Questions we are Frequently Asked
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Some of the frequently asked questions we get asked are presented below:
What
are the marks shown on the head of a bolt?
When
tightening stainless steel bolts - they tend to seize - what's
happening?
I
can't find the shear strength of a fastener in the specification,
can you help?
What
is the best way to check the torque value on a bolt?
What
are the benefits of fine threaded fasteners over coarse threaded
fasteners?
What
methods are available for calculating the appropriate tightening
torque for a bolt.
Does
it matter whether you tighten the bolt head or the nut?
How
do you select a fastener size for a particular application?
Does
using an extension on a torque wrench change the abliity to
achieve the desired torque value?
Is
it okay to use a mild steel nut with a high tensile bolt?
Should
I always use a washer under the bolt head and nut face?
What
is the torque to yield tightening method?
How
do metric strength grades correspond to the inch strength
grades?
What
is the difference between a bolt and a screw?
Are
the use of a thin nut and a thick nut effective in preventing
loosening?
Is
there some standard that states how much the thread should
protrude past the nut?
Some
bolts are deliberately tightened past their yield point. Why
don't they further yield when an external load is subsequently
applied to the joint and come loose?
What are the marks shown on the head of a bolt?
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 organisation that accepts the responsibility that the fastener meets specified requirements. The grade mark is a standardised 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,
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).
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We have a problem when tightening stainless steel bolts - they tend to seize - whats happening?
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.
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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 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 sustain 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 than 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; namely, taking 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. This method is frequently 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 coincide.
For methods 1. and 2. the breakloose torque is normally slightly higher than the installation torque since static friction is usually greater than dynamic friction. In my opinion, 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 in order that dynamic effects on the gauge reading are minimised. 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 process 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 to 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 does 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 also less tendency 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.
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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 that is).
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 underhead and thread friction values and so a single figure cannot be quoted for a given thread size.
The stress that is often 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. Our TORKSense program uses this approach and further details on this is presented in the help file that comes with the demo program that is available for download from our web site. (This program also provides large databases on thread, bolting materials and nut factors.)
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 over come 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. Our Torque and BOLTCALC programs uses this approach and a copy of the demo program can be downloaded from our web site. The help file provided with the demo program does provide additional information on this topic.
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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 and the the contact surfaces are the same (giving the same coefficient of friction). If they are not then it does matter.
Say the nut was flanged and the bolt head was not. If the tightening torque was determined assuming that the nut was to be tightened then if 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 hence being over tightened.
If the reverse was true - the torque determined assuming that the bolt head was to be tightened then if the nut was subsequently tightened - 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. Hence if thread stripping is a potential problem, and 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. Principally these are:
1. How many and what size/strength do the fasteners need to be? Other than rely upon 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 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 minimise 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.
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Does using an extension on a torque wrench change the abliity 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) than:
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 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 are 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).
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Should I always use a washer under the bolt head and nut face?
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 as 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.) Software such as our BOLTCALC program allow for this by basing the design on the lowest anticipated preload that will be achieved in the assembly. 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.
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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 is given in section 3.4 of the standard SAE J (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
If you want to learn more, please visit our website Hebei Bentley Technology.
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.
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What is the difference between a bolt and a screw?
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.
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Are the use of a thin nut and a thick nut effective in preventing loosening?
I had been of the opinion that when two nuts were being used to lock a thread, the thicker of the two nuts should go next to the joint. I had this as one of the 'tips for the day' on some software and a couple of years ago was taken to task that this was wrong. The thin nut he said should go next to the joint.
My reasoning was that nut heights had been
decided by establishing the least height that would ensure
that the bolt would break before the threads started to shear.
So if you wanted to get the maximum preload into the fastener
then the thick nut should go first so that thread stripping
was prevented. If you put the thin nut first, the preload
would be limited by the thread stripping (whose failure may
not be obvious at the time of the nuts were tightened). Putting
the thin nut on top of the thick nut, I thought, would assist
in preventing the thick nut self-loosening. I had also seen
that
using two nuts was a popular method on old machinery - and
the ones that I had seen all had the thin nut on top of the
thick nut.
The correct procedure, I was told, was
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. You have to take care that the thin
nut does not rotate when you are tightening the thick nut.
The tightening of the thick nut would impose a preload on
the joint equivalent to that which would be obtained from
100 - 30 = 70% of the tightening torque (approximately anyway).
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
being jammed (hence the alternative name - jam nut) against
the thick
nut. This helps to prevent self-loosening and improves the
fastener's fatigue performance by modifying the load distribution
within the threads. Doing it the other way, thin nut on top
of the thick nut, does not jam the parts together sufficiently.
