Disc Spring - Inches (for Bolt - DI Series). DI Series Pre - Stressed Disc Springs are used to maintain Load or Tension in Bolted assemblies. Pressure begins at the Outer Radius and Flattens gradually toward the bolt as deflection progresses. Disc Springs exerts uniform pressure that remains constant inspite of Tension Losses caused by Thermal expansion and contraction, compression set or wear of parts. Because pressures are predictable, Disc Springs provide a Simple and Effective means of determining Bolt Tension that is far more accurate that "Torque" readings.