Fejltilstande i snekkegear — 7 årsager, visuelle signaturer og forebyggelse

Failure Mode 1 — Abrasive Wear

Visual signature on the tooth surface: Smooth, uniformly dull surface across the tooth face contact zone. No discrete pits or tears. The tooth profile is progressively shortened — the tooth tips are slightly rounded and the tooth root fillet may be barely perceptible in severe cases. Under magnification (10×–20× loupe), fine parallel scratches are visible in the direction of sliding, like a brushed metal finish.

Root cause: Abrasive particles in the lubricant are cutting microscopic grooves in the bronze tooth surface at every mesh cycle. The particles typically originate from: (1) running-in debris from the initial hours of operation that was never removed by an oil change; (2) external contamination from a failed housing seal; (3) metallic particles from a bearing that began to fail before the gear set; (4) wear products from a previous gear replacement that were not fully flushed from the housing. The particles are too small to see without magnification, but their presence is confirmed by the directional scratch texture on the tooth surface.

Corrective action: Replace the failed wheel and inspect the worm thread surface for similar scratch damage. Drain the housing completely, flush with clean solvent, and inspect all housing seals and vent plugs. Replace all seals found on the housing. Refill with fresh oil confirmed free of contamination. Establish an oil change schedule — at minimum at 50–100 hours (running-in change), and at intervals not exceeding 2,000 hours or 12 months thereafter. If bearing wear is identified as the particle source, replace bearings before installing the new gear set.

Failure Mode 2 — Adhesive Wear (Scuffing / Galling)

Visual signature: Rough, torn, or smeared surface texture. Areas of material transfer — bronze material pulled from the tooth and deposited in adjacent zones, or steel material from the worm transferred onto the bronze tooth surface. The contact zone has a rough, dull appearance with directional tearing marks. In severe cases, tooth surfaces have visible furrows or ridges from material transfer. A distinctive feature: the worm thread surface often shows bronze smearing in the corresponding contact zone.

Root cause: The lubricant film at the mesh contact has broken down, allowing metal-to-metal contact between the bronze tooth and the hardened steel worm thread. The contact pressure and temperature at the point of metal contact momentarily weld the two surfaces together; as sliding continues, the bond tears and pulls material from one surface to the other. The most common causes of film breakdown in worm drives are: (1) sustained operation above the thermal rating of the drive, causing oil temperature to rise above the lubricant’s viscosity stability limit; (2) using EP gear oil whose sulfur additives have chemically attacked the bronze tooth surface, reducing its hardness and increasing its reactivity; (3) running the drive without lubrication after a seal failure or lubricant loss; (4) starting under heavy load before the lubricant has reached operating temperature in cold conditions.

Corrective action: Replace both the worm shaft (which will also show adhesive damage in the contact zone) and the worm wheel — adhesively damaged worm thread surfaces damage replacement wheels at the same rate as the original. Confirm that the replacement lubricant is free of sulfur EP additives. If the drive was running above thermal rating, add forced cooling (fan or oil cooler) or derate the application load. If cold-start adhesion is the cause, specify a lower-viscosity oil grade for winter operation or install a pre-heat element in the housing for cold climates.

Failure Mode 3 — Pitting and Spalling (Surface Contact Fatigue)

Visual signature: Small, approximately hemispherical craters on the tooth contact face, concentrated in the middle third of the tooth height (the pitch zone) where contact stress is highest. Early-stage pitting shows a few isolated pits with smooth, rounded edges — this is the initial pitting phase. Destructive pitting (spalling) shows larger, irregular cavities with sharp edges and loose fragments of tooth material partially detached. The surrounding surface between pits may be smooth and normal-looking in the early stage.

Root cause: Cyclic contact stress at the tooth surface exceeds the fatigue limit of the bronze material. Each time a tooth enters and exits the contact zone, the subsurface stress field cycles from zero to a peak value and back. Over millions of cycles, a crack initiates at a subsurface stress concentration — an inclusion, a pore in the cast bronze, or a machining mark that was not removed during running-in. The crack propagates to the surface and a pit forms when the crack reaches the surface on two faces and the enclosed volume spalls out. The primary causes of premature pitting are: sustained operation above the module’s rated torque, point-contact mesh due to a wheel cut with the wrong hob type (no line contact), and excessive operating speed that prevents the lubricant film from fully forming.

