Anatomy of a Truck Scale
When things go awry with your truck scale, it can be difficult to understand the technical lingo your scale technician might use when diagnosing the issues. Knowing exactly how your truck scale works will make you a better-informed consumer when you need to repair a part on or replace it entirely.
There are many different components in a truck scale that work together to provide an accurate weight. The scale contains a combination of mechanical and electrical components working together to provide a stable weighing environment. Not all of these components are found in every type of truck scale, however. The three main types of truck scales you’ll find today are electronic, hydraulic, and mechanical (although there are fewer and fewer mechanical truck scales left each year).
Mechanical Components
Most of the mechanical components on a truck scale are located outside with the weighbridge. Fifty plus years ago (and older), most of the truck scales installed were completely mechanical pit style truck scales. Fully mechanical truck scales generally consist of the following:
variety of large levers (carbon steel or cast iron)
pivots and bearings
stands and chairs
weighing beam inside the office or scalehouse
If you had an issue with a mechanical truck scale, more often than not a cast part was broken or a weld broke loose on a fabricated part.
Nowadays, most of the truck scales you find installed are either electronic or hydraulic. The only “mechanical” parts on these types of scales consist of the following:
the weighbridge itself (usually split into multiple panels)
base plates, top plates, shims
pins, nuts, bolts, etc (used to prevent motion or keep things tight)
hardware related to checking / position alignment
Load cell system
The load cell system is the most important part of your truck scale and it is often where the most issues occur. To better illustrate our point, we will consider a standard 70’ truck scale. Whether electronic or hydraulic, this length scale will usually have eight (8) load cells and a summing component.
In an electronic truck scale, you will have eight (8) electronic load cells. Each individual load cell will have a strain gauge encased in the body of the load cell. Often, these load cells have more advanced computer boards in the body of the cell as well. In addition to the strain gauge, each load cell will have a cable protruding from one end, generally 30 – 60 feet long. The cable is often made of six (6) conductors, or individual wires, a braided shield, and a PVC jacket that is sunlight and moisture resistant. The heavy-duty jacket ensures good abrasion resistance as cables continually wear. The eight (8) individual cables from each load cell terminate on a summing board which is enclosed in a watertight housing. There can be two summing boards with four load cells each and an interconnect cable connecting the two boards. To learn more about electronic load cells checkout this blog post.
On a 70’ hydraulic truck scale, you will still have eight (1) load cells. These load cells are completely immune to lightning and water. The load cells are connected to a hydrostatic totalizer via 1/8” copper (or stainless steel) tubing. The tubing connects to the load cells and totalizer with special fittings. The totalizer converts hydrostatic pressure from each of the eight (8) load cells into an electronic signal through the single compression style electronic load cell which is installed at the bottom of the totalizer stack. To learn more about hydraulic load cells checkout this blog post.
Homerun cable
The homerun cable is the main artery that connects the scale to the digital weight indicator. Similar to the load cell cable, the homerun cable is often made of six (6) or eight (8) conductors. The homerun cable generally has a braided shield and a PVC jacket that is sunlight and moisture resistant. The lower the gauge of the homerun cable (lower gauge = fatter cable), the longer distance you can have between the truck scale and the digital weight indicator.
At the end of the day, the purpose of the homerun cable is to provide the input voltage (excitation) to the load cell system, and then to return the output voltage (signal) back to the digital weight indicator.
Digital weight indicator
The digital weight indicator is the brains of a truck scale. The indicator provides the input voltage (excitation) to the load cell system; receives the output voltage (signal); and converts the analog signal to a digital signal via the A-D board (A to D or analog to digital). The digital signal is then displayed on the display board showing the weight currently on the scale.
The indicator is also where you connect all other cables into the system. The most common cables connecting to the digital weight indicator include:
Power (usually 120VAC)
Homerun cable
Communications cable (to connect to peripheral devices)
Copper grounding cable
More often than not, truck scale users will identify that they have a problem with their truck scale because the indicator will not be displaying the proper weight. To see what indicator we offer click here.
Peripheral devices
There are a variety of devices you can connect to your truck scale. The list below covers the majority that we see in the field on a day-to-day basis.
- Printers: print inbound / outbound weight tickets for drivers. Includes truck number, date and time, as well as the gross, tare, and net weighments
- Remote displays / scoreboards: the devices are connected to the indicator via two (2) conductor communications cable. The remote display receives a digital RS-232 signal from the indicator and displays an exact match of the weight. Occasionally, truck scale owners will have issues with their scoreboard and not the scale. Scoreboards have a variety of electrical components that are susceptible to failure. Checkout this blog post to learn more about whether you are legally required to have remote display for your truck scale.
- PCs: nowadays everything is connected. Scales are being connected to PCs via serial cables (RS-232), ethernet cables, and USB. The data streaming into the PC must be interpreted by the computer and there must be a good path for the data to travel
- Ticketing software: There are several different types of software available and these are generally industry specific. The software can provide basic ticketing and inventory management. Depending on the package you get, you can also do all of your billing and accounting integration through the software
- Notification devices (horns, stack lights, buzzers, etc): you can connect a variety of devices to the indicator, and these devices can provide an audible or visual alert that there is weight on the scale
- Intercom systems: there are many intercom products on the market today for truck scales. Not many are high quality. Due to noisy truck engines (and surrounding ambient noise), the best intercom systems have noise cancellation features.
- Gates: Gates can be controlled by truck scale indicators based on certain triggers. Once a certain trigger is achieved, the indicator will communicate with the gate to open / close. We often see this type of set up with pay to weigh operations.
- Guard rails: Guard rails are an instrumental part of every truck scale. Most truck scales sit 18” or more off the ground. Drivers are notorious for driving their semis and dump trucks off the side of the scale and that can cause substantial damage to the mechanical and electrical components of the scale. To prevent this from happening, truck scale owners can order their scale with guard rails or install aftermarket ones. Guard rails range from:
Standard 8” high (safety yellow or matching the scale)
24” + dual pipe guard rails (primarily used on sites regulated by MSHA)
DOT style guard rails
Custom guard rails: as an example, American Scale builds guard rails out of 8” schedule 40 pipe. This set up is so stout, it either guides the truck(s) back onto the scale or damages the vehicle, saving the scale.
To see what truck scale accessories and peripheral American Scale offer click here.
How American Scale can help
When functioning properly, these parts seamlessly work together to keep truck scale weighing accurately and precisely, time over time. However, sometimes one or more of these components can malfunction, which can stop the entire process and interrupt your life.
The expert technicians at American Scale have seen it all in our 75 years serving the Southeast and we understand how detrimental a malfunctioning truck scale can be, especially when you have long lines of trucks backed up. If your truck scale isn’t working correctly or you suspect it’s not accurate, contact us today so one of our experienced technicians can diagnose the problem and have your system back up and running in no time!
Finally when it comes to purchase a new truck scale checkout our wide range of options we offer.
Closing Words
At American Scale we strive to bring you informed and useful content on all things scale related. Be sure to check out our legal for trade truck scale system articles about truck scale foundation or how much do truck scales cost. We even have a guide to warranties. We also have articles on scale accessories, common problems to prolong your scales lifespan, weighing applications as well as what scale indicator works best with your junction box. To learn more about bench scales, it would be good to check out our other articles such as “Bench Scale Basics'' and “How Much Does A Bench Scale Cost?”. These articles will help with the basics of bench scale ownership.