Mechanical Movement 7 — Fast-and-Loose Bevel Reverser
How do fast and loose pulleys reverse an upright shaft?
Mechanism No. 7 from 507 Mechanical Movements: the belt from a constantly running drum can be shifted onto one of three pulleys. The middle one is loose: it spins freely on the shaft and transmits nothing. The left one is fast on a hollow sleeve carrying bevel gear B; the right one is fast on the inner shaft carrying bevel gear A. A and B both mesh with pinion C on the upright shaft, from opposite sides, so whichever one is driven, C turns the opposite way. Shift the belt: forward, stop, reverse — without stopping the drum.
How it works, step by step
1. Neutral
The belt runs on the loose pulley, which spins freely on the shaft. No torque reaches either bevel; C stands still.
2. Belt on B: forward
The fast pulley B turns the hollow sleeve and bevel B, which drives pinion C at 449 rpm.
3. Two bevels, one pinion
A and B mesh with C from opposite sides, so they always turn opposite ways. Back to neutral: C coasts to a stop.
4. Belt on A: reverse
Now the inner shaft drives bevel A from the other side of C, so C turns the other way: -450 rpm.
The key idea
B: fwd · A: rev — Shifting the belt between the loose pulley and two fast pulleys starts, stops or reverses the upright shaft.
Shifting one belt across fast and loose pulleys gives forward, neutral and reverse without stopping the drive.
Model and assumptions
Gear ratio: bevels A and B have 30 teeth, pinion C has 20, so C turns 1.5× as fast as the driven bevel. Because A and B both mesh with C, they are always geared together and turn in opposite directions: the idle fast pulley is back-driven.
Frequently asked questions
What is a loose pulley?
A pulley that turns freely on its shaft; with the belt on it, nothing is driven (neutral).
How does the shaft reverse?
Each fast pulley drives its own bevel wheel; the two bevels mesh on opposite sides of the pinion, so they turn it in opposite directions.
Where were these used?
Machine tools driven from overhead line shafts, before clutches and electric motors.
Part of the 507 Mechanical Movements series
This is Movement No. 7 from Henry T. Brown’s 507 Mechanical Movements (1868), redrawn and simulated by BCRUZ. ← Mechanical Movement 6 — Reciprocating Rotary Motion Using a Crossed Belt Mechanism · Mechanical Movement 8 — Stepped Speed Pulleys →
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