Camshaft Chain Drive, Hydraulic Tensioners & Timing Synchronisation

1. Operating Principles & Kinematics of Marine Camshaft Drives

In marine slow-speed two-stroke diesel engines, the camshaft operates at exactly crankshaft speed (1:1 angular velocity ratio). The camshaft must deliver high peak driving torques to overcome the hydraulic resistance of high-pressure fuel injection pumps (injection pressures up to 1000–1200 bar on mechanical engines) and the heavy pneumatic forces of exhaust valve air springs.

The drive transmission is traditionally situated at the aft end of the engine adjacent to the nodal point of the crankshaft to minimize the transmission of torsional vibration oscillations from the running gear into the valve timing train.

What the Camshaft Drive Actuates

• Fuel Injection Pumps: Actuates individual jerk-type fuel injection pump plungers via hardened fuel cams, delivering precisely metered fuel charges at peak pressures.
• Hydraulic Exhaust Valve Actuators: On modern cam-driven engines (such as MAN B&W MC series), the camshaft drives hydraulic exhaust actuator pumps that transmit high-pressure oil pulses to open the poppet exhaust valves against their pneumatic air springs.
• Starting Air Distributor: Drives the rotating distributor disc or pilot valve spindle to time the sequential admission of 30-bar cranking air into the cylinders in firing order.
• Mechanical Cylinder Lubricators: Drives mechanical Alpha / lubricator pump boxes via chain sprockets or eccentric pushrods.
• Mechanical-Hydraulic Governor: Taps off the intermediate shaft to drive flyweight governors (Woodward UG-8 / PGA).

Chain Drive vs. Gear Train Transmission Comparison

2. Metallurgy & Tribology of Heavy-Duty Marine Roller Chains

Marine engine camshaft drives employ precision-engineered duplex (two-strand) or triplex (three-strand) roller chains built to withstand static breaking loads exceeding 1,500 to 2,500 kN and high cyclic dynamic shock loads.

Component Metallurgy & Manufacturing

Hydrodynamic Lubrication Requirements

Centrifugal force at operating speeds (chain linear velocity approx. 6 to 10 m/s) continuously tries to fling lubricating oil off the outer chain surfaces. Effective lubrication depends upon dedicated oil spray pipes and flat-fan spray nozzles:

• Nozzle Aiming: Oil nozzles must be aimed directly into the slack side of the chain immediately before it engages the sprockets, spraying directly into the narrow clearances between the inner and outer link plates.
• Capillary Action: Capillary attraction draws oil between the pin and bush bearing surfaces before entering the sprocket teeth.
• Lubrication Failure: Blocked or misaligned spray nozzles cause boundary starvation, leading to rapid dry galling, pin blueing (thermal discoloration > 300°C), microscopic metal transfer, and catastrophic pin seizure.

3. Chain Tensioning Systems & Hydraulic Tightener Architecture

Proper chain tension is paramount. Excessive tension overloads the camshaft bearings, increases friction, and accelerates pin wear. Conversely, insufficient tension causes the slack chain span to whip transversely, exciting acoustic resonance, damaging rubber guide bars, and risking tooth jumping.

Hydraulic Tensioner with Mechanical Non-Return Ratchet

Modern large two-stroke engines (MAN B&W) utilize an advanced hydraulic-mechanical chain tightener:

• Hydraulic Operating Piston: Engine system lubricating oil at 2.5 to 3.5 bar is piped directly into the tensioner cylinder. Oil pressure acts on a large-diameter piston, applying a constant, cushioned forward thrust against the movable idler sprocket bracket.
• Calibrated Internal Load Spring: A heavy helical compression spring works in parallel with oil pressure, providing base mechanical tension during engine startup before full oil pressure is established.
• Non-Return Ratchet Pawl Mechanism: A precision mechanical ratchet and pawl rack is integrated into the tensioner body. As the chain wears and the piston moves forward, the spring-loaded pawl advances tooth by tooth. The ratchet positively locks the piston against backward movement. This is a critical safety feature:
  • During engine stopping or sudden astern crash stops, inertia generates massive reverse slack-side shock loads that would otherwise push the tensioner back, allowing the chain to jump teeth.
  • In the event of complete engine lube oil pump blackout, the ratchet prevents chain relaxation.

Operational Procedure: The Half-Link Tension Test

1. Engage Turning Gear: Turn the engine Ahead so that the driving strand is tight and the slack strand passes across the longest unsupported free length (between the crankshaft wheel and intermediate idler).
2. Manual Pull-Away Check: At the midpoint of the longest free strand, grasp the chain firmly with gloved hands and pull it away from the vulcanized rubber guide bar.
3. Deflection Limit:
   • Correct Tension: The chain should deflect away from the guide bar by approximately half a chain link thickness (approx. 15 to 25 mm).
   • Under-Tensioned (Slack): If the chain can be pulled away by more than a full link, the chain is slack and must be re-tightened immediately using the tension adjustment screw.
   • Over-Tensioned: If the chain cannot be pulled away at all (zero clearance), loosen the tightener adjusting nut until correct elasticity is restored.

