The following criteria should be considered in the selection of material for a centrifugal pump shaft:
• Endurance limit
• Corrosion resistance
• Notch sensitivity
The endurance limit is the stress below which the shaft will withstand an infinite number of stress reversals without failure. Since one stress reversal occurs for each revolution of the shaft, this means that ideally the shaft will never fail if the maximum bending stress in the shaft is less than the endurance limit of the shaft material.
In practice, however, the endurance limit is substantially reduced because of corrosion and stress raisers, such as threads, keyways, and shoulders on the shaft. In selecting the shaft material, consideration must be given to the corrosion resistance of the material being pumped as well as to its notch sensitivity. Corrosion will substantially lower the fatigue limit of the material. Fatigue cracks will initiate at corrosion pits or other surface
discontinuities that act as stress risers.
In the absence of corrosion, an approximate relationship exist between fatigue endurance limits and mechanical properties. The endurance limit is equal to roughly half the tensile strength of the material. Depending upon the application, the centrifugal pump designer
will usually select the least expensive shaft material that will satisfy the three criteria noted previously. Carbon steel is used when corrosion resistance is not required, and relative low mechanical properties can be tolerated. A low alloy steel, often AISI 4140, is used when the mechanical properties of carbon steel are not adequate. Martensitic stainless steels, usually type 410, are a common choice when some measure of corrosion resistance, combined with reasonably good mechanical properties, is required. The resistance of type 410 stainless to fatigue crack initiation is related to the toughness of the material, which can vary over a wide range in commercial bar stock. The best measure of material toughness is the Charpy impact test, which is not a requirement of most relevant material specifications. In order to ensure the optimum toughness and resistance to fatigue cracking, type 410 stainless should be tempered at a temperature of 1100°F (593°C) minimum.
Stainless steels offering improved corrosion resistance and mechanical properties are also used in centrifugal pump shafting
. These include Nitronic 50, an austenitic grade, and 17-4PH, a precipitation hardening grade.
Several manufacturing issues related to the creation of pump shafts also need to be considered. Difficulty has frequently been experienced when maintaining the stringent tolerances for straightness required for long, thin shafts that are used in multi-stage pumps. This may necessitate intermediate stress relief during the machining process.The shaft may be stress-relieved in the vertical position or, if horizontal, with supports every few feet. Type 410 pump shafting
can be specially heat-treated to eliminate residual stress and maintain straightness tolerances during centrifugal pump operation
Shafts can be plated or coated in specific areas for an improved resistance to wear or corrosion. Chrome plating is commonly used in this manner. The designer needs to be aware that plating reduces the fatigue endurance limit because of the fine micro-cracking associated with this process. An improvement in fatigue life can be achieved on plated surfaces if the substrate is shot peened. Normally, this reduction in endurance limits is not critical, because it is the flat surfaces, rather than the shoulders or keyways, that are being plated. Consequently, the largest stress raisers are elsewhere in the shaft. There is also the potential for the plating process to cause hydrogen embrittlement cracks in high-strength steels.To avoid this problem, the steel should be baked at 300 to 400°F (150 to 200°C) after the plating process.
Finally, the manufacturing process needs to be carefully controlled to avoid the inadvertent introduction of stress raisers, which could shorten the life of the shaft. Abusive grinding has been identified as the root cause of some shaft fatigue failures. Heavy grinding will heat the surface and cause hardenable steels to form a thin layer of untempered martensite. This is a brittle structure and likely to develop fine cracks. These are stress raisers that can be propagated by a fatigue mechanism once the slurry pump
is placed in service. This type of problem is avoided by proper controls on the manufacturing process.