A scissor lift rated for 320 kg is carrying workers and tools when an overload warning appears and lifting is restricted. How does the machine know there is too much weight on the platform?
That depends on its sensing system. A platform load-sensing system does not necessarily weigh the load directly. Some designs infer it from hydraulic pressure and lift position; others use dedicated load cells or a combination of sensor signals.
An overload indicator is therefore not the same thing as a general-purpose weighing scale.
1. Why can lift position matter to load detection?
JLG’s technical documentation describes one scissor-lift load-sensing arrangement that uses two pressure transducers and one angle sensor.
The transducers measure pressure at specified points in the lift hydraulic system. The angle sensor provides information about the scissor mechanism’s position. The controller uses the signals together to determine whether the load is within the permitted range.
Source: JLG, Load Sensing System — Scissors Lift Products, Manual 3124288 (July 2013 revision).
Why not read hydraulic pressure alone? Because the mechanical advantage of the scissor linkage changes with lift position. The same platform load can produce different cylinder forces and pressures at different lift heights.
For that type of system, pressure is only one input to a calibrated relationship. Fitting a pressure gauge to the hydraulic circuit would not, by itself, reproduce the manufacturer’s load-sensing system.
2. Common approaches use different signals
| Sensing approach | Information used | Important consideration |
|---|---|---|
| Hydraulic pressure plus position | Cylinder pressure and scissor angle or position | Load is inferred using a calibrated relationship |
| Load cells | Force measured at a specified structural or support location | Interpretation depends on mounting and system design |
| Combined sensing | Several sensors and controller inputs | Response and diagnostics are model-specific |
None of these approaches is automatically more accurate in every machine. Accuracy depends on sensor design and installation, mechanical condition, calibration and control logic.
Genie service documentation for different scissor-lift variants refers to components such as pressure transducers, load-cell modules and platform-height sensors. See the relevant Genie service manual.
When a quotation simply says “overload sensor”, the better questions are: How does this machine measure or infer load? What does it do when overload is detected? What evidence is available that the system has been checked?
3. Does overload shut down every function?
Not necessarily. The purpose of load sensing is to detect an overload and apply the manufacturer’s prescribed warning and motion restrictions. Those restrictions differ by model.
Genie describes machines that use audible and visual alerts and inhibit normal functions when an overload is detected. Certain designs also provide a limited, prescribed recovery mode that may permit downward movement while preventing other functions. See Genie’s platform load-sense guidance.
The critical qualification is model specificity. A recovery capability described for one machine must not be assumed to exist on another.
Bypassing sensors, altering control settings or silencing the alarm to resume normal work is not a substitute for resolving the condition.
4. Why calibration matters
Sensor readings do not automatically correspond to actual platform loads. The controller needs a validated relationship between the input signals and the permitted load condition.
Manufacturers therefore define calibration and verification procedures. Genie’s technical guidance distinguishes between full-load calibration, which uses the prescribed rated test load and conditions, and zero-load calibration, which works from a no-load reference under the particular system’s procedure.
For the systems discussed by Genie, zero-load calibration is not necessarily a complete substitute for the prescribed full-load process; permissible machine capacity may remain restricted until the required calibration has been performed. See Genie, Load Calibration for Load Sense.
The methods, intervals and circumstances that require recalibration belong to the specific service manual. A successful no-load adjustment does not, on its own, prove the machine’s full rated-load performance.
5. No alarm does not prove that the load is acceptable
If a sensor has failed, the system is miscalibrated or related components are defective, overload detection may be unreliable. Even a working system has a manufacturer-defined detection threshold and tolerances.
Genie cautions users not to treat potential detection tolerance as an allowance to add a percentage above the rated capacity. See its load-sense Q&A.
The platform’s published rated capacity—and any separate limits for the extension deck, operating mode or person count—still apply. Load sensing is a protective measure, not a replacement for managing people, tools and materials.
6. What a buyer should inspect or request
| Item | Evidence or question |
|---|---|
| Sensing system | Is it fitted, and which sensing architecture is used? |
| Rated capacity | Main deck, extension deck and any mode-dependent restrictions |
| Alerts | Audible, visual or displayed overload indications |
| Motion restrictions | Which functions are inhibited; is any prescribed recovery mode available? |
| Calibration | Applicable manual, method and maintenance requirements |
| Acceptance records | Procedure, responsible person, test conditions and recorded result |
A practical supplier request:
See the guide to scissor lift pre-shipment inspection for the relevant checks.
An inspection report should distinguish among the system being installed, an alarm being observed, and the system having passed the required functional verification. Those are not interchangeable findings.