Basic Mechanical Fasteners part II TED 126 Types

Basic Mechanical Fasteners part II TED 126

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.

Types mechanical fasteners… Threaded 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.

Types mechanical fasteners… Threaded 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.

Types mechanical fasteners… Threaded 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 M 10, property class 8. 8 bolt can very safely hold a static tensile load of about 15 k. N

Types mechanical fasteners… Threaded 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.

Types mechanical fasteners… When screws and bolts cannot be used, – – – – nailing, riveting, roll pins, pinned shafts, welding, soldering, brazing, and gluing are some alternatives.

Types mechanical fasteners… Threaded 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.

Types mechanical fasteners… Threaded 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.

Types mechanical fasteners… Threaded 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.

Types mechanical fasteners… Threaded 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 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 nutsas used on certain machine tool system for withdrawing the tool carriers. Wood vise.

Nine Thread Types mechanical fasteners… Threaded ACME Used for power transmission i. e. lathe lead screws. Is easier to manufacture compared to a square thread. It has superior root 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 split nuts. Whitworth Most, if not all, British cars, motorcycles, airplanes and machinery up to the 1970 s – and possibly beyond – used Whitworth thread forms. Worm …simply a screw, captured in place and running in bearings, that accepts the input motion. Its threads engage the teeth of the worm wheel which is basically a disk with partial screw threads running around its circumference. Worm is widely used for speed reduction or increase, torque multiplication and resolution, and accuracy enhancement for positioning systems.

Nine Thread Types mechanical fasteners… Threaded Knuckle …is usually rolled from sheet metal or cast, and it is used for light-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 the strongest of the thread forms. When taking heavy loads on the near vertical thread face this thread is almost as efficient as a square thread form. Car jacks.

Basic Types of Heads mechanical fasteners… FLAT AND OVAL HEADS (UNDERCUT): The standard flat or oval head 80° to 82° counter sunk screw will fit a standard counterbored hole and is particularly adaptable to flush assemblies in thin stock. ROUND HEAD: Not recommended for new design (see pan head). This head was the most universally used design in the past. BINDING HEAD (Straight Side): Most generally used in electrical and radio work because of its identifying undercut beneath the head, which binds and eliminates fraying of stranded wire. A medium - low head with ordinarily sufficient bearing surface. Not recommended as a Phillips Recessed head – see Pan Head for better functional design.

Basic Types of Heads mechanical 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.

Basic Types of Drives mechanical fasteners… Standard / Straight / Slot head 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 drive has 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.

Basic Types of Drives mechanical fasteners… Torx is 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 by minimizing the need to bear down on the drive tool to prevent cam out. Square drive prevents slippage even under maximum torque, and the screws have positive holding power in a variety of materials.

Basic Types of Drives mechanical fasteners… 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 A or standard clutch head resembles a bow tie. These were common in GM automobiles of the 1940 s and '50 s, particularly for body panels. Clutch Type G head resembles a butterfly. This type of screw head is commonly used in the manufacture of mobile homes and recreational vehicles

Spax Screws Patented MULTIhead: Countersinks screw head flush with material. Patented serrations: Quicker and easier fastening. Patented 4 CUT™ Point: No pre-drilling, no splitting. Square end divides fibers.

mechanical fasteners… Installing a wood screw Two holes are drilled Pilot hole – First hole drilled through completely through first piece and into the second. Shank hole – Larger drill bit - drilled completely through the first piece of wood. If needed a countersink is used to put the flat head screws flush with the material’s surface. If using a #4 screw, pilot hole for hard wood is 1/16” and shank hole would be 7/64”. Softwood pilot would be 3/64” and shank hole is the same as the hard wood.

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 screw After the size you will have three letters indicating the head. THE END