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Studs /Stud bolts / Weld Studs /Weld Bolts


Introduction

The notes below relate to studs/stud bolts, and to stud welding using weld studs.    These are widely used engineering components with real advantages compared to other fastener types used for the same applications.     The notes are primarily applicable to carbon steel components to the relevant British Standards.    In fact the studs and weld studs are available in a wide range of materials to various national and international specifications.    Studbolts are often manufactured to very high specifications for the more exotic industries e.g aerospace / off shore applications.    These notes shoud therefore only be considered as introductory.    For actual detail design applications it is recommended that relevant suppliers are consulted.

Studs / Studbolts

Relevant British Standards
BS 4439:1969..Specification for screwed studs for general purposes. Metric series

A stud (stud-bolt) is a round metal bar screwed at both ends or fully screwed.    Engineered studbolts can also have flanges at a set distance from one screwed end which provides a positive collar against which the stud is tightened.


An important application form this type of fastening is for internal combustion engine cylinder blocks.  The cylinder head is normally fastened to the cylinder block using stud bolts. The studs are fitted onto engineered tapped holes in the block.   The projecting studs are convenient guides for locating the associated holes in the cylinder head . The head is fastened down using nuts on the studbolts.   This type of assembly is also used for special flanges, tank lids, pressure vessel inspection covers, and car wheels.  With stud fastening the nut has the ability to "float" and adjust on the nut end threads, which is not possible on a bolt...when fastening into deep tapped hole, this feature becomes even more important.


Studs are used with anchor fixings in civil and architectural engineering and when fastening machinery to concrete flooring.

There are also engineering applications when studbolts ( with two nuts ) are used in place of bolts.   These are generally highly loaded , large dia, long bolted items for which the material control and heat treatment is more economically completed on a stud as opposed to a bolt.  For high load-high duty applications, high strength stud-bolts can provide significant advantages if properly engineered.   The use of special nuts with close tolerances and specially contoured internal threads can allow distribution of the stress.   Studbolts are widely used in the petrochemical and nuclear industry .


The advantages and disadvantages of studbolts joints over conventional bolted joints are listed below:


Studbolt Advantages
  • Improved stress concentration factors are possible
  • Uniform section reduces local material variations
  • Lower costs in high duty applications
  • Facilitates ore convenient assembly and maintenance when using fixed studbolts which act as pilot guides
  • Less clearance required on holes allowing more accurate assemble.
  • Inventory costs can be reduced.
  • For lower duty applications studs can be manufactured on site
  • Studs with two nuts can be tightened from either side of joint
Studbolt Disadvantages
  • An additional threaded item required for each joint
  • More items can loosen
  • The nuts are normally considered the weak link in a joint
Stud Nomenclature, referenced to BS 4439

Metal End-- End screwed into component.    This is generally 1 or 1,5 x the nominal diameter as specified by purchaser.
Nut End-- End which is not screwed into component.
Plain portion - part of stud which is not screwed. This shall be greater than 50% of the nominal dia and shall include thread runouts
Nominal length- The referenced British Standard identifies a range of Nominal lengths from 12mm to 500 mm
d --The refenced British Standard identifies a range of nominal diameters from 3mm to 39mm


Threads Notes

According to the referenced standard the threads on the nut end and on the metal end shall be in accordance with BS 3643 parts 1-3. The nut end shall be to the tolerance 6g (medium fit.). This is as normal for a general bolt.    The metal end shall be to the tolerance of a close fit (4h) or an oversize tolerance as specified in the standard BS 4439. The tapped hole shall be to the tolerance 4H or, if it is not critical application a medium 6H tolerance can apply.



