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The Development of the Dry Couplant Rolling
Probe and its Applications.

Jocelyn Langlois
Gary N. Langlois
Sigma Transducers, Inc.


The dry couplant continuous test rolling probe, also known as the wheel probe, has evolved considerably from the original oil filled rubber tire containing one immersion transducer. Todays' wheel probe is more than an ultrasonic transducer. It is an ultrasonic inspection device in which a variety of transducers can be included. Inspections axially or circumferentially, for various applications including flaw detection and sizing, can be done simultaneously with wall thickness measurements. The wheel probe is a precision instrument with two primary components: The stator (axle), which contains the various transducers; and the rotor (wheel), which rotates about the stator on precision bearings. The replaceable tire which mounts on the wheel provides ultrasonic coupling to the part being inspected. As wheel probe uses became more demanding due to higher speeds and longer distances of operation, a new tire material became necessary. Studies of materials based on attenuation, flexibility, and wear were carried out and materials appropriate to these requirements were discovered. With the addition of new materials, utilization of shaped elements, advances in computers and instrumentation, and new ideas in configuration, the design and development of wheel probes is a continuous process.


The foundation of the wheel probe is the stator. This piece forms the housing for the transducers, and also the fixed mounting for the connectors and cables. Though normally constructed from stainless steel, material selection can be based on the application and requirements for a specific use. The wheel, which rotates about the stator on precision bearings, is constructed from the same plastic used in the manufacture of the transducer delay line, "LOTEN". Loten is capable of withstanding temperatures up to 350 degrees F (by request) and can withstand an accumulated dose > 108 RADS. The total delay of the wheel and the transducer combined, is designed to fit with the application for which the probe is intended. 180 degrees from the transducer face, a pressure regulation diaphragm has been placed for wheel probe use in pressure sensitive environments. Pressures up to 1500psi have been tested with no damage to the probe mechanically, or to the functioning of the transducers mounted within it. Between the transducer and the wheel is a .003 inch liquid couplant layer. The entire probe is assembled under a vacuum and sealed, so that this is a permanent airless layer.


Until recently most wheel probe uses were at slow speeds and the original tire material (plastisol) was adequate and performed well. However, at higher speeds, the plastisol would stretch and walk off the wheel. In order to improve the tire for higher speeds and longer distances, studies of materials based on attenuation, flexibility, and wear were mandated. The results of these studies indicated that rubber should be used.

To facilitate these studies a test set-up was designed using a 6.5 inch O.D. used pipe casing, a machinists lathe, the standard 0 degree wheel probe, and an ultrasonic test instrument. The pipe was chucked into the lathe and aligned, a fixture supported the wheel and probe against the pipe and the wall thickness was monitored during the entire test series. The first natural rubber test tire was a section of a motorcycle inner tube. The material was black black color filled, 50 durometer, .07 inch nominal wall. This tire was run 508 KM at 3.62 M/s with no apparent signal loss. A 40 durometer, natural rubber extruded tire, with a 0.10 inch wall, was tested for 250 KM at 1.42 M/s, and 27 KM at 5.93 M/s. There was no signal loss during these tests, though a gradual slippage of the tire was noted at the higher speed.


DUROMETER- The hardness of the rubber appears to be the speed controlling element. Lower durometer rubbers slipped on the wheel at all test speeds, generating surface heat, which caused the tire to deteriorate. currently 40 durometer natural rubber is being used as the standard wheel probe tire. These tires will operate at speeds up to 5 M/s with no slippage. The effects of slippage above 5 M/s will depend on speed and duration.

ATTENUATION- It appears that any filler used to modify the natural rubber such as that for creating neoprene, butyl, or color, increased the attenuation. Losses of up to 20dB of signal amplitude were found, on thickness measurements from the test probe at its focus, from natural rubber to rubbers with fillers. The Sigma wheel probe traveled 1236 KM, at speeds from 1.42 to 5.93 M/s during this series of tests. The probe was completely disassembled and checked for wear, all parts were still within specifications, the probe was then reassembled and is currently in use.


Although thickness measurement is currently the most widely used application for the wheel probe, it is not its only use, currently or potentially.

Wheel ProbeVideo


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