FAQ

A seal’s service life depends on many operating conditions, including the number of operating hours per day. Running a machine continuously for 24 hours a day can shorten seal life. The number of operating cycles, heat, and the cleanliness of the oil in the system also affect seal life. Because these conditions vary, there is no fixed service-life figure that applies to every seal.

Store seals in a dry, clean, dust-free place at approximately 20°C, away from direct light. These conditions help preserve their quality during storage before installation.

Each material has advantages and limitations depending on the application. Rubber is highly flexible and works well at low pressure. In high-pressure applications, however, polyurethane generally offers greater durability and better resistance to frictional wear. This is why polyurethane is widely used for seals, including low-pressure pneumatic seals. The material hardness must also be suitable for the intended application.

Standard polyurethane seals are generally injection-moulded as rings, although some manufacturers cast them from liquid polyurethane. The specially selected polyurethane stock imported by Specseal is mostly moulded into billets and then machined into finished seals. Some stock sizes must be cast because of size and weight limitations in injection moulding. Manufacturers control the quality of cast stock to make it comparable to injection-moulded material. The material-production methods are therefore similar; machined seals require the additional machining stage.

Many users misunderstand the relationship between seal height and groove size. Most seal profiles are designed with a height smaller than the groove, leaving room for the seal to expand after installation in the cylinder. A seal that is as high as the available groove space may not perform effectively. The appropriate dimensions depend on the seal profile and operating pressure.

When designing a housing for a seal, study the seal manufacturer’s manual carefully and use the groove dimensions specified there. Do not determine groove dimensions simply by measuring the seal and adding your own allowance, as this may lead to leakage.

Seal manufacturers take operating friction into account because it generates heat and contributes to seal wear. A thin oil film on the rod is therefore allowed during operation, but its quantity must remain within the specified limits. Some seal designs can prevent leakage without leaving an oil film on the rod; these require highly wear-resistant materials.

Replacing the complete seal set at the same time is generally more economical. Compared with the labour required to dismantle and reassemble the cylinder, together with the production downtime involved, the cost of a complete seal set is relatively small.

PTFE stands for polytetrafluoroethylene. Teflon is a trade name associated with this material, rather than a different base material. PTFE has low friction and good heat resistance but limited elasticity, so it is often used together with a more elastic material.

U-cup seals are commonly made from polyurethane. Some manufacturers fit a rubber energizer into the U-cup groove to improve elastic recovery. This helps the sealing lips respond more flexibly and improves low-pressure sealing performance compared with a conventional U-cup seal.

The rear groove is designed to collect the oil film carried out on the rod during operation, helping to keep the exposed rod dry. This arrangement requires an oil drain in the housing between the rod seal and the wiper. Without a drain, trapped oil can build up pressure and push the wiper out of its groove. A wiper without this rear groove allows the oil film to pass along the rod and does not require the same drainage arrangement. The choice depends on the intended application.

Wear ring materials have different operating characteristics. Bronze-filled PTFE is suitable for high-speed cylinders but has more limited load capacity. The original guidance recommends it for vertically mounted cylinders without side loading. For applications with high side loads, polyester-fabric or phenolic materials are more suitable. POM wear rings can also carry relatively high loads at an economical cost, but are supplied as finished rings in specified sizes. Select the material according to the operating conditions, including temperature.

Plain rubber seals are more flexible than fabric-reinforced rubber seals, so they seal oil effectively at low pressure. However, plain rubber alone cannot withstand higher working pressures as well. Alternating plain rubber V-rings with fabric-reinforced V-rings therefore suits applications where oil pressure changes frequently. An all-fabric-reinforced V-packing set offers greater strength and durability in applications with steady, consistent oil pressure.

A double-acting seal is designed to operate on both sides, with pressurized oil providing lubrication during operation. A single-acting seal is designed for pressure from one side only. Using a double-acting seal in a single-acting application can generate heat at the unused sealing lip, unnecessarily increasing cylinder temperature and shortening seal life. For the best results, choose a seal designed for the intended application.

A seal designed specifically for a rod should not be used as a piston seal. A rod seal has an inner lip designed to withstand movement and friction against the rod, while its outer lip sits in the housing groove and withstands pressure without sliding movement. Using a seal in an application for which it was not designed can shorten its service life.

Polyurethane seals are increasingly common in pneumatic cylinders. The polyurethane used for pneumatic seals is generally softer than that used for hydraulic seals. Rubber and polyurethane pneumatic seals typically have a hardness of approximately 70–75 Shore A. Polyurethane is increasingly used because it offers better wear resistance than rubber. When rubber is used for pneumatic seals, a self-lubricating compound helps reduce friction.

