What Should Be Checked When Calibrating A 50 Ton Forklift Weighing System?

An onboard weighing system can be useful on a high-capacity forklift, but it should not be confused with the forklift's rated capacity.

A weighing system tells the operator how much the load weighs. The forklift's capacity plate tells the operator under what load-center and configuration conditions that load can be handled.

For example, the Propolift 50 Ton Forklift is rated at 50,000 kg under its specified load-center condition. A weighing display showing less than 50,000 kg does not automatically mean every load is safe, because actual capacity can also be affected by load center, mast, attachment and load position.

So before calibrating a weighing system, the first step is to understand what type of scale is installed and how its manufacturer requires it to be checked.

There is no single calibration procedure for every forklift weighing system.

Common designs include:

• load-cell-based systems;
• hydraulic-pressure-based systems.

A load-cell system may measure force through weighing forks, a carriage plate or another sensor arrangement.

A hydraulic system may calculate load weight from hydraulic pressure during a specified lifting sequence.

Because the measuring principle is different, the calibration procedure can also be different.

Before calibration, confirm:

• scale manufacturer;
• model;
• serial number;
• rated weighing range;
• controller/display model;
• service or calibration manual.

Without this information, it is not reliable to use a generic calibration procedure copied from another forklift.

This is especially important on a 50 ton forklift.

A load may weigh 45 tons and still be unsuitable if its center of gravity is much farther forward than the rated load center.

Attachments can also change residual capacity because they add weight and may move the load farther away from the truck.

For this reason, a weighing result should always be considered together with forklift rated capacity and load center.

The weighing system answers:

How much does the load weigh?

The forklift capacity information answers:

Can this truck safely handle that load in this configuration?

These are related questions, but they are not the same question.

Calibration should be carried out under the conditions specified by the weighing-system manufacturer.

Depending on the system, preparation may include:

• positioning the truck on a suitable level surface;
• checking the forks or weighing carriage;
• inspecting electrical or hydraulic connections;
• placing the mast and forks in the required position;
• confirming the display returns correctly to zero;
• allowing the scale to stabilize or warm up where required.

Do not assume that every scale uses the same mast angle, fork height or hydraulic condition.

The correct setup comes from the actual scale manufacturer's instructions.

A weighing system needs a reliable reference load.

This may be:

• certified test weights;
• traceable calibration weights;
• an approved known load;
• another reference specified by the scale manufacturer.

The test load does not have to follow a universal sequence such as 10, 20, 30, 40 and 50 tons.

Some systems use one span point. Others may use several calibration or linearization points.

The important thing is that the reference load is known and that the procedure follows the scale specification.

Calibration of the scale is not the same as proof-load testing of the forklift.

A 50 ton forklift should not automatically be loaded above 50 tons simply to create a "calibration margin."

If a special proof-load or acceptance test is required, the test condition should be separately defined by engineering and contract requirements.

That process is different from checking the accuracy of an onboard weighing system.

Propolift's heavy-duty forklift testing before delivery includes checks of load performance, hydraulics, steering and braking, but this should not be confused with scale calibration.

Although the exact procedure varies, three checks are commonly important.

Zero

The unloaded system should return to the correct zero reference under the specified weighing condition.

Repeated zero drift may indicate a problem with:

• sensors;
• wiring;
• installation;
• hydraulics;
• controller settings.

Span

The displayed weight is compared with a known reference load and adjusted according to the scale manufacturer's service procedure.

Repeatability

The same known load should produce reasonably consistent readings when measured repeatedly under the same conditions.

If readings vary significantly, the cause should be investigated rather than simply recalibrating the display.

Some weighing systems require the truck to be stopped or the mast to pass through a defined lifting position.

Other systems are specifically designed for weighing during movement or during a controlled lift.

So "dynamic calibration" should not be treated as a universal procedure.

If weigh-in-motion is required, it should be a documented capability of the installed weighing system.

The intended use of the scale also matters.

For internal operations, a company may use the weighing system for:

• load checking;
• inventory;
• production monitoring;
• shipping preparation.

In this case, calibration requirements may mainly follow the manufacturer's instructions and the company's quality procedures.

If the measured weight is used for commercial transactions, local legal-metrology requirements may apply.

In the United States, commercial weighing equipment may be subject to weights-and-measures requirements based on NIST Handbook 44 and local regulations.

Buyers should therefore confirm before ordering:

Is the scale only for internal load checking, or will the weight be used for commercial/legal-for-trade purposes?

There is no universal rule that every forklift scale must be calibrated every six months.

The interval should depend on:

• scale manufacturer recommendations;
• frequency of use;
• operating environment;
• company quality requirements;
• applicable metrology rules.

Additional checks may also be appropriate after:

• scale or sensor repair;
• removal and reinstallation;
• hydraulic repair on a hydraulic weighing system;
• major impact;
• unexplained zero drift;
• inconsistent readings.

If a weighing system is required on a new 50 ton forklift, it is better to define the requirement before production.

Useful information includes:

• internal weighing or legal-for-trade use;
• maximum expected load;
• required accuracy;
• weighing-system type;
• cab display requirements;
• printer or data-logging needs;
• operating environment;
• any special attachment fitted to the truck.

This makes it easier to select the scale together with the forklift rather than adding an unsuitable system later.

A correctly calibrated weighing system is useful, but it does not change the forklift's approved capacity.

The scale tells you the load weight.

The forklift's capacity plate, load center, mast, attachment and actual configuration determine whether that load can be handled safely.

For a 50 ton forklift, keeping those two concepts separate is more important than following a generic calibration procedure.

References

1. OSHA - Powered Industrial Trucks: Nameplate and Capacity. Guidance on forklift rated capacity, load center and capacity information.
2. OSHA - Powered Industrial Trucks: Attachments. Guidance on how attachments can affect capacity and center of gravity.
3. RAVAS - Forklift Weighing Systems and Calibration Guidance. Manufacturer information on hydraulic and load-cell weighing systems.
4. Rice Lake Weighing Systems - Forklift Scale Technical Manuals. Reference information on zero, known test loads and calibration procedures.
5. NIST Handbook 44. Technical requirements relevant to commercial weighing equipment in applicable U.S. legal-metrology applications.