Prestress technology for piezoelectric ceramics
Prestress is the average static stress to which the ceramics of a transducer or converter are subjected during bolt preloading. It aims to maximize simultaneously the operating power and effective contact area between elements, besides avoiding displacements during vibration. In typical ultrasonic transducers and converters, wherein shear forces between the ceramics and the metallic masses are negligible, the pre-tension coincides with the average static pressure on the piezoelectric ceramics.Table 1 - Typical optimal prestress (±14 %).
The optimum prestress depends on the piezoelectric material compressive limit and stress distribution in the piezoceramics (usually, the stress is maximum close to the bolt and minimum at the edge). The typical ideal prestress values are 45 MPa for the PZT-8 and 35 MPa for the PZT-4, as shown on table 1 above. Unlike prestress, the torque varies according to the section area of the ceramic pieces, bolt dimensions and friction coefficients.
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 an innovative technology developed by ATCP in which prestress is measured accurately and in real time during bolt preloading. Usually, prestress control has been limited to torque control or by electrical charge storage, without due concern for the prestress absolute value effectively applied in pressure units (MPa or PSI).
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 retained prestress by simply loosening the transducer bolt with PiezoClamping® connected and configured.