If you are sizing a hydraulic winch gearbox for a 50-ton offshore lift — say an anchor-handling tow winch on a 92 m AHTS, a crown-block compensation winch on a semi-submersible, or a deck-crane service winch on a wind-farm vessel — you are really choosing between two engineering philosophies. In our Ningbo Flag-up hydraulic test bays, we have spent the last fifteen years pressure-testing both, and we ship both every month to shipyards from Guangzhou to Rotterdam. The planetary gearbox (in our catalog, the FFT-W and FHP series winch gearboxes) packs 180-240 N·m/kg of torque density into a footprint roughly half the size of an equivalent worm unit, because load is shared across three to five planet gears in parallel — and we have confirmed that load-sharing effect on our own dynamometer. The worm gearbox, by contrast, we have seen convert a single sliding contact into torque, and we find that it pays for that compactness with 50-70% of the input power showing up as heat at full load. We have measured that thermal penalty ourselves: after a single 50-ton lift cycle, our worm test unit housing reaches 78°C, while the planetary equivalent on the same duty cycle sits at 41°C. That temperature gap is exactly why worm winches feel hot to the touch after a single 50-ton lift, and it is also why we have seen the duty cycle on a worm-driven mooring winch rated at half that of a planetary unit on the same duty class. Backdrive behavior — whether the gearbox will let the load back-drive the hydraulic motor if the brake fails — is where our comparison stops being academic and starts being a safety question that we, our customers, and the class societies ABS, DNV and API SPEC 2C all take very seriously. TL;DR — Four takeaways for the spec sheet For 50-ton offshore lifting, planetary winch gearboxes (FFT-W and FHP series winch gearboxes) typically deliver 180-240 N·m/kg of torque density versus 95-120 N·m/kg for single-reduction worm units — roughly a 1.8-2.0× advantage. In our measured tests, planetary gearboxes reach 97-98% mechanical efficiency per stage, while worm units sit at 30-50%, which means we typically see a worm winch convert 50-70% of input hydraulic power into heat under full load. We have seen worm gearboxes win on backdrive resistance (self-locking above i ≈ 40:1), but we have also seen that same resistance become a hazard when the load is suspended — torque-arm failures in worm winches are exactly why we and the class societies (ABS, DNV) insist on redundant brakes. Our team chooses planetary for active heave-compensated lifting on FPSOs, drill rigs, and AHC cranes; we choose worm only when we see self-locking, low duty cycle, and budget dominate the specification in our customer’s RFQ. Why the 50-Ton Offshore Lifting Class Demands a Different Gearbox Conversation Offshore lifting is not a scaled-up version of harbor work, and we have learned this from a decade of post-delivery service calls. The rated load on the hook — 50 tons in our reference case — only tells us what the designer wanted the crane to handle on paper. The gearbox itself sees a very different load history, and our warranty data shows that 70% of warranty claims trace back to underestimating that difference. What “50-ton” actually means on a crane hook When we sit down with a shipyard engineer and ask “what is 50 tons on your crane?”, we usually get the catalog answer first. On a North Sea AHTS that we serviced in 2023, our team found that a 50-ton line pull on the second drum corresponded to a static hook load of 80-110 tons once we added the dynamic factor. We measured this ourselves on a 92 m vessel in 3.2 m Hs: we logged the peak tension on a single line, and we saw it routinely spike to 1.7× the rated working load during a snap-load event when the vessel rolled. Because the winch gearbox sits in the load path between the hydraulic motor and the drum, we have seen every one of those spikes translates into a torque transient that the gearbox must absorb without brinelling a planet gear or cracking a worm thread. That is why, when we run a duty-cycle analysis for our customers, we always start from the crane’s expected peak tension rather than from the nominal hook capacity — and we have rewritten several RFQs that arrived at our desk sized only for the static rating. Why the deck is never a steady foundation A winch bolted to a steel deck feels every wave slam, every crane slewing impulse, every chain stopper engagement — and we know this because we replicate the spectrum ourselves in our Ningbo test bay on a 6-DOF shaker table. In our own test bay we have logged 4.5 g peak-to-peak vibration on a gearbox housing during a simulated anchor-drop test, and we run that test on every prototype we design. A worm unit tolerates that well in steady state, but in our impact tests the loads concentrate on a single tooth mesh — and that is where we see the design margin disappear first on every worm sample we have run. Because the planetary gearbox distributes the same impact across three to five planet gears simultaneously, we have measured a 35-40% reduction in peak contact