Marine Gearbox Clutch Gears for Shipbuilding

Client Background#

A shipbuilding enterprise required clutch gears for gearboxes used on vessels. Marine gearbox parts work in a demanding environment: moisture, salt and long duty cycles all bear on how long a part stays serviceable. The customer’s requirement was specific — the clutch gears must have seawater corrosion resistance and high reliability in service.

Requirement & Challenge#

• Material 42CrMo with surface chrome plating.
• Precision GB grade 6, with flaw detection on every piece.
• Must comply with classification society rules (CCS/DNV).

For a part of this kind, the difficulty is not only achieving the final geometry. Material integrity has to be proven before machining starts, because a defect inside a forging cannot be corrected later in the process. Corrosion protection has to be applied without sacrificing the tooth accuracy that was produced by machining. And because the rules of the classification societies apply, each individual piece has to be inspected and documented rather than dealt with by sampling. The requirements therefore form one chain — material, coating and inspection — and each link depends on the one before it.

Solution & Process#

1. Material Control: 42CrMo forgings were used as the starting stock, and ultrasonic flaw detection was carried out on every piece so that internal defects in the material would be identified before any machining began.
2. Gear Tooth Surface Chrome Plating: A hard chrome layer of 0.05 mm thickness was applied to the tooth surfaces to improve corrosion resistance in a seawater environment.
3. Full Dimensional Inspection: Magnetic particle inspection, ultrasonic testing and full dimensional inspection were performed, covering surface defects, internal defects and final dimensions.

Result & Feedback#

• The clutch gears passed CCS classification certification.
• No corrosion after a 1000-hour salt spray test.
• The gears have been equipped on 20 vessels.