Overload Protection in Electronic Weighing Systems
by Albert E. Brendel
All weighing systems are designed
so that they exhibit maximum accuracies or resolutions over a defined
loading range. Typical resolving power
of these systems is I%, 0.1% or
0.01% of full scale range. For a high
accuracy system (i.e. O.Ol%), this
means that the full scale capacity of
the system is 10,000 times the
minimum resolvable weight that the
system can measure. For example, if
a system is capable of measuring 1
ounce, and is a accuracy system, the
full scale capacity will probably be
10,000 oz. (or 625 Ibs.). For a low
resolution system (i.e. l%), the full
scale capacity would only be 100 oz.
(or 6.25 Ibs.).
It’s interesting to note that both a
1% system and a 0.01% system are
capable of measuring a given weight
to the same degree of accuracy and
resolution. The real advantage of the
higher accuracy (and much more
costly) system is the range over which
it will make measurements to this
accuracy.
Most electronic weighing systems
utilizing load cells as the sensing
element have two things in common:
they have a structural element that
acts like a spring which deforms when
a force (or weight) is applied and
some form of sensing device that is
capable of measuring this deformation. The resolution of these systemns
is governed by how much deformation
the structural element can tolerate
and the sensitivity of the sensing
device used.
In all of these systems, the structural element is designed to be
compatible with the sensing element
used and normally is designed to
have the maximum possible deformation when the anticipated full scale
load is applied. The sensing element
is the primary part of the system that
determines the obtainable resolution
or accuracy of the system. The
problems associated with overload
Reproduced with permission from Rice Lake Weighing Systems.
oare invariably connected to the
structural element of the system.
Keeping in mind that the structural
element of the load cell deforms as
we apply weight, we can define foru
distinct regions of operation for the
typical weighing system:
The Normal Operating Range - The
structural element deforms proportionally and repeatability to the
applied weight and can be considered
to be perfectly elastic.
The Moderate or Allowable Overload Range - The structure still can be
considered elastic, but the deformation may not be exactly proportional to
the applied load. No detectable
damage will occur to the system from
occasional opoeration in this region.
This region can be further divided into
two sub-regions: the “Stated” and the
“Actual” allowable overload range. The
reason for this is that the upper
border of this region is difficult to
predict with accuracy and therefore
manufacturers apply a ‘factor of
safety” and derate (hopefully) their
stated range. The amount that
different manufacturers “derate” varies
tremendously even among similar
products. the factors that enter into
the derating are those of material
property variations, unknown loading
conditions, potential risk costs, and
unfortunately....sales appeal.
The Severe Overload Range - Here
the structure starts to exhibit signs of
permanent damage. After removal of
the overload, the structure might not
respond repeatedly to any applied
load. Minor excursions into this region
are difficult to detect, but normally are
associated with unexplained changes
in calibration or “zero shifts”. Loads
applied in the upper part of this region
usally show signs of physical damage.
It is sometimes helpful to get the
manufacturer’s estimate of the upper
border of this region, especially if
structural failure cannot be tolerated.
The Destructive Overload Range The structure FAILS. The primary
consideration here, is HOW it fails.
Compression failure is usually
considered “fail-safe” with the load
being automatically transferred to the
support structure of the weighing
system. Precautions should be taken,
however, to protect against outwardly
thrown shrapnel. Tensile failure allows
any suspended weight to fail, possibly
with damaging effects. Needless to
say, this is a region that one never
hopes to be in.
Of these four regions, it is obvious
that we never wish to exceed the
allowable overload range.
In order to protect the stsructural
element in an electronic weighing
system, steps must be taken to ensure
that the element is never loaded
beyond its allowable overload range. If
the anticipated maximum overload
falls within this region, there’s no
problem. However, if there’s any
possibility that a damaging overload
can be applied to a system, Murphy’s
Law will ensure that it will be. Therefore, some added protection for the
weighting system will be required.
The most straight-forward method of
overload protection is simple derating
of the system’s “Normal Operating
Range.” For example: assume that a
100 lb. scale on a production line is
used to weigh components with an
accuracy of l/10 of a pound (i.e. 1 .O%
accuracy). If the stated allowable
overload capacity of the system is
150% of full scale (i.e. 150 pounds),
you can be confident that a 200 pound
operator will use that scale as a seat
every lunch hour.