Two years on and I am still unconvinced. I am still asked the two nut question but I always tend to recommend other more modern ways of locking the threads. I think that the reasons that I am not easy with the method is that it is too reliant upon the skill of the person tightening the joint. There is also the amount of backlash in the threads (you could strip the threads of the small nut if it was a tight fit) and the preload will be down on what it could be as well.
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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 stipulates 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 are 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.
In summary the full height of the nut is to be used if you are to avoid thread stripping. Have a look at information on the website on the BOLTCALC program and thread stripping - there is a tutorial/presentation available from the website.
In terms of maximum protrusion I have not come across any guidelines on this point other then minimise to avoid wasting material.
See shortbolting.htm for more information on this topic.
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Some bolts are deliberately tightened past their yield point. Why don't they further yield when an external load is subsequently applied to the joint and come loose?
When a bolt is tightened into its plastic region, yielding is the result of the combined effects of both tensile/axial stress and the torsional stress exceeding the yield point of bolt material. The tensile/axial stress is the result of the extension/stretching of the bolt and the torsional stress as a result of the thread friction and stretch torque acting on the thread. When the joint sustains an external loading, there are two effects that allow the bolt to be axially loaded without sustaining further plastic deformation:
1. A significant proportion of the torsion in the bolt, typically
50% or so, disappears as soon as the tightening operation
is concluded. There is a change in the torque reaction within
the fastener. For example, if the nut is tightened, there
will be a torsion acting down the shank being driven from
the socket and reacted at the bolt head. When the socket is
removed, the torsion is then reacted between the nut face
and the bolt head with it being reduced by 50% or so. The
remaining torsion is thought normally to disappear as a result
of embedding/relaxation losses.
2. A new yield point forms at the point
on the strain curve that the bolt had been tightened to. This
effect is referred to as the Bauschinger effect (more details
on this effect is available at https://en.wikipedia.org/wiki/Bauschinger_effect).
The net result of these two effects discussed
above is that even with the bolt tightened plastically it
will perform elastically when external loads are applied to
the joint. Obviously there are limits to the magnitude of
the load that can be applied before yielding occurs. In many
applications, joint separation occurs before the bolt yields.
One important exception is for joints consisting of different
materials subjected to a significant temperature change. One
such application is a steel bolt in an aluminium joint. In
such joints the bolt can further yield as a result of differential
thermal expansion subsequent to it being tightened. (The coefficient
of thermal expansion of aluminium is broadly twice that of
steel and so the joint thickness increases with rising temperature
at a greater rate than the steel expands.) Effects such as
a reduction in the yield strength of the material at an elevated
temperature can also play a part. In some of these cases,
for example cylinder head bolts, yielding can occur when the
engine first starts and heats the block but the yielding is
limited and is stabilised in subsequent heat-cooling cycles.
.
Being in the business of selling a carefully selected range of high quality fasteners we are routinely asked some common questions about screw, nuts and bolts. In our latest blog post we endeavour to answer some of these frequently asked questions.
There are many types of screws, nuts and bolts commonly used these days. Screws are one of the most widely used fasteners, available in a wide variety of types and sizes. They are often categorised by their head types:
As noted, the term 'flat head' is also sometimes used to described slotted head screws that require flat-bladed screw drivers. Cross head screws, also called cross-point or cruciform, require a cross-head screwdriver. These are sometimes called Phillips screws and screwdrivers. Pozidriv screws and screwdrivers are an enhanced version of the Phillips cross head type.
Bolts, just like screws, are available in a vast range of designs and types including hex bolts, carriage bolts, anchor bolts and more. As for screws, bolts are often categorised by their head types, along with their application. For example anchor bolts, which are commonly used to attach objects to concrete, stone or brickwork.
Most bolts also require suitable nuts and there is also a wide variety of these including hex nuts, lock nuts, wing nuts and cap nuts. Perhaps the most familiar form of nut and bolt combination is a hex bolt with a hex nut. Both the bolt head and the nut are hexagonal in shape, requiring a spanner to tighten and loosen them. These ubiquitous fastenings have been in common use since the 19th century.
A variety of one-way and two-way security fasteners are also available. Clutch head screws, for example, are a one way security screw. They have a special screw head design that enables the screw to be tightened, with a standard, flat bladed screwdriver, but they can't be loosened.
The size of fastener needed for a job is determined by a number of considerations.
If a fastener is needed to replace a damaged, worn or lost screw of bolt then ideally the replacement should be the same size and type as the fastener that was previously used. But if fasteners are needed for a newly constructed object then the size and type will be determined by the design, the loads the fasteners must withstand and the environment in which the fasteners must remain secure.