4. Chain Elongation Measurement & Wear Criteria

A common misconception among junior engineers is that a marine roller chain "stretches" elastically or plastically like a rubber band. In reality, modern alloy chain side plates do not yield under normal service loads. Chain elongation is purely the cumulative result of microscopic abrasive wear on the outer diameter of the bearing pins and the inner bore of the bushings across every articulated joint.

Even a minute wear of only 0.1 mm per pin-bush interface, when multiplied across a 150-link chain loop, produces an overall increase in chain length of 15 mm!

Measuring Elongation Over 10 to 20 Pitches

Engine manufacturers mandate measuring chain elongation across a specified straight, tensioned section of 10, 15, or 20 pitches using a vernier depth trammel or precision caliper:

Sprocket Tooth Hook Wear

As the chain pitch elongates, the rollers no longer seat centrally in the semicircular tooth troughs. Instead, the oversized pitch forces the rollers to ride up the driving face of the sprocket teeth. This localized contact cuts a sharp notch or hook shape on the tooth tip:

• Inspect tooth profiles using an engine-specific profile template gauge.
• If tooth hooking exceeds manufacturer tolerances, fitting a new chain onto worn sprockets is strictly prohibited; the hooked teeth will destroy the new chain within hundreds of hours. Both chain and sprockets must be replaced together.

5. Camshaft Phasing, Timing Drift & Re-Synchronisation Procedures

The most serious operational consequence of progressive chain elongation is camshaft timing drift (timing retardation).

Because the crankshaft drives the camshaft through the tight strand, any elongation along this driving strand causes the camshaft wheel to lag behind the crankshaft by an angular phase angle Δθ_cam:

Thermodynamic Symptoms of Retarded Camshaft Timing

• Delayed Fuel Injection: Fuel injection occurs late, shifting combustion peak pressure (P_max) further past TDC and lowering peak combustion temperatures.
• Delayed Exhaust Valve Opening: Exhaust valves open late, shortening blowdown duration and increasing residual gas backpressure during cylinder scavenging.
• Elevated Exhaust Gas Temperatures: Late combustion extends into the expansion stroke (afterburning), causing exhaust temperatures after the cylinder valves and before the turbocharger to rise by 20°C to 40°C.
• Elevated Specific Fuel Consumption: Lower thermodynamic cycle efficiency increases SFOC by 2% to 4%, substantially increasing vessel bunker expenditure.

Step-by-Step Camshaft Re-Timing Procedure

1. Set Engine to TDC Cylinder No. 1: Using the electric turning gear, rotate the engine in the Ahead direction until the flywheel TDC pointer aligns precisely with the 0.0° TDC mark for Cylinder No. 1.
2. Inspect Camshaft Timing Witness Marks: Remove the camshaft inspection cover adjacent to the drive wheel. Inspect the pointer mounted on the bearing housing relative to the engraved mark on the camshaft flange:
   • If the chain has elongated, the camshaft mark will lag behind the pointer in the direction of rotation.
3. Relieve Chain Tension: Loosen the lock nut on the chain tightener and back off the tension adjustment screw to provide slack.
4. Loosen Sprocket Hub Coupling: Loosen the hydraulic coupling ring bolts or fitted friction bolts securing the camshaft sprocket to the camshaft driving flange.
5. Rotate Camshaft Independently: Using a specialized lever or turning tool on the camshaft flange, rotate the camshaft independently until its engraved witness mark aligns perfectly with the stationary pointer.
6. Re-Torque Coupling & Tension Chain: Hydraulically pre-tension the hub clamping bolts to exact manufacturer specification; re-adjust chain tensioner to the half-link limit; lock the ratchet pawl; turn engine two full revolutions on turning gear and re-verify timing alignment.

6. Maintenance Routines, Emergency Breakage & Chain Renewal at Sea

Planned maintenance routines must be executed under the vessel's Planned Maintenance System (PMS) to guarantee chain transmission integrity:

Procedure for Drawing In a New Chain at Sea

1. Secure Engine: Post 'DO NOT START' tags; isolate starting air; engage turning gear.
2. Break Old Chain: Position the old chain connecting link at an accessible position on the upper intermediate platform; grind off riveted pin ends or remove cotter pins; press out the link using an extraction clamp.
3. Connect New Chain as a Messenger: Couple the leading end of the new chain to the trailing end of the old chain using a specialized temporary connecting link.
4. Rotate Turning Gear Slowly: Rotate the engine slowly in the Ahead direction using the turning gear. As the old chain is pulled off the sprockets, it gently draws the new chain into position through all sprockets, idlers, and rubber guide bars without losing track.
5. Fit Permanent Master Link: Once the new chain completes the circuit, disconnect the old chain. Fit the permanent master connecting link, press on the outer side plate using a hydraulic clamp, and peen/rivet the pin ends to specified crown dimensions.
6. Tension & Synchronize: Adjust chain tension, lock the ratchet pawl, execute full camshaft timing alignment, and verify oil spray lubrication before returning engine to service.