loadcell
The types of strain gauge loadcells commonly used in platform scales includesingle point, planar beam, bending beam, shear beam, dual shear beam, and Stype
loadcells.
Resistance Strain Guage Loadcells
Figure shows the inside of a column-type loadcell. The positions of the foil type active
and passive strain gauges and their connections can clearly be discerned. These gauges
together with their trimming and temperature compensating resistors are wired into a
Wheatstone bridge network. The column and base are machined from a one piece
forging of special heat-treated high tensile steel.
Since that time, however, strain gauges have proliferated both as mechanical scale components and in
stand-alone load cells. Today, except for specific laboratories where precision mechanical balances are still
used, strain gauge load cells dominate the weighing industry.
Resistance Strain Guage Loadcells
The loadcell is completed by a cast iron case which encloses the gauged portion, being sealed by O-rings at
the top using the machined grooves. The case also carries the output cable socket.
The typical cell having a nominal capacity of 20 tons is about 3.5 inches high including the cap and suffers a
compression of about 0.006 inch at full load to give an output of some 18mV with a standard input of 10V.
An 800 tons cell would be about 17.5 inches high and would compress about 0.025 inch for a similar output.
Combined errors are claimed not to exceed + or- 0.1%.
The problems of non-axial loading almost disappear if the loadcell can be used
in tension instead of compression. In many cases this is not convenient, but
when it is, loadcells of the kind illustrated in fig. are available for the purpose.
Unit- Weight disposition
A simple spring balance or lever type self-indicating Weighing scale is self-constrained
as regards its energy requirements. The disposition of the load provides the energy to
move the indicator by an amount proportional to the deflection of the lever work or
the extension of the spring.Non – self-indicating and semi – self-indicating scales have the
additional energy requirement of the disposition of weights or
counter poises, usually a manual operation, but one which can be
and has been mechanized.
Automatic load balancing, in full or part, by the dispositioning of
weights and counter poises by electrical means is a relatively later
development.
Unit- Weight disposition - Industrial weighers
Figure shows the back of the main cabinet and the master dial. The Unit-weight handling
forks can be seen to be actuated by a sector gear which is locked by a pawl entering an
attached subsidiary square notched sector; the pawl is operated by means of a solenoid.
At a signal from the slave’s notched disk, the solenoid operates, lifting the pawl and closing twin microswitches by means of an attached lever to start the electric motor turning. Drive
is by way of a gearbox and an electro magnetic clutch to a shaft which carries a worm-gear
engaged with the sector gear already mentioned. Drive continues until the unit weights
thereby positioned bring the pointer on to the chart, when the microswitch in the slave unit opens to drop the pawl and stop the motor.
Precision Balances
Until the commencement of series
research in the middle of the 19th
century, accurate weighing had to be done on the conventional swan neckor box end beam scales.In the University of Edinburgh,
in 1755 Dr.Black used precision balance to conduct experiments which
laid the foundation for modern quantitative chemistry. Figure shows
the balance with the glass case removed. The beam is light and well
designed. The bearings for the end knife-edges are planes delicately
balanced and a relieving device raises the central knife edge from its
bearings.
The triple beam balance is an instrument used to measure mass very precisely.Such devices typically have a reading error of ±0.05 grams. Its name refers to its three beams, where the middle beam is the largest, the far beam of medium size and the front beam the smallest. The difference in size of the beams indicates the difference in weights and reading scale that each beam measures. Typically, the reading scale of the middle beam reads in 100 gram increments, the far beam in 10 gram increments, and the front beam can read from 0 to 10 grams.The triple beam balance can be used to measure mass directly from the objects, find mass by difference for liquid and measure out substances.
The triple beam balance is an instrument used to measure mass very precisely.Such devices typically have a reading error of ±0.05 grams. Its name refers to its three beams, where the middle beam is the largest, the far beam of medium size and the front beam the smallest. The difference in size of the beams indicates the difference in weights and reading scale that each beam measures. Typically, the reading scale of the middle beam reads in 100 gram increments, the far beam in 10 gram increments, and the front beam can read from 0 to 10 grams.The triple beam balance can be used to measure mass directly from the objects, find mass by difference for liquid and measure out substances.
How does Electronic Weighing Balance work?
As a means of measuring force, the electromagnetic
balance method utilizes the electromagnetic force
generated from a magnet and coil, whereas the electrical
resistance wire method (load cell type) utilizes the change
in resistance value of a strain gauge attached to a piece of
metal that bends in response to a force.
Comparison between Electromagnetic Type and Load cell Type
Self– indicating Pricing Scales
In the previous weighing news, the necessity for the weighing and pricing scales and the
invention of steelyard type pricing scale were described. The steelyard type pricing scale was non automatic and was therefore slow in use.
