A Chain and Pulley form 3 drive is a hoisting or transmission arrangement where a load chain runs over a pocketed sheave — a wheel cut with shaped recesses that capture each chain link by its geometry rather than by friction. The pockets engage the link barrels and cross-bars in sequence, transferring torque without slip even at very low speeds. The arrangement gives you a compact, slip-proof lifting drive with mechanical advantage set by the differential between two pocketed sheaves, used in chain hoists, theatre fly systems, and overhead trolleys carrying loads from 250 kg up to 5 tonnes.
Operating Principle of the Chain and Pulley (form 3)
The form 3 chain and pulley is the hybrid case — neither a pure flat-belt pulley nor a pure roller-chain sprocket. The wheel is a sheave with milled or cast pockets shaped to the exact link geometry of the load chain. As the sheave rotates, each pocket catches a chain link and drags it around the wrap, dropping the trailing end out the other side. There's no friction grip involved — the chain is mechanically captured. That's why a chain hoist can hold a 2-tonne motor block in mid-air with the operator letting go of the hand chain entirely.
The pocket geometry is where the engineering lives. The pocket pitch must match the chain pitch within roughly ±0.3 mm over a full wrap, or you get progressive walking — each link sits a hair off-centre, the error accumulates around the sheave, and the last engaged link climbs out of its pocket and jumps. On a 6 mm load chain that means pocket centres at 18.0 mm with a tolerance band you can measure with a vernier. Get it wrong and the chain skips under load, which on a hoist means the load drops 50 mm in a heartbeat. The wrap angle matters too — you need at least 3 links engaged simultaneously to share the load, which sets a minimum pitch diameter for any given chain size.
Failure modes are predictable. Worn pockets let the chain seat deeper, raising the effective pitch diameter and de-syncing the engagement — the symptom is a clicking sound under load. A stretched load chain (more than 3% elongation, the standard condemnation limit per ASME B30.16) won't seat at all and will ride up out of the pockets. And if the chain twists entering the wrap, one link cocks 90° and jams the sheave solid. That's why every proper chain hoist has a chain guide and stripper finger built into the housing.
Key Components
- Pocketed Load Sheave: The driven wheel with shaped recesses (typically 4 or 5 pockets per revolution on a hoist sheave) that capture each chain link by geometry. Pocket profile is machined to ±0.1 mm of the link form so the chain seats fully without rocking. Material is usually forged alloy steel, induction-hardened to 55-60 HRC at the contact surfaces.
- Load Chain: Calibrated short-link chain (Grade 80 or Grade 100 alloy) where the pitch is held to the manufacturing tolerance the sheave was cut for. A 6 mm Grade 80 chain has a pitch of 18.0 mm and a working load limit around 1,120 kg per leg. Chain elongation beyond 3% means it must be retired — it no longer fits the pockets.
- Hand Chain and Hand Sheave (on manual hoists): The input side of a differential or geared chain hoist. The hand sheave is a smaller pocketed wheel driving through a planetary or spur reduction (typically 30:1 to 60:1) so a 25 kg pull on the hand chain lifts a 2,000 kg load.
- Chain Stripper and Guide: A fixed finger inside the housing that peels the chain out of the pockets as the sheave rotates past the wrap zone. Without it, chain links can ride past the exit point and bind. Clearance to the sheave is typically 1.5-2.5 mm — close enough to strip reliably, wide enough to clear chain dimensional variation.
- Load Brake (Weston-style mechanical brake): On lever and chain hoists, a friction brake stack in series with the gear train holds the load when input torque is removed. It uses the load itself to generate the holding force — release the lever and the load reaction clamps the friction discs. Holds indefinitely with zero standby power.
Industries That Rely on the Chain and Pulley (form 3)
You see form 3 chain and pulley drives wherever positive (slip-free) lifting matters more than speed. The mechanism shines in low-duty-cycle, high-precision-positioning lifts — situations where a wire rope hoist would be overkill or where the chain's flexibility into a small drum-equivalent footprint is an advantage. The chain hoist, chain fall, and pocketed-sheave overhead trolley are all variants of the same core mechanism, just with different reductions and brake configurations.
- Theatrical rigging: CM Lodestar electric chain hoists used in concert touring — a single Lodestar Classic D8 lifts 1,000 kg of line-array speakers at 4 m/min, and a flown rig might use 24 of them in synchronised motion control.
- Industrial maintenance: Harrington CF series hand chain hoists in factory overhead crane applications, lifting motors, gearboxes, and dies up to 20 tonnes during machine changeovers.
- Shipbuilding and marine: Yale VS+ lever hoists used for tensioning rigging and pulling stubborn fasteners in confined ship-engine-room work where a powered hoist won't fit.
- Construction: Kito CB differential chain blocks hung from steel beams to lift HVAC rooftop units of 500-2,000 kg into position during commercial fitouts.
