Constant Force Spring Explanation - Part 2

Posted by Catherine Tyger on Thu, Jul 19, 2012 @ 09:34 AM

In Springs, constant force

Our last blog discussed the two main types of Constant Force Spring; linear force and torque. As you can see by the following graph, Vulcan is able to change the force of the spring over the travel length to match customer requirements. These springs are referred to as Gradient Positive and Gradient Negative springs.

Chart1 resized 600

Gradient Positive Springs

There are applications that demand a stronger force as the spring is extended. A very popular example of this is for pusher trays in stores. When a product is removed from the shelf, the next one comes forward, keeping the product up front so the customer can see it and buy it. In this situation, there are many products lined up in a row and the spring pushes them forward. As a product is added, the weight required to push increases. Vulcan will customize the spring’s force gradient to adequately increase as the spring is extended. When there are a few products remaining on the shelf, the force of the spring is at its minimum.

A Gradient Positive Spring is also known as a Variable Force “V” Spring and is relatively easy to differentiate from a Constant Force Spring. The coils are open as shown in the following photo.

variable force spring


Gradient Negative

Certain applications may require the opposite action as discussed above. Sometimes the requirement is for the spring to weaken as it is extended. For these applications Vulcan can produce a Gradient Negative Spring. One example of this is used with our Constant Torque Configuration (see part 1 of this blog). We use the Constant Torque Spring in a Pullbox used to hang signs. When the store wants to change the sign they pull the sign down so an employee can access it. Using a standard Constant Force Spring,

if the sign were to stay down when extended the spring would be too weak to counterbalance the sign back up at the ceiling. This would allow the sign to sag or drift downward. Since this is usually not acceptable, Vulcan has engineered a Gradient Negative Spring. This spring is designed to provide the maximum force when the cable is retracted and holding the sign at the ceiling. As the employee pulls the sign down the force of the spring decreases allowing the sign to float and to stay wherever it is placed. For any counterbalancing application a Gradient Negative Spring may be required. Gradient Negative and Constant Force Springs have the same physical appearance.

During the course of this 2 part blog we have discussed Constant Force and Constant Torque as well as Gradient Positive and Gradient Negative Spring designs. The Engineers and Project Managers at Vulcan are prepared to discuss your specific application to see which spring will best fit your design. It is always recommended to contact us as early in your project as possible to allow for alternate designs to be considered.

Please contact a Vulcan Product Manager at 215-721-1721 or at to discuss possible design alternatives.