Going Beyond In Aviation Aircraft Gearboxes With Rotary Lip Seals
Omniseal® Rotary Lip Seals For Dynamic Sealing Applications
Dating back to the 1970s, Omniseal® PTFE rotary lip seals or rotary shaft seals were introduced to bridge the performance gap between conventional elastomer lip seals and mechanical face seals. From the 1970s to 1980s, Omniseal Solutions™ worked closely with customers to quickly qualify these lip seals in applications for critical industries including industrial and aviation. Since this time, our manufacturing and engineering business has been providing the polymer solution to meet the high-performance requirements from customers including leakage and wear life. Performing in hostile environments where elastomer lip seals fail and in some cases where mechanical face seals are used, Omniseal Solutions’ PTFE lip seals are selected as they help to overcome extreme temperatures, aggressive media, high surface speeds/pressure, and lack of lubrication conditions. Against mechanical face seals in application regimes where PTFE lip seals can also work, Omniseal® rotary lip seals are less expensive; therefore, providing significant savings in weight/space as well as installation time. With the functional goal of retaining the system media in and external factors out for dynamic applications with rotating shafts, Omniseal Solutions’ rotary lip seals are engineered “right the first time”, a technology advantage of our significant expertise in design, testing, and simulation as well as experience in customer co-development.
The focus of this technical white paper is to illustrate the high performance and advantages demonstrated by Omniseal® rotary lip seals through technical data and insight generated on selected applications. Three main application areas of polymer lip seals commonly developed within Omniseal Solutions’ sixty years of experience are: 1) auxiliary power unit (APU), 2) low thrust jet engine gearboxes, 3) and helicopter turbo shafts. Typical conditions and requirements of these applications are defined in terms of RPM/surface speed, shaft size, temperature, duration, and lubrication type/level, serving as framework for the white paper.
The following Table 1 represents the details of three test campaigns for Omniseal® rotary lip seals in these critical applications: 1) auxiliary power unit (APU), 2) low thrust jet engine gearboxes, 3) and helicopter turbo shafts.
Test Campaign 1: APU & Low Thrust Jet Engine Gearboxes – Typical Conditions
Test sequences were established to simulate typical aircraft gearbox conditions based on Omniseal Solutions’ extensive experience with large OEM/Tier 1 manufacturers. Such conditions generally include testing at high rotational speed, positive/vacuum pressure, high/low temperature, and dry running at speed.
More specifically, as summarized in Figure 1, Test Campaign 1 was tested under the following conditions:
Speeds ranging up to 14,000 RPM (24.5 m/s)
Environmental temperature up to 135°C and down to -55°C
Positive and vacuum chamber pressures from -41 to +41 kPa
For more than 500 hours of test duration
One minute of dry run sequences at 6,000 and 12,000 RPM
The media used was Mobile Jet Oil 254 HTS (high thermal stability), which meets the specification of MIL-PRF-23699 and provides splash condition by filling up to 1/8th of the shaft diameter at a stationary point. The HTS type oil was selected due to being one of the more aggressive media known to attack seal materials and test their chemical compatibility.
Figures 1 and 2 include three main testing sequences and conditions for Test Campaign 1. Chart A summarizes heat runs and Chart B summarizes cold runs, including the following parameters: 1) Typical work conditions (positive/vacuum pressure, and high temperature); 2) Low temperature (-55°C); and 3) Dry run tests (6,000 RPM and 12,000 RPM without shaft/oil contact for 1 minute).
Testing Suitable Materials: Influences of Contact Pressure & Velocity
Table 2 shows the test setups with different seal configurations and shaft materials, where tests were repeated with two seal configurations and two shaft materials. Seal Configuration 1 (two lips design) is one of the designs Omniseal Solutions™ provides for the application with our proprietary polymer compound materials that exhibit suitable range of mechanical, tribological, thermal properties, and chemical resistance. As a result of our design expertise, the polymer lip seal can achieve extremely low leakage and wear results. Seal Configuration 2 (three lip design) is the design that can be used for extremely low leakage requirement, which compromises lifetime compared to that of Seal Configuration 1.
Two other common shaft materials for aviation applications were considered - namely 4140 and 9310 alloys - with the size being what is considered more common as 33.3 mm diameter in the APU, low thrust jet engine gearboxes, and helicopter main gearboxes applications.
Leakage and wear results are summarized in Table 3, showing extremely low leakage rate and minimum wear over the total duration of the test. The methodology for measuring leakage is that the amount of oil absorbed by a cotton pad installed next to the seal is weighed. To measure wear, the thickness at a non-contact point is compared to the thickness at the midpoint of the wear pattern (see Figure 3). The rig tests two seals on each end of the shaft - noted as Rig Side A and Rig Side B. Leakage of less than 0.001 g/hr and wear of essentially none showed that Omniseal® rotary lip seals can comfortably address what are typically known conditions and requirements for APU and low thrust jet engine gearboxes. Please note that the leakage occurred only during high speed coupled with high temperature phases (*2x 100 hours at 135°C, speed range 8,000 to 14,000 RPM).
Longer test duration of more than 500 hours is when we can typically expect more accurate wear life and thus, the endurance test. Test Campaign 2 will show more insights on wear performance.
