Section 9.3 - Pump Couplings: Design Criteria

Design Criteria 

COUPLING BORES  Couplings are mounted to the pump and driver shafts using clearance fit or shrink fit. They can be fitted with keys or not and can be cylindrical or tapered style depending on the shaft to which they are attached. Standards have been developed to assure communication between the equipment manufacturer and the coupling manufacturer. See the ANSI/AGMA references at the end of this chapter. Shaft standards exist and the coupling bore standards accommodate those whenever possible. It is important that the coupling bore is suitably concentric with the hub OD and that the shaft have little run-out so that extra misalignment or unbalance is not introduced to the system. It is

FIGURE 28  Vertical pump with flexible shafting (Flowserve Corp)

also important that the shaft size, coupling bore, and torque transmission method are suitable for the amount of torque to be transmitted.

Clearance fits with keys are used for low power transmission and easy removal of the coupling hub when maintenance is needed. Tapered shafts with keys can be used for easy removal also. Shrink fits with keys and heavy shrink tapered fits without keys are used for high-power, high-speed applications.

TORSIONAL STRESS   The torsional stress in the shafting may be calculated by the following equations:

The allowable shear stress depends upon the material being used and whether it is subjected to other loads, such as bending or compression. The design safety factor on the shafting should be equal to or greater than those of the other components in the drive train.

CRITICAL SPEED  The lateral critical speed of a drive shaft is determined by the deflection, or "sag," of the shaft in a horizontal position under its own weight. The less the sag, the higher the critical speed. In practical terms, a long, slender shaft will have a low critical speed and a short, large-diameter shaft will have a very high critical speed. The deflec­tion of a simply supported shaft is calculated as follows:

'

Knowing the natural deflection of the shaft, it is possible to calculate te final critical speed from the equation:

which expresses critical speed directly in revolutions per minute of the rotating shaft.

In practice, the critical speed should be placed well away from the operating speed of the shaft. Actual operating conditions, such as imbalance and clearance in bearings and couplings, tend to reduce the critical speed from its theoretical value. The designer has sev­eral options from which to choose to attain the desired critical speed: (1) vary the shaft diameter, (2) use tubular shafting to increase stiffness and reduce deflection, and (3) vary the bearing span.

Varying the diameter of the shaft has a dramatic effect on shaft deflection because the deflection decreases inversely with the cube of the diameter. The weight of the shaft will increase proportionately to the square of the diameter and may consequently impose excessive loads on hearings. When this is the case, tubular shafting may be substituted for solid shafting. The ratio I/w is much higher for a tubular shaft than for a solid shaft of the same diameter, so the shaft deflection is again reduced and critical speed increased.

The bearing span may be varied when the size and weight of a single length is broken into sections, with a single engagement coupling used to connect each section and acting as a hinge joint. A self-aligning bearing is placed on each section immediately adjacent to the coupling. Each section may then be treated as an individual shaft, which will result in a substantial reduction in shaft size. The rigidity of the support at each hearing must be taken into consideration when calculating the critical speed of the shaft.

DYNAMIC BALANCE  Drive systems that operate at high speeds and connect two widely separated shafts must be given special consideration with regard to dynamic imbalance. Care must be taken to ensure that the long, slender shaft used in such systems is not bent, either in manufacture or in installation. These systems quite often require shaft­ing that has been dynamically balanced after manufacture to compensate for these normal errors.

REFERENCES

1. American Petroleum Institute Standard 610. "Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries", 10th ed., 1220 L Street, Washington, DC 20005, 2004.

FURTHER READING

American Gear Manufacturers Association, 1500 King Street, Suite 201, Alexandria, VA, 22314.

ANSI/AGMA 9000 Flexible Couplings - Potential Unbalance Classification.

ANSI/AGMA 9002 Bores and Keyways for Flexible Couplings (Inch Series).

ANSI/AGMA 9112 Bores and Keyways for Flexible Couplings (Metric Series).

ANSI/AGMA 9003 Flexible Couplings - Keyless Fits.

ANSI/AGMA 9004 Flexible Couplings - Mass Elastic Properties and Other Characteristics.

American Petroleum Institute Standard 671. "Special Purpose Couplings for Petroleum, Chemical, and Gas Industry Services." 3rd ed., 1220 L Street, Washington, DC 20005, October, 1998.

Design Criteria 

COUPLING BORES  Couplings are mounted to the pump and driver shafts using clearance fit or shrink fit. They can be fitted with keys or not and can be cylindrical or tapered style depending on the shaft to which they are attached. Standards have been developed to assure communication between the equipment manufacturer and the coupling manufacturer. See the ANSI/AGMA references at the end of this chapter. Shaft standards exist and the coupling bore standards accommodate those whenever possible. It is important that the coupling bore is suitably concentric with the hub OD and that the shaft have little run-out so that extra misalignment or unbalance is not introduced to the system. It is

FIGURE 28  Vertical pump with flexible shafting (Flowserve Corp)

also important that the shaft size, coupling bore, and torque transmission method are suitable for the amount of torque to be transmitted.

Clearance fits with keys are used for low power transmission and easy removal of the coupling hub when maintenance is needed. Tapered shafts with keys can be used for easy removal also. Shrink fits with keys and heavy shrink tapered fits without keys are used for high-power, high-speed applications.

TORSIONAL STRESS