Differentiation between bolt and screw

Eye Bolts
An Eye Bolt is a small screw with a loop on one end and threads on the other end. Eye bolts are commonly used to attach lines to objects, for instance attaching a cable to the back of a painting to allow the painting to hang from a nail on a wall.

Drop forged machinery eye bolts. Machinery eyes are fully threaded. Machinery eye bolts without a shoulder should not be used for angular loads. Wire eye lags (also referred to as screw thread eye bolts, eye screws, or turned/bent eye lags) have a wood screw thread for use in wood or lag anchors. Like wire eye bolts, wire eye lags are intended for light duty applications and should not be used for angular loads.

The working load limits on this page apply only to machinery eye bolts and machinery eye bolts with shoulders. The working load limits for regular forged eye bolts can be found on our Forged Eye Bolt Working Load Limits page. Working load limits for eye bolts are based on a straight vertical lift in a gradually increasing manner.

Angular lifts will significantly lower working load limits and should be avoided whenever possible. If an angular lift is required, a properly seated shoulder pattern machinery eye bolt must be used. Loads should always be applied to eye bolts in the plane of the eye, not at an angle to this plane. Angular lifts must never be more than a 45 degree pull.

A screw
A screw is a shaft with a helical groove or thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. It can also be defined as an inclined plane wrapped around a shaft.

Differentiation between bolt and screw
A bolt passes through a hole of larger diameter than its thread, and is held in place by a nut or similar device; it is not designed to be turned. What is often referred to as a bolt is in fact a cap screw, which is designed to be turned (or screwed). Cap screws may, or may not be used with nuts. The distinction is subtle, but significant in the design of the fastener. If threaded all the way to the back of the head a cap screw becomes a machine screw.

Materials and strength
Screws and bolts are made in a wide range of materials, with steel being perhaps the most common, in many varieties. Where great resistance to weather or corrosion is required, stainless steel, titanium, brass or bronze may be used, or a coating such as brass, zinc or chromium applied. Electrolytic action from dissimilar metals can be prevented with aluminium screws for double-glazing tracks, for example. Some types of plastic, such as nylon or Teflon, can be threaded and used for fastening requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation. Even porcelain and glass can have molded screw threads that are used successfully in applications such as electrical line insulators and canning jars.

The same type of screw or bolt can be made in many different grades of material. For critical high-tensile-strength applications, low-grade bolts may fail, resulting in damage or injury. On SAE-standard bolts, a distinctive pattern of marking is impressed on the heads to allow inspection and validation of the strength of the bolt. However, low-cost counterfeit fasteners may be found with actual strength far less than indicated by the markings. Such inferior fasteners are a danger to life and property when used in aircraft, automobiles, heavy trucks, and similar critical applications.

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