A crane scale is convenient: you weigh a load while you are lifting it anyway. At the same time, it combines two operations that each demand their own attention: weighing and lifting. An unsteady load gives an unreliable indication, and a crane scale used incorrectly can become a safety risk. In practice we see that errors in crane scales are more often caused by the way they are used than by the instrument itself.
This article describes how to weigh accurately with a crane scale, what to look for when inspecting lifting components, what the difference is between a calibration and a load test of the crane, and when legal verification is required. How the calibration of a crane scale is carried out is explained on the service page.
Weighing accurately: a steady load
A crane scale measures the force with which the load pulls on the hook. As long as the load is moving, that force changes constantly. Anyone who reads the display while the load is still swinging or bouncing reads a snapshot that is higher or lower than the actual weight.
- Limit swinging: lift the load slowly and vertically, with the hook as closely as possible above the centre of gravity. A load that is pulled up at an angle will start to swing as soon as it comes free.
- Stabilisation time: wait until the load hangs still and the indication is stable. Many crane scales show a stability indicator; only read the display once it is on.
- Hanging completely free: the load, the slings and the chains must not touch the ground, a rack or other objects.
- Tare of lifting gear: if slings, shackles or a lifting beam hang below the crane scale, zero the indication with that lifting gear attached or deliberately subtract its weight.
- Environment: outdoors, wind can make the indication unsteady. Large temperature differences, for example with hot loads, affect the electronics. Stay within the manufacturer's operating limits.
Shock loading and overloading
A load that is lifted with a jerk or drops into the slings briefly exerts a force much greater than its own weight. That peak can exceed the maximum weighing capacity, even if the load itself is considerably lighter. The result can be permanent deformation of the measuring element. The crane scale then weighs incorrectly on a permanent basis, without this being visible on the display.
- Lift and lower at a steady speed.
- Never let a load drop into the slings and avoid slack chains that suddenly become taut.
- Stay within the maximum weighing capacity and within the working load limit of the crane scale and all other lifting components. The lowest value in the lifting chain sets the limit.
- Do not use the crane scale to pull a stuck load free.
- Many crane scales report or record overloading. If that warning has appeared, have the crane scale checked before you use it again for weighings that matter.
Battery and electronics
Most crane scales run on a rechargeable battery or batteries, often with a wireless indicator. A nearly empty battery can lead to an unsteady indication, a lost connection or a display that fails while the load is still hanging. Charge the battery according to the manual and do not start a series of weighings with a nearly empty battery. After switching on, give the crane scale the warm-up time specified by the manufacturer and check the zero reading with only the lifting gear attached.
Inspection of lifting components
A crane scale is also a link in the lifting chain. A defect in the hook or the eye is therefore not a weighing problem but a safety problem. Check before use, and periodically according to your own inspection regime:
- the hook for deformation, cracks and an opened-up throat;
- the safety latch or locking device on the hook: does it close properly and spring back;
- the top eye or shackle for wear and deformation;
- rotating parts: do they turn smoothly and without play;
- the housing for cracks, dents and loose parts, especially after an impact;
- the legibility of the type plate with weighing capacity and working load limit.
If you have doubts about a lifting component, do not use the crane scale and have it assessed. During a calibration we also look at these components; we do not calibrate a crane scale with a damaged lifting component.
Calibration is not a load test of the crane
The difference between a calibration of the crane scale and a load test of the crane is often overlooked in practice.
| Aspect | Calibration of the crane scale | Load test of crane or lifting gear |
|---|---|---|
| Purpose | Establishing how accurately the crane scale weighs | Assessing whether the crane or lifting gear can safely carry the load |
| Result | Indication errors, repeatability and measurement uncertainty in a certificate | Judgement on the safety and suitability of the installation |
| Framework | Quality system, procedure according to EURAMET cg-18 | Occupational health and safety legislation, manufacturer's instructions and the inspection regime |
A calibrated crane scale therefore says nothing about the condition of the crane, and an approved crane says nothing about the accuracy of the crane scale. Load tests of cranes and lifting installations require test weights of known mass. You can hire these from MASSLAB, including transport and positioning. Read more about moving heavy weights safely in transporting and handling heavy test weights.
When is legal verification required?
Most crane scales are used internally: for stock control, production control or monitoring loads. For that, a calibration with certificate is the usual route. Legal verification under the Dutch Metrology Act is required as soon as you use the weighed weight for a legally regulated application, such as buying or selling scrap and metals by weight or calculating charges. The crane scale must then be a verified instrument, with the metrology marking and intact seals. After a repair to the weighing mechanism or a broken seal, a re-verification follows. Read more about the rules in the Dutch Metrology Act and scales.
For the calibration interval, annually is a common starting point. Choose a shorter interval for intensive use, shock loading or a harsh environment, and always have the crane scale checked sooner after an overload warning, a fall or a repair. See also how often to have a scale calibrated.
How MASSLAB helps you
MASSLAB calibrates crane scales according to the principles of EURAMET cg-18, with calibrated weights from our own laboratory of up to 2,000 kg each and with our own transport and equipment. Smaller crane and hanging scales can also be calibrated in our laboratory. As a Recognised Verifier (Erkend Keurder) under the Dutch Metrology Act, we carry out the re-verification of verified crane scales after repair, once we have checked that we are permitted to verify that instrument. More information can be found at crane scale calibration.