Crane Design

Part of Simple Machines

Designing and building timber cranes for lifting heavy loads in construction and manufacturing.

Why This Matters

Heavy construction — raising roof beams, setting stone blocks, erecting wall sections — requires lifting capability far beyond what human muscle alone provides. A construction crew of ten people can lift approximately 500-600 kg with ropes and brute force. The same crew using a properly designed timber crane and block-and-tackle can lift 3,000-5,000 kg with less effort and far better control.

Cranes multiplied human construction capability so dramatically that they are present in every culture that attempted large building projects. Roman construction sites used treadwheel cranes (powered by workers walking inside a giant wheel). Medieval cathedral construction used similar cranes. Egyptian pyramid builders used ramp-and-lever systems that functioned on the same principles. The underlying engineering — boom, mast, guy lines, pulley, winch — has not changed fundamentally in 4,000 years because it works.

For a rebuilding community, a crane dramatically expands what can be built. Structures with heavy timber frames, stone foundations, and masonry walls all become feasible once you have a means of lifting heavy components into position.

Types of Crane

Shear Legs

The simplest crane structure: two poles leaned against each other at the top, forming an inverted V, with a block and tackle hanging from the apex.

Use: Lifting heavy loads straight up in a fixed location. Cannot move the load sideways.

Construction: Two poles (8-12 cm diameter, 4-6 m long) lashed together at the apex. Spread the feet 1.5-2 m apart. A block and tackle hung from the apex lashing.

Capacity: Depends entirely on pole and rope strength. Two 10 cm hardwood poles can carry 1,500-2,500 kg vertical load.

Limitations: The load hangs directly below the apex, which is above the line between the two feet. Cannot overhang this line without tipping. For loads that must be set into a foundation hole or over a wall, the limited overhang is a serious constraint.

Tripod

Three poles meeting at an apex — provides greater stability than shear legs but still limits the overhang.

Construction: Same as shear legs but with three poles at 120° apart. The load hangs below the center point of the foot triangle. Can be moved as a unit.

Best use: Lifting from the center of a stable base. Wells, pits, and vertical lifts.

Derrick Crane (Most Practical)

A derrick uses a vertical mast and an angled boom to provide useful horizontal overhang — the load can be positioned away from the mast, which means you can lift from one side and place on the other.

Components:

1. Mast: A vertical timber, 4-8 m tall, typically 20-25 cm diameter at the base. The mast is the primary structural element.
2. Boom: An inclined timber, 3-5 m long, 15-20 cm diameter. The boom extends outward and upward from the mast base, carrying the load at its tip.
3. Topping lift: A rope from boom tip to mast top. This rope carries the compressive component of the boom tip force. Changing the topping lift length raises or lowers the boom angle.
4. Guy wires/ropes: Ropes from the mast top to anchors in the ground. These prevent the mast from falling under the lateral forces from the boom.
5. Block and tackle: The lifting pulley system, with the fixed block at the boom tip and the movable block at the load.
6. Winch: On the ground, connected to the hauling end of the block and tackle rope.

Structural Analysis

Understanding the forces in a derrick helps you build it strong enough.

Force in the boom: The boom is in compression. If the load is 500 kg and the boom angle is 60° from horizontal:

• Vertical component of boom force = 500 kg (must support the load)
• Compression in boom = 500 / sin(60°) = 577 kg

Rule of thumb: The boom carries approximately 1.2-1.5 times the load in compression (for typical boom angles of 45-70°). Size the boom for this compressive load, not just the hanging load.

Force in the topping lift: The topping lift carries the horizontal component of the boom tip force. For a 60° boom with 500 kg load:

• Horizontal component at boom tip = 500 × cos(60°) / sin(60°) = 289 kg

The topping lift and its attachment points must carry this force. A single rope from boom tip to mast top also carries significant tension.

Force in the guy wires: Guy wires carry the horizontal forces that the boom transmits to the mast. At minimum, size guys to carry 0.5 × load each, with at least three guys arranged 120° apart.

