Screw

A screw, a pin or bolt which is externally threaded. A compound prepared by a screw is usually a force- and form-fitting and releasable. A basic distinction is between so-called wood and metal screws.

Wood screws are slightly conical and are mainly screwed into wood, where they intersect a mating thread itself. A specific and adapted counterpart as the mother does not exist.

A metal screw is generally cylindrical and is in a counter-thread, which is located in the mother or another involved in the connection part, screwed. They called metal screw, because it was developed in the metal industry and is mainly used.

Both wooden and metal screws consists mostly of metal (primarily steel, brass or sometimes copper), rare plastic.

For screws with head ( cone, disc-or lens-shaped ) the annular surface under the head acts as a stop against the joining with them perforated part. The head contains also elements to form positive contact with tools for rotating the screw: slot ( outer or inner) hexagon or others.

The thread constitutes an oblique plane which is spirally wound on the base body. Due to the wedge effect of an inclined plane, a small applied tangential force is amplified to a greater axial force when tightening the screw. On the other hand, a large movement is converted at the periphery in a small axial movement ( for adjustment applied ).

A screw is also one of the two parts of a helical gear.

In this paper, the properties of metal screws are mainly described.

History

The oldest known helical technical structure is the Archimedean screw, which serves as a pump for liquids. It was described by the Greek mathematician Archytas of Tarentum ( 428-350 BC ), but was used already in the 7th century BC to supply water to the Hanging Gardens of Babylon and Nineveh.

In the 1st century BC screws were common wood in the Mediterranean in boosting oil and wine presses. Here, the principle of the screw is used as connecting element for the first time.

At the beginning of the 15th century metal screws have been made in Europe, but could not prevail because of their high price. Only the industrialization in the 18th century made possible the mass production of cheap and and wide dissemination of screws. The following is a chronology of the modern achievements:

The one to two threaded rods to clamp onto carpenter's benches ( bench ) were manufactured until around 1950 mainly made of wood.

Mechanics of a screw

The thread of a screw can be geometrically interpreted as a rolled-up wedge. The friction ( stiction, self-locking ) to prevent sliding of the wedge or the loosening a screw under load on the wedge surface. Vibrations but it can come for automatically solving, which is why most of a ( positive ) securing screws is necessary in such cases.

The resulting by tightening a screw axial force causes a slight elastic deformation of the screw shaft and the workpiece. The screw shaft is stretched while the workpieces are clamped compressed. The elastically deformed screw connection acts like a stiff spring and is frictionally secured against loosening, as long as this spring preload is not broken, for example by creep (slow plastic deformation ). Wood screw joints are particularly affected because of the creep of wood. A sufficiently durable and safe connection according to this principle is only the particularly soft stretch bolts connection ( with metal screw) that is applied, for example, between the cylinder head and engine block in internal combustion engines. The stem of such a screw is extra long, to act as a soft spring. The Dehnschraubenverbindung is less sensitive to creep and can also be noted that changes their operating power to a greater degree. Within a wide range, intermittent operation force change, the connection is neither repealed nor the screw is claimed on the proof stress out. Dehnschraubenverbindungen come from without additional screw lock.

In the screw shaft are introduced via the annular face under the head and over the thread forces that claim it mainly to train. Because the frictional resistance in the threads and under the head against twisting by tightening he is also subjected to torsion. Shear stress can occur when two parts screwed together to move transversely, bending stress as the area under the head ( and / or the mother) is not square to the shaft. The thread represents a notching the surface, thus the capacity of the shaft is reduced to train, shear and torsion compared to a smooth pin.

Application and installation

There are screws with less than half a millimeter in diameter for movements and also man-high, thigh -thick fasteners to large machines and buildings.

Bolted connections are solvable, they favor the recycling of equipment, machinery and equipment. They facilitate repairs, sorted to separate tags and worn-out equipment and, where appropriate reuse components. Is at odds, however, that device manufacturers more and more difficult, the solubility and thus repairs on purpose to force a rapid initial purchase. It can be selected, for example, unusual screw heads, for which there is not a common connection tool.

