Truck and railroad scale maintenance
November 1, 2007
Scales are the “cash register” and the basis by which crops from the field are weighed, measured and subsequently re-sold to produce the necessities of life, yet they are one of the most overlooked parts of the grain handling process from a maintenance standpoint — until they break down.
As any manager of a grain storage facility will tell you, the accuracy and performance of scale equipment can have a dramatic impact on profits and losses. The technological advancements during the past 40 years with regard to scales have far exceeded what one could have imagined a half-century ago.
Prior to 1969, most truck scales were manufactured from cast iron components. They had hundreds of moving parts and were installed in deep wet pits with timber platforms. Most of these scales required a balance beam or, in later years, a dialing mechanism to accurately weigh trucks. Concrete and steels decks replaced the timber decking used in the first half of the 20th century, but little else changed.
To obtain consistent accuracy, literally hundreds of moving parts had to be kept greased and in perfect synchronization, which was nearly impossible. Moving steel parts meant friction, and friction meant wear, resulting in constant and time consuming overhauls and parts replacement.
These problems were alleviated in 1969 with the advent of the total electronic truck and rail scales with solid state digital weight indicators and extraordinary data processing capabilities. With this development, it became possible to read a truck weight being generated at a facility thousands of miles away in “real time.”
The introduction of self-diagnostic electronic truck and rail scales in the 1990s permitted scales to predict failures before they happened, minimizing costly downtime. Unattended systems permitted weighing of trucks without the involvement of a scale operator by using self-promoting displays or radio frequency identification systems.
Nearly 98% of the truck and rail scales purchased today are of the electronic load cell design.
ELECTRONIC TRUCK/RAIL SCALES
One of the most popular truck scale designs today is the “flat top” model, which is low profile, modular and can be installed in a wide range of foundations, including shallow pier, slab and deep or shallow pits.
Full electronic modular scales are more accurate and dependable in the long term than mechanical type scales, and they have much lower maintenance costs. These models are available in a variety of platform lengths and widths to handle a wide range of vehicles.
The flat top modular design can also be customized to fit existing foundations, often saving thousands of dollars in replacement costs of older models. Options such as grain dumps, safety guide rails, automated controls and other accessories make this design the popular choice.
The most popular electronic rail scale products are the combination truck/ rail scale and the modular rail scale. The combination scale is designed for installation in a conventional deep pit with the platform at grade level. The scale can be installed for weighing both rail and truck traffic, providing maximum flexibility. It is typically 60 to 75 feet (18 to 22 meters) in length and designed for a concrete deck. The combination scales have a nominal capacity of 360,000 pounds (163,260 kg) and an 180,000-pound (81,630 kg) sectional capacity.
Modular type rail scales have increased in popularity during the last two decades, primarily due to the lower costs of installation and foundation construction.
A few common sense maintenance initiatives can help prevent unexpected breakdowns and prolong the life of truck and rail scale equipment. Here are a few preventive steps that can be taken.
• Periodic calibration checks. Every vehicle scale should be calibrated and tested by a state-licensed servicing agency with no fewer than 25,000 pounds of certified test weights. The intervals of testing depend on the number of vehicles being weighed. A minimum calibration and test should be conducted at least every six months. For heavy usage scales, quarterly checks may be necessary.
• Weighbridge maintenance. Proper weighbridge maintenance is essential to prolong the life of equipment and keep it in top operating condition. Removing a scale from the foundation, sandblasting all steel surfaces and painting with an epoxy primer and enamel top coat can dramatically increase the life of this equipment. Depending on the geographic area, type of environment, material being weighed and other factors, this work should be performed every 8 to 10 years. Some manufacturers cover the underside of their weighbridges with emulsified asphalt coating that can substantially reduce rust and corrosion.
• Foundation maintenance. A vehicle scale’s performance is only as good as the integrity of the foundation supporting it. Keeping the foundation clean and free of mud, water and debris will improve the performance of a vehicle scale. A quick visual inspection of sump pumps and drains should be done on a monthly basis. Condition of the concrete and approach coping should be inspected regularly to ensure no problems have surfaced. Cracks in the concrete or heaving due to frost conditions can have an adverse effect on the accuracy and performance of a scale.
• Load cell maintenance. The heart of any electronic vehicle scale is the load cells. They should be inspected for frayed cables, cracked or loose connectors, loose bolts and bridging or binding by mud or debris around the load cell. In canister-type load cells, inspections should also be made to ensure there is no rust or evidence of holes in the load cell itself, as even stainless steel load cells can rust. A transient bypass cable can be used to protect a load cell from lightning or surge voltage damage by routing surges around the load cell.
• Bumper bolt maintenance. On scale models that use bumper bolts, which are normally positioned at the ends of the weighbridge, make sure they are tight so they do not move on their own and yet maintain clearance when the scale is empty or loaded. Bolts should be adjusted with approximately one-fourth to three-eighths of an inch clearance from the end wall plates. Seasonal temperatures and weather changes can cause thermal expansion and contraction of steel and concrete, resulting in potential binds or bolts that are too loose. Bumper bolts that are too tight against the stops can cause inaccurate weights and result in errors that may go unnoticed for extended periods of time. Bumper bolts that are too loose can result in potential damaging side loads on the load cells or, in some cases, a catastrophic collapse of the bridge structure. A bridge that does not swing freely or that results in loud noises as vehicles enter or exit the scale may be too loose.
• Grounding maintenance. Scale manufacturers recommend a variety of grounding procedures. To avoid the existence of multiple zero references that can create havoc with data lines as well as invite lightning damage, simple point grounding is recommended. Checking a single-point ground involves two steps.
First, verify the ground system of the AC power supply. Using a digital volt meter, check the resistance of the AC outlet ground to the actual ground rod of the AC power coming in the building. It should read less than 1 ohm. Then, measure the AC voltage across the ground and neutral of the AC outlet. The result should be 0.0 volts AC, not to exceed 0.5 volts AC.
Secondly, check the scales grounding. Before this is done, make sure all peripherals are plugged into some type of transient protection device such as an uninterruptible power supply. Electronic scales are easily disturbed by any number of voltage distortions, so installing power conditioning equipment is the best first line of defense against power problems. Connecting a bare 10 gauge copper wire to the scale frame and the grounding lug on the junction box is strongly recommended, as well as running the copper wire back to the ground rod provided by the power company. This wire can be buried in the soil from the scale to the AC ground. If your scale also uses an uninterrupted power supply, the device needs to be grounded to this wire as well. Next, measure the resistance between the scale sections and the AC ground. The reading should be less than 1 ohm of resistance. Higher readings are typically caused by corroded connections.
Finally, check the AC power supplied to other peripherals such as remote displays, computers and printers. A remote device may not have the same AC power source as the digital instrument. Therefore, each device may not be grounded to the same point. Measure the resistance between the AC power ground points. The volt meter should read less than 1 ohm.
This article is based on a presentation at GEAPS Exchange 2007 by Bill Murphy, director of sales, heavy capacity products, Rice Lake Weighing Systems, Rice Lake, Wisconsin, U.S. He may be reached at firstname.lastname@example.org .