Contents
Overview
What a load cell is and where it sits
What is a load cell? A load cell is a sensor that measures the force put on it and turns that force into a signal, which an indicator shows as a weight. In lifting, load cells weigh a load from under the crane hook or from beneath it, and some are built into the crane to warn of overload.
In short
- A load link or load shackle in the rigging weighs the load as it comes off the ground, and one on each crane or sling leg shows how the weight is shared.
- Accuracy is quoted as a percentage of the cell’s full capacity, so a cell much larger than the load reads it coarsely.
- LEEA treats a load cell rigged between the crane and the load as a lifting accessory, with the marking and thorough examination that brings in Great Britain under LOLER.
- Calibration checks the reading. The thorough examination separately checks the cell is safe to carry the load.
On a contract lift, our own appointed person plans the lift and writes the lift plan.
How it works
How does a load cell work?
Inside a strain gauge load cell is a metal body that stretches or squeezes very slightly under load. Strain gauges, fine wire or foil laid in a tight zig-zag, are bonded to that body, so they deform with it and their electrical resistance changes in proportion to the load.
The gauges are wired into a circuit called a Wheatstone bridge. With no load the bridge is balanced and gives no output. Load on the cell unbalances it, and the output changes with the load. An indicator turns that signal into kilograms or tonnes, either on a display built into the cell or on a handheld unit or control system it reports to.
Hydraulic and pneumatic load cells are also made, by load cell makers such as HBK.
In lifting
Types of load cell used in lifting
These are the load cells you will meet on a lift. The first three are brought in for the job, and the last is part of the crane.
| Type | Where it sits | What it suits |
|---|---|---|
| Load link, also called a dynamometer | In line under the hook, with a standard lifting shackle at each end | Weighing a load on the hook, and proof load tests |
| Load shackle | In the rigging in place of an ordinary shackle, with the sensor built in | Lifts with little headroom |
| Compression cell | Under the load, which pushes down on it | Weighing large items from below. Wireless sets are sold for finding the centre of gravity as well |
| Load pin or hook block cell | Built into the crane, as a pin that replaces a load-bearing pin or a cell at the load block and hook | Feeding the load indicator in the cab |
Load cells in the crane’s safety devices
L113, HSE’s guidance on LOLER, says lifting equipment should have rated capacity indicators and rated capacity limiters where there is a significant risk of overturning or overloading. An indicator warns the operator as the crane nears its safe limit, and a limiter stops the motion before the limit is passed. L113 adds that rated capacity indicators used to be called automatic safe load indicators or moment load indicators.
A cab indicator can take its load reading from a cell at the hook block or from pressure in the crane’s hydraulic lifting cylinder, and can sound an alarm or stop the lift. Our guide to BS EN 13000 covers what the standard asks of these devices on mobile cranes.
Uses
What a load cell is used for on a lift
Weighing a load
The lift plan starts from the load’s weight. LEEA’s code of practice lists where to find it: a weight marked on the load, its drawings or paperwork, a weighbridge while it is still on its trailer, or an estimate from tables of weights. Table figures are averages, and the real weight can differ.
Where none of those can be trusted, a load link under the hook gives the weight once the crane takes the strain. That reading comes only when the load is already hanging, so the crane and rigging still have to be chosen for the most the load could weigh. Under LOLER a competent person must plan every lift, and BS 7121-1 hands that task to the appointed person on crane lifts.
Checking how the load is shared
When two cranes lift one load, each must be able to carry its part of the weight, allowing for how that can change as the load moves. A load cell on each hook shows the actual split from the moment the load leaves the ground. A sling works the same way: several cells can report to one display, with an alarm for each leg.
Load testing
A proof load test puts a set load on equipment to show it is sound. It is carried out by a competent person, in practice a lifting equipment examiner, and is also part of verifying new equipment and equipment after installation. A load cell in line shows the force actually applied, and one that captures the peak and keeps time-stamped records gives the test report its figures.
Accuracy
How accurate is a load cell?
Accuracy is given as a percentage of full scale, meaning the cell’s full capacity, and the error band is the same width across the whole range. In Tractel’s example, a 5 t unit accurate to 0.2% of full scale reads within 10 kg either way across its range. At 5 t that is 0.2% out at most; at 500 kg it could be 2% out. A cell too small for the load can be overloaded and damaged, so pick one sized to the job.
Figures differ between products, even from one maker. Crosby Straightpoint’s load monitoring catalogue quotes 0.1% of full scale for its Radiolink Plus load link and 1% for its wireless load shackle. Go by the figures for the load cell in front of you and by its maker’s instructions.
