P&Q University Lesson 11: Loadout, Weighing & On-Road Hauling

By |  October 7, 2015

Loadout within any pit or quarry operation is an important stage that needs to be well designed and considered. There is an opportunity to save time and increase profits by understanding the specific needs of a the truck (vehicle) scale and stockpile areas and how the interface between these operations directly affects productivity and profit.

From the time the pit or quarry gates open in the morning to the end of the working day, handling stock is a dynamic and important process that involves many movements of plant and equipment and many interactions between workers and other staff.

Every time plant or equipment touches the sand, gravel or crushed stone product, it increases costs in wages, tire wear, fuel, machine hours, etc. Therefore, it is necessary to minimize interaction with finished products by automating processes where possible and, when manual processes are required, understanding them fully to maximize production output and operational reliability.

This section explores the key issues for stockpile management and load out and discusses the benefits that may be realized in utilizing available technologies.

Photo by Kevin Yanik

Photo by Kevin Yanik

Some of the key issues include:
■ Measuring and improving loadout cycle times.
■ The real costs of delays in loading and at the truck scale.
■ Unnecessary fuel costs from inefficiencies in the loadout process.
■ How automating and streamlining loadout can benefit a business.
■ How to get real efficiencies in mobile plant and equipment.

REDUCING LOADOUT CYCLE TIMES

Loadout cycle times are the measure of how quickly a customer is loaded and ticketed across the truck scale and leaves the site. When customers are kept waiting, they are losing truck utilization and, therefore, profit. This poor performance in loadout cycle times impacts the pit or quarry owner/operator, as unhappy customers shift their business to competitors.

Anything that impacts loadout cycle times will be costly to an operation; and the correct matching of mobile equipment is essential to realize the full efficiencies in this area. To maximize profits, pit and quarry operators must train their employees to ensure established processes and procedures are understood, clearly communicated and repeatable. Where there is a mismatch in loadout equipment, processes and staff knowledge, problems creep in and profits decrease directly as a result.

Photo by Zach Mentz

Photo by Zach Mentz

TRUCK PRODUCTIVITY

Pit or quarry customers operate in a highly competitive environment, where margins are tight, and they are focused on efficiencies in truck utilization. Hauling aggregate is most profitable when the trucks are kept full at all times. Back loads on a truck’s return journey are not always available and, as a result, transport costs climb rapidly as the trucks drive empty.

Most contracts/projects are priced on a set number of loads per day to achieve slim margins to tight construction time frames. When delays occur, especially within a pit or quarry, a customer sees this as unplanned delay that across a shift or day may lead to a reduction in the number of loads completed, decreasing customer revenue.

The average time in a pit or quarry is dependent on a number of factors, most of them outside of customers’ control. Therefore, it must be the aggregate operation’s task to measure time in the yard and put relevant Key Performance Indicators (KPIs) on the different elements of this process with a plan to reduce overall times.

Time in the yard can be broken down into the following:
■ Truck (TARE) weighed/recorded and travel time to relevant stockpile.
■ Truck waiting to be loaded.
■ Truck loaded and application of tarpaulin, if required.
■ Truck travel to the scales and, if required, waiting to be weighed before exiting the yard.

STOCKPILE/TRUCK SCALE LAYOUT

Once a customer has entered the site, effective stockpile layout will minimize congestion and time spent in the stockpile area. Having fast-moving products close at hand or easily accessible allows for quicker loading; slow-moving products should be given a lesser priority for position within the stockpile area. By having an intuitive layout, customers are directed to position their trucks naturally in the best location for quick loadout.

The stockpiling area should be designed to ensure that there is sufficient room for mobile plants and customers’ trucks to operate effectively. Where possible, all vehicles should be able to move past each other with sufficient room. Pinch points should be kept to a minimum or designed out. Where there is not sufficient space for this to happen, implementing a one-way system can be effective in reducing vehicles operating in close proximity.

The design of a one-way system should be intuitive. Once a truck has passed over the scales and entered the stockpile area, the driver’s route around the stockpile should be easily identifiable. Providing adequate signage indicating direction of flow and general traffic rules assists, as does providing well-planned stockpile layout maps at the scale house and other guidance, as required.

Cross roads where traffic merges, especially at the scales, can add to congestion during high or peak periods. One-way systems reduce congestion at these points. Where possible, design the scales so that tickets can be issued via a drive-up window. This means that drivers do not have to leave their trucks to collect their tickets, which offers advantages by reducing drivers’ time on site and, from a safety perspective, reducing the amount of pedestrian activity in the pit or quarry environment.

Vehicle scales

Vehicle scales are available in two basic types with variations on each type. Vehicle scales can be installed over a pit (similar to the basement of a house) or they can be installed above grade on a concrete slab. Pit-type scales are often found in colder climates and offer several advantages. Although generally more expensive than a similar model installed above grade, a pit-type scale provides easy access to the components beneath the scale and does not require ramps for the truck to pull onto the platform. Approaches to a pit-type scale are much easier since the weighing platform is flush with the surface of the ground. On the other hand, a pit-type scale normally requires some means of dealing with the accumulation of water within the pit, whether it be a gravity drain or sump pump, and are significantly more expensive to construct, requiring excavation, forming, and pouring of pit walls and floor. One safety consideration is that with a pit-type scale, a truck cannot fall off the scale if the driver cuts the wheels too sharply.

