Load cells are elastic devices, which can be used for measurement of force through indirect methods, where force or load or weight is converted into an electric signal, using secondary transducers. Load cells utilize an elastic member as the primary transducer and strain gauge as the secondary transducer. When the combination of the strain gauge and elastic member is used for weighing, it is called as a “load cell”.
TYPES OF LOAD CELL
Various types of load cells are,
- Hydraulic load cell
- Pneumatic load cell
- Strain gauge load cell
Hydraulic Load cell
Hydraulic load cell can be used to measure force of very high magnitude (of the order of millions of Newton). Hydraulic load cell consists of a closed container filled with oil and covered with diaphragm as shown in Fig. 1. A pressure measuring device, such as, Bourdon tube pressure gauge is connected to the container. When load (P) acts on the diaphragm, it gets deflected and transmits force to the oil. Due to this, pressure is developed in the oil. The magnitude of pressure developed is indicated by the pressure gauge, which can be converted into force. The pressure gauge can also be directly calibrated in units of force.
Fig. 1: Hydraulic Load cell
Pneumatic Load Cell
Pneumatic load cell is used to measure force up to 20,000 N. Pneumatic load cell consists of a chamber with a nozzle and covered with elastic diaphragm as shown in Fig. 2. Initially, pressurised air is supplied through a pipe to the chamber. As force or load (P) acts on the diaphragm, it deflects and the gap between diaphragm and nozzle decreases, while pressure Pc in the chamber increases. Rise in pressure Pc applies outward force on the diaphragm opposite the force or load p. For any force ‘P’, the diaphragm attains equilibrum under the impact of two equal and opposite forces. At this state, chamber pressure is noted using pressure gauge, which can also be directly calibrated in terms of force or load.
Fig. 2: Pneumatic Load Cell
APPLICATIONS OF LOAD CELLS
- Scales.
- Weighbridge or weighing machine for measuring weight of car, truck etc.
- Force gauges.
- Torque gauges.
- Material handling devices such as crane or AGV.
- Suspension bridges.
- For measuring press loads.
- For measuring pressurization force in case of machines, such as spot welding machines, polishing machines, etc.