A gearbox lets an engine that works best at one speed drive a load at another by trading speed for torque: power stays the same apart from losses, so a 4:1 reduction cuts the speed to a quarter and, ideally, quadruples the torque. A 4:1 reduction turns 9.55 N·m at 3,000 rpm into about 38 N·m at 750 rpm, and the clutch must be released before the ratio is changed.
Part of Internal Combustion
Gear ratios, how torque and speed trade, how multiple ratios are built and shifted, and the alternatives for a stationary engine.
Power Stays The Same, So Speed And Torque Trade
The corpus gives the ratio rule in Gear Ratios: the ratio is the driven gear’s teeth over the driver’s teeth. Gear Ratios and Torque adds that by conservation of energy a speed step-up cuts the torque in the inverse ratio. For a speed reduction the same holds in reverse: the output turns more slowly with more torque. Power is torque times angular speed, and a gearbox changes the first and second while leaving the product (before friction losses) unchanged.
An engine produces useful power only over a range of speed, and its torque at low speed may not be enough to start a load. A gearbox gives the engine a lower gear to start a heavy load and a higher one to run it quickly. A single gear pair is limited: the corpus notes that pairs top out around 6:1 or 8:1 before the size difference becomes absurd, and that 20:1 or 100:1 takes a compound train of stages built in series, in Compound Trains.
How Several Ratios Are Built And Changed
A sliding-gear box has several pairs of gears on two parallel shafts. Each pair gives a different ratio, and a gear is slid along its shaft to mesh with its partner. The gears must be stopped or matched in speed when they come together, or the teeth clash. That is the reason for the clutch in front of the box: release it to take the torque off, shift, and re-engage, as Clutch Design describes. Gear forms are in Gear Types and Spur Gears. Cutting accurate teeth is a precision job described in Gear Cutting.
For a stationary engine driving a machine at a fixed speed, a gearbox is often unnecessary. A belt and two pulleys give a ratio by changing a pulley, absorb shock and slip harmlessly under overload, as the corpus explains in Belt Drive, and Speed Reduction covers the trade-offs between gears, pulleys and compound drives.
What You Need
- Gears of the right form and pitch, hardened where they will wear, or salvaged.
- Shafts and bearings that hold the gears in true alignment.
- A casing that keeps oil in and dirt out.
- Lubricant suited to the gears: Lubricant Types and Lubrication and Failures cover choice and the early signs of a dry gear.
- A shift mechanism with a fork and detents, for a sliding-gear box.
Method: Choosing And Checking A Reduction
Step 1: Fix the engine’s speed and torque at its working point.
Step 2: Fix the load’s needed speed and torque.
Step 3: Compute the ratio. Divide the engine speed by the load speed. Divide it into stages if it exceeds about 6:1.
Step 4: Check the torque against the strength of the gears, shaft and keys, with the shock of starting included.
Step 5: Check the shift. It should engage with the clutch released, with no grinding. A rasping noise points to clashing teeth or a worn fork.
Worked Example: Reduction And A Compound Stage
Take the engine of the clutch example: 3 kW at 3,000 rpm.
- Angular speed: 3,000 × 2π ÷ 60 = 314.16 rad/s.
- Torque: 3,000 ÷ 314.16 = 9.55 N·m.
A 4:1 reduction, ideal and with no losses:
- Output speed: 3,000 ÷ 4 = 750 rpm, which is 78.54 rad/s.
- Output torque: 9.55 × 4 = 38.2 N·m, about 38 N·m.
- Check the power: 38.2 × 78.54 = 3,000 W. ✓.
A 20:1 reduction as a compound train of a 5:1 stage and a 4:1 stage:
- Output speed: 3,000 ÷ 20 = 150 rpm.
- Output torque: 9.55 × 20 = 191 N·m.
Real gears lose some power in every mesh and bearing, so the real torque is a little lower and the engine has to supply the lost power. The loss per mesh is not sourced here; measure it.
Common Mistakes
- Shifting under load. The teeth clash and chip.
- Mismatched shaft alignment. The gears wear on one edge and run noisy.
- A box with no oil. A dry gearbox fails within hours.
- Choosing a ratio from the engine’s power alone. Torque at the load is what starts it.
- Forgetting the starting shock. The starting load can be well above the running load.
Safety
Gears, belts and shafts trap and crush
Guard every gear train, belt and coupling. Never reach into a moving box, never shift with a hand near the teeth, and never clear a jam with the engine running. A load held by a reduction can drive the engine backwards when released, so block it before working.
What’s Next
- Learn next: Converting a Vehicle Engine to Stationary Use — putting the whole engine to work
- Combine with: Cooling and Lubrication — oil and cooling for the gearbox as well as the engine