Photo by Kevin Yanik

Photo by Kevin Yanik

Vehicle scales designed for above-grade mounting — sometimes called low-profile models — can be installed either in a shallow pit or on a slab at grade level. Placing them at or near grade level requires ramps and level approaches at each end of the scale. This type of scale is often equipped with guardrails to keep the truck operator from driving off the scale platform. A low-profile scale also adds to maintenance costs since the area beneath the scale platform needs to be kept free of rubbish and debris that can affect operation of the scale. This cleaning procedure needs to be performed on a regular basis.

Generally, low-profile scales seem to be more popular, probably because of the lower cost of installation. However, before deciding, consider the advantages and disadvantages of both types.

PLATFORM SIZE

Platform size is an important consideration because, once purchased, it is almost impossible to change the size of the platform. Buyers must identify not only the vehicles currently being weighed but also those that may be weighed in the future. Since a typical vehicle scale has a life of 10 years or significantly longer, consideration should be given to what vehicles may be weighed five, seven or 10 years in the future.

Vehicle scales typically come in widths of 10, 11, 12 or 14 ft., or even wider. Lengths range from approximately 20 ft. to more than 200 ft. Of course, as the size increases, so does the cost, so it is best to select the smallest size that will accommodate the vehicles currently being weighed yet is large enough to weigh larger vehicles in the future. A typical size might be 11-ft. wide by 70 ft. long, but specific applications may require a longer and/or wider platform.

When choosing a platform size, also keep in mind the area in which the scale will be located. Most states require a straight and level approach to the scale on both ends. The length of this approach is normally 10 ft., but it could be more. Add to this the room necessary for the truck to maneuver onto the scale and it’s possible that a 70-ft.-long scale requires more than 150 ft. of real estate.

Photo by Kevin Yanik

Photo by Kevin Yanik

WEIGHING CAPACITY

Unfortunately, there has been much confusion when it comes to describing the weighing capacity of a vehicle scale. When examining competitive models of vehicle scales, there are two basic capacity figures: the concentrated load capacity (CLC) and the nominal capacity. There may even be other names for capacity used by some manufacturers such as section capacity or maximum capacity. Keep in mind that there are only two capacity ratings recognized by the National Conference on Weights and Measures — the CLC and the nominal capacity.

What’s the difference between these two capacities? The CLC is just what it says: the concentrated load capacity or, in other words, the scale’s ability to weigh a load concentrated in a relatively small area on the scale platform. A typical truck is supported by multiple axles, each with two or four or even more wheels on a single axle. Each one of these axles places a portion of the truck’s weight on the scale platform. Tandem axles are placed close together and place a larger load in a smaller area on the platform. The CLC rating is based on the maximum load that can be applied in an area approximating that used by a tandem axle. NIST Handbook 44 defines CLC as a capacity rating of a vehicle or axle-load scale, specified by the manufacturer, defining the maximum load applied by a group of two axles with a centerline spaced 4 ft. apart and an axle width of 8 ft. for which the vehicle scale is designed. The CLC rating is for both test and use.

Nominal capacity differs from CLC in that it is the total load that can be weighed on the scale when uniformly distributed over the scale platform. Both the nominal capacity, sometimes referred to as simply “capacity,” and the CLC are marked on the vehicle scale nameplate and on the weight indicator nameplate.

Considering the definitions for the two types of capacity rating, how much capacity is necessary? That is typically a hot topic among scale manufacturers and there is no simple answer. Begin by looking at the trucks that are going to be weighed on the scale. Are the trucks that are going to be weighed within the legal highway load limits (34,000 lb. for tandem axles and 24,000 lb. for a single axle)? If so, a greater CLC may not be required. On the other hand, if the scale will be used to weigh highly concentrated loads common to mining trucks and some dump trucks, a higher CLC rating may be required for the application.




Most vehicle scales have capacities ranging from 120,000 to 200,000 lb. The 200,000-lb. level is the maximum nominal capacity for a vehicle scale when using 20-lb. divisions because NIST Handbook 44 limits a vehicle scale to a maximum of 10,000 divisions. It is possible to achieve a nominal capacity greater than 200,000 lb. by either using a scale with a 50-lb. division or a dual-ranging scale where weights are displayed in increments of 20 lb. up to, say, 100,000 lb., then in 50-lb. increments up to the scale capacity. However, generally the 200,000-lb. value is more than sufficient for the vast majority of vehicle-scale applications.

Photo by Kevin Yanik

Photo by Kevin Yanik

PLATFORM MATERIAL

Normally, vehicle scales are available with one of two platform materials: steel or concrete. Each has its advantages and disadvantages. Although the total cost of a scale with a steel platform is less than one with a concrete platform of the same size, the steel can be slippery when wet and is subject to corrosion.

