1. Rollback Risk on Inclined Conveyors
Any conveyor operating at an incline above the angle of repose of the material being conveyed will experience reverse material movement under gravity when the drive motor is de-energised. For belt conveyors, this is rollback of the belt and load; for screw conveyors, this is reverse rotation of the screw under the weight of material in the trough. The severity of rollback depends on the incline angle, the material bulk density, and the length of the loaded section.
When a loaded inclined belt conveyor rolls back, the belt reverses direction under the weight of material on the carrying side. If the drive is a standard helical or bevel gearbox without a backstop or brake, the rollback speed can reach 2 to 5 times normal belt speed as the load accelerates under gravity. At this speed the belt can jump the troughing idlers, spill material, or snap if the rollback is arrested suddenly by the drive coming back on-line. A NMRV worm gearmotor with ratio 40:1 or above holds the belt stationary instantly when the motor stops, preventing all rollback movement.
An inclined screw conveyor (screw elevator) transporting granular material at angles above 15 degrees will reverse under gravity when the motor stops, discharging material back to the inlet end. At steep angles (30 to 45 degrees), this reverse flow is fast and violent, potentially jamming the inlet with a surge of returned material. The NMRV worm gear reducer self-locking property at 40:1 and above holds the screw stationary when the motor is de-energised, preventing any back-flow of material.
Conveyors on shallow inclines (below 10 degrees) conveying materials with a high angle of repose (coarse aggregate, wood chips, or material on a belt with high sidewall friction) may stop without rollback even without a self-locking drive. However, if the conveyor is restarted under full load without rollback protection, the motor must produce sufficient locked-rotor torque to restart the belt against both the running resistance and the static belt tension — which at shallow inclines is often within the Y2 motor capability without additional measures.
2. Self-Locking Theory of Worm Gears
tan(λ) < μ / cos(α)
where:
λ = worm lead angle
μ = coefficient of friction at worm-wheel interface
α = pressure angle of worm (typically 20°)Lead angle λ decreases as ratio increases — higher ratio = smaller lead angle = more reliable self-locking.
| Ratio 5:1 | λ ≈ 22° — not self-locking |
| Ratio 15:1 | λ ≈ 11° — borderline |
| Ratio 30:1 | λ ≈ 5.5° — usually self-locking |
| Ratio 40:1 | λ ≈ 4.2° — reliably self-locking |
| Ratio 60:1+ | λ < 3° — strongly self-locking |
The self-locking property relies on friction at the worm-wheel contact interface. The coefficient of friction μ between phosphor-bronze wheel and hardened steel worm in normal NMRV gearboxes is 0.08 to 0.15 depending on sliding speed, lubricant viscosity, and operating temperature. At friction coefficient above 0.14 and lead angle below 5 degrees (ratio 40:1 and above), the worm gear is self-locking under all normal operating conditions. Below 0.08 friction coefficient (for example, with a flooded oil bath at high temperature), self-locking may be lost even at high ratios — a key limitation that governs the safe use of worm gear self-locking for inclined conveyor anti-rollback.
3. Gear Ratio Selection for Inclined Conveyor Output Speed
The gear ratio is determined by both the required output speed and the self-locking requirement. For inclined conveyor applications, always select a ratio of 40:1 or above regardless of the required output speed — if the required speed is higher than the minimum achievable at 40:1, the incline angle and load must be reviewed to confirm whether a non-self-locking drive with an external backstop or brake is acceptable.
| Conveyor Type | Typical Output Speed | Min Ratio (Self-Lock) | NMRV Size | Motor Power |
|---|---|---|---|---|
| Inclined belt (light material) | 20–50 rpm | 40:1 min | NMRV 063–075 | 0.55–2.2 kW |
| Inclined belt (medium load) | 15–36 rpm | 40:1 min | NMRV 075–090 | 1.5–5.5 kW |
| Screw elevator (15°–30°) | 30–90 rpm | 40:1 min | NMRV 063–090 | 0.75–4.0 kW |
| Screw elevator (30°–45°) | 20–60 rpm | 60:1 min | NMRV 075–090 | 1.1–5.5 kW |
| Vertical screw (90°) | 15–40 rpm | Brake required | NMRV 075–090 | 1.5–7.5 kW |
Screw diameter = 200 mm. Target throughput = 10 t/h. Material: wheat grain, 780 kg/m³. Screw pitch = 200 mm. Required rpm = (10,000 kg/h ÷ 780 kg/m³) ÷ (π × 0.1² × 0.2 m × 60 min/h × filling factor 0.4) = 12.8 m³/h ÷ 0.094 m³/rev ÷ 60 = approx. 57 rpm. At 4-pole motor 1,450 rpm: ratio required = 1,450 ÷ 57 = 25.4. But to satisfy self-locking requirement, select 40:1 → output 36 rpm. Increase screw diameter or pitch to achieve target throughput at 36 rpm. Select NMRV 075, ratio 40:1, Y2 1.1 kW, 4-pole.
4. Self-Locking Reliability Limits and When to Add a Brake
A worm gear that is reliably self-locking under static conditions can be induced to creep by continuous vibration at the output shaft. If the conveyor drive is subject to significant vibration from the conveyor structure, material impact, or adjacent machinery, the vibratory energy can reduce the effective friction coefficient below the self-locking threshold, allowing slow reverse creep. For applications with significant vibration, add a Y2EJ brake motor in place of the standard Y2 motor on the NMRV gearbox to provide positive holding regardless of vibration conditions.
