Field Manual for Testing Vehicle and
Axle-Load Scales Using a Weight Cart

Prepared by: Tad Tucker and Steve McGuire, Illinois Department of Agriculture

1. Introduction.
2. Purpose. Motorized test weight carts have many variables that affect the calibration for the weight cart such as fuel and hydraulic fluid levels, filters, and battery. Weight carts are also an important part of the scale testing procedure and process. The calibration of weight carts often deteriorates with time and use. Therefore, in order to allow specified confidence in the calibration and to support measurement traceability, the following practice establishes the calibration intervals, the maintenance, and the use of test weight carts.
3. Prerequisites.
1. Calibration history of the weight carts.
2. Test weight standards used in the NIST Handbook 145 S.O.P. 8 procedure for calibrating test weight carts must have a combined and expanded uncertainty of less than one-ninth of the tolerance at the Class F field standard level.
3. The combined and the expanded uncertainty of the calibration of the test weight cart must be less than one-third of the tolerance at the Class F field standard level.
4. Approved method of testing and documented uncertainty.
4. Design parameters.
1. Cart size
2. The width and length of the cart should not exceed the prescribed test pattern as stated in N.1.3.4. Shift Test for Vehicle Scales, Axle-Load Scales, and Livestock Scales with More Than Two Sections subparagraph (b) Maximum Loading.
3. The weight cart’s axle width should be designed to allow the weight cart to be driven over the scales load receiving area. (IE. As wide as the I-beams on the average 10’ wide scale)
4. Gas Tank Size. The gas tank should be of size to assure any change in the amount of fuel will not affect the test. Maximum tank capacity of one gallon.
5. Calibration Responsibility.
1. Metrology Laboratory. Provides the testing and calibration along with the test reports that contain the "as found" and "as left" data with statements of uncertainty and traceability.
2. Weight cart condition when presented for calibration.
3. Any oil or hydraulic leaks must be repaired.
4. Changing the test weight cart’s oil and hydraulic fluids and corresponding filters prior to the weight cart’s calibration.
5. Remove and reinstall a new heavy duty battery. If the battery fails and it is replaced, the test weight cart must be recalibrated.
6. A light coat of paint to be applied.
7. The weight cart’s fuel tank must be filled to the calibration point. This calibration point is where the weight cart’s fuel level should be maintained while in the field.
6. Maintenance.
1. Fluid levels. All fluid levels must be checked each day and the gas level checked prior to each inspection.
2. Fluid leaks or Replacement of parts. Fluid leaks must be reported to the Metrology supervisor to determine when and what corrective action must be taken. Any work on the test weight cart, such as replacing parts and/or seals, will require in the test weight cart to be recalibrated.
3. Safety. The inspector is reminded of the importance of evaluating potential safety hazards priop to an inspection and taking adequate precautions to avoid personalinjury or damage to the device. As a minimum, the safety precautions in this section should be noted and followed during the inspection.
4. Weather conditions. Be aware of the effects of snow, ice, rain and wind on the performance and drivability of the weight cart.
5. Levelness of weight cart. The weight cart must lift and lower level. There is a serious safety concern if the weight cart does not left level. The weight cart may fall from the hoisting system and damage the cart, scale and/or injury the inspector.
6. Scale decks. The inspector must examine the scale deck to assure that it is capable of carrying the weight cart and test weights.
7. Wooden Scale Decks. If a wooden scale deck is not in good condition, IE. rotten or missing planks, no test can be conducted. The scale deck must be repaired before an inspection can be performed.
8. Concentrated Load Capacity (CLC). Use of weight cart and test weights can not exceed the CLC for a vehicle scale. For scales manufactured prior to January 1, 1989, the test load can not exceed the rated sectional capacity.
9. Maximum Test Load. A test load of 100 percent of capacity must be distributed over the entire platform and that you do not exceed any sections CLC.
7. Weight Cart Use
1. Zero Indication. Before placing the weight cart on the scale, the scale indicator is checked to see if it is zero. As stated above, be sure the gas tank on the weight cart is at the calibration point.
2. Initial test. The weight cart is removed from the test vehicle and placed on the scale. If the indicator does not register the weight cart’s calibrated weight, the weight cart is removed and the indicator is re-zeroed. The weight cart is placed on the scale a second time and a reading is taken.
3. Calibrated weights are added to start the build up test. One set of weights should be removed for a decreasing load test. Then the weights should be placed again in the cart for a repeatability test. N.1.1. Increasing-Load Test. N.1.2. Decreasing-Load Test. T.N.5. Repeatability.
4. Shift test. The weight cart is started and driven across the scale to check the scale sections as described below. (Note: Some inspectors may choose to start the cart right after placing it on the scale. Both practices are acceptable.) T.N.4.4 Shift or Section Tests.

