Transmission and Motors
Inside a Gearbox
See how gear arrangements connect a motor to a useful output shaft.
You need to size or verify a real system using “Inside a Gearbox”, with units, duty and limits that match the application.
Follow torque and power through shafts, gears, bearings and housing, then compare parallel-shaft spur or helical, planetary, bevel and worm arrangements without treating their geometry or performance as interchangeable.
- Select or check first
- 1. Identify the arrangement. Record gear family, stage count, shaft relationship, exact ratio, mounting position and rotation direction.
- Controlling assumption
- Load, motion profile, geometry, alignment, duty, lubrication and environment match the selected mechanism and ratings.
- Top risk
- Do not assume a worm gearbox safely holds a load. Back-driving behavior depends on lead angle, friction, lubrication, wear, vibration, temperature and load. A claim such as “self-locking” must come from the exact manufacturer and still may not qualify as a safety brake.
- Proof
- 6. Plan lubrication and inspection. Specify oil or grease, quantity, fill position, breather, seals, intervals and evidence for leakage, wear, vibration and temperature.
Start with the basics Optional foundation before “Inside a Gearbox”
Mechanical-design basics
Mechanical parts must carry loads, guide motion and remain aligned; similar-looking components can behave very differently under real force and duty.
- Force and torque
- Force pushes or pulls along a line. Torque is a turning effect equal to force multiplied by its perpendicular lever arm.
- Speed and acceleration
- Speed describes how fast position changes. Acceleration describes how fast speed changes and often creates the highest inertial load.
- Strength and stiffness
- Strength concerns permanent damage or failure. Stiffness concerns elastic deflection under load; a part can be strong enough yet too flexible for accurate motion.
- Load path
- The load path is the complete route by which force travels through shafts, bearings, fasteners, frames and foundations. Every interface on that route must be checked.
- Tolerance, clearance and alignment
- Tolerance permits controlled variation, clearance provides intentional space, and alignment keeps interacting axes or surfaces in the required relationship.
How to use this article: Sketch the load path and motion direction first, then attach units, worst-case loads, duty and acceptance limits to every calculation.
On this page
A gearbox routes mechanical power from an input shaft to an output shaft through one or more gear meshes. It can reduce or increase speed, change torque, reverse rotation, redirect the shaft axis or split and combine motion. The teeth are only part of the machine. Shafts carry torque, bearings locate rotating parts and react gear forces, the housing holds alignment, seals retain lubricant and exclude contamination, and the lubrication system separates loaded tooth and bearing surfaces.
Trace the load path through every supporting part
Diagram description
A smaller gear on an upper shaft meshes with a larger gear on a lower parallel shaft inside an outlined housing. Yellow arrows show input from the left and output to the right, while labels identify drive gear, driven gear and shafts.
| Part | Primary role | Important checks |
|---|---|---|
| Input and output shafts | Transmit torque to and from the gear set | Torsion, bending, key or spline, shoulders, overhung and axial loads, speed and fatigue |
| Gears | Transfer force at controlled geometry and ratio | Tooth strength, contact stress, material, accuracy, lubrication, speed and load spectrum |
| Bearings | Locate shafts and react radial or axial gear forces | Load direction, life, preload or clearance, speed, lubrication and mounting |
| Housing | Maintains shaft center distances and contains lubricant | Stiffness, bore alignment, mounting distortion, thermal growth, venting and structural reaction |
| Seals and breathers | Control lubricant retention and contaminant ingress | Shaft surface, speed, pressure, orientation, temperature and environment |
| Lubricant | Forms protective films and removes or distributes heat | Specified type, viscosity, quantity, change interval, cleanliness and mounting position |
At the mesh, tooth forces have tangential and often radial or axial components. The tangential component transmits torque; the other components still load shafts, bearings and housing. Helical and bevel teeth can create axial thrust. Worm meshes involve substantial sliding. The selected bearing arrangement and gearbox mounting position must suit those reactions. A drawing that shows only two pitch circles cannot be used to size the supporting system.
Mechanical power at a rotating shaft
P = T × ω
Use consistent units: torque in N·m and angular speed in rad/s give power in watts. This relation applies at input and output; real output power is lower than input power by the gearbox efficiency at that operating condition.
- P
- mechanical power, W
- T
- shaft torque, N·m
- ω
- angular speed, rad/s
A shaft transmitting 10 N·m at 100 rad/s carries 1,000 W of mechanical power. If a gearbox receives that power, its output cannot continuously exceed 1,000 W without another energy source.
