article is the answer to some to and fro about fasteners. I hope
that my poor scrivenings are of use to someone.
torque wrench is the most commonly applied tool for the purpose
of ensuring that screw fasteners are sufficiently tightened. While
the tool is universally used, its purpose is not understood since
many (most?) users believe that the torque (turning against resistance)
of the fastener is what is important to the tightening process.
wrenches measure the torque or turning force applied to a fastener.
When tightening a threaded fastener, the turning effort acts against
the threads to pull the fastener into the opposite threads. The
turning effort is divided into two efforts: friction and tension.
The friction of the threads is not useful to the task but must
be overcome to allow tightening. The tension effort is that which
applies an axial (lengthwise) pull to the fastener and it is this
effort that is useful to the task. As a fastener is tightened,
pressure on the threads increases, thus increasing friction and
tension. What we are doing when tightening a fastener (nut, bolt,
screw) is to apply a clamping force to the components that are
being connected by the fasteners. In other words we are squeezing
them together. In order to squeeze the components together, we
apply a force which attempts to stretch the fastener. It actually
does stretch the fastener in most "nut & bolt" applications.
Thus, it is fastener tension that we are trying to achieve and
not a twisting load (torque) to the fastener. In fact we would
be better off if we could apply this force by applying a straight
tension to the fasteners without any torque at all. This is the
way in which rivets function. If we could avoid the need to turn
the fasteners to apply tension, we could use smaller fasteners
to apply the same loading and this in turn would allow for smaller,
we are torquing a fastener, the specified torque must include
the amount of expected friction as well as the amount of turning
effort that will become tension. Engineers calculate these relationships
and publish torque specifications based on certain assumptions.
They assume that the thread friction will be within a certain
range. If you lubricate when the specs specify dry threads or
if you use a lubricant other than that which is specified (usually
motor oil on vehicles), then you change the relationship of friction
and tension that result. It can be seen from this that if the
friction is reduced, the tension will go up and if the friction
is increased, the tension goes down. Since we require a certain
tension to hold things together, the results can be undesirable.
you know that industrial studies show that small fasteners tend
to be over tightened and large ones under tightened?
you look at tightening practices, you will find that there are
more accurate ways of achieving the right degree to tension (tightening)
than by use of a torque wrench. One method turns the fastener
until its head is in contact with the work surface and then turns
it a specified number of degrees of rotation. Since the angle
of the threads is known and the materials in use are known, the
correct clamping force due to fastener tension results.
method, called “torque turn” in heavy equipment has
been in use to my direct experience for more than 40 years and
is more accurate than the torque wrench method. So why don’t
we use torque turn? Answer: ? Modern automotive head bolts are
usually tightened by torquing the bolts to a given torque and
then use of torque turn (turning the bolts and additional number
of degrees of rotation that allows the bolts to be tensioned into
the beginnings of their yield range. It would be impractical to
use torque for this purpose because of the number of variables
involved. Rats, now I’ll have to find or rewrite the article
on loading of fasteners… (VBG)
While this tome is in process, another point:
When a nut is tightened with a bolt, the force acting on the first
tread is added to that of the second thread and so on. This applies
an increasing tension to the bolt area that is within the nut.
The result would be overloading of the first threads were it not
for the fact that (for any given grade) the nut is of softer material
than the bolt. This means that the tightening process deforms
the nut’s threads and when reaching specified tension (due
to use of specified torque), the threads will be deformed beyond
their elastic range and will therefore not return to their original
position. If the nut & bolt are retightened, the deformation
of the threads will increase the friction component leaving less
of the torque effort to be converted to tension. The result =
less tension. If you repeat this for a second time (third tightening)
or more, the tension that results will be very significantly less
than desired. If we put a new nut onto the bolt and torque, the
resulting tension will be in the normal range.
