Discussion of Permanent Set and Fatigue Life of Q-12 Flow Sensing Blade
Model Q-12 Flow Switch employs a 316 Stainless target blade which is inserted into the fluid flow which produces a drag force on the target blade which is then displaced in a downstream direction. Fig. I illustrates the basic configuration of this design approach.
As shown in the side view of Fig. I, the target blade is bent downstream in a fixed position and shape by flow induced forces. Superimposed on this “fixed position” is oscillatory motion produced by turbulence in the fluid flow. The target blade is subjected to a steady stress level plus a variable stress level caused by turbulence.
Steady state stress can cause slow creep in a bending blade or beam over long periods plus turbulent noise oscillations can cause beam fatigue which can change the response of the beam to flow drag forces which in turn produces a drift in the switch point and may ultimately lead to failure of the target blade due to fatigue.
The target beam design employed in Model Q-12 plus other design features reduce both steady state and turbulent noise stresses to negligible levels thus providing no significant switch point drift or turbulent noise fatigue for periods measured in years. Fig. Ill illustrates the design approach used in Model Q-12 to provide this long term trouble free performance.
As shown in Fig. III, the rigid support post restricts the maximum deflection of the target blade and at the same time eliminates the turbulence induced vibration of the blade, thus simultaneously accomplishing the two important performance requirements for long-term elimination of drift and fatigue discussed above. The maximum stress level in the target beam can be further reduced by employing special shapes which are used in Model Q-12. The maximum upper limit of the stress level in the Q-12 bending blade can be measured using standard bending beam equations and “worst case” geometry. Figure IV illustrates the worst case geometry used in our maximum stress measurement program.
Standard generally accepted engineering practice in the design of all types of springs including the bending target blade used in the Q-12 Flow Switch is to limit the maximum stress level to 50% or less of the yield strength of the material employed. This will provide a bending blade with essentially unlimited life with regard to both steady state stresses and turbulent noise vibration stresses. A calculation of the maximum stress level for the worst case target beam illustrated in Fig. IV gave a value of 23% of yield, stress level. This is substantially less than the generally accepted 50% value. When the Q-12 Flow Switch special tapered target configurations shown in Fig. V is introduced the maximum stress level drops to less than 15% of the yield stress level. Thus Model Q-12 Flow Switch is clearly not subject to fatigue due to steady or vibrational loads.