A concrete platform is more expensive and requires finishing labor and curing time, but is not subject to corrosion. Concrete can, however, deteriorate with age and weather. Generally, a concrete deck is a better choice for high-traffic applications. Another consideration is that a scale with a concrete deck is much more difficult to move at a later date than a scale of similar size with a steel platform.

WEIGHING TECHNOLOGIES

A vehicle scale can be viewed as a platform that is supported by one or more weight-sensing elements, which produce an output proportional to the load placed on the scale. Previous sections have looked at the variations in the construction and size of the platform; this section looks at the different weight-sensing technologies that can be employed. In general, the technologies can be divided into two broad categories: mechanical and full electronic. Mechanical versions use a series of levers to reduce the load to a value compatible with either a mechanical-beam indicator or a load cell. The full electronic version uses multiple load-sensing elements to support the scale platform. These sensing elements differ in type and include both analog and digital load cells employing strain gauges and hydraulic load cells.

MECHANICAL

Mechanical vehicle scales have been around for a long time but are becoming the exception rather than the rule. This is because a mechanical vehicle scale, while an excellent weighing device, is normally more expensive than a model using multiple load cells. The mechanical vehicle scale consists of a series of force-reducing levers where the load on the scale platform is reduced through a series of mechanical levers to a smaller value that is compatible with a mechanical-beam indicator. The output of a mechanical vehicle scale is connected through a rod (called a steelyard rod) to one end of a balance beam. Calibrated weights on the opposite side of the fulcrum (pivot point) are moved away from the fulcrum until a balance condition is achieved. The distance the weights are moved is proportional to the load on the scale platform. It requires a minimal level of skill to properly operate a mechanical vehicle scale but the scale is capable of high-precision weighing.

This type of scale is immune to lightning damage since there are no electrical components used and therefore requires no power. Of course, it is not possible to connect such a scale directly to modern computing peripherals, but it can be done (see the next section). Fewer and fewer scale manufacturers offer fully mechanical vehicle scales, although there is a market for used scales of this type.

ELECTROMECHANICAL

Electromechanical vehicle scales are identical to mechanical scales with one important addition. Electromechanical scales include a strain-gauge load cell mounted within the steelyard rod (the rod connecting the output of the scale lever system to the mechanical beam indicator) to produce an electrical signal proportional to the load on the scale platform.

This addition allows the scale to be connected to a digital weight indicator and, from there, to any number of devices. This type of scale is available from several scale manufacturers. The drawback with this type of scale is that it uses moving parts. Moving parts are affected by friction and wear, eventually requiring service or replacement. Refurbishment of a mechanical or electromechanical scale can be expensive and time consuming, but durability and simplicity of such a scale sometimes overshadow these drawbacks. Normally, a mechanical or electromechanical scale will be mounted in a pit.

ANALOG LOAD CELL

Probably the most common vehicle scale today is one where the scale platform is supported by multiple analog strain-gauge load cells. In this type of scale, a portion of the load placed on the scale platform is applied to each load cell, which generates an electrical signal proportional to the load applied to the cell.  The individual load cell signals are summed in a junction box and the combined signal fed to a digital weight indicator where it is converted to a digital value and displayed for the operator.

Analog load cells used in vehicle scales are available in a number of configurations, but the most common are compression load cells and double-ended shear beam load cells. Both types of load cells perform the same function, but sense the load applied to the load cell in different ways. Vehicle scales employing compression load cells typically have rigid restraint systems to keep the platform from moving, while scales using double-ended shear beam load cells use restraint systems that allow the scale platform to move. When considering a vehicle scale that uses multiple analog load cells, a number of legal metrology requirements must be met. These are more fully described at the end of this section.

Analog load cells offer a number of advantages, but also have some disadvantages. Analog load cells are relatively inexpensive and are a proven technology. They are available from multiple sources and can even be rebuilt or refurbished under the proper circumstances.

Analog load cells used in vehicle scales range in cell capacity from 50,000 to 100,000 lb., with each pair of load cells comprising a section of the scale. This type of load cell uses several strain gauges that consist of a series of thin electrical conductors bonded on a nonconductive backing and cemented to the load-bearing element. Strain gauges, because of their electrical characteristics, are sensitive to lightning damage. Load cell and scale manufacturers take steps to minimize this risk by including surge suppression components to shunt the excessive current from lightning strikes around the strain gauges. In spite of manufacturers’ best efforts, many analog load cells are damaged or destroyed by lightning each year. If a scale uses analog load cells, make certain to follow the recommendations of the scale manufacturer to minimize the risk of damage to the load cells.

Another enemy of analog load cells is moisture. When comparing scales, look for those that use load cells manufactured from corrosion-proof materials, such as stainless steel, and feature hermetic seals and integral cables with moisture barriers. Entry of moisture into the load cell circuit can cause problems ranging from unstable readings to a completely inoperative weighing system.

DIGITAL LOAD CELL

Digital load cells produce a digital output rather than an analog output. Although this can be accomplished in a number of ways, digital load cells used in vehicle scales are typically analog load cells containing signal processing and analog-to-digital conversion circuitry within the load cell enclosure. Digital load cells offer several advantages compared with analog load cells.