If the manufacturer has applied a
sufficient safety factor to his stated
overload range, the scale might
survive as long as the opoerator sat
down gently. If, however, a 200 pound
scale with the same allowable
overload factor was installed, the
operator could eat his lunch and have
a 100 pound secretary join him
without damaging the scale. If the
scale still had to weigh components
with an accuracy of l/10 of a pound,
however, the 200 pound scale would
require twice the accuracy of the
original (i.e. the new scale would
require an accuracy of 0.05%).
The next method of overload
protection is the “Mechanical Stop”
system. Since the structure of a
weighing system deforms as weight is
applied, a mechanical stop could be
installed that would be contacted at a
given force, thereby preventing further
force being carried by the structural
element of the scale. Figure 1 shows
a simple spring scale with this type of
protection.
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1
Figure 1
Figure 2
Although a spring scale was shown,
this type of overload protection is
often applied to “stiff” structural
elements as are found in load cells.
Assume that a load cell deflects ,005”
at its rated capacity and has an
allowable overload factor of 150%.
This would indicate that the load cell
could deflect .0075” before being
damaged. Therefore, if we could
arrange a mechanical stop that would
engage when the applied load caused
the load cell to deflect between ,005”
and .0075”, we could protect the load
cell from damaging overloads. Figure
2 shows this tvoe of overload svstem
designed around a common “proving
ring” structure as is often found in low
capacity load cell designs.
Although simple in concept,
“mechanical stop” overload systems
are often quite difficult to implement in
weighing systems that have very stiff
structural elements. In the above
example, the gap that is required
must be machined (or set) with great
accuracy. If the gap was less than
.005”, the mechanical stop would be
engaged before the full scale capacity
of the scale was achieved, thereby,
giving erroneous readings at high
loads. On the other hand, if the gap
was just slightly greater than .0075”,
the structure would be loaded into a
region possibly causing permanent
damage before the mechanical stop
was engaged. Even if the mechanical
stop system was machined correctly,
a dirt partical that has just the right
size will manage to lodge itself in the
carefully designed gap and lead to
premature gap engagement
(Murphy’s Law).
The normal method of designing an
effective mechanical stop system is to
first design the structural element to
have the largest possible deflection
that is consistent with the sensing
technique used. This might even
involve adding a high deflection
spring in series with the load cell just
to give added deflection to the
system. Figure 3 shows this type of
system.
The other design criterion for this
type of system is to protect the
system from dirt particles either by
protective covers or arranging for
physical inspection and periodic
cleaning of the gaps.
if the weighing system is designed
for high speed operation, the technique of softening the structure in
order to make the mechanical stop
system effective cannot be used. High
speed systems dictate greater
stiffness, not lower ones, and therefore, the mechanical stop system is
extremely difficult to apply with
success. We’ll examine “preloaded
spring” overload systems as a
modified mechanical stop system
which effectively gets us around this
problem.
We previously explored the
possible use of a “soft” spring in
series with a “stiff” load cell in order to
achieve enough deflection to enable a
mechanical overload stop to function
efficiently. We also determined that
the addition of the spring lowered the
natural frequency of the system and,
therefore, also reduced the speed of
the weighing system. For systems that
required high response rates, the
series spring approach for overload
protection is impractical...unless we
could somehow make the spring “stiff”
also, until an overload occurred. The
“preloaded” spring system does
precisely that.
LOAD CELL
db
9
TENSION GAP
COMPRESSION GAP
t
Figure 3
Figure 4
The simplest form of a preloaded
spring is a simple tension spring that
has been wound with a controlled
pretension. Figure 4 (above) shows a
plot of the deflection versus applied
force for this type of spring. For low
capacity tension load cells (i.e. below
about 100 Ibs.), this type of spring
can be readily added to the load cell
and provides tensile and side load
protection for the load cell. A sketch of
this system is shown in Figure 5
(below).
LOAD CELL
TENSION GAP
(PRETENSION LOAD CELL CAPACIN)
OVERLOAD GAP
Figure 5
A compression spring can also be
preloaded, but unlike the tension
spring, requires some additional
hardware to accomplish the task.
Figure 6 shows a simple compression
spring overload system, where the
preload is set by an internal nut.
One directional overload system
using preloaded springs are rather
easy to envision and implement. Twodirectional systems are more
complex, but are still quite practical.
With all of the advantages of the
preloaded spring overload systems,
how come more of them aren’t used?
