A gearbox (gear reducer, speed reducer) is a power transmission element that converts the high-speed, low-torque output of an electric motor into the low speed and high torque a machine needs. Most mining equipment, such as conveyors, screw washers, feeders, agitators, thickener rakes and mills, is gearbox-driven. A gearbox failure often means the whole line it serves comes to a stop.
In this guide we look in detail at what a gearbox is and what it does, gearbox types, selection criteria, lubrication and oil selection, alignment and installation, failure symptoms and maintenance planning.
What Does a Gearbox Do?
Electric motors usually run at around 1,000–3,000 rpm. A conveyor pulley, a screw or a rake, however, must turn at a much lower speed but with high torque. Through its meshing gear stages, a gearbox reduces speed and increases torque in the same proportion (less efficiency losses). For example, a 1:20 gearbox reduces motor speed to one twentieth while increasing torque roughly twentyfold.
This conversion means that:
- Heavy loads can be driven with standard, economical motors.
- Machine speed is matched to requirements.
- Power transmission becomes compact and efficient.
Gearbox Types
| Type | Construction | Efficiency | Typical Use |
|---|---|---|---|
| Helical (parallel shaft) | Helical gears, input and output shafts parallel | High | Conveyors, feeders, pumps |
| Bevel-helical (right angle) | Bevel gear + helical stages; output shaft at 90° | High | Conveyor drives (limited space), screws |
| Planetary | Sun, planet and ring gears | High | Where high torque and compactness are needed, drum motors, mobile equipment |
| Worm | Worm and worm wheel | Low–medium (depends on ratio) | Light and intermittent duties, some adjustment mechanisms |
| Shaft-mounted | The gearbox fits directly on the machine shaft and is restrained by a torque arm | High | Conveyors, screw washers |
For gearbox and pulley arrangements on conveyor drives, see our conveyor pulley and gearbox maintenance guide.
Gearbox Selection Criteria
- Power and torque: Gearbox capacity is determined by multiplying the torque the machine needs by a service factor appropriate to the operating conditions. In crushing and screening plants, an adequate service factor should be allowed because of shock-loaded, dusty operating conditions.
- Ratio: Determined by the motor speed and the required output speed.
- Mounting: Foot, flange or shaft-mounted; the mounting position affects oil level and lubrication arrangement.
- Thermal capacity: Heat can be the limiting factor on continuously running gearboxes; a fan or oil cooler is added where necessary.
- Backstop: On inclined conveyors, prevents the loaded belt running back when power is lost. It is a safety element that must always be checked on inclined conveyors.
- Environment: Dust, moisture and temperature should be considered when selecting seals and breathers.
Lubrication and Oil Selection
Lubrication is the most important factor in gearbox life.
| Topic | Practice |
|---|---|
| Oil type | Gear oil of the viscosity grade (ISO VG) recommended by the manufacturer, usually with EP (extreme pressure) additives; mineral or synthetic |
| Viscosity | Selected by ambient temperature, speed and load; the wrong viscosity weakens the oil film |
| Oil level | Checked at the sight glass or plug for the mounting position; low level damages gears and bearings, high level causes heating and foaming |
| First change | After the running-in period, to remove wear particles |
| Periodic change | According to the manufacturer’s recommendation and operating temperature; can be optimised with oil analysis |
| Breather | Must be clean and working; a blocked breather raises internal pressure and causes seal leaks. A filtered breather is preferable in dusty environments |
| Mixing oils | Different oil types (especially mineral and some synthetics) must not be mixed |
Oil analysis is the most effective way to see inside a gearbox without dismantling it. Iron particles indicate gear and bearing wear, silica dust indicates contamination and water indicates a seal or breather problem. For the use of oil analysis in predictive maintenance, see our predictive maintenance guide.
Alignment and Installation
- Coupling alignment: Alignment between motor and gearbox, and between gearbox and machine shaft, should be done with laser alignment tools. Misalignment shortens bearing and seal life and causes vibration and heat.
- Soft foot: If one of the gearbox or motor feet does not sit fully, the housing is distorted; correct it with shims.
- Shaft-mounted gearboxes: The torque arm must be connected correctly and its bushes must be suitable; if the torque arm is restrained too rigidly, extra load is placed on the gearbox bearings.
- Belt-driven inputs: Excessive belt tension overloads the input shaft bearings.
Failure Symptoms and Possible Causes
| Symptom | Possible Cause | Action |
|---|---|---|
| Overheating | Wrong oil level or viscosity, overload, dirty housing surface, lack of cooling | Oil check, load analysis, clean the housing |
| Oil leakage | Worn seals, blocked breather, worn shaft surface | Replace seals, clean breather, repair/sleeve the shaft |
| Humming and irregular noise | Bearing damage, gear wear | Vibration analysis, oil analysis, inspection |
| Knocking | Broken or pitted gear teeth | Check through the inspection cover, overhaul |
| Increased vibration | Misalignment, loose connection, bearing damage | Alignment, connection check |
| Water or foam in the oil | Water ingress, wrong oil, overfilling | Oil change, seal check |
Maintenance Plan
| Frequency | Check |
|---|---|
| Daily | Noise, temperature (by hand or thermometer), oil leakage, oil level indicator |
| Weekly | Mounting bolts, torque arm, breather, housing cleanliness |
| Monthly | Vibration measurement, coupling check |
| Periodic | Oil analysis and oil change, gear check through the inspection cover |
| Annual / overhaul | Alignment check, seal replacement, bearing and gear overhaul if needed |
Overhaul or New Gearbox?
