Design Guide

In this section:

Metal Exclusion Seal Design Options

For hydraulic and pneumatic cylinders operating in contaminated environment's, metal exclusion seals play a critical role in maintaining integrity and performance by preventing unwanted contaminants from bonding to the rod, or being ingested into the cylinder. Depending on the frequency of cylinder motion and the extent of potential debris accumulation, different seal designs deliver specialized functionality. In the following, we explore three primary types of annular metal exclusion seals—cleavers, scrapers, and wipers—focusing on their respective geometries, material options (including various brass, bronze, and stainless steel alloys), and plating or coating treatments. By understanding the unique benefits of each seal type, design engineers can select the most effective solution for their application’s specific operating conditions.

Cleavers

Overview
Cleaver-type metal exclusion seals are specifically engineered for hydraulic and pneumatic cylinders that operate intermittently or infrequently. In these applications, contaminants have more time to adhere strongly to the rod surface. Cleavers deliver a pronounced, directional force that disrupts and removes resilient particulate or adhesive buildup. Their design excels at lifting bonded debris off the rod, and away from the ingression-zone.

Geometry
Cleavers typically feature a sharp-edged or pointed annular profile. This geometry focuses the contact force into a narrow band, creating a higher stress at the rod interface. By applying concentrated pressure, the cleaver ring cleanly “lifts and breaks” the contaminant bond. A properly selected cross section ensures optimal contact without overstressing either the seal or the rod.

Materials
Commonly used materials for cleaver seals include:.

• 93200 Bronze – Balances wear resistance with ductility, allowing for a longer service life in higher loads.

• 95400 Bronze – Higher strength and hardness than 93200 bronze; suitable for more abrasive environments. May require a higher rod surface specification.

• 30300 Stainless Steel – Resists corrosion and pitting, which is critical for applications exposed to moisture or corrosive fluids. Somewhat less Nobel than alternatives, therefore less likely to trigger a galvanic reaction in housing, retainers etc. Highest machinability of 300 Series Stainless Steels, for reduce potential to work-harden as a result of effect of motion across the rod.

Coatings
Cleaver seals may benefit from specialized coatings that reduce friction and increase durability. Proper coating helps maintain the seal’s sharp edge for a prolonged service interval, ensuring consistent cleaving action. PVD Coatings commonly applied to Cleaver seals include TiN (Gold), TiCN (Rose Gold), AITiN (Dark Grey), AICrN (Blue Grey) and in some exceptional cases, Zirconium Nitride (Pale Gold)

Scrapers

Overview
Scraper-type metal exclusion seals serve dual functions: they remove existing contaminants and deter new contaminant layers from forming. Their moderate, continuous contact with the rod surface helps prevent contaminants from bonding firmly in the first place. Scrapers are well-suited for cylinders operating with moderate to high frequency, where eliminating debris as it appears is paramount.

Geometry
Scrapers typically feature a perpendicular scraping face. This profile ensures consistent contact along the rod’s surface, effectively “sweeping” the debris away with each stroke. The geometry balances the need for enough contact pressure to dislodge debris without causing excessive wear on the rod or the seal.

Materials
Effective scraper seals can be constructed from:

• 36000 Brass – Easily formed into custom scraper edges and provides good corrosion resistance.

• 93200 Bronze – Offers notable durability, with enough ductility to maintain uniform contact over time.

• 95400 Bronze – Higher strength and hardness than 93200 bronze; suitable for more abrasive environments.

• 30300 Stainless Steel – Superior for corrosive or high-humidity conditions where rust prevention is critical.

Plating & Coating
Scraper surfaces often benefit from wear-resistant coatings and plating. These treatments reduce friction, control galvanic interactions between dissimilar metals, and extend seal service life. Selecting the right coating also improves the seal’s ability to shed oily or sticky residues. Some commonly used plating options include:

• Cadmium (required for all MS28776 certified metal rod seals, per MIL-R-5049)

• Zinc

• Nickel

• Copper

• Chrome (not advised for use with chrome rods)

Some commonly used coating option include:

• TiCN

• TAiN

• TiN

• Manganese Phosphate

• Aluminum Phosphate

• Black Oxide

• Nitriding

Wipers

Overview
Metal exclusion seals of the wiper type are designed for cylinders that cycle frequently, especially in high-speed or high-cycle applications. Wipers continuously remove trace amounts of debris, preventing contaminants from developing adhesive bonds with the rod surface, and minimizing particulate accumulation.

