Prestress or preload is the bias pressure applied to piezoelectric ceramics of power ultrasonic transducers during assembly. It aims to maximize simultaneously the operating power and the coupling of the ceramics to the metallic masses, besides avoiding displacement during vibration.Table 1 - Optimal prestress
Essentially, the optimum prestress depends on the maximum mechanical stress supported by the piezoelectric material, unlike the tightening torque, which varies according to the area of the ceramic pieces and to the friction coefficient of the bolt with the metallic masses. The typical values are 45 MPa for the PZT-8 and 35 MPa for the PZT-4, as shown on the table above (values for Medium Grade Ceramics).
Prestress and torque are proportional; however, the proportionality varies greatly depending on the surface finishing, cleanness and lubrification.
Prestress is a key factor for the lifespan, maximum operating power and efficiency of transducers. Nevertheless, the excess of prestress changes the properties of the ceramics and may cause crushing; whilst the lack of it causes the lateral displacement of the ceramics in high power, leading to cracks, electric arcs and short circuits.
Optimum prestress control and application
PiezoClamping® employs a novel technology in which the prestress is measured during the tightening process accurately, steadily and free of variations.
Traditionally, prestress control is limited to the attempt of standardization via tightening torque control or the measurement of the electric charge generated by the piezoceramics, with no concern for the absolute value applied.
Prestress control based on tightening torque control is practical. However, it is indirect and of low accuracy because the correlation with the prestress depends on several factors and varies drastically with the friction coefficient of the materials and with the lubrication. Additionally, when the bolt gets stuck, it may lead to the application of a lower prestress than expected, even when the target torque is reached.
Optimum prestress application using PiezoClamping®
Connect the PiezoClamping® to the converter keeping the bolt loose, entry the ceramic’s parameters and tighten the transducer until the desired prestress is achieved, as shown on the figures above. Tightening can be applied slowly and with pauses without affecting the result. To assemble the transducer, ensure that interfaces, bolt thread and front mass are clean, dry and non-lubricated (only the bolt head seat can be lubricated to reduce the torque required to achieve the desired prestress). PiezoClamping® must be set for the desired prestress and clamping speed, as well as for the number and specific characteristics of the ceramics used.
To protect the bolt if it gets stuck because of thread damaged or contaminations by epoxy bonding, use a torque wrench with the torque set to 120% of the typical torque required to achieve the desired prestress.
PiezoClamping® is also able to measure the prestress that has been applied to the transducer by simply loosening it with the PiezoClamping® connected and configured.