Measurement of Threaded Fasteners

Torque is the most widely used method of measuring the integrity of bolted joints. However variations in friction and alignment mean that measuring the torque does not always give a precise measurement of the applied pre-load (or clamp load). 

In addition some customers require that this pre-load be monitored over the service life of the fastener. While torque measurement methods such as "breakaway" or "back-off torque" exist, these require the fastener to be disturbed, and are time consuming.

In these situations, ultrasonic measurement of the pre-load has proven to be the most reliable and cost effective solution. Ultrasonic measurement of pre-load is made possible by using a predictable decrease in the velocity of sound within the body of the fastener as the stress is increased.

By introducing a sonic pulse at one end of the fastener, and accurately measuring the time required for the echo to return from the opposite end, the ‘ultrasonic length’ of the unloaded fastener is determined. This ‘ultrasonic length’ is very similar to the actual mechanical length although the figures are not interchangeable.

As the fastener is tightened, the increase in stress in the material allows the Norbar USM to calculate the change in the ‘ultrasonic length’. This elongation of ‘ultrasonic length’ is equivalent to the mechanical elongation of the fastener.

Details of the fastener specification can then be used to calculate and display the actual pre-load (clamping force) produced by the fastener.

Data is stored in groups of bolts with alphanumeric labels and comment fields (alphanumeric available on USM-II only), and can be downloaded to a PC and uploaded for future monitoring work. The optional utilities software makes interfacing easy even for new operators. 

There is a 0-10V analogue output, useful for control of a tightening tool such as the range of Norbar Pneutorque‚ torque multipliers. Control interfaces can be supplied on request.

The USM uses state-of-the-art hardware and software to achieve these measurements with maximum automation, and minimizes the need for operator interpretation. The previous generation of ultrasonic bolt length measurement devices required lengthy operator training, but the USM has built over 20 years of knowledge into a clear user interface, suitable for new and experienced users alike.

USM-I

Size: 197mm W x 127mm H x 279mm D Weight: 3.2 kg.

Keypad: Sealed membrane with tactile keys.

Display: 128 X 64 pixels with Backlight. Case: Extruded and cast aluminum.

Line Power: 90-250 VAC 47 to 63 Hz. Operating Temperature: 0º to 40ºC (32º to 104ºF).

Automatic Compensation: for Temperature of Fastener.

Calibration: Automatic verification of bolt material calibration constants.

Data Output: RS232 I/O  0 to 10V Analogue Output.

Resolution: 0.0001 inch (0.002mm).

General: Pitch-catch, pulse-echo, and echo-echo measurement modes. Automatic peak signal and phase selection.

Pulser: 360 volt push/pull tone burst. Selectable burst width. Selectable frequencies at 1.0, 2.25 and 5.0 MHz.

Receiver: Low noise preamplifier. Automatic gain control. Automatic waveform and signal diagnostic display.

Timing: 160 MHz 10ppm TCXO controlled digitizer.

Memory: 2 megabit non-volatile ram. 1000 bolts, 5000 measurements.

USM-2

As USM-1 except: 

Size: 216mm W x 163mm H x 51mm D. 

Weight: 1.6 kg. 

Case: Extruded aluminum with steel end plates. 

Battery Power: 4 ‘C’ Cells. 

Pulser: 160 volt push/pull tone burst. Selectable burst width. Selectable frequencies 1, 2.25, 5, and 10 MHz.

Other sizes and frequencies are available on request.

Ultrasonic Transducers

Magnetic Transducers

This standard style is used with ferrous materials, and consists of a rare earth magnet surrounding the piezo electric transducer. The diameter refers to the nominal diameter of the piezo electric crystal. The annular magnet increases the overall diameter to the value of ‘A’ shown. Temperature range: - 20ºC to + 80ºC ( 0ºF to 180ºF ).

Glue-On Transducers

Allows positive location on non-magnetic fasteners. Also ideal for temporary or permanent monitoring of all joints. Higher repeatability than magnetic transducers because the position of the transducer on the fastener is constant. The cyanoacrylate adhesive acts as a complaint with good sound transmission properties. Operating temperature range is: - 20ºC to + 80ºC ( 0ºF to 180ºF ). Transducers are built to order. Please contact Norbar with your requirements.

SMART Glue-On Transducers

These glue-on units have in-built memory and allow both fast and simple monitoring of joints. Simply attach the transducer to special version of the USM-II and the change in elongation or load since the last inspection can be displayed and recorded. Transducers are built to order. Please contact Norbar with your requirements.

This standard style is used with ferrous materials, and consists of a rare earth magnet surrounding the piezo electric transducer. The diameter refers to the nominal diameter of the piezo electric crystal. The annular magnet increases the overall diameter to the value of ‘A’ shown. Temperature range: - 20ºC to + 80ºC ( 0ºF to 180ºF ).

Allows positive location on non-magnetic fasteners. Also ideal for temporary or permanent monitoring of all joints. Higher repeatability than magnetic transducers because the position of the transducer on the fastener is constant. The cyanoacrylate adhesive acts as a complaint with good sound transmission properties. Operating temperature range is: - 20ºC to + 80ºC ( 0ºF to 180ºF ). Transducers are built to order. Please contact Norbar with your requirements.

SMART Glue-On Transducers

These glue-on units have in-built memory and allow both fast and simple monitoring of joints. Simply attach the transducer to special version of the USM-II and the change in elongation or load since the last inspection can be displayed and recorded. Transducers are built to order. Please contact Norbar with your requirements.