Intelligent washer for measurement of bolt pre-tension

Aircraft Engineering and Aerospace Technology

ISSN: 0002-2667

Article publication date: 1 February 2000




(2000), "Intelligent washer for measurement of bolt pre-tension", Aircraft Engineering and Aerospace Technology, Vol. 72 No. 1.



Emerald Group Publishing Limited

Copyright © 2000, MCB UP Limited

Intelligent washer for measurement of bolt pre-tension

Keywords SKF Engineering Products, Washers, Sensors

An "intelligent" washer that reportedly enables the correct tightness and pre-tension of critical bolted joints to be achieved and monitored is now available from SKF Engineering Products of Luton (Plate 4).

Plate 4 BoltSafe - an "intelligent" washer for critical bolted joints

Called BoltSafe, it is believed to be a breakthrough design. It has been developed by Scan-Sense, Norway's specialist research and development company for high quality offshore and industrial sensors. The device is said to be used as a regular washer yet will measure the compressive force created between the nut and its interface surface to determine the pre-tension in the bolt joint. In addition, BoltSafe is said to allow any changes in the set pre-loading to be monitored throughout the operating life of the installation.

BoltSafe works on the principle that, when a load or pressure is applied to a metal with a magnetic flux signature, then the magnetic flux would change in proportion to the load applied.

Available in two basic range types, the PMS-Series is for manual monitoring of pre-tension readings during routine maintenance cycles and the CMS Series to accommodate continuous measurements or monitoring. In the PMS-Series, a manual probe is docked against a dedicated measurement port on the washer periphery for data collection while the permanently installed CMS version transfers power and data through its cable connection continuously to monitor the installation. With CMS, a "tree" of up to 32 units can be connected by RS-232 or Ethernet type link to a computer or PLC unit.

BoltSafe is said to be suitable for bolt sizes between M12 and M64 and accepts clamping loads up to 2,100kN.

In use the BoltSafe device measures the compressive force between the nut and the joint surface through the washer body. Made from an amorphous ribbon alloy sensor material that is cooled rapidly from a liquid using cryogenic materials at a rate of more than 1,000,000°C/sec. It utilises an application specific integrated circuit (ASIC) to perform signal conditioning, digital network connection and contains a 64-bit individual identification number.

Set on the washer is a circular communications "docking area" that forms the port for the monitoring device to take data from the ASIC circuit via the probe connection. Data available include identification number, temperature, calibration constraints, date and time of reading.

A software package allows printout records to be obtained with comparison against commissioning data. Alarm flags can be installed and a log function can present graphs of pre-tension loss as a fraction of elapsed time.

Details available from: SKF Engineering Products Limited. Tel: +44 (0)1582 496732; Web site:

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