Rotary shaft seals are used in a host of applications, including many that involve harsh environments or strict compliance with FDA standards. But when should PTFE be used?
Industries Where Rotary Shaft Seals Are Used
A good place to start would be looking at some of the industries that depend on the reliable performance of PTFE rotary shaft seals:
- Oil and gas
- Transportation (including motorsports)
- Aerospace and defense
- Medical devices
- Food and dairy
- Fluid and air handling
- Renewable energy
- Chemical processing
Certain characteristics become apparent from this list, such as exposure to corrosive and aggressive chemicals, low friction, reliability, extreme temperatures, and high purity.
When to Consider a PTFE Rotary Shaft Seal
There are certain circumstances under which PTFE is the material of choice for a rotary shaft seal:
- When there is aggressive media involved
- When low friction or dry running is needed
- When applications involve high speeds
- When thermal stability is critical
- When FDA/USDA compliance is necessary
- When high temperatures are involved
When there is aggressive media involved
PTFE is the most chemically compatible seal lip material on the market, with the main exceptions being rare fluorinated compounds and certain alkali metals. In addition, some halogenated and organic solvents can be absorbed by PTFE and cause temporary (and minor) changes in dimension. On the other hand, it is compatible with chemicals such as acetone, hydrochloric acid, sulfuric acid, citric acid, tallow, and sodium peroxide.
When low friction or dry running is needed
The coefficient of friction for PTFE ranges from 0.04 for virgin PTFE(the lowest for any material currently in existence) to 0.19 for 15% Glass / 5% MoS2. In addition, PTFE is self-lubrication, low coefficient of friction, and lack of stick-slip behavior results in significantly reduced breakout torque.
When applications involve high speeds
PTFE rotary shaft seals perform extremely well in high-speed applications with shaft surface speeds up to 35 m/s. PTFE also has the lowest coefficient of friction of both polymers and elastomers, making it perfect for seals that must have an extremely low coefficient of friction. This can be a critical factor in high-speed rotary shaft seal applications where significant heat can be generated between the rotating shaft and the seal lip.
When thermal stability is critical
The maximum service temperature for PTFE is around 500°F and it has the highest melting point of all fluoropolymers, which is why it is used extensively in the oil and gas industry. However, it is often a material of choice for cryogenic applications down to -459°F because of its ability to maintain both its strength and elasticity at low temperatures.
PTFE also possesses a low coefficient of thermal expansion, ranging from 3.8×105 for 25% glass-reinforced PTFE to 5.5×105 for virgin PTFE. Because of this, seals made from PTFE are able to maintain their dimensional stability in operating conditions that can involve significant temperature changes.
When FDA/USDA compliance is necessary
PTFE is available in several different grades that are compliant with strict standards related to food, dairy, and water:
- FDA 21 CFR 177.1550 for fluoropolymers
- (EU) 1935/2004
- 3-A sanitary standards 18-03 and 20-27
- NSF/ANSI standard 61 for drinking water systems
PTFE also holds up extremely well to the intense cleaning and sanitation procedures that such seals may undergo, including hot water, steam, and aggressive cleaning compounds. In addition, PTFE is also hydrophilic, which can prevent water and moisture from being trapped around the seal during cleaning.
PTFE Grades for Rotary Shaft Seals
The most common PTFE grades used for rotary shaft seal applications are:
- Virgin, which works well for slow rotary light duty
- Glass-filled, which enhances strength and wear resistance but should only be used on shafts with high hardness
- Glass MoS2-filled, which increases wear resistance and strength without the abrasiveness of glass-filled
- MoS2-filled, which increases wear resistance and life but should not be used on shafts with low hardness
- Carbon-filled, which will increase wear resistance with less impact on the coefficient of friction
- Carbon and MoS2-filled, which increases wear resistance, enhances high-temperature performance, and maintains the capability of dry running
Other Benefits of PTFE Rotary Shaft Seals
There are other benefits to using PTFE for rotary shaft seals, such as their wider temperature ranges and longer life span when compared to elastomeric seals. They also have low outgassing, good electrical insulation properties, They are also inert to most chemicals and not only compatible with most lubricants but also self-lubricating).
If you are considering a PTFE rotary shaft seal, contact Advanced EMC today. Our team of experienced seal experts will work with you to determine is PTFE is the right material for your application.