Fabricating rigid and lightweight push rods

A fellow club member was instrumental in my decision to go with external dual elevator servos on the Pulse 125 I’m building. During that conversation he shared his way of making rigid and lightweight push rods so I thought I’d pay it forward with a short tutorial.

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Overview

The main component of the push rod is a carbon tube. Fully threaded 4-40 rod is glued into each end for the linkages, and aluminum tube is glued over each end of the carbon tube to prevent splitting.



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Materials

Where appropriate, I’ve included links to the materials I used.

12" Fully Threaded Rods (4-40) - Product number DUB379
Carbon: Pultruded Carbon Tubes: 020001 Pultruded Carbon Tubing: Description O.D.": .1800";Length: 30" special length & price;I.D.": 0.112";O.D.mm: 4.57mm;Wt./gm: 12.9 gm - Product number 020001
36” round aluminum tube, 7/32” - Product number KNS1112*
4-40 Steel Kwik Link (2)*
5-minute epoxy
Masking tape
Paper towels or rags
Acetone

*the manufacturer’s site does not list this product specifically; the link is from my LHS (Great Hobbies)

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Tools (uses)

Rotary tool with cut-off wheel (cutting the carbon tube, aluminum tube and threaded rod)
small file(s) (deburring the aluminum tube)
Ruler (measuring from servo arm to servo horn)
Spring clamps (holding the threaded rod in the tube while the epoxy is setting).

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Preparation

Before beginning, ensure that the servo is installed and electronically centred, and that the chosen servo arm and horn are mounted. Select the holes for use on both the servo arm and servo horn. Secure the control surface in its centre position. Choose the end links you want to use.

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Steps

The length of all the pieces (except the carbon tube) can be decided at the outset - the length of the carbon tube must be calculated as demonstrated in the steps below.
As an aid to understanding I've included general steps (in normal text) and the specific steps I took to create my elevator push rods (in images and italic text). I prefer to work in millimetres when possible.

1. Measurement A - servo arm to horn distance
Measure the distance between the chosen holes on the servo arm and the servo horn. This is measurement A.

Mine was 254 mm (10" exactly).

2. Measurement B - threaded rod lengths
Decide how long you want your threaded rod ends to be, understanding that half will be glued within the carbon tube. The amount protruding is measurement B.

I chose 40 mm in total - 20 mm protruding.

3. End link threading
Decide how far you want to thread your chosen end links onto the threaded rod.

10 mm looked good to me with the 4-40 Kwik Links, giving good space for adjustment in either direction.

4. Measurement C1 - first end link length
Thread your first chosen end link onto a piece of threaded rod the amount chosen in step 3. Measure the distance between the centre of the pin (or hole) on your chose end link and the start of the thread. This is measurement C1.

with 10 mm of thread inside, my 4-40 Kwik Links were 12 mm from pin to threaded rod.

5. Measurement C2 - second end link length
Repeat step 4 for the other end link and record measurement C2.
Since I used 4-40 Kwik Links on both ends, my C1 and C2 were equal.

6. Measurement D - length outside of carbon tube
To determine the length of everything outside of the carbon tube, double B and add it to C1 and C2. This is measurement D (D = 2B + C1 + C2).
For me, D = 2B + C1 + C2 = (2 x 20) + 12 + 12 = 64... 64 mm.

7. Measurement E - carbon tube length
To determine the length of the carbon tube, subtract D from A. This is measurement E.

In my case, A - E = 254 - 64 = 190... 190 mm.

8. Aluminum tube lengths
Decide how long you want your aluminum tube covers to be.

I chose 30 mm.

9. Make the cuts
Cut the carbon tube, aluminum tube and threaded rod to length. Wear eye protection and be sure to use a rotary tool with a cutoff wheel to cut the carbon tube - it works well for the other stuff too.

10. Clean up the parts
To ensure that the ends of the threaded rod are restored, ground or file the burred edge flat - I held it against the rotary tool cutoff wheel (near the centre) and used light pressure. Then taper the edges slightly - I used the cutoff wheel again on a slow setting and rotated the edge of the rod to grind it down.
The aluminum tube will be burred after cutting - use a flat file to remove the burrs on the outside, and a round file to remove the burrs on the inside.

11. Prepare for Epoxy
Wrap some masking tape around the cut threaded rods with one edge at the halfway point. Make sure there's enought tape to create a stop against the carbon tube. Note - the end with the most exposed thread is the one that goes into the carbon tube.

Space two spring clamps apart at the edge of your work surface. The distance between them should be the overall length of the push rod without the end links. This is measurement F (F = A - (C1+C2)). The reason for the spring clamps is to hold the threaded rods in position as the epoxy sets - pressure inside the carbon tube tends to push them out.

In my case, F = A - (C1 + C2) = 254 - (12 + 12) = 230... 230 mm.
Slide one aluminum tube to each end of the carbon tube. Mark the inner point and slide them to the centre.

12. The glue-up (one end at a time)
Mix up some 5-minute epoxy.
Apply a thin coat to one end of the carbon tube between the end and the mark you made at the end of step 11. Slide an aluminum tube from the centre of the carbon tube towards the glued end. When the aluminum tube is in place there may be a buildup of epoxy at the edge - pack as much of it as you can into the carbon tube.
Apply epoxy liberally to the end of the threaded rod and insert it up to the masking tape stop. Clean up the excess epoxy with a rag or a shop towel, first dry and then with acetone applied.
Repeat the above steps for the other end. If necessary, push the threaded rods back in to the stops, and place the assembled rod between the two clamps you spaced apart in step 11.

Once the epoxy has set, peel off the masking tape stops. The push rod is now ready for installation with your chosen links. And they don't weigh much...



... just 6.8g, or slightly less than a quarter of an ounce.