Slide 1
A screw thread turns rotation into travel along an axis
A screw uses a helical thread, spiralling around its axis, to connect rotation with motion along that axis. A mating nut follows the thread.
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Thread lead is the axial distance advanced per revolution. For a single-start thread it equals pitch, while a multi-start thread can have a lead several times its pitch.
Slide 2
A small advance per turn trades distance for force
Lead is the distance a screw advances in one turn.
A small lead trades a large rotational movement for short axial travel, producing substantial axial force from modest torque, or turning effect.
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The ideal work relation compares torque through one full turn with axial force through one lead. Friction reduces the obtainable force and turns part of the input energy into heat.
Slide 3
Some screws resist being driven backward by the load
Some screw mechanisms resist being driven backward by an axial load. This self-locking behaviour depends on thread geometry and friction.
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It is not a universal property of screws. Low-friction ball screws can back-drive readily, so a vertical actuator may require a separate holding arrangement rather than relying on the thread alone.
Slide 4
Thread clearance can delay the response to reversal
Thread clearance allows motion but can create backlash. Direction reversal may move the screw before the nut begins to respond.
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Split or preloaded nuts can reduce lost motion, at the cost of friction or complexity. Position sensing at the load can reveal errors that an encoder on the motor alone misses.
Slide 5
Load, operating speed and lubrication all matter
A screw’s lead, material and lubrication must suit its load and duty. Force capacity alone does not describe useful performance.
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Long slender screws can buckle under compression or vibrate at high rotational speed. Support spacing and operating speed therefore matter alongside thread strength and the requested positioning resolution.
Sources
- Simple Machines
OpenStax
- Basics of Mechanical Design
MIT Center for Bits and Atoms
- What Is the Difference Between Screw Pitch and Lead?
Thomson Industries
- Permissible Rotational Speed
THK
- Questions and Answers: Sizing and Selection of Ball and Lead Screws
Thomson Industries