Fasteners and Sealing
Specifying O-rings for increased life and reliability
Years of experience have been captured in a plethora of O-rings design standards and a wealth of materials is readily available to suit most application requirements. Mick Holland describes how to take advantage of these standards to mitigate risk when specifying O-rings into your fluid sealing system.
W
ithin the O-ring industry, size references are often referred to a ‘dash size’. These numbers specify the size and tolerances applied to the humble toroidal O-ring. Briefly glancing at size reference tables
reveals a myriad of apparently random O-ring dimensions that do not appear to conform to any logical pattern. However, there is reason behind these sizes and a basic understanding can help a design engineer specify a robust solution that minimises through the selection of readily available hardware and seal components and manufacturing time, by minimising the machining requirements.
Hydraulic actuator
Let us imagine that we are a young design engineer tasked with designing a hydraulic actuator with a known system pressure and minimum force requirement. From these basic criteria we can determine the effective area required of
the piston and hence calculate the minimum cylinder bore diameter.
Although the engineer could model the actuator with this
exact diameter, more experienced engineers may ask: “What size of tube and bar does my metal supplier stock?” The engineer may then select the next largest stocked size
of metal diameter - for example, selecting a nominal cylinder bore diameter of 2 inches.
Specifying tolerances
When the engineer buys the tube with a 2 inch inner diameter and a bar of 2 inch outer diameter, the bar will never insert into the tube. This is because the metalwork is sold with a little extra material on to allow the engineer to specify the tolerances (fit) that is required. In our example the engineer has selected a nominal
bore of 2 inches; however the actual dimensions applied to the bore may be 1.992-in/1.994-in and those of the piston
Fig. 1. ERIKS has brought all of these standards together into one easy-to-use online tool. Designers can rapidly identify and detail suitable standard grooves from driving metalwork dimensions.
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