When a gearbox fails, an overhaul (bearings, seals and, if needed, gears) is often more economical than a new gearbox. However, a new gearbox may make more sense if the housing is damaged, many gears need replacing or the model is no longer made. Parts availability, lead time and downtime cost should be weighed together when deciding. Keeping a spare gearbox for critical lines allows production to continue during an overhaul.
Field Example
Suppose that at a washing plant the screw washer gearbox constantly leaks oil and the seals are replaced every few months. Inspection shows that the breather is blocked with mud and pressure is building inside the housing. There is also a wear groove on the output shaft in the seal area. When a filtered breather is fitted and the shaft surface is repaired, the leak disappears. This example shows that replacing seals alone is not the solution; the root cause must be found. For screw washer maintenance see our sand screw washer guide.
Frequently Asked Questions
What is a gearbox?
A gearbox is a gear unit that reduces motor speed and increases torque; it is also known as a “speed reducer” or “gear reducer”.
How often should gearbox oil be changed?
The first change is after running-in; later changes follow the manufacturer’s recommendation and the operating temperature. Oil analysis helps set the change interval correctly.
Why does a gearbox overheat?
Wrong oil level or type, overload, misalignment, bearing damage and dust building up on the housing are the main causes.
Why is a backstop important?
On inclined conveyors it prevents the loaded belt running back when power is lost, avoiding spillage and serious accidents.
What is the difference between a helical and a worm gearbox?
Helical gearboxes are highly efficient and suit continuous heavy duty. Worm gearboxes are compact with high ratios but lower efficiency.
Gearbox Spare Parts
| Part | Reason for Replacement | Note |
|---|---|---|
| Shaft seals (oil seals) | Oil leakage, hardening | Check the shaft surface too; if there is a wear groove, sleeve or repair it |
| Bearings | Noise, vibration, heat | Select to match the original number and suffixes |
| Gears and pinions | Pitting, broken teeth, wear | Usually replaced in pairs |
| Backstop | Slipping, failure to lock | A critical safety part on inclined conveyors |
| Breather / filter | Blockage | Filtered type in dusty environments |
| Oil level gauge and plugs | Breakage, leakage | — |
| Coupling elements | Wear, misalignment | Flexible elements are replaced periodically |
| Torque arm and bushes | Wear, loosening | On shaft-mounted gearboxes |
| Oil cooler and fan | Overheating | For continuous heavy duty |
Send your requirements for seals, bearings, gears, couplings and complete gearboxes for conveyor, screw and feeder drives via our WhatsApp line or our spare parts page. For the other parts of a conveyor drive, see our belt conveyor spare parts page, and for screw washer drives our screw washer spare parts page.
The Overhaul Process
- Initial assessment: Failure symptoms, oil analysis and vibration data are reviewed.
- Dismantling and cleaning: The gearbox is dismantled and the parts cleaned and numbered.
- Inspection: Gears, shafts, bearing seats and the housing are measured, and a damage report is prepared.
- Parts supply: The bearings, seals and gears to be replaced are identified.
- Assembly: Bearing clearances and gear contact patterns are set.
- Testing: Where possible a no-load run test is carried out; noise, temperature and leaks are checked.
- Commissioning: Alignment, oil filling and monitoring of the first run are carried out.
Glossary
- Ratio: The ratio of input speed to output speed.
- Service factor: A safety factor added to gearbox capacity according to operating conditions.
- ISO VG: The viscosity grade of an oil.
- EP additive: An additive that protects gear surfaces under extreme pressure.
- Pitting: Small fatigue-induced pits on a gear surface.
- Backstop: A device that prevents reverse rotation.
More Questions
Can grease be used instead of gearbox oil?
Only in small gearboxes designed by the manufacturer for grease lubrication. Using grease in oil-bath gearboxes impairs lubrication and cooling.
How long does a gearbox last?
Correctly selected, well-aligned and regularly lubricated gearboxes can run for many years; oil quality, load and alignment are the most important factors in their life.
Is synthetic oil an advantage?
It can be advantageous at high temperatures and over wide temperature ranges, and can extend change intervals. When switching from mineral oil, compatibility must be checked and the system cleaned.
CSP Mühendislik Support
CSP Mühendislik supports gearbox selection, installation and alignment, overhaul and spare parts supply for conveyor, screw and feeder drives. You can send your requirements through our service, maintenance and repair services and our spare parts page; for conveyor drive parts see our belt conveyor spare parts page.
