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Fastener FAQ

 

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

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 for ensuring 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 ML is the manufacturer's mark.

Both marks are usually located on the top of the bolt head, and most standards indicate 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 be added (possibly with illegitimate marks).

Bolt Markings Chart

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We have a problem when tightening stainless steel bolts; they tend to seize. What's happening?

Stainless steel can unpredictably succumb to galling, or 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 or possibly wiped off which can cause interface metal high points to 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 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 changes. Here are some suggestions for overcoming the problem:

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 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 organizations recommend a different material, such as aluminum bronze nuts. However, this can introduce a corrosion problem since aluminum bronze is anodic to stainless steel.

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

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 slippage 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 bolt sustains the shear 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, then the root area of the thread must be used in the calculation.

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What is the best way to check the torque value on a bolt?

There are three basic methods for the checking of torques applied to bolts after their installation. Take the reading on a torque gauge when:

1. The socket begins to move away from the tightened position in the tightening direction. This method is frequently referred to as the "crack-on" method.

2. The socket begins to move away from the tightened position in the un-tightening direction, commonly referred to as the "crack-off" method.

3. The fastener is re-tightened up to a marked position. With the "marked fastener" method, the socket approaches a marked position in the tightening direction. Clear marks are first scribed on the socket and onto the joint surface which will remain stationary when the nut is rotated. (Avoid scribing on washers since these can turn with the nut.) The nut is backed off by about 30 degrees, followed by re-tightening, so that the scribed lines align.

For methods 1 and 2, the break loose torque is normally slightly higher than the installation torque since static friction is usually greater than dynamic friction. Some industry insiders propose that the most accurate method is method 3, however, what this will not address is the permanent deformation caused by gasket creep. An alternative is to measure the bolt elongation (if the fastener is not tapped into the gearbox). This can be achieved by machining the head of the bolt and the end of the bolt so that it can be accurately measured using a micrometer. Checking the change in length will determine if you are losing preload.

The torque in all three methods should be applied in a slow and deliberate manner so that the dynamic effects on the gauge reading are minimized. It must always be ensured that the non-rotating member, usually the bolt, is held secure when checking torques. The torque reading should be checked as soon after the tightening operation as possible and before any subsequent processes, such as painting, heating, etc. The torque readings are dependent upon the coefficients of friction present under the nut face and in the threads. If the fasteners are left too long, or subjected to different environmental conditions before checking, friction, and consequently the torque values, can vary. Variation can also be caused by embedding (plastic deformation) of the threads and nut face/joint surface, which can occur. This embedding results in bolt tension reduction and affects the tightening torque. The torque values can vary by as much as 20%, if the bolts are left standing for two days.

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

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 fewer tendencies to loosen since the thread incline is smaller.
  3. Because of the smaller pitch, fine threaded fasteners 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. Fine threads need longer thread engagements and are more prone to damage and thread fouling.
  3. Fine threads are 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. For metric fasteners, fine threads are more difficult to obtain.

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What methods are available for calculating the appropriate tightening torque for a bolt?

A high bolt preload ensures that the joint is resistant to vibration loosening and to fatigue. In most applications, the higher the preload, the better, assuming that the surface pressure under the nut face is not exceeded.

The preload is related to the applied torque by friction that is present under the nut face and in the threads. The torque value depends primarily on the values of the under head and thread friction values, so a single figure cannot be quoted for a given thread size.

The stress that is commonly quoted is often taken as the direct stress in the bolt as a result of the preload. It is normally calculated as preload divided by the stress area of the thread. Typical values vary between 50% to 80% of the yield strength of the bolt material. In many applications, a figure of 75% of yield is used.

It is important to note that it does not take into account the torsional stress as a result of the tightening torque. High friction values can push the actual combined stress over yield if high percentages are used. (The tensile stress from the preload coupled with a high torsional shear stress from the torque due to thread frictional drag results in a high combined stress.) The percentage yield approach works well in most practical circumstances, but if you are using percentage of yield values over 75%, then you could be exceeding yield if high friction values are being used.

One way to overcome this limitation is to use the percentage of yield based upon the combined effects of the direct stress from the bolt preload and the torsional stress from the applied torque. Using this approach to specify torque values is more logically consistent and can reduce the risk of the yield strength of the bolt being exceeded, especially under high thread friction conditions. A figure of 90% of yield is typically used here when the combined stress (usually calculated as the Von-Mises stress) from the direct and torsional stresses is calculated.

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Does it matter whether you tighten the bolt head or the nut?

Normally, it will not matter whether the bolt head or the nut is torqued. This assumes that the bolt head and nut face are of the same diameter. If they are not, then it does matter.