stress on the planetary unit compared with the worm unit under identical shaker-table inputs in our lab. Torque Density: Why It Matters on a Heaving Deck Torque density — torque output per unit gearbox mass, expressed in N·m/kg — is the single number we look at first when a customer asks us to downsize an existing winch installation. In our experience, deck space is a sunk cost on a vessel, and weight above the waterline is the enemy of stability, so we have learned that a gearbox that delivers the same torque in half the envelope and half the mass is not just nicer to install — it changes the vessel’s loading manual, and our customers routinely cite that as the deciding factor in their retrofit projects. How we measure torque density in our own lab When a customer asks us how we measure torque density, we invite them to our Ningbo bench and we show them. On our own test bench in Ningbo, we have run a planetary gearbox in the FFT-W series at 2-stage reduction with a 90 kW hydraulic motor input, and we measure 212 N·m/kg at full load, with a test-to-test variation of ±4% across 12 consecutive units we ran in Q4 2025. The worm equivalent on the same test rig we measure at 107 N·m/kg, with a wider ±7% spread that we attribute to worm thread geometry being more sensitive to machining tolerance. Because the planetary unit shares torque across multiple planet gears, individual gear-tooth errors cancel out statistically in our sample, while a worm’s single-thread contact has no such averaging effect. If you want a head-to-head benchmark rather than a catalog number, we run the same test procedure on every unit: it follows AGMA 2001 gear-class definitions for pitting resistance and ISO 6336 for load capacity, with the input shaft coupled to a calibrated hydraulic motor and the output shaft locked against a torque-load cell. That is the same procedure we run for our FS series industrial planetary gearboxes, and we have confirmed that the numbers are directly comparable across both product lines. Where planetary units pull ahead, and where they do not We have learned that the torque-density advantage grows with reduction ratio. A 2-stage planetary we ship can hit i = 60:1 to 120:1 in a single housing without making the unit impractically long, because we know each stage sits concentrically in our design. A worm gearbox, by contrast, needs a separate second stage — typically a helical or planetary stage — to reach the same ratio, and we see it add length, weight, and a second oil-fill point in every quote we prepare. In our product portfolio, the crossover point where planetary wins on envelope is around i = 25:1; below that, our single-stage worm units can be the more compact solution if self-locking is what the customer is paying for. Backdrive Efficiency: The Safety Lever Most Spec Sheets Gloss Over When we explain backdrive efficiency to our customers, we start with the basic physics: backdrive efficiency is the inverse of forward efficiency, and it is the percentage of output torque that arrives at the input shaft when the output is being driven. In our experience, a worm gearbox with 35% forward efficiency has roughly 35% reverse efficiency — which sounds similar at first, but in the reverse direction we see that 35% mean the gearbox resists being back-driven; above a lead angle of about 4°, we measure the worm as non-overhauling and we find the load cannot back-drive the motor at all. That is the “self-locking” feature we hear quoted as a safety benefit in nearly every sales call we attend. In our engineering view, it is real, but it is also where the safety story splits in two — and we have seen the wrong side of that split on more than one service call. A planetary gearbox, by contrast, we have measured to have very high reverse efficiency — typically 95-97% — because we know all the tooth contacts are rolling. In our risk models, that means if the holding brake fails on a planetary winch, we have to assume that the suspended load will start back-driving the hydraulic motor and the load will begin to descend on our watch. Because the system has no inherent resistance to back-driving, we always tell our customers that the safety case has to be carried entirely by the brake, the counterbalance valve, and the operator’s lockout-tagout discipline. On a worm winch, we find that the gearbox itself provides a static brake effect — useful as a defense in depth in our designs, and we have documented cases where we believe it was the only thing preventing a dropped load. This is why we always cite the major class societies — ABS Crane Rules, DNV-OS-E101, and API SPEC 2C — to our customers up front, because we know they all require a fail-safe holding brake on any offshore lifting winch we ship, regardless of gearbox type. Because a self-locking worm can mask a worn brake until the moment the brake actually fails, we have seen a documented incident profile of “silent brake degradation” in worm winches that we do not see with planetary units in our service records. Failure Modes We Have Seen in the Field (and What They Taught Us) Two field cases from our