MSW Management | Weighing Systems

Be it tanks or containers, liquid or dry goods, short haul or long - each form of truck hauling has a different basis for delivery and billing, but the basic requirement for profitability is the ability to accurately weigh truckloads.

By Daniel P. Duffy

Truck scales are available either as in-ground or onboard. Each type of scale system has its uses, inherent degree of accuracy, superiorities, and drawbacks as described below. In-ground scales can be installed either in pits or on flat ground. The truck drives over the in-ground scale so it can be weighed. Onboard scales allow a truck to directly weigh its load without the need for an exterior scale system. The components of an onboard scale system typically are installed on the framework of the vehicle.

In either case the weight of the truckload is measured by a series of load cells. These cells either deform or have increased interior pressures as a result of the applied load. The amount of deformation depends on the type of material used by the load cell, but in most cases the material is relatively stiff and deforms only slightly under the load weight. Sensors attached to the load cell measure the amount of deformation. The sensors send an electronic or hydrostatic signal to a summing station. The summing station tallies the individual readings from the load cells and displays the total load weight on either a digital indicator or a hard-copy printer.

Load Cells

Load cells are the mechanisms that directly measure the applied truckloads by physical deformation or increased internal pressures. Load cells are attached to the framework of either the in-ground scale or the truck.

There are several types of load cells used by in-ground scales:

At the heart of each load cell is a transducer that measures the deformation (strain) caused by the applied truckload (stress). The amount of deformation is dependent on the stress/strain characteristics of the material used to construct the frame or core of the load cell. For example, standard transducers include strain gauges connected to the core or frame of the load cell. The strain induced in the gauge alters the cross-sectional width of the gauge material. This in turn affects the electromagnetic characteristics (resistivity and frequency) of the gauge. The signal passing through the gauge reflects the new characteristics and alters accordingly.

Each vibrating wire sensor in a vibrating wire load cell consists of a wire tension mounted between two mounting blocks connected to the ends of the cylinder. Deformation of the cylinder under load weight produces relative movement between the mounting blocks, which causes a change in the wire's tension and a resulting change in its frequency of vibration. The new resonance frequency is measured by "plucking" the wire with an electromagnetic coil. A signal cable attaches this coil to a readout device. The readout device's computer translates the frequency resonance change into applied load for purposes of measurement. The average of the measurements is used as the loading on the cell, reducing the effects of eccentric loading.

Air-pressure load cells measure weight by gauging the increase in air pressure as a result of the applied loads. The better systems are highly sensitive to even incremental weight changes and compensate for environmental factors, such as changes in ambient temperature.

The type of construction used for their installation classifies in-ground scales. Concrete-deck truck scales have a roughened concrete surface to minimize the potential for skidding. Less expensive and easier to install are steel-deck flat-top truck scales. Instead of a poured concrete surface of 6- to 8-in. thickness, 12-gauge (or thicker) steel plating is bolted or welded to the scale's main frame. A properly designed frame minimizes the deflection to the frame itself, which can interfere with the accuracy of the load-cell measurements. A good scale will have a deflection strain ratio of no less than 1:850.

In-ground scales can be constructed aboveground or belowground. Aboveground scales can be constructed with either a high profile for ease of maintenance or a low profile for ease of approach. Temporary portable truck scales can also be installed aboveground on a flat grade or pavement. Belowground scales are constructed in pits that allow the surface of the deck to be flush with the ground for greater operational safety.

Massload Technologies in Saskatoon, SK, provides a wide variety of stationary scales. The basic framework assembly of each scale is based on a self-compensating double parallel link suspension. This allows for free movement in all directions, increasing accuracy and reducing the possibility of binding (lock up of the load cell as a result of being pinned by the frame members).

Its basic scale is a portable (temporary) axle scale designed to be installed directly on a flat ground surface. It comes with approach ramps, and several are used, depending on the number of axles being weighed. Each pad is 30 in. wide and measures from 30 in. to 144 in. in length, with a 30,000- to 40,000-lb. capacity per pad. It comes with a checker-plate decking surface to provide a nonslip weighing platform. Massload also provides a series of permanent in-ground scales utilizing the standard self-compensating double parallel link suspension. These come in kits that allow the customers to build them themselves. All materials, blueprints, and wiring diagrams are provided. Several styles are available, such as low profile with side rails, flat-top on piers, and side-rail design on piers.

