Robotic Joint Seals: IP67 Material Selection Guide

A robot joint has both moving and stationary seals. The rotating shaft needs a seal that controls friction and wear, while the housing joints mainly need to exclude water and dust. For an IP67 design, start by identifying the type of movement at each sealing location. Check the material against the lubricant and operating temperature, then evaluate the seal over its expected service life. In most applications:

• NBR, HNBR, FKM, and PTFE are the main candidates for dynamic robot joint seals.
• Silicone and FVMQ are better suited to static housing and environmental seals.
• Polyurethane is particularly useful for wipers and abrasion-resistant contamination barriers.

Understanding these different roles is more useful than searching for a single “best robot seal material.”

What Does IP67 Mean for a Robot Joint?

IP67 is an enclosure protection level defined by IEC 60529.

For a robot joint, it means the complete joint assembly must provide:

• IP6X: protection against dust ingress
• IPX7: protection against temporary water immersion

IP67 is therefore not a property of an individual O-ring, gasket, or rubber compound.

A silicone gasket cannot be described as IP67 simply because silicone is water resistant. The complete sealing system must pass the required ingress test after considering housing geometry, gasket compression, shaft sealing, tolerances, assembly, and other potential leakage paths.

This distinction is especially important in robot joints because several sealing interfaces often exist inside the same axis.

Where Does Water Enter an IP67 Robot Joint?

A typical robot joint may contain a servo motor, encoder, bearings, gearbox, output shaft, cables, connectors, and several housing sections.

Common leakage paths include:

• rotating output shafts
• housing parting surfaces
• motor or encoder covers
• cable exits
• electrical connectors
• fastener holes
• damaged or uneven gasket interfaces

The sealing strategy should therefore divide the joint into dynamic and static sealing areas.

A simplified robot joint sealing system may look like:

External environment → dust barrier or wiper → dynamic shaft seal → bearing / gearbox

while another part of the same joint may use:

Housing → silicone gasket → motor / encoder / electronics enclosure

These two interfaces have very different material requirements.

Types of Seals in Robot Joints

The first step in robot joint seal selection is determining whether relative movement occurs across the sealing surface.

Dynamic robot joint seals

A dynamic seal contacts a shaft or surface that rotates or oscillates.

Typical requirements include:

• low friction
• good wear resistance
• lubricant compatibility
• resistance to shaft runout
• stable sealing force
• long cycle life

NBR, HNBR, FKM, and PTFE are commonly considered for these applications.

Static robot joint seals

Static seals are compressed between two surfaces that do not slide continuously against each other.

Examples include:

• robot joint housing gaskets
• motor cover gaskets
• encoder housing seals
• connector seals
• cable seals

Here, abrasion resistance is less important. Compression set, environmental aging, temperature flexibility, and sealing consistency become more important. Silicone and FVMQ are therefore much more attractive in these locations.

NBR for General Robot Joint Shaft Sealing

NBR is a common starting material for radial shaft seals used around lubricated rotating components.

Its main advantages are:

good resistance to oils and greases, reasonable wear resistance, useful mechanical strength, and relatively low cost.

Typical sealing-grade NBR materials may operate around:

-30°C to +100°C

depending on the compound and operating conditions.

For robot joints with moderate temperatures and conventional gearbox grease, NBR can provide a good balance between cost and sealing performance.

However, NBR becomes less attractive when joint temperatures rise significantly or when longer resistance to heat, ozone, and aging is required.

In these conditions, HNBR or FKM may be more appropriate.

HNBR for Higher-Duty Robot Joint Seals

HNBR is closely related to NBR but provides improved resistance to heat, oxidation, ozone, and long-term aging.

Typical sealing-grade HNBR formulations may operate approximately from:

-40°C to +150°C.

This makes HNBR particularly relevant to compact robot joints where internal seal temperatures can be considerably higher than ambient temperature.

Heat inside a robot actuator may come from servo motors, bearings, gear friction, lubricant shear, and the sealing interface itself.

HNBR can therefore be a useful choice when standard NBR does not provide enough thermal margin but the application does not require the full chemical and high-temperature capability of FKM.

For many industrial robot joints, HNBR represents a practical step up from NBR.

FKM for High-Temperature Robot Joint Sealing

FKM is widely used in sealing systems where temperature and lubricant resistance become more demanding.

Typical FKM sealing compounds may operate over a range around:

-20°C to +200°C,

although the actual temperature range varies considerably by formulation.

