Torsional damping

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 [9],... 

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

Infrared spectroscopy and torsional damping are the most reproducible techniques for the measurement of crystallinity. The former is most flexible in accommodating different sample shapes. Averages of duplicate measurements yield the best results. Orientation introduces significant errors into these methods, thus prohibiting their application for paste extruded PTFE samples however, the void content of most paste-extruded PTFE is usually low. 

Torsional Damping - One of the methods to measure the crystallinity of polytetrafluoroethylene is torsional damping. Other methods include infrared spectroscopy, x-ray diffraction, and ultrasonics. 

This involves the determination of the damping of the oscillations of a torsion pendulum, disk, or ring such as illustrated in Fig. IV-8. Gaines [1] gives the equation... 

Free- Vibration Methods. Free-vibration instmments subject a specimen to a displacement and allow it to vibrate freely. The oscillations are monitored for frequency and damping characteristics as they disappear. The displacement is repeated again and again as the specimen is heated or cooled. The results are used to calculate storage and loss modulus data. The torsional pendulum and torsional braid analy2er (TBA) are examples of free-vibration instmments. 

Fig. 42. Torsion pendulum and typical damped sine wave output. P is the period of the motion and M2 are successive ampHtudes (241).

The solutions to a problem of this magnitude can be found in references [3, 7] and others. Figures 9-16 and 9-17 are torsional mode shape diagrams of some typical systems. While the rigorous solution to the multimass damped system is not within the scope of this book, several interesting points should be made. 

The elastomer compression coupling provides both tuning and damp ing 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. 

A torsional spring of stiffness K, a mass of moment of inertia / and a fluid damper with damping coefficient C are connected together as shown in Figure 3.25. The angular displacement of the free end of the spring is 0 ( ) and the angular displacement of the mass and damper is 6a t). 

Tobolsky, A. V. and DuPre, D. B. Macromolecular Relaxation in the Damped Torsional Oscillator and Statistical Segment Models. Vol. 6, pp. 103 — 127. 

One such case arises in the theory of clocks. As is known, a dock is a mechanism consisting of two parts a torsional pendulum with a small damping, and an escapement mechanism replenishing the energy lost by damping in the torsional pendulum. 

We also emphasize that the MD model does include the vibrational motions of bond, and torsional angles (in the minima of the respective potentials) but, somehow, these small scale fast motions are rapidly damped out in the melt, and do not affect the motion on the nanometer scale (and for corresponding times) significantly. 

Figure 5 Schematic diagram of a torsion pendulum and a typical damped oscillation curve. Modified from L. E. Nielsen,...

A particular question of interest is whether the DNA torsional motions observed on the nanosecond time scale are overdamped, as predicted by simple Langevin theory, and as observed for Brownian motions on longer time scales, or instead are underdamped, so that damped oscillations appear in the observed correlation functions. A related question is whether the solvent water around the DNA exhibits a normal constant viscosity on the nanosecond time scale, or instead begins to exhibit viscoelastic behavior with a time-, or frequency-, dependent complex viscosity. In brief, are the predictions for... 

Note 3 A damping curve is usually obtained using a torsion pendulum, involving the measurement of decrease in the axial, torsional displacement of a specimen of uniform cross-section of known shape, with the torsional displacement initiated using a torsion bar of known moment of inertia. 

Figure 5.80 Torsion pendulum for the determination of shear modulus aud damping as functions of temperature at frequencies around 1 Hz. Reprinted, by permission, from N. G. McCrum, C. P. Buckley, and C. B. BucknaU, Principles of Polymer Engineering, 2nd ed., p. 133. Copyright 1997 by Oxford University Press.

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