Rice Lake Weighing Systems in Rice Lake, WI, manufactures and installs the Survivor series stationary truck scales. The Survivor EZ flat-top concrete deck scale can be installed in a single day and creates a low-profile, 14-ft.-wide weighbridge. Its design minimizes long-term metal fatigue, giving it a longer-than-average life cycle. The EZ also comes in a steel-deck flat-top version. It stands at an overall height of 20 in. and provides easy access and clean-out. The Survivor SR low-profile side-rail truck scale includes a 24-in. main I-beam that limits deflection under very heavy loads. The Survivor PT concrete-deck pit-type truck scale has a frame consisting of 24-in. main I-beams for less flexing and longer life cycles. Rice Lake also provides a series of specialized scales: over-the-road top-access scales (OTR series), all-terrain portable scales (ATV series), agricultural and livestock scales (AG series), and livestock ring scales (LV series).

Cardinal Scale Manufacturing in Webb City, MO, manufactures the SRC line of heavy-duty, long‚life cycle truck scales. These are pit-type electronic scales with heavy-capacity load cells. Platforms are 10 ft. wide and range from 10 to 100 ft. long. Capacities range from 30 to 120 tons. Extra protection against environmental influences is provided by load cells constructed of stainless steel. They are hermetically sealed and coupled with special grounding points and volt surge suppressors.

Emery Winslow Scale Company, headquartered in Seymour, CT, manufactures the Hytronic scale system. Hytronics utilizes hydrostatic signals instead of electronic signals to connect the scale with its meter display. Furthermore, the summing board (the device that tallies the load-weight reading from the scale's various load cells) is located within a protected environment, such as the control room or scale house. The electronic load-cell summing board typically is collocated along with the load cells in the same environment. Hydrostatic transmission tubing, weigh systems, and load cells are not environmentally sensitive. The Hytronic scale utilizes specially designed load cells consisting of cylinders filled with fluid and closed at the bottom with a diaphragm. Connected to the load cell is the output tubing, which sends the hydrostatic signal to the summing board.

Mettler-Toledo, based in Columbus, OH, used the first commercially available digital load cell. Its PowerCell brand transducer is the main component of its Cougar and Jaguar weighing systems. MT's OverDrive vehicle scale management software operates the system and allows for accurate measurement of vehicle use and report writing. MT provides a wide range of rail and truck scales ranging from light commercial to extreme heavy-duty applications. Old or obsolete scales can be upgraded to the latest technology standards with interchangeable conversion kits.

Weigh-Tronix in Fairmont, MN, manufactures a series of stationary truck scales for a variety of uses. Its BridgePort portable truck scales consist of self-contained modular sections that can be customized for particular applications. They can be installed in just two to four hours on any stable surface (bridge planks, concrete piers, concrete blocks, or concrete slab). Each section incorporates built-in Weigh Bar weight sensors, leveling pads, and dedicated conduit runs. Weigh-Tronix's BridgeMont or SteelBridge XT Multi-Platform truck scales combine multiple independent scales in a common foundation. Axle weights and a certified gross weight are provided simultaneously in a single weighing.

Traditionally onboard scaling has never been the standard practice in the trucking industry due to expense, inaccuracy, complicated operations, or sensitivity to environmental or weather changes. Recent advances in technology have changed this situation, however, making onboard weighing competitive with in-ground scales in terms of cost and accuracy.

There are several onboard scale systems. Each uses a type of load cell described above and is designed for mounting on either a truck's wheel axle or structural frame. Two types of load cells based on air-pressure increases due to truck weights are available. The first consists of special air pads constructed in the truck and trailer frame to measure air pressure. The second type measures changes in air pressure in the vehicle's air suspension system. Spring load cells are designed to work with a truck's spring suspension. These are typically used in single-axle vehicles. Trunnion load cells are designed to fit into single-point suspensions. They replace the long trunnion tubes or dead axles in standard suspensions.

