Laboratory manual for the evaluation of non-automatic weighing devices

LG-3.03 Eccentricity

Table of contents


Reference

Sections 9, 10, 11, 12 and 14 of the Specifications Relating to Non Automatic Weighing Devices (1998)

Application

Eccentricity tests are performed on complete devices and weighing elements tested separately. Shift tests are also performed on single point hanging scales if the load receptor is relatively large.

The purpose of this test is to reveal the ability of load cell(s), load cell mounting and check systems of a weighing element to resist or compensate for the torsion effects of non axial loads. This test also ensures that the load cells of electronic scales, the levers of mechanical scales, or both in the case of electromechanical scales, are adequately "balanced" in order to obtain accurate weighing.

The device must be capable of weighing accurately in spite of changes of position of the test load over the load receiving element.

Settings

Procedure for bench, counter or hanging scales, other platform scales, equal arm scales and forklift or hand-truck scales with load receiving elements having no more than four points of support.

  1. Stabilize the device at nominal conditions; zero the device.
  2. Apply a test load equal to 1/3 Max on the center of the platter (Position #1). Record the indication.
  3. Apply the same test load on the device in such a manner that the center of gravity of the test load lies approximately at the center of one of the numbered target boxes in the following illustrations. Record the indication.
  4. Proceed in the same manner with the other points of application. Avoid sliding weights on the LRE. Instead, weights must be picked and placed back down on the LRE. Record the indications.

Note: 25% of Max placed on the LRE over the load cell may also be use to perform corner tests on platform, floor or bench scales having 4 support points.

Number of support points ≤ 4

loading pattern 4 load cells
Full description—Number of Support Points ≤ 4

A four load cell bench scale is shown with supports in each of the four corners. The scale is divided vertically and horizontally into four equal quadrants. The center of the scale is numbered 1, while the four quadrants are then numbered in the centre of each quadrant, clockwise starting from the upper left quadrant, from 2 to 5. These are the locations for applying the test loads.

Platform scale—More than four support points

Procedure

Larger platform scales and other scales with more than four points of support

  1. Zero the device.
  2. Apply a test load equal to 1/(n-1) of Max to the center of the platter (Position number 1). Record the indication. (n = number of support points)
  3. Divide the surface area of the LRE into 1/n segments, each over one of the load support points.
  4. Apply the same test load on the device in such a manner that the center of gravity of the test load lies at the center of each segment. (n = number of support points). Record the indication.
  5. Proceed in the same manner with each of the other segments. The test load should not overhang the edge of the LRE. Record the indication.
Support points > 4
loading pattern more than 4 cells
Full description—Support Points > 4

A six load cell platform scale is shown with supports in each of the four corners as well as two more supports midway along each of the long edges of the platform. The scale is divided vertically and horizontally into six equal quadrants, each with a load cell. The center of the scale is numbered 1, while the remaining six quadrants are then numbered in the centre of each quadrant, clockwise starting from the upper left quadrant, from 2 to 7. These are the locations for applying the test loads.

Forklift or hand-truck scales
  1. Using the appropriate loading pattern below, perform the same tests as above. Weights should be placed upon the largest pallet typically lifted by the forklift or hand-truck. If the forks are adjustable, repeat test at both minimum and maximum fork spacing.
  2. Due to the nature of these pieces of equipment, extreme care must be taken to ensure that the stability of the forklift or hand-truck is not compromised during the test. Testing should always be done with the forks in the lowered position.

Forklift (all configurations)

loading pattern lift truck scale
Forklift
Full description—Forklift (all configurations)

A two load cell forklift scale is shown. The two load cells are in the mast. The load is cantilevered from the mast on the forks. A pallet is placed on the forks to facilitate testing. The pallet is divided vertically and horizontally into four equal quadrants. The center of the pallet is numbered 1, while the four quadrants are then numbered in the centre of each quadrant, clockwise starting from the upper left quadrant, from 2 to 5. These are the locations for applying the test loads.

A picture of a forklift or lift truck.

