What are the differences between Loadshackles and dynamometers?
Why would you choose one over the other?
Getting the right tool for the job is essential.
At the simplest level we could summarise that “Loadlinks are for load measurement and loadshackles are for load monitoring”. In more detail, what does that mean?
Below are some simple criteria to evaluate both options and help you choose the right lifting load cell for your application. At Straightpoint we use the terms ‘loadlink’, ‘dynamometer’ and even 'load indicating device' (LID) interchangeably. They refer to the SP specific name, the generic term and also the term used by ASME B30.26-2010. For the user there is little or no difference between each.
How they work
First a bit about how they work which will help us understand accuracy.
Load shackles work in shear. During manufacturing SP remove the pin of a standard bow shackle, bore a hole down the middle and bond strain gauges through the centre of the pin. Now, the shackle pin has become a loadpin.
Those strain gauges are aligned with two shear grooves that are visible from the outside of the pin. The groove depth is precisely calculated in terms of position and depth to allow the best loadpin performance but stay within our strength and safety guidelines. Advanced finite element analysis (FEA) software is used to model and prove our calculations.
The area between those grooves is the ‘load area’. When load is applied a shear force occurs across those grooves. This shearing effect is measured by the strain gauges which, once the load pin has been calibrated is translated into an actual force or weight reading for the user.
Loadlinks work in tension. Strain gauges are bonded to the body of our aluminium load cells. The process of bonding the strain gauges to the load cell body that is flexing under load is similar to load shackles, but in this case the strain gauges are flexing directly in line with the pull on the load cell, so straight line and in tension.
This direct tension force on the body of the load cell while under load gives us a very clear, linear signal from the strain gauges. This is what gives load links superior accuracy of 0.3% of applied load vs load shackles at +/- 1%.
To achieve the best accuracy with a load pin it is essential to use the full ‘load area’ as described above. You will notice that Straightpoint Wireless load shackles use a bobbin or collar to achieve this
You can read more about the importance of the bobbin in this article here.
Quick guide comparison table
Advantages of loadshackles
A loadlink consists of a top and bottom shackle in addition to the height of the loadlink body itself.
As a result a load shackle typically uses around 1/3 the headroom of a loadlink.
For those who do mobile testing and carry around load cells with them a load shackle gives you a one-piece measuring device and especially for heavier capacities this may offer easier handling.
Familiarity and ruggedness
Riggers like load shackles. They are familiar with size, weight and working load limit (WLL) of each component.
Simple swap to load monitoring
In a rigging setup it is easy to swap standard shackles for wireless loadshackles for a simple load monitoring solution. No change in headroom or configuration.
Multi-point lift monitoring
Once loadshackles are in place the banksman leading the lift has visibility of the individual load on each shackle as well as the total load from one display. It allows them to lead the lift, giving them confidence and control for heavy lifts and when moving bulky or out-of-gauge cargoes.
Advantages of loadlinks
A loadlink (+/- 0.3% of applied load) is around 3 times as accurate as a loadshackle (+/- 1% of applied load).
As such it is often chosen for weighing applications, precise force measurement or to calibrate other devices such as safe load indicators (SLI’s) on cranes.