Shaft Mounted Explosion Proof Helical Worm gearbox

A clarifier bridge in a municipal wastewater works turns at roughly 20 rpm and never stops. Fifty metres from the same basin, a covered sludge channel accumulates methane and hydrogen sulphide above the liquid surface. Any gearbox installed over the channel sits inside a classified hazardous area whether the plant operator has drawn the boundary or not.

The MNAT69-YB0.75-4P-0°-X-65.88-M1-B-Φ40 gearbox from TANHON was configured for exactly the described service envelope. The unit combines a three-stage helical worm gear train, a 40 mm hollow output bore with torque arm fixing, and a 0.75 kW four-pole flameproof motor rated for continuous operation in a potentially explosive atmosphere.

The Engineering Challenge: Low Speed, High Torque, Zero Tolerance for Ignition

Clarifier rake mechanisms, submerged scrapers, flocculation paddles and API separator flight chains share one load profile. Output speed falls below 25 rpm, output torque stays high, and the duty cycle is 24 hours a day, 365 days a year.

Three separate engineering problems converge on the gearbox at the top of the shaft. Each one carries a different failure cost, and each one has to be closed at the specification stage rather than after commissioning.

The first problem is ignition risk. Sewer gas in a wastewater plant and hydrocarbon vapour in a refinery both create Zone 1 and Zone 21 conditions where a hot housing surface or an arcing terminal box becomes an ignition source.

The second problem is corrosion and seal integrity. Aggressive process water attacks the housing from outside while gear oil sits inside, and a failed output shaft seal turns a maintenance job into a reportable environmental incident when oil enters the treated liquid.

The third problem is thermal capacity. A worm stage generates heat in proportion to its sliding losses, and a gearbox running continuously at i=65.88 has no cooling-down window in which to shed the accumulated heat.

Closing the Ignition Risk and Certification Gap

The YB-series 0.75 kW four-pole motor fitted to the gearbox is a flameproof machine, protection type Ex db IIB IP66, with a T4 temperature class limiting maximum external surface temperature to 135 °C. The rating suits methane, hydrogen sulphide and the majority of refinery vapour groups encountered in covered channels, digester galleries and pump halls.

Gas group IIB covers ethylene and the hydrogen sulphide mixtures typical of anaerobic sludge treatment. Where a petrochemical operator carries hydrogen in the risk register, TANHON supplies the same gear case with a IIC-rated motor without altering the gearbox interface dimensions.

The non-electrical portion of the assembly matters as much as the motor. The gear case, the output shaft bearings and the fan cowl are assessed as an ignition source in their own right under ATEX 2014/34/EU, and TANHON builds the unit to category 2G and 2GD requirements for Zone 1 and Zone 21, with category 3GD variants available for Zone 2 and Zone 22 boundaries.

Documentation is supplied with the gearbox rather than assembled afterwards by the buyer. ATEX declarations of conformity, IECEx documentation for projects outside Europe, and HazLoc-NA equivalents for North American sites remove the audit burden from the procurement file. TANHON operates under ISO 9001 and ISO 14001 with CE and EAC marking across the export range.

Corrosion Protection and Sealing Against Oil Ingress

A gearbox bolted over an aeration basin lives in a saturated, chloride-bearing, sulphide-bearing atmosphere. The MNAT69 gearbox is finished to ISO 12944 C4 as standard, with a C5-M system of epoxy zinc-phosphate primer and polyurethane topcoat to a total dry film thickness of 240 µm available for coastal and offshore refinery installations.

The Φ40 hollow output shaft carries the critical sealing interface. TANHON fits dual FKM radial shaft seals in tandem with a grease-packed cavity between the two lips, so a failure of the inboard seal does not immediately release gear oil into the process liquid.

FKM elastomer was selected over standard nitrile for a specific reason. Hydrogen sulphide, chlorinated cleaning chemicals and hydrocarbon carry-over harden nitrile lips within 12 to 18 months, while FKM retains elasticity across a −20 °C to +200 °C range and resists the aggressive media found in both target industries.

A labyrinth deflector ahead of the outer seal keeps abrasive grit and washdown water away from the lip contact zone. On sludge scraper installations subject to daily high-pressure hosing, the arrangement is the difference between a five-year seal life and an annual intervention.

Three-Stage Gear Reduction and the Thermal Argument

A single-stage worm reducer achieving i=65.88 in one bite operates at a lead angle where sliding friction dominates. Efficiency typically falls to 55–65 %, and the lost 35–45 % of input power leaves the gearbox as heat through the housing walls.

