Preventive
Maintenance Is Key To Truck Scale Performance
Ensuring you follow
a thorough preventive maintenance plan can mean the difference between
a five year old scale life or a generation long scale life.
Customers who
purchase a truck scale demand the most return on their investment.
Naturally, their level of satisfaction will depend largely on the
scale’s accuracy, durability, and reliability over time.
Installing a scale
known for its structural integrity is the first step to ensuring dependable
performance, but even a quality scale’s performance can slip
if not maintained properly.
Rice
Lake Weighing Systems Heavy Capacity Scales Group has developed
a basic preventive maintenance (PM) plan for examining components
that are vulnerable to conditions that can cause accuracy errors and
lost calibration. Derived from more than 50 years of experience selling,
installing and servicing heavy capacity scales, these guidelines serve
as a practical reference. Because all scales—particularly those
installed outdoors—are subject to unique circumstances, it’s
impossible to cover every cause of trouble. However, regular checks
similar to those described in the following paragraphs can help ensure
a quality truck scale lasts for decades.
Scheduling
Maintenance Checks
We suggest checking truck scales every six months at a minimum. A
convenient time to run your PM inspection is during the scale’s
regularly scheduled calibration check. You’ll also want to consider
the location and circumstances of the application. It’s a good
idea to inspect a scale just prior to peak usage times, such as harvest
season in agricultural operations.
Visual Inspection
The only tool you’ll need for much of the inspection is a good
pair of eyes. A scale is only as good as its foundation, so that should
be the first element you examine. If the operator has indicated the
occurrence of errors, it may be due to significant foundation settling
caused by poor site preparation or shoddy concrete work. Regardless,
take note of any cracks in the foundation. Cracks that lead to movement
or settling will cause chronic calibration errors. For the scale to
once again read accurately, the foundation will need to be repaired,
which may require removing part or all of the foundation and pouring
a new one.
Next, take a look
at the deck. Rust or crumbling concrete can weaken the scale’s
structure and cause problems if not addressed. You may want to clean
and paint rusted steel, and repair or replace concrete as needed to
maintain the scale’s structural integrity. After examining the
surface, look for debris such as gravel or crumbled concrete caught
between the foundation and scale deck. Binding is a common cause of
inaccuracies and can result from something as small as a single rock.
Installing T-Strip
molding between the deck and foundation can help keep debris from
getting caught or falling down into the scale. In addition, some manufacturers
of above ground scales offer the option of end cleanout plates that
can be removed, allowing access to remove material that builds up
on the foundation’s surface closest to the scale’s end.
Once you’ve
cleaned out debris, be sure that the clearance between the walls and
deck is even and consistent with manufacturers’ specifications.
Bowing and other irregularities caused by pit wall cracks, frost heaving,
or poor concrete work on the deck or wall could result in future binding.
Some scales utilize suspension systems that don’t require checking
devices, eliminating a time-consuming step in your PM protocol. First
inspect check rods to determine if the attachment points are solid,
and then check for binding. It may not be possible to visually determine
if a problem such as binding exists. If there is a problem, weight
readings will not be consistent from section to section. Inspect hardware
to be sure that jam nuts are tight, rods are level and free of foreign
material, and washers are not rusted or distorted. Even if they are
working properly, you should replace corroded hardware components
so they don’t fail in the future.
Unlike check rod
binding, bumper bolt displacement will be easy to see. Bumper bolts
need to be tight so they don’t move on their own, yet maintain
clearance when the scale is both empty and loaded. Seasonal temperature
and weather changes can cause concrete and steel to expand and the
bumper bolts can bind tight against the contact area. If binding occurs,
adjust bumper bolts to maintain the proper amount of clearance for
all conditions.
Next, examine
the load cells
and the area surrounding them. The load cell area may contain a rodent
nest, dirt or other debris that’s built up since the scale’s
last check. Load cells must have clearance to deflect through their
capacity range. The same type of foreign materials that can bind the
scale deck can also keep the load cells and suspension components
from moving freely.
The performance
of a load cell is often dependent on the condition of the attached
cable, which should be the next stop on your inspection. Inspect the
load
cell cable for rodent damage. If rodent damage is present, you
can bet the seal is degraded as well. To combat this occurrence, some
manufacturers offer rigid or flexible metal conduit to run the cable
through, providing a layer of protection that’s nearly impervious
to damage from rodents. Steel overbraid cable provides more protection
from rodent damage than cable sheathed with polyurethane or plastic,
but is still vulnerable and will need to be checked more frequently.
Weather is another enemy of load cell cable. A damaged seal around
the gland where the cable enters the cell can cause water infiltration.
When exposed to moisture, cables can also serve as wicks. Water can
move along the cable’s length by capillary action until it enters
the load cell or junction
box. Once inside, it can not only damage these components but
seep into the inner layer of the cable. Wet cables may not cause signal
distortion immediately, but may slowly corrode the copper wires or
shield beneath the insulation, setting the stage for future problems.
Now move on to
the junction box (j-box). Internal condensation is the most common
moisture problem. This condition occurs from normal air exchanges
from heating and cooling cycles, and over-aggressive washing that
damages sealing components that haven’t been properly maintained.
If left unchecked, wiring terminals and other components in the enclosure
will corrode. Due to its material properties, a stainless steel j-box
promotes internal condensation and are therefore more vulnerable to
this problem. If stainless steel is not required, the best practice
is to install a new Tuff
Seal j-box.
Quality Counts
Whether you own a pit-type, above ground, siderail or portable vehicle
scale, a portion of your preventive maintenance plan rests in the
hands of the manufacturer. It’s not uncommon for scales that
are functioning correctly to collapse without warning. The culprit?
Rust. Many scale understructures are left untreated and vulnerable
to corrosion. Steel deck scales that have a welded bottom plate also
promote corrosion because water that seeps into the scale cannot run
into the ground and is less likely to evaporate. In these cases you
can and should protect the scale components mentioned earlier; however,
there isn’t a solution for protecting the understructure. If
you are considering a truck scale purchase, you may want to look at
a scale that includes a corrosion-resistant undercoating or does not
include a bottom plate. Depending upon the scale’s location,
special features, and the material it is weighing, your scale inspection
may or may not require all of the steps mentioned above. However,
these guidelines should give you a good start for developing a preventive
maintenance plan for your truck scales. Portions
of this article courtesy of Mark Johnson, Jr. is the Business Unit
Manager for heavy capacity scales at Rice Lake Weighing Systems.
Call (919) 776-7737
or click above for a quote
