Bolts are used for joints, which are required to be removable.
Bolts transfer loads through tension or shear. In either
application, it is important not to change bolt specifications
as you do your maintenance - replace type with type. Don't
substitute.
From where does the strength of a bolted joint come?
If the joint is in shear - i.e. the parts are trying to
slide past one another - then the friction between the
bolted parts produced by the bolt tension allows the joint
to hold more than the shear strength of the bolt itself.
'Friction joint' might be a better name than 'bolted joint'.
Builders and maintainers may be fooled into thinking
they are automatically getting the full nominal shear
value of the bolt regardless of torque applied. This is
far from the case. Not only would full strength not be
developed, but the fatigue life of the loosely bolted
joint is much reduced from what might be expected under
ideal conditions.
Job number one is to obtain a torque wrench, and to
use it, following recommended values for the fastener.
"Knuckle White" tight is not an acceptable standard.
If you don't believe this, tighten a bolt using your method.
Then check it with a torque wrench. 95% of bolts installed
this way are over-tightened. The problem is particularly
acute when tightening a nut or bolt against aluminium.
Aluminium is softer, more plastic, and 'gives', thinning
out the material under the fastener.
As you pull on a wrench, and the bolt turns, two simple
machines are at work; the 'Lever' of the wrench, and the
'Inclined Plane' of the pitch on the bolt. The mechanical
advantage produced by each is not additive, but multiplicative.
In English this means that if the Mechanical Advantage
of the wrench on the bolt is 30, and that of the pitch
is 40, then the Ideal Mechanical Advantage of the system
is 1,200! In other words, if there were no friction, the
tension produced by a 10 lb. pull on the wrench would
produce a force of 12,000 lb. in the bolt. Snap. Luckily
there is friction!
You should know that:
When you wrench a bolt it experiences both tension and
torsion. Only the tension remains after wrenching, so
if it hasn't snapped during the torquing it won't fail
in service if the correct size bolt is installed.
If at all possible, turn the nut, not the bolt. If nothing
else, turning the bolt will remove the protective Cadmium
finish on the bolt, enabling corrosion
As you torque a bolt, about 90% of the effort is used
to overcome friction. This has the effect of protecting
the bolt from being over-stretched during the tightening
operation. Observe whether the torque value you are using
is for 'wet' or 'dry' installation, or for plated hardware.
If 'dry', it implies no lubricant - oil or other additive.
Installation in 'wet' conditions may lead to over-stretching
of the bolt. A residual film of oil under a nut may also
make it easier for it to come loose again. Normally threads
should be clean and free of all contaminants.
When using a table of torque values notice that bolt
values are higher than nut values. The reason for that
is that bolts experience greater friction. Again, turn
the nut, not the bolt.
As a bolt is tightened, it stretches and becomes a little
thinner. Thus its clearance in the hole is further increased.
It is important that bolts are pre-stressed during the
torquing to values near the load they are expected to
carry. This assures maximum clamping pressure so that
any 'necking down' of the bolt diameter won't allow relative
movement between the clamped parts.
As you apply the final torque, hold that torque for 4
seconds to make sure that there is no further stretching
or 'following'.
Nuts have to be matched to bolts so that as tension
builds, the stretch between the threads of the bolt are
matched by 'give' in the threads of the nut. Otherwise
undue stresses are built up in some of the threads of
the bolt. Ideally each engaged thread should carry its
share of the total load.
As a bolt stretches and a nut takes the load, there is
a tendency for the nuts to dilate, exposing the forces
to a smaller cross section of the triangle making up the
threads. This leads to stripping of threads at the weaker
tips of the triangle. It is therefore a good practice
not to re-use nuts and bolts in highly loaded joints.
The pitch diameter limits on a 1/4" nut are measured
in ten thousandths, 0.0001", the permissible range
being 0.0003".
If you torque a castellated nut, and the holes don't line
up, back off, change nut or washer thickness and try again.
Don't over torque to get a fit.
Understand that if you add more washers than required
because you don't have a bolt of the right grip length,
the longer bolt will stretch more for a given load. In
other words, a longer bolt will 'give' more, making it
more likely to 'work' and to come loose. Good practices
dictate no more than 2 washers. Beyond that you should
change bolt length. If the bolt is too long, it is possible
for the nut to bottom out before producing the required
tension, giving a false reading of the proper torque having
been applied.
The best guarantee against a fastener coming loose is
proper tension. The friction produced by tension prevents
rotation. Once a nut turns on a spring washer, it is only
a matter of time before the nut loosens itself totally.
That is why we safety-wire in the manner we do. It absolutely
prevents rotation.
In the light structures used in aircraft, concern is
not only with static joint strength, but the effect vibration,
differential expansion rates and materials' differing
elasticities.
If you re-use a bolt in an aluminium structure, it is
likely that the protective Cadmium plating will have been
scratched, exposing the corrosive steel to the aluminium.
With moisture, corrosion will result. How do you know
that the bolt you have removed was not already over-stretched?
Don't re-use bolts. Don't substitute, even if it is a
'stronger' bolt.
Elastic stop nuts are ideal fasteners in applications
where temperatures stay under 150¼F. These nuts
tend not to scrape the Cadmium plating off the bolt as
do the 'pinched metal' type of self-locking nut.
After you finish torquing a fastener it is a good idea
to put a dab of witness paint on it to remind yourself
of what has and has not been done.
It is absolutely essential that appropriate torques are
applied to nuts and bolts so that the design strength
of joints is maintained.
Frank Hofmann, AME_Retired Prof. of Aircraft Maintenance_EAA
Technical Counsellor.