Gear families arrange shafts and tooth contact differently
Diagram description
Four labeled panels show two external gears on parallel shafts, a sun with planets and internal ring, a screw-like worm over a wheel, and two conical gears meeting at an angle.
| Family | Geometry | Design implications |
|---|---|---|
| Spur or helical | External gears commonly connect parallel shafts | Spur teeth are straight; helical teeth are angled and can create axial thrust. Multi-stage units extend the ratio range |
| Planetary | Sun, planet gears, carrier and ring share a common axis | Which member is input, output or fixed determines ratio and direction; load sharing, bearing support and manufacturing accuracy are critical |
| Bevel | Conical gear pitch surfaces connect intersecting axes, often near a right angle | Changes shaft direction; tooth form, mounting distance, bearing thrust and mesh adjustment matter |
| Worm | A worm drives a meshing wheel on typically non-intersecting perpendicular axes | Can provide high ratio in one stage and right-angle packaging; sliding contact affects efficiency, heat, wear and lubrication |
Spur and helical are not synonyms
Both can connect parallel shafts, but tooth orientation changes how contact develops and what forces arise. Helical gearing can provide smoother overlapping engagement, while its helix angle produces axial force that the bearings and housing must react. Noise, efficiency and capacity depend on tooth accuracy, speed, load, lubrication and the complete design—not solely on choosing an angled tooth.
Choose by the complete duty and interface
| Input | Questions | Affected parts |
|---|---|---|
| Speed and ratio | Input range, required output, overspeed and reversing? | Gear stages, bearings, lubricant and motor operating point |
| Torque and load spectrum | Continuous, effective, peak, impact and starts per hour? | Teeth, shafts, keys, bearings, housing and service factor |
| External shaft loads | Radial or axial force, lever arm, direction and attached transmission? | Output shaft and bearing life |
| Accuracy and dynamics | Backlash, torsional stiffness, reflected inertia, reversal and settling? | Gear quality, preload, ratio, coupling and controls |
| Thermal duty | Ambient, airflow, mounting surface, duty cycle and permissible temperature? | Efficiency, lubricant life, seals and continuous torque |
| Mounting position | Which face, shaft direction and orientation changes occur? | Oil level, bearings, seals, breather and churning loss |
| Environment and life | Dust, water, corrosion, hygiene, expected hours and maintenance? | Housing, protection, coatings, seals, lubricant and inspection plan |
Read a gearbox datasheet as a power-path record
- 1. Identify the arrangement.Record gear family, stage count, shaft relationship, exact ratio, mounting position and rotation direction.
- 2. Match speeds and torques.Check input and output operating ranges, effective or continuous demand, peaks, reversal and the manufacturer's service method.
- 3. Check all shaft forces.Include coupling, belt, chain, screw or process forces with their lever arms and direction at the actual shaft interface.
- 4. Close the thermal balance.Use duty, ambient, ventilation and mounting conditions to verify permitted continuous load and lubricant temperature.
- 5. Check motion quality.Include backlash, torsional stiffness, reflected inertia, efficiency variation and control response in the system model.
- 6. Plan lubrication and inspection.Specify oil or grease, quantity, fill position, breather, seals, intervals and evidence for leakage, wear, vibration and temperature.
A gearbox must connect cleanly to the shafts and couplings explained in What Shaft Couplings Actually Do. Gear Ratio, Speed and Torque in Real Machines next quantifies the trade and compares direct, geared and belt-driven layouts. Inside the housing, every useful output depends on a continuous path through teeth, shafts, bearings, lubricant, structure and interfaces.
One-minute questions
Questions to answer before you apply this lesson
What should I select or check first for Inside a Gearbox?
1. Identify the arrangement. Record gear family, stage count, shaft relationship, exact ratio, mounting position and rotation direction.
Which assumption can change the conclusion?
Load, motion profile, geometry, alignment, duty, lubrication and environment match the selected mechanism and ratings.
What can go wrong if I copy the example directly?
Do not assume a worm gearbox safely holds a load. Back-driving behavior depends on lead angle, friction, lubrication, wear, vibration, temperature and load. A claim such as “self-locking” must come from the exact manufacturer and still may not qualify as a safety brake.
How do I verify the result in the real system?
6. Plan lubrication and inspection. Specify oil or grease, quantity, fill position, breather, seals, intervals and evidence for leakage, wear, vibration and temperature.
Sources and further reading
- Gear Units — Project PlanningSEW-EURODRIVE GmbH & Co KG · 2026-10-12Primary manufacturer documentation reviewed for ratio, torque, mounting position, thermal rating, lubrication losses and manufacturer-specific service factor.
- Data for Drive Selection and DesignationSEW-EURODRIVE GmbH & Co KG · 2026-10-12Current manufacturer definitions reviewed for ratio, torque, speed, effective duty, efficiency, braking work, protection, altitude and ambient inputs.
- Calculating the Basic DataSEW-EURODRIVE GmbH & Co KG · 2026-10-12Manufacturer planning guidance reviewed for gear ratio, efficiency losses, load spectra, peak checks and bearing-life context.
- Service FactorSEW-EURODRIVE GmbH & Co KG · 2026-10-12Primary source reviewed for the warning that gearbox service factors are not standardized or directly comparable across manufacturers.
- R, F and K Gear UnitsSEW-EURODRIVE GmbH & Co KG · 2026-10-12Current product documentation reviewed for stage-dependent helical, parallel-shaft and helical-bevel efficiency context; values are not generalized in the manuscript.
- Gearboxes and Geared Motors — Explanatory NotesGetriebebau NORD GmbH & Co. KG · 2026-10-12Primary catalog reviewed for shaft tolerances, radial and axial loads, ratio, torque, speed, efficiency and mounting data.