mechanical fasteners - part ii

8/13/2019 Mechanical Fasteners - Part II

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Basic

Mechanical Fasteners

part IITED126


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Types

mechanical fasteners ThreadedTwo very basic types of machine-threaded fastenersare

nuts & boltsand screws.Their great advantages include the ease of assembly, which generally requires no special

equipment, as well as

the possibility to de- and re-assemble without (much)damage to the joint.

They may be used for most materials.


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Types

mechanical fasteners ThreadedScrews and boltsare 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 acoating such as brass, zinc or chromium applied.

Electrolytic actionfrom dissimilar metals can be prevented

with aluminum screws for double-glazing tracks...

Some types of plastic, such as nylon or teflon, can be threaded and

used for fastening requiring moderate strength and great resistanceto corrosion or for the purpose of electrical insulation.


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Types

mechanical fasteners ThreadedScrews and boltsare usually in tension when properly fitted.

In most applications they are not designed to bear large shear

forces.

for example, when two overlapping metal bars joined by abolt are likely to be pulled apart longitudinally,

the bolt must be tight enough so that the friction between thetwo bars can overcome the longitudinal force.

If the bars slip, then the bolt may be shearedin half, orfriction between the bolt and slipping bars may erode andweaken the bolt (called fretting).

For this type of application, high-strength steel boltsareused and these should be tightened with a torque wrench.


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Types

mechanical fasteners ThreadedHigh-strength boltsusually have a hexagonal head with anISO strength rating (called property class) stamped on thehead.

The property classesmost often used are 8.8 and 10.9.

The number before the point is the tensile ultimate strength in MPadivided by 100.

The number after the point is 10 times the ratio of tensile yield strengthto tensile ultimate strength.

For example, a property class 5.8 bolt has a nominal (minimum) tensileultimate strength of 500 MPa, and a tensile yield strength of 0.8 times

tensile ultimate strength or 0.8(500) = 400 MPa.

Tensile yield strength is M10, property class 8.8 bolt can verysafely hold a static tensile load of about 15 kN


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Types

mechanical fasteners ThreadedA screwis a shaft with a helical grooveor thread formedonits surface.

Its main uses are as a threaded fastener used to hold objectstogether, and

as a simple machineused to translate torque into linearforce.

It can also be defined as an inclined plane wrapped around ashaft.

Screws can normallybe removed and reinserted without

reducing their effectiveness. They have greater holding power than nails and permit

disassembly and reuse.


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Typesmechanical fasteners

When screws and boltscannot be used, nailing,

riveting,

roll pins,

pinned shafts,

welding,

soldering,

brazing, and

gluing are some alternatives.


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Types

mechanical fasteners ThreadedNuts & boltsare generally used with

pre-drilledholes.

In the case of a nut and bolt, a simple hole will do.

Access from both sides of the component during

assembly is generally required,

although this may not be necessary if captive or welded nuts

can be used.

Because of their relatively high cost, nuts & boltsare mainly used for thicker sections, where they are a

well-established technique.


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Types

mechanical fasteners ThreadedNuts & bolts

Typical use is for structural assemblyin a wide

range of engineering applications, particularly where

high strength is required.

They can also be found in smaller form in for

example components where de- and re-assembly

may be required.


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Types

mechanical fasteners ThreadedScrews

Self-tappingor thread-forming screws, on the other hand,

require no nuts or tapped holes.

Mostly used with pre-drilled holes although self-drilling screwsare available,

the screw forms a thread in the materials being joined when

inserted, avoiding the need for tapping of the hole or for access to

both sides.

Flow drilling(causing the material around the hole to beextended beyond the normal material thickness) usually

provides enough material for thread engagement, although if

required an additional nut or clip may be used.


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Types

mechanical fasteners ThreadedScrews

Self-drilling screwsmay be used without the need forpre-drilled holes.

In thin materials, a screw with a special tip can beused to flow drill the hole in the material, providingadditional thread engagement.


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Basic Types

mechanical fasteners ThreadedMachine Bolt - a bolt with a square or hexagonal head on one end

and a threaded shaft on the other end; tightened with a wrench; used toconnect metal parts.

Lag bolt / screw - refers to a large wood screw with a hexagonalhead, driven by a wrench rather than a screwdriver.

Hanger Bolts - have a machine thread on one end, woodthreads on the other. These bolts are frequently found infurniture.

Stove Bolts - similar to a carriage bolt, but usually used in metal. Itrequires a square hole in the metal being bolted to prevent the bolt fromturning.


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Basic Types

mechanical fastenersThreaded

Carriage Bolts - has a domed or countersunk head, and the shaft istopped by a short square section under the head. The rib neck carriagebolt has several longitudinal ribs instead of the square section, to gripinto a metal part being fixed.

Machine screws - has a cylindrical shaft, threaded its entire length,and fits into a nut or a tapped hole.

Traditional Wood Screw - has a tapered shaft, allowing it topenetrate un-drilled wood.

Self-tapping Screw - Sheet metal - have sharp threads thatcut into a material such as sheet metal or plastic. They are sometimesnotched at the tip to aid in chip removal during thread cutting.


