Question

(a) What are the key factors that influence the choice between a gear drive and a chain drive for a large marine engine. (6)

(b) Define backlash and explain how backlash is measured and corrected in a gear drive of large marine engine. (5)

(c) Discuss the effects of increased backlash on the performance and reliability of a large marine engine. (5)

Answer 1

Part (a)

Power/ torque transmission: Gears transmit higher power and torque more efficiently and reliably than chains; for large marine engines (camshaft drive, auxiliary drives), gears are preferred for high power.
Speed ratio and accuracy: Gears provide precise, fixed speed ratios and accurate timing (essential for camshaft/ valve timing); chains can stretch and lose timing accuracy.
Reliability and life: Gears are more reliable and have a longer life under continuous heavy duty; chains wear, stretch and require more frequent adjustment/ replacement.
Space and layout: Chains can transmit power over longer distances and around obstacles (flexible layout); gears require the shafts to be close/ in line.
Maintenance: Gears require less frequent maintenance (lubrication, inspection); chains require regular tensioning, lubrication and replacement.
Noise and vibration: Gears are generally quieter and smoother than chains at high speed.
Cost: Chains are cheaper initially but have higher maintenance cost; gears have higher initial cost but lower running cost.
Lubrication: Gears require a reliable lubrication system; chains require oiling.
Alignment: Gears require precise alignment; chains are more tolerant of misalignment.
Application: For the main engine camshaft drive and critical timing, gears are used; for auxiliary/ less critical drives, chains may be used.

Part (b)

Backlash is the amount of clearance (play) between the meshing teeth of two gears - the gap between the non-driving faces of the teeth when the gears are in mesh. It is necessary to allow for thermal expansion, lubrication, and manufacturing tolerances, and to prevent jamming, but excessive backlash causes noise, impact and wear.
Measurement: Backlash is measured by holding one gear stationary and rocking the other gear, measuring the angular/ linear movement of the teeth (using a dial indicator on the gear or a feeler gauge between the teeth). It can also be measured by the lead/ feeler method or by a backlash gauge.
Correction: If the backlash is too large, the gears are brought closer together (by adjusting the centre distance/ shims under the bearing housings) to reduce the backlash. If too small, the gears are moved apart. The backlash is set to the maker's specified range. If the backlash is excessive due to tooth wear, the gears may need to be renewed.

Part (c)

Increased noise and vibration (rattling/ hammering of the teeth).
Impact loading on the teeth, causing accelerated wear, pitting and possible tooth breakage.
Loss of timing accuracy (for camshaft/ valve drives), affecting engine timing and performance.
Increased wear of the teeth and bearings.
Reduced reliability and risk of gear failure.
Possible damage to the driven components (e.g. camshaft, pumps) due to the impact and irregular motion.
Increased maintenance and downtime.

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