You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
Pfeiffer <strong>Vacuum</strong><br />
Page 122<br />
6<br />
<strong>Vacuum</strong> <strong>Technology</strong><br />
Valves and Components<br />
6.1 General<br />
A vacuum system includes a variety of different components, such as recipients, vacuum<br />
pumps, measurement instruments, shut-off devices, filters, separators, etc., that must be<br />
joined together to form a unit. A distinction is made between detachable connections, which<br />
are equipped with seals, and non-detachable connections. The various types of connections<br />
that have to be taken into consideration in configuring a system and selecting components for<br />
it are described and presented in the sections below.<br />
6.2 Seals<br />
When vacuum technology components are detachably joined, seals must be used to prevent<br />
ambient air from flowing into the vacuum. There are different designs for this purpose, depending<br />
upon the application and pressure range in question.<br />
O-rings / round rubber rings<br />
O-rings are the most frequently used of all seals. O-rings are available in a variety of materials,<br />
usually elastomers with hardness ranging from 65 to 80 Shore. Their suitability as good<br />
vacuum seals stems from their ability to adapt to the minute unevenness of the mating<br />
surfaces. The surface of the o-ring must be free of releasing grease or talcum, smooth and<br />
crack- or scratch-free. In the low vacuum range the o-ring can be coated with a thin film of<br />
a low vapor pressure grease (silicon grease, Fromblin grease or mineral oil-based grease),<br />
depending upon the application in question. In the case of dry installation, particular attention<br />
must be paid to surface quality, the cleanliness of the mating surfaces as well as to the sealing<br />
material. The cross section diameter (thickness) of the o-rings can be 2 to 12 mm. 5 mm<br />
thicknesses are used for many joints, while rings with thicknesses of 8, 10 or even 12 mm<br />
are used only for very large seals. The o-rings should be seamlessly pressed. The parting line<br />
of the compression molding die is in the plane of the cross section diameter and is usually<br />
removed by abrasion.<br />
Generally speaking, o-rings are used as static seals. If dynamic stress is involved, precision<br />
o-rings that are manufactured especially for this purpose should be used. A discussion on<br />
how to dimension the grooves for this purpose will not be presented here. O-rings can also<br />
be used in axial or radial grooves, in addition to being employed in conjunction with centering<br />
rings or sealing washers. In most cases, o-rings are placed in grooves and pressed between<br />
flanges, with one flat flange and one grooved flange typically being used. The grooves must<br />
be carefully dimensioned in accordance with the following criteria:<br />
Compression, i.e. the ratio (width / height)-1, should be a maximum of 30 % for o-ring<br />
thicknesses (cross sections) of less than 3 mm, and 20 – 15 % for thicknesses of 5 – 10 mm<br />
The groove fill factor should be between 79 and 91 %<br />
The inside diameter of the groove should be equal to or only slightly larger than the<br />
inside diameter of the o-ring<br />
The outside diameter of the groove may be larger than the outside diameter of the o-ring<br />
in its compressed state<br />
www.pfeiffer-vacuum.net
Change language