Eccentricity & Side Loading
It is fairly straightforward to design a O-rings as radial seals, but if the hardware is not machined as designed on the print, the result can be damaged seals or drastically reduced seal life. This is due to the seal being overcompressed in one position, and undersqueezed in another. This condition is typically called eccentricity or sideloading.
The easiest way to describe eccentricity is to imagine looking down into the piston, and visualize the groove is a circle inside a second circle, which is the bore. Perfectly concentric circles have the same center. Eccentric circles do not have the same center. The impact to sealing is increased squeeze on at one position between the piston and bore, and decreased squeeze on the exact opposite side of the piston and bore.
Hardware which is out of round can have the same impact on the seal as eccentricity. Out of round refers to a condition where the circle is not round, rather it has more of an elliptical or egg shape. The impact to the seal is creased squeeze on at one position between the piston and bore, and decreased squeeze on the exact opposite side of the piston and bore.
A third scenario with the same impact will be a result of sideloading. Sideloading is when the piston shifts off center inside the bore.
Designs using O-ring have a suggested Eccentricity based on the O-ring cross sectional thickness, as detailed in the O-ring Handbook. When the hardware exceeds these conditions, there are specialty seals available outside of an O-ring:
