Screw Mechanisms

8 min read

Parent
📐
Inclined Planes
Ramps, wedges, screws
Current
🔩
Screw Mechanisms
Rotary to linear motion

Screw Mechanisms

Part of Simple Machines

How screws convert rotation to linear force, enabling presses, vises, jacks, and precision positioning.

Why This Matters

The screw is the simple machine with the highest mechanical advantage — and also the most useful property of any simple machine: self-locking. A screw press generating 5,000 kg of force remains in that state the moment you stop turning the handle. It doesn’t spring back, doesn’t require continuous effort, doesn’t need a ratchet. The thread friction that limits the screw’s efficiency is precisely what makes it hold position without input.

This makes screw mechanisms indispensable for:

  • Presses (cider, olive, grape, cheese, paper) — apply enormous force, hold it while the material slowly yields
  • Vises and clamps — hold workpieces under force for long operations
  • Lifting jacks — raise buildings, machinery, and vehicles to a fixed height
  • Precision positioning — move tools and workpieces small, controlled distances

The screw’s other unique property is precision. One turn of a screw with a 2 mm pitch advances it exactly 2 mm. This makes screws the basis of all precision measurement and precision positioning tools: micrometers, lathes, milling machines, and astronomical instruments all use screw mechanisms for their fine adjustment.

The Physics of a Screw

The screw as inclined plane: A screw thread is an inclined plane wrapped around a cylinder. The pitch is the height of the inclined plane per revolution. The circumference of the drive circle (2π × handle radius) is the length of the inclined plane per revolution.

Mechanical advantage:

MA = (2π × drive radius) / pitch

For a screw with 5 mm pitch and a 200 mm drive radius:

MA = (2π × 200) / 5 = 1,257/5 = 251:1

Applying 10 kg of force at the handle produces: 10 × 251 = 2,510 kg of linear force at the screw tip.

The self-locking condition: A screw is self-locking (will not backdrive under load) when:

Thread lead angle < arctan(coefficient of friction)

For most practical screws with metal threads and coefficient of friction ≈ 0.1-0.3: any pitch smaller than about 30% of the thread circumference is self-locking. This is almost all practical power screws.

Efficiency: Because thread friction is what enables self-locking, it also limits efficiency. Self-locking screws are typically 20-50% efficient. This is acceptable because the screw’s purpose is force application, not power transmission.

Thread Forms

Different thread profiles have different properties:

Square thread:

  • Flat-topped teeth, flat root
  • Best for power transmission (presses, vises, jacks)
  • Highest efficiency among self-locking threads
  • Most difficult to cut accurately by hand
  • 50% of the force goes to the face of the thread, 50% to friction

V-thread (triangular):

  • Standard fastener thread form
  • Good self-locking properties
  • Slightly lower efficiency than square thread (more friction on angled faces)
  • Easiest to cut with a file or by hand

Buttress thread (asymmetric):

  • One vertical face, one angled face
  • Efficient in one direction only (the force direction)
  • Used in screw presses where force is always applied in the same direction
  • Stronger than square thread for unidirectional force

Rope thread (very coarse, rounded):

  • Very coarse pitch (10-20 mm)
  • Easy to cut by hand
  • Suitable for low-precision applications (wooden presses, simple clamps)
  • The rounded profile reduces stress concentration

Making Wooden Screws

Wooden screws are the most accessible screw type for a rebuilding community. They require only woodworking tools and patience, produce useful results, and were the basis of most pre-industrial screw presses.

Tools Needed

  • Screw box: A cutting die with a V-shaped cutter that cuts the thread groove as the rod is turned through it. Can be made from hardwood with an embedded iron cutter.
  • Tap: A tapped rod used to cut internal threads in the nut. Made by cutting external threads on a rod and then hardening the surface.
  • Alternatively: cut threads by hand with a chisel and file (laborious but no special tools required)

Making a Wooden Screw by Hand

  1. Select the rod: Straight-grained hardwood (oak, hickory, boxwood) free of knots. Turn or whittle to a uniform cylinder — precision matters; even 1-2 mm variation in diameter will cause the thread to cut unevenly.

  2. Mark the helix: Wrap a strip of paper around the rod diagonally. The paper edge traces a helix. The angle of wrapping determines the pitch. For a vertical rise of 1 cm per wrap of a 3 cm diameter rod: pitch = 1 cm.

  3. Score the helix line: Scratch the helix line into the wood with a marking knife.

  4. Cut the groove: With a V-chisel or small gouge, cut along the helix line to the thread depth (typically half the pitch depth for a square thread, or follow the V profile for a V-thread). Work in multiple passes, deepening gradually.

  5. Clean up the profile: File the thread faces flat and smooth. Test by turning against the mating nut.

Making the Nut (Internal Thread)

The nut is harder than the screw because you cannot see inside the bore to work.

Method 1: Split nut:

  1. Cut a square block of wood
  2. Drill a hole through the center, matching the screw root diameter
  3. Saw the block in half through the center of the hole
  4. With the two halves held together around the screw, force the screw through while turning — the screw itself cuts the thread
  5. Once threaded, bind the halves with iron straps or wrap with wet rawhide (which shrinks as it dries)

Method 2: Scrape-thread nut:

  1. Drill the bore to the major diameter (outside of thread)
  2. Mark the helix on the bore interior through the hole with a bent wire scriber
  3. Cut the groove with a narrow hooked chisel, working blind
  4. Test frequently against the screw

Building a Screw Press

A screw press is the most immediately useful application of a wooden screw mechanism.

Frame construction:

  1. Two heavy upright posts (15 × 15 cm minimum), 1.5-2 m tall, set firmly in the ground or bolted to a heavy base
  2. A cross-beam at the top connecting the uprights — this carries the reaction force from pressing
  3. The cross-beam has a round hole through its center, threaded to match the screw

The screw:

  1. A large-diameter wooden screw (5-8 cm diameter, 10-15 mm pitch) threaded through the cross-beam
  2. At the bottom of the screw: a pressing plate (a flat block slightly larger than the material being pressed)
  3. At the top of the screw: a cross-bar handle (60-80 cm arms extending from the screw head)

The base platen: A heavy hardwood platform or stone slab at the bottom, between the two posts, on which the material to be pressed rests.

Operation:

  1. Place material between base platen and pressing plate
  2. Turn the cross-bar handle (both arms, for balance) clockwise
  3. The screw advances downward, pressing the plate against the material
  4. Continue turning until desired pressure is achieved
  5. Allow the material to yield under pressure (time varies: 30 min for cheese, 4-12 hours for wood billets)
  6. Turn the handle back (counterclockwise) to release

Screw Jacks

A screw jack lifts heavy loads by the same principle as a press, but oriented vertically.

Construction for a simple bottle jack:

  1. A heavy iron or hardwood base plate (supports the reaction from lifting)
  2. An iron sleeve welded or forged to the base plate (the body)
  3. An iron screw threaded through the sleeve (the jack screw)
  4. A swivel head at the top of the screw to contact the load without biting in
  5. A cross-bar for turning

For wooden jack:

  1. Heavy timber base with iron pad on top
  2. A female-threaded block mounted on the base
  3. The wooden screw threads through this block
  4. A hardwood pad on the screw head

Capacity: A wooden jack with 8 cm diameter screw, 12 mm pitch, and 50 cm handle arms:

MA = 2π × 50/1.2 = 262:1
Applying 20 kg at each handle end (40 kg total): 40 × 262 = 10,480 kg

In practice, thread friction reduces this to perhaps 25-40% efficiency: 2,600-4,200 kg actual lift force — sufficient to raise a heavy structural beam.

Maintenance

Wooden screw maintenance:

  • Apply tallow or linseed oil to the thread surfaces before each use — dry threads wear rapidly
  • Inspect for cracks along the thread helix (common in dry conditions as wood shrinks) — a cracked wooden screw must be replaced before it fails catastrophically
  • Store with the screw extended (not fully retracted) to avoid trapping moisture in the nut

Metal screw maintenance:

  • Keep threads clean of grit (grit accelerates wear dramatically)
  • Apply graphite grease to thread surfaces
  • Inspect for rust pitting — pitted threads have reduced strength and increased friction