Pre-tensioning

Summary¶

Pre-tensioning is the deliberate introduction of compressive or tensile stress into a joint before any operational load is applied. In libdrone's CF rod and arm architecture, pre-tension is created by interference fit: the arm's rod channel is designed slightly smaller than the rod diameter, so inserting the rod deforms the channel slightly and loads the joint with clamping force that exists before any propeller thrust or crash load arrives. This passive pre-load increases joint stiffness, reduces micro-movement at the interface under vibration, and extends fatigue life without adding mass.

Concept¶

Why joints are the problem¶

A structure fails where stress concentrates. In a drone frame, stress concentrates at joints — the transitions between dissimilar materials, at fasteners, and at the rod-arm interface. A joint under cyclic vibration with no pre-load will develop micro-slip: the surfaces move fractionally against each other under each vibration cycle. Micro-slip generates fretting wear, transfers vibration through the structure rather than damping it, and eventually loosens fasteners or cracks the printed material around the bore.

Pre-tension eliminates micro-slip by ensuring the joint surfaces remain in contact and under compressive load even when the vibration load would otherwise separate them. The pre-load must exceed the maximum separation force from operational loads for this to work.

Interference fit in the CF rod channel¶

The arm rod channel in libdrone is designed with a bore diameter 0.1–0.15 mm smaller than the CF rod nominal diameter. When the rod is pressed into the channel, the printed material deforms elastically — PETG at its yield boundary — generating a clamping force distributed around the rod circumference. This force is the pre-tension.

The magnitude of pre-tension is controlled by the interference amount and the Young's modulus of the printed material. PETG has a Young's modulus of approximately 2 GPa — stiff enough to maintain meaningful clamping force, compliant enough to be pressed in without cracking. PC-CF (used on Pro arms) has a higher modulus and requires tighter interference tolerances to avoid cracking during assembly.

Pre-tension and vibration isolation¶

The floating motor mount on libdrone arms uses an O-ring that applies a separate pre-tension to the motor interface. The O-ring's elastomeric pre-load decouples the motor's vibration from the arm at frequencies above the O-ring's natural frequency. This is a different mechanism from the rod channel interference — the O-ring provides both pre-tension and vibration isolation, where the rod channel interference provides pre-tension and stiffness only. See → floating-motor-mounts and → vibration-isolation-theory.

Reference¶

Interference fit tolerances (libdrone V2.4.x): - Rod nominal diameter: 3.00 mm - Arm channel bore: 2.85–2.90 mm - Interference: 0.10–0.15 mm

Procedure¶

Verify pre-tension on rod insertion¶

1. Dry-fit the rod into the arm channel without adhesive. The rod should require finger pressure to insert but not require a mallet — if a mallet is needed, the interference is too tight; if the rod slides freely, it is too loose.
2. The rod should not rotate in the channel when the arm is held and a 0.5 N·m torque is applied to the rod. If it rotates, reprint with tighter bore tolerance.
3. After full assembly, verify no rod movement is visible when the frame is flexed gently by hand.

Acoustic ping verification¶

The acoustic ping is the field test for rod pre-tension. It requires no tools.

1. Power off the drone. Hold the frame at the body — not the arm.
2. Tap the CF rod near the arm tip lightly with a non-metallic tool (a pen cap or fingernail works).
3. Listen for the resulting tone. A correctly pre-tensioned rod produces a clear, high-pitched ring in the 2.2–2.6 kHz range — audible as a musical "ping" that sustains for 0.5–1 second.
4. A dull thud or very brief low tone indicates the rod has lost pre-tension — it is no longer gripped tightly in the channel. This happens after crashes that deform the channel slightly, or as PETG creep reduces clamping force over many flight hours.
5. If the tone is dull: tighten the pinch slit bolt gradually (¼ turn at a time) until the ring tone returns. Do not exceed the point where the tone goes very high — this indicates over-stress risk.
6. A smartphone spectrum analyser app (e.g. Spectroid) can confirm the exact frequency if the tone is ambiguous to the ear.

Perform the acoustic ping check at the maintenance intervals in → scheduled-maintenance and after every hard landing or crash.

Rationale¶

Pre-tensioning via interference fit was selected over a separate fastener (set screw or clamp bolt) at the rod-arm interface because it adds zero mass and zero parts count. A set screw adds a stress concentration point at the rod surface; a clamp bolt adds a fastener that can loosen under vibration. The interference fit is passive — it cannot loosen, does not require maintenance, and is established correctly or incorrectly at print time. The print tolerance is the quality gate.

Connections¶

requires: - frame-structure-overview - cf-rod-architecture related: - floating-motor-mounts - vibration-isolation-theory - exact-constraint-design - sandwich-structure - petg - bolt-torque-reference leads_to: - floating-motor-mounts - exact-constraint-design - bolt-torque-reference