Pivot Pins and Bushes
 

Im researching pivot pin and bush materials in construction and earthmoving equipment but am also keen to learn about the technologies used in other industries. I am looking for information about pin and bush materials and common problems experianced with them in aircraft.

I am aware that pivot pins are used in the articulating landing gear of aircraft, with common materials being chromed pins and bronze bushes and that they are lubricated with grease. Common problems being corrosion of the pin or fatigue cracking of the pin.

Can anyone give me any more details on the subject. Any help much appreciated!

James

Big problems involve excluding environmental moisture from the pin/bearing surface and also the bush/housing contact area.

On something like the ERJ 145, Embraer appeared reluctant to use anti corrosion assembly materials (like corban, mastinox, ca1000) at build which caused problems with corrosion in the trunion housings (machined alu alloy).

Hygroscopic tendencies in some selected greases for lubrication (synthetic & clay based) can induce moisture into the exact area it's not welcome.

So,

Metal material selection, environmental seals, lubrication and assembly materials, paint finishes, effective maintenance and inspection intervals.....what else anyone?

Thankyou for the quick reply.

When you say metal material selcetion, what are the preferred options? Also are you aware of anyone using polymer or composite fibre bushes?

James

Don't really know what the engineering and metalurgical genii are doing, but tried and tested brass bushings on cadmium plated pins is still pretty much the standard. At the end of the day its down to the techs 'n mechs doing the maintenance, especially the lube jobs, to the best of their ability, and not trying to satisfy yet another rush to finish and get the aircraft airborne.

A good example of rushed/incomplete/incorrect maintenance is the MD crash off the California coast several years ago. Difficult to access the brass ball-nut on the horizontal stabilizer, and it was overlooked for a couple of servicings. Next thing the ball-nut got chewed up and slipped off the screwjack.

I know that some pins are coated with Chrome. Has this been/will it be banned soon? I believe it was used due to high hardness and an ability to retain lubricant.

James

Some bushings in aerospace uses have Teflon lining for it's self lubricating dry characteristics but it has a tendency to migrate under higher load and requires replacement frequently. It also does not get flushed with regular lube and so once environmental particles get in - they're there for good - accelerating wear.

I doubt "Hard Chroming" will be banned anytime soon throughout engineering.....

.....Unless you can provide some info?

Banned ...?? not likely. Hard-chroming is used primarily for hydraulic jacks and oleo struts. It forms a super-smooth finish, that is extremely hard and almost corrosion-proof. Yes, teflon in low-load areas, but more often as a seal back-up in hyrdraulics. Mastinox, etc, mostly used in long-term mounting bolts.

In all the aircraft I've worked, I cant off-hand think of seeing chromed pivot pins; they were all cadmium plated with brass or steel-alloy bushings .....

Landing gear pintle pins & trunions are chromed for example

Composite fibre bushes are used extensively in some Messier-Dowty landing gear and actuators. Supposed to be maintenance free, no lubrication required. Works great until you have to disassemble something then the bush falls to bits when you push the pin back in and you have to send the component back to Messier because they are bonded in and not field replacable (mostly).

Composite and Carbon bushes are utilised on VBV and VSV shafts of GE engines, they have low failure rates, actually, they almost never fail!!!!
however, they are used on air systems.

On Landing gear systems, the best precaution is systematic and periodic servicing and inspection.

Anyone know who manufactures the composite bushes for Messier-Dowty? Or any other aero application?