Since higher cycle rates will inevitably result in greater wear on the seal, wipers are designed to satisfy two competing goals: for durability to ensure longevity,  but at the same time, they must be sacrificial, to minimize the possibility of causing wear on the rod surface.  The use of coatings and plating can significantly improve lubricity, thus minimizing friction as a cause of wear, and in minimizing breakout or static friction to also conserve energy.

Geometry
Metal Wipers typically have one of two annular profiles. 

1. A singular, medium cross-section body either solid or containing a groove for a tensioning spring, with a perpendicular face. These wipers are designed for longevity and maximum service life in high frequency, high cycle-rate and speed applications.

2. One or more flexible angled scraping elements. These may resemble a section of a cone, or a compound radius profile, often contained within a metal cage.

The geometry is optimized to maintain close contact while reducing friction and drag on the rod. These shapes effectively “wipe” away contaminants in each stroke, ensuring consistent rod cleanliness and uninterrupted seal function, as well as interrupting the formation of bonds between contaminants and the rod surface.

Materials
Wipers are commonly made from:

• 36000 Brass – Holds a resilient wiping edge and provides good corrosion resistance. Brass is highly ductile, resulting in ideal rod-conformance.

• 93200 Bronze – Offers notable durability, with sufficient ductility to maintain uniform contact over time and in the face of wear.

Plating & Coating
A low-friction plating is often specified to extend lifespan, and may also include corrosion-inhibiting properties to help prevent galvanic reactions between the seal and rod, especially important in high-humidity or salt-laden environments. Additionally, coatings such as TiCN containing a lubricity additive e.g., PTFE or other polymer-based finishes) may be employed on wiper seals to minimize energy loss and heat buildup. 

Selecting the Right Metal Exclusion Seal

When specifying cleavers, scrapers, or wipers for a hydraulic or pneumatic cylinder application, consider:

1. Contaminant Type – Evaluate whether contaminants are likely to form strong adhesive bonds with the surface of the rod, or occur in moderate amounts that can be consistently swept away. 

2. Cycle Frequency – Intermittent use often requires the forceful action of a cleaver, while moderate use suits scrapers, and high-frequency operation typically benefits from wipers.

3. Material Properties – Match your choice of brass, bronze, or stainless steel to the conditions of load, temperature, and corrosiveness.

4. Plating & Coating Requirements – Select the appropriate surface treatments to mitigate galvanic corrosion, reduce friction, and extend service life.

Simplistically, a cleaver design may be required to break strong adhesive bonds such as might form on cylinders operating intermittently. A scraper may deliver the right combination of price and performance for medium bond strengths, typically on cylinders operating at more frequent cycle-rates; and a wiper design may provide the economy and resilience needed on continuously operating cylinders. By carefully balancing geometry, material selection, and surface treatment, you can ensure reliable contaminant exclusion that preserves rod integrity and prolongs the life of hydraulic and pneumatic cylinders.

Specifying Conventional Seals with Metal Exclusion Seals

The following sectional is informational, only. It contains observations intended only as input into the process of specifying the elements of  mechanical systems and is not intended as a definitive guide.

Exclusions Seals are most commonly located at the aperture, or outboard-most-position in the series of seals and packings designed into the gland, sleeve or stuffing box of a mechanical system. As such, they provide the 'first line of defense' against contaminant ingression. Many of the environments in which exclusion seals are employed are extreme in nature, and often this includes extremes in operating temperatures. Consideration should therefore be given to specifying appropriate materials for other seals in an assembly - additional elastomeric exclusion seals such as wipers, u-cups etc. rod seals, buffer and backup seals, piston and wear rings and orings, including static as well as dynamic seals. Some of the design considerations for such seals might include the following.

