Industrial Skills

Fasteners

Fasteners are used in manufactured products for several basic purposes:

They simplify manufacture.

They simplify repairs.

They provide safety.

When selecting a fastener for a particular use, consider these factors:

Strength: Will it hold the loads and pressures?

Security: Will it remain attached?

Cost: Realistic?

Installation: Appropriate for situation?

Skill: Is specialized training needed?

Equipment: Is specialized equipment needed and available?

Appearance: If the fastener shows, which kind looks best?

Nails – Most common method of fastening one wooden member to another.

Simplest & quickest. May not result in the

strongest of joints. Many different sizes

and various shapes of heads, points & shanks.

Each type designed for a particular purpose.

Drive nails at angles slanting toward or away from each other to get best holding power.

Nails are designated by “penny” size,originally a termwhich related to priceper hundred but now signifies length. The symbolfor penny is the lowercase letter“d”. Nail diameter increases withlength. Nails are now sold by the pound.

Screws – A large and important family of fasteners.

The most common types of screws are:

Sheet-metal screws

Machine screws Set screws

Wood screws

“Mechanical devices for fastening things together. Essentially a cylindrical or conical piece of metal threaded evenly around its outside surface with an advancing spiral ridge and commonly having a slotted head: it penetrates only by being turned, as with a screwdriver.”

Use Categories

Wood ScrewsServe much the same purpose as a nail, but:

Provide greater holding power than a nail.Screws can be easily removed and replaced.Screws are neater in appearance and offer more decorative

possibilities.In addition to fastening pieces of wood together – the

most common use of wood screws would be to anchor objects (hardware) to a wood surface.

Sheet-Metal ScrewsAlso called “Tapping Screws” or “Self-Threading Screws”.Used to fasten light pieces of metal together or to attach covers,

panels and other light parts.These screws have sharp threads that can cut their own grooves

into metal.They come with “coarse” or “fine” threads and are usually case

hardened to cut threads and withstand hard twisting forces.Distinguishable from wood screws in that they are threaded all

the way from the point to the head.

Machine ScrewsUsed for the assembly of metal parts and usually are driven

into “threaded” holes rather than drawn tight with nuts.Like all screws, there are many head designs to choose from.Machine screw threads are also designated by the number of

threads per inch, just like bolt threads:A 6-32 machine screw has a #6 body diameter and 32 threads per

inch of length. Most machine screws are fully threaded to the head.

Set ScrewsFrequently used to hold a knob, collar, pulley or gear to

a rotating shaft.There are a variety of “head” or “point” styles, each best

suited for its job.Generally made of high-strength material and are heat

treated.Not an especially strong type of fastening – depend on

friction and “shear” to hold parts together.

Once you have decided to use screws, in addition to the use category, you must consider four things before ordering.

Type of head

Material made of

The length

The diameter

The “type of head” should include both the “shape” and the “style”.

Pan HeadTruss Head Hex HeadFlanged Hex Head

Screw Head “Shapes”Screw Head “Shapes”Screw Head “Styles”Screw Head “Styles”

The most common “material screws are made of” is steel.

If the fastener is exposed to the “weather” – steel alone does not offer much protection against the harmful effects of corrosion.

Coatings offer more protection.Steel BluedZinc CoatedChromiumGalvanizedNickelSilver PlateGold Plate

Marine Applications may require different metals and/or materials.

Stainless SteelAluminumCopperBrassBronzeSynthetic Materials

(Plastic or Nylon)

The “length” of screws commonly range from ¼ inch to 4 inches. Shorter or longer lengths are generally special order items.

(Metric lengths are also available)

Screw diameters can be expressed by the “gauge” number or by the fraction of an

inch. (Metric diameters are expressed in mm)

American Screw Gauge Sizes are designated by length in inches

(millimeters). Diameters less than ¼ inch (6 mm) use gauge number. Diameters greater than ¼ inch use fractions of an inch. Wood screws are an exception to this rule in that they

generally go up to a #20 gauge (21/64”).

Bolts, Cap Screws, Nuts & WashersNomenclature of bolt-type fasteners tends to be confusing.Bolts are usually used in plain holes drilled through the parts being

fastened.Bolts are generally held in place with a mating nut.When the nut for any bolt is turned down on wood, always use a flat

washer under the nut.Cap screws are normally used in threaded holes, without a nut.

Machine Bolts/Cap ScrewsMachine bolts have square or hexagonal heads.

Installed with a wrench.Usually used if the parts to be joined are made of metal.

Cap screws generally look just like a machine bolt.Slightly different application.Screwed into threaded holes rather than being used with a nut.

Round-Head BoltsCommonly used to fasten wood parts.Most have a square neck under the head.

Also used to fasten steel parts with square punched holes.

A Carriage Bolt is the most common type of round-headed bolt to be used when working with wood.A Plow Bolt has a flat, tapered head that fits into a countersunk hole – primarily used with metal parts.

