Given: 2-tonne Chain Block

Worm gear ratio: 30:1. Worm: single-start, lead = 1 × axial pitch = π × module = π × 5 = 15.7 mm. Worm reference diameter d₁ = 50 mm. Lead angle λ = arctan(15.7 / (π × 50)) = arctan(0.100) = 5.71°.

Friction Angle

Lubricated bronze-on-hardened-steel friction coefficient μ = 0.08–0.12. At μ = 0.10 (ISO VG 220 worm oil, warm): friction angle ρ = arctan(0.10) = 5.71°. Self-locking condition: λ ≤ ρ. Here λ = ρ = 5.71° → borderline. At ratio 30:1 Korea Ever-Power uses d₁ = 45 mm: λ = arctan(15.7 / (π × 45)) = 6.32° > ρ — NOT self-locking.

Correct Specification

For reliable hoist self-locking, target λ ≤ 4.5° (safety margin below ρ): at ratio 40:1, λ = arctan(lead / (π × d₁)) = arctan(12.57 / (157)) = 4.57° < 5.71° ✓. Korea Ever-Power recommends ratio 40:1 minimum for safe self-locking in chain block and crane hoist worm gear drives without reliance on a marginal friction condition.

TYPE 01

OVERHEAD CRANE
WIRE ROPE HOIST

Worm gear specification: Ratio 30:1–60:1, PB2 bronze worm wheel, 42CrMo4 worm, lead angle 3°–5° (confirmed self-locking), FEM duty group M3–M5 for general workshop crane, centre distance 80–160 mm depending on lift capacity. Overhead crane hoists (0.5–20 tonne, span 5–30 m) are the highest-volume worm gear hoist application. The crane hoist worm gear must meet FEM 1.001 rating for the applicable duty group, with the mechanical life factor applied to both the worm shaft (steel fatigue) and the worm wheel (bronze contact fatigue). Korea Ever-Power supplies hoist worm gear sets to FEM M3, M4, and M5 ratings with calculation certificate confirming compliance. EN 14492-2 crane directive requires that the hoist brake — separate from the worm gear self-locking — is capable of holding 1.5× the rated load with the motor de-energised.

TYPE 02

CHAIN BLOCK
& LEVER HOIST

Worm gear specification: Ratio 20:1–40:1, PB2 bronze, compact cast iron housing, lead angle 4°–5°. Electric chain block hoists (0.25–5 tonne, hook height 3–30 m) use compact worm gear reducers between the electric motor and the load chain sprocket. The chain block must self-lock reliably in all orientations — horizontal, vertical, and inclined — so the lead angle must be verified at the minimum expected friction coefficient (cold lubricant, worst-case contamination: μ = 0.07 for contaminated worm oil). Korea Ever-Power recommends specifying the lead angle for μ = 0.07 rather than the nominal 0.10, giving a self-locking safety margin that accounts for lubricant degradation over service life. Manual chain blocks (hand-operated) use the same worm gear geometry as electric hoists, with the operator hand-chain replacing the electric motor as the input.

TYPE 03

STAGE &
THEATRICAL HOIST

Worm gear specification: Ratio 40:1–80:1, PB2 bronze, very quiet operation required, EN 17206 (Entertainment technology lifting standard), electronic position control, emergency stop braking <0.3 s. Stage and theatrical hoists carry scenery and lighting equipment above performers and audience — the most demanding safety classification of any hoist type. EN 17206 requires hoists carrying loads above people to have two independent braking systems; the worm gear self-locking contributes one layer of this redundancy. The very high ratio (60:1–80:1) used in stage hoists provides low lifting speed for precise position control (10–50 mm/s typical) and extremely reliable self-locking at lead angles below 2°. Korea Ever-Power supplies stage hoist worm gear sets with individual efficiency measurement (to calculate braking torque contribution) and self-locking verification report on request for EN 17206 compliance documentation.

TYPE 04

GARAGE DOOR
& PLATFORM LIFT

Worm gear specification: Ratio 20:1–40:1, low cost PB2 bronze or thermoplastic worm wheel for residential garage door operators, higher-rated PB2 for commercial platform lifts and scissor lifts. Residential and commercial garage door operators use small worm gear reducers (ratio 20:1–30:1, M1.5–M2.5) between the AC motor and the door drive sprocket or lead screw. The worm gear self-locking holds the door at any intermediate position without a separate brake. Commercial platform lifts (0.5–2 tonne capacity) use larger worm gear sets (ratio 30:1–50:1, M4–M6) to the scissor lift hydraulic pump or mechanical screw drive. Korea Ever-Power supplies worm gear sets for all common garage door operator models and for commercial platform lift replacement and OEM production.