Self– indicating Pricing Scales:
It was soon followed by the Dayton automatic computing Scale with suspended pan.
The swinging pan was replaced by a goods fitting on a Roberval
system. In 1907 Henry Pooley & Son Limited, by arrangement with
the Dayton Computing Scale Company,
successfully submitted to the British
Board of Trade, a scale of the
type illustrated in the figure.
One of the chief modifications to this type was the introduction
of a pendulum resistant unit in place of springs, by the Toledo
Scale Company of U.S.A. Figure shows the first scale of
pendulum type brought to England, sold through the agency of
W.& T. Avery Limited.
These cylinder type of Scales usually had a capacity of
twenty pounds, with a minor division on the chart of half –
an – ounce. Since the Board of Trade Regulation required,
on verification, an accuracy of a subdivision, the Scale
should be accurate to 1 in 1280. Great skill and extreme
accuracy in manufacturing made it possible for the
commercial production of reliable scales to this standard.
Industrial Self – indicating Scales
Many ideas for heavy capacity scales of the self-indicating type were tried and several of these
became obsolete.
In the beautifully illustrated book “A Short History of Weighing” written by L.Sanders and published in 1947 by history employers W. & T. Avery Ltd., there is a reproduced page from a weighing machine catalogue issued by Henry & Son Ltd., in 1859 showing a spring dial platform scale in use in railway parcels office. Mr. Sanders said – and no one could speak with greater authority – that this was probably the first self-indicating platform scale used for industrial purposes. Henry Pooley constructed Hydrostatic balances as early as 1874 and some remained in use for many years.
In the beautifully illustrated book “A Short History of Weighing” written by L.Sanders and published in 1947 by history employers W. & T. Avery Ltd., there is a reproduced page from a weighing machine catalogue issued by Henry & Son Ltd., in 1859 showing a spring dial platform scale in use in railway parcels office. Mr. Sanders said – and no one could speak with greater authority – that this was probably the first self-indicating platform scale used for industrial purposes. Henry Pooley constructed Hydrostatic balances as early as 1874 and some remained in use for many years.
In 1907 Henry Pooley& Son Limited, introduced
their “quadrant indicator” a massively constructed
pendulous lever, carrying at one end a quadrant
with a specially designed chart having an inclined
vernier scale enabling small subdivisions of weight
to be read. In the indicator shown in the figure, the
chart and window are hidden behind uprights.
An important invention, known as the Aerostat, was made by Mr. Stickig at the beginning of
the 20th century. This mechanism operated on a principle which formed the basis of many
dial weighing machines.
The Aerostat mechanism was applied to platform Scales and weighbridges by W. & T. Avery Limited, in 1906.
The Aerostat mechanism was applied to platform Scales and weighbridges by W. & T. Avery Limited, in 1906.
In 1924, Avery introduced a semi – self-indicating platform scale
which was marked with very small subdivisions of weight. Figure
shows one variation of this model, the invention of C.Mc.G.Sykes.
Essentially there was a steelyard with sliding poise for balancing
the major units of load and a subsidiary pendulous resistant unit
which gave automatic indication of the minor divisions.
Semi self – indicating counter scales
Disadvantage of the spring scales is that with repeated use, the spring in the scale can be permanently stretched, which tends to bring less accuracy.
The cam was introduced around 1906 to enable chart graduations of equal width to be obtained. As the pendulum moves outwards, under the influence of the load, to a position of equilibrium, the indicator traverses the chart and indicates the correct weight. Figure shows a semi – self-indicating scale of cam and pendulum type with Beranger lever system.
These are based on Levers, Knife-edges & Bearing System for durability, sensitivity and consistent accuracy. The lever based Self and Semi-self indicating Counter Scales are suitable for quick and accurate weighing in retail shops and in numerous fields of industries.
The cam was introduced around 1906 to enable chart graduations of equal width to be obtained. As the pendulum moves outwards, under the influence of the load, to a position of equilibrium, the indicator traverses the chart and indicates the correct weight. Figure shows a semi – self-indicating scale of cam and pendulum type with Beranger lever system.
These are based on Levers, Knife-edges & Bearing System for durability, sensitivity and consistent accuracy. The lever based Self and Semi-self indicating Counter Scales are suitable for quick and accurate weighing in retail shops and in numerous fields of industries.
S e m i – s e l f – i n d i c a t i n g c o u n t e r s c a l e s were designed to prevent the use of weights of small
denomination when weighing goods in varying quantities to facilitate repetitive manual
weight of a particular loading, precisely or within tolerance.
