Specifying O-rings for increased life and reliability

Paul Boughton

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.

Within 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.[Page Break]

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.[Page Break]

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 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.

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. 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 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.[Page Break]

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. [Page Break]

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 seals, O-rings, static seals, hydraulic and pneumatic systems, bellows, bespoke sealing products and vibration control technology.l

Mick Holland is General Manager of ERIKS Sealing Technology, Halesowen, West Midlands, UK. www.eriks.co.uk

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