Heavy wear in a particular area of the inner circumference of a packing ring may be caused by （）.

The output volume of a positive fixed displacement pump can be changed only by （）.

A grease gun is generally used to lubricate a （）.

If the thermal bulb of an expansion valve is incorrectly placed at the center of the coil instead of the end, it will （）.

The general purpose of a heat exchanger is to （）.

The major source of chemical contaminants in hydraulic fluid is （）.

Which of the problems listed would came the discharge temperature of an R-12 refrigeration compressor to increase?（）

When the steering gear is in operation, you should （）.

Which of the following problems could cause the high pressure cutout switch to shut down the compressor in a refrigeration system?（）

The amount of fluid friction or internal friction inherent in a mineral lubricating oil is reflected as a measure of its （）.