Load Measuring System Basics

A lot of the failures in load cells installed on overhead bridge cranes stem from improper installation, which is caused by a lack of understanding and knowledge of how they work. The load cell, typically in the form of a “load pin”, is often an integral part of the axle supporting the equalizing or running pulleys or “sheaves” on a wire rope hoist. Because the pin is often perfectly round and often can support the load from many angles, it’s a common mistake to assume “the load cell is just a pin with a scale in it."

To understand load cells, one needs to know about a Wheatstone Bridge. The Wheatstone Bridge diamond-shaped circuit was developed by Charles Wheatstone and can be used to accurately measure unknown resistance values, or as a means of calibrating measuring instruments, voltmeters, ammeters, etc., using variable resistance and a simple mathematical formula.

Following is a quick video explaining what it is:

Now that we understand what a Wheatstone Bridge is, let’s see how it’s used in a load cell. This video explains the different types of load cells used in weight measurement.

The load measuring pin is a transducer used on a lot of overhead crane applications. It is basically a hollowed axle pin used on sheaves or dead ends, with a terrifically thin grid of wires (strain gauges) inserted and installed internally. A small amount of current is passed through the strain gauges, and as the load weight value rises or falls, the pin deflects, and the resistance of the wires changes. The current going in and out is measured, and the change in current is converted by a microprocessor and assigned a numerical weight value in tons, pounds, or kilograms. Load-measuring pins are strain gauge-based transducers that are used to measure load, overload, or tension conditions. Often used to replace an existing axle or shaft within a structure, load pins offer a very convenient and compact location for load sensing, in that they do not normally require any change to the mechanical structure being monitored.

This video explains the different types of load cells used in weight measurement.?

This is all great, but the great equalizer is gravity, and gravity only works in one direction. Straight down. If the load is applied off-center, the load cell accuracy is affected. If the load shifts or moves, the values change. If the load cell is misaligned, it might not work at all. Given the accuracy demanded of such an instrument, and it is a “precision instrument,” and should be treated as such, if the load cell is prevented from deflecting by bad installation or failure in the equipment design, it will fail to deliver as promised! Given all the challenges they are simple devices that require a bit of understanding to function correctly. They are precision instruments, so don’t use a hammer or sledge during installation, and if there is a lot of “noise” from the power system, or equipment around them, it may affect the signals from the transducer to the controller which equates to improper values on the readout.

With over 40 years of experience in the crane industry, Unique Group’s Water Weights? has extensive knowledge of customer requirements and desired functionality. Building on this experience, we design and build crane load monitoring systems that are high quality, reliable, and easy to install, calibrate and maintain.

Unique Group's Water Weights??systems provide real-time measurement of critical components and assemblies to support either the daily operation or long-term maintenance of the crane.

Applications include:

·?Above the hook (sheaves, drums. trolleys)

·?Below the hook (links, shackles, canisters, beams)

·?In the hook block (trunnion, sheave pin, hook)

·?Rope dead end (A2B, wedge socket, clamp on, line rider)

·?Cabled or wireless telemetry

Load Accuracy Throughout the Range

Crane monitoring systems utilizing a two-point calibration make it impossible to ensure that the load indicated is accurate at any point in the envelope, other than at zero and 100% capacity.

Water Weights?? takes calibration a step further by employing a multipoint calibration over the span of the crane capacity. By calibrating at least 6 points in the range of the crane this method accounts for any nonlinearity in the load cell, wire rope, or crane support structure itself. This method does ensure that the monitoring system is accurate across the full range of the crane capacity.

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Learn more: https://www.uniquegroup.com/product/ug-crane-load-measurement-monitoring-systems/

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