This document introduces a sensing washer system for monitoring bolted joints. The system uses sensing washers on each bolted joint connected via transmission cables to individual transmitters, which are connected via an RS485 cable to a LORA gateway and tablet. Each joint is tightened to a target preload of 60kN manually, and the system displays the preload and torque during tightening. It monitors for preload above or below thresholds of 54-66kN and alerts if out of range. Observations showed attenuation rates of around 5% for the bolted joints over several days under static conditions.

1. Introduction of Sensing Washer System
⚫ M20 Bolt + SW-20→Bolted Joint A～D
⚫ Bolted Joint A～D via Transmission Cable to Transmitter A～D individually
⚫ Transmitter A～D via RS485 Transmission Cable to RS485 Connection Hub→LORA Gateway
⚫ LORA Gateway via LORA 2.4G to LORA Dongle and Tablet/Laptop with APP for displaying and monitoring
the bolted joints status continuously or periodically
Bolting with Sensing Washer
⚫ Tightening each bolt joint with SW sequentially to its target preload 60KN (preferable upper limit 66KN due
to the fact of attenuation) with manual torque wrench.
⚫ The screen shows the preload generated on the bolt joint during the bolting process and the torque
equivalent applied on each joint.
⚫ Set the threshold of high and low limit of each preload to alert with buzz and Status OK or NG.
⚫ Since the preload will attenuate in a short time after the tightening is completed. The most of the attenuation
occurs in the first 5-10 minutes, accounting for about 80% of the total attenuation and then, it will enter steady
status after 20-30minutes or couple hours depends on the joint characteristic especially hard or soft.
⚫ Above description is under static condition. If the bolted joint is under transverse vibration, the reduction
rate of the preload under 12.5Hz and attitude 1mm should be less than 20% as DIN 65151 specified.
⚫ In the embodiment above, set the threshold as 54-66KN for controlling and monitoring the preload on each
bolt joint. Due to the characteristic of attenuation, the tightening preload can be close to the high limit even
higher a little bit for the ease of tightening control.
Monitoring with Sensing Washer
⚫ Tightening Bolted Joint C intentionally to make the preload over high limit 66KN, it will be alert with buzz and
shown the status as NG.
⚫ Loosening Bolted Joint D intentionally to make the preload under low limit 54KN, it will be alert with buzz
and shown the status as NG.
⚫ The ALARM button turn to GREEN, the alert function with BUZZ is ON.
⚫ The ALARM button turn to RED, the alert function with BUZZ is OFF.
⚫ The SW Monitor turn to BLUE, it’s under monitoring function.
⚫ The SW Monitor turn to RED the monitoring function is discontinued.
Observation about the attenuation of the clamping force on bolted joint
The screenshot recorded the status of the preload on each bolted joint A～D from 2021-04-15 14:10:58PM till
2021-04-19 10:07:48 AM. The attenuation of each bolted joints are as Bolted Joint A～5%, Bolted Joint B～3.4%,
Bolted Joint C～4%, Bolted Joint D～4.5%. All under static condition.