FKM also offers strong resistance to many oils, lubricants, hydrocarbons, and industrial chemicals.

For robot joints, FKM becomes particularly useful when the application combines:

• elevated internal temperatures
• long operating cycles
• gearbox lubricant exposure
• demanding chemical environments
• long maintenance intervals

However, FKM is not automatically better than NBR or HNBR. It generally costs more, and some FKM compounds have weaker low-temperature flexibility.

The correct choice therefore depends on the actual robot joint operating temperature rather than simply selecting the material with the highest temperature rating.

PTFE for Low-Friction Robot Joint Seals

PTFE is especially relevant when seal friction becomes part of the robot’s motion-control problem.

Unlike NBR, HNBR, and FKM, PTFE is not a conventional elastic rubber.

Its advantages include:

• very low friction
• low stick-slip behavior
• excellent chemical resistance
• wide temperature capability
• good wear performance with suitable compounds

These properties are particularly valuable in collaborative robots, precision manipulators, force-controlled robots, and low-torque actuators.

Why Seal Friction Matters in Robot Joints

Seal friction creates resistance against joint motion.

The approximate seal friction torque can be expressed as:

Tseal ≈ μ × FN × r

where:

μ = friction coefficient
FN = sealing contact force
r = effective seal radius

This relationship is important because robot joint seals often operate at relatively large diameters.

As joint radius increases, the same friction force produces greater resisting torque.

Excessive dynamic seal friction may affect:

• breakaway torque
• low-speed motion
• servo efficiency
• torque sensing
• positioning consistency

A PTFE-based robot joint seal can therefore be attractive where low friction is more important than using a conventional elastomer shaft seal.

PTFE Seals Often Need an Energizer

PTFE has much lower elastic recovery than rubber.

For this reason, many PTFE sealing structures use an elastomer or spring to maintain contact pressure.

A typical design may use:

PTFE sealing ring + NBR, HNBR, or FKM energizer

The PTFE provides the low-friction sliding surface while the energizer maintains sealing pressure.

This means PTFE and elastomers are often complementary materials rather than direct competitors.

Silicone in Robot Joints

Silicone is widely used in robots, but it is generally not the first material to consider for a continuously sliding radial shaft seal.

The main reason is wear resistance.

Silicone provides excellent flexibility and environmental stability but generally has lower abrasion and tear resistance than materials commonly used for dynamic shaft sealing.

Repeated contact with a rotating shaft can therefore produce excessive wear, especially when dust reaches the sealing interface.

This does not mean silicone is unsuitable for robot joint sealing. It means silicone belongs in a different part of the sealing system.

Silicone is particularly suitable for static environmental seals such as:

• robot joint housing gaskets
• servo motor cover gaskets
• encoder enclosure seals
• connector gaskets
• cable-entry seals
• protective boots
• flexible dust covers
• custom molded sealing components

These applications allow silicone to use its strongest properties without being limited by continuous sliding wear.

Important advantages include:

• good compression sealing
• excellent ozone resistance
• excellent weather resistance
• broad temperature flexibility
• good electrical insulation
• complex molded geometries

For robots that operate outdoors or experience large temperature changes, these characteristics can be particularly valuable.

Silicone Gasket Compression for IP67 Robot Joints

Static IP67 sealing depends heavily on gasket compression.

For a simple gasket:

Compression (%) = (t0 − t1) / t0 × 100

where:

t0 = original gasket thickness
t1 = installed gasket thickness

For example, if a 3.0 mm gasket is compressed to 2.4 mm:

Compression = (3.0 − 2.4) / 3.0 × 100 = 20%

The correct compression should be determined according to:

• silicone hardness
• gasket cross-section
• groove geometry
• housing stiffness
• manufacturing tolerances
• temperature
• expected compression set

More compression is not necessarily better. Excessive gasket compression can cause permanent deformation, excessive assembly stress, housing distortion, and reduced long-term sealing reliability.

The objective is to maintain sufficient contact pressure around the complete sealing perimeter.

When FVMQ Is Better Than Standard Silicone

Some robot joint gaskets are located close to gearboxes, bearings, or lubricated mechanical components.

In these locations, standard silicone may not provide enough resistance to oils or hydrocarbon-based fluids.

FVMQ, or fluorosilicone, can provide a better balance.

Compared with standard VMQ silicone, FVMQ offers improved resistance to many:

• mineral oils
• hydrocarbons
• fuels
• lubricants

while retaining useful silicone characteristics such as low-temperature flexibility.

