Hinge Play and Hinge Wear
My first set of stock MS hinges did not last very long. The inherent play combined with the torsional vibration produced by the engine caused the aileron hinges to get hammered by bouncing ailerons. Note that this is not related to flutter. The entire mass balanced aileron would move vertically as the wing responded to the power pulses of the engine. Over time, the aluminum hinge holes were elongated vertically. A review of the MS20001 hinge specification reveals an allowable ID of .093 to .098" for the hinges we are using on the Lancair. The MS hinge pin is .089 +/-.001. Given this range, one is guaranteed lots of play.
Figure 1, Graphic Representation of Tolerance Difference
As the OD of the pin and the ID of the hinge spread, local stresses rise, wear rates climb very rapidly and, in a vibration environment, the hinge gets hammered by the mass of whatever is attached to the other side. Going the other direction, as you approach a perfect line to line fit, loads are distributed and the relative motion between the hinge halves vanishes, eliminating the hammering, play and wear. The MS20001, per its specification, is simply poorly suited for applications where a close tolerance hinge is desired. The above figure graphically shows the magnitude of the difference.
I began investigating alternatives that would eliminate both play and wear.
Over time, three options were examined:
1. Sleeved Teflon Pins (Gary Hall kit)
3. Reamed Aluminum MS Hinge with a larger Stainless Steel Pin
The following clip compares four types of hinge material in new condition: MS hinge with an MS pin, MS hinge with the Teflon sleeved pin, Carbinge, and reamed MS hinge with welding wire.
Reamed MS Hinge
The reamed hinge turned out to be the successful alternative best meeting the goals of a wear resistant hinge without play. It also has the best strength.
The ‘reamed hinge’ is a piece of stock MS20001 hinge that is reamed with a .0955 chucking reamer. The hinge pin is 3/32”, 308L stainless steel welding rod. The welding rod is nominally .09375”. In used hinge material, the rod may fit without reaming. This however indicates that there could already be some internal wear. It is best to start with new hinge material. Even with new hinge material, not much metal is removed. The reaming generally only removes material near the entrance and exit of each hinge element. The result is a very clean fit. This good fit has both eliminated the relative movement and wear observed previously on the stock MS hinges. It has now been several years since I switching out all of my hinges.
This following clip shows a reamed hinge after about 10 years of use and 850 hour of flight time as an aileron hinge on the Lancair 360. I clean, lube the hinges with LPS2 at each annual condition inspection. Thus far, it looks like they will last the life of the airframe.
Teflon Sleeved Hinge Pin
The other two alternatives, Teflon sleeves and Carbinge, fell short in a few areas. This was primarily due to the non-metallic bearing material in each. The Teflon Sleeve kit was actually installed on my plane for about a year. Piano hinges act much like shears when under load. Teflon has very low strength (1500-3000 psi) and is relatively soft. The highly loaded flap hinges nearly cut all the way through the Teflon sleeve. As this was occurring over time, the flaps rode ever higher, until finally making contact with the upper wing skin. Even when new, the Teflon sleeve approach had the worst dimensional control of all the options.
Figure 2, Damaged Teflon Tubing (Inner Flap hinge tubing could not be removed)
Carbinge was another alternative examined. The stock hinge, as supplied, had more play then desired. While one can use a better fitting third party pin, a few other undesirable characteristics remained. First Nylon is subject to creep. That is, under load, it will slowly flow. Technical Data regarding Nylon creep can be obtained from “International Plastics Handbook, Hanser”. I tested this characteristic by loading a sample of Carbinge for one month. When the load was removed, there was increased play in the direction the load had been applied. The pin was slowly pushing through the Nylon.
Nylon is also hydroscopic. It will absorb moisture and swell when exposed to high humidity. The magnitude of the dimensional change needs to be considered in design. In this case it was equal to the total clearance I wanted to achieve. Therefore, dimensional stability could not be sufficiently controlled.
The primary drawback was that, Nylon has very low stiffness when compared to metals. Aluminum is ~ 10,000,000 psi. Nylon starts at ~400,000, but drops sharply with humidity. In Carbinge, Nylon replaces aluminum as the bearing material. This dramatically lowers the stiffness of the hinge. In other words, for the same load, Carbinge has much greater deflection. The following video compares deflection of Carbinge and aluminum. Even with five times the load, the aluminum has far less deflection.
Finally the strength of Carbinge was far below that of the aluminum reamed hinge. The following clip shows the results of a simultaneous pull test of Carbinge and reamed MS20001 hinge material. The test showed that caution is called for when considering Carbinge in high load applications. Here one can also see the difference in stiffness (deformation) during the load application.
Figure 3, Hinge Sections after Pull Test
Note the S-shape of the Carbinge pin after the pull test. This is a by-product of the soft bearing material. Since the pin is not infinitely stiff and the Nylon cannot sufficiently support the hinge pin, the forces will try to bend the pin into a slalom S-shape. The figure below illustrates this deformation.
The deformation of the pin serves to concentrate the loading at the very edges of each hinge segment. If the hinge material is not able to withstand this local stress it will begin to deform. Thermoplastic bearing materials are particularly susceptible to this load concentration allowing a greater deformation. This, in turn, exaggerates the S-shape of the hinge pin. The net effect, even for light to moderate loads, is relative hinge movement, even with the best pin to bearing material fit at installation.
The following clip shows what appears to be a loose fitting hinge pin. In fact, the pin was a snug press fit. The yielding of the Nylon results in a lot of movement even with this relatively mild load of 10-15 lb/in.
The search for a precision hinge with high load bearing capability led back to all metal construction. Along with a larger pin, reaming improved the tolerance and strength of the final hinge assembly. This produced a precision fit that eliminated the wear seen earlier. The best part is that the solution was easy and inexpensive to implement.
Straightened stainless steel wire of all sizes has become more available in recent years. This provides the opportunity to use a pin diameter larger than the 3/32” welding wire described above. Since 2011, I have been testing a section of hinge material reamed to .098”, the maximum permissible in the manufacturing tolerance paired with a pin of .096 +.001/-.000. This approach provides a guarantee of a tightly controlled fit even if a batch of hinge material is used that falls on the upper end of the tolerance band. Reaming to 0.098” will generally remove much more material and requires more time to ream the hinge material.