The fact that the output is digital rather than analog means that the signal does not degrade as it travels from the load cell at the scale to the weight indicator. Further, the output from a digital load cell is normally optically coupled, which provides the digital load cell with a much greater level of immunity to lightning damage. Errors common to analog load cells (nonlinearity, hysteresis and creep) often can be reduced through compensation algorithms contained within the load cell itself.

Unlike analog load cells, there are no standards regarding the output of digital load cells, hence the format of the data output differs from digital load cell to digital load cell. This means that it can be difficult to impossible to replace a digital load cell without buying it from the original manufacturer or its representative. Analog load cell replacements are available from a number of sources.

Because the output format from digital load cells differs from manufacturer to manufacturer, normally the instrumentation that comes from the digital load cell manufacturer must be used. While normally that is not a problem, special applications may require features not available with digital load cell instrumentation.




Yet another consideration is the effective sample rate of digital load cells. Although getting better, digital load cell systems may not always be a good choice when doing any automatic filling on a vehicle scale. The time needed to read the weight data from all of the digital load cells in a scale is greater than the time needed to perform a single analog-to-digital conversion in a multiple analog load cell scale of the same type. An analog load cell scale provides weight data quicker and therefore decisions regarding the flow of material can be made sooner, resulting in more accurate material control.

HYDRAULIC LOAD CELL

Another type of load cell is the hydraulic load cell. This load cell uses a hydraulic fluid to transmit the force applied to the scale platform to a pressure transducer, where it is converted to an electrical signal. The hydraulic load cell operates entirely on hydraulic pressure and uses no electrical components, making it a good choice for hazardous areas or areas prone to lightning damage. Hydraulic load cells used in vehicle scales are normally of the compression type and can range in capacity from 25,000 to 100,000 lb.

When comparing hydraulic load cells, make certain that the load cell is listed on a current National Type Evaluation Program (NTEP) Certificate of Conformance and is constructed from a corrosion-proof material such as stainless steel.

The individual pressure lines from the hydraulic load cells must be combined into a single output in order for it to be displayed by the weight indicator. Sometimes the combined signal is achieved by stacking pressure actuators, one for each load cell, thereby mechanically summing the forces. The combined force is then applied to a single pressure transducer. Another method employs a pressure transducer for each hydraulic load cell then combines the outputs electrically.

A vehicle scale using hydraulic load cells is usually significantly more expensive than one using analog load cells. This is because of the added manufacturing costs associated with the hydraulic load cells and associated pressure totalizing system. This increased cost can sometimes be offset because of the inherent reliability of this type of system and because of its immunity to damage from lighting and voltage surges.

Photo by Kevin Yanik

Photo by Kevin Yanik

PERIPHERAL DEVICES

After determining, in consultation with the scale-manufacturer representative, which scale technology, platform size and type best fit an application, it is time to decide what, if any, peripheral equipment will be required. There are all types of equipment to transmit, display, store and process the weight data from scales, and identifying what is required can sometimes be difficult. Following is a brief overview of the most common types of peripheral equipment.

Power-conditioning equipment: Depending on the operation’s location and the quality of the power the electrical utility provides, power-conditioning equipment may be appropriate. The most common power problems include low voltage levels, electrical noise, and line transients caused by lightning or switching of inductive loads. Weight indicators operated from the power line are required to operate correctly down to 85 percent of the nominal line voltage, so low line voltage is not normally a problem. If it is, an inexpensive voltage-regulating transformer will take care of the problem.

If the scale is in a remote location that experiences power outages on a regular basis, or if continual scale operation is extremely important, consider adding an uninterruptible power supply (UPS).

A UPS supplies power to the scale for a limited time should there be a disruption in the utility power. Some of these devices power the scale continually from batteries, recharging the batteries when utility power is present, while others automatically switch the scale from the utility power to battery power when a power outage is detected. For most applications, the former type of UPS is a better solution. The size and number of batteries is determined by the length of time the UPS needs to be able to power the scale and by the total load attached to the UPS. In many cases, the UPS is a good investment and even adds a layer of protection against power line surges.

There are a variety of devices on the market used to protect electrical equipment against damage from power transients. Some are quite effective while others leave much to be desired. Review the specifications on the device and, if in doubt, check with the scale manufacturer’s representative or local utility company for their recommendation.

Weight-Indicating Instrument: There are many models of weight-indicating instruments available that can be used with vehicle scales. The best place to start is again with the scale manufacturer’s representative, who can explain the features offered by each model. Make certain that the selected indicator has an NTEP Certificate of Conformance and is compatible with the selected scale. Not every indicator may be capable of powering the 10 load cells in an analog load-cell-based vehicle scale. It is usually a good idea to use a weight indicator manufactured by the same company that manufactures the vehicle scale. Doing this ensures that there are no compatibility problems and provides a single point of responsibility.