To measure a screw, nut or bolt, use a caliper to determine the diameter of the shaft, across the widest part of the threaded section. Then measure the length from the underside of the screw or bolt head to the tip. For countersunk screws and bolts the length measurement is taken from the top of the countersunk head. And then determine the thread pitch using a thread gauge.
Screws, nuts and bolts are available in a wide range of materials including:
There is a great deal of confusion over metric and imperial fasteners. The basic difference is that metric fasteners are measured in millimetres while imperial fasteners are measured and defined in inches.
This means the diameters, lengths and threads are defined differently for metric and imperial bolts and screws. Importantly, metric and imperial bolts are not interchangeable. Metric threads will not mate with imperial and vice versa.
So how can you tell if a bolt is metric or imperial? A notable difference between metric and imperial fasteners is that metric fasteners have a coarser thread than imperial. If a bolt has lines on its head then it is likely to be imperial. And if a bolt has numbers on its head then its metric.
In the UK the metric system was introduced in . But there are many old cars and legacy pieces of machinery that continue to use imperial fasteners.
In the UK one of the key standards for fasteners is called 'British Standard' or BS. There is a wide range of these standards such as BS, the specification for precision hexagon bolts screws and nuts.
Another important collection of standards come from the International Organisation for Standardisation or ISO. This is a globally recognised standards authority that applies to a wide range of industries. ISO 898-1, for example, is the standard for bolts, screws and studs made from carbon steel or alloy steel.
Another important standards authority is Deutsches Institut fur Normung or DIN. This is the German institute for standardisation, responsible for a wide number of standards used throughout Europe. DIN 134 is a standard for nuts and bolts.
Deciding on the right fastener for a project can be difficult. Often, the best option is consult with an expert. The factors that need to be considered are:
There are often additional considerations such as the need for security fasteners, that can't be undone without specialist tools, along with aesthetic requirements.
If you need help deciding what form of fastener is right for your project why not give our friendly, helpful team a call: .
Questions about how to prevent screws or bolts from loosening over time often arise. In some environments, such as on machinery, fastenings are subjected to a lot of vibration, which can shake them lose. Options to prevent fasteners from loosening include:
Its always important to use the right size fastener and to tighten as required.
Screws and bolts can become stuck for a variety of reasons. Corrosion is a common cause of bolts and screws becoming very difficult to remove. This problem can usually be effectively overcome by applying some penetrating oil or WD40 and allowing this sit for a while. Gently tapping the bolt head can help loosen the threads and if that doesn't work, applying a little heat, to expand the metal, might be cautiously attempted.
Stripping the thread from a bolt is another common problem. When the thread is stripped the bolt might rotate, but doesn't undo. What can work is to apply extraction force to pull the bolt, using grips or pliers, while rotating the bolt with a spanner. If the bolt head can be raised a little then a flat bladed screwdriver or chisel can be used to exert extraction pressure, under the bolt head.
Another option is to use a bolt or screw extractor. These tools generally require a small pilot hole to be drilled into the head of the offending fastener, to insert the extractor tool. This same technique and tool can be used to remove the shaft of fasteners when the head has broken off.
When selecting fasteners for outdoor use corrosion resistance is a primary consideration. If the outdoor environment is anywhere near the sea then there is the additionally corrosive effect of the salty atmosphere that must be considered.
A4 Stainless steel fasteners are the best option for marine, outdoor environments. A2 Stainless steel would be OK in most situations well away from salt water. Brass or nylon fasteners might be used for low load applications.
There are a wide variety of excellent sources for high quality fasteners in the UK. These vary from the well known, huge DIY stores, located in most towns and cities, to smaller local hardware stores that still exist in some towns. There are also many excellent trade outlets that sell to the general public.
Specialist suppliers, such as Insight Security, offer carefully selected ranges of high quality fasteners for specific applications, along with valuable guidance on fastener use.
When shopping for fasteners its important to remember that product quality can vary. That's why its essential to select a reputable supplier that has a verifiable background in supplying reliable fasteners. Another important consideration is whether the supplier provides excellent help and guidance.
Working with screws, nuts and bolts requires a variety of tools. For screws, a range of both flat-bladed and cross-head screwdrivers, to suit various screw sizes, should be in the tool kit.
For bolts, spanners, including socket and adjustable spanners, are a valuable toolbox addition. Also, mole grips and pliers can be helpful.
As well as standard screws and hex bolts, Allen bolts are increasingly common these days. It therefore makes sense to include a set of Allen keys in a toolkit. And if there is a need to deal with more specialised fastenings, such as Torx head, then appropriate tools are needed.
