Torsional Damping and Resilient Coupling

Many believe that couplings are only torque transmitters between different pieces of equipment with a secondary function of handling ever present misalignment.

While couplings are typically sized and chosen based on the before mentioned requirements, they can take on another role, that of a torsional fixer

Once the driver and driven equipment have been chosen and it is deter mined that none of the items will be subject to any lateral vibration problems, the system torsional analysis is performed. If a calculated torsional natural frequency coincides with any possible source of excitation (Table 9 2), the system must be de-tuned in order to assure reliable operation A good technique to add to the torsional analysis was presented by Doughty [8j, and provides a means of gauging the relative sensitivity of changes in each stiffness and inertia in the system at the resonance in question.

Typically, the couplings in a turbomachinery train will be the softest torsional elements in the system. As a result, they represent the controlling factor in the system's overall stiffness since the total system spring rate cannot exceed the stiffness of the softest spring.

Should analysis indicate that a coupling is in a sensitive position, then a small amount of custom design in a relatively standard coupling can accommodate the de-tuning of the critical in question. One note of caution: while changes in stiffness or inertia may de-tune a given resonance, their effect on the other criticals must also be determined, since any change in the system will result in a new set of resonant frequencies.

If modification to coupling stiffness cannot effectively de-tune the system, or if unanticipated excitation frequencies are encountered in the field, the designer has another option to desensitize the train. This "last resort" or "shotgun" approach involves the resilient or torsionally damping coupling. These couplings add damping to the system in order to dissipate vibrational energy and effectively reduce twist amplitudes during a resonant condition. While this does not eliminate the source of the problem, it does allow the manageable operation of the unit under resonant conditions,

Once again, a resilient coupling applied during a resonant condition must be at a location sensitive to the applied damping. Since damping is a function of the relative velocities of the coupling hubs, little would be gained by placement at a node or points of small angular velocity changes.

Another alternative to consider if operation on a known resonance is anticipated is a stress analysis. It is possible that the stresses at resonance may be within an acceptable level permitting the compressor train to operate without a problem.

Specification

A guideline for specification of torsional damping and/or resilient couplings can be found in Appendix B of API Standard 671, Special-Purpose Couplings for Refinery Service 19].

The majority of torsional resilient or damping couplings currently in use can be classified into five major categories:

L Quill shaft

2, Metal-metal resilient

3. Elastomer, compression

4, Elastomer, shear

5. Fluid

The torsionally soft or resilient coupling such as the quill shaft and the metal-metal resilient transmit torque can handle misalignment through springs, metal strips, coils, disks and diaphragms. They will tune the system by changing the spring constant K.

The elastomer compression coupling provides both tuning and damping to the system. In some cases, the two functions interact, that is, the stiffness K or damping C may be a function of the other. The elastomers are torsionally softer than the metal-metal resilients, but will introduce higher levels of damping into the system.

There are two major types of elastomer compression couplings. One is the torus type in which the elastomer is attached directly to the coupling hubs (see Figure 9-18). The other is the compression type with the elastomer held in compression by the hub geometry (see Figure 9-19).

Probably the obvious question that arises after the preceding discussion is, "Why not use a soft, highly damped coupling in every case and sidestep the torsional problem altogether?" The solution is often compromise. On the good side, these various couplings provide greater latitudes in the selection of coupling characteristics to solve the torsional problems mentioned earlier. Furthermore, if analysis fails to predict a torsional problem and one arises in the field, these couplings are a quick and inexpensive means of bringing the unit back on line.

On the bad side, many of the elastomeric types are highly nonlinear in their characteristics. The elastomeric compression-type couplings are very soft at small wind-ups under low loads, but once the elastomer has filled the available squeeze space, the coupling is effectively rigid. This makes prediction of system response difficult unless the load and coupling characteristics are well defined prior to installation.

The elastomeric couplings generally do not have a life factor equivalent to a gear or flexible element coupling. This is further complicated by the fact that if the coupling is to provide damping, the dissipated vibrational energy is converted to heat, which can further shorten the life of the ele-

Figure 9-19. Elastomeric type coupling, commonly used tor torsional system tuning. (Courtesy of Kop-Flex Power Transmission Products)

Figure 9-18. Torus type coupling. (Courtesy of Falk Corporatiorfi

Figure 9-19. Elastomeric type coupling, commonly used tor torsional system tuning. (Courtesy of Kop-Flex Power Transmission Products)

ment. Likewise, any elastomer will degrade with time, resulting in a coupling with characteristics that are nonlinear with both load and time of service.

Most torsional vibration problems can be flagged prior to installation of a compressor system by performing a thorough torsional analysis. Typically, the system can be de-tuned by varying the torsional stiffness of a spacer type coupling with very little modification to a standard design. However, if de-tuning with coupling stiffness alone turns out to be ineffective, a stress analysis can predict a problem, or if field operation results in a system failure, a resilient damping type coupling may be the answer.