Stud Strength Classes

The strength grade of the stud can be any one of the grades 4,8  /  5,8   /  6,8  /  8,8   /  10,9  /  12,9   / 14,9 The first digit is the ultimate strength Rm in kgf/mm2. the second digit after the comma is the ratio of [the yield strength (Re) or the 0,2% proof stress (R0,2 ) for 8,8 grades and above ] to the the ultimate strength. e.g 4,8 is a steel with an ulitmate strength of 4 kgf/mm2 and a yield strength o 3,2 kgf/ mm2


Weld studs

Relevant British Standards

BS 7670-1:1993..Steel nuts and bolts for resistance projection welding. Specification for dimensions and properties

BS 7670-2:1997..Steel nuts and bolts for resistance projection welding. Specification for welding of weld nuts and weld bolts

BS EN ISO 14555:2006... Welding. Arc stud welding of metallic materials

BS EN ISO 13918:2008..Welding. Studs and ceramic ferrules for arc stud welding

Application Notes

For low duty applications for thin plate and sheet steel assemblies the weld stud provides a convenient reliable method of fastening flanged joints and removable covers.   The stud welding procedure simply involves positioning a special stud against the flange face using a special welding gun. The gun is operated and the stud is automatically welded to the face..Typical Applications include... Material Handling and Conveyors, Lifts, Metal Furniture, Transportation, Construction and General Machinery.


The advantages and disadvantages of the welding stud joint are listed below..


Advantages
  • No Need for Holes -- no cost of making them -- or of sealing them
  • Superior Strength -- greater than that of (sheet) base metal.
  • Faster Installation -- than comparable one-sided fasteners
  • Total Flexibility -- weld studs anywhere, at any time
  • Design Freedom -- improved appearance, no exposed fasteners
  • Low installation cost when for medium to high quantities
Disadvantages
  • Special equipment required
  • Not possible for medium /high carbon materials
  • Limited to thinner sections
  • More preparation required

The two main stud welding processes are Capacitor Discharge method (CD) and the Arc Stud Weld Process


Capacitor Discharge method

Capacitor Discharge (CD) stud welding is generally used to weld smaller diameter fasteners to thin base metals.  Because the weld cycle is completed in milliseconds, welds can be made without distortion or burn-through .   As long as one end of the fastener is designed for CD welding, CD studs can be manufactured in various shapes.    To use CD Studwelding, the parent material must be clean and flat.    The sheet may be Mild Steel, Stainless Steel, Aluminium Alloy, Pure Aluminium or Lead Free Brass.

  • The fastener is placed against the workpiece.   Most CD studs have a special tip which provides precise consistent, automatic welds.
  • The weld gun is activated and stored energy is discharged through the fastener melting the full diameter of its base and a portion of the parent material.
  • Spring-loading forces the fastener into the molten metal.
  • The fastener is momentariliy held in place as the molten metal solidifies and instantly creates a high quality fusion weld.
Arc Stud welding.ref.BS EN ISO 13918:2008

Arc stud welding is generally used to weld large diameter fasteners to rougher and thicker base metals.   Arc studs may be almost any shape and there are literally hundreds, however, they must have one end of the fastener designed for Arc welding.   Mild steel, stainless steel, and aluminum are applicable materials for Arc stud welding.

  • A fastener and ceramic ferrule are firmly placed against the work surface under spring tension.
  • Upon triggering, the weld gun automatically lifts the fastener from the base metal and initiates a controlled electric arc which melts the end of the fastener and a portion of the base metal.
  • A ceramic arc shield concentrates the heat and retains the molten material in the weld area for maximum weld strength and reliability.
  • At the precise moment the fastener and the parent metal become molten, the fastener is automatically plunged into the work surface.   The metal solidifies and a high quality fusion weld is completed.

Arc stud welding is generally used to weld large diameter fasteners to rougher and thicker base metals.   Arc studs may be almost any shape and there are literally hundreds, however, they must have one end of the fastener designed for Arc welding.   Mild steel, stainless steel, and aluminum are applicable materials for Arc stud welding.

Weld Bolts - ref BS 7670 parts 1& 2.

Weld bolts and weld nuts are welded to the base plate using the arc process and the process is facilitated by have projections on the face to be welded. See figure below.



Sites Providing Information On Studs
  1. Advanced Studwelding Co uk..Supplier .Site includes lots of relevant information
  2. Studwelding.com..American supplier with lots of relevant information and videos
  3. HBS - Stud welding ..Site includes lots of useful info

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Last Updated 24/01/2013