   
   N.1.3.4. Vehicle Scales, Axle-Load Scales, and Livestock Scales With More Than Two Sections. - At least one shift test shall be conducted with a minimum test load of 12.5% of scale capacity and may be performed anywhere on the load-receiving element using the prescribed test patterns and maximum test loads specified below. (Two-section livestock scales shall be tested consistent with N.1.3.8.) (Amended 1991 and 2000)

   (a) Prescribed Test Pattern. The normal prescribed test pattern shall be an area of 1.2 m (4 ft) in length and as wide as the scale platform. Multiple test patterns may be utilized when loaded in accordance with Paragraph (b). (Part (a) Amended 1997)

   (b) Maximum Loading. When loading the scale for testing, one side of the test pattern shall be loaded to no more than half of the concentrated load capacity or test load before loading the other side. The area covered by the test load may be less than 1.2 m (4 ft) x the width of the scale; for test patterns less than 1.2 m (4 ft) in length the maximum loading shall meet the formula: [(wheel base of test cart or length of test load divided by 48 in) x 0.9 x CLC]. The maximum test load applied to each test pattern shall not exceed the concentrated load capacity of the scale. When the test pattern exceeds 1.2 m (4 ft), the maximum test load applied shall not exceed the concentrated load capacity times the largest "r" factor in Table UR.3.2.1. for the length of the area covered by the test load. For weighing elements installed prior to January 1, 1989, the rated section capacity may be substituted for concentrated load capacity to determine maximum loading. An example of a possible test pattern is shown below:

   
   

   (Part (b) Amended 1997)

   (c) Multiple Pattern Loading. To test to the nominal capacity, multiple patterns may be simultaneously loaded in a manner consistent with the method of use.

   (d) Other Designs. Special design scales and those that are wider than 3.7 m (12 ft) shall be tested in a manner consistent with the method of use but following the principles described above.
   (Amended 1988)

5. Continued build-up test. The weight cart is driven to the test vehicle to continue the build-up test. Upon completing the build-up test, the weight cart is again driven across the scale to conduct a shift test.
6. Zero-Load Balance Change. The weight cart is removed to see if the scale will return to zero.  N.1.9. Zero-Load Balance Change.
7. Strain Test. Table 4. Whenever practical, a strain load test should be conducted to the used capacity of the scale. The test vehicle is driven on the scale and its weight is recorded. Then the weight cart is driven back on the scale. The combination of test vehicle, weight cart and calibrated weights is recorded and compared to the indicator reading.
8. Strain Test Tolerance. The tolerance on a strain test is only applied to the known amount of calibrated weights. See Table 6. in Handbook 44.
1. Example: 4,000 lbs. weight cart with 16,000 lbs. of calibrated weights
2. Test truck is 35,000 lbs.
3. Total strain test: 55,000 lbs.
4. Maintenance tolerance allowed: 2d on a 20 lbs. Scale increment.
5. (20,000 lbs./20 division/500 = 2d)
9. Post test duties. Return weights and weight cart to the test vehicle. Be sure to secure the weight cart to the test vehicle.

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Date created: December 2001
Last updated: October 1, 2002

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