Assemblies comprise one of the conventional counter scales of Roberval, inverted Roberval,
Beranger or Phanzeder pattern associated with a variable resistant of the cam or eccentric
pendulum, tension spring or torsion strip type.
We have already seen that Roberval principle has formed the basis of many types of counter
scale and for many applications, it was the most convenient form of lever system.
In one of its variations the
stays were arranged above
the beam,
known as Imperial Scale.
In the middle of the 19th century, Joseph Beranger,
a French scale maker, invented a system of levers
by his name. This design was very reliable and
accurate. The Beranger design permitted several
variations and in which one the Phanzeder became
popular in the continent of Europe.
Self-indicating counter scales
Self-indicating weighing machine means a weighing instrument in
which the whole or part of the weight of the goods being weighed is
indicated by a pointer moving over a scale or chart graduated in
units of mass, or a graduated chart moving in relation to a fixed
pointer, or a digital display, or by means of a printed record which
avoids Physical or mental effort in weighing and pricing of goods.
The idea of making the scales to indicate value of the goods, as well as weight, occupied the
minds of inventors in the 19th century.
One of the earliest of the price indicating scales to be made on a
commercial basis was the Stimpson steelyard computing scale,
which made its appearance in 1897, mostly in American market.
The figure shows such a scale, the steelyard carries a flat
rectangular chart graduated in units of weight and with price
computations.
It was soon followed by the Dayton automatic computing scale with suspended pan.
It was soon followed by the Dayton automatic computing scale with suspended pan.
Hydrostatic balances
Leonardo da Vinci’s, universal genius of the 15th
century notebooks contain sketches and a description
of two self – indicating weighing mechanisms, one
being semi – circular in form, the other triangular.
The models were constructed by the research department of W.&T. Avery Ltd working from the sketches from Leonardo ‘s notebooks, preserved in Paris.
The models were constructed by the research department of W.&T. Avery Ltd working from the sketches from Leonardo ‘s notebooks, preserved in Paris.
Although Leonardo da Vinci’s sketches of these mechanisms were essentially practical in
conception, no attempt seems to have been made to exploit the invention until 9th century.
A simple pendulum balance patented in 1863 consisted of a
pendulum made from flat sheet brass and suspended from shackle
as shown in the figure. The load was applied by a cord and ring.
The circular pendulum weight formed a graduated scale at the
centre. When a load was suspended from the instrument, the
pendulum swung outwards to a position of balance and the pointer
indicated the correct weight. These little balances were made for
weighing letters and postal packages.
In chronological order, Richard Salter of
George Salter & Co.Ltd.,West Bromwich is
known to have been made a barrel spring
balance in the year 1770. At first, a
compression spring was used and later spring
balances incorporated tension springs.
Augustus Siebel invented the bow spring balances in 19th century and
subsequently the elliptical spring balances in partnership with
H.Marriott.
The latter form of resistant and indicator used in platform scale for many years and supplied to Railway companies by Henry Pooley.
Hydrostatic balances were constructed as early as 1874 by Henry Pooley and some remained in use for many years.
The latter form of resistant and indicator used in platform scale for many years and supplied to Railway companies by Henry Pooley.
Hydrostatic balances were constructed as early as 1874 by Henry Pooley and some remained in use for many years.
The indicator consisted of a beam or lever, one end, which
was connected to a conventional load carrying system of a
platform scale or weighbridge as in the figure. From the
other end, a counter poise was suspended dipping in a tank
of water.
When a load was applied on the platform scale, the
counter poise was raised out of the water, losing part of its
buoyancy until a position of balance was found
automatically. In one version of the scale, an indicator was
driven from motion of the lever by means of a chain and
drum and the weight was thus read on a graduated chart.
COMPOUND LEVER BALANCES
Thaddeus and Erastus Fairbanks’ agents took out a British patent for a compound lever scale in 1833.
However, it was declared invalid when English scale makers proved that the invention had been in use
for a number of years.
In the year 1730, in the Birmingham street called Digbeth, there lived one James Ford who made steelyards. All his work was done by hand; machinery was at the time unknown in this trade. In this humble domestic workshop, the business then conducted by W.&T.Avery Limited began.
In the year 1730, in the Birmingham street called Digbeth, there lived one James Ford who made steelyards. All his work was done by hand; machinery was at the time unknown in this trade. In this humble domestic workshop, the business then conducted by W.&T.Avery Limited began.
