Fastening & Joining
Joints measured by ultrasound
Direct measurement of bolt clamping force is much better than monitoring torque. Tom Shelley reports
Ultrasonics proves to be the best way of measuring tension in steel rods and clamping force in bolted joints, rather than the more traditional method of measuring bolt torque.
Applications include the construction and monitoring of process plant, spacecraft, building frames and better control of plastic injection moulding machines.
The USM-1 measures tension in metal bolts and rods. As the metal is tensioned, the increase in length increases the time required for a pulse to travel from the end with the transducer to the far end and back again. At the same time, the increased stress reduces the velocity of sound in the metal, which further increases the echo time.
The instrument produces a tone burst, which can be adjusted by varying the number of synchronised impulses sent to the transducer. The receiver is designed to produce a minimal amount of internal noise, and has automatic gain control. Digital signal analysis optimises the measurement process and warns of potential problems.
Vendor Norbar says it is important to select the correct transducer for an application, ensure it seats correctly and make sure the reflecting end is perpendicular. Misalignment of more than two degrees can cause significant errors. This is most likely to arise from bent bolts, a particular problem with pipe flanges or joints with partial gaskets. Bolts whose heads are rough, rusted or have specifications proud of their surface may have to be ground flat for the technique to work. Recessed marks can be filled if they cause trouble.
Low frequency transducers may be needed to produce echoes in long bolts or those made of metals with high attenuation. At the same time, low frequencies produce more noise and tend to spread, causing stray reflections from the sides. Higher frequency pulses tend to travel more directly down and back up the centre line of the bolt, with less noise and distortion.
Despite these problems, ultrasonic tension measurement is much more accurate than relying on tightening torque. Torque is related to tension, but the exact relationship depends on degree of lubrication and friction, and also geometry.
Applications, mostly in the US, include monitoring nuclear reactor flange bolts and fasteners for spacecraft. The technique is also used for auditing steel building frames for bolt relaxation after construction. A non-bolting application is the tension measurement of plastic injection moulding machine tie rods. The rods, generally four in number, need to be of equal tension to give an equal closure of the mould tool. Norbar has written a special software version to control this process. The UK list price of the unit is £6,400 plus £700 to £800 for a suitable transducer.
Norbar Torque Tools STimms@Norbar.com
* The USM-1 precisely measures the load, tensile stress or elongation in rods and fasteners of almost any material from 2.5 to 6,000mm in length
* Choosing the correct transducer for the application is crucial
* Bolts whose heads are rough or have specifications proud of their surface may have to be ground flat for the technique to work
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