Increasing Speed in Linear Actuators Through Sealing Material and Design

Electromechanical actuators are used in a variety of applications for their ability to convert electrical energy into mechanical motion where precision, flexibility and compactness are required. Linear actuators are a type of electromechanical actuator used where straight-line movement is needed in certain industrial automation, automotive and aerospace applications.  Can we go faster? Achieving high and consistent speeds in linear actuators can be a challenge and is determined by the motor, gearbox, shaft size, thread pitch, bearings, lead screws and seals. Seal material and design is often a limiting factor for speed and a common failure point for linear actuators. Speed is a function of stroke length and duty cycle with stroke length being how far the rod travels and duty cycle being how often within a minute it travels that length. Short stroke lengths with high duty cycles are the most difficult operation for a seal to withstand because a seal’s friction creates heat which is transferred to the rod. If the rod cannot dissipate the heat fast enough, it will cause the seal to overheat and break down. Another failure point effecting speed in linear actuators is side load. Side load is somewhat inevitable and occurs following improper installation of the linear actuator when loose connections put excessive strain on the seal and bearings. If the seal is damaged due to side load, the rod will not be able to achieve full speed. The right seal design can absorb some side load, decreasing the maintenance and downtime required to replace a seal while allowing the rod to operate at maximum speeds.  Finally, stick-slip is an additional problem when it comes to speed and seals in linear actuators. Stick-slip can be thought of as a vibration. As the rod travels across the seal and speed increases, it can develop a vibration due to the friction of the seal contact point. At higher speeds, this vibration becomes too much for the system to operate properly causing premature seal failure. What’s the deal with the seal? When it comes to choosing a seal design and material that can help prevent these failure points, there is no single industry standard. This is because seals used in linear actuators must withstand different levels of contamination depending on the application. Many linear actuators require ingress protection (IP) ratings which help to protect the inner workings of the actuator from contaminants as well as prevent grease and oil from exiting the actuator. The higher the required IP rating, the slower the top speed of an actuator due to increased friction and pressure on the rod from the sealing element. Application-specific needs such as temperature and force are also factors that determine the seals used in linear actuators. Original equipment manufacturers (OEMs) frequently opt for U-Cups and/or double-acting scrapers because of the ability to keep the necessary fluids in and contaminates out. U-Cups are reliable, cost effective and readily available. There are other higher-performing linear seal solutions that offer increased speed which would typically be employed in high-precision or more critical applications.

Who’s the expert here? Tribology is the study of friction, wear and lubrication of interacting surfaces and their relative motion. At Trelleborg, scientists study tribology to determine the best seal material and profile design for increased speeds in linear actuators. Trelleborg’s Zurcon DA22 is a double-acting seal with scraping lip in injection molded polyurethane. Although plenty of double-acting scrapers exist for use in linear actuators, the Zurcon material has excellent friction characteristics and high wear resistance. It can also operate in temperatures from -31 °F to +270 °F/-35 °C to +130 °C, has a low compression set and is FDA compliant for use in food and beverage applications. The Zurcon DA22 can be used stand-alone or in conjunction with a U-Cup. The combination solution helps with high ingress protection needs.  High-performing seals that reduce friction in linear actuators also help contribute to energy savings. If a seal can offer increased performance and efficiency by even a small percentage, it offers monetary savings for end users. A component partner like Trelleborg offers digital design and finite element analysis capabilities that help with design, planning preventative maintenance and reducing unexpected downtime.  What’s the gist?  The speed of a linear actuator is determined by several factors, but one of the main culprits of speed limitations is the seal and its material. No single industry standard exists because of application-specific contamination considerations. Many OEMs default to a standard U-Cup, but other high-performing seal designs can combat heat from friction, the effects of side load and stick-slip. This can also significantly contribute to system efficiencies, reduce downtime for maintenance and create energy savings.