Miniature seals tutorial.
* Tolerance stack-up.
In any sealing application, correct O-ring selection depends on a number of design considerations, including dimensions, chemical compatibility and the ability to resist pressure and temperature extremes, in microminiature applications, the importance of the aforementioned considerations increases as the size of the O-ring decreases.
Microminiature designs require tighter tolerances and higher-precision, leaving less room for inaccuracies. It is helpful to pay particular attention to some basic sealing concepts when selecting a microminiature O-ring.
The first concept to evaluate is sealing pressure. Sealing pressure is the force exerted by compression of the seal between the mating surfaces forming the gland itself and external forces (i.e. the fluid being sealed) transmitted through seal. A physical barrier is formed when the force from compressing the seal and pressure applied by the fluid causes the seal to fill at all leak paths.
The second concept to look at is low-pressure sealing. In a low-pressure sealing application, system pressure does not cause a noticeable increase in the seal's sealing force. Because of this, all sealing force must come from the ability of the seal to deform and create a barrier by compression of the seal. Surface finish on both the seal and gland are critical to an effective seal.
Thirdly, attention should be paid to high-pressure sealing. In high-pressure sealing applications, system pressure causes a noticeable increase in the seal's sealing force to fill all leak paths. Because this pressure activates the seal, the compression of the seal and surface roughness are not as important as in low-pressure sealing.
Finally, tolerance stack-up should be evaluated. Tolerance stack-up is an important factor to consider because the tolerances account for a large percentage of the nominal O-ring size. In a sealing application, the tolerances of all the parts in contact with the O-ring must be considered in order to create an effective seal. The combination of these tolerances is the tolerance stack-up.
Apple Rubber Products' MicrOring seal, an example of a true microring seal, measures less than 1 mm in either inside diameter or cross section. Applications for these seals, which are made of composites, include precision instruments, medical devices, aerospace and fiber optics.
Circle 242--Apple Rubber Products, Lancaster, NY. www.rsleads.com/405df-242