Powered driver tools, especially cordless, can be enormously helpful, especially when assembling or disassembling equipment that has many fastenings. Another potentially useful tool is a torque wrench which is used to tighten bolts or nuts to a specific torque value, avoiding both over and under tightening.
Various security fastener require special tools. Snake Eye screws, also known as two hole screws, pig nose screws and pig nose bolts, have a distinctive head type that presents two holes for the required tool. These screws and bolts are popular for their aesthetic appearance and commonly used to secure objects in public environments.
Security Hex bolts and screws have a Allen style socket head type, but there is a pin in the centre of the recess which means a standard Allen key will not work. These fastenings require a Security Hex Driver tool or driver bit.
Standard Torx screws and bolts have a head type much like Allen screws and bolts except the recess is a six pointed star shape that requires a Torx driver. But tamper proof Security Torx fasteners have an additional security pin in the centre of the star shaped recess which means conventional Torx drivers will not work.
Another form of Torx security fastener that's growing in popularity is the 5 Lobe Torx Screw. Like their 6 Lobe Pin Torx counterparts, these tamper proof Security Torx fasteners require a matching security driver tool for installation and removal.
Shear Nuts are a clever one-way security fastener. The unique design of these nuts means that, once tightened to a predetermined torque force, the breakaway section of the nut shears off leaving a highly secure, tamper resistant fixing.
Security Scroll Nuts are another form of clever two-way security fastening. These attractive nuts are also known as wave nuts due to the wavy nut design that requires a special tool to tighten and loosen them. They are conical in shape making it virtually impossible to remove them without the required tool.
People are often understandably confused by the distinction between screws and bolts. Broadly speaking, screws are intended to be used in either preformed or threaded holes, or are capable of forming their own threaded hole (self-tapping screws). Screws are tightened by using an appropriate driver tool to apply torque to the screw head.
Another key distinction is that screws tend to be threaded over the full length of the shaft, whereas bolts are often only partially threaded. And bolts are generally intended to mate with matching nuts, which can require appropriate tools (spanners) to be used for both the nut and bolt, to tighten them.
But there are exceptions. For example, Atlas Bolts. While these excellent fasteners are called bolts, strictly speaking they should be called self-tapping concrete screws, as the thread extends over the full length of the fastener shaft and they cut their own thread into a substrate when screwed in.
Fastener threads have a number of key attributes. The gender terms, male and female, are used to refer to holes and what goes into them, for obvious reasons. Bolts and screws have a male thread while the hole in nuts has a female thread.
Threads can also be both right-handed and left-handed. Right-handed threads are the standard form of conventional thread that's tightened by turning clockwise.
Thread pitch and threads per inch are also important thread characteristics. Thread pitch is the distanced between the crest of one thread to the next, in millimetres, for metric fasteners. And threads per inch (TPI) refers to the number of threads in an inch, generally applied to imperial fasteners.
The major and minor diameters refer to the fastener shaft diameter. The major diameter is measured from the peaks or crest of the threads and the minor diameter is measured from the dips or root between the threads.
The flank angle is another thread characteristic. This is a measure of the angle of the threads, against a perpendicular line.
There are a confusing variety of threads including:
ISO Metric threads are the best known and most commonly used in Europe. Thread measurements are in millimetres. As well as the 'M' metric thread there is also the 'MF' thread, which refers to metric fine. This is a finer gauge of metric thread, commonly used in watches and high precision applications.
NPT pipe threads are a standard used for connecting pipes and plumbing. NPT stands for National Pipe Tapered. These threads are more commonly used in the USA.
Whitworth threads are named after British inventor Joseph Whitworth. He is responsible for the introduction of the world's first thread standardisation in . British Standard Whitworth (BSW) is a standardised imperial thread that's still used on some older vehicles and machinery.
UNC stands for Unified National Coarse thread and UNF refers to Unified National Fine thread. Mainly used in the USA, this is an American equivalent to the Metric ISO thread.
To determine the thread type of a bolt you need to measure the bolt shaft major diameter, determine the thread pitch and compare these metrics with a thread chart. There are also some useful thread checking tools that can be used to accurately determine the thread of a nut or bolt.
People sometimes ask if there are differences between the fasteners used by professionals compared with those available for domestic DIY users. The answer is no, both professional and DIY users all have access to the same ranges of fasteners.
But, there are some forms of specialist fasteners, such as certain security fasteners, that require special tools, as previously noted, which most people would not have in their tool kits.
Storing fasteners in an organised way that ensures they are protected against dirt and moisture is always sensible. Using organised, labelled containers is often the best option.
Fasteners might be organised by size and type in a collection of repurposed coffee jars, for example. Importantly, they should be kept in a dry environment and sachets of silica gel might be included to absorb moisture.
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