Ford died in 1761 and the business passed through the hands
of James several family connections until it came under the
control of William and Thomas Avery in 1818. In their
hands, it began to grow from a purely local concern trading
with Midland people and a few travelling merchants into a
nation wide and finally world-wide organization. New types
of scales were made and new markets found. In 1894, the
business was formed into a Public Limited Liability
Company. The Avery business in India was originally
conducted through London export houses and in 1947, this
organization which had been operating as branch of W.&T.
Avery Limited was established as a separate Indian company
under the title of The Avery Company Limited.
Platform scales were originally fitted with loose – weight steelyards and
with these, the major portion of the load is balanced by loose proportional
weights and a small sliding poise serves to balance and indicate the odd
pounds and ounces.
No-loose weight steelyards were introduced later and fitted to platform scales and weigh bridges. In these, a major poise balances the greater part of the load such as tons and the smaller sliding poise serves to balance the remaining smaller portion of the load.
No-loose weight steelyards were introduced later and fitted to platform scales and weigh bridges. In these, a major poise balances the greater part of the load such as tons and the smaller sliding poise serves to balance the remaining smaller portion of the load.
COMPOUND LEVER BALANCES
A combination of Roberval system and steelyard was used frequently for
commercial weighing. Basic structure of two such weighing machines is shown
in figures.
In the first figure, equal arm beams are employed and in the second, unequal
arms are used to obtain mechanical advantage.
The systems used for the scales, so far mentioned, is for the non – self
indicating scales.
Non-self – indicating weighing Scales are weighing instruments for which the
equilibrium position is obtained totally by the operation of the user.
The progression of the weighing scales obviously didn’t stop with the non – self indicating scales.
SELF-INDICATING SCALE
A self-indicating scale is capable of balancing a load automatically and giving an indication of its
weight, without the operator having to manipulate weights, proportional weights or sliding poise.
These scales, in many forms and now in digital display, are indispensable to modern commerce.
The first records of the invention
of the self-indicating scale were
made by the universal genius of the
15th century,Leonardo da Vinci,
whose paintings ” The Last Supper ”
and ” Mono Lisa” are world famous.
His notebooks which were compiled
over a period of forty years,
contain sketches and description of
two self-indicating weighing mechanisms,
similar in principle, one being semi-circular
form and the other triangular.
COMPOUND LEVER
Hitherto, as we have seen in our last issue, the use of the Beam and counter scale (single lever) all of
which incorporate an equal armed lever or beam, yield no mechanical advantage, loads counter balance
by weights. Also the Bismar and Roman steelyard can weigh substantial loads by means of relatively
small weights (resistant).
COMPOUND LEVER
A system (as in various weighing scales) of two or more levers arranged to transmit motion or force by linking an arm of each lever to an arm of the next lever in the train.
Although the discovery of the principle of lever as a means of weighing is
attributed to Archimedes (287-212 BC), it is a fact that common steelyard was
in use throughout the Orient and India many centuries before him which we
have seen in detail under the heading Bismar and Roman steelyards. But it is
believed only Archimedes defined the principle of lever mathematically.
COMPOUND -LEVER SCALES
In compound lever scales (platform scales) more
levers are linked, each affording a mechanical
advantage providing a more compact assembly
by suitable arrangement of the levers. The
Roman principle also forms the basis of
steelyards fitted to platform scales.
The Turnpike Act of 1741 authorized road trustees to erect at toll gates “any crane, machine or
engine, which they shall judge proper for the weighing carts wagons or other carriages” and charged
them to levy toll according to weight and to apply revenue to the repair of roads.
Although this Act increased the number of cart steelyards, the inconveniences of weighing by them spurred inventors to devise some less laborious and expeditious manner of weighing.
Although this Act increased the number of cart steelyards, the inconveniences of weighing by them spurred inventors to devise some less laborious and expeditious manner of weighing.
It has been claimed by Eayre and Yeomans that they invented
the platform scale, but John Wyatt constructed the first
compound lever platform scale around 1741 and his first
weigh bridge was erected at the Birmingham Work house.
Wyatt’s genius and the enormous value of his invention were
not recognized in his time.
The only scale in use in the United States at that time were the even balance and the Roman steelyard.
Weighing heavy and bulky products on the steelyard was a slow and laborious operation. When an
enthusiasm for growing hemp swept over Vermont, the problem became acute.
In 1824, Erastus joined Thaddeus’ lucrative business. The two brothers formed
the E & T Fairbanks Company in St. Johnsbury, Vermont. Once in business
together, the two brothers realized that the current weighing system yielded
inaccurate results. So, Thaddeus decided to invent a new,
more dependable weighing scale. Innovation found in the form of
Levers. Now his scale was not only accurate, but very stable. In
1830, Thaddeus built his first real scale and applied for a patent.
