Understanding how scales work and the options available will help users make the proper choice to meet company goals.
Byron Mucke
Electronic on-board scales are not new. They were introduced over 30 years ago into trucking applications where monitoring gross vehicle or payload weight was necessary, but where platform scales were not readily available. Over the years, improvements were made to these early electronic on-board scales. Load cells were improved and specialty load cells were developed for fifth wheels, center hangers, single points and most other types of spring suspensions. Strain gauge based air sensors were added for an increasing number of air ride suspensions and suspension transducers for spring suspensions. Hydraulic sensors were designed for vehicles equipped with hydraulic lift cylinders. Deflection transducers were developed as a low-cost solution for measuring gross vehicle weight. Today, on-board scales can be installed on any truck or trailer with air, spring or mixed suspensions.
On-board scales on refuse vehicles and refuse fleets allow drivers to know current vehicle weight at all times thereby minimizing liability due to accidents while simultaneously promoting hauling efficiency and reinforcing positive driving habits. On-board scales allow managers to set operational weight limits for every vehicle in the fleet so the driver can act on that weight limit based on the continual, real-time feedback from the in cab weight display meter. This is the most basic weight management/monitoring system that requires no back office software, additional personnel or monthly fees to realize multiple benefits.
Load path, secondary measurement, weigh in motion, etc. These terms are used on a daily basis in the process of developing different solutions to meet the various objectives of the refuse and other industries. But what do these terms mean to the end user, fleet manager, or purchasing department? The answer is that it is most important to first understand and define client operational objectives and then apply the proper system to meet those objectives to meet expectations and avoid the frustrations of applying the wrong system for the wrong application. Following is a summary of how scales work and different configurations to allow users to make informed decisions about which weighing solution is right for each specific application.
Modern Load Cells
Modern load cells do not have moving parts, and use bonded strain gages as the sensing element. The load cells undergo a slight deformation when loaded which produces a voltage signal that is proportional to the load. The signal is then used to report a weight, force or deflection. This technique is used to design quality weighing devices that protect this measurement so that the sensors perform with high degrees of accuracy and repeatability in harsh environments such as the refuse industry.
Today, on-board scale systems available for the refuse industry include: body scales, suspension scales, front fork scales, roll-off scales, deflection transducers and air sensors. Some solutions can only be used for one type of application, but in many cases a variety of solutions are available for a particular application and it is important to apply the proper solution to meet application objectives.
Body scales, suspension scales, front fork scales and shear pin load cells used on roll-off scales directly measure the load and are proven to be the most accurate and reliable over an extended period of time. They work by being installed on the vehicle in different configurations to directly measure the load.
Body Scales
Body scales are often installed between the body and chassis of rear loaders, side loaders or front loaders to provide optimal performance. This solution is also occasionally used on roll-off or dump truck systems where the requirements justify this high-performance solution. In these applications the body weight is transferred directly through the load cells allowing the strain gauges to measure a voltage change that relates to the load being added. Though they require a more intensive installation process, once installed they are very accurate, reliable and need little maintenance.
Fork Scales
Fork scales are second solution for a front loader where the principle is essentially the same. As the front forks pick up a container the weight is transferred through the fork itself. In this application, strain gauges installed in the front fork load cells measure a voltage change indicating bin weight that the operator can monitor, record and use for decision-making. For optimum results, fork scales require the driver to follow a simple but consistent procedure to obtain accurate results during the weighing process.
Suspension Scales
Suspension scales of all types exist to provide the full benefits of a load cell scale system. In the refuse industry these are most often applied in transfer station environments using fifth wheel load cells on transfer tractors or yard tractors to measure weight directly. For transfer trailers a variety of load cell solutions exist such as center hanger suspension scales, single point load cells and Chalmers suspension scales. All of these spring solutions use direct load measurement and can be mixed and matched. Similar to the body scales, once installed, they provide accurate and reliable weight data in the harshest environments.
Roll-Off Scales
Roll-off scales are another way to scale roll-offs using load cells, and are the most common method to obtain optimal performance. In this specific application load cell shear pins are installed in the rear hinge assembly to measure the rear of the box. The weight of the front of the box is often obtained by measuring the hoist cylinder pressure with a hydraulic pressure sensor, or sometimes by installing load cells or load shear pins under the hoist assembly depending on configuration and application requirements. This configuration has been proven to be accurate and low maintenance for roll-offs and dump truck applications.
Secondary Measurement Devices
Secondary measurement devices such as deflection transducers measure the deflection of a part of the vehicle such as a suspension member or axle. These devices are more susceptible to outside influences such as loading environment and driver performance so require more stringent adherence to specified loading practices. An example of a secondary measurement device is a deflection transducer. They are mounted on suspension equalizing beam or axle of the vehicle and measure the deflection of the axle or suspension under load to measure the weight. Using proper weighing procedures they can provide a useful measurement when the vehicle is fully loaded.
Secondary Pressure Sensors
Secondary pressure sensors such as air sensors measure changes in air bag pressure to correlate to weight. To obtain the best performance monitoring air pressure it is important that operators adhere more carefully to loading steps and guidelines since the repeatability is more susceptible to outside influences. Mixed air and spring scale solutions are often the best fit for a variety of application requirements depending on configurations, cost and user objectives.
After Installment
So now measurement devices are installed on your vehicle. How is the weight displayed or recorded? First you need to calibrate your system. Systems using current technology allow simple calibration where you only need a full and empty weight from a certified platform scale to set up your system. From that point system electronics obtain the load cell signals and provide digital signals to an in-cab display providing drivers with the information they need to make decisions including gross vehicle weight, axle weights, etc. This information can also be transmitted to on-board computers or telematic systems including GPS for additional monitoring to help with decisions when the truck is in route, or for planning purposes.
When specifying and purchasing an on-board weighing system the first step is to determine your objectives and requirements. Then identify venders with multiple solutions and understand each so that the proper solution can be selected for your specific application requirements. Understanding how scales work and the options available will help users make the proper choice to meet company goals.
Byron Mucke is the Marketing/Communications liaison for Vulcan On-Board Scales (Kent, WA). For 20 years, Byron has been committed to informing the trucking industry about the benefits of on-board weighing, and believes the better informed you are, the better you are able to make positive strategic decisions when it come to your needs and goals. To learn more about on-board weighing, e-mail [email protected].