Reaper and tunnelling shield: what they have in common

1. Step 1. Reaper — evolved into — Combine harvester. Reaper + thresher.
2. Step 2. Combine harvester — has part — Belt drive. Belt drives.
3. Step 3. Belt drive — is used in — Conveyor belt. The belt runs around the drive and tension drums.
4. Step 4. Conveyor belt — is part of — Tunnel boring machine. Spoil is removed by a conveyor.
5. Step 5. Tunnel boring machine — evolved from — Tunnelling shield. A development of the tunnelling shield.

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