Question 1

How do I calculate the gear ratio from tooth count?

Accepted Answer

Gear ratio = Number of teeth on driven gear / Number of teeth on drive gear. Example: Drive gear has 20 teeth, driven gear has 60 teeth → Ratio = 60/20 = 3:1. This means the driven gear turns 3× slower than the drive gear. For a multi-stage gearbox, multiply the individual stage ratios: 3:1 × 4:1 = 12:1 overall. For helical or worm gears the formula is the same — based on tooth count (or thread count for worm gears).

Question 2

How does gear ratio affect torque and speed?

Accepted Answer

Speed and torque are inversely related through gear ratio (assuming 100% efficiency): Output speed = Input speed / Ratio. Output torque = Input torque × Ratio × Efficiency. With a 5:1 reduction gearbox at 97% efficiency: If input is 1480 rpm and 100 N·m → Output = 296 rpm and 485 N·m. The power is reduced only by the efficiency loss (3% in this example). This is why gearboxes are used to match high-speed motors to low-speed, high-torque applications like conveyors and agitators.

Question 3

What is a compound gear train?

Accepted Answer

A compound gear train has multiple stages where an intermediate shaft carries both a gear driven by the first stage and a gear driving the next stage. Overall ratio = Stage 1 ratio × Stage 2 ratio × ... × Stage n ratio. Example: Stage 1 = 4.5:1, Stage 2 = 3.2:1 → Overall = 14.4:1. This allows very high reduction ratios (100:1 to 10,000:1 for worm-worm compound), impossible with a single stage. Compound gearboxes are used in elevators, hoists, and heavy machinery. Overall efficiency = η₁ × η₂ (typically 0.97 × 0.97 = 0.94 for helical).

Question 4

What are the different types of gearboxes used in industry?

Accepted Answer

Common industrial gearbox types: Spur gear: Simple, efficient (96–99%), loud at high speeds. Helical gear: Quieter, higher capacity (96–99%), used in most industrial gearboxes. Bevel gear: Right-angle drive, 94–98% efficient. Worm gear: High reduction (10:1 to 100:1), 50–90% efficient (self-locking). Planetary (epicyclic): Compact, high torque density (94–97%), used in final drives. Cycloidal: Very high reduction, high shock load capacity. South African applications: SEW-EURODRIVE, Bonfiglioli, Flender, Radicon (Renold) brand gearboxes are widely used in mining and materials handling.

Question 5

How do I calculate belt drive ratio and belt length?

Accepted Answer

Belt drive ratio = Driven pulley diameter / Drive pulley diameter. Output speed = Input speed × (Drive diameter / Driven diameter). For V-belt/chain centre distance C and pulley diameters D (large) and d (small): Approximate belt length L = 2C + π(D+d)/2 + (D-d)²/(4C). Speed range limits: V-belt pitch line velocity should not exceed 30 m/s. Chain drives: Pitch line velocity ≤ 15 m/s for roller chain. Synchronous belts: Up to 60 m/s. Use anti-clockwise or tensioner setup for long centre distances to prevent slack side vibration.

Question 6

What gearbox efficiency should I use in calculations?

Accepted Answer

Standard gearbox efficiency values: Spur gear pair: 98–99%. Helical gear pair: 97–99%. Bevel gear pair: 95–98%. Worm gear: 50–90% (depends heavily on lead angle; single-start worm = lower efficiency, multi-start = higher). Planetary stage: 95–97%. Overall gearbox (multi-stage): Multiply per-stage efficiency. 2-stage helical: 95–97%. Chain drive: 96–98%. V-belt: 93–97% (tension dependent). Important for motor sizing: always calculate required motor power as P_motor = P_load / (gearbox efficiency × coupling efficiency).

Question 7

What is service factor and how does it apply to gearboxes?

Accepted Answer

Service factor (SF) accounts for load variations and duty conditions beyond rated steady-state torque: SF = 1.0: uniform smooth load, 8 hrs/day. SF = 1.25: moderate shock, continuous. SF = 1.5: heavy shock, 24 hrs/day. SF = 2.0+: very heavy shock / mining / quarrying. Required gearbox rated torque = Operating torque × Service factor. Example: Conveyor requires 500 N·m, heavy-duty duty: Select gearbox rated at 500 × 1.5 = 750 N·m. AGMA and ISO 6336 define service factors. South African mining operations typically require SF ≥ 1.5–2.0 per SANS standards.

Question 8

When should I use a VFD instead of a gearbox for speed reduction?

Accepted Answer

Gearbox vs VFD comparison: Fixed speed ratio needed → Use gearbox. Variable speed needed → Use VFD (or VFD + gearbox). High torque at low speed → Gearbox is more efficient and reliable. Energy saving on fans/pumps → VFD (power ∝ speed³ for centrifugal). Gearbox advantages: No harmonic distortion, lower cost for fixed-speed, more reliable long-term. VFD advantages: Adjustable speed, soft starting, energy saving, built-in protection. Best practice for mining conveyors: Use gearbox for fixed final ratio + VFD for speed control. South African note: Eskom large power users must demonstrate efficiency measures — VFD on >30 kW variable loads is effectively mandated.

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