If you are in a hurry and need to undo a nut or bolt, chances are it will refuse to budge. You’ll find yourself growing red in the face and speaking in many tongues! At such times it’s worth keeping in mind the saying that it never pays to let an inanimate object know that you’re in a hurry, because it won’t fail to frustrate you.
A nut can usually be chiselled off or loosened with heat, whereas a bolt or stud should be doused with a thin penetrating oil and left for some hours before working it backwards and forwards until it is free. A stud can usually be removed by locking two nuts together on the protruding thread.
When it’s time to reassemble the parts, you have to ask yourself some important questions: Can you use the same fasteners again? If you buy new ones, how can you be sure they’re strong enough? How do you know if the bolt or nut is tight enough, and what are the chances of either loosening in service?
This article aims to answer these questions, and also to clear up some misconceptions about the way bolts should be employed, tightened and locked.
WHAT IS A BOLT?
If you habitually read handbooks or workshop manuals you’ll soon discover
that there is no uniformly-accepted definition for bolts, screws and studs.
However, most people trained in South Africa will agree with the following
definitions:
- a bolt is a headed fastener, designed to be used with a nut. It usually
clamps parts together, with the portion of the shank that passes through the
material left unthreaded.
- a screw also has a head, but has to enter into a tapped hole or may be designed
to tap its own hole, so that it is threaded for its full length.
- a stud is headless, and one end screws into a threaded hole. The other end
is threaded to take a nut, but the shank has an unthreaded portion.
These definitions are not cast in stone, but are nevertheless a useful guide. Furthermore, please note that any reference here to a bolt will also be applicable to a screw or a stud.
BOLT STRENGTH
One of the most important considerations when choosing a bolt, either for a particular
task or to replace a bolt that’s no longer worth using, is its strength. Bolts
manufactured in SA are classified according to the ISO (international Standards
Organisation) code and the bolt head is usually marked.
On mild or alloy steel bolts, the markings consist of two numbers separated
by a dot. This is not a decimal point, but merely a separator. The first number,
multiplied by 100, is the ultimate strength, in megapascal (one MPa = one newton/mm2).
This is the stress at which theoretically the bolt should break. The second
number, multiplied by 10 times the first number, is the yield strength, ie
the stress at two per cent permanent set, in MPa. These grades start at 3.6,
and increase in seven steps to 8.8, then continue through 10.9, 12.9 and 14.9.
The grades from 8.8 upwards are the only ones that are normally marked, so
that an unmarked bolt will have an ultimate strength of less than 800 MPa.
For example, if the bolt is marked 10.9, then the ultimate strength is 10
x 100 = 1 000 MPa, while the yield strength is 9 x 10 x 10 = 900 MPa.
Since the stress in a bolt is equal to the clamp load divided by the cross-sectional
area, the above values can be used to calculate the maximum clamp load. This
is simply the stress multiplied by the cross- sectional area of the bolt. However,
for most applications these calculations can be ignored, because the numbers
on the bolt are a good guide to quality. All you have to remember is that an
unmarked bolt is weaker than a marked one, and the higher the numbers, the
stronger the bolt.
The automotive industry uses two metric grades of stainless steel bolts. Grade
A2 is the most popular because it is corrosion resistant, but grade A4, which
is acid resistant, may be found on battery clamps. The heads of such bolts
will be marked A2-50, A2-70 or A2-80, (or A4-50, A4-70 or A4-80) where the
number after the dash is one-tenth of the tensile strength in MPa.
In the case of nuts, the marking consists of a single number, and if this
number matches, or is higher than, the first number on the bolt, then the nut
is strong enough.
But if the number is lower, then the bolt/nut combination is only as strong
as indicated by the lower number.
Bolts originating in the US have markings as shown on the accompanying chart.
In most cases, the more lines there are, the higher the tensile strength.
COATINGS
Automotive bolts are usually made from mild steel, alloy steel or stainless
steel. It’s worth knowing that a plain black finish, known as black oiled,
is the best from a strength and preload point of view, because any form of
plating may cause embrittlement unless the bolts are treated after plating.
This is especially true of chromium plating applied to high-alloy steels,
and such bolts should not normally be used in high-stress applications. However,
unplated bolts will rust, so bolts are often coated with a very thin layer
of zinc or cadmium, followed by a chromate coating to lock-in the finish.
Such treatment is less harmful on steels of grade 8.8 and less, which is
why coated bolts are often of a lower strength grade than black bolts.
The plating also affects the friction between the bolt and the joint surfaces,
so the recommended torque values will change. Whenever you change from a
black bolt to a coated bolt you should find out what the torque should be.
For
example, zinc plating increases the friction by up to 40 per cent, and stainless
steel doubles the frictional coefficient, but cadmium plating reduces the
friction by about 25 per cent.
THREAD LUBRICATION
Thread lubrication is another variable that needs to be considered, and for
high-stress applications one should follow the instructions in the workshop
manual. In general, a light oil or a good anaerobic coating (thread locking
compound) will reduce the required torque values by about 10 per cent,
but special anti-seize lubricants may mean a
reduction of about 20 per cent.
