If a spring is activated by 0.5 mm and it generates a 
force of 150 gm, the load deflection rate of spring 
would be:
STIFFNESS OR LOAD DEFLECTION RATE 
This is the force magnitude delivered by an appliance and is proportional to the modulus of elasticity (E). Low stiffness leads to an ability to apply lower forces, a more constant force to be delivered over time and greater ease and accuracy in applying a given force.
75 gm/mm
150 gm/mm
175 gm/mm
300 gm/mm