A seal is any device that prevents the passage of a fluid (gas or liquid) or fine particles. Because it plays such an important role, it’s useful to know the various types of seals commonly used. Whether static, oscillating, rotary, or something different, seal knowledge is important to know for any application.
Static Seals
These components are placed between two surfaces that don’t move relative to each other, making them stationary. They’re categorized as either axial or radial, depending on the direction in which the o-ring’s cross section (the ring’s thickness) is squeezed. Axial seals are squeezed at the top and bottom of the o-ring’s cross section and lack an extrusion gap (the space between parts that fit and contact each other, which lets seal material be forced into the gap under pressure). No extrusion gap means fewer design steps and easier tolerance control.
Static radial seals are different. They’re squeezed between the o-ring’s inner and outer surfaces and are often used in setups where the o-ring seals the outside of a component or plugs a seal inside an opening; this is known as cap-and-plug-type applications. Because radial seals have an extrusion gap, they must be designed with precision to prevent o-ring failure under pressure. This is because radial seals must account for tolerances on the outer and inner o-ring diameters. Both axial and radial static seals are frequently made with nitrile (NBR), fluoroelastomer (FKM), ethylene propylene diene monomer (EPDM), or silicone.
Reciprocating Seals
A dynamic seal (meaning motion is present between a sealing element and a hardware component), reciprocating seals are used in applications involving a rod or cylinder and a moving piston. They’re critical for maintaining pressure and preventing entry of environmental contaminants like dust, dirt, and water. These seals typically use materials that can withstand high wear, pressure cycling, and friction: as such, polytetrafluoroethylene (PTFE), polyurethane (PU), NBR, and FKM are frequent choices.
Oscillating Seals
This seal is also dynamic and designed for components that rotate back and forth or reciprocate rather than rotating in a single direction. It aims to maintain a tight barrier between the parts in motion. Medium-hard and hard o-rings are typically used to reduce the possibility of the seal’s shaft (the rotating rod that passes through the seal and against which the seal forms a leak-tight contact) twisting. Too hard of an o-ring can increase wear and reduce sealing effectiveness, so the specific hardness of the o-ring should be carefully considered. Oscillating seals are often made with durable, low-friction materials like NBR, FKM, or PTFE to handle high wear.
Rotary Seals
These components stop external fluids from leaking between a rotating shaft and non-moving parts. Seals maintain lubrication (like grease or oil) and keep it inside the application to ensure smooth, continuous operation. There are many factors to consider when designing rotary seals: temperature limits, since their use is not recommended for operations below -40°F and above 250°F; frictional heat buildup, which requires o-rings to be made with compounds that are highly resistant to heat and have minimal friction-generating properties; no stretch over the shaft; and a minimal amount of squeeze. Rotary seals are usually found in pumps and motors and are made with PTFE, NBR, FKM, and silicone.
Pneumatic Seals
These low-friction components are used in rotary or reciprocating motion to prevent gas or air leakage and maintain proper system pressure to ensure efficient operation. Pneumatic seals also protect machine parts from wear and tear that can come from dust or moisture. They are engineered and optimized for low-pressure, high-speed air and for reducing friction in moving parts. Pneumatic seals are used in valves and cylinders. Materials that provide good wear and chemical resistance, as well as handling high temperatures, are used for these seals; NBR, FKM, PU, and PTFE are a few examples.
Seat/Valve Seals
Lastly, these stop leaks by sealing gaps, forming an extremely tight barrier when one part presses against another to stop gases and fluids from escaping. Seat seals are usually located in valves between the movable part of the valve and the valve body. Closing a valve causes that movable part to press against the seat seal, forming a barrier that stops gas or fluid from passing. This makes sure that these substances don’t flow when they’re not supposed to. These seals are typically made of EPDM, silicone, and PTFE.
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