Hand-truck (3 support)

loading pattern hand truck scale
Hand truck
Full description—Hand-Truck (3 support)

A three load cell hand-truck or pallet jack scale is shown. The three load cells are at or near the wheels. A pallet is placed on the forks to facilitate testing. The pallet is divided vertically and horizontally into four unequal equal quadrants. From the handle to the tip of the forks, the pallet is divided into 2/3 and 1/3 sections respectively. Side to side, the pallet is divided equally in half. The point at which the dividing lines cross is numbered 1, while the four quadrants are then numbered in the centre of each quadrant, clockwise starting from the upper left quadrant, from 2 to 5. These are the locations for applying the test loads.

Picture of a pallet jack or hand-truck

Monorail scales
  1. Zero the device.
  2. Apply a rolling load corresponding to the usual rolling load, the heaviest and the most concentrated one which may be weighed, but not exceeding 80% of Max at different points of the load receiving element.
  3. Record the indications.

During this test, observe the rail to detect any inappropriate motion, deflection, binding or friction that could adversely affect the scale performance.

Monorail scale
loading pattern monorail scale
Full description—Monorail Scale

An overhead rail scale is shown. The load cells are not explicitly located, but generally will be at or near each end of the live rail. The left end of the live rail is designated L1, the middle of the live rail, L2 and the right end of the live rail is L3. These are the minimum locations for applying test weights to the device.

Tank or hopper scales
  1. Zero the device.
  2. Use a load of at least 10% of Max without exceeding 25% of Max. Apply the load to each point of support. Care must be taken to keep the center of gravity of the load between the supporting points to prevent cantilevering the scale.
  3. Record the indications.

Note: The ratio between the test load and e must be such that it will be possible to establish the error and apply the limits of error.

Tank/Hopper scale
loading pattern hopper & tank scale
Full description—Tank/Hopper Scale

A four load cell hopper scale is shown. The supports may be below or above the hopper assembly and are typically located in the corners if the LRE is square, or equally spaced around the perimeter if the LRE is round. There may be fewer or more supports in a given installation. Each support is sequentially numbered starting from 1 and proceeding until all have been designated. Each of these locations should be tested with test weights.

Section / Shift tests on vehicle scales
Vehicle scale (3 section)
loading pattern vehicle scale
Full description—Vehicle Scale (3 section)

A four section vehicle scale is shown in plan and profile. On the scale rests a truck used for conducting shift tests. The scale is zeroed and then loaded starting from right to left. The center of gravity of the vehicle is placed over each of the scale supports or stands. After a reading is taken, the vehicle is advanced to the next location. This continues until the final location. The vehicle is stopped immediately before exiting the LRE and a final reading is taken. The entire procedure is then repeated from left to right.

Maximum concentrated load

The axle (or set of axles) weight of the vehicle used must not be greater than 75% Max in the case of a two section scale, and not greater than 50% Max in the case of scale with more than two sections.

Procedure

  1. Zero the device
  2. Drive the loaded vehicle onto the weighbridge and position the center of the heaviest set of axles over the first section; record the indication.
  3. Move the concentrated load over the second section. Record the indications.
  4. Repeat the procedure for each of the other sections.
  5. Enter the weighbridge in the opposite direction and test each section again. At least two complete sets of shift tests should be conducted over each section of the scale. This is to determine the repeatability of the scale.

Note: The last section in each direction, before leaving the scale, cannot be adequately loaded with a typical test truck. The final stopping position should be just before the first set of axles leaves the weighbridge.

Shift test—Deflection

The concentrated load must also be placed between the sections to determine if any deflection of the deck or understructure is causing inaccuracies.

Deflection
mid span deflection
Full description—Deflection

A vehicle is shown sitting on a vehicle scale with the center of gravity placed midway between the supports or stands. This test ensures that deflection between section is not significant enough to cause measurement inaccuracies.

Note: When using a loader to conduct a shift test, the center-of-gravity (CoG) should be position over the load cell(s). This position will change depending upon the configuration of the machine. In addition, loaders, when used as strain or shift test loads should have their buckets / grapples in lowered positions if possible. This lowers the CoG and reduces the "sail" effect from the wind.

