NG – Torque/Tension Testing of Bolts with Epoxy Primer

Brett Reges, Connor King, Gaby van Brunt, Wyatt Downing

Project Description

torque testing example
  • The MIL-PRF-23377 epoxy primer is used in aerospace applications to prevent corrosion
  • The nut factor, K, is required to find the proper installation torque, T, given a bolt preload, F, and bolt diameter, D.

T = K*F*D

Design Requirements

  • Aluminum and steel joints
  • 1.25 safety factor above yielding
  • Limited noise and reliable data
  • Large sample size

Performance Review

  • Calibration of load collars performed with Tinius Olsen Tensile Testing Machine
  • Signal decay of output observed through repeated load cycling
  • Calibration performed after steady behavior of loading, verified by loading following testing
  • Significant misalignment of calibration results observed following testing
  • Decaying signal could indicate plastic deformation of load collar
pressure tester

Design Description

  • Setup a threaded plate with a load collar, washer compressed under bolt head
  • Design load collar with strain gauges in full wheatstone-bridge configuration to measure compressive preload of bolts
  • Plates are designed with enough holes to accommodate many tests for each bolt size and plate material
  • Repeated testing will provide accurate epoxy nut factor
3d rendered model
3d rendered model cut view


system laid out on a desk
  • Nut factor from empirical data is higher than would be expected for a thread lubricant/epoxy such as MIL-PRF-23377
  • Testing should be repeated with tensile testing machine with verified calibration
  • Epoxy cannot be considered a locking feature for bolts without further investigation
  • Bolts larger than 1/4-in. diameter fail at interface with installation tool rather than in tensile stress
Test Plate Material Bolt Size Nut Factor Breakaway Torque (In lb) Failure Torque (in lb)
Aluminum 3/16 0.2840 31.9 144.2
Steel 3/16 0.3225 29.1 172
Aluminum 1/4 0.3825 29.4 N/A*
Steel 1/4 0.3267 32.1 312.9
Aluminum 3/8 0.2629 66 N/A*
Steel 3/8 0.2120 54.4 N/A*

*Failure occured as stripping in bolt head, not in axial tension

College of Engineering UtahStateUniversity

Special Thanks to our Industry Mentors Chris Schmit and David Rothstein from Northrop Grumman