The load capacity of a classic 3-groove mount is limited by the point contacts that are formed between the spheres and the V-grooves. When placed in the same grooves, the Spherolinder eliminates these single-point contacts, without degrading the kinematicity of the mount.
To the left is a stress plot of both a sphere and a Spherolinder based V-groove mount. The stress concentration in the sphere mount is quite evident.
Below is a comparison of Spherolinder and Sphere based 3-groove mounts. Both connectors and mating surfaces are Stainless Steel 440C. The load is evenly distributed, and the figures are for the entire 3-connector mount, and include a 1.4 safety factor for each of material and load.
| Connector size | Sphere based mount | Spherolinder based mount |
| 0.2" (5 mm) | 4.7 lb (2.1 kg) | 463 lb (210 kgs) |
| 0.4" (10 mm) | 18 lb (8.2 kg) | 1854 lb (843 kgs) |
| 1" (25 mm) | 117 lb (53 kg) | 12,744 lb (5.8 tons) |
| 2" (50 mm) | 470 lb (210 kg) | 49,720 lb (22.6 tons) |
The improvement in load capacity depends on the engineering material. For Stainless Steel 440C (ball bearing steel) the factor is about 100. We included the design calculations at the end of this page, and have also created a pair of web calculators for your convenience.
- Pre-seating
In a 3-groove mount, the mediating connectors (spheres or Spherolinders) are assumed to self-locate to their nominal positions. In most calculations, the friction model is simplistic - F=µN. For the small micron-level motions that we're interested in, however, it is important to note that under hertzian contact conditions, the local elastic deformations are at the same order of magnitude, or even larger than, the relative motion of the parts. - Trapped particulate contamination
Particulate contaminators can come in a variety of sizes and levels of hardness. Particles can cause direct misalignment, can cause damage to the mating surfaces that will show up on further re-assemblies, or can cause harmless elastic deformations. Since Spherolinders have much smaller radii of curvature than comparable load-capacity spheres, they present less surface area that is non-mating, but is still susceptible to contamination. As with spheres, and any precision assembly, clean air is our friend. - Surface contamination layer
Surface contamination layers have a more subtle effect on precision assemblies. They do not damage mating surfaces, and can actually aid with lubrication. Their biggest problem is with slow creep - a greasy layer will actually 'float' a mating connector, and will slowly with time (and depending on load) creep away. These effects are highly dependent on the radius of curvature, and as noted above, are mitigated by the much smaller radius of curvature allowed by the Spherolinder. - Geometrical imperfections
While kinematic mounts allow loose tolerances on fabrication of the mating bodies, the mating features themselves still need to be tightly toleranced: Spheres must be highly spherical, cones rounds, and v-grooves flat. Surface quality is of utmost importance, in particular in light of use-dependent wear-and-tear degradation which places additional restrictions on fabrication processes used while making mating components. Since the Spherolinder is more geometrically complex than a sphere, there are more tolerances to watch - sphericity, cylindricity, concentricity... Deviations in concentricity (eccentricity) are mitigates by the small degree of angular motion allowed by the retaining bolt. Small deviations in cylindricity and sphericity are mitigated by the averaging nature of the line contact, all within the bounds of elastic deformations.