Shafts are made of material ranging from medium carbon to low alloy steel and are usually heat treated. Shafts were originally made of forgings for the compressors in process service. But because of the availability of high quality material, hot rolled bar stock has been used for shafts up to 8 inches in diameter. Bar stock shafts are given the same heat treatment and quality control as forgings. Many of the process users prefer a low alloy, chrome-moly-nickel material for shafting, particularly for compressors in critical service.

Shafts require a good finish that can be achieved by machining. Honing, or sometimes grinding, is used to improve the finish in selected areas. Since proximity probes are used with most process compressors, the probe area must receive extra attention to minimize mechanical and electrical runout. On the whole, the shaft is the foundation for good mechanical performance to keep the rotor dynamics in control and maintain good balance. The requirements are that the shaft must be round and all turns must be concentric to the journals. As simple as it sounds, it is not easy to accomplish. The tighter the tolerance, the closer to perfection, the more expensive that particular manufacturing step. However, some added expense at this point will save time in subsequent rotor balancing providing the user with a rotor that can be more easily maintained. By using CNC machine tools for manufacturing shafting, the cost should come down, quality improve, and the product should become more consistent.

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