Make sure that the type of display used by the weight indicator is legible under the lighting conditions where it will be installed. Vacuum fluorescent types of displays can be difficult to read in direct sunlight, while reflective-type LCD displays can’t be read when the lighting level is low.

Try to choose a weight indicator that has at least two serial interfaces that will enable it to interface with a printer or computer or remote display. The most common interfaces include RS232, RS485 and USB types. The indicator should also come with a keyboard tare feature to allow weighing of net loads.

Pick a weight indicator that has a type of enclosure appropriate for the location in which it will be installed. An office type or NEMA 12 enclosure is not a good choice if the indicator is going to be used outside or where it will be exposed to water. If planning to install an indicator outside, ensure that the enclosure has a rating of NEMA 4 or NEMA 4X or equivalent so that it is protected from dust and water.

Including the scale manufacturer’s representative in the decision on a weight indicator is a good idea. He or she can show a number of models with special features that will make the operation and use of a new scale quick and easy.

Printers: Most every vehicle scale should have a printer to record the weight from the scale. There are three basic types of printers commonly used with vehicle scales — the ticket printer, the tape printer and the full-sheet or report printer. In most instances, it is a good idea to buy the printer with the weight indicator and scale. This ensures that the indicator and printer are compatible and that the correct interface cable is obtained. You might be able to save some money by buying a printer at a local discount store and connecting it to the indicator yourself, but you run the risk of having a printer that is not compatible with the indicator.

Some indicators are equipped with a feature that will allow formatting the printed information as desired. This is a good feature and allows equipment to handle future print layouts as needs change.

Remote displays: A remote display is good when needed to show the weight at more than one location. For example, depending on the application, you may be required to provide a weight display for the vehicle driver. In these instances, a remote display can be connected to the weight indicator and provide a visual display of the scale weight.

Remote displays can range in character size from 0.5-in. high to more than 6-in. high and can be read in all lighting conditions. Look for displays that are housed in weatherproof enclosures that can be interfaced directly with the weight indicator. Interfaces can range from RS232 or current loop types to wireless interfaces. Again, the scale manufacturer’s representative is a good source for information concerning the use of remote displays.

Vehicle-management software programs: Vehicle-management software programs are available to accept weight from the weight indicator and keep track of the weight of commodities shipped and/or received. These software programs are great management tools that allow operations to keep track of production and costs. They can even be interfaced with an accounting system. A local scale manufacturer’s representative can provide information about available packages and their features. However, be sure to ask if the system being considered has an NTEP Certificate of Conformance. If using the system to generate invoices or receipts or to charge by weight, an NTEP Certificate is required.

Legal metrology issues: Most vehicle scales are used in commerce and therefore are required to meet certain minimum requirements to ensure that the weight readings are accurate. As previously mentioned, the NTEP is used to evaluate weighing instruments and their components and, when found to be in compliance, issues a Certificate of Conformance attesting to that fact. The complete vehicle scale requires three NTEP Certificates of Conformance — one for the scale structure or “load-receiving element,” one for the load cells used in the scale (if it is not a mechanical scale) and one for the weight indicator. View all existing NTEP Certificates of Conformance on the NCWM website at www.ncwm.net. The certificate number will also appear on the nameplate of each device.

A few states also require that the scale have a state-issued certificate of conformance for these same components. A local weights and measures representative can tell you whether your state is one of those that require this certification and, if so, where it can be obtained. The scale manufacturer’s representative can provide this information, as well.

Most state or local jurisdictions also require purchase of a permit or license for a new scale. This step ensures that the weights and measures officials have a record of the scale and that, after initial inspection, it can be placed into commercial use. This is usually indicated by placement of an official state sticker on the scale or weight indicator showing that it is authorized for commercial use.

After purchasing and beginning to use a new vehicle scale, there are a few things to keep in mind to ensure continued compliance with legal metrology regulations:

■ Be sure to maintain the scale in good condition and to renew the scale license when it becomes due. The expiration date is located on the official sticker.
■ Should it become necessary to replace one of the load cells, remember that the replacement cell must be of the same type (compression, double-ended shear beam), have an equal or lesser load cell verification interval (Vmin marked on the cell and on accompanying documentation), and have a current NTEP Certificate of Conformance. Be wary of rebuilt or refurbished load cells. While less expensive, they often are not acceptable for use and do not have an NTEP certificate.
■ If replacing the weight indicator, make certain that the new indicator has a current NTEP certificate and is compatible with the vehicle scale.

Photo by Kevin Yanik

Photo by Kevin Yanik

SCALE SITE CONSIDERATIONS

Location: Although sometimes there is little choice about where to place a vehicle scale, there are some things to keep in mind. The selected site should have easy access both onto and off of the scale. Trucks should be able to pull on and off in a straight line. No maneuvering should be necessary until a truck is completely off of the scale platform.

The scale should be positioned such that the scale operator can view the truck on the scale. This is necessary so that the operator can ensure that the truck is fully on the scale platform and not have a wheel partially off the scale. The scale should also be located such that the scale operator can communicate with the truck driver. In some installations, an electronic intercom must be installed between the scale and the weight indicator to allow driver-to-operator communications.

