Grades 3–5 · Machines
How does a screw turn rotation into travel?
A screw can turn a large movement of the hand into a small, controlled advance. Its thread is a ridge that winds around it. Matching threads in a nut guide that ridge, so turning the screw moves it along its length when the nut is kept from turning.
In the diagramA screw thread links turning and travelTurning handleFixed nut1 full turn2 mm / turn2 mm travelThread and nut must engage to guide travel.Turning and engaged threads guide lengthwise travel.
The distance advanced in one complete turn is called the lead. The pictured screw has a lead of 2 millimeters: three full turns advance it 6 millimeters. A screw with a 4-millimeter lead would advance twice as far per turn. Lead describes travel per revolution, rather than just the width of a ridge.
The thread works like a sloping path wrapped around a cylinder. Turning follows a long path around the screw while producing a shorter movement along it. With the same turning arrangement and negligible friction, a smaller lead can exchange more turning distance for a greater pushing force. It cannot supply more energy than goes into turning it.
This slow advance is useful when a clamp needs to close gradually or a mechanism needs a carefully controlled position. Real threads have friction and can deform under load, so travel per turn alone does not establish the force a screw can carry.
About this explanation
Starting knowledge: Follow a spiral path and compare one turn with forward movement.
Sources
- OpenStax, Rice University — Simple Machines
- OpenStax, Rice University — Chapter 9 Key Equations