MD6250 Rotary Drill Systems Caterpillar

Air Compressor Drive

Illustration 1	g06266741
(1) Engine (2) Air compressor (3) Viewing cover

The air compressor (2) is bolted to the back side of the engine (1) with the use of a rubber drive coupling. The coupling can be viewed and inspected through the viewing cover (3).

Air Compressor Control Circuit

Low-Pressure Compressor (If Equipped)

The compressor unit is a single stage positive displacement, flood lubricated type rotary screw air compressor. As the rotors turn, air is drawn axially into the compressor (1) when the rotors turn past the intake port. The air is trapped between the rotor lobes and the compressor housing. With the rotors turning, the lobes reduce the volume in the cavities, compressing the trapped air where the air is then discharged.

Air Compressor Oil

Upon start-up, the fluid temperature is cool and routing to the cooler is not required. The fluid first enters the thermal valve and then flows on to the compressor unit, bypassing the cooler. As the compressor continues to operate, the temperature of the fluid rises to 77° C (170° F) causing the internal thermal valve to move. This movement causes a portion of the fluid entering the valve to be directed to the cooler. The fluid is cooled and returned to the thermal valve. The cooler is a radiator type cooler that works with the fan. The fan draws air through the cooler removing the heat of compression from the fluid. The pressure bypass valve, insures a supply of fluid to the compressor during periods of excessive pressure drop across the cooler. After the fluid passes through the thermal valve, the fluid is then directed through the main fluid filter. The fluid is filtered in preparation for injection into the compression chamber and bearings of the compressor unit. The filter has a replaceable element and a built-in bypass valve. The bypass valve allows the fluid to flow even when the element becomes plugged and requires changing, or when the viscosity of the fluid is too high for adequate flow. After the fluid is properly filtered, the fluid lubricates, seals, and cools the compression chamber as well as lubricates the bearings and gears. The fluid stop valve functions on shutdown when the valve shuts off the fluid supply to the compressor unit and keeps compressor from pressurizing and filling with fluid. The fluid stop valve is held open by a pressure signal from the compressor unit. At shutdown, the pressure signal is lost and the fluid stop valve closes, isolating the compressor unit from the cooling system.

Compressor Oil Cooling Circuit

Illustration 2	g06289970
Compressor Oil Circuit (A) Air/oil (B) Cool oil (C) Hot oil (D) Scavenge Circuit (1) Air compressor (2) Compressor High Discharge Temperature Switch (3) Discharge Temperature Gauge Sender (4) Compressor Oil Temperature Transducer (5) Oil Pressure Transducer (6) Flow to Shutdown Blowdown Valve N/O (7) Oil Stop Valve (8) Oil Distribution Manifold (9) Compressor Oil Filter (10) Thermostatic Valve (11) Oil Cooler (12) Air Receiver Tank (13) Orifice (14) Sight Glass (15) Strainer (16) Dry Side Transducer (17) Wet Side Transducer (18) Minimum Pressure Valve (19) Inner Separator Element (Dry Side) (20) Outer Separator Element (Wet Side) (21) Safety Relief Valve (22) Poppet Intake Valve (23) Sight Glass

The air compressor (1) is cooled and lubricated by oil. Oil in the receiver tank (12) flows to the thermostatic valve (10). If the oil is cool, the oil will flow to the oil filter (9). If the oil is hot, the thermostatic valve opens and directs oil to the oil cooler (11).

The oil cooler is mounted as part of the radiator core and is cooled by the engine fan. Oil then flows to the oil filter (9) and then to the air compressor (1).

High-Pressure Compressor (If Equipped)

The compressor unit is a two stage positive displacement, flood lubricated rotary screw air compressor. As the rotors turn, air is drawn axially into the compressor when the rotors turn past the intake port in the first stage. Air is then trapped between the rotor lobes and the compressor housing. With the rotors turning, the lobes reduce the volume in the cavities, compressing the trapped air and discharging the air into the second stage. This process happens again in the second stage of the compressor where the air is further compressed to obtain a higher pressure.

Air Compressor Oil

The compressor cooling and lubrication system is designed to provide adequate lubrication and maintain the proper operating temperature of the compressor. For lubricant life, run the lubricant and discharge temperature as low as possible. However, depending on the inlet temperature and relative humidity of the air entering the compressor inlet and the discharge pressure of the compressor, the compressed air can have water condensed in the fluid separator.

Upon start-up, the fluid temperature is cool and routing to the cooler is not required. The fluid first enters the thermal valve and then flows on to the compressor unit, bypassing the cooler. As the compressor continues to operate, the temperature of the fluid rises to 115.5° C (240° F) causing the internal thermal valve to move. This movement causes a portion of the fluid entering Port A to flow out of Port C to the cooler. The fluid is cooled and returned to the thermal valve at Port B. The fan forces air through the cooler removing the heat of compression from the fluid. From the cooler, the fluid is routed back to the thermal valve, entering at Port B. The pressure bypass valve insures a supply of fluid to the compressor during periods of excessive pressure drop across the cooler which may occur in cold weather. After the fluid passes through the thermal valve, the fluid is then directed through the main fluid filter. There the fluid is filtered in preparation for injection into the compression chamber and bearings of the compressor unit. The filter has a replaceable element and a built-in bypass valve which allows the fluid to flow even when the element becomes plugged and requires changing, or when the viscosity of the fluid is too high for adequate low. After the fluid is properly filtered, the fluid lubricates, seals, and cools the compression chamber as well as lubricates the bearings and gears. The fluid stop valve functions on shutdown when the valve shuts off the fluid supply to the compressor unit and keeps compressor from pressurizing and filling with fluid. The fluid stop valve is held open by a pressure signal from the compressor unit. At shutdown, the pressure signal is lost and the fluid stop valve closes, isolating the compressor unit from the cooling system.

Compressor Oil Cooling Circuit

Illustration 3	g06354014
Compressor Oil Circuit (A) Air/oil (B) Cool oil (C) Hot oil (D) Scavenge Circuit (1) Air compressor (2) Poppet Intake Valve (3) Sight Glass (4) Orifice (5) Air Receiver Tank (6) Compressor Oil Filter (7) Thermostatic Valve (8) Oil Cooler

The air compressor (1) is cooled and lubricated by oil. Oil in the receiver tank (5) flows to the thermostatic valve (8). If the oil is cool, the oil will flow to the oil filter (7). If the oil is hot, the thermostatic valve opens and directs oil to the oil cooler (9).

The oil cooler is mounted as part of the radiator core and is cooled by the engine fan. Oil then flows to the filter and then to the air compressor .