We’ve recently completed the instrumentation of the largest sheave assembly we have had come through our shop. Last year, we were approached by Tidewater to build a hanging sheave assembly for a workboat A-frame that would accommodate the bending radius of 1.5” Amsteel Blue rope and 60,000lbs, the peak expected tension of the application.
We worked with Supreme Integrated Technologies (of Harahan, LA) and Skookum, an Ulven Forging company (or Hubbard, OR), to build a 6 foot tall and 3.5 foot wide sheave. To provide payout and speed monitoring, we needed to install targets on the side of the sheave and a sensor on the side plate. We installed 40 magnets equally spaced at 6 inch intervals on the side of the sheave. The single barrel, dual channel Hall Effect sensor will provide an accurate quadrature or square waveform when it crosses a magnet. Our LCI-90i display converts these dual channel (90 degrees out of phase) square waves into speed, payout, and direction. If there is no slipping of the rope through the sheave, the 6 inch spacing of the magnets will provide 3.4” resolution at all distances…100’ or 10,000’. To instrument this sheave for tension (or load) we developed a load pin that fit the bale of a standard Skookum shackle.
The shackle was required to be installed in an upset orientation to minimize the stack height of the sheave assembly on the A-Frame. This will also allow the sheave assembly its full range of motion and provide accurate tension readings. Changes in the rope wrap angle can cause errors in the tension readings. Instrumented hanging block sheave assemblies are notorious for this.
To mitigate the changing wrap angle error, we had to calibrate the instrumented shackle for two angles (e.g. fully deployed and a fully onboard A-frame positions). A simple external selector switch integrated to the LCI-90i, will allow the operator to easily change tension calibration coefficients. This is exactly the kind of interesting, non-standard project that our engineers like to dive into from time to time.
If you have questions about instrumenting existing sheaves or interesting tension monitoring applications, give us a call.
