How to Correctly Install, Test and Remove an Aerospace Spherical Plain Bearing
Whether you are a component manufacturer or an MRO station, bearing installation is a critical part of a completed product. You may source this step to an outside vendor which can be costly and time consuming. You may already perform these installations on-site but struggle with inspection failures, time consuming procedures and scraped expensive parts. Bringing this process in-house gives you complete control over quality and urgency while utilizing the cutting edge techniques will save you time and money. Erick Sloan from Cater Manufactured Limited outlines the bearing installation and inspection process which intends to educate and inspire.
Bearings can be found all over the aircraft for example: landing gear, door systems, airframe structures, engine mounts and more. Some installations are required on the aircraft itself while others on individual components. Each area has its own unique challenges and design geometry considerations; however, the process is similar.
To start, the bearing must be inserted into the housing. If the bearing is not properly aligned, this process can damage the housing which must be avoided. The tool used should allow the bearing to be gradually drifted into the correct position. A two stage process is used. The primary stage ensures full concentricity and position while the secondary stage ensures the bearing is accurately centered within the housing width.
|Stage 1 – Primary Installation||Stage 2 – Secondary Installation|
If the installation calls for the use of a sleeve, a similar process should be used prior to bearing installation.
There are many methods of bearing swaging. These include anvil staking, two-wheel swaging, tri-roller swaging, ball staking, segment staking, reverse swaging, among others. We will focus on tri-roller swaging as it is generally recognized within the industry as the superior method. If you are interested in these other methods, Carter will be happy to support you as they are all within our production portfolio.
The two methods of tri-roller swaging is with a portable hand-held tool or a drill press style tool. Both have their advantages and disadvantages but the concept is the same.
|Portable Tri-Roller Swaging Tool||Drill Press Tri-Roller Swaging Tool|
A series of 3 angled rollers rotate within the v-groove of the bearing which gradually extend the bearing lip onto the housing chamfer. When done on both sides, this should safely secure the bearing within the housing. There are two stages to this process: primary swage and secondary swage.
The primary swage using a locating feature, primary anvil, which ensures the bearing continues to be perfectly centered in the housing. The secondary swage is performed on the opposite side of the bearing and uses a second locating feature, secondary anvil. This contacts the bearing v-groove thus keeping the bearings original centered position.
The geometry of this tool is tightly control by either the US government or manufacturer’s specifications such Boeing or Airbus. Improper compliance can yield a non-conformance when audited by the customer, a failure during inspection or even a premature failure in application.
You may now ask yourself, how do I know if this bearing has been properly installed? Beyond basic visual inspection and a feeler gauge, there are three common tests: Proof Load Test, Breakaway Torque Test, and Swivel Test.
Proof Load Test
Manufacturers need tangible data driven proof that the bearing has been successfully installed and will not be dislodged from the housing if an axial force is presented. The proof load test is designed to apply a designated axial force onto the bearing and measure its linear displace with respect to the housing. The test is performed on both sides of the bearing and determines if the swage is safe and successful, or not. The approval criteria is normally defined on the process specification; however, Carter can advise based on extensive industry knowledge.
There are two common methods of proof load testing: anvil style on a hydraulic press and portable style. Many customers prefer the portable style as it is compact, quick to set up, repeatable and easily adaptable. It utilizes a threaded nut and wrench for force application to the bearing which is measured by a calibrated load cell connected to a digital load reader. The bearing displacement is monitored by a DTI gauge which is contact with a sprung loaded piston moving along with the bearing. The image below shows this test in action on a display assembly.
|Portable Proof Load Kit|
The proof load test can be challenging to accomplish on components with space restrictions or restrictive geometry like blocker doors. Custom Unasis tools have been engineered for these specific circumstances.
Breakaway Torque Test
The breakaway, or rotational, torque test determines the torque required to initiate ball rotation as shown in the Figure 16. This torque is closely monitored as it has a direct impact on the mechanical system. An improper staking technique, due to inaccurate tooling, can negatively impact this torque.
The test consists of a tool inserted through the bearing bore causing an interference fit and an electronic torque reader to accurately measure the torque. Use extreme caution if your tool allows for over compression of the ball from use with a threaded bolt and nut assembly as this could damage the bearing.
Swivel, sometimes called tilt movement, torque is defined by the torque required to initiate perpendicular movement to the axis of rotation as shown in Figure 16. Ask Carter about its custom swivel torque tool. This test is standard on many Airbus specifications and is catching traction with the other big manufacturers and also the motorsport industry.
In the event that the bearing fails visual or test inspection or a component repair is required, a cutting tool is necessary.
The Unasis cutting tool is precisely designed to cut a portion of the bearing v-groove causing a crease on the lip. When done correctly, this crease allows the v-groove to fold into itself when extracted from the housing using an install remove tool. This eliminates the possibility of housing damage and ensures the housing component is still flight worthy.
|Portable Cutting Tool||Drill Press Cutting Tool|
Bearing installation is a dauntingly important process. Improper methods can damage incredibly expensive and hard to source bearings or housings. Having the tools that have been designed and continuously improved to optimize repeatability, minimize time and save money is the best option. Knowing a warehouse of standard tools are in stock and custom Engineering and design support is on-call is a weight off your shoulders. Carter is the supplier with decades of experience and application knowledge ready to immediately assist you with the projects in your pipeline. Get in touch with Carter now to discuss your technical requirements.