Used in the marine industry for attaching cutters and other parts to dredges.

Plow Bolt

Carriage Bolts

Stove Bolts are available with the same types of head designs as wood screws, in diameters from 5/32” to 1/2” and in lengths from 3/8” to 6 inches.

Studs are another type of threaded fastener, which have no head at all and is merely a steel rod with threads on both ends.

One end is screwed into a part, other parts are assembled over the studs and screwed in place with a nut.

Other “ Bolts”Some fasteners are referred to as bolts

but are actually screws.A Lag Bolt is really a “heavy-duty screw”

with a square or hex head.Designed to be driven with a wrench.Available in lengths from 1” to 6” and diameters

from 1/4” to 1/2”.

A Hanger Bolt is a fastener that has wood screw style threads on one end and machine threads on the other.

No head on this type of “bolt”.Designed to be a hidden fastener.

Threads – External helical ribs on the body of a bolt at the end opposite the head.

The diameter of a bolt is determined by the diameter of the crest of the threads.

The length of most bolts (or machine screws) is determined by the measuring from the bottom of the head to the end of the threads.

Flat head bolts are measured from the threaded end to the top of the head.The Head Size determines what size wrench or socket must be used to

turn or hold the bolt or nut.A square or hexagonal bolt head is measured across the “flats”.Example – A ¾” wrench is needed to turn a ½” diameter bolt head.

Sizes and Descriptions

ThreadsThreads – External helical ribs on the body of a bolt. Usually a

bolt “mates” with internal threads of a nut.The top of the rib is called the crest, or thread tip.Bottom of the groove is called the thread root.

Threads are measured by counting the number per inch. (Metric threads are measured by the distance between threads – pitch – in mm)

Thread gauges are available that match threads against those on the gauge.

The type of threads that are used for most applications are coarse with deep grooves.

Some threads are finer, with shallower grooves. Bolts with fine threads are used only under special conditions – such

as when the parts being fastened have thin walls.

Bolts and screws normally have right-hand threads.Turned to the right (clockwise) when tightened.

Occasionally, bolts, screws and nuts with left-hand threads are needed.

Turned to the left (counter-clockwise) when tightened.

American National Standards Institute( ANSI )

There are carefully controlled standards for threading bolts and nuts.

ANSI establishes such things as the angle of threads, the depth of the root and the manufacturing tolerances (fit) which are referred to as the “Unified Screw Thread Standards”.

International Organization for Standardization( ISO )

Established standards for classifying metric bolts and screws.

Grades and Head Markings – Inch-SeriesThe kind of steel bolts and screws are made of and the

treatment they receive during manufacture determine their strength.

The Society of Automotive Engineers (SAE) has established standards for classifying inch-series bolts and screws into grades, based on their tensile strength.

Markings consist of radial “slashes”.High-quality inch-series bolts and screws ¼” and larger have

them.Grade 5 or better hardware should be used in most situations.

Grades and Head Markings - MetricThe kind of steel bolts and screws are made of and the

treatment they receive during manufacture determine their strength.

The International Organization for Standardization (ISO) has established standards for classifying metric bolts and screws into property classes based on strength.

Numbers on the head indicate property class.High-quality metric bolts and screws 4mm and larger have them.Class 8.8 or better hardware should be used in most situations.

Nuts and WashersNuts have coarse or fine internal threads that correspond to

the threads of a bolt and are designed to “screw” onto the bolt to fasten it in place.

A great variety of shapes and sizes for standard and special applications.

Threads per inch or distance between threads can be determined with a thread gauge just like bolt threads.

Nuts have three important dimensions:ThicknessDistance across the flatsInside diameter (same as that of the bolt with which it is to be used).

Jam Nuts – Used to lock a threaded part in place. Castellated and Slotted Nuts – Secure a nut in place so it

can’t possibly come loose.Self-Locking Nuts – Stay firmly in place even with constant

vibration.Many other types of nuts available for special uses.

Hex and Square NutsThe most common nuts, generally made of steel and are hexagonal or square in shape.

A plain washer is simply a steel disk with a hole through the center.

Like bolts and nuts the may be manufactured with a variety of different materials

They help distribute the load over an area larger than the head of the bolt or nut, thus reducing the stresses that would otherwise exist.

Plain washers are identified by their outside diameter and the diameter of the hole, which is the bolt size rather than the actual diameter of the hole.

The washer thickness varies with the size of the washer.

Lock washers are frequently used to keep nuts and bolts tight, especially when they are subject to vibration.

Helical Spring Washers – Made of tough spring steel. They are split and one end of the split is bent up. When the nut is tightened, the section of the washer that is bent up “bites” into the nut and the fastened part.

Toothed Lock Washers – Has many sharp, heat-treated teeth to dig into the surfaces pressing against it.

Many other types of washers are available for special uses.