Fixture Holes Explanation
Figure 1. General description of a wire rope isolator and its components
A wire rope isolator (WRI) has a simple design composed of the following components: wire rope steel cable, four bars and
clamping screws. As shown in Figure 1, out of the four bars, two bars are called inner bars and the other two are called outer
bars. The inner and outer bars are attached together using clamping screws, and the number of these screws depends on the
model of the WRI. In order to attach the WRI to the equipment it is protecting, there are three different fixture holes options:
through hole, countersunk hole (82°), and threaded insert. To adapt the different types of fixture holes, only the inner bars
fixture holes are adjusted, while the outer bars fixture holes are kept as through holes. In Figure 1, a full helical wire rope isolator
is shown and it generally has four fixture holes on the top and four fixture holes on the bottom. The goal of this document is to
inform the customer about the different types of fixture holes, and the consequences of choosing each one.
a) Through Hole Option
Figure 2. Through hole option for the fixture holes
Figure 3. Through hole option for the fixture holes with bolts used to attach the WRI to the equipment
With the through hole option for fixture holes, all four bars have the same through holes. The disadvantage with this option is
that due to the bolts head protruding between the two inner bars, the maximum distance the WRI is allowed to deflect in
compression is reduced. This means that the maximum shock value that the WRI is able to sustain without bottoming out is
reduced. So the customer should be aware that when using the through holes option the WRI cannot be used to its full shock
value rating.
Note also that for small loop WRI models (i.e. XX##-20), through holes are not recommended. This is because the distance
between the inner bars in a small loop WRI is small, and usually will not fit a bolt long enough to provide adequate thread
engagement with the equipment (at least half the diameter of the bolt). An example of the FH92-20 is shown in Figure 4, where
the minimum length bolt does not fit between the inner bars.
Figure 4. FH92-20 where the minimum length bolt does not fit between the inner bars
b) Countersunk Hole (82°) Option
Figure 5. Countersunk hole option for the fixture holes
Figure 6. Countersunk hole option for the fixture holes with countersunk screws used to attach the WRI to the equipment
With the countersunk hole option for the fixture holes, the two inner bars are adjusted with an 82° countersunk hole. The two
outer bars are kept as through holes. This fixture hole option is ideal because the it does not reduce the space between the
inner bars (i.e. the maximum allowable deflection of the WRI).
Although the standard countersunk hole is 82°, 90° countersunk holes can also be provided.
c) Threaded Insert Option
Figure 7. Threaded insert option for the fixture holes
Figure 8. Threaded insert option for the fixture holes with bolts (threaded from above) used to attach the WRI to the
equipment
With the threaded insert option for the fixture holes, threaded inserts are added to the holes of the two inner bars. The two
outer bars are kept as through holes. The equipment is attached to the WRI in the space between the bolt heads and the outer
bars, as shown in Figure 8. This fixture hole option is ideal because the it does not reduce the space between the inner bars (i.e.
the maximum allowable deflection of the WRI).
d) Fixture hole combinations
All examples discussed above showed the same fixture hole option for both inner bars. Customers can also have a WRI with a
combination of fixture hole options; i.e. inner bar 1 and inner bar 2 each having a different fixture hole option. There can be
three such combinations: inner bar 1 with trough holes and inner bar 2 with threaded inserts (Fig. 9), or inner bar 1 with
countersunk holes and inner bar 2 with threaded inserts (Fig. 10), or inner bar 1 with through holes and inner bar 2 with
countersunk holes (Fig. 11). So depending on the interface design between the WRI and the equipment it is supporting, the
customer should select any of the six fixture hole combinations that best fits the application. The six fixture hole combinations
are summarized in Table 1.
Figure 9. Through holes for inner bar 1 and threaded inserts for inner bar 2
Figure 10. Countersunk holes for inner bar 1 and threaded inserts for inner bar 2
Figure 11. Through holes for inner bar 1 and countersunk holes for inner bar 2
Table 1. Summary of all six fixture hole combinations and their nomenclature under the imperial units system
Fixture holes
combinations
Inner bar 1
Inner bar 2
Through
Holes
Countersunk
Holes (82°)
Threaded
Inserts
Through Holes
T2
N/A
N/A
Countersunk
Holes (82°)
TC
C2
N/A
Threaded
Inserts
TI
CI
I2
Note that metric fixture holes are also possible, in which case a letter M is added in the nomenclature as shown in Table 2.
Table 2. Summary of all six fixture hole combinations and their nomenclature under the metric units system
Fixture holes
combinations
Inner bar 1
Inner bar 2
Through
Holes
Countersunk
Holes (90°)
Threaded
Inserts
Through Holes
TM2
N/A
N/A
Countersunk
Holes (90°)
TMC
CM2
N/A
Threaded
Inserts
TMI
CMI
IM2