PCB Load & Torque, Inc.
Toll-Free in USA 866-684-7107
716-684-0001
www.pcbloadtorque.com
DUAL BRIDGE LOAD CELL OPERATION MANUAL
7
The strain gages used are made with Modulus Compensated
Modified Karma Alloy. The alloy steel gages have a safety
factor against fatigue of approximately 2, and the aluminum
gages’ is approximately 4 as shown in Figure
15.
Dual bridge fatigue-rated load cells are capable of surviving
exceptionally high overloads. Occasional loads up to 300% of
the rated capacity (due to accidental overload) have a safety
factor against yield of the strain gage sections of
approximately 1.5 for both alloy steel and aluminum as shown
in Figure 16.
9.0 SHUNT CALIBRATION DESCRIPTION
Shunt calibration is used to simulate a known tension or
compression load on a load cell. The calibration certificate
will indicate which leg of the bridge to apply the shunt resistor
to for both tension and compression load simulation. Typically
tension is simulated by inserting the shunt resistor between the
+P and +S connector leads. Compression loading is simulated
by inserting the shunt resistor between the
+S
and
–P
connector leads.
9.1 Resistor Value
Dual bridge load cells have a nominal 2.0 mV/V full scale
output. For a 350 ohm strain gage bridge the precision shunt
resistor is, 60,000 ohms
±
0.01%, simulates an output of
approximately 73% of the full scale output for the load cell.
The calibration values for each bridge are found on the
calibration certificates supplied with each load cell.
9.2 Shunt Calibration Process
To perform the shunt calibration, use the following procedure:
1.
Stabilize all forces on the load cell. If possible,
remove all loads.
2.
Set the load indicator display to read exactly 00.000.
3.
Connect the shunt resistor to the terminals specified
in the calibration certificate, and adjust the span or
gain until the display reads the force value stated on
the certificate.
4.
Repeat steps 1-3 to verify that a valid calibration
setting has been obtained.
5.
If possible, apply a known load to the measurement
system to further verify that the calibration has been
accurately set up.
9.3 Estimating Shunt Resistor for a Given Load
The following formula can be used to estimate the
approximate value of shunt resistor required to simulate a
mechanical load.
R
cal
= (
25
* R
b
) / (Output
FS
* L
cal
)
Where:
R
cal
= Shunt Resistor (K ohms)
R
b
= Bridge Resistance (ohms)
Output
FS
= Full Scale output of the load cell (mV/V)
L
cal
= Load to be simulated, % of Load Cell Capacity
10.0 MAINTENANCE
Routine maintenance of the dual bridge load cell should
include cleaning the electrical connectors, housings, and
mounting surfaces with solutions and techniques that will not
harm the physical material of construction. Make sure liquids
are not allowed to migrate into devices that are not
hermetically sealed. Such devices should only be wiped with
a damp cloth, and never be submerged or have liquids poured
on them. Never use a pressure washer on the load cells.
Steel Safety Factor > 2.5
Aluminum Safety Factor
≈
2
Note: Aluminum alloy does
not have an endurance limit.
Figure 14
- S-N Curve (Load Cell)
Figure 15
- Strain-N Curve (Strain Gages)
Figure 16
- Shear Stress/Strain Curves (Load Cell)
10
2
10
3
10
4
10
5
10
6
10
7
10
8
10
9
Endurance Limit
Number of Load Cycles (Fully Reversed)
Shear
Stress
(ksi)
120
100
80
60
40
20
S.F. Steel
S.F. Alum.
Aircraft Quality Steel Alloy
Aluminum Alloy
10
2
10
3
10
4
10
5
10
6
10
7
10
8
10
9
5000
4000
3000
2000
1000
Number of Load Cycles (Fully Reversed)
Steel (2 mV/V)
Microstrain
Aluminum (4 mV/V)
Modulus Compensated
Modified Karma Alloy
Strain Gages
Strain Gage Safety Factors
Steel S.F.
≈
2
Aluminum S.F.
≈
4
Design Stress
Steel = 25 psi
Aluminum = 8 psi
25
125
100
75
50
150
Strain
Shear
Stress
(psi)
Yield Alloy Aluminum
Yield Alloy Steel
Safety Factor
≈
1.5 @ 300% Overload
300% Overload Steel
