If I tighten a bolt/screw to yield, can I use it again
or is it broken??
This question is on the minds of a lot of fastener engineers
and people who use fasteners all the time. Lets examine what
happens when you tighten a bolt into the yield area of a steel
material. First of all lets look at the fastener close up
A threaded fastener is usually a straightened piece of steel
wire of a given diameter and some length. It is then sent
through a sizing die and drawn to the final diameter and then
sent to a header that shapes and forms the head by cold heading.
Now we have a cylinder with a hex form on one end for driving
the finished fastener to tighten it. It next goes to a thread
rolling station where the threads are rolled on the opposite
end. It then goes to heat treat for hardening (or the bar
may have been hardened prior to thread rolling and the threads
are rolled on the hardened stock.) When the threads are rolled
they physically yield and upset the steel to the final thread
form to the measurements designed.
This method of making threads is known as cold forming and
makes for very strong threads. The resulting dimensions are
the finished outside diameter (OD) of the fastener, the pitch
diameter (PD) and the thread root diameter (RD). The OD is
self-explanatory, the PD is the diameter where the pitch of
the threads is measured and also the theoretical point of
contact with the mating threads of the nut or threaded hole.
The RD is the smallest diameter on the shank of a standard
bolt. (Fig.3 below)
OK lets now look at a thread on the bolt and a look at the
yield process. Unwrap a single thread form a bolt and you
end up with a right triangle. (Fig 2)The shortest side will
represent the pitch, the hypotenuse will roughly represent
the PD and the side adjacent to the pitch will roughly represent
the RD. Remembering that the RD is the smallest diameter on
the bolt it is logical to assume that the bolt will yield
in the RD (which it will).
If we mount the bolt in a fixture and pull the bolt in tension
until it yields you will see the yield stresses show on the
surface of the RD and the yield value will be some load number.
The yield point is defined as the point where the load / elongation
curve leaves the linear portion and enters the plastic range
(Hookes law - Fig 1). The load is noted to be some number
in lbf (pounds force). This point, an average, found after
several samples have been loaded to the yield point, is known
as the tensile yield strength of the fastener. If we then
use a fixture that measures the clampload during tightening
of the fastener with a nut or tightening into a block we will
find the yield load to be somewhat lower than the tensile
test. This load will be typically about 5-10% lower, depending
on several factors including friction. The main reason for
the difference is the addition of the shear forces induced
by the rotation of the bolt during tightening and the friction
in the joint. All this points to the fact that a bolt that
is tightened to the yield point does not develop the tensile
yield load. The load combination of torsion, creating shear
stress and the tensile load developed by stretching the bolt
will cause the bolt to yield in the thread root at a load
lesser than the tensile yield. (When I worked in the fastener
lab, this is one of the factors that never correlated and
my equipment was always blamed for the discrepancy even though
I had proven the calibration).
Now let us look at the thread root diameter and find out
how the yield stress is distributed. The stressed length (with
concern for yield) of the bolt is perceived as the length
of the bolt from the first fully formed thread to the first
fully engaged thread of the nut plus ½ thread due to
thread bending under load. (see Fig 4). Actually, because
the first yield occurs on the surface of the root diameter,
the length is much longer that the above-mentioned length.
Looking at the triangle of the unwrapped thread we can see
the length of the root diameter exposed is longer than the
pitch. So, in reality the yield length becomes the root diameter
circumference times the number of pitches from the first fully
formed thread to the first thread engaged by the nut. I know
that this explanation sounds like drivel, but consider a coil
spring being stretched out. Each coil is being stressed equally
therefore the spring can stretch much farther than a solid
bar with the same starting length. It is not quite the same
thing happening because the coils are free and the threads
are not but I think you can visualize the idea.
At the beginning I mentioned thread rolling after heat-treating
the bolt. This calls for special thread forming dies and results
in very strong threads due to the residual stress left due
to cold working. These types of rolled threads are stronger
than the regular rolled threads that are heat-treated after
rolling and are used for special applications that require
strength and smaller size. For example connecting rods in
engines. These fasteners will not usually fail in the threads
rather the shank or transition area of the first formed thread,
when tested in a tensile machine.
Another note to consider is the difference in shear stresses
induced by tightening the bolt from the head end as opposed
to tightening the nut. I expect this is where the variation
of installed yield strength comes from.
I ran some tests years ago and found that if I set up a test
where I would yield a bolt by driving the head and then turn
an additional 20 degrees I could find out whether or not the
bolt was reusable. I set up my equipment to automatically
tighten to some load that was the average of several tests
to yield and then turn an extra 20 degrees, I then loosened
the bolt and repeated the test again using the same bolt until
it failed. The average times of tightening the bolts were
29 times to failure. There fore I concluded that I could reuse
the same bolt at least 3 to 4 times before throwing it out.
Of course the decision to reuse the bolt depends a lot on
the type of joint it is used in, for example connecting rod
bolts are a safety problem so I would not reuse them more
than twice. Head screws or headbolts perhaps 2-3 times. Other
places where bolts are tightened to yield could be cam bearing
straps, oil pump drive gear fastener, flywheel to clutch plate
assembly and crankshaft to clutch assembly as well as the
vibration dampener on the crankshaft and the crankshaft bearing
cap to block bolts.
Home work assignment: Battery cable connector end bolts.
George Lorimer, RETIRED GM POWERTRAIN