Shift test—Wide deck

If the width of the weighbridge exceeds 3 meters (or when the inspector deems it necessary), perform a first series of tests with the vehicle shifted on the right side of the deck and then shifted to the left side.

Warning: Wide Deck shift test should not be attempted on wooden deck scales if the test means driving the vehicle off of the longitudinal timbers intended to support the tires. The transverse mounted decking may not have adequate strength to support the concentrated load of the vehicle.

Wide deck
loading pattern wide vehicle scale
Full description—Wide Deck

A four section wide vehicle scale is shown in plan. On the scale rests a truck used for conducting shift tests. The scale is loaded starting from right to left staying to the left or right side of the deck. The center of gravity of the vehicle is placed over each of the scale supports or stands. After a reading is taken, the vehicle is advanced to the next location. This continues until the final location. The vehicle is stopped immediately before exiting the LRE and a final reading is taken. The entire procedure is then repeated from left to right.

Shift test—Modular scales

In the case of weighbridge made of modules (multi-deck vehicle scale), shift tests must also be conducted by placing the load so that it straddles the connection between the modules. At least one shift test is to be conducted on the scale with the test load place on one side of the connection line of the module, then on the other side of the connection line.

Modular vehicle scale
loading pattern modular vehicle scale
Full description—Modular Vehicle Scale

Two, two section vehicle scales are shown in plan and profile. The two scales are placed end to end to increase the effective length of the LRE. On the scale rests a truck used for conducting shift tests. The CoG of the vehicle is stopped at the joint between the two decks. First on the end of the first deck, then straddling the joint between the two decks and finally on the end of the second deck. The entire procedure is then repeated in the opposite direction.

Use of hydraulic jack

The use of weight truck hydraulic jack beam is prohibited for safety reasons.

Vehicle scale mounted side by side

Procedure

Vehicle Scales mounted in non-traditional fashion. Often used for weighing of off-road mining equipment or large logging equipment. These scales usually consist of two scale decks mounted side by side, however other configurations are possible including a "T" configuration.

  1. Zero the device.
  2. If possible, test eccentricity on each scale separately using the loading patterns for vehicle scales as above. Then continue with the following tests.
  3. Drive a loaded vehicle onto the weighbridge and position the center of the heaviest set of axles over the first support point/section; record the indication.
  4. Move the concentrated load over the second load point/section. Record the indications.
  5. Repeat the procedure for each of the other load points/sections.
  6. If possible, enter the weighbridge in the opposite direction and test each load point/section again.
  7. Testing should approximate normal use loading patterns as much as possible.
Side by side
loading pattern side by side scales
Full description—Side by Side

Two, two section vehicle scales are shown in plan. The scales are installed adjacent to one another in order to increase the effective width of the LRE. The scales are loaded starting from right to left and straddling the two decks. The center of gravity of the vehicle is placed over each of the scale supports or stands. After a reading is taken, the vehicle is advanced to the next location. This continues until the final location. The vehicle is stopped immediately before exiting the LRE and a final reading is taken. The entire procedure is then repeated from left to right. If the decks are wide enough, a typical shift test may also be conducted on each of the decks without straddling the joint.

Interpretation of the results

The difference between the results for different positions of the load must not exceed the absolute value of the In-Service Limit of Error for that load.

Each individual result must also be within the applicable limits of error for the test.

Revision

Rev. 1 (November 2004)
  • Change reference from "load cell" to "support point" to cover all scale types.
  • Added eccentricity drawing for Forklift, single support.
  • Corrected number of support points reference in platform scales (≤ 4).
  • Added procedure for platform scales with more than four support points.
  • Added procedure for "side by side" deck vehicle scales.
Rev. 2 (April 2005)
  • Modified & simplified forklift/hand truck loading requirements.
Rev. 3 (September 2009)
  • Modified test weight to 1/3 capacity
  • Removed diagram for bench/counter scales with a single support point (test point now same for all devices up to 4 support points)
Rev. 4 (March 2010)
  • AZSM renamed to AZT
  • Formatting changes
Date modified: