OM6009 1/04 Payload Meter III - Section 60 Page 60-11 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 deter- mined by a series of calculations made while the truck is traveling to the dump site. SOURCES FOR PAYLOAD ERROR Payload Error The number one source of error in payload calcula- tion is improperly serviced suspensions.
The pay- load 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. 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. 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 accu- rately represent the force necessary to support that portion of the truck. Measurement Accuracy Payload measurements are typically repeatable within 1%.
Accuracy for a particular scale test depends on specific combinations of pressure sen- sors and payload meters as well as the specifics of each scale test. Comparisons from different scale tests are often made without considering the differ- ences 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 indi- vidual 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.
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 can- not support the load, the suspension will collapse and make metal-to-metal contact. In this case, the pressure inside the cylinder does not accurately rep- resent the force necessary to support that portion of the 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 2 or 3 empty truck weights as an empty tare for the entire scale test will introduce sig- nificant error when comparing scale weights to PLMIII weights. Low oil height can also introduce errors by not cor- rectly supporting a loaded truck. This is why the cor- rect 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.