Question 1

What is torque and how is it calculated?

Accepted Answer

Torque (T) is the rotational equivalent of force — it measures how much a force causes an object to rotate about an axis. T = F × r × sin(θ), where F is force (N or lbf), r is the perpendicular distance from the axis (m or ft), and θ is the angle between force and lever arm. When force is perpendicular to the arm (θ = 90°), sin(θ) = 1, giving T = F × r. Torque is measured in Newton-metres (N·m), kilonewton-metres (kN·m), or foot-pounds (ft·lb).

Question 2

How do I calculate shaft torque from motor power?

Accepted Answer

T = (P × 9,550) / N in metric units, where P is power in kW and N is speed in RPM. The result is torque in N·m. In imperial: T = (HP × 5,252) / RPM gives result in ft·lb. Example: A 37 kW motor at 1,450 RPM produces T = 37 × 9,550 / 1,450 = 243.7 N·m. Always add 20–25% service factor margin when selecting couplings, gearboxes, and shafts.

Question 3

What is the relationship between torque, power, and RPM?

Accepted Answer

P = T × ω (power = torque × angular velocity). Since ω = 2π × N / 60: P (kW) = T (N·m) × N (RPM) / 9,550. Or P (HP) = T (ft·lb) × N (RPM) / 5,252. This relationship is fundamental to motor and gearbox selection. Higher RPM at the same power means lower torque (and vice versa). Variable-frequency drives use this to control speed by adjusting power and frequency.

Question 4

How do I calculate torsional stress in a shaft?

Accepted Answer

Torsional shear stress τ = T × r / J, where T is torque (N·mm), r is outer radius (mm), and J is polar moment of inertia. For a solid circular shaft: J = π × D⁴ / 32. So τ = 16T / (π × D³). Maximum allowable shear stress for steel shafts: typically 40–80 MPa depending on material and surface finish. Always apply a safety factor of 2–3 for rotating machinery.

Question 5

What torque units are commonly used in engineering?

Accepted Answer

Metric: Newton-metre (N·m) — standard SI unit. kN·m for large structures. N·mm common in shaft calculations. Imperial: Foot-pound (ft·lb) — US standard. Inch-pound (in·lb) for smaller fasteners. Conversions: 1 N·m = 0.7376 ft·lb = 8.851 in·lb. 1 ft·lb = 1.356 N·m. 1 kgf·m = 9.807 N·m. South African and European engineering uses N·m as standard.

Question 6

How do gear boxes affect torque?

Accepted Answer

A gearbox multiplies torque by the gear ratio while reducing speed: Output torque = Input torque × Gear ratio × Efficiency. Example: 10 kW motor at 1,450 RPM through a 10:1 gearbox (95% efficiency): Output speed = 145 RPM. Output torque = (10,000 × 9,550 / 1,450) × 10 × 0.95 = 625 N·m. Gearboxes convert speed to torque for conveyors, mixers, crushers, and other low-speed, high-torque applications.

Question 7

What is the difference between static and dynamic torque?

Accepted Answer

Static torque: Torque required to hold a load stationary or break it loose (starting torque). Often 1.5–3× running torque due to static friction. Dynamic (running) torque: Torque required to maintain steady motion against friction and load. Peak torque: Maximum instantaneous torque during acceleration or load impact. Motor starting torque must exceed load breakaway torque. Gearboxes and couplings should be rated for peak torque with appropriate service factor.

Question 8

How much torque can a bolt handle?

Accepted Answer

Bolt torque depends on bolt grade, diameter, and lubrication. General guide (dry condition): M12 Grade 8.8: ~85 N·m, M16 Grade 8.8: ~210 N·m, M20 Grade 8.8: ~410 N·m, M24 Grade 8.8: ~710 N·m. Lubricated (with anti-seize or oil): reduce by 30%. Grade 10.9 bolts: multiply by 1.4. Grade 12.9: multiply by 1.7. Always use a calibrated torque wrench and follow equipment manufacturer specifications. Use our Bolt Torque Calculator for detailed calculations.

Material	Tensile Strength (MPa)	Shear Yield Strength (MPa)	Notes
Mild steel (EN3B)	430–570	120–160	General shafting
Medium carbon (EN8)	700–850	200–250	Good strength shafts
Alloy steel (EN19)	850–1,000	250–300	High-duty shafts
Stainless 304	520	100–130	Corrosion resistant
Aluminium 6061-T6	310	80–100	Light weight
Cast iron	200	80–120	Brittle — avoid torsion

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