Helical Gearbox — Full Mechanical Design

Project Brief

Design a single-stage helical gearbox transmitting 8.5 kW at a ratio of 5.5:1 — from functional requirements to a fully specified assembly. The methodology follows AGMA standards throughout.

Gear Design

Key Parameters

| Parameter | Value | |-----------|-------| | Power | 8.5 kW | | Ratio | 5.5 : 1 | | Material | 35CrMo4 (case-hardened) | | Helix angle | 15° | | Module (normal) | 3 mm | | Centre distance | 156 mm |

AGMA Calculations

• Bending stress (Lewis + AGMA correction factors): verified for pinion and gear
• Contact stress (Hertzian): pitting resistance over required service life
• Safety factors: Sf ≥ 1.2 on bending, Sh ≥ 1.1 on contact

The helical geometry was selected to achieve contact ratio > 1.6 and reduce noise versus equivalent spur gears.

Shaft Design

The gear mesh produces tangential, radial, and axial forces on each shaft. Bending moment diagrams were constructed in both planes and combined with torsion.

Shaft diameters (Tresca criterion, fatigue safety factor applied):

• Input shaft (pinion): Ø 35 mm at gear seat
• Output shaft (gear): Ø 55 mm at gear seat

Bearing Selection — SKF

Bearings selected from the SKF catalogue for required life L10 ≥ 20,000 h. Fixed-floating arrangement on each shaft: fixed bearing (duplex angular contact) reacts axial loads; floating bearing (cylindrical roller) accommodates thermal expansion.

Assembly Connections

• Gear-to-shaft: interference fit (H7/p6) + key-and-keyway
• Bearing inner rings: interference fit (k6 on shaft)
• Housing cover: threaded fasteners + O-ring seal + drain/fill plugs
• Lubrication: oil splash — level set to 1/3 tooth depth immersion on large gear

What This Demonstrates

Every value in this design connects to the next: gear forces → shaft loads → shaft diameter → bearing bore → SKF reference. This end-to-end traceability — not isolated calculations — is what AGMA-based mechanical design requires.