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Besides not addressing deck deflection, the "r" factor is of limited use for certain applications with
vehicles not designed for travel on highways. Most problems of truck scale suitability are for trucks that
do not typically travel on highways. For example, the Rice Lake Weighing Systems SURVIVOR series
trucks scales (and their foundations) have been designed for a 90,000 lb 4' tandem axle load, hence their
"r" factor is 2.65. The trucks that are most often the biggest problem for vehicle scales are mining trucks
not limited by legal highway load limits. These vehicles may have a single large axle that usually exceeds
34,000 lbs on that single axle, and may exceed 90,000 lbs. Bridge formula table "B" does not address this
type of single-axle loading configuration.
Span Deflection Ratio
The best method of determining weighbridge strength is the one chosen by the railroads many years ago.
This is the standard span deflection ratio for railroad scale decks at a specific load. As an example, a
railroad weighbridge should not deflect more than 1/1200 of the span distance. A 20' span (240") should
not deflect more than 0.2" (1/1200 x 240") when loaded to the maximum load that the scale will see (for
railroad track scales, E-80 equals 80,000 lb loads every 5 feet along the scale).
Deflection in this case is defined as a ratio of span to the overall deflection of the weighbridge when
loaded in the worst-case position on the weighbridge with the highest designed load (CLC or dual axle
loads).
The SURVIVOR siderail truck scale (SR-series) deflects no more than 1/900 at its rated load of 90,000
lbs on 4' axle spacing (CLC rating and dual axle load rating are considered the same for this calculation).
This means that the scale weighbridge will not deflect more than 0.27" over a 20-foot span. The less
deflection, the less bending stress in the weighbridge and the longer the life cycle. The figure below is
exaggerated to illustrate span deflection action.
Typical truck scales deflect 1:500 – 1:600
This final piece of the puzzle, long term life cycles, is critical when looking at a truck scale purchase.
Summary
We have three methods of reviewing a truck scale:
•
CLC to determine weighing accuracy,
•
"r" factor for the methods of loading, and
•
span deflection ratio for comparing deflection and the life cycle of competitive truck scales.
Each is important, each is a valuable measuring method, and each provides a unique perspective to the
end user making a decision to purchase a vehicle scale.
SURVIVOR SR truck scales deflect only 1:900
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