Corrective action: Replace the wheel and inspect the worm thread for matching fatigue marks. Verify the actual operating torque against the rated continuous torque for the existing module — if the drive is consistently overloaded, upsize the module. Verify that the replacement wheel was cut with a worm-profile hob (confirming line contact) by inspecting the contact pattern under marking compound at assembly. If overload is occasional (start-up torque spikes), check whether a soft-start drive controller can reduce the peak stress.

Failure Mode 4 — Corrosive Wear

Visual signature: Rough, etched surface texture on the tooth face — not the smooth polished appearance of mechanical abrasive wear, but a chemically attacked surface with a grainy, matte appearance and possible discoloration (greenish or dark brown for bronze, rust-colored for steel). The attack may be concentrated in crevice areas — the tooth root, the bore keyway, or any surface recess where corrosive fluid accumulates. In severe cases, material is visibly missing from corroded areas that have dissolved rather than worn away mechanically.

Root cause: Chemical attack on the tooth surface, either from: (1) sulfur or chlorine EP additives in the gear oil reacting with the copper and tin content of the bronze wheel — this is the most common corrosive failure mechanism in bronze worm wheels and is entirely preventable by lubricant selection; (2) water contamination of the gear oil from a failed seal in a humid or wet environment — water carries dissolved oxygen that causes direct metal corrosion; (3) acidic or alkaline process fluids contacting the gear in food, chemical, or agricultural equipment. EP oil corrosive attack on bronze is particularly insidious because it progresses slowly and invisibly — the tooth surface roughens gradually, abrasive wear accelerates, and the drive eventually fails from what appears to be standard abrasive wear but originated from chemical softening of the tooth surface.

Corrective action: Replace the wheel and immediately change to a lubricant confirmed free of sulfur and chlorine EP additives. For wet or washdown environments, replace all housing seals and confirm the housing IP rating is appropriate for the environment. For process fluid contact, specify SS316 worm gear components and food-grade lubricant. When the new drive is installed, schedule a first oil analysis at 500 operating hours to confirm no corrosive contamination is occurring with the new lubricant.

Failure Mode 5 — Tooth Fracture

Visual signature: One or more teeth broken off, leaving a clean fracture face at the tooth root. The fracture surface character identifies the loading mechanism: a dull, fibrous fracture surface with visible deformation at the edges indicates ductile overload — the tooth bent and tore under a single extreme load event. A bright, granular, crystalline fracture surface with no deformation indicates brittle fracture — the tooth separated cleanly without bending, typically in a material that has become brittle from improper heat treatment or from operating at extreme low temperature. Beach marks radiating from an initiation point at the tooth root fillet indicate fatigue fracture — the tooth cracked progressively over many load cycles before the final fracture.

Root cause: For bronze wheel tooth fracture: ductile overload from a sudden impact load exceeding the tooth’s ultimate strength — machine jam, obstruction hit, or start-up shock. Fatigue fracture in bronze indicates the tooth root stress has been cycling above the material’s fatigue limit, typically from a slight contact alignment problem that concentrates load at the tooth root rather than the tooth face. For hardened steel worm shaft thread fracture: brittle fracture at the induction hardening case-core boundary under shock loading (change to through-hardened 40Cr material), or fatigue fracture from repeated overload cycling.

Corrective action: Replace both components — a tooth fragment from a fractured wheel will typically damage the worm thread before it is expelled, and the worm thread damage will destroy the replacement wheel quickly. For ductile overload: identify and eliminate the overload source — add torque limiting clutch, reduce impact loading, or upsize module. For fatigue fracture from alignment: verify worm shaft axial play, check housing bearing wear, and confirm the contact pattern is centered on the tooth face. For brittle steel shaft fracture in impact applications: change to 40Cr through-hardened worm material — see agricultural machinery section for this specific failure pattern.

Failure Mode 6 — Misalignment and Edge Loading

Visual signature: The contact pattern is shifted to one side of the tooth face or concentrated at the tooth tip or root rather than centered on the mid-face. The worn zone does not extend across the full theoretical contact area — one edge of the tooth face shows heavy wear or pitting while the opposite edge is barely touched. In severe misalignment, edge loading produces a line of heavily worn or pitted material running parallel to the tooth width at one end of the face, while the opposite end shows no contact marks at all.

