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Screws and bolts.

Several factors should be considered when selecting a screw for a particular job: finish, length, diameter, head style and slot style.

All threaded fasteners are externally threaded to fit into holes in assembled parts. The difference is in method of tightening. Screws are tightened by turning the head and letting the threads tighten into the material. Bolts require a nut that is turned to tighten the fastener.

A bolt or screw is made up of some or all of these elements: head, driving recess, shoulder or neck, unthreaded shank, threaded shank and a point.

HEADS

Screw and bolt heads can be divided into two general groups. The most familiar are those with a driving recess--slotted and Phillips being the most common. Others are designed to be driven or held by a tool gripping the outside of the head, such as the square and hex types.

In addition to the standard slotted head for conventional screwdrivers, other recessed heads are designed for use with special screwdrivers, bits or keys.

The profile of the head differs depending upon the application. In many cases, final appearance dictates choice. Flat countersunk screws, for instance, can be driven flush with or even below the surface of the material. With an oval head, only part of the head is countersunk.

The type of head has a bearing on the measurement of a bolt or screw. Generally, the length does not include the head. However, when the head extends into the material, it is included in the length. Hence, the length of a flat-head countersunk screw would include the head. The oval-head, where only part extends into the material, would be measured up to and including the part that countersinks.

Generally, threaded fasteners are measured from the largest diameter of the bearing surface to the extreme end of the fastener.

Diameters are measured on screws smaller than 1/4" in numbers from 0-10. Screws larger than 1/4" are measured in increments of 1/16" up to 1/2", then 1/8" increments up to 2" long, and finally 1/4" increments up to 3" long. Beyond that, special order sizes are required.

SHOULDERS OR NECKS

Some threaded fasteners have shoulders to perform a function such as preventing the turning of a bolt during tightening. These may be square, ribbed, fin neck, round or oval. They are often referred to by the neck as round-head, square-neck carriage bolt or oval-neck connector bolt.

SHANKS

Headed fasteners often have an unthreaded portion called the shank. When enlarged, this is referred to as the shoulder or neck.

Others have a full-diameter shank, equal to the major diameter of thread. This is characteristic of machine bolts and cap screws. Still others, such as machine screws, have undersized shanks equal to the pitch diameter of the thread.

THREAD FORMS

The Unified Screw Thread System for commercial bolts, nuts and screws sets the standards in the United States and Canada.

Classes of thread are distinguished from each other by the amount of tolerances and allowance specified. External threads or bolts are designated with the suffix "A"; internal or female nut threads with "B."

* Classes 1A and 1B: For work of rough commercial quality where a loose fit spin-on-assembly is desirable.

* Classes 2A and 2B: The recognized standard for normal production of most commercial bolts, nuts and screws.

* Classes 3A and 3B: Used where a closer fit between mating parts for high-quality work is required.

* Class 5: For a wrench fit. Used principally for studs and their mating tapped holes. A forced fit requiring the application of high torque for semi-permanent assembly.

* Coarse threads are used more than fine threads because they are easier to assemble. They are recommended for threading into materials of a lower tensile strength and for certain applications. They are considered stronger than fine threads in sizes 1" and larger.

POINT STYLES

A variety of point styles are used, especially with set screws. Among them are flat, oval, cup, dog, half-dog, machine, gimlet and nail. Each is designed for a special purpose.

SCREWS

* Sheet Metal Screws--Sheet metal screws fasten thin metal to thin metal. Threaded the entire length, they have flat, oval, round or binding heads, usually in lengths from 1/8"-2". Starting holes, either drilled or punched, should be slightly smaller than the screw diameter.

* Machine Screws--Machine screws come with four head styles: round, oval, flat and fillister. Round is most commonly used; flat head is used when the top must be flush with the surface.

Oval is used in a countersunk hole so that only a slight extension appears above the work surface. A fillistar head, which is used in counter-bored holes, is cylindrical with a semi-elliptical top.

* Set Screws---Set screws prevent bolts from loosening due to vibration. Four types are thumb screws, tightened by hand; headless set screws, tightened with a screwdriver; square-head set screws, tightened with a wrench; and socket set screws, tightened with a hex wrench.

* Tapping Screws Partial tapping screws are used where thread cutting is necessary. They can be used in deep holes. Self-tapping screws can be used in thicker materials.

* One-Way Screws--Can be tightened but not removed. They are used to install security devices.

* Dowel Screws--Dowel screws are threaded on both ends to provide end-to-end connections.

* Wood Screws--Common wood screws are made of unhardened steel, stainless steel, aluminum or brass. Threads run from the point along three-fourths of the length and heads are slotted. Steel screws come in a choice of several coatings: bright-finished, blued, or zinc-, cadmium- or chrome-plated.

* Deck/Drywall Screws--These are coated for use with decks and wood fences. They prevent rust when drywall compound is applied.

* Lag Screws--Lag screws (or bolts) are similar to wood screws but slightly stronger. They are useful when ordinary screws are too short or too lightweight o and when increased gripping power is needed. They are used for wrenching into wood surfaces or for inserting into lag shields in masonry.

* Cap Screws--Cap screws are used where strong holding power is essential, such as in machine tools, engines, pumps, etc. Cap screws have three types of heads: hex, flat and button.

SCREW HOOKS

Screw hooks are used for specific purposes. A cup hook is fitted with a stop cap for uniform extension when the hooks are used in rows. An ordinary screw hook is used to hang tools and utensils. It has a sharp point for self-starting and can be driven to the depth required. Eye and ring combinations take snap hooks of the type used on leashes. A screw eye is formed from a single piece. A square bend screw hook is commonly used for curtain rods and hanging kitchen utensils.