service records are worth describing, because they are the reason our engineering team now default-recommends planetary units for offshore 50-ton lifting unless the customer has a specific reason to want worm. Case 1 — Worm winch on a SE Asian deck crane, 2019, dispatched by our service team. A single-start worm in a deck-crane service winch developed a 0.4 mm wear step on the thread flank after 1,800 hours, and we were the team that received the call when the operator noticed. The operator reported to us that the load drifted 80 mm during a 30-second hold before the service brake engaged. We traced the root cause to material selection: the worm had been case-hardened to 58 HRC, but the wheel was only through-hardened to 320 HB, and we found the hardness differential was insufficient for the combined sliding-rolling contact. We fixed it by upgrading the wheel material to a nickel-alloy bronze with a case-hardened steel core, and we retrofitted a second service brake on top of the gearbox. The gearbox itself we scrapped, and we now apply the upgraded material pair as standard on all our worm-built winches we ship from Ningbo. Case 2 — Planetary winch on a Chinese-built AHTS, 2022, which we delivered and which we later serviced under warranty. A 3-stage planetary in the anchor-handling winch developed planet-bearer spalling after 4,200 hours in tropical waters — and we were the ones who had to send a service engineer to the vessel. When we arrived on site, our investigation showed that the original sealing stack used a paper gasket between the planet carrier and the housing; we found salt-water ingress had reached the needle bearings. Our corrective action — and the one we now standardize on in our factory — was to switch to a dual-lip radial seal plus an O-ring piloted flange, which we have now made standard across our FFT-W series. The gearbox survived, and we kept the customer, but the lesson we took back to our design team was that planetary units are not maintenance-free — they are differently maintained, and our service manuals now say so explicitly. When a Worm Gearbox Still Earns Its Seat at the Table There are three offshore-related scenarios where we still recommend a worm gearbox, and our sales engineers are trained to identify them up-front rather than upsell planetary by default. Vertical-axis mooring winches with long static-hold duty. When we configure a mooring winch that holds a line for hours or days with minimal cyclic activity, our engineers lean on the self-locking feature because it eliminates the need for a continuously-energized holding brake — we have measured that this saves hydraulic power and reduces heat build-up in the machinery space, which is why we still ship worm units for this exact duty profile. Deck-crane service winches under 10-ton rated load. When the reduction ratio is low, we have found that a worm unit is cheaper, more compact, and the lower duty cycle means the thermal penalty is acceptable. We have shipped worm-driven service winches for shipyard gantry cranes for this exact reason, and we will keep doing so. Subsea ROV winches with extremely low line speed. We have shipped worm units for subsea ROV winches where the single-stage high reduction ratio, combined with inherent self-locking, gives the slow line speeds and positional stability that ROV operators tell us they prefer — and we agree, because it removes the brake-management complexity that a planetary system would impose on a subsea package. If none of those three conditions apply to your 50-ton offshore lifting application, our default recommendation — and we make it deliberately — is a planetary unit, specifically the FFT-W series for line-pull winches, or the FHP variant for high-speed winch applications. We have made this recommendation roughly 80% of the time over the past five years, and our post-delivery data shows that those customers we have onboarded have seen fewer warranty events than the worm-driven minority. FFT-W series planetary winch gearbox, the workhorse for 50-ton offshore line-pull and active heave-compensated lifting applications. Image courtesy of Ningbo Flag-up Hydraulic. Side-by-Side Engineering Comparison The table below summarizes the engineering trade-offs for a 50-ton offshore lifting winch, based on our own test data and on industry-standard references including AGMA 6035-B92 for worm-gear rating and ISO 13628-7 for offshore lifting appliances. We have used this same comparison table with our own shipyard customers in their RFQ reviews, and we are sharing it here so you can pressure-test any supplier’s proposal against the same numbers. If you want the underlying spreadsheet and the full test report, you can request gearbox torque density comparison data from our engineering team, and we will send it within one business day. Parameter Planetary (FFT-W / FHP) Worm (single-stage) Typical torque density (N·m/kg) 180-240 95-120 Forward efficiency (per stage) 97-98% 30-50% Reverse (backdrive) efficiency 95-97% (free-rolling) 30-50% (self-locking above i≈40) Heat rejection at full load 2-3% of input power 50-70% of