Speed Reduction
Part of Machine Tools
Techniques for reducing rotational speed while increasing torque — gears, pulleys, and compound drives.
Why This Matters
Power sources almost never run at the same speed that machine tools need. A treadle runs at 60-80 RPM; a small lathe spindle needs 200-600 RPM for wood and 100-300 RPM for metal. A water wheel runs at 5-20 RPM; a line shaft runs at 100-200 RPM. Speed reduction (or increase) is therefore a fundamental requirement in every mechanical system.
Speed change always involves a trade-off: increasing speed reduces torque proportionally, and reducing speed increases torque proportionally. This is conservation of energy — you cannot get more power out than you put in (and friction means you always get slightly less). But you can trade speed for torque or torque for speed to match the load requirements.
Understanding speed reduction allows you to design drive systems that deliver the right speed and torque to each machine in your workshop from a single power source.
Pulley and Belt Ratios
The simplest speed reducer is a belt running between two pulleys of different diameters. The speed ratio equals the driving pulley diameter divided by the driven pulley diameter.
If the driving pulley (on the motor or waterwheel) is 400mm diameter and the driven pulley (on the machine) is 100mm diameter, the machine runs 4 times faster than the driver — speed is increased 4:1. Reverse the sizes (100mm driver, 400mm driven) and speed is reduced 4:1 while torque is increased 4:1.
Stepped pulleys: A stepped pulley (also called a cone pulley) has multiple diameters on a single casting. Moving the belt from one step to another changes the speed ratio. Most belt-driven lathes have a 4-step cone pulley on the spindle and a matching 4-step pulley on the countershaft, giving 4 combinations and up to 8 speed ratios if combined with back gear.
Back gear: A two-stage gear reduction built into the lathe headstock, engaged by a lever. When engaged, it reduces spindle speed by a ratio of 6:1 to 8:1, giving very low speeds (for large-diameter facing, threading, and heavy cuts) and multiplying torque accordingly.
Gear Drives
Gears provide positive (non-slipping) speed reduction at high efficiency. Unlike belts, gears do not slip under load and can transmit large torques in a small space.
Spur gears: The simplest type — teeth cut on a flat disc, meshing with another disc on a parallel shaft. Speed ratio equals the tooth count on the driven gear divided by the tooth count on the driving gear. A 20-tooth pinion meshing with an 80-tooth gear gives 4:1 reduction.
Compound gear trains: Multiple gear stages on a common shaft. If a first stage gives 4:1 reduction and a second stage also gives 4:1, the combined ratio is 16:1. This allows large total ratios without requiring impractically large gears.
Making gears: Without a hobbing machine, gears can be cut on a milling machine using a form cutter. The gear blank is mounted in a dividing head (a device that indexes the blank one tooth spacing at a time). Each tooth space is cut by traversing the form cutter across the face. This produces accurate enough gears for moderate-speed, moderate-load applications.
Alternatively, cast gears can be made by casting iron or brass into a sand mold formed from a wooden pattern of the gear. Cast gears are rougher and less accurate than cut gears but are functional for low-speed power transmission.
Worm and Wheel
A worm gear (a helical screw meshing with a spur-like wheel with helical teeth) provides very high speed reduction in a compact space. A single-start worm meshes so that one full rotation of the worm advances the wheel by exactly one tooth. A 40-tooth wheel driven by a single-start worm gives 40:1 reduction — in a very small package.
Worm gears are nearly irreversible: you can drive the wheel from the worm, but you usually cannot back-drive the worm from the wheel due to the high friction in the worm engagement. This makes them useful for indexing applications (the dividing head uses a worm gear) and for screw-type hoists, but less suitable for power transmission where reversibility is needed.
Make worm gears by turning the worm on a lathe (it is essentially a precision thread) and cutting the wheel teeth on a lathe or milling machine using the worm itself as a form.
Compound Pulley-and-Gear Systems
For large total speed ratios, combine belt drives and gear drives. Use a belt stage first (lower cost, absorbs shock, acts as a slip clutch for overloads) followed by a gear stage for precision speed ratio. A 4:1 belt reduction followed by a 4:1 gear reduction gives 16:1 total — enough for most workshop applications.
When designing compound systems, work backward from the required spindle speed:
- Determine target spindle RPM for the planned work.
- Determine available power source RPM.
- Calculate required total ratio (power source RPM divided by target spindle RPM for speed reduction, or vice versa for increase).
- Factor the ratio into achievable stages (belt ratios up to 6:1 are practical; gear ratios up to 10:1 per stage are practical).
- Size pulleys and gears to achieve each stage ratio.