As global industry standards continue to evolve, load sensing systems (LSS) are now required on all MEWPs (mobile elevating work platform), a provision that mandates that a MEWP include a means of monitoring platform weight.
This has challenged MEWP manufacturers to adopt, deploy and develop new technologies to help equipment operators determine if the platform has been overloaded beyond the machine’s safe limits.
There are several ways in which a MEWP (formerly known as aerial work platforms) can be overloaded, including by adding too much weight to the platform on the ground, adding weight to the platform while elevated, a sensor failure, lifting into overhead structures or from debris falling into the elevated platform.
Specific to scissor lifts, an overload condition results in the disabling of all vehicle motion until the excessive platform weight is cleared. There are many approaches to determining platform load, but the industry has converged around a pressure-based load prediction system.
A pressure-based load sensing system for scissor lifts is used as a regulatory safety system and is designed to monitor the load of persons, materials, and tools in the platform, ensuring vehicle capacity ratings are not exceeded. Apart from the basic requirement stated above, this system also has other functional, calibration and diagnostic requirements to ensure operator safety.
Functionality
For scissor lifts, the system is designed to monitor the force acting on the main lift cylinder, as well as platform height. To determine a platform load, sensor data is measured (both elevation and cylinder pressure) against data recorded in the calibration process, then the scissor lift determines if the load is greater than the maximum allowable operating capacity of the machine when operating indoors or outdoors.
In the event of an overload, the scissor lift notifies the operator by sounding an alarm sequence and indicator lamp in the platform and ground controls. The scissor lift then disables all drive functionality, all steer functionality and prevents further elevation until the overload state is clear.
In the event of an emergency, a manual descent system or the ground control station may also be used to rescue an elevated and incapacitated operator.
Calibration
Scissor lifts with a load sensing system require periodic recalibration due to wear or component failure. During a routine service interval, when a sensor fails or when a machine fails to pass an annual inspection, a technician must perform an LSS calibration so the system can learn the machine’s new mechanical characteristics.
During a calibration, most scissor lifts will require the operator to load the platform with weights to maximum platform capacity. The control system then uses both pressure and height to generate a dynamic pressure calibration curve which can be used to determine if an overload condition exists.
With zero load technology, or empty deck calibration, the process typically requires only a diagnostic tool, without the need for platform weights. After successfully completing an empty deck calibration, the machine will return to an operable state.
Diagnostics
In the case of an overload situation occurring, to remove the overload condition and resume normal operation, the operator must remove weight from the platform until the overload condition no longer exists or a technician may have to repair the machine. Both the platform and ground visual indicators will display when overload is active in addition to when the platform is no longer over capacity.
JLG’s scissor lift load sensing system
Today’s load sensing systems for JLG scissor lifts take all the lessons from prior systems and improve the calibration, functional and diagnostic performance of the machine. For example, JLG now uses a pressure transducer on the rod side of lift cylinders, in addition to the head side of the cylinder, in order to compensate for hydraulic fluid viscosity in low temperatures. The hydraulic cylinders also contain oil within the rod side of the barrel to lubricate the cylinder seals, reducing wear. As the rod extends, this fluid is displaced back to the main reservoir. When lifting down, the system diverts the fluid trapped on the barrel side of the cylinder through the remaining rod side cavity contained within the barrel. Any excess fluid escapes through the rod cavity and back to the reservoir. The weight of the platform provides the force needed to retract the cylinder.
An advantage of JLG scissor lifts’ load sensing technology is that it will allow operators to elevate with an overload to a certain distance; however, when the overload light comes on, it will not prohibit the operator from descending.
JLG has also developed a means to verify the integrity of the load sensing system’s calibration called Load Verification. This is done by running a single lift cycle up and down from the ground station with an empty deck, verifying whether the machine characteristics have changed significantly from their last calibration. If the lift cycle data differs beyond limits established in the last calibration, the machine will need to be recalibrated with an empty deck calibration to return partial functionality or go through a full calibration with test weights. The advantage of this is that service technicians can process a fleet of vehicles very quickly, isolating the out of tolerance vehicles from the healthy vehicles. The technician can choose to partially or fully recalibrate the vehicles once the necessary equipment is available such as an overhead crane and the correct collection of test weights.
The updated ANSI and CSA requirements in North America continue to challenge MEWP manufacturers to provide modern load sensing systems on scissor lifts that limits an operator’s ability to put him or herself in an unsafe condition while operating a machine. With this technology, scissor lift operators will not only benefit from the enhanced safety but also from productive operation.
For more information about how JLG MEWPs comply with industry standards, click here. To learn more about JLG scissor lifts, click here.
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