If possible, also consider weather conditions when selecting a site for the scale. A location where the effects of wind on the vehicle and scale and where less snow and rain accumulates is preferable to one where the wind can blow on the platform and wind-driven snow collects on the scale.

Site requirements: The soil at the selected site must be of sufficient load-bearing capacity to hold the scale foundation and scale structure plus the loads being weighed. The scale manufacturer’s representative can provide the minimum soil bearing capacity for the selected scale. It may then be necessary to have the soil tested to ensure that it meets this minimum capacity.

Foundation: The scale’s foundation is critical to its successful operation. Assuming that there is sufficient soil bearing capacity, the scale foundation must be of sufficient strength to hold the scale and the maximum load that is weighed on it. The scale manufacturer can provide a set of drawings that detail construction of the scale foundation. It is crucial that the foundation be constructed in conformance with these drawings. An improperly positioned pier or insufficient reinforcing steel can ruin the installation and cause operational problems for what would be an otherwise excellent scale.

Most local scale-service companies either handle their own foundation work or use contractors who are experienced with scale foundations. Generally, it is best to have the local scale-service company handle the scale foundation. As with any type of foundation, take care to ensure proper drainage to prevent collection of water in or around the foundation.

Instrumentation location: As previously mentioned, the location for installation of the weight indicator should be carefully evaluated. Obvious considerations include available space and scale operator access, but there are other items to consider. Most manufacturers recommend that scale instrumentation be on a separate circuit from the electrical-distribution panel. In many cases, it will be necessary to install a new circuit for the scale. Make certain to include sufficient outlets for the indicator and for each peripheral connected to it, such as a printer, intercom system and/or computer. When adding this circuit, make sure to follow all of the applicable local electrical codes.

Further, there needs to be a means of communication between the scale operator and the driver of the vehicle being weighed. The scale operator must be able to tell the vehicle driver if he or she needs to reposition the vehicle and when to pull off the scale. In many instances, communication is provided by an intercom system; a traffic light may be used to signal the driver when to exit the scale.

Onboard scales

Pits and quarries often adopt onboard scales because of the need to optimize truck payload, process, and efficiency and to avoid over-loading fines. By tracking payload data (product, tonnage, customer, operator, time, date stamps, etc.) operations can use this data to improve cycle times, truck turnaround or payload supply.

When selecting onboard scales, consider the budget, existing requirements and future requirements. Most operations have an adoption curve that starts with payload optimization, and as comfort with the technology increases, the additional features are used. Equipment operators also may have a preferred option that they have experience using or find easy to operate. Using a similar technology across machine brands and types will also make training and data management easier.

Standard workflows allow for payload measurement on loaders, excavators and conveyor belts in the extraction, processing and loadout phases. A truck scale/scale house is also used to verify payload before a truck leaves the site. 

Key considerations are:
■ Accuracy.
■ Extended features.
■ Data management needs.
■ Connectivity.
■ Support.

ACCURACY

Most loader scales achieve +/-1 percent accuracy or better; excavators will have 1 to 3 percent accuracy, and conveyor scales better than 0.5 percent accuracy. When considering the number of loads, a small change in accuracy can add up quickly and impact profitability. Just 1 percent accuracy will quickly add up to thousands of dollars for a busy pit or quarry.

Modern technology also allows for changes in site slope, temperature and payload measurement during vehicle movement (e.g., while driving or rotating an excavator).

In Canada, Europe and Australasia, pits and quarries may also add an option for “Legal for trade,” whereby certified scales can be used for invoicing the customer, without the need for truck scales.

EXTENDED FEATURES

While extended features have many names, according to the manufacturer, they perform similar roles. An operation’s workflow may benefit from a particular feature or combination of features. Features may include the ability to total payload for a day or longer period, automatically add bucket payload without operator intervention (via a preset lift arm/boom position), and multi-attachment selection.

Onboard scales also can perform basic math for the operator, such as count up/down to a total or adjustment of the last bucket to a specific target payload.

DATA MANAGEMENT

Printers are often the first option chosen for data management. Mounted in the cab, printers provide the operator the ability to immediately print a docket with information for the truck driver or office to track individual trucks and daily or weekly loadout.

While all scales provide basic weighing, pits and quarries are able to realize productivity improvements by analyzing their operations data. Scales allow tagging of data fields such as product, operator, customer, project, etc. to track material through the operation. For analysis, manufacturers may offer download to a spreadsheet, real-time access to preformatted reports or a secure page of a web browser. For less critical data, a USB module may be sufficient for a small site.

Improvements in telematics now provide payload data in the same environment as energy consumption, maintenance and location data, allowing for a detailed picture of productivity by a number of variables.

CONNECTIVITY

With the importance of process improvement, connectivity between machines, the scale house, the office and mobile devices is increasingly valuable as it allows for near-time visibility of downtime or potential issues. Standard options include Wi-Fi, cellular or radio connectivity. Using the scales as an indicator of under-production, the pit or quarry manager can make adjustments to operations to return to optimal production. Recent developments in onboard scales will present customer orders, placed at the scale house, directly in the cab.