input power Reduction ratio in one housing 3.5:1 to 120:1 (multi-stage) 5:1 to 80:1 (single-stage practical) Shock-load tolerance High (load shared across planet gears) Moderate (single-thread contact) Maintenance interval (typical) 2,000-4,000 h (oil + seal inspection) 500-1,500 h (worm + wheel wear check) Relative unit cost (same torque class) 1.0× (baseline) 0.6-0.8× Typical 50-ton offshore fit Default choice Niche (mooring, ROV, light service) Spec Checklist Before You Sign the PO When our team receives an RFQ for a 50-ton offshore lifting winch gearbox, we run through the same nine-point checklist that we have refined over a decade of offshore deliveries. We are sharing it here so you can pressure-test any supplier’s proposal — including ours — against the same criteria that we use internally. Confirm the dynamic load factor. We always tell our customers to multiply the rated hook load by at least 1.5 for offshore service, and we ask them to get the actual peak line tension from their crane maker’s duty-cycle analysis before they send us the RFQ. Specify the duty class. We always reference API SPEC 2C duty classifications (e.g., Class 1-1-1 for heavy offshore lifting) and we ensure the gearbox we propose is rated accordingly — that is the first cross-check our application engineers run. Require redundant braking. Our standard specification calls for two independent service brakes, each capable of holding 1.5× the static hook load, regardless of gearbox self-locking status — we have never approved a single-brake design for offshore lifting, and we do not intend to start. Ask for measured torque density, not catalog claims. We always ask suppliers for the test procedure, sample size, and standard deviation before we accept a torque-density number. We provide ±4% on our planetary units; anything above ±10% should be questioned in our view, and we have walked away from RFQs that could not produce those numbers. Verify the class society route. We always confirm the supplier has current type-approval certificates from ABS, DNV, Lloyd’s, or BV — each has its own witness-test protocol. We have been burned in the past by suppliers who only provided material certificates, and our purchasing checklist now treats that as an automatic disqualifier. Check the sealing stack for marine service. Our minimum standard is IP65, with dual-lip seals on all rotating shafts and O-ring-piloted flange joints on the housing split lines — we changed our corporate standard to this configuration after Case 2 in 2022, and we have not gone back. Specify oil type and change interval. We always specify synthetic PAO gear oil rated for marine use, with a 2,000 h initial change interval and on-board oil sampling ports — that is the configuration we ship from our factory, and it is what we recommend to every customer we work with. Confirm thermal rating at ambient. We tell our customers to specify the maximum ambient temperature for the installation (typically 45-55°C in engine rooms or on tropical decks) and we always require a thermal rating curve from the supplier. We have rejected gearboxes that quoted only the nameplate rating without the derating curve. Request failure-mode data. We expect every gearbox maker we deal with to publish MTBF figures and field-failure statistics by duty class. If the supplier cannot produce them, we treat that as a red flag in our supplier audits, and we have replaced two suppliers in the last three years on this criterion alone. Field-Proven Product Pairings from Our Factory Floor Below I list three configurations we ship regularly for offshore lifting customers in the 50-ton class. Each one we have validated against our internal duty-cycle test rig, and we keep the test reports on file for every serial number we ship out of our Ningbo factory. FFT-W series winch gearboxes — this is our workhorse for line-pull winches, and we have shipped it for ten years running. We configure it as 2-stage planetary, ratios from 35:1 to 120:1, peak torque up to 110,000 N·m, and we pair it with our HPMW-series high-speed piston motor winches for active heave-compensated lifting on offshore construction vessels — we have seen it perform on rigs from the North Sea to the South China Sea. FS series industrial planetary gearboxes — we recommend these for the auxiliary service winches on the same vessel, where we find the duty is lighter and where we see our customers want to standardize on a single gearbox family for spares commonality. We maintain common gear designs across both series we build in our Ningbo factory, and our customers routinely tell us their mechanics only need to learn one set of replacement procedures we provide in our service manuals. FHP series winch gearboxes — we developed these for high-line-speed applications (towing, anchor-handling) where we know the gearbox input speed exceeds 2,500 rpm and where we need a specialized high-speed bearing set. We pair them with our HPVW piston-motor winch family, and we have run them at sustained 3,000 rpm input without thermal trip in our own endurance tests. If you would like the underlying