The screw heads are provided with a connection geometry ( slot, hex, and others) for contact with an assembly tool (screwdriver, wrench), with the tightening torque is transmitted. To selectively produce a specific value of the tightening torque, a torque wrench is used. The optimal value is required especially in Dehnschraubenverbindungen. The screws for mounting a car wheel of steel be tightened using a torque wrench with a specific torque. The wheel bolts are not stretch bolts, but the environments of the rim holes give way elastic, so that self-locking screws compounds are formed the same as with expansion bolts. An additional safeguard against loosening is due to the friction forces between the screw and the rim, which are increased as a result of the conical shape of the contact surface.

Production

For the production of head bolts, there are now two main production processes:

The cold extrusion process on a multi-stage press for large quantities and diameters up to current maximum M 36 The starting material is delivered as a wire wound on spools and uncoiled and directed into the upstream plants. Modern extrusion presses work stages, that is, in a stroke of several operations are carried out in succession, for example, shear blank hex head preforms, ready to dive, deburring and reduce threaded part. In the subsequent process, the thread having a thread rolling machine is made without chip removal ( rolling of the thread groove into the surface of the screw shaft ). Press and rolling machine form a unit, the so-called " Boltmaker " usually. Depending on the diameter and length of the screws to achieve such plant production levels of more than 300 pieces per minute.

The hot -pressing method to forging a forging press for small to medium volumes and diameters up to 200 M starting material is round material in rod form. After cutting the blanks are heated fully or partially to forging temperature ( depending on the material up to 1,250 ° C) and pre-formed in a press. Completion is performed by cutting, usually by ( CNC turning, threading). Bolts with higher strength in the thread are made without cutting after forging the raw form on a thread rolling machine ready.

The earlier production of screws ( and nuts) on automatic lathes is cost-and material-intensive and is used only in small quantities. For small diameters, the thread with the help of taps are made ( for female) and dies ( for external threads ). These two tools simultaneously with multiple intersecting cutting parts are also used for making smaller thread diameter by hand. For large diameter a specially shaped turning tool is used (only one cutting part ).

Corrosion protection

Corrosion arms or stainless screws are made of stainless steel, nickel alloys, copper alloys, plastics, occasionally aluminum, titanium or recently also made of carbon fiber reinforced plastic ( "Carbon" ). Simple steel bolts do require a protective coating (eg, anti-corrosion paint ), if they are not used on corrosion-free or - poor conditions. Possible methods of surface treatment are:

Blue finish: (also called blackened screws), while a thin, permanent mounted dark brown to black oxide ( Eisenoxiduloxid - Fe3O4) is produced by immersion in a heated, highly alkaline salt solution. Standard to: . DIN 50938 The obtained corrosion protection is weak, he is more aesthetic reasons.
Phosphating thereby produced chemically or electrochemically, a phosphate layer on the surface. Weak corrosion protection, the adhesiveness eg plaster used primarily for requirements (eg drywall screws drywall ) or in the automotive industry for engine bolts that are already exposed to corrosion, such as connecting rods and cylinder head bolts. Advantageous in this case the adjustability of coefficient of friction and high load-bearing capacity of the phosphate layer is exploited. Standard, see: EN 12476
Zinc: EN ISO 10684 ) and galvanizing ( " HDG " ) ( standard for this purpose: This hot dip galvanizing ( " tZn " ) standard for this purpose are EN ISO 4042, EN ISO 2081 also ) distinguished. The inexpensive and most common by far is the zinc galvanizing. Large screws ( from M 16 ) for outdoor use are usually galvanized.
Chromate ( environmental and health protection reasons because of the content of hexavalent chromium decreasing) (standard for this purpose: EN ISO 4520, EN ISO 4042 )
Tin to make a good solder surface area ( electrical industry )
Cadmium plating ( adjusted for environmental reasons ) ( standard for this purpose: EN ISO 2082 )
Zinc flake coating: free of hexavalent chromium ( Cr -VI ) with high corrosion resistance produced no hydrogen embrittlement ( this standard: EN ISO 10683, VDA 235-104 )
Zinc -nickel coating: Approved in the automotive industry ( see EN ISO 4042, VDA 235-104 ) Cr -VI -free, friction adjustable, temperature- resistant up to 300 ° C, the highest corrosion protection ( > 1000 h according to EN ISO 9227 )
Sherardizing: In Europe, little-known diffusion galvanizing, corrosion resistance is comparable to that of hot-dip galvanizing, good adhesion.