- Wind energy: Service chain hoists permanently mounted inside wind-turbine nacelles ��� typically a Demag or Stahl 1-tonne unit used by technicians to lift gearbox components and tooling 80-120 m above ground level.
- Automotive workshops: Snap-on and OTC engine pulling cranes using 2-tonne chain hoists at the boom tip to lower engines onto stands with millimetre-level positioning control.
The Formula Behind the Chain and Pulley (form 3)
The defining number for a differential chain and pulley hoist is the mechanical advantage — the ratio between the load lifted and the hand-chain pull required. The formula tells you what input force you need for a given load, but more usefully, it tells you what *speed* you sacrifice to get that mechanical advantage. At the low end of the typical hoist range (10:1 ratio) you pull a lot of hand chain to move the load a little, but you can lift heavy with bare hands. At the high end (60:1) a single operator can lift 5 tonnes, but they're pulling 60 m of hand chain to raise the load 1 m. The sweet spot for general-purpose 1-2 tonne hoists sits around 30-40:1.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| MA | Mechanical advantage (load lifted ÷ input force) | dimensionless | dimensionless |
| Dload | Pitch diameter of the load (large) sheave | mm | in |
| Dhand | Pitch diameter of the hand (small) sheave on a differential hoist | mm | in |
| η | Mechanical efficiency (typical 0.30-0.50 for differential, 0.65-0.85 for geared) | dimensionless | dimensionless |
| Fin | Hand-chain pull force = W<sub>load</sub> / MA | N | lbf |
Chain and Pulley Form 3 Interactive Calculator
Vary load, engaged links, and pitch error to see link load sharing and pocket tolerance use on a pocketed sheave.
Equation Used
The calculator treats the hanging load as a static force and divides it equally across the number of chain links engaged in the pocketed sheave. It also compares the pocket pitch error with the allowable pitch tolerance to show how close the geometry is to the stated limit.
- Static vertical hoist load
- Engaged links share load evenly
- g = 9.81 m/s^2
- Pitch error is compared against the allowable pitch tolerance
Worked Example: Chain and Pulley (form 3) in a museum exhibit installation chain hoist
You are sizing a manual differential chain hoist hung from a roof truss to lift a 680 kg restored steam-locomotive bell crank into place inside a railway heritage museum gallery in York. You have a single operator on the hand chain, a 4 m lift height, and you need to position the casting within ±5 mm vertically while two riggers guide its rotation. You're choosing between a 20:1, 40:1, and 60:1 hoist.
Given
- Wload = 680 kg (≈ 6,670 N)
- Lift height = 4 m
- η (geared chain hoist) = 0.75 dimensionless
- Single operator pull, sustained = ≈ 250 N (25 kg comfortable)
Solution
Step 1 — at the nominal 40:1 ratio (the typical sweet spot for a 1-tonne class hand chain hoist like the Kito CB010 or Yale VSIII 1000), calculate the required hand pull:
That's about 22.5 kg of pull — a sustainable one-handed effort for a single rigger. The hand chain travel needed to lift the bell crank 4 m is 4 × 40 = 160 m, which sounds dramatic but the hand chain is in a continuous loop, so you're pulling the same 1.5 m loop repeatedly.
Step 2 — at the low end of the range, a 20:1 hoist:
That's 45 kg of sustained pull. One operator can do it in short bursts but their grip will fail before they complete a 4 m lift. You'd be reaching for a second rigger inside 30 seconds. Hand chain travel drops to a more manageable 80 m total though — useful for a short 200 mm trim lift, useless for the full installation.
Step 3 — at the high end, a 60:1 hoist:
15 kg of pull — almost effortless, perfect for fine positioning. But hand chain travel is now 240 m total. For a 4 m lift you're standing there pulling for 8-10 minutes per metre. The 60:1 makes sense if you only need to creep the load the last 50 mm into a tight bracket — not if you're doing the whole 4 m.
Result
The 40:1 hoist is the right answer at 222 N hand pull and roughly 4 minutes of pulling for the full 4 m lift. At 20:1 the operator runs out of grip before the lift completes; at 60:1 the lift is effortless but takes nearly three times as long, and the riggers guiding the casting have to hold their position the whole time. The 40:1 sits in the engineering sweet spot — fast enough that the hold-and-guide window is reasonable, light enough that a single operator finishes without stopping. If you measure a hand pull noticeably above the predicted 222 N, suspect three things: (1) chain twist entering the load sheave, which adds drag and in extreme cases jams the stripper finger, (2) a load chain that's elongated past the 3% retirement limit and is no longer seating cleanly in the pockets, or (3) gearbox grease that has thickened in cold storage — η drops from 0.75 toward 0.55 below about 5 °C until the hoist warms up.
