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Frequently asked questions

How do you select a fastener size for a particular application?

posted by Kunal K on 5/6/2013

When selecting a suitable fastener for a particular application there are several factors that must be taken into account. Principally these are:

  1. How many and what size/strength of fasteners to be used
  2. The bolt material to resist the environmental conditions prevailing. This could mean using a standard steel fastener with surface protection or may mean using a material more naturally corrosion resistant such as stainless steel.

The general underlying principle is to minimize the cost of the fastener whilst meeting the specification/life requirements of the application. Each situation must be considered on its merit and obviously some detailed work is necessary to arrive at a detailed recommendation.

What is the difference between a bolt and a screw?

posted by Moiz S on 17/6/2013

Historically the difference between a bolt and a screw was that the screw was threaded to the head whereas the bolt had a plain shank. However I would say that now this could cause you a problem if you made this assumption when specifying a fastener. The definition used by the Industrial Fastener Institute (IFI) is that screws are used with tapped holes and bolts are used with nuts.

Obviously a standard 'bolt' can be used in a tapped hole or with a nut. The IFI maintain that since this type of fastener is normally used with a nut then it is a bolt. Certain short length bolts are threaded to the head - they are still bolts if the main usage is with nuts. Screws are fastener products such as wood screws, lag screws and the various types of tapping screws. The IFI terminology and definition has been adopted by ASME and ANSI.

Is there some standard that states how much the thread should protrude past the nut?

posted by Natarajan N on 4/7/2013

There are some building codes that stipulate that there must be at least one thread protruding through the nut. However it is common practice to specify that at least one thread pitch must protrude across a range of industries. Typically the first few pitches of the thread can be only partially formed because of a chamfer etc.

Nut thickness standards have been drawn up on the basis that the bolt will always sustain tensile fracture before the nut will strip. If the bolt breaks on tightening, it is obvious that a replacement is required. Thread stripping tends to be gradual in nature. If the thread stripping mode can occur, assemblies may enter into service which is partially failed, this may have disastrous consequences. Hence, the potential of thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved. When specifying nuts and bolts it must always be ensured that the appropriate grade of nut is matched to the bolt grade.

In cases of when a threaded fastener is tapped into a plate or a block it is usually the case that the fastener and block materials will be of different strengths. If the criteria is adopted that the bolt must sustain tensile fracture before the female thread strips, the length of thread engagement required can be excessive and can become unrealistic for low strength plate/block materials. Tolerances and pitch errors between the threads can make the engagement of long threads problematical.

How do metric strength grades correspond to the inch strength grades?

posted by S Pillai on 15/7/2013

Some details on conversion guidance between metric and inch based strength grades is given in section 3.4 of the standard SAE J1199 (Mechanical and Material Requirements for Metric Externally Threaded Steel Fasteners).

Metric fastener strength is denoted by a property class which is equivalent to a strength grade. Briefly:

  • Class 4.6 is approximately equivalent to SAE J429 Grade 1 and ASTM A307 Grade A
  • Class 5.8 is approximately equivalent to SAE J429 Grade 2
  • Class 8.8 is approximately equivalent to SAE J429 Grade 5 and ASTM A449
  • Class 9.8 is approximately 9% stronger than equivalent to SAE J429 Grade 5 and ASTM A449
  • Class 10.9 is approximately equivalent to SAE J429 Grade 8 and ASTM A354 Grade BD

For information there is no direct inch equivalent to the metric 12.9 property class.

Should I always use a washer under the bolt head and nut face?

posted by Shetty Y.R on 20/7/2013

Our opinion is that plain washers are best avoided if possible and certainly, a plain washer should not be used with a 'lock' washer. It would partly negate the effect of the locking action and secondly could lead to other problems (see below). Many 'lock' washers have been shown to be ineffective in resisting loosening.

The main purpose of a washer is to distribute the load under the bolt head and nut face. Instead of using washers however the trend has been to the use of flanged fasteners. If you compute the bearing stress under the nut face it often exceeds the bearing strength of the joint material and can lead to creep and bolt preload loss. Traditionally a plain washer (that should be hardened) is used in this application. However they can move during the tightening process (see below) causing problems.

Research indicates that the reason why fasteners come loose is usually caused by transverse loadings causing slippage of the joint. The fastener self loosens by this method. When using impact tightening tools there is a large variability in the preload achieved by the fastener. The tightening factor is between 2.5 and 4 for this method. (The tightening factor is the ratio of max preload to min. preload. Because of changes in the thread condition itself - different operators etc. it could be that lower values of preload are being achieved even though the assemblies may appear to be identical. 

One problem that can occur with washers is that they can move when being tightened so that the washer can rotate with the nut or bolt head rather than remaining fixed. This can affect the torque tension relationship.

