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Insight - How Expandable Screw Works

Table of Contents:

Written By: 

Bijal Parikh


In my latest conquest of DIY i.e. Do It Yourself mission, I wanted to learn about a SCREW and not just our simple screw but most recent addition in the family of screws i.e. Expandable Screws. I found out recently that the air conditioner on my second floor window is placed using these screws. So, when I started researching more I learnt that these screws were specifically designed for installing heavy objects on concrete walls or for masonry purpose. Some common examples are installing dish antenna on the roof, holding the shade to cover your car or open area, tightening a rope to climb the wall etc .A normal screws gains their capacity to hold loads  by creating an interlock with the part in which they are inserted. For example a screw is inserted in a wall or in a wooden plank etc. The threads on the screw imparts grip and strengthen the hold. However, this interlocking force weakens in concrete walls due to cracking, holes or surface irregularities. This loosens the grip of the screw and it will come out easily .But the specific design used in expandable screw solves the problem. However, my ever so inquisitive brain was still spinning with the questions like what makes these screws special? How do they work? What is so special about their design?

So, the curiosity to know more about them forced me to buy one from the market.  The image below shows a typical expandable screw.

Expandable Screw

From the image we can say that expandable screw has a bolt at the center with a conical shape nut on one end and surrounded by three metal plates which act as sleeves. A twisted wire wound which acts as a spring at the center of the screw holds them together. 

After observing the outer structure, I tightened the screw to see what exactly happens? As the screw was tightened the conical shaped nut placed at the end of the bolt climbs on the threads and moves in between the sleeves and this force the sleeves to move radially outward. 

At this point I got the general idea about the working of the screw but my aim was to build the screw conceptually with designer’s perspective. Understanding the principle behind the screw will help me visualizing the design clearly.

Let’s think about a normal screw. It looks like a small rod with threads running along its sides. It is inserted in the drilled hole in between two objects that will be held together and a nut is turned on the screw’s thread until it is tightened and cannot be turned further. Expandable screw is built on the same concept with change in design that makes the expandable screws compresses against the wall of the drilled hole to transfer the loads which provides a tightened grip on the wall. Let’s put the concept in simple terms.We drill a specific size hole, insert the screw and radially increase the diameter of the screw larger than the hole so it gets a strong grip inside the hole.

With this idea in mind I opened the entire screw assembly so that each part can be assembled again by understanding their need in working of expandable screw.