- Practice · 6 marks
Power screw threads, standards and materials
Short answer
Square threads (0° flank, highest efficiency, jacks/presses) follow IS 4694. Acme/trapezoidal (29°/30°, split nut for wear, leadscrews/valves) ASME B1.5/ISO 2904. Buttress (7°/45°, one-way loads, vices) ASME B1.9. Thread root sharpness causes Kf~2-3; first nut threads take most load. Screws: medium‑carbon/alloy steel (hardened); nuts: phosphor bronze/cast iron (low friction).
Compare the square, Acme (trapezoidal) and buttress threads used in power screws. State where each is used and the standards. Explain stress concentration in screw threads and the choice of materials for the screw and nut.
Power screws convert rotary motion into linear motion with high force (screw jacks, lathe lead screws, presses, valves).
Thread forms
| Point | Square | Acme / trapezoidal | Buttress |
|---|---|---|---|
| Thread angle | 0 degree flanks | 29 degrees (Acme), 30 degrees (metric trapezoidal) | 7 degrees load face, 45 degrees back |
| Efficiency | Highest, lowest friction | Lower, because with | High in the load direction |
| Strength of thread | Weaker root | Stronger root | Strong in one direction |
| Wear take-up | None | Split nut can take up wear | Not applicable |
| Manufacture | Difficult (milling) | Easier (thread rolling, cutting) | Moderate |
| Use | Jacks, presses | Lead screws of lathes, valves | One-way loads: vices, presses |
Standards: square threads to IS 4694; metric trapezoidal threads (Tr, 30 degrees) to ISO 2904 / IS 7008; Acme to ASME B1.5 (29 degrees); buttress to ASME B1.9 or the relevant ISO/IS standard.
Stress concentration
The thread root has sharp corners and a sudden change of section, giving stress concentration factors of 2 to 3. Also, the load is not shared equally by the threads in the nut: the first threads at the loaded face carry most of the load (often one third in the first thread). Failure of the nut thread or screw begins there and is more dangerous under fatigue and varying load. Remedies: root fillets, a larger root radius, rolled threads (compressive residual stress), and a tapered nut for more uniform load distribution.
Materials
- Screw: medium-carbon steel (C45, 40C8), or alloy steels (e.g. 40Ni3, 40Cr1) for heavy duty, to resist wear and torsion; the thread surface is hardened or ground.
- Nut: a softer material with a low friction coefficient, to protect the costly screw: phosphor bronze, gun metal, cast iron or aluminium bronze.
- Lubrication with oil or grease, and sometimes a second safety nut for jacks.
✓ Answer checked by Claude Opus 5.5 (high effort)
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