Studies have shown that corrosion protection coatings containing hexavalent chromium - eg chromated and zinc-electroplated layers - can be carcinogenic. Need according to the ELV Regulation 2000/53/EC, therefore, be all new vehicles from 1 July 2007 free of hexavalent chromium ( Cr -VI). Therefore, more and more coatings, for example, be converted to the zinc flake coating. In addition to the automotive sector is of it, for example, the electronics industry affected, which also must change the Cr- VI-free method (see RoHS).

The corrosion resistance of coatings is determined by the salt spray test, that is coated parts (eg screws ) must withstand a test chamber in a continuous salt fog over, for example, 240 or 480 hours without exhibiting corrosion. Thus one simulates the life cycle of a component relative to its corrosion resistance.

Other surface treatments, which are also used in addition to corrosion protection for decoration or to the better electrical contact, are the silver-plating, copper plating, the brass coating, chrome plating, nickel plating and gilding.

Marking / strength classes

The identification of the hexagon and hexagon socket screws from M5 takes place on the screw head, on which the manufacturer identifications and the strength class are indicated in stainless steel bolts in addition A2 or A4.

The full name of all relevant data are given, an example is:

ISO 4014 - M10 × 60 - 8.8 - A2E

The designation 4014 (metric ISO thread or coarse thread ) is due to the ISO standard ISO herauszulesen that there is a hex screw with shaft and a nominal diameter of 10 mm and a length of 60 mm and the following declared class 8.8. The screw has a galvanic coating with the coating metal zinc ( A), layer thickness 5 microns (2 ) blank with gloss, no color (E); designated in accordance with EN ISO 4042nd

ISO 8765 - M20 × 2 × 60 - 8.8

The identification of these hex bolt is still added to the slope 2.0 mm. This is not a standard thread ( pitch 2.5 mm) but a fine thread. Additional parameters, such as edge and core diameter, and core -voltage cross-section and angle of inclination can be determined using the DIN 13.

From the strength class of steel screws, the tensile strength Rm and yield strength Re can be calculated. As an example of the strength class 8.8:

Rm is calculated by the first number is multiplied by 100: 8 N / mm ² x 100 = 800 N / mm ² minimum tensile strength,
Re by two numbers multiplied together and the result is again multiplied by ten: (8 × 8) N / mm ² = 64 N / mm ², 64 N / mm ² x 10 = 640 N / mm ² minimum yield strength. In this case, the tensile strength at 80 % (in this case starts the constriction of the screw and the maximum transmittable power is here the greatest ) the yield strength (from here occur irreversible deformations ) achieved and deformation proceeds from the elastic to the plastic range beyond. When the screw is loaded beyond, the material is permanently deformed and the bolt is permanently elongated. This should be avoided.

According to the Standards for mechanical and physical properties (EN ISO 898-1 ) property classes 4.6, 5.6, 5.8, 6.8, 8.8, 10.9 and 12.9 are used.

In industry, very often on the Class 8.8 for use, the classes 4.6, 5.6 and 5.8 are apart of flanged joints, only rarely encountered. 10.9 and 12.9 are mainly used for calculated and defined pre-stressed bolted connections.

In hardware stores, however, the strength class 4.6 is offered in many cases.

