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) Hole (82 ) Option Figure 5. hole option for the fixture holes Figure 6. 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) Insert Option Figure 7. insert option for the fixture holes Figure 8. 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. 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 2 Through Holes Inner bar 1 Holes (82 ) Through Holes T2 N/A N/A Holes (82 ) TC C2 N/A 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 2 Through Holes Inner bar 1 Holes (90 ) Through Holes TM2 N/A N/A Holes (90 ) TMC CM2 N/A TMI CMI IM2