COMPRESSION LOAD CELL
The difficulties of a compression measure
Three fundamental difficulties arise:
- the materialisation of the load cell effort axis
- the contact surface permitting the application of the force being measured to the load cell
- the effect of the structure reaction on the load cell
Materialisation of a compression load cell effort
Materialisation of a compression load cell effort axis is less evident than that of a tension load cell. If the transducer has a flat base fitted with a good mechanical quality bore centred on the geometric axis of the proof body, it is known that the transducer effort axis is perpendicular to its bottom plate going through the centre of this bore.
In this case, the application point of the force to be measured on the opposite plate of the load cell must be located on this axis. The bore on the bottom plate can be replaced by a machining of exactly the same external diameter as this plate if it is cylindrical.
The contact surface
Taking the resistance of the material into account, it is evident that the application of the force being measured to the load cell cannot be fixed on one point. It requires a contact surface which must be as small as possible and avoid, as much as possible, the generation and transmission of interfering forces and torques.
The best solution is to fit the load cell proof body with a spherical cap at one end, on which a flat support surface is placed. This system produces very effective decoupling. The main difficulty relates to selecting the radius of the spherical cap and the mechanical characteristics of the support piece, specifically its resistance to compression and its hardness.
The effect of the structure’s reaction on the force sensor
Contact with the force sensor is then reduced to a point which becomes a surface during the application of the force, in an reversible process. The geometry of the support piece must allow for it to be precisely positioned on the structure generating the force being measured. If the load cell is not fitted with a spherical cap, it is usually possible to apply one with appropriate dimension, for which the assembly conditions must be determined.
Very high-capacity compression load cells are sometimes equipped, at the top, with axial rod ends of very large radius. If such equipment is useful for limiting the influence of the non-parallelism of facing surfaces, it is not a real filter for parasitic forces.
According to the laws and principles of mechanics, the bottom plate of a compression load cell placed on a surface receives an equal opposite force to that applied to the top of the load cell. This point is often overlooked and causes several measurement errors. The supporting surface of the compression transducer significantly conditions the strain on the proof body and used to measure the force applied to the transducer. The load cell must be placed on a base which is in contact with the whole of the supporting surface.
This piece must be perfectly flat and as rigid as possible. The hardness of the material used must be comparable to that of the load cell. Also, the precise positioning of the load cell on this base often ensures that the force being measured goes through the transducer effort axis. If the load cell is clamped by screws on a specific mounting base or a support surface, the tightening torque of this screw must be defined and respected.
NOTE : Spanish version /en/tecnologia/sensores-de-fuerza/sensor-de-fuerza-en-compresion