Ultrasonic bolt tension monitors

Ultrasonic bolt tension monitors are non-destructive instruments for the precise measurement of the preload (axial pretensioning force) applied to bolts and tie rods in critical mechanical connections. They use the propagation of longitudinal ultrasonic waves in the bolt material to measure the elastic elongation induced by tightening: knowing the elastic modulus of the material and the geometry of the bolt makes it possible to calculate the axial force precisely, giving considerably more accurate control than torque control alone. They are used in mechanical engineering where the failure of a bolted joint has serious consequences: turbines, generators, centrifugal pumps, chemical reactors, petrochemical and nuclear plant, and structural works.

The UT measuring principle

The principle rests on the relationship between the elastic elongation of the bolt under load and the propagation time of the ultrasonic wave in the material: T = T₀ · (1 + ΔL/L), where T is the time of flight after tightening, T₀ the time with the bolt unloaded, L the nominal length of the bolt and ΔL the elastic elongation (related to the force F by Hooke’s law, F = E·A·ΔL/L). By measuring T before and after tightening, the axial force applied to the bolt is obtained. Measurement precision is typically better than 5% on the force (considerably better than torque control, which can be in error by 25-30% because of the variability of the friction coefficient).

Operating procedure

The procedure involves: preliminary characterization of the bolt material (elastic modulus, ultrasonic velocity — generally declared by the bolt manufacturer or determined on a master bolt); preparation of the bolt head surface (cleaning, with couplant where needed and with a marked reference point for repeatability); measurement of the initial time of flight T₀ (bolt unloaded, before tightening); tightening of the bolt according to the prescribed procedure; measurement of the time of flight T under load; and automatic calculation of the elongation and the force by the instrument. The measurement typically takes 30-60 seconds per bolt, with accuracy far above that of torque control.

Features of the instruments

Ultrasonic bolt tension monitors combine: a contact UT transducer, typical frequency 5-10 MHz; automatic couplant for stable coupling (gel applied to the bolt head); acquisition electronics with a high sampling rate (50-100 MS/s) for precise time-of-flight measurement; an A-scan display for viewing the echo and diagnosing anomalies (internal defects, choosing the correct return echo); a bolt database with preloaded parameters (length, material, geometry, elastic modulus); automatic force calculation functions; and an archive giving traceability for every tightening operation.

Critical applications

Applications include: tightening the stud bolts of wind turbines (tower flange studs, blade-to-hub studs); electrical power generators; high-pressure centrifugal pumps; chemical reactors and pressure exchangers; high-pressure piping in oil & gas; large water-main pipelines; the heads of large combustion engines; safety-system flanges in nuclear power stations; and geotechnical anchorages on major works. In all these cases, preload control by UT guarantees the integrity of the bolted joint throughout its operating life, with periodic checks during preventive maintenance.

Reference standards

The reference standards are: ASTM F2168 (UT procedure for measuring bolt preload); ISO 16047 (determination of the friction coefficient of threads, complementary to UT); ASME PCC-1 (Pressure Vessel Component — guidelines for critical bolting); VDI 2230 (calculation of bolted joints). Bolts used with UT must be specially prepared (marked reference head, characterized material) in order to achieve the declared accuracies. Qualification of personnel as UT operators to ISO 9712 (level 1 minimum for preload measurement) is recommended.