3.1 Harness Routing & Securing
Strain relief, bend radius, and fastening methods.
Harness routing is where electrical intent meets real-world motion, heat, and vibration—and it quietly decides long-term reliability. The aim is simple: protect connectors and jackets from strain while keeping signals calm for EMC (electromagnetic compatibility) and noise-free operation. Clear rules of thumb—like “first clamp before first bend” and sizing bends by cable OD (outer diameter)—turn tricky spaces into predictable, repeatable paths, even for FFC/FPC (flat flexible cable/flexible printed circuit). Good routing also manages NVH (noise, vibration, harshness), steering bundles away from fans, hinges, and hot zones so nothing buzzes, chafes, or cooks. With the right anchors and a little discipline, wiring fades into the background—and the box ships solid and stays that way.
3.1.1 The goal (in one line)
Make cables easy on the connectors and invisible to trouble—no strain, no chafe, no heat soak, no noise—so the unit survives shipping, use, and time.
3.1.2 Golden rules you can build by
- First clamp before first bend: place a cushioned clamp 50–80 mm from the backshell/connector.
- Bend radius (minimum):
- General cable, static: ≥ 6× OD; dynamic: ≥ 10× OD
- Shielded/stiff jackets: ≥ 8× OD static
- Coax: ≥ 10× OD
- FFC/FPC: follow maker; no crease at latch
- Keepouts: ≥ 10 mm from sharp edges or heatsinks; add edge guards anywhere you cross a metal line.
- Separate & cross smart: keep power/switching runs ≥ 100 mm from low-level signals; if you must cross, do it at 90°.
- Hug metal: route near chassis rails and clamp every 200–300 mm to shrink loop area and cut EMI.
- Labels live free: never under clamps/boots; place 50–80 mm from backshell, readable after install.
3.1.3 Fastening options (pick by environment, not habit)
Tips: rounded-head ties only; trim flush. No bare zip-ties on insulation—use over sleeves or saddles.
3.1.4 Surface prep for adhesive anchors (why some “fall off”)
- Wipe land with IPA (or approved cleaner); dry.
- Warm surface to ≥ 15–20 °C.
- Press with firm roller for 3–5 s; hold position (no load) while adhesive dwells—initial grip now, full bond in ~24 h.
- Avoid silicone residues; don’t place on paint seams or texture too rough to wet.
3.1.5 Strain relief at connectors (save the solder and pins)
- Back shells/boots installed; first clamp 50–80 mm away.
- Route the first bend after that clamp—never right at the cup/latch.
- For heavy cables, use two-point relief (backshell + P-clamp line).
- Use lacing tape or tie over sleeve, not bare insulation; no cutting grooves into jackets.
3.1.6 Routing order (simple, repeatable)
- Landmarks in first: install edge guards, grommets, and the first P-clamps at datum holes.
- Lay the trunk: run the main bundle along chassis rails; loosely clip in every 200–300 mm.
- Branch & loop: set service loops to drawing (e.g., 80 ±10 mm) with a go/no-go gauge (20.3/19.5).
- Tighten & trim: final torque on clamps, trim ties flush; add NVH tape where parts might buzz.
- Verify: check bend radii, door/cover pinch, fan clearance, and label visibility.
3.1.7 Heat & moving parts (where harnesses go to die)
- Keep away from >85–100 °C surfaces; if you must pass close, add heat sleeve or reflective wrap and a standoff bracket.
- Maintain ≥ 10 mm from fan blades and vents; don’t drape across airflow.
- Hinges/doors: route on the hinge-neutral axis; add strain-relief loop that survives full swing; no rub at max open/close.
3.1.8 EMC-friendly routing (quiet by geometry)
- Short, close returns: run power and its return together; avoid big loops.
- Bond early: shields bond at the enclosure entry (23.4/19.4); avoid long internal pigtails.
- Chassis hugs: clamping along metal reduces radiated area; cross noisy nets at 90° to sensitive harnesses.
3.1.9 Acceptance cues (fast eyes, no debate)
3.1.10 Common traps → smallest reliable fix
3.1.11 Quick tests that pay off
- Door/cover swing test: full motion; no scrape or tug.
- Shake & listen: light rattle test; add NVH tape/clamps where needed.
- Thermal sniff: after a 5–10 min load run (22.1/21.2), touchless IR—no cable segment should exceed spec by routing.
- Tug @ clamp: firm pull shows clamp takes the load, not the connector.
3.1.12 Pocket checklists
Before routing
- Edge guards & grommets installed; lands clean
- Clamps picked (size/spacing); heat sleeves staged
- Loop lengths and clamp points on drawing
During
- First clamp ≤80 mm from backshell; no bend before it
- Radius meets rule; crossings at 90°; away from heat/fans
- Clamps every 200–300 mm; ties trimmed flush; labels visible
After
- Door swing clear; no pinch; shake test quiet
- IR spot check near hot zones; OK
- Photos (route, clamps, labels) attached to SN if required