Captivated Screws | Spring-Loaded Screws

Overview

A spring-loaded, captivated screw involves all the same features associated with a simple screw with one additional component. The spring sits between the shoulder of the retainer and the head of the screw, keeping it extended when not engaged with a mating thread. Depending on the application, this spring can accommodate a smaller retainer and surround it or sit inside a larger retainer. The tradeoffs associated with these options are further discussed in the context of knob cap screws.

Application

The mechanical function of the spring is to fully disengage the screw from its mating thread when loosened, with some spring-loaded screws receding past the bottom surface of its panel completely. While this has some benefits for cosmetic requirements such as minimizing the risk of scratching with the screw tip, the primary benefit is the increased ease of mounting and dismounting.

Cross-section schematic of enclosed and exposed spring varieties of panel fasteners

For example, elevators that stop working need to be fixed quickly. Using a spring-loaded screw with a Phillips or flat head drive, service technicians can quickly remove and replace the cover plate to access the elevator controls and make the repair. They also have the added benefit of the recessed screw head, whose flush installation when fully engaged can discourage disengagement by hand.

Spring-loaded panel fasteners with a recessed, self-clinching retainer inhibit hand-tampering in an access panel

A spring-loaded screw with a retainer allows for quick engagement & disengagement of an elevator access panel.

Spinning clinch bolts retain themselves in a panel with a small clinching feature that still allows for free rotation

Simple screws may limit cosmetic options, but offer a cost-effective solution for an access panel with no loose hardware.

Alternative Solutions

Loose screws, though less expensive, lead to the possibility of lost screws. More complex hardware like a latched cover with a hinge might be even easier to access but creates more dimensional requirements for the assembly as a whole. This potentially complicates the production process on the side of the elevator manufacturer.

However, while it may be easiest with the included spring to access the elevator controls, the increased cost associated with the assembly of the screw, spring, and retainer can be circumvented with a simple screw and retainer with no spring, perhaps even without a retainer.

Common Attachment Technologies

Spring-loaded captivated screws are among the most common captivated fasteners with the most options available for specific features and customizations. So it’s no surprise they’re leveraged in many applications using all of the attachment technologies previously discussed.