Root cause: The center distance or angular alignment between the worm and wheel shafts is not correct. The most common causes in field conditions are: worn housing bearings that allow the worm shaft to deflect under load (increasing the center distance dynamically), a housing that was damaged and then repaired with incorrect bearing bore position, corrosion of the bearing seats that has slightly displaced the shaft centerlines, or installation error where the housing was reassembled with incorrect shims or bearing preload settings. Note that slight contact pattern offset (10–20% off-center) is normal and does not indicate a problem — only contact that is completely absent from one side of the tooth face warrants investigation.

Corrective action: Replace the worn gear set and perform a contact pattern check with marking compound at assembly before final housing bolt-up. Adjust center distance and worm shaft axial position until the contact pattern covers at least 50–60% of the tooth face width, centered on the tooth face. Replace any housing bearings that show measurable play. If the housing bearing bores have been damaged or corroded beyond recovery, the housing should be replaced — installing a new gear set in a distorted housing will cause the same edge-loading failure within months.

Failure Mode 7 — Seal Failure and Lubricant Loss

Visual signature: The gear set itself may show any of the failure modes above — particularly adhesive wear from dry running, or corrosive attack from water ingress. The distinguishing diagnostic is the housing and shaft condition: oil staining on the exterior housing surface around the output shaft or input shaft seals, white emulsified oil if water has entered, or a housing that is almost entirely dry of oil when opened despite being filled at the last service interval. The gear failure is secondary — the primary failure is in the sealing system.

Root cause: Lip seal failure on the worm shaft or wheel shaft is the most frequent sealing failure mode in field worm gear drives. Lip seals fail from: shaft surface wear in the seal contact zone (creating a circumferential groove that the new seal cannot seal even if the old seal is replaced), installation damage to the seal lip during assembly, oil temperature above the seal’s rated limit causing rubber compound degradation, or shaft runout that causes the seal lip to lose contact during each revolution. Blocked housing vent plugs cause internal pressure buildup that forces lubricant past the seals faster than normal lip seal wear would allow — always check the vent plug condition when investigating seal leakage.

Corrective action: Inspect the shaft surface in the seal contact zone before installing the replacement seal — if a visible groove has been worn into the shaft by the old seal, the replacement seal will not seal correctly at the same shaft position. Either install the new seal at a slightly different axial position using a seal-fitting sleeve, or replace the shaft section. Replace all seals during the gear replacement procedure — do not attempt to reinstall the old seals even if they appear intact. Verify the vent plug condition and replace if blocked. Confirm the replacement oil viscosity grade is within the seal’s rated temperature range.

Daily / Per-Shift Visual Inspection

◆ Check housing exterior for oil seepage around shaft seals

◆ Check oil sight glass or dip stick — level should be within marked range

◆ Listen for any change in noise character — new tonal content or increased amplitude indicates developing tooth damage

◆ Check housing temperature by hand — uncomfortable hot (above approximately 60°C) indicates lubrication problem or overload

◆ Check vent plug is clear — insert a pin if any doubt

Monthly Operational Check

◆ Drain a small oil sample — check color (dark brown or black = overheated, milky white = water), smell (acidic or burning), and particle content (run a clean white cloth over the drain spout)

◆ Check output shaft backlash — mark a position and measure angular play with a dial indicator at a known radius

◆ Check output shaft radial runout — indicates bearing wear

◆ Confirm motor current draw is within normal range — increasing current at same load indicates increasing friction from gear wear or oil degradation

Annual Internal Inspection

◆ Drain and inspect oil — metallic particles are evaluated (bronze or iron), volume of particles estimated against previous year

◆ Open inspection cover (if provided) or measure output shaft torque vs baseline to estimate gear wear state

◆ Replace all lip seals and vent plug — treat as scheduled replacement items, not inspect-and-reuse items

◆ Check bearing end-play and radial play — replace bearings approaching limits

◆ Refill with fresh oil of confirmed bronze-compatible grade — note the oil brand and grade in the maintenance record