During earthquake strong motions, seismic devices are subjected to large horizontal deformations combined to high axial loads. For elastomeric bearings, the most widespread technology used to this aim, it is necessary to value critical load capacity under this combined effect, to ensure their stability in service. In this paper, elaborations of data from experimental campaign on full-scale rubber bearings will be compared to results form a brief theoretical treatment, to evaluate their stabilty limits. To examine the effect of device geometrical characterization on the mechanical behaviour, instability mode and interaction vertical pressure - shear deformation, three different types of full-scale devices have been considered (ϕ500, ϕ600, ϕ700). Experimental data refers to "soft" natural rubber bearings, having a shear modulus G= 0.4MPa (at γ=100%), an equivalent damping factor ξ = 10÷15and a secondary shape factor S₂ varying in the range [2.8÷3.4]. Data derive from static shear tests, at increasing levels of shear strain (up to γ = 250%) combined to growing compressive stress (from 6 MPa to 20 MPa). Their elaboration has been accompanied by the use of dimensionless parameters, in particular the ratio γ/S₂, that results an effective design parameter to evaluate deformation limits for rubber devices. On the base of an analytical formulation proposed by authors, that define the critical load as function of both shape factors, the experimental results have been plotted in stability domains, showing the effectiveness of this formulation in defining device stability condition.