Case

Case Highlights
A boat drive manufacturer producing outboard gearboxes for mid-power engines was facing complaints about gear whine at cruising speed and pitting on the spiral bevel gear set in salt-water use. The lower unit on a marine engine runs fully immersed, exposed to salt water and corrosion; if protection is poor, the housing and gears can suffer severe damage over time.

DD Gear engineered a hardened spiral bevel gear pair for the outboard gearbox, using marine-grade alloy steel, fine grinding and improved surface protection. Spiral bevel gears with curved teeth engage gradually, giving smoother and quieter operation and high torque capacity in right-angle drives—ideal for marine propulsion. After field trials, the OEM saw noticeably lower gear noise, better contact pattern stability, and longer service intervals on heavily used boats.

Customer Background & Project Scope

• Customer industry: Manufacturer of outboard motors and lower-unit gearboxes for leisure boats.

• Application: Spiral bevel gear pair in the outboard’s right-angle gearbox (engine crankshaft → vertical driveshaft → spiral bevel set → propeller shaft). Bevel gear shafts are critical components in marine propulsion, transmitting power from engine to propeller.

• Operating profile:

  • Frequent acceleration and deceleration, including hard gear shifts.

  • Long hours at partial and full throttle in salt-water conditions.

• Project goals:

  • Reduce gear whine at typical cruising rpm.

  • Improve pitting resistance and contact fatigue life of the spiral bevel pair.

  • Maintain the existing housing and shaft layout to avoid redesign of the lower unit.

Challenges

1. Gear noise in the lower unit

   • Existing gears generated a tonal whine at mid-range rpm.

   • The previous spiral bevel set had less-than-ideal tooth contact; misalignment and surface finish issues can increase vibration and noise, shortening gear life.

2. Wear and pitting in salt-water use

   • After several seasons in salt water, some gear pairs showed pitting and micro-spalling at the contact zone.

   • Spiral bevel gears for marine work must withstand high torque, impact loads and a corrosive environment, so material and surface treatment choice is critical.

3. Tight packaging & alignment sensitivity

   • Spiral bevel gears need accurate alignment and tooth geometry to perform well; they are manufactured to very exacting specifications.

   • The new gearset had to fit the existing cone clutch, bearings and seals without changing mounting dimensions.

DD Gear Solution

1. Spiral Bevel Gear Design

• Tooth geometry & contact pattern

  • DD Gear refined the tooth geometry, spiral angle and contact pattern using 3D modeling and contact analysis, so the load sits near the central region of the tooth and remains stable under deflection. Modern design methods for spiral bevel gears use static, dynamic and thermal load calculations to ensure durability.

  • The spiral angle was chosen to balance smooth, quiet meshing with acceptable axial loads on tapered roller bearings; a larger spiral angle improves smoothness but raises axial force.

• Pattern-matched pinion & gear

  • Each pinion–gear pair is matched so that contact patterns remain correct after assembly, similar to other marine drives that use pattern-matched spiral bevel gears to minimize gear noise.

2. Materials, Heat Treatment & Corrosion Protection

• High-strength alloy steel

  • Gears produced from high-strength, wear-resistant alloy steel (e.g., SCM4xx / 20MnCr5 grade), consistent with recommendations that spiral bevel gear components use high-strength alloy or stainless steels for long-term, high-speed service.

  • Carburizing + quenching used to form a hard case over a tough core, raising contact fatigue strength.

• Ground tooth flanks

  • Tooth flanks are precision ground, which improves accuracy, lowers roughness, and supports high efficiency in spiral bevel gear drives.

• Corrosion and oil protection

  • Marine-grade surface protection (phosphate or similar) is applied before oil fill; internal surfaces are fully flooded with gear oil to protect against corrosion, as long-term storage and operation require good oil coverage to prevent rust.

3. Manufacturing & Quality Control

• Accuracy & inspection

  • Spiral bevel gears manufactured to high accuracy class (ISO / JIS equivalent Grade 5–6); stock examples of hardened spiral bevel gears show similar module ranges and precision classes used for demanding right-angle drives.

  • Gear tooth profile and contact patterns are measured on dedicated gear measuring equipment; blue-contact checks verify mounting distances and tooth contact position.

• Assembly & validation testing

  • Lower units with DD Gear sets were tested on a water-cooled dyno under varying rpm and torque.

  • Vibration and sound pressure were measured near the transom; gear oil samples were monitored for metal content during endurance runs.

Results

• Noise reduction

  • Test data and sea trials showed noticeably lower gear whine at typical cruising rpm; the tonal component from the bevel mesh decreased when compared with the previous design. This matches the known benefit of spiral bevel gears with optimized geometry—smooth, quiet operation in high-speed right-angle drives.

• Durability & reliability

  • After extended salt-water operation, the improved surface hardness and tooth geometry showed no early pitting within the OEM’s validation window.

  • The OEM reported fewer lower-unit failures related to bevel gears in warranty data for the updated models.

• Drop-in integration

  • The DD Gear spiral bevel pair fit the existing lower-unit housing, bearings and cone clutch, allowing the OEM to update the gearbox without changes to casting or propeller shaft layout.

Typical Technical Specifications

Representative values; DD Gear customizes to engine size and gear ratio.

Case Summary

By combining marine-grade alloy steels, precision-ground spiral bevel geometry, and careful corrosion protection, DD Gear helped this outboard OEM:

• Reduce gear whine at cruising speed.

• Improve contact fatigue life and reliability in salt-water service.

• Upgrade the gearbox as a drop-in spiral bevel gear set without redesigning the lower-unit housing.

This approach suits outboard motors, sterndrives and other marine right-angle gearboxes that need compact, quiet and durable spiral bevel gears.

Designing or upgrading a marine outboard gearbox?

Contact our engineers to discuss spiral bevel gear sets tailored to your lower-unit layout and sea-water conditions.