Cadmium Substitution - garteur

GARTEUR LIMITED
ANNEX I
Tribological properties
I.1 Introduction
The tribological properties of the coatings are an important consideration when used on
fasteners or on threaded parts. In the tightening up of a bolt most of the effort expended
is used in overcoming the frictional constraints. Cadmium has a low coefficient of friction
(~0.2) so that a comparatively low torque need be applied to achieve a given level of preload.
During the assembly of aircraft structures it is often necessary to bolt parts into
place and then remove them to make adjustments or modifications. This process may be
repeated several times and it is important that the torque-tension characteristics of the
coated bolts used remain unaltered. In order to determine the suitability of the different
coatings for use on fasteners the current programme has determined the coefficient of
friction of several of the coatings and has compared the torque-tension characteristics of
coated fasteners after repeated tightening and untightening.
I.2 Torque - tension measurements
I.2.1
Hi-Lok fasteners
Two systems were evaluated:-
(1) steel pins and steel collars
(2) titanium pins and steel collars
For each system the preload obtained and the locking torque was recorded. Tables I1
and I2 summarise the data for tests made on steel and titanium pins respectively. The
figures given represent the average of 7 measurements. The results obtained indicate
that the level of preload and locking torque values are dependent on the coating system.
The normal requirement is that the preload should be in the range 4 to 9kN. As indicated
in tables I1 and I2 the minimum preload of 4kN was exceeded with all of the coatings
examined.
I.2.2
Coated steel bolts
Results of the torque - tension tests conducted by SAAB on steel bolts and nuts coated
with cadmium and various alternative coatings are given in table I3. Data are also
included in the table previously obtained at SAAB [I1] and at DERA [I2,I3].
I.3 Reusability tests
Tests were made to determine the effect of repeated tightening up and untightening on
the performance of coated nuts and screws. The work undertaken by DBAA has
concentrated on fasteners coated with cadmium and Delta-tone. Data obtained are
presented in figure I1 showing the effect of repeated loading and unloading on the
coefficient of friction (μ). Different combinations of cadmium and Delta-tone plated steel
bolts and nuts have been examined. The results indicate that when the bolt and nut are
both cadmium plated there is an increase in the coefficient of friction as the combination
is repeatedly tightened and untightened. When either the nut or bolt is coated with Deltatone
there is only a small change in the value of μ. Data are presented in figure I2
showing the effects of repeated loading on the preload to torque-off ratio. This shows
that the largest decrease occurs with the cadmium plated bolt-cadmium plated nut
combination.
GARTEUR SM/AG17 TP128 Page 97