What is the material commonly used for packing in a stuffing box?

The material commonly used for packing in a stuffing box is typically metal with oil running through it. This design helps to ensure an effective seal that prevents leakage while accommodating the movement of the shaft. The metal packing provides durability and resistance to wear, while the oil helps to lubricate the moving parts, minimizing friction and heat generation. This combination enhances the efficiency and longevity of the equipment by maintaining a secure seal and reducing the risk of thermal degradation and wear. The use of oil also assists in dissipating heat, which is crucial in mechanical systems where components operate under high temperatures.

In contrast, other materials and combinations presented in the options do not offer the same level of effectiveness for this application. For example, plastic with refrigerant could compromise the integrity of the seal and may not withstand high pressure or temperatures. Rubber, while effective in some applications, may not provide the necessary strength and durability when exposed to extreme conditions typically present in machinery with a stuffing box. Lastly, copper without lubrication would likely lead to increased friction and premature failure of the packing due to lack of necessary lubrication to facilitate smooth operation.