JOINT TIGHTNESS
How do we know when a bolt is tight enough? Experience has shown that a bolt
will not lose its grip if it is applying a greater clamping load than the
load the part is experiencing during service. The correct bolt for any
application is one whose material and dimensions allow it to distort less
than the part
being clamped. Tightening the bolt compresses the faying surfaces (ie,
the surfaces being joined) but tensions the bolt. The proportions of the
stresses
borne by each part obviously depend on the design, but may be as high as
90 per cent taken by the faying surfaces, and 10 per cent taken by the
bolt. During service, the joint experiences tensions whose effect will
be to relax
the compression on the faying surfaces and increase the tension in the
bolt, in proportion to the percentages carried by each component. In a
correctly
designed and tensioned joint, the faying surfaces will relax more than
the bolt will tension, keeping the joint intact. However, if the bolt tension
is too low, the faying surfaces will relax so much that the joint opens.
In this case the bolt will carry all the stress, and the joint will fail.
LOCKING DEVICES
As a general rule, the best way to prevent a nut from loosening is to tighten
the bolt to the correct tension. Even locking devices become ineffective
if the joint is too loose, because either vibration or stress reversal
will eventually destroy the locking device. In fact, boltheads are wired
together
on aircraft and racing cars not to prevent the bolts loosening, but to
present visible proof that the bolts are tight, because nobody will wire
a loose
bolt.
TORQUEING THE BOLT
In many applications the bolt is deemed to be tight enough when it has been tightened to a specified torque, and the motor industry goes to great lengths to publish torque values for all important applications.
However, it is possible for a joint to fail even if it has been tightened
to the correct torque. This happens because the clamping load, ie the force
transmitted by the bolt to the joint, is more important than the actual torque
reading, and in practice it’s very difficult to know exactly what load the
bolt is carrying. There is no direct relation between the tightening torque
and the stress in the bolt, which is an indication of the clamping load. Experiments
have shown that about 50 per cent of the tightening torque is used to overcome
friction at the bearing face of the nut, and a further 40 per cent is used
to overcome friction between the mating threads, leaving only 10 per cent to
increase the axial load in the shank, which is directly related to the clamping
load, and hence the stress.
These are average values using clean components,
so that one can easily have a situation where the frictional resistance is
so high that the shank does not get stressed at all. This happens, for example,
when the components are dirty, but it could also happen when the bolt is
used without the designed washer, thus allowing the base of the head to dig
into
any softer material, such as aluminium. This means that every time you tighten
a bolt you’re actually taking a chance.
USING STRETCH BOLTS
This dilemma explains why so-called stretch bolts are often used for important
applications, such as cylinder head bolts, or big-end bolts. These bolts
are normally tightened until they just start to feel some resistance, and
then rotated through a fixed angle, say 90 or 180 degrees. This guarantees
that a fixed percentage of the rotation causes stress in the bolt, ie the
friction does not affect the tightness of the bolt. They are called stretch
bolts because they’re designed to take on a permanent set, whereas ordinary
bolts should (in theory) return to their original length when the load
is removed.
These bolts should normally not be used more than once, because the fact that
they have lengthened implies that the second time the same tightening procedure
is used will not result in the same clamp load, so the joint may fail even
if the bolt doesn’t. However, since there are no hard and fast rules in
engineering, some workshop manuals advise you to measure the bolt length.
If it is below a given value, the bolt can be used again. PRECAUTIONS
There are a number of precautions to take when installing bolts, and these
become especially important if you’re tightening ordinary bolts, ie bolts that
have a given torque reading, but no angles. It is most important that the threads,
the load-bearing surfaces, and the washers, are clean. In fact, it is vital
that the correct washer is used, as any change in the material will affect
the crushability and hence the relationship between the torque and the clamping
load.
USING A TORQUE WRENCH
A torque wrench should be treated with care, and not thrown around, because
the shock loads may damage the reading mechanism. A bolt should be torqued
in a steady movement, because any jerking will result in a false reading.
If the scales are not marked in Newton.metres (N.m), or the workshop manual
gives the torque in pounds-force.feet (lbf.ft) or kilogram.metres (kg.m),
herewith a conversion table:
One lbf.ft x 1,36 = one N.m
One N.m x 0,735 = one lbf.ft
One kg.m x 0,1 = one N.m
One N.m x 10 = one kg.m
If the torque wrench doesn’t read to a high enough value then you can use an extension, but in this case the readings on the scale should be increased in proportion. For example, if the extension doubles the length of the wrench then the correct torque will be given by double the scale readings.
CHECKING THE TORQUE
How do you check the torque of a bolt that has been tightened long ago? Any
attempt to apply a torque with a wrench until the bolt just starts to move
will give you a false reading, because of friction. All you can do is to
mark the position of the head, undo the bolt, and rotate it to the same
position, while taking note of the torque reading. If it is less than the
specified value, you have to conclude that the bolt was not torqued to
the correct value.
REPAIRING A STRIPPED THREAD
All of us strip threads from time to time, and the repair may cost a fortune.
Bolts can usually be replaced, but the threads in a blind hole are a real
problem. The most elegant solution, and one whereby the original bolt can
be used, is to have a thread insert installed. In most cases, the repair
will be as strong as the original, if not stronger. Another option is to
drill a slightly bigger hole and tap a bigger diameter thread into the
material, but this will mean that a bigger diameter bolt will have to be
used, and this may bring complications. A further possibility, especially
when the bolts do not carry a heavy load, is to repair the old thread with
an epoxy-like material. A thread-repair kit includes a release compound
and full instructions.
