CONTROL LEVERS

Most scrapers have three basic control levers located on the right side of the operator's seat. The first lever is the bowl lever (fig. 10-38, view A). The bowl lever raises and lowers the bowl. The middle lever is the apron control lever (fig. 10-38, view B). The apron control lever opens and closes the apron, allowing the required amount of material to enter or be ejected from

Figure 10-38.-Scraper control levers.

the bowl. The third lever is the ejector lever (fig. 10-39, view C). The ejector lever forces the dirt out of the bowl. For maximum hydraulic cylinder operating efficiency, the engine should be operated at maximum rated speed and the control levers moved to their extreme operating position.

SCRAPER OPERATIONS

The greatest engine power is available when the engine is running at top governed speed. The proper transmission gear ratios must also be engaged to obtain maximum engine power output. When the transmission is placed in a too high a gear ratio for full engine power, the result is a stall condition in the transmission converter. Stalling the converter prevents the engine from operating at maximum efficiency and results in rapid overheating and premature wear of the converter or transmission.

The transmission must be downshifted correctly while the scraper is in motion to prevent damage to the power train. Improper downshifting overspeeds the transmission and engine and usually results in premature wear and unnecessary transmission breakdowns.

When moving the scraper from a full stop, always start with the transmission in low gear, depressing the throttle for the degree of acceleration required. A wide, open throttle provides the fastest acceleration under full-load conditions.

When running downgrade, avoid overspeeding the engine by keeping the scraper speed at, or below, the maximum speed for the transmission range engaged. As a general rule, downhill scraper speed should not exceed 5 mph more than that attained on level ground in the transmission ratio engaged.

When the selected transmission ratio is too high, slow the scraper with the service brakes until the transmission can be properly downshifted to the required range for the grade.

Downhill speed can be slowed, if necessary, by lowering the scraper bowl until the cutting edge drags enough to slow the scraper to the required speed to permit proper downshifting or stopping.

Do not fan the brakes by repeated depressing and releasing. This practice can reduce air pressure below the point required for proper breaking. The air pressure system should indicate 105 to 125 psi on the air pressure gauge for effective braking. When the gauge indicates a pressure drop below 105 psi for a long time, shutdown the scraper until the trouble is corrected.

Figure 10-39.-Entering the cut.

Figure 10-40.-Loading the bowl.

Figure 10-41.-Loaded bowl.

The scraper work cycle has four phases of operation: loading, hauling, spreading or unloading, and returning to the cut.

Loading

When loading, enter the cut with the ejector positioned at the rear of the bowl, open the apron enough to allow material to enter the bowl (normally 4 to 8 inches above the leading edge of the bowl), and then lower the bowl to cut a depth of 1 to 1 1/2 inches. The gear that the transmission is engaged in depends on the nature of the material being cut. For light, loose material, a relatively high gear can be used. For heavy compacted material, a low gear is used; however, to obtain a full load, you should use a lower gear, even in loose material. As the scraper proceeds through the cut, the material is loosened by the scraper cutting edges and forced into the bowl by the forward motion of the scraper (fig. 10-39).

NOTE: Avoid spinning the scraper tires during cut operations. Spinning the tires is nonproductive and causes expensive premature wear of the tires, differential, and transmission.

The material entering the bowl boils back against the ejector and forward against the apron (fig. 10-40). When the bowl is filled to capacity (commonly know as heaped), close the apron, and at the same time, raise the bowl 1 or 2 inches above the ground (fig. 10-41). On scrapers equipped with diverter valves in the apron hoist system, the bowl automatically starts raising, while the apron control lever is held in the lower position. After the scraper is fully loaded and the bowl is raised, continue to travel out of the cut with the scraper bowl at a height that spreads out the material that piles in front of the cutting edges.

The push cat supplies extra power to the scraper during loading operations. When a push cat is used, it should be positioned about 45 degrees off the lane to be cut. The scraper should start loading before the push cat makes a smooth contact with the rear push block (fig. 10-42).

Figure 10-42.-Push block.

NOTE: The push cat operator should be extremely cautious in NOT hitting the rear scraper tires with the dozer blade.

The push cat operator must ensure that the reinforced section of the dozer blade is centered on the scraper rear push block. Additionally, the push cat operator must be alert to turns made by the scraper that might cause the push cat to apply unequal pushing force. This could result in the dozer blade contacting the scraper rear tires or causing the scraper to jackknife. The push cat should continue pushing after the scraper has a full load to give the scraper a boost in leaving the cut.