Leaf Chain and Hoist Chain: AL/BL Series, Forklift Mast Drives and Safe Working Loads

An investigation into a forklift mast chain failure at a Incheon logistics terminal in 2024 found that the AL1022 leaf chain on the mast had been replaced at 3% elongation — the same threshold the maintenance team applied to the drive roller chains on the same vehicles. This was incorrect. For leaf chain in lifting service, ASME B29.8 requires retirement at 2% elongation for standard service and 1.5% elongation for applications where the chain is exposed to corrosion or shock loads. The chain that failed had been measured at 2.1% elongation and returned to service. The failure was a fatigue crack through a link plate — not a single-event overload, but the cumulative result of running a chain at cyclic loads above the fatigue limit for several hundred additional cycles after the retirement threshold had been passed.

Leaf chain failures in lifting equipment are not failures of the chain material or the chain manufacturer’s specification. They are almost always failures of the inspection and retirement programme. Understanding why leaf chain has different retirement criteria from roller chain — and why those criteria exist — is the foundation of a safe hoist chain maintenance programme.

What Leaf Chain Is — Structure and Why There Are No Rollers

AL vs BL Series: The Naming System and What Each Series Represents

ASME B29.8 defines two leaf chain series — AL (even lacing) and BL (balanced lacing). The letter prefix encodes the lacing pattern, which determines the plate distribution across the pin. The number that follows encodes first the pitch group and then the lacing count.

In AL-series chain, the plates are arranged in sets of equal count on each side of the centre line — 2×2, 3×3, 4×4, and so on. In BL-series chain, the centre set has additional plates that are not mirrored on both sides — the lacing is asymmetric through the chain cross-section. BL chain is generally heavier and stronger per pitch than AL chain of similar designation, and it is the standard specification for forklift mast chains in medium and large industrial trucks. AL chain is more commonly found in lighter industrial hoist and balancer applications.

How Leaf Chain Fails: Fatigue Fracture vs Overload — and Why This Changes Everything About Inspection

Overload failure — where a chain is pulled to its minimum break load in a single event — is not the dominant failure mode in forklift and hoist chain applications. Statistical analysis of leaf chain field failures consistently shows that more than 85% of failures in properly rated lifting systems are fatigue failures — crack initiation and propagation under repeated cyclic loads that are individually well within the chain’s structural capacity.

The practical implication is profound. An overloaded chain gives visible warning before failure — the links deform plastically and the elongation becomes visible before fracture. A fatigue crack in a leaf chain link plate is typically 0.2–0.5 mm wide on the plate surface, oriented perpendicular to the chain axis, and nearly invisible until it has propagated to approximately 50% of the plate cross-section — at which point remaining static strength may be reduced to near the working load and fracture is imminent. By the time the crack is visible to an inspector performing a routine visual check, it may have been propagating for several hundred lift cycles.

This is why the elongation threshold for leaf chain (2% for standard service) is lower than for roller chain (3%), and why visual inspection for cracks, corrosion, and plate deformation is mandatory in addition to elongation measurement. The elongation alone does not reveal the fatigue state of the plates — a chain can be within the elongation limit and still have fatigue cracks developing in the plate sections adjacent to the pin holes.

Leaf Chain Inspection Requirements: ASME B29.8 and EN 14659 Mandated Checks

Both ASME B29.8 (North American standard) and EN 14659 (European standard, adopted in Korean OEM equipment documentation) specify minimum inspection content for in-service leaf chains. The inspection interval is typically defined by the forklift OEM in the maintenance manual — most Korean and Japanese forklift OEM service documentation specifies annual inspection as a minimum, with more frequent inspection for high-cycle operations (chains performing more than 200 lift cycles per day should be inspected every 6 months).

Elongation Retirement Limits: 12-Link Measurement Reference Values

The 12-link caliper method used for roller chain also applies to leaf chain. Measure pin-centre to pin-centre across 12 links and compare to the nominal value. Retire when the measured span reaches or exceeds the values in the table below.

Leaf Chain Lubrication: Why This Is More Critical Than for Roller Chain

Leaf chain has no bushing — the pin surface bears directly on the inner plate hole. This means the pin-hole interface has no intermediate wear component that can absorb damage before the structural element (plate) is affected. In roller chain, the bushing wears before the link plate hole enlarges. In leaf chain, the plate hole is the direct bearing surface. If this surface runs dry, the hole enlarges rapidly through abrasive wear, accelerating elongation and — more critically — creating stress concentrations at the hole edge that initiate fatigue cracks.

The correct lubrication for forklift mast chains is a chain-specific oil applied to the inner plate surfaces at each lubrication event — not grease. Grease is too viscous to penetrate the pin-plate interface by capillary action and instead builds up on the plate outer surfaces where it collects contamination and provides no benefit at the actual bearing point. SAE 30–40 mineral chain oil, or an equivalent synthetic PAO oil for cold store applications, applied with a brush or spray to the inner plate faces at each weekly maintenance event, is the correct method. ASME B29.8 recommends lubrication at intervals not exceeding 250 operating hours under normal conditions and 50 operating hours in environments where the chain is exposed to contamination.

For forklift operations in cold storage environments (−20°C to −10°C), standard mineral oil thickens to the point where it provides no penetrating action at the pin-plate interface. A synthetic PAO-based chain lubricant specified for sub-zero operation (typically rated to −40°C pour point) must be used for cold store forklift mast chains. The service life of leaf chain in cold store applications without correct low-temperature lubrication is typically 40–60% of that in ambient temperature service, and the elongation rate accelerates sharply when the chain warms up from −20°C to operating temperature during the first 30 minutes of each shift (the temperature cycling itself causes differential thermal expansion at the pin-plate interface that creates abrasive fretting).

Forklift Mast Chain Replacement: What Must Always Be Done in Pairs

On forklifts with two mast chains — which includes virtually all counterbalance forklifts with rated capacities from 1 t to 10 t — the two chains must always be replaced simultaneously, never individually. This requirement appears in every major forklift OEM service manual and is a mandatory requirement under EN 14659 for new chain installations.

The reason is differential elongation. A new chain installed on one side of the mast will gradually elongate, while the older chain on the other side (if not yet retired) will elongate at a different rate due to its accumulated wear history. This differential elongation causes unequal load distribution across the two chains — the shorter (newer) chain carries a disproportionate share of the lift load as the tilt geometry of the carriage shifts. In the worst case, a forklift with one new and one worn chain may load the newer chain at 110–130% of the rated capacity while the worn chain carries 70–90%, accelerating the fatigue of the new chain significantly.

Simultaneously with chain replacement, check and replace the mast sheaves if any wear grooves are visible on the sheave contact surface. For drive sprockets on electric forklift traction chains, matched sprockets for forklift drive and traction systems are available in standard and custom bore configurations. A worn sheave groove concentrates the chain load at a narrower contact arc than the design geometry, increasing the per-plate stress at the wrap points and accelerating the fatigue of the new chain. Sheave condition is directly linked to chain service life — replacing chains without checking sheaves is the second most common cause of premature leaf chain failure after incorrect lubrication.

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