Calibration
Calibration checks the cell’s reading against a testing machine of known accuracy. LEEA’s guidance on load measuring devices says they need recalibrating every 12 months. For the Radiolink Plus, the user manual asks for the same in normal use and monthly to quarterly in severe use.
Calibration and thorough examination are separate records, and LEEA warns that an in-date calibration certificate on its own does not meet the legal requirement. Keep both with the cell and check the dates before it goes on the hook.
The law
Load cells and LOLER
The Lifting Operations and Lifting Equipment Regulations 1998 (LOLER) define an accessory for lifting, in regulation 2, as work equipment for attaching loads to machinery for lifting. LEEA’s guidance treats a load cell placed between the crane and the load as a lifting accessory, so it falls under the same rules as the shackles fitted to it. LOLER covers Great Britain. Northern Ireland works to the Lifting Operations and Lifting Equipment Regulations (Northern Ireland) 1999.
Marking and thorough examination
Under regulation 7, an accessory must be clearly marked with its safe working load. The Difference Between SWL and WLL sets out what that figure means.
Under regulation 9, an accessory exposed to conditions that cause deterioration liable to lead to danger needs a thorough examination by a competent person (in practice, a lifting equipment examiner) at least every 6 months. A written examination scheme, drawn up by a competent person, can replace that interval with its own. It also needs one each time something exceptional happens that could affect its safety, such as an overload. For load measuring devices, LEEA recommends no more than 6 months between examinations, or up to 12 months under a scheme where the use allows. If the Radiolink Plus display shows an overload, its manual says to remove the load at once.
From our lifts
We check the load cell is in date, rated for the load and fitted in a straight line. Then we take the strain slowly and lift just clear of the ground, with a visual check, before trusting the reading.
Questions
Common questions
What is a load cell used for?
On a lift, a load cell weighs a load whose weight cannot be trusted from its paperwork, and shows how that weight is shared between cranes or sling legs. It also records the force applied in a proof load test. Built into the crane, a cell at the hook block can feed the overload warning in the cab.
What are the four types of load cells?
The four main categories named by Flintec, a load cell maker, are beam, single point, compression and tension. On a lift you will mainly meet tension cells, such as load links and load shackles, which the crane pulls on. Compression cells are set under a load to weigh it from below.
What are the typical sizes of load cells?
Lifting load cells are sold by capacity, and one maker’s range runs from 1 t to hundreds of tonnes. In Crosby Straightpoint’s catalogue, the Radiolink Plus load link runs from 1 t to 300 t and the wireless load shackle from 3.25 t to 500 t. Choose a capacity close to the load, since accuracy is quoted against full capacity.
How heavy is a load cell?
It depends on the capacity. In Crosby Straightpoint’s catalogue, the 1 t Radiolink Plus load link weighs 1.5 kg and the 300 t model 118 kg. Count the cell and its shackles in the weight lifted: L113 says that in some circumstances, such as with a mobile crane, accessories and the hook block should count as part of the load.
How do you check a load cell is working?
Switch it on with nothing hanging from it and check it reads zero, then check its calibration and thorough examination are in date. The Radiolink Plus user manual says a cell comes out of service if it is visibly damaged, such as a crack or bend in a load-bearing part, or if its display cannot be read.
What are the common problems with load cells?
Shock loads and large twisting forces can damage a lifting load cell beyond repair, according to the Radiolink Plus user manual. It also asks for the load to sit centrally in the shackle bow, which it says gives the best performance. Tractel warns that an undersized cell risks overload and damage, so match its capacity to the load.
What are the disadvantages of using load cells?
A load link adds length to the rigging, which costs lifting height where headroom is short. Crosby Straightpoint sells its load shackles for low headroom lifts. Readings can also shift with temperature. Tractel states its Dynafor accuracy figures at 21°C, and the Radiolink Plus manual says to wait 20 to 30 minutes after a rapid change in temperature before use.
Get help
Lifting a load when its weight is uncertain
Call us on 0800 008 6096 or email office@synergy-lifting.co.uk. Include the load’s dimensions with any weight shown on its paperwork or drawings, and a few photos. From those we can plan the lift around the heaviest it could be.
For the crane itself, see crane hire and contract lifting. Where old plant has to be taken apart before it can be lifted out, see industrial plant dismantling.
Get a quote
Ready for a quote?
Send us the details and we will come back with a quote, usually the same day for a desktop survey.
Or call 0800 008 6096
