Module C: Stress, Strain, Force and Load

Question 1

How do load cells relate to mass measurements?

Answer 1

Changes in strain of load cells is related to mass measurement, where mg = kE

[pneumatic and hydraulic load cells convert mass measurements to pressure signals]
[strain also relates to voltage changes]

Question 2

MASS MEASUREMENT - Spring Balances + limitations

Answer 2

Spring balances - relating mass to spring extension (x):

mg = kx

Spring characteristics change over time or with changes in environmental conditions

Question 3

MASS MEASUREMENT - Mass Balances + 2 types

Answer 3

• Null-type instruments
• used by comparing the mass of one body to another
• independent of gravitational forces
• Weight beam balance: robust*
• *Electromagnetic balance: high accuracy**

Question 4

MASS MEASUREMENT - Electromechanical balance vs electromagnetic force compensation balance

Answer 4

Electromechanical balance: Load cells with strain gauges

• suitable for large masses
• slower response time

Electromagnetic force compensation balance: Applied electromagnetic force instead of weights

• can measure small masses accurately
• fast response time, stable

Question 5

Vertical vs horizontal forces

Answer 5

Vertical forces: weights (measured using load cells and balances)

Horizontal forces:

• accelerometers (F=ma ; acceleration is measured for constant known mass)
• vibrating wire sensors (resonant frequency depends on applied force)
• plastic deformation (small element is permanently deformed by a force beyond its elastic limit; shape and size of deformation can be calibrated)

Question 6

Torque measurement

Answer 6

Torque is a rotational force, where T = F x r
F = force, r = radius of the shaft

Strain gauges can equally be used to measure torque

Question 7

Challenges to calibration of mass, force and torque sensors

Answer 7

• Changes in acceleration due to gravity with latitude and altitude
• upward force generated by ambient air