SUPPORT

Onboard scales should be installed by a skilled specialist technician. Machine manufacturers are starting to pre-wire for scales, which provides a better integrated, standardized installation.

While leading brands of scales are very reliable, there is value in regular maintenance. Don’t underestimate the peace of mind that comes from having an experienced and responsive local support function for questions of installation, calibration and training. Many operations also consider extended warranties a key concern when selecting scales.

SCALE USE

Operation of onboard scales is easy, and few operators will have difficulty, even if they have never used one before. A local support team should offer good training to ensure getting the most from onboard scales.

A unit will normally be mounted in the cab in easy view. Using the scale involves three steps: Add, Clear, Zero.

Add: When the bucket is lifted (without material falling out) the operator can add the payload to the total using a button on the indicator screen or a remote button (normally found on the controls). Weighing takes less than a second and can be done on the move, without slowing operation. Additional bucket loads can be added to provide a running total or count down to the desired amount. In the cab, this display helps the operator make decisions about adding more material to meet requirements.

Clear: When the required payload is reached, the operator can then clear the total to begin another truck-loading cycle.

Zero: At the start of a shift or day, the operator may reset the system to a zero total by a button push. This will tare any build-up of materials in the bucket.
The operator may pre-program the scales for other features, or create selections in the cab.

On-road hauling

On-road hauling of sand, gravel and crushed stone is handled by haul/dump trucks and trailers. Dump trailers include end dump, side dump, bottom dump and live bottom trailers. The choice of which is best for the customer is typically dictated by what product is being hauled and how it is delivered. Tractors used to pull dump trailers are somewhat similar, but vary depending on the weight of the load, the terrain and the grades. Which dump truck to use, on the other hand, can vary greatly, depending on where the trucks will run. Each state and province has differing weight and length laws, which dictate how the truck is equipped, the length it should be, the number of axles needed and the axle weight ratings. As a result, customers should first determine whether a truck or a trailer is needed and the weight and length laws in the areas these trucks will be working.

Generally speaking, states in the northern U.S. allow shorter trucks with more heavily rated axles. States in the south and west use bridge formula laws requiring a longer length and sometimes more axles to spread the weight over a greater distance and more evenly. Bridge formula equations and further explanation is available online at www.fhwa.dot.gov. When additional axles are needed, the use of lift axles, which are raised and lowered using pneumatics, should be considered.

Applications in Canada typically are divided by the use of lift axles, twin steer (two front steering axles) or tridrive, which is three rear driving axles instead of two. All of these options are used to spread the weight over a greater distance and to allow more points on the ground.

Photo by Kevin Yanik

Photo by Kevin Yanik

BUILDING TRUCK SPECIFICATIONS

Part of determining truck specifications is knowing what body the dump truck will use. Lengths, weights, materials and shapes vary according to a customer’s needs — hauling sand, gravel, asphalt, ore or debris. Steel bodies are tough and should be considered in applications with big rocks and debris that will damage lighter-weight materials such as aluminum. However, the lighter the truck, the more load it can haul, and the more it hauls per load, the greater payback per trip. A longer body hauls more load, but it must adhere to legal length and weight limits.

Truck components are another consideration. Once again, lightweight components allow a customer to haul more payload, but consideration must be given to where the trucks are going and what they are hauling. Then a customer must evaluate whether the components will survive, or at least survive long enough, to get a return on the investment. Will the truck run mainly on the highway and only carry gravel or sand? If so, an aluminum body with lighter-weight components may be used to maximize the payload.

Suspensions and engine size are main contributors to the weight or weight savings of a truck, adding or subtracting hundreds of pounds. But there are always tradeoffs. Will a smaller engine offer enough horsepower or torque to carry what it needs to? Will a lightweight suspension be durable enough for the payload or jobsite? Hauling big loads short distances through harsh terrain may require a more substantial suspension with large steel camelback springs or steel walking beams, which are durable, reliable and have great articulation. Hauling long distances on-highway may require lighter-weight components to maximize the load because the number of trips per day will be limited.

DRIVETRAIN

As mentioned, engine size can greatly affect weight. Everyone wants more horsepower, but 375 or 405 horsepower is plenty for most dump applications. The 400-horsepower range allows a customer to stay with an 11-liter engine instead of going to a 12- or 13-liter engine. This will save money through lower fuel consumption and make money by saving hundreds of pounds of weight, allowing the vehicle to carry more payload.

Once the drivetrain and the number of lift axles are determined, weight distribution needs to be considered. Be sure each axle is utilized to the maximum allowable capacity so that the maximum payload can be carried to make the most money per trip. Dump trucks come in axle-forward and axle-back configurations. On an axle-forward model, the front axle is positioned as far forward as possible.

This configuration may be required in a bridge law location because it provides greater length between the front and rear axles, spreading the ground loads as far as possible. An axle-back model has the front axle moved farther back from the bumper. Generally, this is 49 in. from the front of the bumper to the center of the axle. This popular set-up shortens the wheelbase of the truck. The shorter the distance between the front and rear axles, the better the truck’s turning radius. The axle-back position also allows for easier loading of the front axle. A few calculations can help to achieve the optimum weight loading.