Q 01

What is the difference between self-locking and self-braking in worm gear hoists, and which do I need?

Self-locking is a static property — under a sustained static load with no motor input, the worm gear cannot be back-driven because static friction at the tooth contact exceeds the reverse driving force. This is the condition calculated from lead angle vs friction angle. Self-braking (also called self-sustaining) is a dynamic property — the worm gear decelerates and stops a descending load without the motor applying braking torque, relying on friction to absorb the kinetic energy of the moving load. A worm gear that is statically self-locking may not be dynamically self-braking under shock loading or at high lowering speeds, because the dynamic friction coefficient is lower than the static friction coefficient (typically μ_dyn = 0.07–0.09 vs μ_static = 0.10–0.12). For hoist applications: self-locking worm gears (lead angle <5°) reliably hold static loads and are the basis of worm gear hoist drive design. Mechanical brakes (failsafe spring-applied, electromagnetically released) are required by EN 14492-2 and EN 17206 for all electrically-driven hoists as the primary braking mechanism, with the worm gear self-locking as secondary. Korea Ever-Power designs all hoist worm gear sets for reliable static self-locking and provides documentation confirming this property.

Q 02

How do I determine the FEM duty group for a worm gear hoist replacement?

FEM duty group (also referred to as ISO 4301 mechanism group) is determined by two factors: (1) The load spectrum class (L1–L4) — how often the hoist lifts its maximum rated load vs partial loads during a typical working period. L1: rarely at max load; L4: almost always at max load. (2) The utilisation class (T1–T8) — the total number of operating hours over the design life of the hoist mechanism. Combining spectrum class and utilisation class from the FEM or ISO tables gives the duty group M1–M8. For replacement purposes: if you have the original hoist manufacturer’s duty group designation from the nameplate or manual, specify the same duty group for the replacement worm gear set. If the hoist has been upgraded (heavier loads, more cycles per day than original design) you may need to specify the next higher duty group. Korea Ever-Power can calculate the required duty group from your operating data (rated load, actual load distribution, lifts per hour, and annual hours) and confirm the worm gear design to meet it — contact Korea Ever-Power with these parameters for a compliance calculation before ordering the replacement worm gear set.

Q 03

My crane hoist worm wheel shows progressive surface pitting on the tooth flanks — is this a lubrication problem or a gear specification problem?

Worm wheel bronze tooth flank pitting in hoists has three root causes with different remedies: (1) Contact stress exceeds bronze fatigue limit — the hoist is being operated at loads above its FEM duty group rating, or the original gear was under-specified. Remedy: replace with a larger-diameter worm wheel or higher-strength AB2 aluminium bronze wheel, confirm the replacement to the actual operating duty group. (2) Lubricant film breakdown — the worm oil viscosity is too low for the actual operating temperature, or the oil change interval was exceeded, allowing the EP additive package to deplete. Remedy: drain and flush, refill with ISO VG 460 worm gear oil (heavier grade), and reduce change interval to 2,000 hours or annually. (3) Misalignment between worm and wheel — the contact pattern is shifted to one side or the toe of the tooth, concentrating stress in a small contact area below the full-face capacity of the wheel. Remedy: check centre distance and shaft parallelism; the contact pattern under blue marking test should cover the central 50–65% of the tooth face. Korea Ever-Power can advise on replacement specification based on inspection photographs of the failed worm wheel — contact Korea Ever-Power with high-resolution photographs of the tooth surface showing the pitting location and pattern.

Spur Gears

M1–M20 · C45, 20CrMnTi, 316L

Helical Gears

Low noise · high contact ratio · gearbox

Ingranaggi conici

Right-angle drives · straight & spiral

Worm Gears

Self-locking · hoist · valve · solar

Ring Gears

Internal · planetary · wind, excavator

Planetary Gears

Compact · high torque density · servo, EV

Plastic Gears

POM · MC Nylon · UHMWPE · FDA