Payload Meter III - Section 60 OM6009 8/11
Page 60-8
Load Calculation
The final load calculation is different from the last
swingload calculation. The accuracy of the swing
load calculation depends on loading conditions and
the position of the truck during loading. The last
swingload calculation is not the value recorded in
memory as the final load. The final load is
determined by a series of calculations made while
the truck is traveling to the dump site.
Carry Back
Carry back is calculated as the difference between
the current truck tare and the clean truck tare. The
clean truck tare is calculated using the PC software.
When the suspensions are serviced or changes are
made that may affect the sprung weight of the truck,
a new clean truck tare should be calculated.
Measurement Accuracy
Payload measurements are typically repeatable
within 1%. Accuracy for a particular scale test
depends on specific combinations of pressure
sensors and payload meters as well as the specifics
of each scale test. Comparisons from different scale
tests are often made without considering the
differences introduced by the specific installation and
operation of the scales for each test. In addition,
each pressure sensor and payload meter introduces
it's own non-linearity. Each truck becomes an
individual combination of sensors and payload meter.
Errors from these sources can introduce up to a ±7%
bias in the payload meter calculations for a specific
scale test, for an individual truck.
Because the PLMIII calculates a new empty tare for
each payload, a detailed scale test must weigh the
trucks empty and loaded for each haul cycle. Using a
simple average of two or three empty truck weights
as an empty tare for the entire scale test will
introduce significant error when comparing scale
weights to PLMIII weights.
SOURCES FOR PAYLOAD ERROR
Suspensions
The number one source of error in payload
calculation is improperly serviced suspensions. The
payload meter calculates payload by measuring
differences in the sprung weight of the truck when it
is empty and when it is loaded. The sprung weight is
the weight of the truck supported by the suspensions.
The only method for determining sprung weight is by
measuring the pressure of the nitrogen gas in the
suspensions. If the suspensions are not properly
maintained, the payload meter cannot determine an
accurate value for payload. The two critical factors
are proper oil height and proper nitrogen charge.
If the suspensions are overcharged, the payload
meter will not be able to determine the empty sprung
weight of the truck. The suspension cylinder must be
able to travel up and down as the truck drives empty.
The pressure in an overcharged suspension can
push the suspension rod to full extension. In this
case, the pressure inside the cylinder does not
accurately represent the force necessary to support
that portion of the truck.
If the suspensions are undercharged, the payload
meter will not be able to determine the loaded sprung
weight of the truck. The suspension cylinder must be
able to travel up and down as the truck drives loaded.
If the pressure in an undercharged suspension
cannot support the load, the suspension will collapse
and make metal-to-metal contact. In this case, the
pressure inside the cylinder does not accurately
represent the force necessary to support that portion
of the truck.
Low oil height can also introduce errors by not
correctly supporting a loaded truck. This is why the
correct oil height and nitrogen charge are the most
critical factors in the measurement of payload. If the
suspensions are not properly maintained, accurate
payload measurement is not possible. In addition,
suspension maintenance is very important to the life
of the truck.
To Next Page
Loading...
