235/320/360 Over-Center Link wear
Posted: Wed Apr 30, 2025 4:18 am
I am always on the look out for long term wear issues in our planes. One system I closely examine every annual is the over-center link on the gear. I remove the parts, disassemble, and measure. For some 22 years wear stayed down around .001”. One year wear jumped to .003 unexpectedly. I can’t explain the sudden jump. The bearing surfaces have never run dry so perhaps some sort of fine abrasive was introduced in the prior year. Whatever the cause, there are a few weaknesses in the stock parts from a design standpoint. The first is that the bearing material is the aluminum part itself. This makes the link a wear item. The second thing I noticed during the last annual is that one of the press fit bearings had loosened. I had seen this on other planes. This is the reason one should have a large area washer under the bolt head. Below is a photo taken by one very lucky owner. The bearing had loosened the link slipped off. It was just hanging on to the bolt head.
One theory that could explain the loosening of the press fit on the bearing over time involves the very small cross section produced when the hole is added for the roll pin. If this small cross section is yielding (stretching) in the slightest it would loosen the grip on the bearing. It doesn’t take much. For this size bearing the interference is between .0000 and .0013”(Aurora data). The hoop stress can be substantial while the material cross section up against the bearing is only .049” thick.
With the annual complete, I set out to address these two issues with some design changes.
The first item to tackle was the pivot at the ¼” bolt. I machined some custom high strength journal bearings that would make the bolt the sacrificial wear element. Initially just the bearing material was cycle tested under load. Two approaches were taken. First assuming regular maintenance, i.e. regular cleaning and lubrication, and secondly assuming no maintenance. I assumed 270 flights a year with annual maintenance for 10+ years. Every 270 cycles I removed the bolt, cleaned and lubricated the joint, then reassembled.
The second approach involved performing the entire 10 year duration with no maintenance. That is 3,000 cycles without cleaning or lubricating.
To ensure concentration of the wear, the bolt was restrained such that it could not rotate in the assembly i.e. the same area of the bolt and bearing would make contact for each cycle.
As hoped the result showed that all measurable wear occurred on the bolt. Replacing the bolt restored the original 0.001” clearance. Worth noting is that I use only close tolerance bolts, AN174 series as opposed to AN4, in these pivot joints. The AN174 series is held to a tighter tolerance, 0.2492-0.2487”. The standard AN4 bolts can vary quite a bit more, from .246 to .249”.
New OC links were fabricated to accommodate the three small journal bearings. While made of a stronger alloy, 7075, a slight bump out was added so that the bushing volume would not reduce the material cross section. Similarly, material was added where the roll pin hole is drilled to maintain the same nominal material cross section.
The prototype was again run though durability testing. The bearings performed just as before. I ran 5,400 cycles straight through with no cleaning or re-lubrication. The bolt showed signs of wear as before. The bearings looked new. When the bolt was replaced with a new one, operating tolerance were restored.
Here is a video comparing the effect of wear on the old links (2700 cycles) and the new links after (5400 cycles)
https://www.youtube.com/watch?v=RGQGRdrF3Hw
Here is an entertaining video of bearings being cooled for installation.
https://www.youtube.com/watch?v=swe_UFQ ... QJAYcqIYzv
I’ll add these to the classified section.
One theory that could explain the loosening of the press fit on the bearing over time involves the very small cross section produced when the hole is added for the roll pin. If this small cross section is yielding (stretching) in the slightest it would loosen the grip on the bearing. It doesn’t take much. For this size bearing the interference is between .0000 and .0013”(Aurora data). The hoop stress can be substantial while the material cross section up against the bearing is only .049” thick.
With the annual complete, I set out to address these two issues with some design changes.
The first item to tackle was the pivot at the ¼” bolt. I machined some custom high strength journal bearings that would make the bolt the sacrificial wear element. Initially just the bearing material was cycle tested under load. Two approaches were taken. First assuming regular maintenance, i.e. regular cleaning and lubrication, and secondly assuming no maintenance. I assumed 270 flights a year with annual maintenance for 10+ years. Every 270 cycles I removed the bolt, cleaned and lubricated the joint, then reassembled.
The second approach involved performing the entire 10 year duration with no maintenance. That is 3,000 cycles without cleaning or lubricating.
To ensure concentration of the wear, the bolt was restrained such that it could not rotate in the assembly i.e. the same area of the bolt and bearing would make contact for each cycle.
As hoped the result showed that all measurable wear occurred on the bolt. Replacing the bolt restored the original 0.001” clearance. Worth noting is that I use only close tolerance bolts, AN174 series as opposed to AN4, in these pivot joints. The AN174 series is held to a tighter tolerance, 0.2492-0.2487”. The standard AN4 bolts can vary quite a bit more, from .246 to .249”.
New OC links were fabricated to accommodate the three small journal bearings. While made of a stronger alloy, 7075, a slight bump out was added so that the bushing volume would not reduce the material cross section. Similarly, material was added where the roll pin hole is drilled to maintain the same nominal material cross section.
The prototype was again run though durability testing. The bearings performed just as before. I ran 5,400 cycles straight through with no cleaning or re-lubrication. The bolt showed signs of wear as before. The bearings looked new. When the bolt was replaced with a new one, operating tolerance were restored.
https://www.youtube.com/watch?v=RGQGRdrF3Hw
Here is an entertaining video of bearings being cooled for installation.
https://www.youtube.com/watch?v=swe_UFQ ... QJAYcqIYzv
I’ll add these to the classified section.