Quick Answer
A gearbox housing has three jobs: hold bearing bores in alignment under load, damp gear noise, and keep oil in. Gray iron (A48 Class 30–40) remains the default — its damping and machinability are unmatched at the price. Ductile iron steps in for shock loads; A356 aluminum where weight matters. The specification battleground is not material, though — it is datum strategy for bore machining and leak-tightness policy, and both belong in the RFQ.
Material Choice
| Material | Pick when | Watch out for |
|---|---|---|
| Gray iron A48 Cl.30–40 | Default: stationary industrial gearboxes | Section sensitivity in heavy walls; see the A48 class guide |
| Ductile iron A536 65-45-12 / 80-55-06 | Shock loads, mobile equipment, safety factors | ~15–25% cost premium over gray; slightly worse damping |
| Aluminum A356-T6 | Weight-critical: vehicle, robotics, portable drives | Thermal expansion 2× iron — bearing fits need design attention; see the A356 guide |
| Cast steel | Extreme torque, welded frames | Cost, damping loss — rare choice for housings |
The Bore Alignment Problem — and the Datum Answer
Gear mesh quality lives or dies on center distance and parallelism of bearing bores, typically toleranced at 0.02–0.05 mm over hundreds of millimeters. The casting cannot hold that; machining must — and machining needs honest datums:
- Cast datum targets (3-2-1 pads) defined on the drawing, used by foundry layout and machine shop alike — the single best defense against “the casting is wrong” disputes
- One-setup line boring of coaxial bores wherever geometry allows — setups, not castings, cause most misalignment
- Machining stock 3–5 mm on bores and joint faces, agreed at DFM — enough to clean up cast variation, not so much that stress release distorts
- Stress relief before finish machining for large or asymmetric housings — cheap insurance against post-machining movement
Leak-Tightness: Specify It, Don’t Assume It
Oil finds connected microporosity that visual inspection misses. If the housing holds lubricant:
- State a pressure or vacuum test: e.g., 0.5 bar air under water for 60 seconds, no bubbles — on 100% of parts or an agreed sample
- Define the impregnation policy: vacuum impregnation of minor porosity is industry-standard practice — agree up front whether it is allowed, banned, or requires notification
- Machined joint faces: flatness callout plus DPT on sealing lands for critical drives
Typical Manufacturing Route
Housings run through sand casting (gray/ductile iron, larger aluminum) or gravity casting (smaller aluminum), then stress relief, then CNC machining of joint faces, bores and mounting features, then pressure test and dimensional inspection. Split housings machine as matched pairs — keep halves together with match marks from first machining onward.
FAQ
Why not weld a housing from steel plate instead?
Fabrication suits one-offs and very large boxes. Castings win from tens of pieces up: integrated bosses and ribs, better damping, less machining, lower unit cost.
What surface finish do bearing bores need?
Machined to the bearing maker’s specification — commonly Ra 1.6–3.2 µm with tight roundness. The casting only needs to deliver clean stock there.
Can ribs be added after tooling exists?
Pattern modifications to add ribs or bosses are routine and cheap in sand casting — one of its underrated advantages during design evolution.