Too often we find that the springs we manufacture are mistakenly used as stopping mechanisms in a design. This design flaw can occur in all types of springs including Constant Force Springs, Constant Torque Springs and Power Springs.
As an example, consider a simple cable extension product where the spring retracts the cable back into a housing. If the spring is fully wound prior to the cable being fully unwound, the spring will act as a stopping mechanism. This action will damage the spring and the function of the assembly and should be avoided at all cost. Here are some helpful hints when considering stopping mechanisms.
Constant Force Spring Application: Initial designs often show the inner end of a Constant Force Spring attached to its' spool. This is not necessary. If the spool is larger than the natural spring diameter, the spring will automatically fit tightly on the spool. In the case when a screw is attached to the spool, additional concerns arise. If the spring is pulled tight on the screw, a hump will result that will not only make the retraction difficult but will cause a fracture point and reduce life cycles. To avoid this, we recommend that the spool maintain 1-1/2 wraps of spring material at the furthest extension. If these additional wraps are not practical in the design, another method of stopping the rotation is required.
Constant Torque Spring Applications: This type of motor spring is commonly used to retract a cable or to keep a constant torque rotational force on a spool. It is extremely important not to use the end of the spring as a stop. The cable should be fully extended with at least 1-1/2 wraps on of spring material remaining on the storage spool. Likewise, the spring should be pre-wound so that when the cable is fully retracted there is still one or more wraps on the output spool. If this is not practical in the design, another method of stopping the rotation is required. An example of how to accomplish this is shown below.
Power Spring Applications: Similar to the constant torque spring, a power spring, or a pre-stressed power spring, will a retract cable or provide rotational torque. There are commonly two points of attachment for a power spring: the case and the arbor. Again, it is necessary to avoid operating the spring near either end. The spring should be pre-wound at least one full turn and not allowed to unwind past that point. If not pre-wound the recommended amount, the spring will perform poorly and ultimately fail. Winding the spring completely will cause constant pulling on the outer end. In kind, the spring will not function properly or even fail when allowing it to unwind. This may be avoided by stopping rotation a turn or two before the spring is fully wound.
To optimize your design, it is recommended you contact a Vulcan Spring engineer. Please contact a Vulcan Product Manager at 215-721-1721 to discuss possible design alternatives. Design criteria and options can be found on-line at .
Written by: Bob Gustafson