THEORY OF OPERATION
This work package explains how the systems and components of the machine work together. A functional description is
provided for each major component and system.
The 621B Scraper is a wheeled machine used for the transport, self-load and self-unload of varying types of soils for use
in construction of roadways and fill areas. Power is by a Caterpillar in-line, six-cylinder, direct-injected, turbocharged
diesel engine. The low and extra-wide scraper bowl operates by high-speed hydraulics, with cutting edge near center of
bowl for minimum material travel. A power-closing apron with ejector allows positive materials placement. The 621B
Scraper is capable of full 90-degree left or right articulated steering. Cantilever-mounted wheel assemblies are equipped
with 28 ply rating (earthmover-rock tread service) tires. Sound-suppressed ROPS cab has insulation, heater and win-
dows for all-weather operation. Air-cushioned seat provides additional operator comfort.
The scraper is equipped with a Caterpillar 3406 turbocharged, direct-injected, valve-in-head, in-line six cylinder diesel
engine. Parallel manifold porting is used with two intake and two exhaust valves per cylinder. An ether injector is connected to
the air intake manifold system to assist in cold weather starting. An engine oil sampling valve, located at front-right side of
engine, next to oil fill access, is supplied for sampling engine oil condition. Refer to LO 5-3805-248-12 and TB 43-0210 for oil