OM6007 2/99 Optional Equipment - Section 6 Payload Meter II Page 6-7 Brake Lock The Brake Lock only applies the rear brakes. This allows the front wheels to rotate slightly as the truck is being loaded. This is important because the payload meter assumes that the front wheels can rotate freely. As the truck is being loaded, it will begin to squat down on the suspensions. This will change the wheel base dimension of the truck. This freedom of movement prevents additional binding and friction in the front suspensions. Typical Data From Service Check Mode The incline of the grade on which the truck is loading is measured by the inclinometer.
This helps determine the incline factors that are applied to the front and rear sprung weights. The tire forces on the road surface that hold the truck on grade affect the suspension pres- sures. If the front and rear brakes are locked, the effect on the suspension pressures cannot be determined. If only the rear brakes are applied the effect is predict- able and the incline factors accurately account for the forces on the tires. If the service brake or park brake is used and depend- ing on the incline and other factors, the payload meter can overestimate or underestimate the load. It is im- portant that only the Brake Lock be used while loading the truck. Figure 6-4.
A sample data set is shown in Figure 6-4. This data was taken in the laboratory and is used in Figure 6-5 to calculate the final load. Note that the front suspen- sion pressures were converted into the front sprung weight using the formulas above Figure 6-5. The front sprung weight is then multiplied by the front incline factor and the front linkage factor. The same is done with the rear sprung weight. The front and rear sprung weights are then summed. This number is multiplied by the Gt gain potentiometer value. This value should be 1.000. The calibration load is subtracted from the total to produce the final load.
The load displayed on the meter is this final load (item #15) multiplied by the UP gain factor. Sources of Error Suspensions Poorly charged suspensions can lead to systematic error in the calculation of payload. The error is most obvious when the oil level is low. When there is too little oil in the suspensions, the cylinder may compress completely under load. The weight of the truck will be carried by the metal to metal contact within the sus- pension. Not only will the ride of the truck and the life of the tires be affected, but the pressure in the cylinder will not truly represent the load on the truck.
In the under-charged condition the payload meter will typi- cally weigh light and under report the load. Note - This screen is the only place that the value of the Gt gain potentiometer can be checked. THIS GAIN SHOULD BE SET TO 1.000. ANY OTHER SETTING CAN PRODUCE SYSTEMATIC ERRORS IN THE PAYLOAD MEASUREMENT. Note - There are two gain factors that can be applied to the payload measurement. The first is the Gt gain factor and the other is the UP gain factor. They are not applied uniformly to all payload calculations. Both gain factors should be set to 1.000. See the warning on page 16 for more information.
If the G t gain factor displayed in the Service Check Mode is not 1.000, adjust the gain to exactly 1.000. Number Data Description 1 13:09 Current Time 2 749.4 Front Left Pressure (Psi) 3 848.9 Front Right Pressure (Psi) 4 863.2 Rear Left Pressure (Psi) 5 1049.0 Rear Right Pressure (Psi) 6 106.0 Front Weight (Tons) 7 75.1 Rear Weight (Tons) 8 -1.85 Inclinometer (Degrees) 9 0.95 Incline Factor - Front Wheels 10 0.984 Incline Factor - Rear Wheels 11 1 Link Factor - Front Wheels 12 1.539 Link Factor - Rear Wheels 13 70.6 Calibration Sprung Weight (Tons) 14 1.000 Gain Adjustment 15 143.8 Current Load (Tons) 16 3.9 Battery Backup Voltage (Volts)