Vibrating Wire Centre Hole Load Cell

Applications

The measurement and control of load including: Load in rockbolts, tie backs, foundation anchors and tunnel support Tension in cable anchors and multistrand tendon anchors

Operating Principle

The vibrating wire load cell comprises a set of three or six vibrating wire gauges, mounted parallel to each other equally spaced in a ring in an alloy steel cylindrical housing. The method of construction results in a very robust instrument suitable for use where high performance, longevity and mechanical strength are important. A heavy gauge, multicore, sheathed cable connects the cell to the readout unit. The mode of operation is by electrical plucking of the vibrating wire. The change in load acting on the load cell is proportional to the difference of the squared frequency of vibration for the corresponding load. The readings are taken on EKE 871, EKE 875 or EKE 878 Dataloggers

Advantages

Accurate, robust and very good long term stability Negligible temperature effects compared with oil filled load cell

Fast response time
Accuracy unaffected by cable length
Suitable for remote readings
Capable of reading eccentric loads
Common readout for all vibrating wire type instruments

Installation

Tensile loads in rockbolts and cables are measured by installing Centre Hole load Cells between the rock/concrete face and the tensioning nut of the bolt or anchor. For the most reliable results, a concrete bearing pad should be cast, with its upper face flat and perpendicular to the bolt or cable. A bearing plate is placed beneath the cell to spread the load and to take up any residual nonalignment. A further bearing plate is placed between the cell and anchor. The assembly is centralised and held in place by applying a small tension to the bolt or cable. Fault in centralisation or alignment will be apparent from differences in load readings at each of the vibrating wire gauges in the cell. In situations where speed of installation is important, the bearing pad may be formed using high alumina cement or resin grout. Readings of the vibrating wires are averaged to obtain the bolt or cable tension. In multi-strand cable anchors, it is possible to tension the strands uniformly, by monitoring and balancing the load reading for each of the vibrating wire gauges.