Shaft-to-Hub Joints

The following illustration (Figure 10.9) shall explain the basic coherences for the dimensioning of a bonded shaft-to-hub joint. Here, a limitation to the essential geometrical and mechanical parameters is required, since the reduction factors, viscoelastic adhesive layer behavior, stress development, surface geometry of the adherends and the like, do not allow for a detailed consideration at this point (Figure 10.9). 

The calculation of shaft-to-hub bonded joints is based on the following parameters ... 

This rule refers to circular bonded joints, for example, tubular bonded joints or shaft-hub joints, with heat or hot-curing reactive adhesives, if different metal materials are to be bonded, Figure 11.5. 

Interference Fit - A mechanical fastening method used to join two parts, such as a hub and a shaft, in which the external diameter of the shaft is larger than the internal diameter of the hub. This interference produces high stress in the material and must be determined carefully to avoid exceeding the allowable stress for the material. Stress relaxation can occur in interference fits, causing the joint to loosen overtime. Also called press fit. 

Press Fit. In press or interference fits, a shaft of one material is joined with the hub of another material by a dimensional interference between the shaft s outside diameter and the hub s inside diameter. Press-fit joints can be made by simple application of force or by heating or coohng one part relative to the other. This simple, fast assembly method provides joints with high strength and low cost. 

One must also bear in mind the effect of differences in the coefficient of linear thermal expansion between the two parts. If the coefficient of linear thermal expansion is greater for the shaft than it is for the hub, the gap will tend to close at elevated temperatures and become greater at low temperatures. Whether or not this is desirable depends on whether the joint is intended to be fixed or sliding at elevated temperatures. In the case of a fixed joint, it may be desirable to elevate the temperature of the hub (only) for assembly in order to assure that the joint remains fixed at elevated temperatures. However, this practice can lead to stress cracking of the hub at low temperatures if it is not strong enough to withstand the added stress under such conditions. 

The interference fit is a common technique to join cylindrical parts together. The most common joint geometry consists in a shaft fitted into a hub or a ring. In order to guarantee the coupling, an interference d exists among the hub and the shaft. 

The pull-out strength of a solely interference fit joint depends on the pressure P between the hub and the shaft, the friction coefficient p, and the contact area A, according to the equation (Croccolo, De Agostinis, Vincenzi, 2010 Croccolo, de Agostinis, Vincenzi, 2011 Kleiner Fleischmann, 2011) ... 

In order to test the behavior of different adhesive systems in the hybrid joint and to analyze the influence of the interference contribution, hub/shaft laboratory samples were created. 

The joint geometry of our samples surely implied the presence of spew fillets, being the shaft press-fitted into the hub. The external spew fillet was visible after the coupling and it was removed before the adhesive cure. The internal one was observed after the breaking of the joint, but the geometry of the sample made impossible to remove it. Indeed, according to the pin and collar test method, a cord shaped adhesive spew must remain in one of the collar ends (Abenojar et al., 2013). 

The second set of experiments was carried out to identify the interference contribution in the hub/shaft samples and its interaction in the hybrid joints bonded with the FT-EP adhesive (Gallic et al., 2014). The design of the experiment is depicted in table 2, a minimum of 4 specimens were tested for each sample type. 

Unbonded interference-fit samples were prepared by using the press-fitted hub/shaft joints and tested in static axial pull-out in order to understand the strength contribution of the interference-fit alone (Gallio et al., 2014). Assuming the same friction coefficient between steel and steel for all the... 

The load after break in the hybrid joints mainly depended on the radial contact pressure present between the hub and the shaft due to the interference fit. The important contribution due to the friction and wear phenomena observed in the unbonded samples was absent in these joints, thus the load after break can be most likely referred to the system described by the analytical equations. According to this hypothesis, the load after break can be more easily related to the contribution provided by the interference level. In order to obtain an estimation of this additional load, the mean value between the first peak and first valley of the stick and slip state was calculated. The obtained values are plotted in figure 7 and compared to those of the unbonded interference samples. 

The R-EP was the only adhesive that seemed to be negatively affected by the interference. This result was surely affected by the high data dispersion obtained with the samples bonded with this adhesive. This recorded dispersion was probably due to the quantity of this adhesive able to remain inside the joint during the press-fit coupling. This hypothesis was confirmed by the analysis of the fi acture surfaces of every hub and shafi after the decoupling. In the majority of cases, the shafts and the hubs presented some cured adhesive residues on the mating surfaces. In the case oiFT-EP, AC and PU samples, the cured residues were present in a similar morphology on both the hubs and the... 

Fi Interference contribution to the axial release force of the joint [N] Fad Adhesive contribution to the axial release force of the joint [N] Fcoupi Pushing in force during shaft-hub coupling (peak value) [N]... 

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