How Do Hollow Wall Anchors Work?

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How Do Hollow Wall Anchors Work?
Some wall anchors expand when you screw into them. (Jupiterimages/Photos.com/Getty Images)

Drywall doesn't hold screws, so when you need to attach something to it, you need a wall anchor. Plastic and metal anchors are available, and they work in one of three ways. They either expand when you drive a screw to lock in place, they have threads like a screw or they have wings that lock against the back of the wall. Different types are rated for different loads.

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Expansion Anchors

The most lightweight plastic anchors expand against the drywall when you drive a screw. They are usually conical in shape, and to use one, you need to predrill a hole in the drywall into which it can fit tightly. When you drive a screw into it, it expands outward and wedges against the sides of the hole. Smooth conical anchors are prone to slipping out of the hole when an outward force is applied, so expansion anchors often have serrations to increase stability. These types of anchors are appropriate for hanging pictures and other light applications.

Screw Anchors

The design of anchors with threads that make them resemble screws gives them more holding power than expansion anchors. You usually don't have to predrill a hole to install one of these anchors. Instead, you simply drive it clockwise into the drywall as if you were driving a screw. The wide threads lock into the drywall material and hold the anchor and the weight it supports in place. Screw anchors have the extra advantage of being easily removable. You simply back them by screwing counterclockwise in the same way you would to remove a screw.

Winged Anchors and Toggle Bolts

Plastic winged anchors have still more holding power than screwable ones because they don't rely on the holding power of the gypsum inside wallboard for their stability. Instead, they have wings that expand outwards and lock against the back of the drywall when you insert them into predrilled holes. To get the wings to expand, you usually have to insert a pointed probe. Metal versions of winged anchors, called toggle bolts, have a threaded sleeve with wings that similarly expand against the back of the drywall when you screw in the bolt that fits in the sleeve.

Molly Bolts

Molly bolts are similar to toggle bots and can support almost as much weight. They have a metal sleeve that you tap into a predrilled hole until the opening is flush with the drywall. The sleeve has cuts along its length, and, when you screw in the bolt, it draws the far end of the sleeve towards the back of the drywall, forcing the metal along the cuts outward to anchor the sleeve firmly to the drywall. Once a molly bolt sleeve has been anchored, it is permanent, and removing it can be difficult.

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