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Nine Thread Types


Sharp V thread

not used much as a thread design.

American National

similar as a sharp but stronger.

Unified Thread Standard UTS (1949)

current standard.unified with the U.S., United Kingdom andCanada. UNF, UNC, UNEF etc.

Square

this form is used for power/force transmission i.e. linear jacks,clamps. The friction is low and there is no radial forcesimposed on the mating nuts. The square thread is the mostefficient conventional power screw form. It is the most difficult

form to machine. It is not very compatible for using split nuts-as used on certain machine tool system for withdrawing thetool carriers. Wood vise.


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Nine Thread Types


ACME

Used for power transmission i.e. lathe lead screws. Is easierto manufacture compared to a square thread. It has superiorroot strength characteristics compared to a square thread.The acme screw thread has been developed for machine tool

drives. They are easy to machine and can be used with splitnuts.

Whitworth

Most, if not all, British cars, motorcycles, airplanes andmachinery up to the 1970sand possibly beyondused

Whitworth thread forms.

Worm

simply a screw, captured in place and running in bearings,that accepts the input motion. Its threads engage the teeth ofthe worm wheel which is basically a disk with partial screwthreads running around its circumference. Worm is widelyused for speed reduction or increase, torque multiplicationand resolution, and accuracy enhancement for positioningsystems.


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Nine Thread Types


Knuckle

is usually rolled from sheet metal or cast, and it is used forlight-bulb bases, bottle caps, and glass jars.

Buttress

A strong low friction thread. However it is designed only to

take large loads in on direction. For a given size this is thestrongest of the thread forms. When taking heavy loads onthe near vertical thread face this thread is almost as efficientas a square thread form. Car jacks.


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Basic Types of Headsmechanical fasteners

FLAT AND OVAL HEADS (UNDERCUT):The standard flat or oval head 80to82counter sunk screw will fit a standard counterbored hole and is particularlyadaptable to flush assemblies in thin stock.

ROUND HEAD:Not recommended for new design (see pan head). This head wasthe most universally used design in the past.

BINDING HEAD(Straight Side): Most generally used in electrical and radio workbecause of its identifying undercut beneath the head, which binds and

eliminates fraying of stranded wire. A medium - low head with ordinarilysufficient bearing surface. Not recommended as a Phillips Recessed headseePan Head for better functional design.


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Basic Types of Headsmechanical fasteners

HEXAGON HEAD(TRIMMED): This is the standard type of wrench-applied

hexagon head, characterized by clean, sharp corners trimmed to close

tolerances. Recommended for general commercial applications.

SQUARE SHOULDER SCREWS: An adaptation of the standard carriage bolt

design. Possesses a truss head on a square shank, which resists rotation when

located or driven into place.


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Basic Types of Drivesmechanical fasteners

Standard / Straight / Slothead has a single slot, and is driven by a flat-bladed

screwdriver. The slotted screw is common in woodworking applications, but is

not often seen in applications where a power driver would be used, due to the

tendency of a power driver to slip out of the head and potentially damage the

surrounding material.

Phillips screw drivehas slightly rounded corners in the tool recess, and was

designed so the driver will slip out, or cam out, under high torque to prevent

over-tightening.


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Torxis a star-shaped "hexalobular" drive with six rounded points. It was designed to

permit increased torque transfer from the driver to the bit compared to other

drive systems.

TORX is very popular in the automotive and electronics industries due to resistance

to cam out and extended bit life, as well as reduced operator fatigue byminimizing the need to bear down on the drive tool to prevent cam out.

Squaredrive prevents slippage even under maximum torque, and the screws have

positive holding power in a variety of materials.


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Allen / Hex Key There are six contact surfaces between bolt and driver and

torque is constrained by the length and thickness of the key.

Clutch Type Aor standard clutchhead resembles a bow tie. These werecommon in GM automobiles of the 1940s and '50s, particularly for body

panels.

Clutch Type Ghead resembles a butterfly. This type of screw head is

commonly used in the manufacture of mobile homes and recreationalvehicles


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Spax Screws

Patented MULTIhead:

Countersinks screw head flush

with material.

Patented serrations:Quicker and easier fastening.

Patented 4CUT Point:

No pre-drilling, no splitting.

Square end divides fibers.


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mechanical fasteners Installing a wood screw

Two holes are drilledPilot hole

First hole drilled through completely through first piece

and into the second.Shank hole

Larger drill bit - drilled completely through the first pieceof wood.

If needed a countersink is used to put the flat head screws

flush with the materials surface.If using a #4 screw, pilot hole for hard wood is 1/16 and shank hole wouldbe 7/64. Softwood pilot would be 3/64 and shank hole is the same as the

hard wood.


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mechanical fasteners Sizes

Machine Screws and Bolts 1/4 x 20

First number is the diameter

Second is the number of threads per inch

Wood Screws #6 x 3

First number is the diameter

Second is the length of the screwAfter the size you will have three letters indicating the head.

THE END

mechanical fasteners - part ii

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