Field Inspection Manual — Non-Automatic Weighing Devices

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Part 3, Section 15: STP-15 Eccentricity Test

Reference

Sections 11 and 13 of the Specifications Relating to Non-Automatic Weighing Devices (1998).

Purpose

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 electro-mechanical 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.

General

Eccentricity testing of a multi-interval device shall be conducted at the load specified below and based upon Max of the device. If this results in a load which spans two intervals, the testing and evaluation should proceed in the smallest interval.

Eccentricity testing of a multiple range device shall be conducted at the load specified below once for each range of the device. In this case, Max is considered to be Max for the selected range.

Procedure

1. Bench, Counter, Platform & Equal Arm

All devices with four or fewer bearing or support points.

Procedure

  1. Zero the device.
  2. Apply a test load of ⅓ Max (between 30% and 35% is acceptable) on the center of the platter (position number 1 on the appropriate graphic below). Record the indication. This position establishes the Maximum Permissible Error (MPE) applicable to the load. Loading of this position is mandatory when the load used is not comprised of known standards (unknown load) - optional when the load is known.
  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 each of the other numbered target boxes. The test load should not overhang the edge of the Load Receiving Element (LRE). Record the indication.
  5. Most LRE's will be rectangular, however regardless of the shape of the LRE, it should be divided into quarters as illustrated and the appropriate test load applied in the approximate center of each quarter.
Figure 1: Number of Support Points ≤ 4
loading pattern 4 load cells
Full description—Figure 1: Number of Support Points ≤ 4

A four loadcell 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.

⊕ identifies support point (lever chair, load cell stand, flexure element, etc.)

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

Two Pan Scale or Balance

Loading for two pan scale or balance is the same as for the bench or platform scale. The test procedure is also the same, but the following loading pattern shall be observed.

Figure 2: Two Pan Scale or Balance (equal or unequal arm)
loading pattern equal arm scale
Full desctription—Figure 2: Two Pan Scale or Balance (equal or unequal arm)

Two round scale pans are shown. The first pan is is divided diagonally into four equal quadrants. The center of the first pan is numbered 1, while the four quadrants are then numbered in the centre of each quadrant, clockwise starting from the top quadrant, from 2 to 5. These are the locations for applying the test loads. The second pan is labeled with an L in the exact centre. This is the location for the counterweight. Once the left pan has been tested, the procedure is repeated for the right pan.

Note: When testing two pan scales of the 'pan over beam' type, the test loads should be applied first to one LRE then the other LRE of the device. A suitable counterweight (L) should be placed in the center of the opposite LRE.

Larger Platform Scales and other Scales with more than Four Points of Support

Procedure

  1. Zero the device.
  2. Apply a test load equal to 1⁄(n -1) Max to the center of the platter (position number 1). Record the indication. This position establishes the maximum permissible error (MPE) applicable to the load. (n = number of support points)
  3. Loading of this position is mandatory when the load used is not comprised of known standards (unknown load) - optional when the load is known.
  4. Divide the surface area of the load receiving element (LRE) into 1⁄n segments, each over one of the load support points.
  5. 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. Record the indication. (n = number of support points).
  6. 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.
Figure 3: Support Points >4
loading pattern more than 4 cells
Full description—Figure 3: Support Points >4

A six loadcell 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.

⊕ identifies support point (lever chair, load cell stand, flexure element, etc.)

2. Forklift or Hand-Truck Scales

Using the appropriate loading pattern below, perform the same tests as for platform scales 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.

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 lowest possible position.

Figure 4: Forklift (all configurations)
loading pattern lift truck scale Forklift
Full description—Figure 4: Forklift (all configurations)

A two loadcell forklift scale is shown. The two loadcells 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.

Figure 5: Hand-Truck (3 support)
loading pattern hand truck scale Hand truck
Full description—Figure 5: Hand-Truck (3 support)

A three loadcell hand-truck or pallet jack scale is shown. The three loadcells 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

⊕ identifies support point (lever chair, load cell stand, flexure element, etc.)

3. Monorail Scales

Procedure

  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 on the LRE. At a minimum, the ends and middle of the LRE shall be tested.
  3. Record the indications.
Figure 6: Monorail Scale
loading pattern monorail scale
Full description—Figure 6: Monorail Scale

An overhead rail scale is shown. The loadcells are not explicity located, but generally will be at or neareach 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.

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

4. Tank or Hopper Scales

It may be difficult or dangerous to supply the required amount of test standards when testing tank or hopper scales with a Max greater than 10 000 kg. Since these devices are not significantly affected by eccentricity errors, the loading requirements may be reduced. For these larger capacity devices a load of between 50-100d may be used instead of 10%-25% Max. This load will allow the inspector to ensure that each of the load cells is live and is contributing to the overall weight indication. If the load consists of test weights that are contained within the centre of gravity (CoG) of the device, the load is subject to the applicable limit of error (LOE). However, if the eccentricity load is outside the CoG, cantilevering may occur and the load should be considered as diagnostic only and not subject to the otherwise applicable LOE.

Procedure

  1. Zero the device.
  2. Use a load of at least
    1. 10% of Max without exceeding 25% of Max for a device with a Max ≤10 000 kg.
    2. 50-100d for devices with a Max>10 000 kg
  3. 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.
  4. Record the indications.
Figure 7: Tank/Hopper Scale
loading pattern hopper & tank scale
Full description—Figure 7: 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 nuymbered starting from 1 and proceeding until all have been designated. Each of these locations should be tested with test weights.