SCREW WASHERS

Screw washers are small metal circles that provide a hard surface against which you tighten a screw. They match the size of the screw they are being used with, and come in flat, countersunk or flush shapes.

POWER-DRIVEN FASTENERS

There have been a number of fasteners designed to be installed with power equipment. Several characteristics are common to fasteners that have been designed for them.

* Drive "Styles"--The old-fashioned slotted screw is simply inadequate. Under power, a slotted driver blade will never maintain a consistent grip. It frequently slips; causing damage to the screw head and the surface of the material that is being fastened. Although there are many others, the most common styles are Phillips and hex.

* Engineered Threads--In most cases, fasteners designed for power drivers are self-drilling and tapping.

* Special Purpose Designs--Each category has a unique combination of design characteristics that makes it suited to specific applications, such as deck screws, particleboard screws, self-drilling screws, cabinet screws, wood trim screws, masonry screws and drywall screws.

With the popularity of metal studs growing, new fasteners have been developed specifically for securing them to lightweight materials, such as foamboard sheathing and housewrap.

Other evolving features of power-driven fasteners include nibs that provide a neat, flush finish. Newer ceramic deck screws have sharp points that eliminate wandering.

Deck screws cannot be given a hot-dipped finish, since it would clog the treads. Manufacturers coat galvanized screws with waterproofing resins. Sometimes the coating is colored, but some manufacturers use a clear coating. Look on the box for words such as "special weather-resistant coating."

* Accessories--The basic attachment needed for installing fasteners with power drivers includes common sizes of Phillips, hex head and slotted tips, a spring-loaded bit holder and a portable friction clutch. These are available as single items or in kits.

BOLTS

Bolts are designed to fasten metal to metal. Most bolts can only be turned with a wrench. Unlike screws and nails, their ends are blunted, not pointed. Their "machine" threads require a nut to tighten against a surface. The diameter of a bolt is listed in inches.

* Carriage Bolts have a square shoulder under the head that pulls into soft materials such as wood and prevents the bolt from turning while the nut is being tightened. They have coarse, partial threads and a smooth, rounded head.

* Machine Bolts come in regular, square, hex, button or countersunk heads. Square heads fasten joints and materials where bolt requirements are not too severe; button heads work best where smooth surfaces are necessary; and countersunk heads are recommended for flush surfaces. Countersunk and button heads can be tightened only by wrenching the nut.

SPECIAL BOLTS

Continuous threaded rods are available in different diameters and lengths and are used for jobs where extra long bolts are required. They can be cut to any length and can be bent to make U-hells, "Eye" belts and J-bolts.

* Stove bolts hold light metals or wood. Heads can be flat, oval or round and slotted for a screwdriver.

* Expansion belts are used to hold heavy, hanging objects and are good in masonry.

* Turnbuckles are used for tightening wire, such as clotheslines or bracing doors.

* Hanger bolts feature large screw threads on one end and bolt threads on the other. They are used to mount fixtures in the ceiling.

NUTS

Nuts screw onto bolts to help tighten the bolt against whatever surface it is being fastened. Most common are hex and square nuts, which are also called full nuts. Wing and knurled nuts are used where frequent adjustment or disassembly is necessary. Locknuts have a self-locking feature that allows them to be locked into position without additional lock washers, cotter pins or locking wire.

COTTER PINS

A widely used, versatile fastening device, cotter pins are made of ferrous and nonferrous wire in various diameters and lengths ranging from 1/32" x 1/2" to 1/4" x 18". When inserted into a hole in a bolt, shaft or similar part, an eye on one end prevents the pin from going through, while prongs at the other end are bent back to lock the pin in place.

HITCH PIN CLIPS

A variation of cotter pins; hitch pin clips are formed from oil-tempered spring wire and act as a quick fastening device. Internal hitch pin clips are inserted through a hole in a shaft, while external hitch pin clips snap into grooves on a shaft. Sizes range from 1/8"-1/4" shaft diameter.

WIRE HARDWARE

Wire hardware includes eyebolts, U-bolts, cup hooks and various threaded wire configurations. Two such important products are lag thread and machine thread eyebolts. Lag thread eyebolts are similar to lag bolts but are used to support or suspend objects from wood surfaces. Nut eyebolts are used to hang, support or anchor objects.

The machine threads allow flexibility in attaching to practically all surfaces.

DECK CLIPS

These L-shaped fasteners are first nailed to the side of the decking, and then nailed to the joist. They are particularly secure, and eliminate nails or screws on the surface of the deck so there are no hammer dents. They also prevent water puddling on nail heads and surface rust stains. By providing an unbroken deck surface, they make sanding and resurfacing the deck easier.

RIVETS

Rivets are a reliable way to securely fasten something that can be reached from just one side. Multi-grip rivets expand to fill oversized and irregular holes and self-adjust for misaligned holes. Multi-grip rivets can be used in metal, plastic and composite materials and are ideal for projects such as installing gutters and drop ceilings or repairing large appliances, lawn mowers and boats. They are available in 1/8", 3/32", 3/16" and 1/4" body diameters and dome, countersunk and large flange head styles.
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Copyright 2006 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Hardware & Fasteners
Publication:Hardware Retailing
Geographic Code:1USA
Date:Aug 1, 2006
Words:2070
Previous Article:Screening materials.
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