Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 73)

INSTALLATION & SERVICE TIPS
4-10
Load Cell Sizing
Selecting Replacement Load Cells
Mechanical Conversions
In mechanical truck scale conversions, it is necessary to select a
load cell that is large enough to carry the scale’s dead load as well
as the scale’s live load while still providing adequate signal voltage.
Dead load identifies the weight that is always present on the cell,
such as the weight of the platform structure and the levers,
transferred through the lever system. Live load describes the
weight applied to the cell when weight or material is placed on the
scale. In a conversion, the scale’s live load comes through the
existing mechanical lever system, so the multiple must be deter-
mined prior to sizing a cell.
Example: A mechanical truck scale has a 400 to 1 multiple and a
scale capacity of 100,000 lb utilizing 20 lb graduations. To find the
live load, we divide the scale’s total capacity by its multiple:
100,000 lb capacity
400 multiple
To find the dead load at the steelyard rod, use a 1000 lb load cell
and your standard indicator calibrated at 1000 lb x 1 lb. Install the
load cell in the steelyard rod and power up the indicator. It will
read your actual dead load. For this example let’s assume the
indicator displayed 200 lb.
Now we combine the live and dead loads to determine total load
capacity.
200 lb dead load capacity
+ 250 lb live load capacity
450 lb total load capacity
As load cells are not normally constructed at 450 lb capacity, we
move up to the next highest increment, 500 lb. With a 500 lb
capacity load cell we will be using about 50% of the cell for live load,
providing us with plenty of live load signal output. Be careful not to
simply overrate load cell capacity to ensure against overloads. In
mechanical scale conversions, more cell capacity is often not better.
See “Determining Microvolts Per Graduation” on page 4-3.
NOTE: To determine the dead load of a mechanical floor scale, use
a small, hand-held tubular scale or a fish scale, and hook onto the
transverse lever. Pull up until the weight of the of the empty scale
is being read by the scale. This is the dead load at the load cell. Use
the same formulas to determine live and dead loads for determining
floor scale load cell size.
Electronic Replacements
In fully electronic truck, railroad track or tank scales, load cell
sizing procedures are different. Here we are not using a multiple, as
all of the dead load is resting on the load cells. For this example:
100,000 lb capacity truck scale with an 80,000 lb deck and 8-60,000
lb 3 mV/V canister load cells. The indicator will supply 10 VDC
excitation and reads 100,000 lb by 20 lb.
1. Determine total scale capacity:
80,000 lb = dead load capacity
+ 100,000 lb = live load capacity
180,000 lb total capacity needed
2. Determine load cell capacity:
8 load cells
x 60,000 lb capacity
480,000 lb total load cell capacity
In our example we have more than enough capacity to handle the
live load, but we maybe in danger of reducing the live load signal too
far by overrating the load cells. For further analysis, let’s determine
if we have enough signal voltage to properly operate our scale.
1. Divide live load by total load cell capacity:
100,000
÷
480,000 = .208 or 20.8%
This means only 20.8% of the total capacity will be used to
measure live load.
2. Determine live load output of the load cells:
3 mV/V (rated output) x 10 VDC excitation = 30 mV full scale
Multiply 30 mV by the 20.8% live load usage:
30 mV x .208 live load = 6.24 mV of full load signal
3. Determine scale graduation by dividing total capacity by read-
out increments. In this example we have a 100,000 lb scale
reading by 20 lb increments, yielding 5000 graduations.
100,000 lb capacity
÷
20 lb = 5,000 graduations
4. Determine signal per graduation by dividing the total gradua-
tions by our full load signal:
6.24 mV
÷
5,000 graduations = 1.25
µ
V per graduation
1.25
µ
V (microvolts) is an extremely small signal and may be too
small for your digital weight indicator to process accurately. Check
your indicator specifications to determine if this signal is within
specifications. For more information on determining microvolt per
grad, see “Determining Microvolts Per Graduation.”
= 250 lb live load.
NOTE: If it is a legal-for-trade application and NTEP load cells are required, there are several other considerations; consult the NTEP
section for a detailed discussion, or consult your Rice Lake Weighing Systems Authorized Distributor.