Other things to consider are tires, fuel capacity, air cleaners, power take-off (PTO) requirements, frame rails, cross members and frame rail space. In a nutshell, tires should be chosen for the application and the work the truck will do. Long on-highway runs may require a fuel economy tread, while off-road applications may require traction tires or may require a tread that can do both.

Fuel capacity for dump trucks should be large enough to last one full day. If too much fuel is carried, load capacity is lost. If not enough is carried, time is lost while refueling. For most, this capacity is 75 to 100 gallons, and once again, there is a choice of aluminum or steel fuel tanks.

Under-hood air cleaners are the best choice for dump trucks. The dusty conditions of job sites do not lend themselves well to external air cleaner cans. Pre-cleaners and a filter minder to indicate when the air cleaner is in need of changing is worth adding on.

A PTO driven off of the transmission in order to raise the dump body will be needed, so make sure the transmission is equipped for this and talk with the body installer for what will be used. Cost will vary based on the transmission.

Last but certainly not least is the frame system and frame packaging of the truck. For the most part, the goal is to build the truck with a single frame rail if the expected loads and wheelbase allow.




Most truck makers offer several frame thicknesses to fit this need. This can help save weight and prevent possible corrosion. It’s also important to order the correct cross members. Highway and medium-duty cross members are meant for hauling freight trailers and medium-duty loads.

Choose heavy-duty cross members, especially behind the cab where the dump hydraulic hoist will be lifting the body and load. Finally, look at frame rail packaging space. Lift axles and additional equipment require clear rail space.

DRIVER COMFORT AND ENVIRONMENT

Driver comfort and ergonomics in the cab should always factor into the truck purchasing decision. The layout of the dashboard and the reach and vision to controls, gauges and switches are critical to keeping the driver focused on the road. Visibility through the windows and over the hood and dashboard play a key role in helping to ensure safety. Although customers may not want the highest level interior because of dust and dirt on a jobsite, the driver’s comfort and the ease of driving the vehicle should be considered because it helps reduce driver fatigue, which equates to better driver retention and improved driver safety.

HEAVY HAUL

Construction equipment and onsite rigs and plant components are also part of on-road hauling. These will mainly be transported by heavy-haul tractors using lowboy or flatbed trailers. These tractors are typically equipped with a big-block engine (14 liters or greater), 18-speed transmission with or without auxiliary transmission, and in most cases, one pusher axle. These are big-power, big trucks for large loads. Many of these loads range from 110,000 lbs. to 250,000 lbs., with some special loads being even greater.

Customers must consider their hauling needs prior to meeting with a salesperson to develop tractor specifications best suited for the application. Understanding the options available and asking the right questions of the salesperson can help customers choose a truck that is reliable, built correctly for the job they need it to do, and one that will maximize the return on investment.


Scale maintenance

As with most things, a good maintenance program will help ensure maximum return from a scale investment with a long service life and superior performance. It is a good idea to establish a maintenance agreement with a local scale-service technician to ensure that the scale adjustments and calibration are regularly checked and maintained. Also make certain that the scale and the area around it are kept clean and free of debris. Drains, if any, should be kept open and clear to prevent the accumulation of water beneath or around the scale.

It is also a good idea to periodically give the scale a good visual inspection. Check for damaged guardrails, if any, and have them repaired as quickly as possible. Look for missing or chipped paint and use touchup paint (available from a local scale-service company or scale manufacturer) to keep the scale protected from corrosion. Look also for any obstructions such as rocks or other debris that may have become lodged between the scale platform and surrounding structure. These should be removed to ensure proper performance of the scale. In snowy regions, it’s a good idea to remove salt or other ice-melting chemicals from the scale platform.

A scale and weight indicator should come with a warranty against defects in material and workmanship issued by the manufacturer or manufacturers. Some scales even come with warranties covering lightning damage to the load cells. As with any warranty, take the time to read the fine print to determine what the warranty covers and the owner’s responsibilities to keep the warranty in effect and to file a warranty claim. Most warranties are for a year, but some are longer. All of this information will be contained in the warranty. Keep purchase records in a safe place to easily establish the purchase date should a warranty claim ever become necessary.


 

Sources:

Contributors to this chapter include the following, in alphabetical order:

Cardinal Scale Manufacturing Co.

Stu Russoli
Vocational Segment Manager
Mack Trucks

Trimble Loadrite


Lesson 11 Quiz

1. What can be defined as the measure of how quickly a customer is loaded and ticketed across a truck scale and leaves the site?

2. Can aggregate-loaded railcars be weighed on the track while in motion?

3. Where can vehicle scales be installed?

4. Why is platform size important when installing a vehicle scale?

5. How does nominal capacity differ from concentrated load capacity (CLC)?

6. What are some of the most common complaints quarry operators receive from town neighbors?

7. What is a benefit of conveyor belt scales?

8. Name four scale site considerations.

 

Click here for the quiz answers.

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