The following Figure 3 shows the tested elements of Seal Configuration 1 on the rig side A.
Test Campaign 2: APU & Low Thrust Jet Engine Gearboxes – Endurance Test
In this campaign, the same Seal Configuration 1 - together with AMS6265, 9310 steel alloy shaft - was tested under the same shaft size and lubrication type/level as performed in Test Campaign 1. Since this case represents a more general design, Seal Configuration 1 and 9310 alloy as the shaft material were selected as they are more commonly used in the application in comparison to 4140 alloy. The purpose here is to test the system in successive speeds for longer duration compared to Test Campaign 1 with the maximum speed holding the same level at 14,000 RPM (24.5 m/s) as was the case in Test Campaign 1. The rotary lip seals were also subjected to pressure cycling between vented to atmospheric pressure and +41 kPa. Further, heat was added to reach 120°C of environmental temperature when pressure was added to the system as shown in the below Figure 4.
In this endurance test that targeted the same APU and low thrust jet engine gearboxes application, Omniseal® rotary lip seals demonstrated extremely low leakage and wear, successfully proving their ability to address conditions in longer duration, have higher confidence in leakage, and more importantly low wear results compared to Test Campaign 1. Wiper element is an excluder that keeps out external environmental factors; therefore, the wear on this component is not critical for leakage performance. Wear is expected and has little impact on performance because the actual sealing performance is ensured by the primary element.
As seen in Table 4 and Figure 5, the primary element wear is about 0.005 mm on the Rig Side A at the end of 900 hours. At 50% reduction in the lip seal contact width, typically a reduction in lip loading is started to be observed in many applications and hence serves as the reference point in which the wear life estimation is calculated. Please note that wear is overestimated as the wear is higher during initial steps while it follows a linear low in later stages. Using this assumption, the wear life estimation comes out to be 710,000 hours with the Omniseal® rotary lip seals as the solutions for the given application with no measurable leakage detected in this test segment.
Some polishing of the shaft occurred during the sealing life as well as the PTFE material transfer to the shaft, which protects the shaft. As seen in Table 5, the Ra of the shaft before and after test shows polishing occurring from 0.124 to 0.074 on the drive side and from 0.175 to 0.057 on the idle side.
Test Campaign 3: Helicopter Main Gearboxes
In this test campaign, the same Seal Configuration 1 was used along with the same sized 9310 alloy shaft and lubrication type/level. As seen in Figure 6, this test campaign was designed to test higher speed at 18,000 RPM, or the surface speed of 31.5 m/s maximum at 135°C maximum temperature, with cycling between the pressure addition of +41 kPa and vented to atmospheric pressure, building on the test protocol of previous Test Campaign 2.
Once again, extremely low leakage rate of less than 0.01 g/hr and low wear of 0.02 mm on the primary element (see Table 6) was observed at the end of this high-speed test of 500+ hours. Leakage was only observed during high speed coupled with high temperature phases (*175 hours at 135°C, speed range 14,000 to 18,000 RPM). Wear life estimation was assessed at more than 10,200 to 13,500 hours depending on which side of the test rig the seal was tested. Once again, please note that wear is overestimated as the wear is higher during initial steps while it follows a linear low in later stages. As expected, as seen in Table 6, the shaft was polished during the sealing life and the transfer of PTFE material from seal onto the shaft protected the polished surface.
Discussion & Conclusion
Based on the three test campaigns, Omniseal Solutions’ PTFE double lip and triple rotary lip seals were proven to show very low leakage and wear performances in conditions upwards of 18,000 RPM (31.5 m/s) in speed, 135°C and -55°C in temperature, and +/- 41 kPa in pressure, and even a brief dry run all in splash conditions with Mobil 254 Jet Oil (HTS oil). At the specified conditions, observed leak rate was a few mg/hour (double lip) to less than 0.1 mg/hour (triple lip), and the wear rate after 500 hours was still too low to evaluate the lifetime at the moderate speed. Also, the Omniseal® polymer seals were found to maintain its performance after dry run sequences.
One important outcome of this testing would be that the results should be interpreted more generally based on the application conditions. The results in this white paper demonstrate a small part of our vast experience as well as the key technical expertise tied to multiple successes from close collaboration with customers at large OEM and tier 1 manufacturers. Omniseal Solutions’ engineers can define the designs based on hands-on expertise, knowledge, and key technical advantages that are continuously developing within many successful applications, making Omniseal SolutionsTM an invaluable partner to co-develop solutions “right the first time” for your application needs. In other areas where the conditions closely resemble the precision testing presented here, these results should be considered for potential new applications with Omniseal® rotary lip sealing solutions such as last generation of larger accessory gearbox (AGB) / helicopter main gearboxes and many others where applicable.
In upcoming white papers, our global technical team will present data showing the performance of Omniseal® rotary lip seals with larger diameter and higher speeds. Explaining how factors such as the heat dissipation mechanism can impact performance of the seals and how we are able to manage these challenges are important elements that need to be considered. By exploring different levels and type of lubrication, our team is going beyond the boundaries of possible with Omniseal® rotary lip seals, understanding the general performance of PTFE lip seals in areas where they can be used to help critical applications run durably and protect the aviation industry.