Building a Derrick Crane

Step 1: Mast Preparation and Stepping

1. Select a straight, sound timber: 20-25 cm diameter, 6-8 m long
2. Debark and allow to dry if time permits (green wood works but is heavier)
3. Dig a mast pit: 60-80 cm square, 1.0-1.5 m deep
4. Stand the mast in the pit, using props and ropes to hold it vertical while the pit is backfilled
5. Backfill with large stones tightly packed, then smaller stones, then compacted earth
6. Check vertical with a plumb bob on all sides before final compaction
7. Attach the guy anchors before releasing the props

Guy anchors: Bury heavy timber deadman anchors (logs set horizontally underground) 1-2 m from the mast base, at 120° apart. Attach guy ropes to the deadmen. Each anchor must withstand 500+ kg of pulling force.

Step 2: Rigging the Guys

1. Attach three or four guy ropes to the mast top (use a strong loop lashing)
2. Run each guy out at 45° from vertical down to its anchor
3. Use a rope tensioner (a turnbuckle made from two opposing hooks on a twisted loop, or a simple Spanish windlass) to bring all guys to equal tension
4. Test tension by pushing the mast top sideways — it should feel solid and return to vertical immediately

Step 3: The Boom

1. Select a straight timber for the boom: 15-20 cm diameter, 4-5 m long
2. Attach the boom foot to the mast base with a pivot fitting — a simple iron U-strap through which the boom can be raised and lowered
3. The boom foot must carry the full compressive load while allowing the boom angle to be adjusted

Pivot fitting options:

• Iron pin through the boom end and through a bracket on the mast
• Rope lashing (the boom end is lashed to the mast with enough slack to allow angle adjustment)
• Mortise and tenon with a loose fitting (the boom tenon can pivot in the mast mortise)

Step 4: Topping Lift

1. Attach a rope from the boom tip to the mast head
2. This rope sets the boom angle — shorter topping lift = higher boom angle
3. Add a block and tackle in the topping lift to allow easy adjustment of boom angle
4. The crane operator changes boom angle by adjusting the topping lift tackle

Step 5: The Lifting Tackle

1. Attach the fixed block of your block and tackle to the boom tip
2. Run the rope down to the movable block (load hook), back up, and so on for your required MA
3. Run the hauling end down the boom and mast to the winch at ground level
4. The load hook hangs freely below the boom tip

Step 6: Slewing (Rotation)

If the crane needs to rotate (to lift from one position and set at another), the mast must be able to rotate.

Simple rotation: Set the mast in a bearing socket at the bottom (a heavy wooden or stone cup with a grease-filled bearing surface) and a guide at the top of the mast support. The mast can then be rotated by hand.

Guy wire adaptation for rotating mast: If the mast rotates, the guy wires must be attached to a rotating crown at the mast top. This is more complex — one approach is to attach the guys to a non-rotating collar that bears on the rotating mast.

Operational Safety

Pre-operation checks:

• Inspect all ropes for fraying, cuts, or deterioration
• Check all pin and lashing connections
• Verify guy wire tension
• Test the winch ratchet
• Check the boom pivot is secure and free to move as intended

Danger zones:

• Directly below any suspended load — absolutely clear of people
• Below the radius of the boom — if the load swings, it can reach this area
• Within fall radius of the mast — if a guy fails, the mast could fall

Load limit: Never exceed 70% of your calculated capacity. Cranes fail at load extremes where dynamic effects (swinging loads, jerking movements) add to static load.

Wind: Even moderate wind (15-20 km/h) adds significant lateral force to a suspended load. Lower the boom angle or cease crane operations in wind over 30 km/h.

Crane Operation Requires Two People Minimum

One person operates the winch and controls lifting. A second person manages the load — guiding it into position, preventing swinging, and watching for obstructions. Never operate a crane that carries people or is lifting over workers with only one operator.