Capturing Captive Fasteners

Introduction

A captive fastener is a very simple type of fastener with a very important feature that manifests itself in many different designs and configurations across screws and nuts. The fastener and components are engineered to ensure that the fastener stays captured within the joint and the fastener components remain in place, even when the fastener is unfastened. You probably encounter dozens of different captive fasteners without a book-level knowledge of the definition of “captured fastener”. In this way, you may not be aware of the nuance behind the engineered components that are designed to capture the fastener and prevent the fastened joint from complete liberation, even when components are unthreaded. 

The simplest example of a captive fastener is as partially threaded metallic screws that are designed to be drilled into a pre-drilled pilot hole. What makes the screw captive is the screw is designed with an unthreaded portion of the shank that is a smaller diameter than the threaded portion of the screw. The shank is unthreaded and typically has a locking washer or other feature mating with it has a larger diameter than the top layer hole which prevents the screw from being liberated from the top joint. This means that the fasteners can be unthreaded from the lower joint material while the top layer of the joint remains captured to the screw. In this way, the screw can be unfastened from one layer of the joint without liberating the fastener from the joint. The unthreaded, reduced-diameter shank captures the screw remarkably well.

Types of Captive Fasteners

Captive Screws

As discussed previously, a captured screw can be as simple as a screw or a bolt with an unthreaded portion of the shank and a locking washer to prevent the removal of the screw. However, the captivating mechanism of the joint can also be achieved by welding the bolt, press fitting the bolt through the top joint hole, or broaching. No matter the method employed, as long as the screw or bolt is contained within one of the mating joint layers, the fastener is considered to be captured.

Weld Nuts

Weld nuts are typically made of low-carbon steel or stainless steel. They are designed as their name suggests to be welded to another item of metal material which reduces spaces and gaps. This provides a strong bond between the weld nut and the object it is being welded to and is ideal for heavy-duty applications.

Captive Fasteners in Real Applications

The captive nature of the fastener means that the use of these components is restricted to specific applications and joints that have been specially designed to the capture features of the nut or fastener. These products typically have joints where one part of the fastener must remain in place for safety considerations. Many ordinary consumer products utilize captured fasteners or nuts in at least one safety-critical connection. For example, the doors of refrigerators are fastened to the body of the refrigerator with captive bolts while automobile trunks and most residential doors are fastened using captured metal and/or wood screws.

If you can understand the key features and limitations of captive fasteners, you will be much better able to incorporate them into your machine designs. MISUMI has thousands of captive screws, bolts, and nuts available to fit the most unique fastened joint applications. Explore the selection here.

About the Author

Carlicia Layosa

Carlicia is a Product Marketing Engineer at MISUMI. She holds a bachelor's degree in Mechanical Engineering and a master's degree in Energy Engineering from the University of Illinois at Chicago. She is a Certified SOLIDWORKS Associate, Marketo Certified Expert, and is passionate about education and training.

Leave a Reply

Your email address will not be published.

*

You may also like these