Fasteners and Sealing
1.988”/1.999.”. A logical nominal groove depth is then applied to the metalwork, eg 1
/8.”-ring size Once the nominal groove dimensions have been
determined a number of criteria are applied to determine the correct size of O-ring: Compression (Squeeze) – An O-ring of greater cross
section than the radial gap of the groove is installed. This utilises the elastomeric properties of the O-Ring to provide an initial reaction (sealing force). The amount of compression applied varies, subject to the type of application, eg static or dynamic.
Standard gland fills are therefore typically below 85 per cent to allow for differential rates of thermal expansion and swelling of the O-ring as media goes into solution within the elastomer. Correct application of these basic principles is critical to
maintain the integrity of the seal and maximise life. These basic principles are applied to various scenarios where O-rings may be used, eg rod, piston or face seal grooves and a number of standards have evolved that assist the design engineer to rapidly specify seals into standard metalwork dimensions (Fig. 3).
British standards include BS1806 and BS4518, designed
for imperial and metric nominal sizes respectively. Both of these standards offer engineers the freedom to
apply their own tolerances to grooves; however BS1806 does apply low levels of squeeze to small cross section O-Rings and therefore does not always offer the best solution for piston and rod seals. Both of these standards offer good solutions for Face seal grooves. The Society of Automotive Engineers in the US has two
standards for Piston and Rod seals, the AS5857 and AS4716 for static and dynamic seals. These standards are the most widely used of all and offer
comprehensive robust hardware specifications matched to AS568 O-rings. The International Standards Organisation ISO3601
Fig. 2. Correct application of basic principles is critical to maintain the integrity of the seal and maximise life.
Stretch - Although on metal work, inner diameters generally have plus tolerance and outer diameters a minus tolerance; as O-rings are used in numerous types of location then they have a plus/minus tolerance. Therefore in piston applications a small permanent stretch is applied and in rod applications a small permanent interference is applied on the outer diameter. This ensures a snap fit to assist with assembly. Gland fill - Although initial compression provides an
initial low pressure seal pressure energisation of the O-ring is required to maintain the seal at high pressures. To ensure this the groove width must be sufficient to allow pressure into the groove, ensuring that the centre of pressure energises the seal.
PISTON SEAL
standard also defines grooves in a comprehensive manner. Fortunately, the AS568, BS1806 and ISO3601 dash sizes for O-rings correlate, allowing engineers to specify or purchase; for example, to buy a -224 O-ring and know that they will receive a 1.734-in x 0.139-in O-ring for their 2-in cylinder bore application.
Online tool
To save design engineers time, ERIKS has brought all of these standards together into one easy-to-use online tool. Unlike other tools that allow the engineer to design and analyse grooves with a free hand, the simple-to-use tool freely available (
http://oring-groove-wizard.eriks.co.uk), rapidly identifies and details suitable standard grooves from driving metalwork dimensions. This tool maximises the chances of specifying a standard
seal correctly, optimising the supply chain and manufacturing benefits of the engineers design. Once the correct dimensional solution is achieved then
the engineer should identify the correct material based upon thermo-chemical, mechanical and legislative requirements/
Distributor partnership Meanwhile, Simrit and ERIKS have formalised their
partnership in the UK by signing an approved distributor agreement. Simrit supplies companies in all sectors with radial shaft
ROD SEAL FACE SEAL
Fig. 3. These basic principles are applied to various scenarios where O-rings may be used, eg rod, piston or face seal grooves and a number of standards have evolved that assist the design engineer to rapidly specify seals into standard metalwork dimensions.
38
www.engineerlive.com
seals, O-rings, static seals, hydraulic and pneumatic systems, bellows, bespoke sealing products and vibration control technology. l
Enter 38 or ✔ at
www.engineerlive.com/ede
Mick Holland is General Manager of ERIKS Sealing Technology, Halesowen, West Midlands, UK.
www.eriks.co.uk
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