The Trouble with Torque in Electrical Connections
Torque and force are not the same
Norman Shackman, P.E. http://earth.prohosting.com/elecon/
Posted 08-20-03
The secret to making and keeping reliable electrical connections
is contained in two elements: start with clean contact surfaces,
and apply high force.
Clean contact surfaces are a function of cleaning procedures,
including joint compounds, and will be covered in a future
article. Application of high force is the subject here.
The trouble comes about because the terms "torque" and "force" are
incorrectly used interchangeably. Force is NOT torque. Force
is a function of torque. The expression which describes the
relationship is
F = T/K
Note that the equation has a variable, K, that includes the
coefficient of friction. The higher the friction, the lower
the force for the same torque. Torque is a convenient way to
get at force and is usually specified in making an electrical
connection. Force is considered inconvenient to measure.
Torque can be misleading
Consider the following. Suppose you are given a torque value
for an electrical connection and suppose that the connection
is frozen due to corrosion, arcing, etc. Obviously, the recommended
torque will not assure a good connection. Thus, relying on
torque to judge the quality of an electrical connection can
be misleading.
Levels in uncertainty in the accompanying section, "Force
Variations by Methods of Tightening Connections," are
taken from mechanical engineering sources and represent a
rough estimate of the percent variation encountered when
trying to tighten a connection using different methods.
You can see there is a wide variation in accuracy depending
upon the method and that many of them are fairly inaccurate.
In fact, when considering life safety, torque values are rarely
mentioned.
What is the correct force? When a connection is tightened,
the joint electrical resistance drops as the force increases,
up to a certain point. Beyond that certain amount of force,
a marked decrease in resistance no longer occurs; the resistance
remains fairly constant even with increased force. That certain
amount of force is the minimum value of force needed.
In bolted connections, I have found that the forces associated
with SAE Grade 5 hardware produce this correct value.
Applying proper force
To assure you are applying the proper force in a connection,
there are a few methods which can be utilized:
- Low and consistent K factor by the use of lubrication.
You can produce repeatable, high forces in the connection.
To safely use lubricants, run tests in the shop before applying
on the job.
A well-lubricated fastener is stressed to a higher force
for the same torque than an unlubricated one. Check that
the fastener does not fracture at the higher force. Having
conducted tests, then apply the selected torque to the lubricated
threads.
- Belleville washers. These are not always required in electrical
connections and are often questionable. The washer must flatten
at the proper force and many applications do not use a high
enough force. In addition, since the bow in the washer is
difficult to see, Bellevilles are sometimes installed upside
down. If a proper high force is utilized in the connection,
I have found that a Belleville is usually not necessary.
But a Belleville is an excellent force indicator and therefore
can solve the force/torque dilemma. If you choose a Belleville
that flattens at the desired force, you then can proceed
with implementing the connections and not worry about a torque
value.
- Direct tension indicators. As mentioned previously, in
mechanical connections where life safety is a subject of
concern (e.g., buildings, bridges, etc.), torque is not mentioned.
Instead measurement of force is required.
A common procedure is the use of direct tension indicators.
These are washer-like devices that feature protrusions (bumps)
which flatten as a function of force applied to the connection.
A feeler gauge is used to announce when the proper force is
reached. Later inspection is simple through the use of a feeler
gauge. Since the indicators are designed for use with steel,
make sure the bumps are put against a hardened steel washer,
not a copper or aluminum bus.
These devises are available for 1/2 in. hardware and larger.
It is possible to use the 1/2 in. indicator with smaller hardware
by requesting the force/gap characteristics from the manufacturer
and then selecting the proper feeler gauge for the desired
force. Make sure the gauge is narrow enough to fit between
the bumps.
Norman Shackman, P.E., is based in Kent. CT. He conducts
in-house seminars on electrical connections and can be reached
at (860) 927-4067.
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