For screws made of stainless steel, the quality and strength class is specified on the screw head. These are, for example, A (for austenitic steel), 1-5 (variety ) and 50 ( soft), 70 ( work-hardened ) or 80 ( high-strength), for example, A2-70 or A5 - 80th Further mechanical properties of stainless screws are described in ISO 3506-1.

Mainly the qualities are generally A2 and A4 for increased corrosion stresses used. These qualities are colloquially even today named coined by Krupp factory names " V2A " and " V4A ". In special cases, screws come from the materials to the material numbers 1.4439 or 1.4462 is used, for example, in the offshore area. " Stainless " (actually low-corrosion ) screws have a silver- matte finish and are often (if austenitic ) non-magnetic.

Material of screw

Steel, structural steel from the cheap way to non-magnetic stainless steel
Brass and other non-ferrous metal alloys
Titanium
Plastic
Ceramics
Composite ( composite materials )
Aluminum (e.g., in conjunction with magnesium components )

Thread forms

Right - or left-hand thread
ISO metric screw threads (fine, standard, coarse)
Inch threads (UN thread and Whitworth thread )
Pipe thread
Wood screw thread ( the carrying capacity of the timber adapted)
Self-tapping or self- defining thread for metal or plastic
Tapping screw thread with a thread similar to that of wood screws
Drywall screws thread with multiple thread or large pitch angle
Self-tapping wood screws thread, for example as a synonym brand name SPAX (by: chipboard screw with cross slot "x").
Nail screws, these are injected using a pneumatic device shot into wood. In contrast to conventional drywall screws, they are linear and non-rotating introduced into the workpiece, which simplifies the manufacturing step. The advantage to a pure nail connection is the slight solubility of the compound.
Machine screws with metric threads or the American UNF and UNC threads

The pleonastic tradename screw ( each screw has a thread ) commonly referred to screws with thread forms, which are provided for receiving a mating internal thread, so for example, machine screws with metric or imperial thread, but no wood or sheet metal screws or self-tapping.

Screw head shapes

Flat - head
Round - head
Hexagon head
Countersunk head
Countersunk round head ( lens head)
Cylinder head

Screw head drives

The most common additional forms on the screw head to the recognition of a screwdriver or wrench are:

Outside hexagon
Exterior square
Allen (Allen)
Hexagon Round ( Torx )
Internal serration ( triple square )
Square female ( Robertson, distributed in North America)
Head Phillips ( Phillips and Pozidriv)
Head - slot

To complicate unauthorized access, screws are used with exceptional drives on the head:

Allen TR, Allen with a central mandrel
Pentalob, used by Apple in the iPhone 4 and Macbook Air
Torq-Set, profile with offset cross, widely used in the aerospace industry
Torx TR ( Tamper Resistant = tamper proof Torx with central spike )
Tri-Wing, used in the aerospace industry

The corresponding screwdriver offered after some time in the trade, so that again there is cause for the introduction of a new unusual shape.

Screw heads with slots, the left face is oblique, only can be screwed, but not solve (example: one- slot ).

Manually operable screw

Screws
Thumbscrews
Screws on the regulation of mechanical quantities

Combinations

There are also screw heads that combine multiple profiles. The picture on the left shows a cross-head screw with additional outside hexagon. Are widely used Phillips screws, with which one of the two slots is carried out over the edge of the head also, so that they can be operated with ( different sizes ) head screwdrivers (see picture below: common computer screws). For such screws there are now special screwdrivers, which are usually offered as a " plus-minus screwdriver ". They allow a very tightening as with screws while providing the danger of slipping the Phillips screws. Another combination exists for Torx and slotted screws.

