LPS2 will unfortunately not be able to help the general fleet.
The Mil Std Specification allows a clearance between the hinge and pin that ranges from .004” to .010” with .008” being the most predominant. This cavernous gap between the hinge and MS pin allows all sorts of bad things to get inside: air, moisture, abrasives etc. They simply don’t stand a chance.
The reason SS welding rod has proven so successful is closely tied to the very tight tolerance that is achieved. This provides protection against the intrusion of foreign matter between the hinge and pin while at the same time retaining lubrication. The close fit also improves load bearing capability of the hinge without incurring wear. Contact stresses between the parts are reduced. A gap of 1 to 2 % of the diameter is a good target in this application. The attached image shows graphically what the typical hinge play looks like when compared to a reamed hinge. Enlarging to this scale gives a better relative and intuitive view.
Reamed hinges will have a diametrical tolerance of about 1%. All other options examined were around 10% (8-15%). A close fit can be achieved by selecting a reamer .001” over the pin diameter. Reamers are available at McMaster Carr for about $14, SS extension tubing is about $9.
The search for a better hinge started with the need to remove play from trim tabs and ailerons. Reamers, by definition, provide exceptional dimensional control (.0002”). Nothing else on the market could come close.
The result is a hinge that has shown no signs of wear in nearly 1,000 hours and ten years of flying. All indications are that the hinges will last the life of the air frame. Problem solved.
I didn't pursue alternate pins for Carbinge for a few reasons.
First, it would have been a challenge to find the right size pin to guarantee a one to two thousandth clearance. Since reaming is not an option, the pin size must be adjusted to a hinge of unknown size and variability.
Nylon is hydroscopic. This means Nylon absorbs moisture and changes dimensionally as it does so. In many applications this can be accommodated, but here a change of just one percent would turn a nice close fit into a no-fit. Many papers have been devoted to quantifying the relationship between relative humidity, moisture content and dimensional part growth. Nylon is not well suited for applications where dimensional stability is needed. Extra dimensional tolerance must be allowed in design to accommodate this characteristic.
Finally, the resulting hinge would have only been good for lightly loaded applications (i.e. evenly distributed) due to the low bearing strength of Nylon. Thermoplastics such as Nylon exhibit a behavior known as creep. When placed under continuous load, Nylon will flow and slowly deform. When the load is finally removed, it does not return to its original shape. This behavior is accelerated by elevated temperature and moisture.
The following clip shows the creep that occurs over time when Nylon is under load. A sample hinge section was loaded to 400 lb/ft for one month and then re-examined for play. The hinge pin slowly displaced the Nylon. The result was an elongated hole with approximately twice the clearance in the loaded direction than in the non-loaded direction.
The L360 has a few highly loaded hinges (inner hinge of the outback flaps, inboard gear door, etc.), These are asymmetric load cases that place a much higher percentage of the load on the very first hinge segment. The hinges with thermoplastic bearing materials did not handle the concentrated loads very well and showed signs of creep Therefore hinges with Nylon or Teflon were abandoned.
Some suggestion for improving Carbinge:
1. Use a non-thermoplastic bearing material that can match the strength of aluminum
2. Move towards a larger hinge pin diameter - greater bearing area and easier to hold a close tolerance
3. Use SS pins to eliminate corrosion
Chris Zavatson
N91CZ
360std
From: John Barrett <jbarrett@carbinge.com>
To: lml@lancaironline.net
Sent: Thursday, December 1, 2011 10:05 AM
Subject: [LML] Re: Legacy vibrations resolved
Crhis Z’s videos and his observations are interesting and productive. He does not follow through with Carbinge to the same stage he did with the aluminum extruded hinge. You can match fit pins to each hinge and acquire a very snug fit with Carbinge as easily as you can use a reamer to achieve a matched fit with the aluminum and SS welding rod. I am admittedly biased in that my company produces Carbinge but I think the sample that he showed indicates greater play in the stock aluminum hinge vs the Carbinge sample. I suspect that this could vary somewhat sample to sample, but we purposely strive to make our tolerances tighter than the aluminum hinges.
It is quite possible that using the LPS2 lubricant could reduce or remove the risk of corrosion with the aluminum hinges. I have not seen examples of this or tried it out. However I have seen many, many airplanes, my own spam cans included, that have dark streaks on the airframe aft of every aluminum hinge on the airplane whether it is in a trim tab, a flight control surface or a door. The experience in the fleet is that these hinges do fret and corrode. It’s nice to think that the above mentioned lubricant could eliminate that problem across the entire aircraft fleet.
Thanks for the tip Chris.
John Barrett
Leading Edge Composites