Cushion seals slow or stop a fast-moving piston before it strikes the cylinder end cap, helping prevent impact damage. They are therefore needed to provide smooth cushioning during pneumatic cylinder operation.

The seal at the end of a pneumatic cylinder may look like a simple dust wiper, but its inner lip also retains pressure. It therefore performs both pressure-sealing and dust-wiping functions in a single component.

Pneumatic systems generally do not have a complete lubrication system, so materials with good wear resistance and self-lubricating properties are essential. Seals and guide rings should use materials containing lubricating additives, such as carbon-filled PTFE or PE. This helps extend their service life.

Hydraulic and pneumatic seals may look similar, but their materials differ, particularly in hardness. Pneumatic seals are generally softer and may contain lubricating additives. Using a hydraulic seal in a pneumatic application can cause faster wear and a shorter service life.

O-rings were often used as piston and rod seals because they were readily available, inexpensive and easy to replace. However, friction during reciprocating movement can twist an O-ring within its groove, causing tearing and damage. Seals designed for dynamic applications are therefore increasingly used instead.

Machinery imported into Thailand comes from many countries. Knowing the country or region of manufacture can help identify the O-ring standard used, although availability and cost also influence the machine manufacturer’s choice. Common standards include the American Standard AS Series and the Japanese Standard JIS Series. Consult Specseal’s O-Ring Handbook for dimensions.

O-rings with a hardness of 70 Shore A are common in general applications. For hydraulic systems operating above 100 bar, this guidance recommends 90 Shore A O-rings because they withstand higher pressure without requiring a smaller clearance between components, which can be difficult to maintain within machining tolerances.

Rubber materials may share the same name while differing in manufacturing methods and compound formulations. These differences affect elastic recovery after compression. An O-ring with good recovery can regain much of its round cross-section after removal, whereas rubber with poor recovery may retain the shape of its housing groove.

Fluorocarbon, also known as FPM, is a generic material name, while Viton is a trade name. These materials have similar properties. The original source identifies Viton as a DuPont trade name and states that products bearing the name must use the corresponding branded material.

First measure the actual operating temperature with an appropriate instrument, such as a thermometer in the oil. Do not estimate it by touch or personal impression. Every rubber material has advantages and limitations. The source describes fluorocarbon or Viton as withstanding temperatures up to 230°C but having comparatively limited elastic recovery. Using this material where its heat resistance is unnecessary may add cost without providing a useful benefit.

An O-ring groove generally has a rectangular cross-section, leaving room for the O-ring to deform when compressed between mating components. Consult Specseal’s O-Ring Handbook for the correct relationship between groove dimensions and O-ring cross-section, as the dimensions depend on the application.

Both types retain oil around rotating shafts. A double-lip oil seal has an additional outer lip to keep dust out, making it suitable for dusty conditions such as outdoor machinery. The dust lip requires regular lubrication; otherwise, direct friction against the shaft can generate heat and wear both the seal and shaft. Single-lip oil seals suit similar oil-retention duties in sheltered, dust-free locations or inside machinery.

A V-ring is generally fitted outside an oil seal or bearing to keep dust and water spray out of the system. It grips the shaft and rotates with it. Its lip runs against a metal face to exclude dust and water without causing wear on the shaft.

Commonly available oil seals can withstand a pressure of 0.5 bar, which is sufficient for general rotating-shaft applications. Special designs are available for higher pressures. Clearly explain the operating conditions to the supplier before ordering.

Continuous contact between the sealing lip and shaft can wear a circular groove in the shaft. An initial remedy may be to use a thinner oil seal so that the lip contacts an unworn part of the shaft. After the contact position has been moved several times, shaft repair may be necessary and can take several days. A more economical alternative is to fit a ready-made stainless-steel repair sleeve over the worn area. Contact Specseal for advice on suitable sleeves; installation can take only a few minutes.

Some users fit double-lip oil seals as wipers in hydraulic cylinders. However, oil seals are not designed for reciprocating motion, so their durability and performance are lower than those of wipers specifically designed for that application.

Fluorocarbon oil seals are commonly selected for heat resistance or chemical-fluid applications. Designs with a fully rubber-covered inner surface can serve both purposes. Designs with exposed metal on the inner surface may be used in hot applications, but the source advises against using them with chemical fluids because the chemicals may react with the exposed metal.

The spring around the sealing lip increases contact force against the shaft to retain liquids that can escape easily along it. Springless oil seals are used where less contact force is needed, particularly for retaining grease.