单项选择题

If a reciprocating air compressor has a knock occurring in frequency with its operating RPM, the cause is probably （）.

A.misalignment or worn main bearings
B.insufficient cylinder lubrication
C.defective or poorly fitted valves
D.all of the above

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单项选择题

Why are removable sleeves installed on centrifugal pump shafts?（）

A.They make it easier to replace the pump shaft packing
B.They can be economically replaced as they wear out
C.They can be removed when it is necessary to lighten the weight of the pump
D.They increase the strength of the shaft

单项选择题

In reference to air conditioning, when air attains the maximum amount of moisture it can hold at a specific temperature, it is said to be （）.

A.superheated
B.saturated
C.condensed
D.convected

单项选择题

An axial轴向 piston pump differs from a radial piston pump as the pistons of an axial piston pump are positioned （）.

A.radially from the shaft
B.parallel to each other and to the shaft
C.parallel to each other but at a right angle to the shaft
D.at an angle to each other and to the shaft