COUPLING SELECTION AND DESIGN
In order to decrease the time required to specify a coupling for an application, significant analysis is devoted to determining ratings for standardized sizes. A torque capacity is determined for the disc pack operating at a given speed, angular and axial misalignment. This is referred to as the maximum continuous torque (MCT) rating and the coupling is designed for infinite life if it is operated at or below these limits. In order to determine the coupling rating, the torque, axial and angular misalignment, and speed are used in combination to determine the Safety Factor. The relationship between mean and alternating stresses are plotted using a modified Goodman diagram as seen below (Image 1).
Image 1 – Goodman diagram
Image 2 – Reduced Moment High Performance Disc Coupling with interlocking hubs
Additionally, axial thermal growth values the coupling will experience are required so the coupling can be designed to accommodate the thermal growth. The customer can request special features to be included such as an integrated torque meter, shear section for torque overload protection, and electrical insulation to name a few.
For a given application, some uncertainty in operating conditions will remain. For this reason, API 671 requires a Service Factor to be applied to the nominal steady state torque of the equipment. The Service Factor can account for any unknown vibratory torques, transient torques or any other abnormal operating conditions which could be more demanding on the coupling than anticipated
Once the design has been completed for a given application, the mass elastic data (M.E.D) is provided on the coupling drawing. The data included is the half weight and center of gravity (CG) location, the moment of inertia as well as coupling stiffness – axial, angular, and torsional. M.E.D. is used when performing the lateral and torsional analyses on the drivetrain.
Current turbomachinery design trends force compressors to operate at higher speeds to achieve increased efficiency. This highly engineered equipment can achieve the same pressure ratios as larger equipment and can therefore perform the same process in a smaller package. A reduction in casing size results in the use of smaller shafts and therefore smaller bearings. Long slender shafts with increased bearing spans operated at higher speeds are significantly more sensitive to rotordynamic issues.
Image 3 – High Performance Diaphragm Coupling
Image 4 – Hub (L-R) keyed straight bore, polygon bore, splined, keyless tapered keyed tapered hub
These rotordynamic issues can be mitigated through the use of a smaller lighter weight coupling. A lighter weight coupling also lowers the imbalance, providing better dynamic stability and improving the system response. The flexible elements of a reduced moment style coupling are mounted on the coupling hubs which decrease the load on the bearing when operated at high speeds. Since the center of gravity is moved closer to the bearing, the overhung moment is reduced which directly effects the lateral critical speed (LCS) of the train. The larger the overhung moment, the lower the LCS.
Given the adjustability of the coupling design, it is typically the most convenient and lowest cost location to tune the drive train. Because the coupling is typically the least torsionally stiff component of the drive train, its stiffness greatly affects drive train performance and makes it the perfect component to modify. Whether this means using a larger, stiffer tubular spacer, a softer, smaller solid spacer, or changing the coupling component material to lightweight titanium, all of these options can be used in order to achieve optimal drivetrain performance.
Coupling design continues to advance as the modeling and testing technologies improve. Kop-Flex has developed proprietary software (Coupling Automation Program) to assist with streamlining coupling selection for our engineers. Based off the required inputs discussed previously (DBSE, Speed, HP, Bore Sizes and Interface connections), the software selects the coupling and creates a drawing which can then be fine-tuned by our engineering team as necessary. While fairly simple in theory the actual steps required to complete a selection and design can be extensive. As equipment continues to increase in horsepower and speed, requiring a reliable design is of even greater importance.