring is then set by the diameter of a certain thickness of steel. See our previous blog concerning life cycles for more information on the subject.

A Constant Force Spring can be utilized to apply a linear extension force. This type of spring is used to push or pull an object or series of objects, or to counterbalance an object throughout its travel length.

When discussing a linear extension we will specify the force in lbs. (Kg, N, etc.) of pull force. Vulcan’s trademark name for this type of spring is Conforce. Please view the following video to get an idea of a typical set up for this type of spring:

An alternate version of this is called a Constant Torque Spring. This spring is wound onto an output spool providing rotational torque. This type of spring is utilized to wind up cable, provide torque in toys and other applications that require rotational torque.

When discussing the torque spring we will work in inch-lbs. (NM etc.) of torque. Vulcan’s trademark name for this type of spring is Contorque. Please view the following video to get an idea of the configuration of this type of spring:

By designing a Constant Force Spring into a product, the engineer can save space and provide for the correct amount of force required for the design. Please contact a Vulcan Product Manager at 215-721-1721 or at to discuss possible design alternatives.