Screw standards (selection)

Hex head Hexagon screw with shank ISO 4014 (previously DIN 931)
Hexagon bolt with threaded up to the head ISO 4017 (previously DIN 933 )
Hexagon screw with shank, fine thread 8 × 1 to 100 × 4, ISO 8765 (previously DIN 960)
Hexagon screw with fine thread 8 × 1 to 100 × 4 to the head, ISO 8676 (previously DIN 961 )
Hexagon fit bolt for steel structures ( small hole game ) DIN 7968
Hexagon screw for steel structures DIN 7990
Allen -HV - screw for steel structures ( high-strength connection ) EN 14399 (previously DIN 6914 )
Hexagon head wood screw DIN 571
Hexagon head tapping screw ISO 1479 (previously DIN 7976 )

Square head bolts with collar DIN 478
Square head bolts with short dog point DIN 479
Square head bolts with DIN 480 tip approach

Cylinder head screw with hexagon socket ISO 4762 (previously DIN 912)
Cylinder head screw with hexagon socket, lower the head, with keyway to DIN 6912
Cylinder head screw with hexagon socket, low head DIN 7984
Slotted cheese head screw ISO 1207 (previously DIN 84 )
Cylinder -head tapping screw with slot ISO 1481 (previously DIN 7971 )
Cylinder head screw with hexagon socket with metric fine pitch thread EN ISO 12474

Countersunk screw with hexagon socket ISO 10642 (previously DIN 7991 )
Countersunk Slotted ISO 2009 (previously DIN 963 )
Countersunk wood screw with slot DIN 97
Countersunk flat head tapping screw with slot ISO 1482 (previously DIN 7972 )
Countersunk Phillips ISO 7046 (previously DIN 965 )
Countersunk wood screw with cross recess DIN 7997
Countersunk flat head tapping screw with cross recess ISO 7050 (previously DIN 7982 )

Raised countersunk head with slot ISO 2010 (previously DIN 964 )
Raised countersunk head wood screw with slot DIN 95
Raised countersunk head tapping screw with slot ISO 1483 ( old: DIN 7973 )
Raised countersunk head with cross recess ISO 7047 (previously DIN 966 )
Raised countersunk head wood screw with cross recess DIN 7995
Raised countersunk head tapping screw with cross recess ISO 7051 (previously DIN 7983 )

Slotted round head wood screw with slot DIN 96
Slotted round head wood screw with a Phillips head DIN 7996

Lenses - tapping screw with cross recess ISO 7049 (previously DIN 7981 )

T-head bolts DIN 261, with square to DIN 186 and DIN 7992 with nose
T- slot bolts DIN 787

Truss-head bolt with square neck DIN 603

Set screw with hexagon socket and flat point ISO 4026 (previously DIN 913 )
Set screw with hexagon socket and top ISO 4027 (previously DIN 914 )
Set screw with hexagon socket and pin ISO 4028 (previously DIN 915 )
Set screw with hexagon socket and cup point ISO 4029 (previously DIN 916)
Set screw with flat point DIN 551 slot
Screw with slot and tip DIN 553
Screw with mortise and tenon ISO 7435 (old: DIN 417)
Set screw with cup point DIN slot and 438

Eye bolts DIN 444
Thumb screws DIN 316
Thumb screws high type DIN 464
Screws low form DIN 653
Eye Bolts DIN 580
Locking screw with hexagon socket, taper thread DIN 906
Locking screw with hexagon head, taper thread DIN 909
Screw plug with collar and hexagon socket, cylindrical thread DIN 908
Screw plug with collar and external hex, heavy duty, cylindrical thread DIN 910
Screw plug with collar and outer hexagon, light duty, cylindrical thread DIN 7604
Thread - cutting screw DIN 7513
Studs, screw-in ~ 2 d DIN 835
Studs, screw-in ~ 1 d DIN 938
Stud bolts, threaded end ~ 1.25d DIN 939
Studs, screw-in ~ 2.5 d DIN 940

There are a variety of special screws that are designed for special applications for which no standard screws can be used and are mainly distinguished by their head or tapping (eg drilling screw with tapping screw thread DIN 7504 )

Depending on the shape of the head needs to appropriate wrench or screwdriver for rotating the screw. In many compounds, it is necessary to use a torque wrench.