Air-Weigh, based in Eugene, OR, manufactures a scale that converts air pressure in the tractor and trailer air springs into an onboard weight. The number of trucks with air suspension has increased from 20% of new production in 1992 to 70% in 1999. Trucks with air suspension have a control valve that maintains a constant vehicle ride height. The Air-Weigh system electronically senses changes in the air pressure required to support a load at a constant height. Once calibrated, its accuracy is between 200 and 300 lb. The scale system operates in the cab via a display module and allows the driver to weigh as he loads, making for more efficient bulk-hauling operations. Weighing while loading eliminates commercial scale fees, reduces the cost of overweight fines, reduces the number of underweight loads, reduces wasted time, and reduces the cost of driving out of route (where no in-ground scale is available).

Vulcan On-Board Scales in Kent, WA, also manufactures an air sensor for use with a truck and trailer air suspension. It has single or dual port sensors, depending on the number of height control valves used to level the ride height. It is compatible with other types of load cells and can be used in conjunction with spring or trunnion-type cells. The air sensors are manufactured from high-strength stainless steel and have a maximum air-pressure capacity of 120 psi. Their accuracy is within 1% of ground vehicle weight.

Vulcan also makes mechanical (non‚air pressure) onboard load cells. These include a rear spring hanger bracket for use with its Super-Beam load cell and a trunnion load cell that uses standard-suspension bushings and parts and is designed for rugged high-capacity applications. Vulcan load cells are compatible with most electronic and data systems. They have a maximum capacity of 75,000 lb. and are accurate to within 1% of ground vehicle weight.

Weigh-Right of South Hutchinson, KS, manufactures three types of onboard scales. Its air scale system uses air bags mounted on the vehicle frame and axle that provide direct measurement of vehicle load by measuring changes in air pressure. The air bags are connected to a display meter by means of a hydrostatic air line. The meter reads and displays the weight on each suspension group.

Weigh-Right's spring scale system can be transferred quickly from truck to truck (it can be bolted on in less than four hours). It has an optional alarm system for when the load reaches its allowable weight, and it can come with automatic relays to cut off hydraulic load functions when weight is exceeded. The spring scale's transducer is connected to the display meter by means of a three-wire cable. As with all Weigh-Right systems, it can be combined with air-pressure or hydraulic load-weighing systems for increased accuracy.

A simpler front-axle scale is also provided by Weigh-Right. Front-axle scales are mounted directly to the steer axles of the truck using the frame rail mounting as a reference for positioning. They can be mounted to either the driver or passenger side and are especially useful for measuring weight under unlevel loading conditions.

Loadrite Scales offers a system designed for front-end loaders and other earthmoving equipment. The advantage of this system is that dump loads can be tracked instead of (or as a check to) truck scale operations. The system includes an optical trigger mounted on the lift arm frame that determines the point at which the load is weighed. Hydraulic transducers then convert hydrostatic pressure into equivalent load weight. An onboard management information system provides data updates directly to the driver/operator either via display screen or hard-copy printout. The information can be downloaded to an office computer for complete analysis and report writing.

LoadMan in Renton, WA, provides a variety of suspension and fifth-wheel load cells for over-the-road transport. These include single-point load cells, four-spring suspensions (Hutch transducers), four-spring suspensions (Reyco transducers), fifth-wheel load cells (Holland and Fontaine transducers), and air-ride suspensions (air-pressure transducers). For waste hauling, LoadMan has the Onboard Fork Weighing System for solid waste hauling trucks. This system can weigh dynamically (on the fly) and therefore does not require the typical stop-and-go method utilized by front fork scales.

Software programs typically are used to create weight data, reports, and billing activities. Software also exists to regulate automated truck loading systems and allow for automated truck loading.

