⚙Mechanical & Machinery Principles · 7 min read

Gears & Gear Ratios

Types, ratios, direction of rotation — and why worm gears matter

Gears are one of the most heavily tested mechanical concepts on the EIAT. You need to understand how different gear types work, how to calculate gear ratios, and how those ratios affect speed and torque. Worm gears deserve special attention — they are the gear type used in geared traction elevator machines.

On the Test

Gear questions on the EIAT are comparison and direction problems — no calculator needed. Expect scenarios like "Gear A drives a larger Gear B — which one turns slower?" or "which direction does the output gear rotate?"

•Bigger driven gear = slower output, more torque. Smaller driven gear = faster output, less torque.
•Two directly meshing gears always rotate in opposite directions. An idler gear between them reverses direction again — without changing the ratio.
•Worm gears are used in geared traction elevator machines and are self-locking: the worm drives the wheel, but the wheel cannot back-drive the worm.
⚠Common trap: Assuming the larger gear always produces more speed. The larger gear is the slower one — it produces more torque, not more speed.

How Gears Work

Gears transfer rotational motion and torque between shafts. The driver gear is connected to the power source (motor). The driven gear is the output. When two gears mesh, their teeth interlock — as one turns, it pushes the other.

The fundamental rule: two directly meshing gears always rotate in opposite directions. If you need the output to rotate in the same direction as the input, you add an idler gear in between (it cancels the reversal but does not affect the ratio).

Gear Ratio

Gear ratio = number of teeth on the driven gear ÷ number of teeth on the driver gear.

If the ratio is greater than 1: the driven gear turns slower than the driver, but with more torque (speed reduction, torque multiplication). If the ratio is less than 1: the driven gear turns faster, but with less torque.

Example: Driver has 20 teeth, driven has 80 teeth. Ratio = 80 ÷ 20 = 4:1. The driven gear turns at 1/4 the speed of the driver, but with 4× the torque. If the motor runs at 1,200 RPM, the output shaft runs at 300 RPM.

Speed and torque are always inversely related through a gear set — what you gain in torque, you lose in speed, and vice versa. No gear set creates power; it only redistributes it.

Types of Gears

Spur gears: Teeth are straight and parallel to the shaft axis. The simplest and most common type. Used when shafts are parallel. Can be noisy at high speeds due to abrupt tooth engagement.

Helical gears: Teeth are cut at an angle to the shaft. Multiple teeth engage simultaneously, so they run quieter and can carry more load than equivalent spur gears. Used in many gearboxes including some elevator machines.

Bevel gears: Cone-shaped gears that transmit motion between shafts at an angle — typically 90°. Used when you need to change the direction of a drive shaft. Miter gears are a special case: two identical bevel gears at 90°.

Worm gears: Consist of a worm (a helical screw thread) driving a worm wheel (circular gear with curved teeth). Worm gears produce very high reduction ratios in a single stage — 20:1 up to 100:1 or more. The defining characteristic: the worm drives the wheel, but the wheel cannot back-drive the worm. This is called self-locking. In an elevator machine, self-locking means that if power fails, the gear set itself resists the car load — the brake provides the final hold, but the worm gear helps prevent runaway. This is why worm gears are chosen for geared elevator machines.

Rack and pinion: A circular pinion gear meshes with a straight rack (a flat linear gear). Converts rotational motion to linear motion. Used in some construction hoists, car parking systems, and some hospital elevators.

Gear Trains & Direction Rules

When multiple gears are chained together, the total gear ratio is the product of all individual ratios. Example: three stages each 4:1 → total = 64:1.

Direction of rotation: count the number of external gear meshes. An odd number of meshes means the final output rotates opposite to the input. An even number means same direction. Idler gears add a mesh without changing the ratio — just the direction.

Key Takeaways

✓Worm gears are used in geared elevator machines. Memorize this — it will appear on the test.
✓Worm gears are self-locking: the worm drives the wheel, never the reverse.
✓Gear ratio = Driven ÷ Driver. Bigger driven gear = slower output, more torque.
✓Two meshing gears always rotate in opposite directions.
✓Gear ratio > 1 = speed reduction + torque multiplication. Ratio < 1 = speed increase + torque reduction.

Key Terms

Driver gear

The input gear connected to the power source.

Driven gear

The output gear turned by the driver.

Gear ratio

Driven teeth ÷ Driver teeth; describes the speed and torque relationship.

Spur gear

Simplest gear type with straight parallel teeth; shafts must be parallel.

Helical gear

Angled teeth for quieter, higher-capacity operation.

Bevel gear

Cone-shaped gear for transmitting power at an angle (usually 90°) between shafts.

Worm gear

Screw-type driver meshing with a worm wheel; very high ratio, self-locking.

Worm wheel

The circular gear driven by a worm.

Self-locking

Property of a worm gear: the wheel cannot drive the worm backward.

Rack and pinion

Circular gear (pinion) on a flat linear rack; converts rotation to linear motion.

Idler gear

An intermediate gear that reverses rotation direction without affecting the overall ratio.

RPM

Revolutions per minute — unit for rotational speed.

Torque

Rotational force; measured in ft·lbs or N·m.

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