FVMQ can therefore be considered for specialized static robot joint seals where environmental sealing and lubricant exposure occur at the same interface.

However, improved oil resistance does not automatically make FVMQ the preferred material for continuous rotary shaft sealing.

Dynamic wear requirements still need to be evaluated separately.

Robot Joint Seal Material Comparison

The table should be used as an initial material-selection guide rather than a substitute for application testing.

Seal geometry and operating conditions can change material performance significantly.

Robot Joint Motion Type Affects Seal Selection

Robot joints do not all move in the same way. An industrial robot arm commonly performs oscillating or bidirectional motion.

A mobile robot wheel may rotate continuously.

A collaborative robot may move relatively slowly but require very low friction and accurate torque measurement.

These different movement patterns produce different sealing requirements.

For a continuously rotating shaft, surface speed can be estimated using:

v = πdn / 60

where:

v = shaft surface speed in m/s
d = shaft diameter in meters
n = rotational speed in rpm

For example, an 80 mm shaft operating at 300 rpm has a surface speed of approximately:

1.26 m/s.

For oscillating robot joints, however, rpm alone does not adequately describe the application.

Engineers should also consider:

• joint angle
• acceleration
• reversal frequency
• duty cycle
• accumulated sliding distance
• lubrication condition

A seal repeatedly oscillating ±30° may behave very differently from a continuously rotating shaft even if the peak surface speed is similar.

Shaft Runout Can Cause Robot Joint Seal Leakage

Dynamic seal performance depends on maintaining contact with the shaft.

Actual seal interference can change because of:

shaft runout

• bearing clearance
• housing concentricity
• seal groove tolerance
• seal dimensional variation.

If the combined radial movement exceeds the seal’s ability to follow the shaft, one side of the sealing lip may lose sufficient contact pressure.

This creates a potential ingress path.

For IP67 robot joints, runout and concentricity should therefore be treated as sealing parameters rather than only general mechanical tolerances.

The seal should be selected together with the shaft, bearings, housing, and groove tolerances.

Shaft Surface Finish Is Part of the Robot Joint Seal

The mating shaft directly affects dynamic seal wear and leakage.

A rough shaft can accelerate seal wear.

Deep scratches can create direct leakage channels.

Directional machining marks may transport lubricant or water through the sealing interface.

For this reason, a dynamic robot joint seal specification should consider:

• shaft roughness
• machining lead
• shaft hardness
• coating
• runout
• concentricity

The required surface condition may differ between elastomer radial shaft seals and PTFE sealing systems.

Selecting a high-performance seal material without controlling the mating surface can still result in premature leakage.

Do Not Select Robot Joint Seals Based Only on Ambient Temperature

Robot joint seal temperature may be considerably higher than surrounding air temperature.

Heat can be generated by:

• servo motors
• gear friction
• bearings
• lubricant shear
• seal friction

For example, a robot operating in a 40°C environment may have considerably higher temperatures inside a compact joint actuator.

Material selection should therefore use the estimated local seal temperature rather than ambient temperature alone.

This can directly affect whether NBR, HNBR, or FKM is the better choice.

How to Specify a Robot Joint Seal

A seal supplier needs more information than:

“IP67 robot joint seal required.”

A useful robot joint sealing specification should include at least:

Once these conditions are known, the seal material becomes much easier to narrow down.

For example:

HNBR or FKM may be considered for the primary radial shaft seal.

PTFE may be evaluated when friction torque is critical.

PU may be added as an external wiper in abrasive environments.

Silicone or FVMQ may be used for the stationary robot joint housing gasket.

This is a more reliable design approach than selecting one polymer for the entire robot joint.

Validate IP67 After Robot Joint Durability Testing

Passing an IP67 test on a newly assembled robot joint does not necessarily prove long-term sealing reliability.

Dynamic seals change during operation because of:

wear, frictional heat, shaft movement, contamination, lubricant migration, and material aging.

A more useful validation sequence is:

Initial sealing test → robot joint cycle testing → seal and shaft inspection → repeat IP67 testing

Engineers may also monitor:

• seal friction torque
• breakaway torque
• local temperature
• leakage
• seal wear
• shaft wear

Outdoor joints also need evaluation after environmental exposure. Moisture and temperature changes can affect the assembled seal, while sunlight and ozone may age exposed rubber. The goal is to retain the required ingress protection throughout service, not just pass IP67 testing when new.