Screw-mount assemblies

OFTEN NEGLECTED INFLUENTIAL FACTORS

The information available in specialised literature on this input data does not factor in the sheer diversity, which is quasi systematic, of the parameters exerting an impact on results :

  • screw materials (steel, stainless, titanium, etc…)
  • lubrication (grease, Molykote, etc…)
  • washer type and materials (steel, stainless, permaglass, flat, onduflex etc…)
  • thread type and materials (steel, stainless, aluminium, carbon, helicoil etc…)

The tests we have been performing for several years, however, highlight differences in preloads amounting to double-figure percentage points for the same torque applied.

 

KNOWING MEANS TESTING

Having already been confronted with this lack of information in our mechanical design practice, we have developed several test sets used to perform characterisations such as :

 curve plotting the relation between preload and applied torque plus averages, standard deviation, A value and B values.

  • drift in applied preloads following multiple tightening/untightening operations
  • drift in applied preloads over time (creep, composite materials)
  • determining friction coefficients under screw heads and threads
  • determining optimal tightening procedures for complex assemblies.
The tests are performed on sample batches totally representative of the target assemblies. The number of specimens per batch is varied to meet characterisation needs.