Method and apparatus for tightening threaded fastener assemblies

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

• driving means for imparting torque and rotation to the fastener to tighten said assembly, the actual torque-rotation curve for said assembly having a non-linear tightening portion followed by a substantially 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 calculated gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• means for storing (1) a plurality of gradient signals representative of the gradients of the substantially linear tightening portions of a plurality of possible torque-rotation curves for said assembly, and (2) a plurality of tightening torque signals representative of a family of tightening torque curves which define the relationships between a plurality of preloads which may be induced in said fastener and said gradients of the substantially linear tightening portions of said possible torque-rotation curves;
• selection means for selecting a desired preload which is to be induced in said fastener;
• means responsive to said selection means, said calculated gradient signal, and said stored gradient signals for deriving from said plurality of torque signals that tightening torque signal representative of the torque defined by the relationship of said selected desired preload and said stored gradient which is closest in magnitude to said calculated gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• and control means responsive to said derived tightening torque signal for stopping said driving means.

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

• driving means for imparting torque and rotation to the fastener to tighten said assembly, the actual torque-rotation curve for said assembly having a non-linear tightening portion followed by a substantially 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 calculated gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• means for storing (1) a plurality of gradient signals representative of the gradients of the substantially linear tightening portions of a plurality of possible torque-rotation curves for said assembly, and (2) a plurality of tightening torque signals representative of a family of tightening torque curves which define the relationships between a plurality of preloads which may be induced in said fastener and said gradients of the substantially linear tightening portions of said possible torque-rotation curves;
• selection means for selecting a desired preload which is to be induced in said fastener;
• means responsive to said selection means, said calculated gradient signal, and said stored gradient signals for:
• (1) comparing said calculated gradient signal with said stored gradient signals to identify which of said stored gradient signals is closest in magnitude to said calculated gradient signal, and
• (2) deriving from said plurality of torque signals that tightening torque signal representative of the torque defined by the relationship of said selected desired preload and said stored gradient which is closest in magnitude to said calculated gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• and control means responsive to said derived tightening torque signal for stopping said driving means.

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:

• selecting a plurality of possible torque-rotation curves for said assembly;
• determining the gradients of the substantially linear tightening portions of said plurality of possible torque-rotation curves;
• establishing the tightening torques required to induce a desired preload in said fastener when said fastener is tightened according to said possible torque-rotation curves, one such tightening torque being associated with each gradient of said substantially linear tightening portion of said possible torque-rotation curves;
• imparting torque and rotation to said fastener;
• calculating the gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• comparing said calculated 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 calculated substantially linear tightening portion of said actual torque-rotation curve;
• deriving the tightening torque associated with said gradient closest in magnitude to said calculated gradient of said substantially linear tightening portion of said actual torque-rotation curve;
• determining when said torque imparted to said fastener is equal to said derived tightening torque;
• and ceasing to impart torque and rotation to said fastener when said torque imparted to said fastener is equal to said derived tightening torque.