torque-density test data, the AGMA-class gear-rating calculations, or a side-by-side TCO comparison for your specific 50-ton application, we invite you to request gearbox torque density comparison data directly from our engineering team. We commit to responding to technical RFQs within one business day, and we routinely run feasibility studies for shipyards and crane OEMs at no charge — that is how we have built our offshore customer base over the last decade, and we would like to do the same for you. Frequently Asked Questions What is a “good” torque density for a 50-ton offshore winch gearbox? In our book, anything above 180 N·m/kg for a planetary unit in the 50-ton class is competitive; above 200 N·m/kg we consider it excellent. For worm units we ship and test in our lab, we view 100-130 N·m/kg as the realistic envelope at this torque level. The ratio matters as much as the absolute number, in our view — we see a gearbox that delivers 150 N·m/kg at i = 30:1 as engineering differently from one that delivers 150 N·m/kg at i = 100:1, and we believe the second one is mechanically more impressive. Can a planetary winch gearbox ever be self-locking? No, and we have to say this clearly to customers who ask us. All-rolling contact gear geometries we work with every day cannot be self-locking by definition — we always point out to customers we meet that this is a feature unique to worm gearing, where the sliding friction angle exceeds the lead angle. For every planetary winch we ship from our Ningbo factory, we ensure the safety case is carried by the brake and the counterbalance valve, and never by the gearbox itself. Why do ABS and DNV require redundant brakes if a worm gearbox is already self-locking? We get this question often. Our short answer to the customer is always that self-locking is not fail-safe. We explain that the self-locking torque depends on the worm-wheel friction coefficient, which we know changes with temperature, lubrication, and wear — we have measured the friction coefficient drop on a worn worm wheel ourselves, and it is not small. A brake that is “backed up” by a gearbox with declining friction is a brake with a single point of failure that the class societies do not accept for offshore lifting. How often should the gearbox oil be changed on an offshore planetary winch? For the FFT-W series we ship, our standard specification is a 2,000-hour initial change interval, with subsequent changes every 4,000 hours if on-board oil sampling shows particle counts within limits. In tropical water service we shorten the interval to 1,500 hours because of salt-air contamination risk. What reduction ratio do I need for a 50-ton line-pull winch? In our standard configurations that we ship every quarter, a 50-ton line-pull at 0-15 m/min line speed pairs with a reduction ratio of 40:1 to 80:1 and a high-speed piston motor at 1,500-2,500 rpm. Below 40:1 we find the line speed becomes too fast for safe load control; above 100:1 we have to oversize the motor to compensate for the efficiency penalty, and we generally do not recommend it in our customer quotes. Does a worm winch gearbox really convert 50-70% of input power into heat? Yes, at full load and at typical lead angles, and we have measured this on our own test bench in Ningbo. A worm with 35% forward efficiency is dissipating 65% of the input power as heat in the gear mesh and bearing losses. That is why we see worm winches feel hot after a single lift cycle, and why we always specify an external oil cooler or a larger oil sump than a planetary unit of the same torque rating. Can I retrofit a worm winch with a planetary gearbox? Usually yes, and we have done several such retrofits for our customers. Expect three engineering items we always walk our customers through: (1) the hydraulic motor may need to change because the planetary unit free-rolls in reverse and we have to update the counterbalance valve logic; (2) we re-verify the brake sizing because the planetary unit no longer contributes any holding torque; (3) we work with the class society for re-approval of the modified winch, which typically means a witnessed load test. We can share a typical retrofit timeline on request, and our engineering team will scope it for free. FFT-W series planetary winch gearbox, the workhorse for 50-ton offshore line-pull and active heave-compensated lifting applications. Image courtesy of Ningbo Flag-up Hydraulic. About the author: Oliver is a member of the Technology Department at Ningbo Flag-up Hydraulic Co.,ltd., a hydraulic system specialist established in 2010 in Ningbo, China. The company operates a 20,000 m² factory with 200+ employees, a 20-person R&D team, and US$30 million in annual sales. The R&D team is organized into three specialized centers covering pilot joysticks and electronic control, winches and gearboxes, and cartridge valves and manifold systems. Verified author profiles: LinkedIn · YouTube · Instagram Engineering contacts: Request gearbox torque density comparison data · FFT-W and FHP series winch gearboxes · FS series industrial planetary gearboxes Post time: Aug-20-2026
