3
2.0
Operation
2.1
Portable Weighing Procedures
The PW 400/PW 500 Portable Wheel Load Weigher is specifically designed to indicate the weight of an
individual wheel resting at any position within the weighing area. The operator must follow the basic prin-
ciples of portable weighing in order to obtain accurate and reliable results. Improper weighing procedures can
induce errors into the weighing process that are no fault of the weigher.
Accurate vehicle axle or gross weight cannot be arrived at by measuring a single wheel, a single side, or a
single axle within an axle group. An axle group is defined as any group of axles that share a common linkage
between them, whether it be mechanical, hydraulic or pneumatic. Both sides of any axle must be weighed at
the same setting. In the case of a tandem axle group, all four wheels must be weighed at the same setting.
The PW 400 and PW 500 weighers are precision weight indicators. By using correct techniques, excellent and
dependable results will be achieved.
2.2
Transient Weight Shifting
The largest single factor determining the accuracy of portable weighing is transient weight shift within the
vehicle itself.
Transient weight shifting can best be defined as weight which is transferred from one axle to another due to:
•
Placing one axle in an unusual attitude in relation to another by lifting off a normal plane.
•
Placing the vehicle in a non-level position due to poor approach or poor choice of weigh site.
•
Physical contact of tire with ground.
Axles are naturally lifted off a normal plane any time that the weigher platform is above ground level. The
amount of transient weight shift is a direct function of the distance that the axle is lifted. Using the correct
number of weighers reduces transient weight shift to a minimum. Hence, reducing the number of consecutive
weighments required will result in a more accurate weighing. Axles spaced within 12 feet or closer should
always be weighed at the same setting.
The balance of the transient weight shift may also be compensated for by placing simple blocks, approxi-
mately 3 inches in height, under the conflicting axle so as to create a level plane.
Example: When single or tandem drive axles are weighed, the steering axle should be elevated to the same
level as the weigher platform. When the steering axle is weighed, drive axles should be elevated to platform
level. This also applies to coupled trailers with short axle spacing.
The transient shift between drive axles and most trailer axles is normally insignificant and no blocks need be
used.
2.3
Selection of Weigh Sites
Selection of a suitable weigh site is a prerequisite to accurate weighing. Ideal weighing sites should be flat,
level and hard surfaces. Approaches should be long enough for a straight-line movement of the vehicle prior
to and during the weighing. Realizing the problems of finding ideal weighing surfaces, the PW 400/PW 500
weigher is designed to weigh on a side slope, not exceeding 3 degrees, and on any relatively flat surface
capable of holding the weigher above ground. The site chosen should not be up- or downhill, in a hollow or
on a crest, as these conditions will redistribute the weights of the axles.
