Method and apparatus for tightening threaded fastener assemblies

Apparatus for tightening an assembly including a threaded fastener comprising:

• driving means for imparting torque and rotation to said fastener to tighten said assembly, the actual torque-rotation curve for said assembly having a non-linear tightening portion;
• torque sensing means responsive to said driving means for developing a first torque signal representative of the torque imparted to said fastener;
• angle sensing means responsive to said driving means for developing an angle signal representative of the rotation imparted to said fastener;
• gradient calculating means responsive to said first torque signal and said angle signal for developing a calculated gradient signal representative of the gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• means for storing a second torque signal representative of the theoretical tightening torque on the theoretical torque-rotation curve for said assembly required to induce a desired preload in said fastener when said assembly has been tightened to a desired degree;
• means for storing a plurality of correction factor signals and a plurality of gradient signals defining a curve representative of the relationship between a plurality of correction factors and the gradients of the substantially linear tightening portion of a plurality of possible torque-rotation curves for said assembly, each of said correction factors being related to the difference between said desired preload and a projected possible preload induced in said fastener when said theoretical tightening torque is applied to said fastener along one of said plurality of possible torque-rotation curves;
• means for comparing said calculated gradient signal with said stored plurality of gradient signals and for deriving the correction factor signal related with the stored gradient signal which is closest in magnitude to said calculated gradient signal;
• means responsive to said stored second torque signal and said derived correction factor signal for developing a third torque signal representative of a calculated tightening torque equal to the product of said theoretical tightening torque and the correction factor represented by said derived correction factor signal;
• comparison means responsive to said first torque signal and said third torque signal for comparing said torque imparted to said fastener with said calculated tightening torque and for developing a control signal when said torques represented by said first and third torque signals are equal;
• and control means for supplying said control signal to said driving means for causing said driving means to cease to impart said torque and rotation to said fastener.

A method for tightening an assembly including a threaded fastener to a desired preload comprising:

• establishing a theoretical tightening torque of a theoretical torque-rotation curve for the assembly required to induce the desired preload in the fastener when the assembly has been properly tightened;
• imparting torque and rotation to said fastener to tighten said assembly;
• calculating the instantaneous gradient of the tightening region of a torque-rotation curve for the joint assembly being tightened;
• adjusting said theoretical tightening torque in response to said instantaneous gradient;
• controlling said torque and rotation imparted to said fastener according to said adjusted theoretical tightening torque and ceasing to impart said torque and rotation to said fastener when said torque imparted to said fastener is equal to said adjusted theoretical tightening torque.

A method for tightening an assembly including a threaded fastener to which torque and rotation are imparted to induce a desired preload when said assembly has been tightened to a desired degree, the actual torque-rotation curve for said assembly having a non-linear tightening portion followed by a substantially linear tightening portion, said method comprising:

• establishing a theoretical tightening torque on the theoretical torque-rotation curve for said assembly required to induce a desired preload in said fastener when said assembly has been tightened to the desired degree;
• selecting a plurality of possible torque-rotation curves for said assembly;
• calculating the gradients of the substantially linear tightening portions of said plurality of possible torque-rotation curves;
• calculating the gradient of the substantially linear tightening portion of said theoretical torque-rotation curve;
• developing a plurality of correction factors, one such correction factor associated with one of said gradients of said substantially linear tightening portions of said plurality of possible torque-rotation curves and related to the difference between said associated gradient and said gradient of said substantially linear tightening portion of said theoretical torque-rotation curve;
• imparting torque and rotation to said fastener;
• calculating the gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• comparing said gradient of said substantially linear tightening portion of said actual torque-rotation curve with said gradients of said substantially linear tightening portions of said plurality of possible torque-rotation curves and determining which of said gradients of said substantially linear tightening portions of said plurality of possible torque-rotation curves is closest in magnitude to said gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• deriving the correction factor associated with said gradient closest in magnitude to said gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• calculating a tightening torque by multiplying said theoretical tightening torque by said derived correction factor;
• determining when said torque imparted to said fastener is equal to said calculated tightening torque;
• and ceasing to impart torque and rotation to said fastener when said torque imparted to said fastener is equal to said calculated tightening torque.

A method for tightening an assembly including a threaded fastener to which torque and rotation are imparted to induce a desired preload when said assembly has been tightened to a desired degree, the actual torque-rotation curve for said assembly having a non-linear tightening portion followed by a substantially linear tightening portion, said method comprising:

• establishing a theoretical tightening torque on the theoretical torque-rotation curve for said assembly required to induce a desired preload in said fastener when said assembly has been tightened to the desired degree;
• selecting a plurality of possible torque-rotation curves for said assembly;
• calculating the gradients of the substantially linear tightening portions of said plurality of possible torque-rotation curves;
• calculating the gradient of the substantially linear tightening portion of said theoretical torque-rotation curve;
• developing a preload versus gradient curve defining the relationship between said gradients of said substantially linear tightening portions of said plurality of possible torque-rotation curves and a plurality of projected possible preloads induced in said fastener when said theoretical tightening torque is applied to said fastener along said plurality of possible torque-rotation curves;
• developing from said preload versus gradient curve a correction factor versus gradient curve defining the relationship between a plurality of correction factors and said gradients of said substantially linear tightening portions of said plurality of possible torque-rotation curves, each of said correction factors related to the difference between said desired preload and one of said projected possible preloads;
• imparting torque and rotation to said fastener;
• calculating the gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• comparing said gradient of said substantially linear tightening portion of said actual torque-rotation curve with said gradients of said substantially linear tightening portions of said plurality of possible torque-rotation curves and determining which of said gradients of said substantially linear tightening portions of said plurality of possible torque-rotation curves is closest in magnitude to said gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• deriving the correction factor associated with said gradient closest in magnitude to said gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• calculating a tightening torque by multiplying said theoretical tightening torque by said derived correction factor;
• determining when said torque imparted to said fastener is equal to said calculated tightening torque;
• and ceasing to impart torque and rotation to said fastener when said torque imparted to said fastener is equal to said calculated tightening torque.