High Temperature Environments

For high-temperature applications, several materials may offer good performance, though the specific choice depends on the exact temperature range and other operating conditions. It's important to consider that prolonged exposure to high temperatures can cause irreversible changes in tensile strength and elongation and can lead to hardening and cracking. When selecting a material for a high-temperature application, it's crucial to ensure that the chosen compound can withstand the specific operating temperatures without degradation. Material properties can oftentimes be enhanced or diminished through compounding, so testing final formulations under actual service conditions is always recommended. The following is a brief assessment of some materials a design engineer may consider for specifying inboard wipers, rod seals, orings etc., intended for use in cylinders operating in a high temperature environments:

1. Silicone* - is known for its ability to resist high temperatures, with some grades being able to withstand temperatures from -103° to 437° F [-75° to 225° C]. It also offers low compression set.

2. Fluorocarbon (FKM), also known as fluoroelastomer, is another choice suitable for high-temperature applications. Standard FKM compounds can generally withstand temperatures ranging from -15° to +400° F [-26° to 204° C], and some specialty compounds can extend the low temperature limit down to -22° F [-30°C] for dynamic seals and about -40° F [-40°C] in static applications.

3. Perfluoroelastomer** (FFKM) is another option, offering resistance to high temperatures. One particular compound, SC1070, is viable up to 572° F [300° C], and SC1071 offers comparable high temperature resistance.

4. Hydrogenated Nitrile Butadiene Rubber (HNBR)** is suitable for applications involving heat and can withstand temperatures ranging from -25° to +300° F [-32° to 149° C].

5. Tetrafluoroethylene Propylene (TFE/P)** resists high temperatures, with short exposures up to 450° F [232° C], and maintains its chemical resistance in high temperatures.

6. Polyurethane can withstand temperatures up to 225° F [107° C] in its cast, thermoset form, and a new injection-molded PPDI-based compound can withstand temperatures up to 275° F [135° C].

7. Ethylene Propylene (EPDM) can be used in high temperatures as well.

*Silicone is noted as resisting high temperatures, with a range of -103° to 437°F, depending on grade.

**Fluorosilicone also resists high temperatures, though specific temperature ranges aren't given.

Low Temperature Environments

For very low temperature applications, several materials can be considered, each with its own specific temperature ranges and properties. Low temperatures can cause an elastomer to become harder and less flexible, which can lead to leakage if the seal does not maintain sufficient contact with the mating surface, especially under low pressure. In addition, the material may shrink and require a smaller gland. When selecting a material for low-temperature applications, it is crucial to consider both the service temperature and the system pressure. The chosen material must remain sufficiently soft and resilient to ensure an adequate seal.

As mentioned previously, material properties can be enhanced or diminished through compounding, so testing final formulations under actual service conditions is always recommended.

1. Silicone offers excellent low-temperature performance, with some grades able to withstand temperatures down to -103°F [-75°C]. Certain silicone compounds can operate effectively at very low temperatures, making them suitable for extreme cold environments. Some silicone compounds can perform well at temperatures as low as -103°F [-75°C].

2. Fluorosilicone also exhibits good resistance to low temperatures. Although specific minimum temperatures aren't always provided, they are generally suitable for low-temperature conditions. Some fluorosilicone grades are rated down to -90°F [-68°C].

3. Nitrile compounds with a low ACN (acrylonitrile) content can maintain flexibility down to -65°F [-54°C]. However, it's important to note that standard nitrile compounds may become hard and lose their sealing properties at very low temperatures.

4. Hydrogenated Nitrile Butadiene Rubber (HNBR) is another material with good low-temperature performance, though specific minimum temperatures are not always specified.

5. Polyurethane has good low-temperature flexibility, with some formulations performing down to -65°F [-54°C].

6. Ethylene Propylene (EPDM) is another option for low-temperature sealing. EPDM has a temperature range down to -65°F [-54°C].

Static vs Dynamic Seals

Specialty Fluorocarbons (FKM), such as Viton GFLT, can be used at low temperatures down to -40°F [-40°C] in static applications, with brittle points reaching -50°F [-46°C] through compounding. In addition, some specialty FKM compounds can extend the low temperature limit down to about -22° F [-30°C] for dynamic seals.