The MNAT69 gearbox takes a different route to the same overall ratio. Two hardened helical stages handle the first portion of the reduction at 96–98 % efficiency per stage, leaving the worm stage a far lower single-stage ratio and a much more favourable lead angle.

Measured combined efficiency for the three-stage arrangement sits in the 82–86 % band. At 0.75 kW input, heat rejection drops from roughly 300 W for a comparable single-stage worm unit to approximately 115 W — a reduction of more than 60 % in the thermal load the housing has to dissipate.

The margin is what makes maintenance-free continuous duty realistic. Oil sump temperature stabilises well below the 90 °C threshold at which synthetic lubricant degradation accelerates, and the gear case surface stays within the T4 temperature class limit even at 40 °C ambient.

Charged with PAO synthetic gear oil to ISO VG 220, the gearbox reaches a 15,000 to 20,000 hour lubricant interval against the 3,000 hours typical of mineral oil fills. For a clarifier running 8,760 hours a year, the difference is one oil change every two years instead of three per year, each one requiring a confined-space permit.

Service Factor and Load Reserve

Applied torque at the output shaft is approximately 285 N·m at 1400 rpm input. The size 69 gear case carries a catalogue continuous rating in the region of 600 N·m, giving a mechanical service factor above 2.0 against the nominal motor loading.

The reserve is not over-specification. Clarifier rakes encounter grit build-up, ragging and seasonal sludge density swings, and a scraper breaking free from a settled deposit imposes a starting torque of two to three times the running value.

The three-stage gear train handles the peaks without recourse to the shear pin protection often needed on marginally sized units. Where a plant requires positive overload protection regardless, TANHON fits a torque-limiting coupling at the hollow shaft interface as a factory option.

Hollow Shaft Mounting and Torque Arm Fixing

The Φ40 hollow bore slides directly onto the driven shaft, eliminating the coupling, the bedplate, the alignment procedure and the four foundation bolts a foot-mounted arrangement would require. On a rotating clarifier bridge where every kilogram of deadweight loads the centre bearing, the saving is structural as well as economic.

The torque arm absorbs the reaction moment through a single anchor point. Fitted with an elastomer buffer, the arm isolates the gear case from the shock loading transmitted back through the rake arms when a scraper meets an obstruction.

Mounting position M1 with shaft arrangement B places the motor at 0° relative to the output centreline. The geometry keeps the flameproof terminal box accessible from the walkway side for periodic inspection, an important consideration when the enclosure joints require verification under a hazardous area inspection regime.

Shrink discs, keyless bushings and a plain keyed bore are all available on the same Φ40 interface. For petrochemical mixer applications where the driven shaft must be withdrawn for internal inspection, TANHON supplies the hollow shaft with an extraction thread machined into the outboard end.

Application Fit

The MNAT69 gearbox suits primary and secondary clarifier rakes, thickener bridges, flocculators, screw conveyors handling dewatered cake, bar screen rakes and grit classifiers throughout a wastewater treatment works. Every one of the listed machines operates in the 15–30 rpm output band the i=65.88 ratio delivers directly.

In petrochemical service the same configuration handles API separator flight chains, sludge scrapers in oily water basins, small agitators in additive dosing skids and slop tank mixers. Hazardous area classification is normally the binding constraint, and the flameproof motor pairing addresses it without a separate enclosure or purge system.

TANHON has manufactured gearboxes in Ruian City, Zhejiang Province since 1989, holding 98 independent patents across 4 invention, 62 utility model and 32 design registrations. Production capacity reaches 80,000 units per month across a power range from 0.18 kW to 4,000 kW, with more than 30 series and over 100,000 configuration variants available to project engineers.

Specifying the Correct Variant

Confirm four items before releasing an order. The hazardous area zone and gas group determine the motor protection type; the driven shaft diameter and length determine the hollow bore and any bushing requirement; the ambient temperature and washdown regime determine the coating class and seal material; and the peak breakaway torque determines whether the standard 2.0 service factor is adequate.

Supply frequency also affects the outcome. At 60 Hz the same gearbox delivers 26.6 rpm and a proportionally lower output torque of approximately 228 N·m, a point worth verifying against the rake manufacturer’s speed requirement before a North American installation.

TANHON’s engineering team reviews the load profile, duty cycle and certification requirement and returns a dimensioned proposal with the thermal rating calculation attached. For plants standardising a fleet of clarifier gearboxes, a single configuration across multiple basins reduces the spare parts holding to one gear case and one seal kit.