The aftermarket automotive industry has extensively utilized exotic filaments for 3D printed components with great success for many years. The intent of this article is to explore the application of common easily printed filaments for sacrificial wear surfaces on experimental aircraft printable on most consumer grade FDM 3D printers.
I am not ashamed to admit I love Delrin.
I use Delrin (POM) everywhere that requires stable compressive load handling and low low coefficient of friction. In relative terms and in small applications, it is cheap and machines amazingly well.
I use it for rudder cable pulley retainers: (white)
I use it for tailwheel bushings: (black)
I use it anywhere I need a bearing surface and I am always amazed at how well it handles the dirty filthy abrasive nature of our beloved backcountry.
Aluminum, Delin, POM, Acetal, Nylon, and a host of other materials are relatively cheap when measured in aviation monetary units. On the other hand, CNC time is expensive and very time consuming.
What if we could replace certain applications of Delrin with other common printable plastics such as ABS or PETG for non-consequential bearing & bushing surfaces? 100 hours of testing later, here are some examples of just that.
First, some context.
Below is the 3D model of the control stick torque tube assembly in a highly Experimental PA-11-like Cub. It is an over-engineered celebration of CNC'd 6061 aluminum and 4130 chromoly:
The control stick itself rotates on sealed 618 ball bearings, however there are a variety of wear surfaces that, in typical Cub fashion, are left metal on metal. One such area is the torque tube retaining cap, and the other the traveler tube utilized on J3 & PA-11 type designs which runs the full interior of the torque tube and actuates the elevators.
Note in the above image the torque tube is captured by a 4130 steel cap with grease often being utilized rather than a true bearing surface. This design is lightweight and easy to fabricate, but it is often sloppy in operation and leads to a control stick that feels like a loose tooth.
Another area of metal on metal in early Cub designs is the traveler tube to torque tube. The traveler tube provides the push-pull force on the elevators via a bell crank situated behind the rear seat, and in some (bad) cases can ride along the ID of the torque tube when at full fore/aft travel. The following cross section illustrates the potential interference:
In the above image, teal is the traveler tube with red being the control stick. At full fore/aft stick movement the traveler tube is pulled to the top of the centerline of the torque tube. When the control stick is perpendicular to the torque tube, the traveler tube rotates below the centerline of the torque tube.
The potential front metal-metal interference can be eliminated by a correctly designed control stick pivot position, however it is common to find wear at the rear of the traveler tube. The rear of torque tube must allow for the relative traveler tube angle to change yet needs to limit any metal on metal contact, and this is a perfect candidate for a Delrin bushing. Note the black plug bushing encompassing the traveller tube in the ID of the torque tube:
In section:
Due to my love of Delrin, my first reaction was to simply turn a part in my lathe. This turns out more complicated due to the width of the traveler tube connection tab at the rear bell crank. The optimal bushing has a slot for the wide 4130 tab and a cone-shape in the ID (as seen in the above section):
How do we retain a tight retention of the OD of the traveler tube without having to set up a time consuming CNC two-sided operation to machine this out of Delrin?
As it turns out, this is a thirty minute 3D print job in PETG. The resulting bushing is lightweight, inexpensive, and has excellent wear characteristics. The part is easily replaced during inspection and is non-consequential in operation. (If it were to fail, there would be no consequence in this specific instance).
100 HOBB hours later, here how the test part looked post removal:
This part showed almost no wear and caused no wear marks on the traveler tube. It had limited UV exposure due to its location however it did experience two hot Colorado summers and one very cold Colorado winter. PETG has relatively good heat, chemical, and UV resistance and this part was still firmly pressed into the torque tube when removed.
I concede there is limited load on this part and I suspect PLA might even withstand use in this specific application. It is also important to note I utilize 3D printed parts throughout my entire aircraft in a variety of applications and have a strong understanding of what to expect. This test was specific to utilizing PETG as a wear surface in my standard environmental conditions, and with this baseline information I am confident testing the next PETG wear component.
The above is a torque tube retaining cap with black being 3D printed PETG and silver being CNCd 6061 aluminum. Delrin could easily withstand the minimal compression forces exerted by the two clamp through bolts, however I intend to utilize two internal aluminum sleeves to act as compression members with the PETG simply acting as a sacrificial wear surface.
Will this allow me to eliminate all torque tube slop and stop wiggling the stick like a loose tooth? Check back in 100 more HOBB hours to find out!