Choosing the wrong gearbox can cripple your production line, waste energy, and drain your maintenance budget. The right selection depends on understanding torque requirements, speed ratios, efficiency ratings, and mounting constraints—not just picking the cheapest option. This guide breaks down the essential specifications you need to compare before making a purchase or sourcing from a supplier.
Understanding Torque and Power Requirements
Torque is the rotational force your system needs to move the load. Start by calculating the torque required at the output shaft, measured in Newton-meters (Nm) or foot-pounds (ft-lb). Multiply the force needed to move your load by the distance from the rotation center.
Power is equally critical. Gearboxes are rated in kilowatts (kW) or horsepower (HP). Your motor's power output must exceed the mechanical load by at least 10–15% to account for friction and heat losses. If your application requires 15 kW, don't buy a 15 kW gearbox; look for 17–18 kW minimum.
Speed Ratio and Gear Selection
The speed ratio determines how much the input speed decreases relative to the output torque. A 10:1 ratio means a 1,500 RPM motor input becomes 150 RPM output, with ten times the torque multiplication.
Common gearbox types serve different needs:
- Helical gears: High efficiency (92–98%), smooth operation, better for continuous duty
- Spur gears: Lower cost, moderate efficiency (85–92%), acceptable noise levels
- Worm gears: Highest torque multiplication, lowest efficiency (50–90%), self-locking capability
- Bevel gears: Right-angle drive, moderate efficiency, compact mounting
- Planetary gears: Compact design, high efficiency (90–97%), excellent for tight spaces
Match the gear type to your duty cycle. Continuous industrial applications favor helical or planetary designs, while intermittent loads tolerate spur gears at lower cost.
Efficiency and Heat Dissipation
Gearbox efficiency directly impacts operating costs. A 90% efficient unit dissipates 10% of input power as heat; at 85% efficiency, that jumps to 15%. Over 5,000 annual operating hours, this difference translates to real energy bills and cooling requirements.
Check the manufacturer's efficiency curve across your operating range—efficiency varies with speed and load. Most quality industrial gearboxes operate in the 88–96% range. Budget-tier units may drop to 80–85%, requiring larger motors and more cooling investment.
Heat buildup shortens bearing and seal life. If your gearbox runs hot (above 80°C), you'll need active cooling or larger casings. Allow 20–30% oversizing for applications in hot environments or with continuous duty cycles.
Mounting, Shaft, and Bearing Specifications
How the gearbox connects to your motor and driven equipment matters. Standard mounting options include:
- Foot-mounted (bolted to a frame)
- Flange-mounted (direct motor coupling)
- Hollow-bore output (shaft passes through)
Shaft specifications include diameter, keyway size, and spline options. Mismatched shafts mean custom couplings—an unnecessary expense. Request exact drawings or CAD files from suppliers before purchase.
Bearing type affects lifespan and maintenance intervals. Deep-groove ball bearings suit lower-speed applications; tapered roller bearings handle higher radial and axial loads. Industrial-grade gearboxes use sealed, lubricated bearings rated for 10,000–20,000 operating hours between overhauls.
Lubrication and Maintenance Access
Oil type and viscosity (typically ISO 68–220) depend on operating temperature and speed. Synthetic oils cost 30–50% more but extend drain intervals from 2,000 to 5,000 hours.
Check for accessible fill, drain, and sight-glass ports. A gearbox without a sight glass requires scheduled disassembly to check oil levels—a poor design for remote or high-frequency monitoring applications. Budget an extra £200–500 for condition-monitoring sensors if downtime is costly.
Typical Price and Lead-Time Ranges
Standard industrial gearboxes range from £500 for small spur units up to £5,000+ for large planetary or helical designs rated above 30 kW. Custom or specialty gearboxes (compact, high-ratio, food-safe sealing) push into the £8,000–15,000 range.
Lead times vary: stock items ship in 1–2 weeks, engineered units take 4–8 weeks. Rush orders incur 15–25% premiums.
Mercoly simplifies sourcing by letting you compare specifications, pricing, and availability from trusted Bearings & Power Transmission suppliers in one place—saving time and helping you avoid costly mistakes.
Frequently Asked Questions
Q: How do I know if my gearbox is oversized or undersized? An undersized gearbox will overheat, slip, or seize within weeks; an oversized unit wastes capital and floor space but operates cooler with longer bearing life. Check that your peak torque demand stays within 80–90% of the gearbox's rated output—this leaves margin for shock loads and aging.
Q: What's the difference between rated and peak torque? Rated torque is the continuous output the gearbox handles all day; peak torque is a brief overload (typically 1–2 seconds) it can survive without damage. Most applications need peak torque 20–30% above rated—check your load profile with your supplier.
Q: Should I buy a sealed or open gearbox? Sealed units cost 10–15% more but eliminate contamination and extend service intervals from 2,000 to 5,000+ hours. For food, pharmaceutical, or outdoor applications, sealed is mandatory; for clean factory floors, standard sealed designs are industry standard.
Use Mercoly to compare gearbox options side-by-side from certified suppliers near you.