By Josh Cosford, Contributing Editor
Urethane (or polyurethane) seals are among the most versatile and durable materials used in fluid power sealing. Urethanes are cast into billets or molds to create the seals used in many of the hydraulic components you see every day, such as pump shaft seals, cylinder pistons and rod seals. The material benefits from countless possible chemical formulae, providing a wide range of sealing characteristics to suit nearly every application.
Urethane seals provide superior resistance to extrusion due to their combination of hardness and toughness. Available anywhere from a soft 20 durometer Shore A to 85 Shore D, urethane spans hardness from a soft nitrile to nearly plastic. Somewhere between those extremes lies many seal materials suited for the demanding requirements of hydraulics.
An excellent hydraulic seal must resist shearing and tearing. The energy of hydraulic fluid under pressure easily overcomes inferior technology, especially when internal gaps and clearances are not tight. Urethane undergoes a change in shape under pressure, which ensures positive sealing without leakage, but its high strength nature returns the seal to its original form after pressure relaxes.
Contamination is a common symptom in many hydraulic systems. When particles’ abrasive effect gets out of hand, the soft seals inside hydraulic components are at risk of early failure. Urethane benefits from naturally high resistance to abrasion, especially compared to other polymers such as Buna Nitrile and Viton.
When the heat gets hot, urethane is the go-to choice for high-pressure hydraulic applications. With urethanes rated for over 250°F while also scoffing aside 5,000 psi or more, this combination of extremes has put urethane at the top of the class for hydraulic cylinders. Viton easily handles over 400°F but is often limited to 2,000 psi or less. Other technologies such as PTFE and polyester plastics may handle pressure with ease but do not provide urethane’s superior sealing.
Because of polyurethane’s machinability, this material suits customers with economical seals even at one-off quantities. Rather than pour or mold from a liquid base, round urethane billets are spun on a CNC lathe to provide reproductions of even the most complex U-cup profile. Most high-volume seal manufacturers support low-volume manufacturing, and you’d be surprised at how economical a small run of customs seals is.
As advanced polymer recipes and manufacturing techniques progress, new high-performance seals take your hydraulic applications to the next level. For example, many older urethanes were prone to hydrolysis when exposed to water, especially with concomitant heat. Cutting edge urethane polymers now resist hydrolysis when exposed to water, ameliorating what was previously a downside. As urethane technology advances, expect this material to cover nearly every hydraulic sealing application, no matter how extreme.
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