When to Use a Chain and Pulley (form 3) and When Not To
The form 3 chain and pulley is one of three common ways to lift a load on a hook. Compared against wire rope hoists and lever-arm pneumatic balancers, it has a specific sweet spot: medium loads, low duty cycle, where slip-free holding and compact head-room matter more than lift speed.
| Property | Chain & Pulley Hoist (form 3) | Wire Rope Hoist | Pneumatic Balancer |
|---|---|---|---|
| Typical lift speed | 0.5-8 m/min | 4-30 m/min | 30-60 m/min |
| Load capacity range | 250 kg - 50 tonnes | 500 kg - 500 tonnes | 20-500 kg |
| Positioning precision | ±2-5 mm (manual hand chain) | ±10-25 mm (electric only) | ±1 mm (float mode) |
| Standby power to hold load | Zero — mechanical brake holds | Zero — disc brake holds | Continuous air supply needed |
| Headroom (drive height above hook) | Compact, 250-500 mm | Bulky, 400-800 mm + drum length | Compact, 200-400 mm |
| Cost (1-tonne class, 2024) | $300-$1,200 manual / $2,500-$6,000 electric | $3,500-$15,000 | $4,000-$10,000 |
| Duty cycle suitability | Light to medium (M3-M5) | Heavy (M5-M8 continuous) | Heavy continuous (assembly line) |
| Reliability / failure mode | Predictable wear at chain pockets, gradual | Wire fraying — sudden if undetected | Hose and seal leaks, frequent |
Frequently Asked Questions About Chain and Pulley (form 3)
That's the Weston load brake unloading and reloading. The brake stack only clamps when the load reacts back through the gear train. If you ease off the hand chain too quickly, the friction discs separate momentarily before the load reverses direction and re-engages them — and during that window the chain free-falls a few links, which is the drop you feel.
It gets worse with worn friction discs or oil contamination on the brake faces. Pull a brake inspection cover and look for glazing or oil sheen on the discs. A clean brake holds within one chain link of where you stopped pulling; a contaminated one slips 3-5 links every time.
Yes. Clicking under load almost always means the chain is no longer seating fully in the pockets. The two common causes are load chain elongation past the 3% retirement limit (measure 11 links and compare against the manufacturer's table) and a worn pocket profile from years of service. Both let the link rock in the pocket before it fully seats — that rocking is your click.
Stop using the hoist until you've identified which one. A chain riding up out of pockets under load can jump completely free, which means the load drops uncontrolled.
Wire rope, almost certainly. Chain hoists in the 2-tonne class are rated M3 or M4 duty (intermittent), and a 4-hour continuous-lift shift will overheat the motor and accelerate chain wear. Wire rope hoists at M5-M6 are designed for that workload and lift 3-4× faster, which means each shift cycle completes quicker and the duty rating is comfortably under spec.
The exception is if positioning precision matters more than speed — a chain hoist places loads to ±5 mm easily, where a typical wire rope hoist needs an inverter drive to match that. For die changes and machine setups, the chain hoist still wins.
Efficiency loss in the gear train. A new geared hoist runs at η ≈ 0.75-0.80; a hoist that's been sitting in cold storage with old grease, or one that's been overloaded in the past and has galled gear teeth, can drop to η ≈ 0.50. That alone explains a 50% increase in hand pull.
Quick diagnostic: lift a known small load (say 100 kg) and measure the hand pull. Compare to Fin = 100 × 9.81 / (MA × 0.75). If your measured pull is 1.4× or more above predicted, the gearbox needs servicing — usually re-greasing with the correct EP grade fixes it. If not, you've got pitted gears.
Only if it's specifically a lever hoist (chain puller) — not a hand chain hoist. The difference is in the load brake design. Lever hoists like the Yale VS+ or Harrington LB are built with side-load-rated hooks, brake stacks that work in any orientation, and a free-chaining mechanism. Hand chain hoists assume the load hangs vertically below the load sheave and the brake is gravity-assisted.
Pulling sideways on a hand chain hoist can let the load chain skew in the pockets, jamming the stripper finger, and the brake may not hold reliably. The chain block manufacturer's manual will state the maximum allowable side-pull angle — typically 0° for hand chain hoists and up to full horizontal for purpose-built lever hoists.
The hand chain loop length is wrong, or the hoist has been hung at a height where the loop is folding over itself. The hand chain wants to hang as a single clean loop with both legs near-vertical. If the loop is too long for the hanging height, the slack accumulates and can drift into the hand sheave the wrong way, occasionally jumping a pocket.
Trim the hand chain so the loop ends about 600 mm above operator-floor level. If you can't shorten it (because the hoist gets used at multiple heights), fit a hand chain bag — a fabric pouch that catches the loop and stops it tangling.
References & Further Reading
- Wikipedia contributors. Hoist (device). Wikipedia
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