The new scale was a hit. There after the Fairbanks brothers
manufactured many different kinds of scales, including delicate
instruments for chemists and jewelers as well as huge platform
scales for railroads and canal weigh locks.
- Their agents took out a British patent in 1833. The later, however, was declared invalid when English scale makers proved that the invention had been in use for a number of years.
- It is only fair to add that the introduction of Fairbanks’designs led to great improvements in the manufacture of weighbridges and platform scales throughout the world.
Platform scale may be portable or dormant. Platform
scales include those of low capacity, frequently
referred to as bench platform scales, which can be
carried from one position to another, and those which
are moveable.
Dormant platform scales are essentially immobile, as their name implies and, in many features of construction, they resemble weighbridges. The name weighbridges was applied to relatively high capacity dormant platform scales used for weighing wheeled traffic i.e. road vehicles and rail trucks.
Dormant platform scales are essentially immobile, as their name implies and, in many features of construction, they resemble weighbridges. The name weighbridges was applied to relatively high capacity dormant platform scales used for weighing wheeled traffic i.e. road vehicles and rail trucks.
Weighbridges are used all over the world, both small and large companies,
as well as transport companies, weigh trucks that transport all types of
cargo, from food products to metals, as well as durable consumer goods or sold.
COUNTER SCALES
Crude weighing beams having a nail for centre pivot and wire rings passing through holes near
ends of the suspension of strings carrying wooden pans or platters were used in farm houses for
weighing butter up to the end of the 19th century.
The needs of the alchemists and the early assayers for accuracy in the work probably encouraged the inventors during the 16th and 17th centuries and finally the true knife edge emerged.
The needs of the alchemists and the early assayers for accuracy in the work probably encouraged the inventors during the 16th and 17th centuries and finally the true knife edge emerged.
Figure shows stages in the evolution of knife edge, and of the methods of forming or fixing it in the varioustypes of beam ends, such as the swan -neck, the box end, and the dutch end.
COUNTER SCALES
In 1669 Gilles de Roberval invented what is known as the “ Static Enigma”
which puzzled the greatest mathematicians of the day, although an average
engineering student of the 20th century would have no difficulty in explaining
mechanics of device. In this the beam, legs and stay forming th system make
perfect parallelograms.
Not until the beginning of the nineteenth century did the scale makers
apply this principle to the balance , but since then the Roberval enigma
has formed the basis of many counter scales
The beam scale, counter scale all of which incorporate an equal armed
lever or beam, yield no mechanical advantage, loads counter balance
by weights.
BISMAR STEELYARD
Bismer balances were first developed either in India where
they appear in writings and art,or around the Mediterranean
where they subsequently made their way to India.
The Bismar is one of the simplest and yet least known of all weighing instruments. It is really a counter- weighted lever used to balance the load to be weighed, and an idea of how the device was used may be obtained from the figure.
The Bismar is one of the simplest and yet least known of all weighing instruments. It is really a counter- weighted lever used to balance the load to be weighed, and an idea of how the device was used may be obtained from the figure.
The loose loop of cord forming the fulcrum is moved along the lever until the position of the
balance is reached. In old bismars, nail heads served to mark the graduation.
Both the place and time of origin of the Bismar are obscure and its distribution was very
wide. Among the countries where it has been used are India, Burma, Malaysia, Denmark and
Scandinavia. The Danish invaders introduced it into Britain in the 8th century, and it was in
use for the trade purposes until made illegal in the reign of Edward lll.
Like the crude forms of the balance, the Bismar has remained in use throughout the centuries,
The principle of using a sliding poise on the arms of a beam to
counter-balance a load was probably discovered by the Greek-speaking
people of Campania, and led to the evolution of the Roman Steelyards at
a time somewhat later than the invention of the Bismar.
The earliest Roman steelyards were fitted with ring and hole pivots. Many centuries passed after the fall of Roman Empire before a properly constructed pivot or knife edge was fitted to these steel yards.
A steel yard balance or steelyard is a straight – beam balance with arms of unequal length. It incorporates a counter weight which slides along the longer arm to counter balance the load and indicate its weight. A steelyard is known as a Roman steelyard or Roman balance.
The earliest Roman steelyards were fitted with ring and hole pivots. Many centuries passed after the fall of Roman Empire before a properly constructed pivot or knife edge was fitted to these steel yards.
A steel yard balance or steelyard is a straight – beam balance with arms of unequal length. It incorporates a counter weight which slides along the longer arm to counter balance the load and indicate its weight. A steelyard is known as a Roman steelyard or Roman balance.
Very large suspended steelyards were on use until the end of 18th
century for weighing carts, until the invention and general
introduction of the compound lever platform scale.