The coefficient of friction at the worm-wheel contact falls significantly as lubricant viscosity decreases with temperature. At gearbox oil temperatures above 70°C (which can occur at high duty factors or in high ambient temperature environments), the friction coefficient may fall below the self-locking threshold even at ratios of 40:1 or above. Monitor gearbox oil temperature during commissioning and in hot ambient conditions. If oil temperature approaches 80°C, either reduce duty factor, improve ventilation, or add a Y2EJ brake motor to provide positive holding independent of the self-locking friction state.
Specify a Y2EJ brake motor on the NMRV gearbox rather than a standard Y2 when any of the following apply: (1) conveyor incline above 30 degrees where rollback force is high; (2) personnel or product could be injured or damaged by a rollback event; (3) application is subject to significant vibration; (4) ambient temperature above 40°C or expected gearbox oil temperature above 70°C; (5) ratio between 20:1 and 35:1 where self-locking is not reliable. The Y2EJ brake motor adds fail-safe positive holding at no increase in gearbox size or conveyor mechanism complexity.
5. Thermal Duty Considerations for Inclined Conveyor Drives
Inclined conveyor drives typically operate at higher duty factors than horizontal conveyors because the gravitational component of the belt load does not reduce to zero even when the conveyor is lightly loaded — the drive must always overcome the weight of the belt on the return side incline. This increases the average running torque compared to an equivalent horizontal conveyor at the same throughput.
The NMRV gearbox thermal rating is based on continuous S1 duty with the stated input power. Inclined conveyor drives operate continuously (S1) at an elevated average torque compared to horizontal conveyors. Apply a service factor of 1.25 to 1.5 to the calculated gearbox input power when selecting the NMRV size for an inclined conveyor application to ensure the gearbox thermal rating is not exceeded during extended continuous operation under maximum load.
When an NMRV gearmotor is mounted at an angle to drive an inclined conveyor, the standard oil fill quantity and plug positions may not be appropriate for the inclined mounting orientation. Always specify the mounting orientation angle when ordering the NMRV for an inclined conveyor application so that Korea Ever-Power can supply the gearbox with the correct oil fill and plug positions for the specific mounting angle. Incorrect oil fill in a non-standard orientation leads to either oil starvation of the worm mesh (underfill) or oil leakage through the shaft seal (overfill).
6. NMRV Gearmotor Specifications for Inclined Conveyor Drives
Korea Ever-Power NMRV worm gearmotors for inclined conveyor applications are supplied as matched and assembled combinations of Y2 motor or Y2EJ brake motor with NMRV 050 through 090 worm gear reducer at ratios from 40:1 to 100:1. The right-angle output configuration allows the gearmotor to mount at the drive pulley or screw end without requiring a 90-degree bevel gearbox stage, reducing drive complexity and cost. All NMRV sizes accept both Y2 and Y2EJ motor input with no modification to the gearbox. The complete range is available in the gearmotor product section. Contact Korea Ever-Power for inclined mounting orientation specifications and oil fill confirmation.
| Self-locking ratio | 40:1 and above |
| Ratio range | 5:1 to 100:1 (select 40:1+) |
| Motor options | Y2 or Y2EJ brake motor |
| Power range | 0.06–22 kW |
| Output speed (4P, 40:1) | 36 rpm |
| Output speed (4P, 100:1) | 14.5 rpm |
| Output shaft | Solid or hollow bore |
| Mounting | Confirm orientation for oil fill |
7. Inclined Conveyor Applications
Grain and Feed Screw Elevator
Inclined screw conveyors (15 to 45 degrees) transferring grain, feed pellets, rice, and flour in grain handling facilities and feed mills. NMRV 075–090 with Y2 or Y2EJ at 40:1 to 60:1. Screw speeds 20 to 60 rpm. Self-locking at 40:1 prevents reverse flow of grain when motor stops. Hollow bore output option allows direct mount onto screw shaft without separate coupling and coupling guard. |
Inclined Belt Conveyor (Light to Medium Material)
Inclined belt conveyors at 10 to 30 degrees handling light to medium bulk materials (sugar, salt, plastic pellets, grain, coffee beans, lightweight aggregate). NMRV 063–090 with Y2 at 40:1 to 60:1 provides self-locking anti-rollback and slow conveyor speed (24 to 36 rpm at drive roller) suitable for product integrity. For steeper inclines or heavy aggregate, upgrade to Y2 with inline helical gearbox plus backstop device rather than relying on NMRV self-locking alone. |
Municipal solid waste sorting lines use inclined belt conveyors at 20 to 30 degrees to feed material to sorting stations. NMRV 075 at 40:1 to 60:1 with Y2 4-pole 0.75 to 2.2 kW. Self-locking prevents waste material from sliding back and jamming the infeed when the conveyor stops between production runs.
Inclined belt or slat conveyor elevating growing trays between floor levels in vertical farm and greenhouse facilities. Very light load but steep incline (30 to 45 degrees). NMRV 040–050 at 60:1 to 100:1 with Y2 0.12 to 0.37 kW. Self-locking essential to prevent tray descent between cycles.
Biomass and wood chip inclined screw conveyor feeding boiler combustion chamber. Steep incline 30 to 60 degrees. NMRV 075–090 with Y2EJ brake motor at 40:1 to 60:1 — Y2EJ brake motor specified because vibration from the boiler and conveyor chain could release worm self-locking; fail-safe brake provides positive holding between feed cycles.
Inclined belt or slat conveyor in retail display gondola or automated vending system. Very compact installation in store fixture. NMRV 025–040 at 60:1 to 100:1, Y2 or YS 0.06 to 0.18 kW. Self-locking prevents product from sliding back on display when conveyor pauses between customer purchases.
8. Frequently Asked Questions
Edited by Cxm