Moral - you must not re-use nuts in critical
locations. If you didn't know this before, I'll bet you don't
like to hear it now. Check it out, take a new nut and inspect
the threads. Torque it & remove inspect again --waadiditellya?
can check this out in another way if you have access to a hollow
centre hydraulic cylinder. Screw a pressure gauge into the cylinder
and bleed out as much air as possible. Put an adapter plate on
either side to allow a nut and bolt to bear against the piston
on one side and the body of the cylinder on the other. Tighten
the nut & bolt to the specified torque. Read the resulting
pressure (calculate bolt tension if you want). Loosen the nut
and re-torque to the same spec. The pressure is significantly
less isn't it? Repeat - the pressure resulting is really down
you’re doing this, read the next lines very carefully! Do
not, (repeat) DO NOT, tighten the fastener until it breaks! The
tension will result in stretch of the fastener and compression
of the metal of the nut and hydraulic cylinder. IN addition, it
will result in the compression of the hydraulic fluid (if you
were told that fluids don’t compress then you were lied
to, same with Santa, Easter Bunny and honest lawyer. Sorry! VBG)
The resulting storage of potential energy may result in a lethal
projection of the fastener ends when it breaks. Translation: the
two pieces may come out like bullets!
the icing on the cake, so to speak:
If there is insufficient clamping force to prevent leakage (i.e.
cylinder head or oil pump) you will have a leak. No big deal.
There's usually enough surplus clamping force built in to the
design. With newer designs this isn't always the case. Ford require
replacement of head bolts when reinstalling the cylinder head
on some engines due to the change in thread friction and the degree
of tension used.
occur when you have a reciprocating or varying load on a fastener.
If the load on the fastener is less than the tension on the fastener,
the load will cause the fastener to stretch. If the load varies,
the fastener with stretch and relax over and over. If the amount
of cycling of the fastener is sufficient, the fastener will "metal
fatigue" and break. General Motors did some testing with
under tightened connecting rod bolts and had breakage due to "fatigue”.
The more under tightened, the sooner they broke. Most often when
a con rod bolt breaks, the person doing the repair will state
that it was likely a faulty bolt or they may say that the bolt
was over tightened. Think about the physics of the loading- if
the bolt were over tightened it will break right away while under
tightened will break later due to gradual fatigue. I don't like
to reuse nuts when reinstalling critical items such as steering,
suspension or brakes. See why?
Some more points:
1) Nuts, bolts and cap screws (a capscrew is
called a bolt if it is used with a nut) are rated by Grade.
2) American and ISO (metric) grade systems both
use numbers but you had better not confuse a metric grade with
a US grade. (i.e. Metric Grade 8.8 is closer to a US grade 5 in
strength than it is to a US Grade 8)
3) Always use the same grade nut and bolt together.
4) If you mix nuts and bolts of different grades
you will have to go by the specs for the lower grade component.
5) Learn how to read nut and bolt grades and
keep a reference sheet around if you don't work with them a lot.
6) Make sure that you use hardened flat washers.
If you use a soft washer, particularly with Grade 8 (Metric 9.6)
or up, the washer may/will compress over time removing the clamping
force. This = loose bolt.
7) Don't use split lock washers (these are the
kind which are a ring with the ends of the split spread in opposite
directions. They work by biting into the nut or bolt head and
are not reliable with Grade US 5/ Metric Grade 8.8 (the minimum
you should use) or above, because they aren't able to bite into
the material. They have a tendency to break and fall out. Auto
Manufacturers stopped using them 30 years ago.
8) If you stop turning a fastener while torquing
in the top 20% of the torque range, back it off and start over.
This is because the torque (twisting force) necessary to start
it moving is a lot more than is required to keep it turning.
9) Use the lubrication (or none) specified.
10) Recheck your torque wrench every few years
and don't leave the tension on "click" type torque wrenches
because it will weaken the spring. How would you like to have
the head off of a bulldozer out in the bush several times due
to blown head gaskets and have me suggest your torque wrench might
be at fault? Yes.
11) Just because "and I haven't gotten killed
yet!" hasn't happened YET doesn't mean that you were right,
maybe just lucky. If you had gotten killed, we wouldn't be doing