Quite frankly, I don’t know. Maybe
because load cell manufacturers don’t
have to replace an obviously overloaded load cell under warranty.
Seriously, there are some drawbacks to the preloaded spring
systems. They tend to be limited to
the smaller capacity systems (under
10,000 Ibs.), they add some mechanical complexity to the weighing
system, and they also can have
strange effects on systems that are
sensitive to relative deflections of
components.
OVERLOAD GAP
COMPRESSION SPRING
OVERLOAD ADJUSTMENT
LOAD CELL
Figure 6
An example of the last problem
would be found in “steelyard” conversions using load cells. If in the
conversion, a preloaded spring
overload protection system was also
incorporated, every time that the
overload system was activated, it
would “reset” itself to a possibly
different overall assembly length. This
would result in an apparent “zero shift”
in the load cell. The key point is that
load cell applications such as the
steelyard conversion also requires
that the installed length of the system
be constant or else differing tare
weights will be suspended on the load
cell. In the preload spring system,
reset tolerances can be on the order
of plus/minus ,020 due to hysteresis
losses between coils of the spring or
“seating problems of the various
components. The test of the relative
suitability of a preloaded spring
system would be to determine the
effect of adding a variable and
possilby non-reatable length load cell
to the system.
For weighing systems that can
tolerate physical separation of the
load cell link, a simple shear pin
system might be a low cost method of
protecting a high price load cell.
Thomas Register lists over a dozen
suppliers under the category of Pins,
Shear.
All of our discussion of overload up
until now as dealt with those loads
that have been relatively slowly
applied.
Have you ever seen the act where a
man places a massive stone block on
his stomach and then has an accomplice break it in half with a sledge
hammer? Or have you ever tried to
drive a nail through a thin sheet of
plywood that didn’t have any support
behind it? If so, you have either
witnessed or observed at first hand,
the control or miss-control of shock
loading.
Shock loading is a significant factor
in determining whether or not a
weighing system will survive in the
environment in which it’s placed. The
very feature that makes electronic
weighing systems so attractive (their
high operating speed) makes them
especially prone to damage from
shock loading. In order to better
understnad the effects of shock
loading, let’s examine a fictional case
of shock load damage on a 50 pound
scale.
A ten pound box of nails is accidentally dropped from a height of 10
inches on a 50 pound (full scale)
counting scale. After the incident, the
scale has experienced a shift in zero
reading that can no longer be nulled
out. In order to determine what
happened, let’s slow the action down
and repeat the accident.
As the box of nails is dropped, it
begins to accelerate gathering
momentum. One quarter of a second
later, moving now at 5 miles per hour,
it contacts the top surface of the
scale. Since the scale is very stiff, the
box of nails must now come to zero
velocity in a very short distance
(typically .005”). This means that the
mass must undergo tremendous
deceleration, which is on the order of
160 times the acceleration due to
gravity. The forces generated by this
acceleration (or deceleration), could
build to 1600 pounds if nothing
happened to relieve the force.
However, the cardboard container
starts to crush and many of the nails
inside the container start to shift,
which absorbs some of the energy of
the fall. Since the box starts to crush,
the effective distance over which the
weight is accelerated also increases,
which further reduces the acceleration forces. While all of this is
occurring, the upper surface of the
scale, which has a certain amount of
mass, produces a resisting force,
which is directly proportional to the
amount of mass it contains, thereby
tending to protect the load cell that is
mounted directly below. However, the
mass of this platform has been
purposely minimized in order to
reduce the tare load on the load cell
and enhance its high speed performance. This is one reason why large
truck scales with their huge platform
masses are not too susceptible to
shock loads; the mass of the platform
tends to absorb them.
The shock load force is now
attenuated by the container crushing,
the nails shifting, and the resisting
force of the upper platform to the load
cell. The two surfaces held together
by the bolts in this connection are
stable because of the friction that
exists between them. In the original
process of tightening, these surfaces
were preloaded in some manner and
the surfaces deformed, which was
then maintained by the frictional
forces. The shock load force entering
the bolted connection can now serve
to change the original amount of
energy stored in the connection.
If the load cell is properly designed,
the conditions of bolting should have
only minor effects on the operating
zero point of the load cell. However,
because of space limitations, the load
cell was designed in such a manner
so as to utilize the structural stiffness
of the attached members, which is all
right provided the attached members
remain attached in the same manner
at all times. Although hard to visualize, joint shifting is a prime cause of
unexplainable zero shifting in a scale
system that is subjected to shock
loading conditions.
After having passed the bolted
connections, the force now enters the
highly stressed member of the load
cell that is used to measure the load.
If the force has not been sufficiently
attenuated by this time, the stresses
may be high enough to yield or break
the load cell. If the load cell does
yield, further deflection is added to the
system, which in turn serves to
reduce the acceleration forces.
After passing the load cell, the force
again passes through another bolted
connection, causing the same
problems as were previously mentioned, and arrives at the base
structure of the scale. If the base is
massive, it reacts to the applied force
by resisting acceleration, or if light
weight tends to pass the shock force
on through the rubber mounting feet,
further tend to attenuate the shock
wave to the point that the table upon
which the scale is placed, carries little
more than the additional IO lb. load
that would exist if the nails had been
applied normally to the scale. In fact,
many times, the scale can be
assumed to be the protection device
for the table upon which it sets.
From this fictional story of a shock
loading incident, some real insights
should be evident. If we can somehow
increase the distances over which the
suddenly applied load is stopped, the
forces produced by this deceleration
are greatly reduced. The bumpers on
the newer model cars are prime
examples of this design concept.
Another possibility for minimizing the
effects of shock loading is in the
careful management of the internal
masses of the scale itself. For
example, a preloaded spring overload
system can be effective for shock
loads as long as the load cell does
not have to contend with large inertia
loads. The proper use of elastomer
mounts also can be an effective
method of controlling shock loads
again by adding to the effective
distance over which the load is
decelerated and also by converting
some of the energy to heat in the
elastomer itself.
Earlier we dropped things on scales
and attempted to slow down the
action so we could visualize how the
force was channeled through the
structure, with the shock or impulse
being absorbed or modified by means
of either the relative masses of the
scale’s components or generated as
heat in elastomer (rubber like)
elements.
It turns out that for effective control
over shock loads, the scale designer
must pay attention to the masses of
the various components of the scale
and their relative positions in the force
path of the scale. Two types of spring
preload overload protection systems
are shown in Figure 7 (below).
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7 (4
While both of the designs shown
will protect against normally applied
overloads, the system shown in
Figure 7 (B) will also offer relatively
good protection for shock loads, while
7 (A) will not.
The reason is that a shock force
applied to the input platform is
reacted by the acceleration forces
produced by the upper platform, the
housing of the laod cell, and the lower
platform which is spring loaded
against internal “stops” in the housing
of the overload protector. In this
system, only the inertial force of the
upper platform helps absorb some of
the shock load, while the inertia
forces generated by the load cell
housing and the lower platform serve
only to prevent the applied shock load
from reaching the spring, which is
supposed to collapse and protect the
load cell. Since the inertia of the load
cell housing and lower platform react
to the applied shock load, the force
which is the very thing that the
overload protector was supposed to
protect in the first place.
The dseign in Figure 7 (B) on the
other hand was basically only one
moving part under shock loading
conditions...the upper platform,
whose inertia tends to react to the
applied force in a helpful manner to
begin with.
Figure 8 shows two analogous
overload protectors for tensile shock
load protection. Again, Figure 8 (A) is
poor for shock loads and Figure 8 (B)
is all right.
I
8 (4
Figure 8
8 (B)
There are virtually unlimited ways to
design efficient spring pre-load
overload systems, working in either
tension, compression, or both. The
one inherent feature in all designs
with regard to their suitability to shock
loading can be recognized by
examining the location of the overload
system with respect to the housing of
the load cell. In general, shock
protection is only afforded by those
designs where the base of the load
cell is directly connected to the frame
of the machine or in some way is
prevented from accelerating and
producing damaging reaction forces.
Shock loads normally don’t stop
having damaging effects after simply
being channeled through overload
stops. Consideration should also be
given to the shock produced as a load
is suddenly removed from a scale. In
these cases, the mass of the upper
platform which helped in the normal
overload situation, suddenly becomes
a moving mass which can apply
damaging forces to a load cell if not
considered. To guard against this
condition, the best approach would
seem to be to keep the upper platform
as light as possible, to reduce the
return force (after an overload has
been removed),
Prevention of the shock load from
damaging the surfaces of the overload gaps themselves, or attenuating
the shock load as it by-passes the
load cell is probably best handled by
the judicious use of elastomer
elements, either in the overload gaps
themselves, or somewhere else in
series with the by-pass force path.