REG NASA-LLIS-2839-2010 Lessons Learned Reduced Torque Output from Electro-Mechanical Actuators.pdf

1、Public Lessons Learned Entry: 2839 Lesson Info: Lesson Number: 2839 Lesson Date: 2010-05-4 Submitting Organization: KSC Submitted by: Annette Pitt Subject: Reduced Torque Output from Electro-Mechanical Actuators Abstract: Reduced torque output of the orbiters electromechanical actuators has been a p

2、roblem for the Shuttle Program. It has resulted in unplanned work to test, remove, and refurbish the actuators. Disassembly and inspection has revealed that the torque reduction is a result of problems with the torque limiters and the motor clutch/brake assemblies in the actuators. Refurbishment is

3、a lengthy process because of the difficulty involved in setting the torque limiter to operate through the required temperature range. The next generation of actuators should have a better design, or the mechanical system should be capable of withstanding the maximum output loads of the actuator. Des

4、cription of Driving Event: The electro-mechanical actuators for several of the orbiter mechanical systems have exhibited torque output degradation over the life of the shuttle program. In some cases, even serviceable spares that had been in logistics storage showed significant degradation, despite t

5、he fact that they had not been operated and had been maintained in a controlled environment. This resulted in the need to periodically test and trend the performance of some of the actuators installed in the Orbiters, to ensure the torque output level was sufficient to operate successfully in flight

6、. In some cases, Ground Support Equipment (GSE) had to be developed to enable the test to be done with the actuator still installed in the vehicle. It also resulted in unplanned work when the actuator needed to be replaced, and often required additional re-rigging of the system after the replacement

7、 actuator was installed. These actuators all contain torque limiters and motor clutch/brake assemblies, which were usually the cause of torque output degradation. Lesson(s) Learned: The torque limiters of the External Tank door actuators were redesigned in the early 1990s because of unacceptable deg

8、radation of the disk friction surfaces. The new design was a multidisk ball-ramp setup. When the torque reached a certain value, the ball-ramp design reduced the compression load on the disk stack and allowed the disks to rotate relative to each other (slip). The disks in this particular actuator we

9、re made of steel and Vespel. After several years of service with the new torque limiter design, most of the actuators exhibited reduced torque output. Investigation determined that the torque limiter degradation was due to grease contamination of the disk surfaces. Inspection confirmed that excess g

10、rease was applied to some of the torque limiter components (bearings) during assembly and eventually migrated to the disk surfaces. However, even after the actuators were refurbished, excess grease was removed, and the grease was cleaned from the disks, the actuators often continued to exhibit torqu

11、e degradation and show evidence of grease contamination of the disks. Another problem was the amount of effort required to set the torque limiter to slip in the required range when the actuator was being serviced. The value at which the torque limiter slips depends somewhat on the temperature, and t

12、he actuator is required to operate with a specified torque output through a large temperature range (over 300 F). Several rounds of adjustments were needed, each followed by days of testing. In addition, there was degradation of the motor clutch/brake assemblies in these actuators. Each actuator has

13、 two AC motors, each of which interfaces with the gear box differential through a clutch/brake assembly. For redundancy, the actuator will operate on only one motor. When a motor is not operating, the clutch/brake assembly is in the brake mode. A friction disk is spring-loaded against a stator, whic

14、h locks the input to the differential and disconnects it from the motor. This is necessary so that during single-motor operation, the nonoperating motor input is not back-driven through the differential. When the motor starts up, the assembly switches to clutch mode. A solenoid pulls the disk away f

15、rom the stator, separating the brake surfaces and engaging the clutch surface of the disk to the motor output. The motor then drives the differential input through the clutch. During tests of the actuators on the vehicle, the torque output varied significantly by motor mode. Since there is a single

16、torque limiter, the point at which it slips should be the same regardless of which motor is operating. This indicated that the clutch/brake assembly was slipping prematurely. Disassembly and inspection confirmed that both the clutch and brake surfaces were worn. There was evidence of scoring on the

17、interfacing surfaces and metallic debris in the assembly. Recommendation(s): Future mechanical system designs should attempt to eliminate the need for a torque limiter and any type of clutch/brake assembly. This is especially true for any actuator that will have a long service life. Ideally the hard

18、ware should be designed to withstand the maximum output loads of the actuator. Even with torque limiters, many of the orbiters mechanical systems were not completely protected for jam conditions. In a system with an over-center linkage design, there is a huge mechanical advantage as the linkage reac

19、hes on-center. As such, the loads in the linkage are high for a relatively low input torque. Analysis showed that the ET door uplock latch system was not well protected by the torque limiter. A jam near the end of the latching cycle would cause the hook to yield before the torque limiter would slip.

20、 For future programs/designs where a torque limiter is necessary, new torque limiter designs should be considered for improved reliability and serviceability. Evidence of Recurrence Control Effectiveness: N/A Documents Related to Lesson: Click here to download ET Door Drive Actuator Anomalies Presen

21、tation Click here to download ET Door Drive PDU Stall Max Torque Data Mission Directorate(s): Exploration Systems Space Operations Additional Key Phrase(s): 1.Review systems and boards Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,- 1.Orbiting Vehic

22、les 1.Entry Systems 1.Long term sustainability and maintenance planning 1.Engineering design and project processes and standards 1.Ground Operations 1.Spacecraft 1.Launch Vehicle 1.Ground processing and manifesting Additional Info: Project: Space Shuttle Program Approval Info: Approval Date: 2010-06-24 Approval Name: mbell Approval Organization: HQ Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-