20 Sealing
Choosing the right O-rings Selecting the correct O-ring for an application can be critical, but to achieve this many factors need to be considered. To help users make the right choice, fluid sealing product specialist, M Barnwell Services, looks at key aspects of an O-ring’s design.
E
lastomers are different compared to any other material used by engineers. For example, plastic or metal components are most likely to be failing when they are visibly deformed. When it comes to an O-ring, for it to function correctly, it must be deformed. An O-ring that is not stretched and squeezed is the wrong O-ring for an application. An O-ring is a doughnut-shaped article; a closed circle usually made of elastomeric rubber. The O-ring seal’s purpose is to prevent an unwanted leakage or loss of gas or fluid, and it is the most popular form of a seal. The opposite faces of an O-ring are compressed between the faces of the gland. The consequent zero clearance within the gland guarantees an efficient seal, preventing the fluid or gas from flowing through the gland. O-RING MANUFACTURING There are two standard processes for the manufacture of rubber O-rings. In the compression moulding process, the material is manually placed into a mould before the upper and lower parts of the mould are closed. This process is particularly suitable for larger dimensions and smaller quantities due to it being very time-consuming. With the injection moulding process, the material is automatically injected into a mould (otherwise known as a tool), which has various O-ring cavities. Injection moulding is a process suitable for small dimensions and large quantities. ELASTOMERS Elastomers are polymers, which are described as large molecules (or macromolecules) formed of Quarter 3 2021
an O-ring retains its shape; if it were stretched too much, it would cause the bonds to break.
many sub units. In Greek, polymer means ‘many parts (poly, ‘many’ + mer, ‘parts’). The polymers are linked to each other to compose a network with cross-connections. Consequently, they demonstrate the typical elastic and rubbery
A properly designed sealing system includes some level of initial O-ring squeeze. features. The raw product is rubber and is either acquired from plants that produce the substance or developed synthetically. When applied to an elastomer, vulcanisation leads to the formation of chemical crosslinks of polymer chains. This is to ensure that after the force is removed, the molecules return to their original shape. This is how
HOW AN O-RING WORKS A properly designed sealing system includes some level of initial O-ring squeeze. Only the elasticity of the squeezed O-ring ensures the seal at atmospheric pressure. Nevertheless, the O-ring is pressed against the low-pressure side of the gland as system stress activates the seal. The O-ring fills the diametrical clearance and prevents any additional leakage. Pressure and many other considerations determine the efficiency of a seal.
O-Ring ID =
Groove Diameter % of stretch wanted +1 (Between 1% and 5%)
Formula for calculating the optimal O-ring internal diameter (ID)
www.bpma.org.uk
> 22
