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Truck Scale Evaluations Once you have decided that your operation requires a truck scale, the difficult tasks begin. Now you have to make decisions on where the truck scale will be located, what size scale you will need, if it will be installed in a pit or above ground, and finally the capacity or strength of the scale. SITE SELECTION )
Where you place your scale will have a huge impact on your operation. Your scale consultant should be able to actually lay out your scale to ensure that it fits in the area that best suits your process. Now would be a good time to consult your fleet managers or truck drivers to ensure your plans are functional solutions for all parties involved.ABOVE GROUND OR IN A PIT Until the 1970's, all truck scales were mechanical, lever based installed in pits. Times have changed and for a number of reasons, most truck scales today are installed above ground. The Table below provides a quick reference comparison of Pit and Pitless (above ground) scales.
Based on the information above and other application information, pit scale applications are rare today; however there will always be instances where pit scales actually are a better solution. Some of the conditions that may require you to install a pit scale are; overhead obstructions, height restrictions or not enough real estate to install a pitless scale. SCALE SIZE The length and width of your scale will be dictated by the size of the vehicles your scale will be weighing. To comply with Handbook-44, your scale must be able to weigh the entire truck at one time. Dual draft weighing is not an acceptable means of capturing a legal for trade gross weight of a vehicle. A typical 18 wheeled tractor trailer is 8' wide and 60' long. A good rule of thumb is to get a scale that is 20% wider and 20% longer than your vehicles. Using this rule, the size scale needed for most 18 wheeled tractor trailers is 70' long by 10' wide. You should be leery of salesmen who try to push 11, 12, or 14 foot wide scales for applications to weigh typical tractor trailers in legal for highway configurations. Bigger is not always better. You could find it a waste of your money and real estate in the end. In most above ground scale applications, rub-rails are recommended. These are used to help guide the trucks on and off the scale to speed up your process and help prevent accidents and damage to your scale. Rub-rails should be a bright and easy to see color such as safety yellow to help prevent them from blending in with the scale platform. Scale Capacity Per Handbook-44 scale manufactures must rate their scales based on a Concentrated Load Capacity (CLC). H-44 defines CLC as "A capacity rating of a vehicle, axle-load, or livestock scale, as specified by the manufacture, defining the maximum load concentration for which the weighbridge is designed. In the case of vehicle and axle-load scales, it is the maximum axle-load concentration (for a group of two axles with a centerline spaced 4 feet apart and an axle width of 8 feet) for which the weighbridge is designed as specified by the manufacture. The concentrated load capacity rating is for both test and use. CLC ratings range from the lower end of 60K that is ideal for typical volume, legal for highway vehicles, to the higher end of 100K, that is ideal for higher volume applications that are weighing trucks too heavy for highway travel. Be sure that you buy the correctly sized scale for your application. Buying an undersized scale will cost you more than the scale built for your application. Shown below is a simple usage chart based on number of legal for highway trucks your scale will weigh in one day.
Gross or Nominal Capacity The term gross or nominal capacity is the total weight that can be evenly distributed over the entire scale. The gross or nominal capacity of a truck scale is limited by H-44 using the following equation [Gross Capacity = {# of Sections - .5} X CLC]. Most truck scale manufactures rate the Gross Capacity of their 70'X10' truck scale at 100 tons. Some of the higher end scale will be rated at 125 to 150 tons. SCALE DECK TYPE The deck of a scale is the part that the truck actually drives on and is typically a part the scale structure as well. There are two types of decks for truck scales, steel and concrete. Each have their advantages and disadvantages. Concrete Deck In above ground concrete deck scale the concrete is typically poured inside the modules once they are set in place. This type of construction is called Field Pour. Each manufacture has a unique way of reinforcing the concrete to minimize cracking and account for the tension below the neutral axis. We will advise that you choose a manufacture that uses similar design principals and construction standards that are used in highway bridge construction by the American Association of State Highway and Transportation Officials (AASHTO) and the Federal Road and Bridge Department. Pre-cast Concrete Some scale manufactures actually have the ability to pre-cast concrete modules at their factory. This practice usually eliminates a lot of headaches for all concerned, however it may increase the shipping cost for your scale. Pre-cast concrete does eliminate the need to pour concrete in the field in less than controlled conditions. Steel Deck Steel deck scales are usually comprised of a deck plate ranging from ¼" to a ½" thick, supported by some type of structural supports. You should choose a scale manufacture that utilizes extruded steel I-beams as structural members. This type of design is consistent with the design standards used by the American Association of State Highway and Transportation Officials (AASHTO) and the Federal Road and Bridge Department. Ensure that the structural support members in your steel deck scale are not fabricated or bent pieces of steel. Using fabricated or bent pieces of steel creates undo stresses in your structural members prior to them even carrying the weight of a truck. Also ensure that the bridge design you choose does not have enclosed areas. These enclosed areas facilitate spaces for moisture to collect and initiate corrosion from the inside out. Steel Deck vs. Concrete Deck
Shown in the table below is how the steel deck and concrete deck compare.
EQUIPMENT OPTIONS You have many different types of equipment options. Most manufactures have trademark features that differentiate themselves from others. Some of these features are truly unique and actually add value, reliability and durability to their products. Let's take a closer look at specifics. Mechanical Fully mechanical scales have been around since 1830. These scales are comprised of levers that transfer a ratio of the weight to a mechanical indicator such as a beam or dial style indicator. The Mechanical scale has many variations of levers types. Most levers manufactured for truck scales are fabricated out of structural pipe and plate steel. These parts are more susceptible to corrosion than a cast part would be. There is one manufacture that still has the levers for their scale cast.
Mechanical Electronic combination The fully electronic scale has only been around for about 25 years but all truck scale manufactures have one to offer. The total load on the entire scale is transferred through multiple analog load cells that use strain gages to detect and gage weight. All load cell signals used to gage weight are analog; however some manufactures convert this analog signal to digital once it leaves the strain gages. Some manufactures convert the signal inside the load cell while others convert the signal away from the load cell. Where the signal gets converted is not as important, as long as the signal does get converted to digital before it is routed to the indicator. Load cells - There are many different types of load cells that are manufactured and used in truck scales today. Most manufactures claim their load cells are hermetically sealed. This term has been used very loosely by many manufactures to mean just about anything. Digital -vs- Analog Comparison PDF
Hermetic Sealing: Some have said that a load cell is a load cell. We know that is just not true. All load cells are not created equal. Some load cells are 'potted', some are hermetically sealed only where the strain gages are located and some load cells are totally hermetically sealed, hermetically sealed at the strain gage location and hermetically sealed at the cable entry. So what does hermetically sealed really mean?
You may have noticed in Webster's definition it has been pointed out that the term has been and continues to be misapplied. A simple potted seal is not the same as hermetically sealed. Truly hermetically sealed load cells, feature welded enclosures and gauge chambers filled with nitrogen gas. This method produces the absolute best load cell available.
Read more about Rocker Column vs DESB load cells DATA INTERFACE )
With the development of computers and software, information from the weighing process is being used to manage other aspects of many business. Due to this fact, interfacing your weighing equipment to your office computers is getting to be more important. The information obtained by the scale can be used for inventory control, payroll, general efficiency, and billing. Ensure that if you plan on using this information to help manage your business, talk to your scale consultant to see how his equipment accomplishes these tasks. |
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