Tension meters for wire ropes and cables

Tension meters for wire ropes and cables are instruments dedicated to measuring the tension applied to steel ropes, tie rods, stay cables, structural cables, suspensions and anchorages. They are used on civil construction sites and infrastructure (checking bridge stays, tower bracing, the stays of tensile-structure roofs), in the maintenance of industrial plant (lifting gear, cranes, hoists), in marine applications (shrouds, halyards) and in special structures (cableways, aerial ropeways). Periodic checking of the tension is essential to guarantee structural safety and to plan maintenance work.

Measuring technologies

The main methods are: by deflection (with three-roller instruments similar to wire tension meters but of high capacity, up to hundreds of kN); by vibration (the sonic method, used in particular for bridge stays and structural cables, in which an accelerometer measures the natural frequency of the cable, related to the tension through mass and length parameters); by fitting load cells between the cable and the anchorage (for permanent high-precision measurement on structural tie rods); and by local strain (with clip-on strain gauges applied to the cable, for high-precision real-time measurement).

Deflection tension meters

Deflection tension meters for cables are robust and portable, with high-capacity strain-gauge load cells (up to 100-200 kN), a typical accuracy class of 1% of full scale, a digital display and data output. The mechanical structure comprises two fixed lateral rollers and a movable one connected to the load cell, dimensioned for wire ropes (hardened steel rollers of appropriate hardness). Application requires bringing the tension meter into contact with the cable for the duration of the measurement, which is short (a few seconds). For permanently installed cables, the instrument is easily carried along the infrastructure for periodic checks.

The vibration method

For bridge stays and structural cables of considerable length (tens or hundreds of meters), the vibration method is particularly effective: an accelerometer temporarily fixed to the cable records its response to an excitation (a manual tap, a shake induced by traffic or by wind); spectral analysis of the signal identifies the natural frequency (or the first harmonics), from which the tension is calculated once the linear mass and the effective length of the cable are known. The method is non-destructive, requires no intervention on the structural cable, and allows periodic checks throughout the life of the structure. Dedicated software handles corrections for the bending stiffness of the cable (important for highly tensioned structural cables).

Structural applications

Applications include: monitoring the stays of cable-stayed bridges (long-span bridges, such as tower structures with stays); checking post-tensioned tendons in prestressed reinforced concrete works; checking the suspensions of tensile-structure roofs; checking the cables of antennas and of electricity and telecommunications towers; monitoring crane and hoist ropes; checking geotechnical anchorages (rock-face bolting, slope anchorages); and checking the ropes of ski-lift installations (gondola lifts, chairlifts, drag lifts). Continuous monitoring (with permanent load cells) is standard practice on critical engineering works.

Standards and calibration

Cable tension testing procedures are referred to in sector standards: EN 1993-1-11 (the Eurocode for the design of structures with tension components); EN 1996 (for wire ropes and their components); EN 12385 (terminology and classification of wire ropes). Calibration of portable deflection tension meters is carried out with master load cells calibrated to ISO 17025, in accredited laboratories. Vibration systems use high-class accelerometers calibrated to ISO 5347.

Selection criteria

The choice depends on the tension range required (tens of N to hundreds of kN), on the diameter of the cable (which governs the roller geometry or the case for non-contact methods), on the accessibility of the cable (for contact methods), on whether continuous monitoring or periodic measurement is needed, on the availability of dedicated software for spectral analysis (for the vibration method), and on the data interfaces for archiving and historical analysis of the checks.