Control valves stiffness

A nearly infinite stiffness can be attained using controlled restrictors such as servo valves, diaphragms, or spool valves (Morsi 1969 Rowe 1969). However, these systems suffer from dynamic instability due to the resonance of control valve components. The orifice restriction and the constant flow supply system are also sensitive to lubricant viscosity change. When lubricant... 

Eressure difference—see Fig. 8-71). Dynamic stiffness is usually igher with piston designs than with spring and diaphragm actuators see Positioner/Actuator Stiffness. Failsafe action, if necessary, is achieved without a spring through the use of additional solenoid valves, trip valves, or relays. See Valve Control Devices. ... 

Recanalization of tight occlusions of the left innominate vein, which has a long mediastinal course, as may be caused by mediastinal tumor or fibrosis, can require a combined femoral and axillary or a bilateral axillary approach (Fig. 5.6). When the guidewire has crossed the occlusion, it is grasped with a snare, a basket, or a flexible forceps on the other side, and retrieved through the haemostatic valve sheath at the second percutaneous entry point. This makes it possible to insert a PTA catheter or any other stent delivery catheter over a stiff Amplatz-type guidewire, which is controlled and straightened at both ends. 

There are many uses for such shape memoiy alloys. The curved shape in a dental brace, for example, can be formed at high temperature into the curve that the teeth are desired to follow. Then at low temperature, where the metal is flexible, it can be shaped to fit the mouth of the wearer of the braces. When the brace is inserted in the mouth and warms to body temperature, the metal passes into the stiff phase and exerts a force against the teeth as it attempts to return to its original diape. Other uses for shape memory metals include heat-actuated shutoff valves in industrial process lines, which need no outside power source. Inserted into the face of a golf club, diape memory metals are said to impart more spin to the ball and greater control of the dub. 

Employing data taken from patients (Hess et ai, 1984) with aortic stenosis (AS), aortic insufficiency (AI) and with mixed lesions (AI -I- AS) pre-operatively and following successful valve replacement (ave. 17.5 yr), the myocardial stiffness-stress relations are evaluated for each group and compared with that for the group of controls (Figure 5). 

At common levels of stress, the stiffness-stress relations pre-operatively were not significantly different in valve disease patients compared to controls. Following surgery, intrinsic myocardial stiffness was significantly increased in the AS group (p<.04, paired t-test). These results are in qualitative agreement with those obtained by Hess et al. (1984) who employed a more complex model. 

Figure 5. Myocardial stiffness-stress relations for groups of patients with AS, AIAS and AI pre- and post valve replacement compared with controls (C). Pre-operatively, there were no significant differences compared with controls. In AS and AIAS there were increases in the intrinsic stiffness following surgery and the increase was significant in the AS group (p<.04, paired t-tcst). Mean values SEM are given at end diastole.

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