CHOOSING LOAD CELLS FOR INDUSTRIAL WEIGHING
overall performance of any scale or weighing system can only be as good as:
1 The original design and build procedure.
2 The quality and performance of key components.
3 The installation and commissioning procedure.
4 On going maintenance and regular calibration.
majority of modern weighing systems rely on strain gauge load cells for the
conversion of weight or load change into usable electrical output.
the development of modern electronics has dramatically outpaced the changes
in load cell development it should not be forgotten that the overall performance
of any system is still dependent on the primary transducers providing stable
and accurate weight data.
electronics can certainly enhance system operation and performance but the basic
mechanical design of a weighing system together with the right choice of key
components is of paramount importance.
the selection of the correct load cell for a particular application is vital
and should be the first consideration when designing any weighing system.
how can design engineers be sure they make the right choice of load cell and
what are the key factors that should be considered?
this, once the correct choice of load cell has been made, what are the main
factors for consideration regarding fitting, commissioning and ongoing maintenance?
main points to consider are:
is wide choice of load cell types available and selecting which type of load
cell to use may, at first sight, seem a daunting task. However once the size,
type and mode of operation of the weighing system is determined then choosing
the type of load cell becomes very straightforward.
load cells operate in two basic modes.
weighing vessel (or similar) either sits on one or more load cells
-compression mode or hangs from one or more load cells -tension mode.
tension applications are relatively easy to set up and gravity ensures optimum
load introduction, weighing vessel design and safety considerations usually
limit the practical load cell capacity range, typically to 5 tonne and below.
the mode of operation has been decided, system capacity usually determines which
type of load cells should be used.
Overview of load cell types
(Individual Load Cells)
Load Cell Type
Ended Shear beam
Double Ended Shear beam
Ended Shear beam
Ended Shear beam
Bending beam load cells require particular care when mounting to ensure good load introduction and to prevent damage from side or non axial forces. Fully weld-sealed stainless steel low capacity beams down to 5kg are now available offering excellent solutions to low capacity weighing in harsh environments.
ended shear beams
ended shear beams provide the ideal solution for medium capacity weighing. Their
ability to withstand 100% side load without problems makes them particularly
suitable for weighing systems with mixers and agitators. However these units
become costly and cumbersome to mount above 5 tonnes.
Double ended shear beams
medium to high capacity applications, the double ended shear beam offers a number
of advantages over other designs. Units like the one shown opposite have built-in
jacking bolts which can be used to support the weighing structure during installation
thus allowing the live load cells to be fitted just prior to commissioning,
avoiding damage. The load cell is designed to rock on its mounting base to accommodate
misalignment in mounting surfaces whilst still being retained. The mounting
arrangement also permits limited movement to allow for thermal expansion and
contraction as well as lift off protection.
Canister load cells
canister load cells have stood the test of time and provide a compact and cost
effective solution for many high capacity weighing systems. Some compression
cells can be damaged by relatively small side loads and therefore it is advisable
to use proprietary mounting hardware to ensure correct load introduction and
protection from side loads. Canister load cells can be fully weld sealed and
stainless steel construction is now an industry standard.
Bending Ring Load Cells
Bending ring load cells are a relatively new concept in load cell design and provide an excellent low profile solution for a wide range of weighing applications. Unlike standard compression load cells, which can readily suffer from off-axis loading problems, bending ring load cells are loaded through a centrally located annular ring. By using a central floating pin to transmit load to this ring, optimum load introduction is assured and off-axis loads up to three or four degrees can be tolerated without loss in performance. Proprietary mounting assemblies, which offer excellent versatility, are now available for both process weighing applications and platform scales. The compact design of the bending ring readily lends itself to all stainless steel welded construction.
point load cells
originally designed for platform scales these versatile load cells are now being
used in a wide range of industrial applications. Improvements in sealing levels
and increased capacity allows these units to be used in the harshest of environments.
Single point load cells are designed so that a platform or similar can be fixed
directly to the load cell they provide accurate weighing wherever the load is
placed. This simplifies scale design and reduces cost. Both bending and shear
versions are now available.
load cells are now becoming more available for industrial weighing in a number
of formats. Although they offer certain distinct advantages over analogue devices,
their overall performance is still dependent on sound mechanical installation
procedures. Some of their advantages are:
complex subject needs careful consideration when designing a weighing system.
Load cell performance parameters can be split into three main groups, namely
those that are:
In practice, these individual parameters cannot be individually isolated and the ultimate system performance will be a function of a combination of these parameters together with other weighing system effects. It can therefore be very misleading to try to calculate weighing system performance from individual load cell data.
trying to establish expected performance criteria, the engineer should consider
how the system will operate and what the worst case operating conditions may
be - the widest temperature range, the smallest weight change required to be
measured, the worst environmental conditions (flood, tempest, seismic activity)
and the maximum overload conditions.
is also little sense in selecting load cells with the optimum performance if
the manner in which they are designed and fitted into the system is substandard.
is important to remember that the method of calibration will also determine
the optimum accuracy which can be achieved. The uncertainty of measurement of
the calibration method should be three times better than the required system
accuracy. For high capacity weighing systems this may limit the system accuracy
to 0.75% at best.