L Jackson, Charles, The Practical Vibration Primer, Houston, TX: Gulf Publishing Company, 1979,

2. Feldman, S„ Unbalance Tolerances and Criteria, Proceedings of Balancing Seminar, Vol. IV, Report No. 58GL122, General Engineering Laboratory, General Electric, Schenectady, N.Y., 1958.

3. ISO 1940/1, Mechanical Vibration—Balance Quality Requirements of Rigid Rotors—Part 1: "Determination of Permissible Residual Unbalance," First Edition, International Organization for Standardization, Geneva, Switzerland, 1986.

4. Harker, Ralph J., Generalized Methods of Vibration Analysis, New York; John Wiley & Sons, 1983.

5. Den Hartog, J. P., Mechanical Vibrations, Fourth Edition, New York: McGraw-Hill Book Company, 1956.

6. Marks, Lionel Simeon, Ed., Standard Handbook for Mechanical Engineers, Eighth Edition, Baumeister, Theodore, et al, Eds., New York: McGraw-Hill Book Company, 1978, pp. 5-73.

7. Myklestead, N. O., "A New Method of Calculating Natural Modes of Uncoupled Bending Vibration of Airplane Wings and Other Types of Beams," Journal of the Aeronautical Sciences, Vol. II, No. 9, April 1944, pp. 153-162.

8. Prohl, M. A., "A General Method for Calculating Critical Speeds of Flexible Rotors," ASME, Journal of Applied Mechanics, September 1945, pp. 142-148.

9. Kerr Wilson, W., Practical Solution of Torsional Vibration Problems, Vols. 1 & II, London: Chapman & Hall, Ltd., 1956.

10. Doughty, S., "Sensitivity of Torsional Natural Frequencies," ASME 76-WA/DE-18, New York: American Society of Mechanical Engineers, 1976,

11. API Standard 671, Special-Purpose Couplings for Refinery Services, Second Edition, Washington, D.C.: American Petroleum Institute, 1990. Reaffirmed 1993.

12. Brown, Royce N,, "A Torsional Vibration Problem as Associated with Synchronous Motor Dri ven Machines," ASME 59-A-141, Journal of Engineering for Power, Transactions of the ASME, New York: American Soci ety of Mechanical Engineers, 1960, pp. 215-225.

13. Brown, Royce N., Torsional-Damping—Transient and Steady State, Proceedings of the Thirteenth Turbomachinery Symposium, Texas A&M University, College Station, TX, 1984, pp. 203-207.

14. Chapman, C. W„ "Zero (or Low) Torsional Stiffness Couplings," Journal Mechanical Engineering Science, Vol. II, No. 1» 1969, pp. 76-87.

15. Eckert, Joachim, "Transient Torques and Currents in Induction Motors Resulting from Supply Changeover and Other Transient Conditions," System and Equipment, pp. 103-106.

16. Hafner, K. E., "Torsional Stresses of Shafts Caused by Reciprocating Engines Running through Resonance Speeds," ASME 74-DGP1, New York: American Society of Mechanical Engineers, 1974.

17. Hizume, A., "Transient Torsional Vibration of Steam Turbine and Generators Shafts due to High Speed Reclosing of Electric Power Lines," ASME 75-DET-71, New York: American Society of Mechanical Engineers,

1975.

18. Holdrege, J. H„ Subler, William, and Frasier, William E., "A.C. Induction Motor Torsional Vibration Consideration—A Case Study," IEEE Paper No. PC 1-8J-2, pp. 23-27.

19. McCormick, Doug, "Finding the Right Flexible Coupling," Design Engineering, October 1981, pp. 61-66.

20. Pollard, Eniest I., "Synchronous Motors . . . Avoid Torsional Vibration Problems," Hydrocarbon Processing, February 1980, pp. 97-102.

21. Pollard, Ernest L, Torsional Vibration Due to Induction Motor Transient Starting Torque, undated manuscript.

22. Pollard, Ernest !., "Transient Torsional Vibration Due to Suddenly Applied Torque," ASME 71-VIBR-99, New York: American Society of Mechanical Engineers, 1971.

23. Porter, B., "Critical Speeds of Torsional Oscillation of Geared-Shaft Systems Due to the Presence of Displacement Excitation," ASME 63-WAS, New York: American Society of Mechanical Engineers, 1963.

24. Söhre, J. S., "Transient Torsional Criticáis of Synchronous Motor Driven, High-Speed Compressor Units," ASME 65-FE-22, New York: American Society of Mechanical Engineers, 1965.

25. Wallis, R. R.f "Flexible Shaft Couplings tor Torsionally Tuned Systems" ASME 70-PET-38, New York: American Society of Mechanical Engineers. 1970.

Living Off The Grid

Living Off The Grid

Get All The Support And Guidance You Need To Be A Success At Living Off The Grid. This Book Is One Of The Most Valuable Resources In The World When It Comes To When Living Within The Grid Is Not Making Sense Anymore.

Get My Free Ebook


Post a comment