The length refers generally to the length of the head ( ie thread plus possibly stem ), but with countersunk screws on the total length including head.

Thread pitch

The pitch describes the way that a thread, such as a screw, is screwed in one revolution. The slopes of threads is usually fixed for the corresponding nominal diameters in standards (for example ISO metric thread).

For metric screw is between " Metric coarse thread " 13-11 differed according to DIN 13-1 and " Metric fine thread " according to DIN 13-2 to DIN. The " Metric coarse thread " there is a fixed assignment of the slope to the nominal size of the screw, while the fine thread (typically from M 8) between different slopes (eg: M 10 x 0.75, M 10 x 1.0, M 10 x 1.25 ) can be chosen depending on nominal size.

Screws with fine thread in the area of M 12 to M 42 are available (among other upgrades ) with a uniform slope of 1.5.

Metric screws without marking the slope (eg, hardware store goods) have " coarse thread ".

For imperial threads the number of threads per inch is typically specified for the slope. Hence the number of revolutions per inch screws axial screw path.

Special designs

Screws without head can pin screws, set screws, set screws or studs to be, with a two-part or continuous threaded. An application example for a threaded pin of the fixing pin for the door handles, for example, a pin screws are the screws for connecting the cylinder head to the engine block in piston engines. The expansion behavior ( material and cross-section) of the cylinder head bolts is adapted to the conditions.
Drilling screws have a tip with two cutting bits, which is suitable in the not too thick and hard materials (usually sheet metal) to drill a hole so that the screw can be screwed in one operation without pre-drilling and tapping.
Self-tapping screws ( self-tapping screws ) forms itself a metric thread into a blind hole or a through hole in metal without producing a chip. They are suitable for soft materials. When screwing in a linearly increasing torque and a higher torque is required. The screws are usually manufactured as tempered, more expensive than standard metric screws, but costs can be reduced in the overall process, there is no need for a thread on the counter component. A molded thread is more durable than a cut, as it fits better to the screw and due to work hardening is often harder. Such self-tapping screws have a tapered thread - start, some as dislodging an easy three angular profile.
Tapping screws and screws for plastic blind holes also form a thread, but resemble a wood screw with a small slope. Self-tapping screws have grooves like a tap.
Turnbuckles ever need a screw eyelet / hook with right and left hand threads. They are used for tensioning of wires and ropes. Turnbuckles (see turnbuckle ), however, have at each end a left - and a right-hand thread.
Wiener quarter turn for wood, for example, serves for easy closing of windows

See also: anchor, motion screw, hanger bolt, standard parts, Archimedean screw, threaded

Screw locks

→ Main article: Thread Locking

Anzieh-/Montageverfahren

The following tightening are now used in the industry for the assembly of bolted joints:

Torque- controlled tightening, eg with electric torque wrench
Torque - rotation angle controlled tightening, eg with torque angle wrench
Yield point controlled tightening, eg with torque angle wrench
Hydraulic prestressing
Thermal tightening ( cf. hot rivets)
Hydraulic screw turning

The screw should be tightened so that the operating force to be transmitted (e.g., the drive or braking moments of a wheel bolt ) of the friction in the link can be reliably transmitted. This is not the case and the screw is subjected to shear forces, there is the danger of the automatic Losdrehens or breakage of the screw.

For the calculation and design of bolted connections made at this point on the VDI guideline VDI 2230-1: " Systematic calculation of high duty bolted joints - one cylindrical bolt " referenced.

Rare metallic threaded joints are well greased. Tube fittings, which carry oxygen must not be greased. At leaks outflowing oxygen can lead to spontaneous combustion of the fat.

Compounds in food lines must remain free of debris to avoid contamination.

The general rule is that all screw connections of materials should be made with the same thermal expansion coefficient as possible so as to avoid mechanical stress change in temperature.

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