Seldom will the ceramic component of a composite transducer have adequate tensile strength to withstand the high mechanical stress associated with the power demands for ultrasonic cleaning applications. The tensile strength of the ceramic elements can be supplemented by mechanically pre-stressing the elements along the direction of polarization. Pre-stress is introduced by incorporating a single, large, central bolt or several smaller, peripherally arranged bolts into the design of the transducer. The single central bolt design offers slightly higher efficiency than the multiple peripheral bolt design, but manufacturing costs can be higher, assembly can be more difficult and, physically, the transducer will be significantly longer.

How Does An Ultrasonic Transducer Work

For the conditions under which ultrasonic cleaning devices are operated, pre-stress of less than approximately 30 MPa usually is sufficient to protect the ceramic components of the transducer. On the other hand, if the prestress is too low, excessive mechanical losses at the ceramic / metallic interfaces can reduce efficiency. Pre-stress can be estimated by using a torque wrench, calibrated against charge, to tighten the bolts. This measuring method is simple, but it is not the most accurate, and consequently it is recommended primarily for production-run transducers for which the variations among corresponding components are, hopefully, minimal. The more accurate way of measuring pre-stress is to measure the charge generated in the ceramic elements under short circuit conditions. A capacitor connected to the transducer’s electrical terminals and to a direct current voltmeter facilitates measurement of the charge as each bolt is tightened.