5. Vehicle Scales—Section / Shift Tests

Figure 8: Vehicle Scale (3 section)
loading pattern vehicle scale
Full description—Figure 8: Vehicle Scale (3 section)

A three 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.

⊕ identifies support point (lever chair, load cell stand, flexure element, etc.)

Maximum Concentrated Load

The heaviest 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 a scale with more than two sections. Subject to the above maximum, the heaviest and most concentrated load available should be used.

Warning: Tracked vehicles should never be used on a scale deck—rubber tire vehicles only.

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, typically the steering axle, 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.

Figure 9: Deflection
mid span deflection
Full description—Figure 9: 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) of the machine should be positioned over the load cell(s). This position will change depending upon the configuration of the machine. Front end loaders, when used as strain or shift test loads, should have their buckets or 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 metres (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.

Figure 10: Wide Deck
loading pattern wide vehicle scale
Full description—Figure 10: 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.

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.

Shift Test—Modular & Multi-Deck 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 placed on one side of the connection line of the module, then on the other side of the connection line. This test may be impractical if the modules are separated with non-sensing areas.

(See STP-26 Weighing Systems with Multiple Weighing Elements for more requirements)

Figure 11: Modular Vehicle Scale
loading pattern modular vehicle scale
Full description—Figure 11: 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.

Vehicle Scale Mounted Side by Side.

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.

Procedure

  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.

(See STP-26 Weighing Systems with Multiple Weighing Elements for more requirements)

Figure 12: Side by Side
loading pattern side by side scales
Full description—Figure 12: 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.

6. Section / Shift Tests—Railroad Scales

Loading patterns to be the same as for vehicle scales. Each section and each end shall be loaded in turn. The tests shall be run in both directions.

Maximum Concentrated Load

The bogey weight of the test car 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 a scale with more than two sections. Subject to the above maximum, the heaviest and most concentrated load available should be used - usually a suitable short rail test car will be available for this test. Alternately, section tests can be run with a track-mobile if a heavy enough one is available.

Procedure

  1. Zero the device
  2. Position the loaded test load onto the weighbridge centered over the first section. Set the brakes lightly on the test car and uncouple and remove the power unit (track-mobile, locomotive, etc.) if used.
    Record the indication.
  3. Repeat the procedure for each of the other sections.
  4. 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.

Warning: Do not allow a locomotive to enter a scale deck unless authorized by the owner of the scale.

Use of Hydraulic Jacking Beams

The use of weight truck hydraulic jacking beams or other weight concentrating apparatus is prohibited for safety reasons. Likewise, stabilizers on crane/boom trucks should not be lowered onto scale decks unless adequate provision is made to distribute the concentrated load. Use of these stabilizers on scale approaches may also require some type of weight distribution measures to be implemented.

Use of Motorized Mobile Weight Carts

Motorized mobile weight carts may be used to test vehicle scales only under the following conditions:

  1. The cart must follow the same loading patterns and restrictions applicable to the use of any other mobile concentrated load such as a truck or loader. The cart must only be moved lengthwise on the deck. The cart must only be positioned on areas of the deck designed to support a vehicle. In no case shall the cart be positioned on wooden aprons or transversely mounted decking timbers outside of the area normally used to support a vehicle. Carts should not normally be used on other platform scales.
  2. Extreme caution shall be taken when moving the cart on or off of the scale. Ensure that the approaches are of sufficient strength to handle the concentrated load and that the transition is smooth to facilitate cart movements.
  3. The cart must be designed and loaded so as to exert a concentrated loading of no more than 14 kg/cm2 or 140 000 kg/m2 (200 lb/in2).
Equation 1
Formula for calculating Maximum Concentrated Load is equal to the total weight of the cart plus the weight of the standards on the cart all divided by the total contact area of the tires. The total contact area of the tires is found by multiplying the individual contact area by the number of tires.

where:

  • CW = total weight of cart in kg (lb).
  • Std = total weight of standards in kg (lb).
  • Tires = number of tires/wheels contacting the road surface.
  • Contact = total contact area of one tire/wheel in cm2 (in2).
  • = width of tire in cm (in) × length of loaded contact patch in cm (in).

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 5.

  • added clarification of requirements for multi-interval multiple range device.
  • make loading position #1 optional for selected platform scales tested with known standards.
  • change loading criteria for large (>10 000 kg) tank & hopper scales.

Rev 4.

  • harmonize load requirements and patterns for scales with not more than 4 points of support.
  • remove note from railroad scale test procedure.
  • update cart procedures to allow movement on/off load receiving element while loaded.

Rev 3.

  • added motorized cart requirements and load restrictions.
  • added railroad scale test procedures.
  • added minimum recommended test loads for vehicle scale eccentricity testing.
  • added maximum recommended length of test load for vehicle scale eccentricity testing.
  • change vehicle scale graphic from 4 section to 3 section to facilitate test load length depiction.
  • added rationale for applying test load to center or number 1 position on platform scales.
  • bulletin reorganized to group common inspection types.
  • correct references to Specifications Relating to Non-Automatic Weighing Devices (1998).

Rev 2.

  • modified & simplified forklift / hand truck loading requirements.

Rev 1.

  • 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.

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