There are a number of recognised ways of calculating the required load cell capacity for particular application.
the load cells must be capable of supporting:
important point, often over looked, which has a major impact on overall system
performance is "load cell output per unit load change". It is vitally important
that the engineer fully understands this when designing any system.
is very straightforward to calculate the output per unit load change provided
the engineer understands how any weigh system will be operated. In other words,
consideration should be given to the minimum load change that has to be measured
and relate this back to load cell output and the ability of the electronics
to discriminate effectively changes of this magnitude.
key phrase here is 'minimum load change'. Often, for a number of practical reasons,
weighing systems have a significantly larger capacity then their actual operating
capacity. In these situations the load cells are chosen to accommodate the overall
capacity and are then required to provide weight data over a much smaller range.
As an example consider a 90 kg vessel supported on four 30 kg load cells which
is used to deliver batches of material of 10 kg anywhere over the 90 kg range.
If we assume the load cells have 2 mV/V rated output and are connected to electronics
supplying 10 volts, then :
Full scale system output (at 90 kg) =90x2x10/4x30 = 15 mV or 15000uV
Actual output over 10 kg range =10x15000/90 = 1667 uV
is the output that the electronics has to process and the resolution of the
weighing system must be related back to this figure.
the electronics has a minimum requirement of 1V per scale increment, then the
resolution that could be expected is 10x1000/1667= 6 gm
the scale increment must be a multiple of 10 (ie 1, 2 or 5, or decimals thereof)
and so the best working resolution in this example would be 10 gm giving a working
resolution of 1 part in 1000.
should be noted that this calculation is based on the minimum signal level required
by the electronics. Consideration must be given to the performance of the load
cells within the working conditions of the system to try to relate this resolution
to overall accuracy or uncertainty of measurement, Relating this figure of 10
gm back to the load cells themselves gives a required resolution of :
10/30x4000x4 or 1 part in 12,000
that we are talking about resolution here and this should not be confused with
load cell or system accuracy.
cells are usually specified as having a safe overload of 50% of rated capacity
This overload capability should be used as a safety valve, never as part of
the normal operating range.
there are no moving parts within a strain gauge load cell, fatigue can cause
failure and it is important to understand the limitations especially in applications
where high frequency operations or shock loading is expected.
can occur in the metallic element of the load cell, the bonding of the strain
gauge or in the materials of the strain gauge itself.
key to reliable and high performance weighing is to ensure optimum load introduction
even under adverse conditions. Load cells are designed and tested to measure
load through their primary axis. Any irregular loading which introduces off
axis forces as a result of poor mounting disciplines will almost certainly introduce
unwanted errors and can cause permanent mechanical damage.
way to minimize these effects is to use proprietary mounting hardware, designed
specifically for a particular load cell.
only lower capacity load cells have been available with complete mounting hardware
assemblies, relying on engineers to design their own fittings for high capacity
systems -a situation which often resulted in cumbersome arrangements and poor
weighing companies have recognised these problems and loading assemblies specifically
designed for higher capacity systems are now readily available.
units usually incorporate both side and lift-off restraints and are designed
to accommodate limited vessel movement resulting from thermal expansion and
contraction. In parallel they are designed to resist the effects of agitators
and mixers whilst still permitting accurate weighing.
applications where loading situations can exceed the rated capacity of the load
cells, overload protection should be designed into the system. Load cell deflections
are very small and therefore direct mechanical overload stops can be difficult
to set up with sufficient accuracy and can also be a cause of problems in dirty
alternative method is to use `relative motion' overload stops. Here a stiff
elastic material or special springs are used between the load cell and the weighing
system to provide additional deflection thus allowing easier setting up of the
stops. Note that this elastic material should be between the load cell(s) and
the weighing structure, not between the load cell(s) and the support structure
However, any problems arising from this additional deflection must be considered . As well as providing over load stops in the normal direction it is prudent to provide `lift off ' protection in certain applications especially on outdoor systems where wind can be an important factor.
is important to ensure that the load cells chosen for a particular application
will meet the required sealing levels to prevent premature failure. Consideration
should be given to:
IP ratings are used by load cell
manufacturers, such ratings do not fully define environmental compatibility.
Fully welded stainless steel load cells usually provide the best protection
but remember that the stainless steel used is not 316 and can corrode under
certain conditions especially if chlorine is present.
weighing systems are to be installed in designated Hazardous
Areas, then expert advice must be sought. Intrinsically safe load cells
are readily available which meet CENELEC requirements for use in hazardous areas.
If care is taken in the choice of the right load cell for a particular application, then you are well on the way to achieving a reliable and accurate weighing system. However correct installation and commissioning procedures are vital to ensure a high performance weighing system.