Mechanical Fasteners
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Basic
Mechanical Fasteners
Types
mechanical fasteners… Threaded

Two very basic types of machine-threaded fasteners are
– nuts & bolts and 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.
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 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 resistance to corrosion or for the purpose of electrical insulation.
Screws and bolts are 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 a bolt are likely to be pulled apart longitudinally,
– the bolt must be tight enough so that the friction between the two bars can overcome the longitudinal force.
– If the bars slip, then the bolt may be sheared in half, or friction between the bolt and slipping bars may erode and weaken the bolt (called fretting).
– For this type of application, high-strength steel bolts are used and these should be tightened with a torque wrench.
High-strength bolts usually have a hexagonal head with an ISO strength rating (called property class) stamped on the head.
– The property classes most often used are 8.8 and 10.9.
– The number before the point is the tensile ultimate strength in MPa divided by 100.
• The number after the point is 10 times the ratio of tensile yield strength to tensile ultimate strength.
• For example, a property class 5.8 bolt has a nominal (minimum) tensile ultimate 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 very safely hold a static tensile load of about 15 kN
A screw is a shaft with a helical groove or thread formed on its surface.
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.
– Screws can normally be removed and reinserted without reducing their effectiveness.
– They have greater holding power than nails and permit disassembly and reuse.
When screws and bolts cannot be used,
– nailing,
– riveting,
– roll pins,
– pinned shafts,
– welding,
– soldering,
– brazing, and
– gluing are some alternatives.
Nuts & bolts are generally used with
– pre-drilled holes.
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 & bolts are mainly used for thicker sections, where they are a well-established technique.
Nuts & bolts
Typical use is for structural assembly in 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.
Screws
Self-tapping or thread-forming screws, on the other hand, require no nuts or tapped holes.
Mostly used with pre-drilled holes although self-drilling screws are 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 be extended beyond the normal material thickness) usually provides enough material for thread engagement, although if required an additional nut or clip may be used.
Screws
Self-drilling screws may be used without the need for pre-drilled holes.
In thin materials, a screw with a special tip can be used to flow drill the hole in the material, providing additional thread engagement.
Basic Types
mechanical fasteners… Threaded

Machine 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 to connect metal parts.
Lag bolt / screw - refers to a large wood screw with a hexagonal head, driven by a wrench rather than a screwdriver.
Hanger Bolts - have a machine thread on one end, wood threads on the other. These bolts are frequently found in furniture.
Stove Bolts - similar to a carriage bolt, but usually used in metal. It requires a square hole in the metal being bolted to prevent the bolt from turning.
Basic Types
mechanical fasteners… Threaded

Carriage Bolts - has a domed or countersunk head, and the shaft is topped by a short square section under the head. The rib neck carriage bolt has several longitudinal ribs instead of the square section, to grip into 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 to penetrate un-drilled wood.
Self-tapping Screw - Sheet metal - have sharp threads that cut into a material such as sheet metal or plastic. They are sometimes notched at the tip to aid in chip removal during thread cutting.
Nine Thread Types
mechanical fasteners… Threaded
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 and Canada. 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 forces imposed on the mating nuts. The square thread is the most efficient 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 the tool carriers. Wood vise.
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