Is it okay to use a mild Steel nut with a high tensile bolt?

posted by A Karim on 28/7/2013

Nut thickness standards have been drawn up on the basis that the bolt will always sustain tensile fracture before the nut will strip. If the bolt breaks on tightening, it is obvious that a replacement is required. Thread stripping tends to be gradual in nature. If the thread stripping mode can occur, assemblies may enter into service which is partially failed, this may have disastrous consequences. Hence, the potential of thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved. When specifying nuts and bolts it must always be ensured that the appropriate grade of nut is matched to the bolt grade.

The standard strength grade (or Property Class as it is known in the standards) for many industries is 8.8. On the head of the bolt, 8.8 should be marked together with a mark to indicate the manufacturer. The Property Class of the nut matched to a 8.8 bolt is a grade 8. The nut should be marked with a 8, a manufacturer's identification symbol shall be at the manufacturer's discretion.
Higher tensile bolts such as property class 10.9 and 12.9 have matching nuts 10 and 12 respectively. In general, nuts of a higher property class can replace nuts of lower property class (because as explained above, the 'weakest link' is required to be the tensile fracture of the bolt).

What are the benefits of fine threaded fasteners over coarse threaded fasteners?

posted by Salomon R on 8/8/2013

The potential benefits of fine threads are:
  1. Size for size a fine thread is stronger than a coarse thread. This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter).
  2. Fine threads have also fewer tendencies to loosen since the thread incline is smaller and hence so is the off torque.
  3. Because of the smaller pitch they allow finer adjustments in applications that need such a feature.
  4. Fine threads can be more easily tapped into hard materials and thin walled tubes.
  5. Fine threads require less torque to develop equivalent bolt preloads.

On the negative side:

  1. Fine threads are more susceptible to galling than coarse threads.
  2. They need longer thread engagements and are more prone to damage and thread fouling.
  3. They are also less suitable for high speed assembly since they are more likely to seize when being tightened.

Normally a coarse thread is specified unless there is an over-riding reason to specify a fine thread, certainly for metric fasteners, fine threads are more difficult to obtain.

I can't find the shear strength of a fastener in the specification, can you help?

posted by Jayraj P on 23/8/2013

Bolted shear joints can be designed as friction grip or direct shear. With friction grip joints you must ensure that the friction force developed by the bolts is sufficient to prevent slip between the plates comprising the joint. Friction grip joints are preferred if the load is dynamic since it prevents fretting.

With direct shear joints the shank of the bolts sustains the sheer force directly giving rise to a shear stress in the bolt. The shear strength of a steel fastener is about 0.6 times the tensile strength. This ratio is largely independent of the tensile strength. The shear plane should go through the unthreaded shank of a bolt if not than the root area of the thread must be used in the calculation.

We have a problem when tightening Stainless Steel Bolts - they tend to seize - whats happening?

posted by A Patil on 4/9/2013

Stainless steel can unpredictably sustain galling (cold welding). Stainless steel self-generates an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the contacting and sliding, thread surfaces, protective oxides are broken, possibly wiped off, and interface metal high points shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing - the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out.

If galling is occurring than because of high friction the torque will not be converted into bolt preload. This may be the cause of the problems that you are experiencing. The change may be due to the Surface roughness changing on the threads or other similar minor change. To overcome the problem - suggestions are:

  1. Slowing down the installation RPM speed may possibly solve or reduce the frequency of the problem. As the installation RPM increases, the heat generated during tightening increases. As the heat increases, so does the tendency for the occurrence of thread galling.
  2. Lubricating the internal and/or external threads frequently can eliminate thread galling. The lubricants usually contain substantial amounts of molybdenum disulfide (moly). Some extreme pressure waxes can also be effective. Be careful however, if you use the stainless steel fasteners in food related applications some lubricants may be unacceptable. Lubricants can be applied at the point of assembly or pre-applied as a batch process similar to plating. Several chemical companies, such as Moly-Kote, offer anti-galling lubricants.
  3. Different combinations of nut and bolt materials can assist in reducing or even eliminating galling. Some organisations specify a different material, such as aluminium bronze nuts. However this can introduce a corrosion problem since aluminium bronze is anodic to stainless steel.

What are the marks shown on the head of a bolt?

posted by A Karim on 5/9/2013

Usually fastener standards specify two types of marks to be on the head of a bolt. The manufacturer's mark is a symbol identifying the manufacturer (or importer). This is the organization that accepts the responsibility that the fastener meets specified requirements. The grade mark is a standardized mark that identifies the material properties that the fastener meets. For example 307A on a bolt head indicates that the fastener properties conform to the ASTM A307 Grade A standard. The bolt head shown at the side indicates that it is of property class 8.8 and SI is the manufacturer’s mark.

Both marks are usually located on the top of the bolt head, most standards indicating that the marks can be raised or depressed. Raised marks are usually preferred by manufacturers because these can only be added during the forging process whereas depressed marks can subsequently added (possibly with illegitimate marks).