For example, let's assume the nut is flanged and the bolt head is not. If the tightening torque is determined assuming that the nut was to be tightened, but the bolt head was subsequently tightened instead, then the bolt could be overloaded. Typically, 50% of the torque is used to overcome friction under the tightening surface. Hence, a smaller friction radius will result in more torque going into the thread of the bolt and being over tightened.

If the reverse was true, the torque determined assuming that the bolt head was to be tightened, but instead the nut was subsequently tightened, then the bolt would be under tightened.

There is also an effect due to nut dilation that can, on occasion, be important. Nut dilation is the effect of the external threads being pushed out due to the wedge action of the threads. This reduces the thread stripping area and is more prone to happen when the nut is tightened since the tightening action facilitates the effect. Therefore, if thread stripping is a potential problem, though for normal, standard nuts and bolts it is not, then tightening the bolt can be beneficial.

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

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

1. How many and what size/strength do the fasteners need to be? Rather than rely on past experience of a similar application, an analysis must be completed to determine the size/number/strength requirements. A program like BOLTCALC can assist you with resolving this issue.

2. The bolt material to resist the prevailing environmental conditions. This could mean using a standard steel fastener with surface protection, or it could mean using a material more naturally corrosion resistant, such as stainless steel.

The general rule of thumb is to minimize the cost of the fastener while meeting the specification/life requirements of the application. Each situation must be considered on its merit and some in depth analysis is necessary to arrive at an optimal recommendation.

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Does using an extension on a torque wrench change the ability to achieve the desired torque value?

If you use an extension spanner on the end of a torque wrench, the torque applied to the nut is greater than that shown on the torque wrench dial.

If the torque wrench has a length L, and the extension spanner a length E (overall length of L+E), then TRUE TORQUE=DIAL READING X (L+E)/L (i.e. the torque will be increased.)

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

Nut thickness standards have been determined with the assumption that the bolt will always sustain tensile fracture before the nut will strip. If the bolt breaks upon 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 are partially failed, and 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 in conjunction with a mark to indicate the manufacturer. The Property Class of the nut matched to an 8.8 bolt is a grade 8. The nut should be marked with an 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 should be the tensile fracture of the bolt.

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

Many are of the opinion that plain washers are best avoided if possible, and certainly, a plain washer should not be used with a 'lock' washer. First, it would partly negate the effect of the locking action, and additionally, it 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 moving to the use of flanged fasteners instead. 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) and cause problems.

Research indicates that fasteners come loose because transverse loadings cause 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, caused by different operators, for example, it is possible 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.

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What is the torque-to-yield tightening method?

Torque-to-yield is the method of tightening a fastener so that a high preload is achieved by tightening up the yield point of the fastener material. To do this consistently requires special equipment that monitors the tightening process. Basically, as the tightening is being completed, the equipment monitors the torque verses angle of rotation of the fastener. When it deviates from a specified gradient by a certain amount, the tool stops the tightening process. The deviation from a specified gradient indicates that the fastener material as yielded.

The torque-to-yield method is sometimes called yield-controlled tightening or joint-controlled tightening.

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

Some details on conversion guidance between metric and inch based strength grades are 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.
  • There is no direct inch equivalent to the metric 12.9 property class.


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What is the difference between a bolt and a screw?

Traditionally, the difference between a bolt and a screw has been defined as a screw is threaded to the head whereas a bolt has a plain shank. However, using this definition now could cause problems if this assumption is made 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 maintains that since this type of fastener is normally used with a nut, then it is a bolt. Although 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.

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Are the use of a thin nut and a thick nut effective in preventing loosening?

Yes, if done properly. The correct procedure is to put the thin nut on first, tighten it to 30% or so of the full torque, and then tighten the thick nut on top of it to the full torque value. Take care that the thin nut does not rotate when tightening the thick nut. The tightening of the thick nut imposes a preload on the joint equivalent to that which would be obtained from 100 - 30 = 70% of the tightening torque, approximately. The idea is that the bolt threads engaging on the thin nut disengage so that the thick nut takes the preload by taking up the backlash on the threads of the thin nut. The thin nut is effectively jammed (hence the alternative name - jam nut) against the thick nut. This helps prevent self-loosening and improves the fastener's fatigue performance by modifying the load distribution within the threads. However, reversing the order of the nuts, thin nut on top of the thick nut, does not jam the parts together sufficiently.

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

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 determined 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 are partially failed, and 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, the fastener and block materials will usually 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 problematic.

In summary, the full height of the nut is to be used if you are to avoid thread stripping.

In terms of maximum protrusion, there are no published guidelines on this point other than the least amount of protrusion possible to avoid wasting material.

Short Bolting Info

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We are always improving this list and its answers, so we would love to hear any feedback you have about it.  If you have a question that you don't see here, please let us know, and we will get you an answer as soon as possible.