Gland Design
A seal assembly is made up of an O-ring and a gland, and prevents the leaking of fluids and gases. The gland supports the O-ring in its place, and provides the necessary deformation. An O-ring must be squeezed and stretched for proper functionality. Generally, an O-ring design should maintain a 10-40% squeeze for static applications, and no more than 30% for dynamic. Additionally, the O-ring volume should not exceed 90% of the minimum gland void to allow the O-ring to deform. There are two main O-ring gland application types:
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Static – There is little or no relative movement between the gland and the O-ring. Static seals are categorized as axial and radial, according to the direction of the squeeze that is applied to the O-ring.
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Dynamic – There is relative movement between the gland and the O-ring, which causes friction.
Consideration must be given to the following factors when designing the appropriate gland for your application:
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Squeeze – There are two ways that O-ring squeeze occurs, Axial and Radial. Axial squeeze is when the squeeze is applied to the top and bottom of the O-ring. Radial squeeze is when the squeeze is applied on the inner and outer surfaces of the O-ring.
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Stretch – In order for the O-ring to fit well, the O-ring must be stretched circumferentially. The standard range is for the O-ring to be stretched between 1% to 5%.
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Reciprocal Motion – Relative backward and forward motion between the sealing surfaces.
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Rotary Motion – Circular motion between the sealing surfaces.
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Seal Swell – Liquid additive used to cause elastomer parts to swell to stop leaking.
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Lubrication – Lubricating the O-ring is done to reduce the effects of friction. An internal lubricant compound can be added to the elastomer before molding. Alternatively, a lubricant can be applied to the external surface of the part to help with installation. Another form of lubrication is done with the application of an external compound, such as Teflon™ TM coating.
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Thermal cycling – The amount of alternating heating and cooling of a material in the application.
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Materials Elastomers – Choosing the proper elastomer to be used is important, and all of the elastomer properties must be considered (i.e. compression set, elongation, hardness, etc.).
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Chemical Compatibility – The chemical compatibility of the O-ring material can be crucial in the design, and can differ for static and dynamic seals.
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Temperature Limits – Different types of seal assemblies work better than others depending on the application temperature limits. For instance, at room temperature rotary shaft seals effectively seal longer than at extreme high or low temperatures.
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Friction – Friction and heat are inevitable with rotary seal applications, and so it is suggested that O-rings be composed of compounds featuring maximum heat resistance and minimum friction generating properties. Friction can also cause hydraulic pressure to rise to high levels causing the breakdown of the O-ring.
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System Pressure – The levels of pressure distributed within the cross-section of the O-ring can cause distortion. The elastomer material, the use of backup rings, and the diametrical clearance gap can effect the ability of the O-ring to stand up to system pressure.
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Surface Finish – Surface finish is the nature of a surface defined by its characteristics of lay, surface roughness, and waviness. It is one of the important factors that control friction.
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Backup Rings – The number of backup rings that will be used in the application impacts the rest of the design.
Gland Design Types
Dynamic Radial O-ring Gland Design
In dynamic radial applications the O-ring is squeezed radially, and experiences intermittent or continuous reciprocating motion.
Dynamic radial O-rings are subjected to friction between the sealing surfaces. O-rings and seals in dynamic radial applications should consider:
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Temperature
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Motion or vibration
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Squeeze
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Stretch
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Friction
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Surface finish
Static Radial O-ring Gland Design
In static radial applications the O-ring is squeezed between its inside diameter (ID) and outside diameter (OD), and experiences no relative motion between sealing surfaces of the gland. Static radial O-rings and seals should consider:
- Temperature
- System pressure
- Gas or liquid contact
- Backup ring usage
- Surface finish
Dynamic Reciprocating O-ring Gland Design
In dynamic reciprocating applications there is a reciprocating motion along the shaft, between the inner and outer sealing surfaces of the gland. Dynamic reciprocating O-rings and seals should consider:
- Temperature
- Motion or vibration
- Squeeze
- Stretch
- Friction
- Lubrication
Static Axial O-ring Gland Design – Internal or External Pressure
In static axial applications with internal or external pressure the O-ring is squeezed on the top and bottom of the cross-section, like a gasket. Static axial O-rings, such as face seals, should consider:
- Temperature
- Motion or vibration
- Squeeze
- Stretch
- Friction
- System pressure
- Surface finish
When there is internal or external pressure on the O-ring, then it should be seated against the low pressure side of the groove. This will help to minimize movement and friction.
Dynamic Rotary O-ring Gland Design
In dynamic rotary applications the O-ring becomes a seal when a turning shaft protrudes through the ID. Dynamic rotary O-rings should consider:
- Surface finish
- Shaft speed
- System pressure
Dovetail O-ring Gland Design
In dovetail applications the O-ring is squeezed primarily in an axial direction, with a valve exerting force on the top and bottom of the O-ring sealing surfaces. Dovetail glands are used in static or slow moving dynamic applications, and require strict tolerances that are difficult to control.
Static Crush O-ring Gland Design
Crush seal applications are so named because the O-ring is completely restricted and pressure deformed, or crushed, within a 45° triangular gland. Static crush seals are easy and inexpensive to machine.