Gain Factor Sources of Error Gain factor is part of the formula the payload meter uses to calculate the payload. The gain factor is a tool that compensates for variations between actual weight of the payload and the calculated weight from the payload meter. The gain factor should only be adjusted after several payloads have been weighed on a scale. The gain should never be adjusted based only on one payload. It takes at least 10 scale weigh-ins to deter- mine an average variation. A worksheet is located at the end of this chapter to assist in calculating the necessary gain adjustment. 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 a load. The weight of the truck will be carried by the metal to metal contact within the suspension. Not only will the ride of the truck and the life of the tires, and other components be affected, but the pressure in the cylinder will not truly represent the load on the truck. Under-charged suspensions typi- cally cause the payload calculations from the payload meter to be lower than the actual payload. There are two types of gain adjustments, G t gain and UP factor. It is recommended that only the G t gain be adjusted.
This adjustment is made using the potenti- ometer on the side of the payload meter. Refer to "Adjusting the Gain". Typical Data From Service Check Mode A sample data set is shown in Figure 20-4. This data was taken in the laboratory and is used in Figure 20-5 to calculate the final load. Note that the front suspen- sion pressures were converted into the front sprung weight using the formulas above Figure 20-5. 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 G t gain potentiometer value.
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. 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.
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. (Parking brake on 330M/HD785). Figure 20-4. M20-8 Payload Meter II M20007.1 03/02 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)