Solid Height - Spring Manufacturers Institute

A standoff, such as these two types, is used to safely and
easily change an element by directly connecting the new
one to the old one and threading them through the oven.
Many inline ovens have elements wired in series,
deltas or other configurations. This means a group of
elements is working together. More often than not, a
bad element will drag down the others directly connected
to it. Therefore, if you find a bad element, you
should suspect any other element with a direct connection
to it. Elements do not slowly deteriorate until
they go bad; they either “work” or “don’t work.” So,
although it is possible to find only one element that
has actually gone bad, be aware that other elements
connected to it will probably go bad soon, too.
Replacing an element is straightforward. Clamshell
ovens afford the opportunity to open the
chamber to assist threading the element through an
oven. However, there is a risk of touching an element
or other part of the heat chamber that is still hot.
The easiest and safest way to change an element is
with a standoff. A standoff directly connects the new
element to the old one and, as you push the new one
in, the old one will come out the other side. See the
above photo of two types of standoffs. After replacing
an element, make sure all the wire connections
are tight and the terminals are snug. Anything loose
or sticking out can cause arcing when electricity is
returned to the oven.
Thermocouples
The next item that should be considered is the
thermocouple. Various types of thermocouples exist,
and each inline oven company will use a style most
suited to its particular oven design. No matter the
style, you should know that typical heat-treatment
temperatures for springmakers range from 400°F
to 950°F, and J-type thermocouples are the fastest
reacting and most accurate type in this temperature
range. A thermocouple is basically two wires of different
materials attached at one end. It is at this tip
where the oven temperature reading takes place.
The temperature reading is the most important
function in an inline oven, and three things can
22 SPRINGS July 2006
happen to a thermocouple that can cause problems.
First, if the fused connection (where the two metal
wires are welded together) breaks, the thermocouple
cannot take a reading, and the thermocouple will
need to be replaced. Similarly, if either of the two
wires has any break or disconnection between the
thermocouple and the temperature controller, the
signal to the temperature controller will be lost.
Luckily, almost every temperature controller on the
market will display something if there is any sort of
discontinuity. For example, instead of displaying a
temperature, it may display “- - - -.” So, the tip of
the thermocouple is the first thing to check if the
display indicates discontinuity. If the tip is OK, trace
back the wire connections
all the way to the temperature
controller to find the
source of discontinuity.
The second common
issue related to a thermocouple
is the physical
displacement of the thermocouple
itself. The tip of
the thermocouple must
extend into the furnace
chamber, but sometimes
it can get pushed up into
the insulation, either by
an airborne object hitting
it or by a mistaken posi-
The fused (good) tip of a
thermocouple.
tioning when a thermocouple is replaced. If the tip
of a thermocouple is inside the insulation, the heat
from the elements will not reach the thermocouple.
As a result, the temperature controller will think
the oven is cooler than it should be, and will tell
the heating elements to go on in order to increase
temperature in the chamber. If this happens, the
chamber can easily reach higher than 1,000°F, while
the temperature display may only show 700°F. This
can cause a serious situation, such as burning out
the elements or starting a fire.
The third thermocouple issue is perhaps the
easiest to prevent but seldom checked until a severe
situation develops. This is when oils and other particles
build up on the tip of the thermocouple. All
day long, oily mists burn off the surface of the wire
being heat treated and, due to thermodynamics, a lot
of it ends up at the top of the heat chamber. This is
where most thermocouples are located. If a buildup
occurs, it is similar to having the thermocouple positioned
in the insulation. The temperature readings
will become sluggish, thus affecting the reactivity of
the oven. The oven will begin to operate erratically
and in a wider range of temperatures than desired.
Even if most of your applications use light oil or you
only run light springs through an oven, it is recommended
that you check thermocouples as part of