Quebec-based Xactec is a firm specializing in the design and development of innovative computerized management systems for the environmental industry. Its software provides both operational flexibility and high precision. It provides load management software primarily for the waste and bulk-hauling industry in conjunction with municipalities and waste collection companies. Its load management system is based on a dynamic onboard scale system equipped with sensors and a radio frequency identification system for weighing, tracking, and identifying loads. Onboard computer and wireless communication links record data and transfer them from the truck to either its destination or home office. The software packages provide the following apparatus:

Other types of software allow an operator to do more than just track hauling operations; they can control the actual loading process. Compro Systems, for example, has developed its Truck Loading System to provide the bulk-handling industry with a method of precisely loading trucks.

These systems are integrated into the batch-systems loading device to allow for automatic, unattended batching systems and automatic, unattended systems that utilize truck scales. The unattended system that utilizes a truck scale is actually two systems combined into one. It simultaneously loads and weighs each truck. This allows a loading facility to dispense with a second truck scale. The unattended batching software reduces manpower needs because the driver can interface directly with the loading system by way of a card reader/keypad/display unit located in the cab of each truck. The driver can initiate the load sequence and set the weight parameters for loading. A ticket printer provides hard-copy records of the transaction. The entire system is tied into an overall management information system network and report-writing database software.

A simpler system allows for unattended operations and data collection. While it does not actually operate the scale, the unattended system allows for automated updates of truck hauling data by using a radio frequency card reader/display/keyboard unit. This allows for near-instantaneous driver identification while entering the data into the management system. This automated scale system includes optional features such as traffic control lights, scale-mounted vehicle-position sensors, and remote scale displays. Security systems and coded access are integrated with the automation software. Also included is an optional radio frequency tag-reader antenna mounted on the scale. With radio frequency tags attached directly to the truck, human error is eliminated during scale operations. The overall system and associated report-writing software is Windows-compatible.

National company PC Scale also makes software for unattended truck weighing. This system includes camera interface software, an unattended terminal, a transaction screen, and a process controller. Identification is by embedded swipe-card reader and swipe cards or by radio frequency ID units. Information can be read by bar code, insert reader, or windshield tags. Traffic control systems are also provided to regulate truck movements into and out of the scale area.

For bulk hauling, PC Scale offers a number of software packages, including AGG 2002 for aggregate. It allows for three-tier pricing (material specific, customer specific, or order specific) that is automatically recorded with each scale transaction. It allows for unlimited data storage of truck information (e.g., load and vehicle designation). It can charge automatically, billing by the ton, bag, gallon, item, and so on. It charges delivery fees as a flat fee or by the mile, ton, hour, and so on, depending on purchase arrangements. It maintains a customer-history database for returned products, invoices, communications, delivery terms, and more. It can record tare expiration dates for each truck, organize corporate taxes, determine the optimum tax grouping (local and state), determine which orders are tax-exempt, track single or multiple shipping, and give driving directions. Its report-writing system provides detailed formats, summary formats, and time of delivery.

Simpler still are attended truck scale data collection systems. With a human in the loop, the system has greater flexibility. Attended systems can be tailored for many different bulk-handling industries. Compro also makes Autopro 2000 software that is compatible with many operating systems and can be directly interfaced with a wide range of LANs and other networks. The data are added manually into a Windows-compatible software package.

Mettler-Toledo provides an automatic identification and data capture system that allows the entire logistics chain to use identification, weight, and dimensions to track, count, control the contents, determine capacities, optimize packing, manifest, determine costs, and determine loads. This system is applicable for all mailing, packaging, containerized shipping, and bulk transport systems. It allows for direct interface with applicable warehousing systems.

PC Automation in Waterloo, ON, provides a software package called Geoware. This software is specifically designed for the solid waste industry but can be modified to manage any bulk-loading and transport operation. Geoware completely automates the management data collection and management operations of a waste truck collection fleet. The software allows the customer to select a task-specific module to fit the needs of a particular waste hauling operation. The various modules are able to share data with other parts of the suite - for instance, the Scalehouse Module - eliminating redundant activities and reducing the opportunity for entry error.

Daniel P. Duffy, P.E., is a professional environmental engineer in Cincinnati, OH.