page_title: Glossary of mill machinery
section_title: Windmill machinery
hierarchical_section_title: Glossary of mill machinery / Windmill machinery
caption_attribution_description: Ballycopeland windmill, Millisle, Northern Ireland. Credit - carolinehornet on en:Flickr.
context_page_description: This glossary of mill machinery covers the major pieces of machinery to be found in windmills, watermills and horse mills. It does not cover machinery found in modern factories.
context_section_description: Bedstone
  The Bedstone is the bottom of a pair of millstones. It does not move. The upper stone is called the Runner Stone.
  Brake Wheel
  The Brake Wheel is the main driving wheel in a Smock or Tower mill, and in some post mills. It is carried on the Windshaft and drives the Wallower on the Upright Shaft
  Buck
  The Buck is an East-Anglian term for the body of a post-mill.
  Centrifugal governors
  Governors are used to regulate the distance and pressure between millstones in windmills in the 17th century.
  Crown Tree
  The Crown Tree is the central, single baulk of timber, usually oak, that rests on top of the post in a post mill. Attached to it are the side-girts and the rest of the frame of the buck.
  Crown wheel
  In a windmill, a Crown Wheel is an auxiliary gear on the Upright Shaft.
  Fantail
  A fantail is a small windmill which is used to keep a windmill facing into the wind automatically.
  Great Spur Wheel
  The Great Spur Wheel is carried on the Upright Shaft It drives the Stone Nuts. Millstones driven by the Great Spur Wheel can be either Overdrift or Underdrift.
  Head Wheel
  The Head Wheel is carried on the Windshaft in a Post Mill and has a brake around its circumference. It drives a Stone Nut, Millstones driven by the Head Wheel are always Overdrift stones.
  Middling
  See Stock
  Overdrift
  Millstones driven from above are known as Overdrift stones.
  Pintle
  The pivot centering a post mill on top of the main post.
  Runner Stone
  The Runner Stone is the topmost of a pair of millstones. It is driven by the Stone Nut. The lower stone is called a Bedstone. 
  Sails
  The Sails are the source of power in a windmill. They are carried on the Windshaft. Most windmills had four sails, although some had five (Boston), six (Waltham, Lincs) or eight sails Heckington, Lincs and there is one recorded twelve sailed windmill (Cottenham, Cambs).
  Common Sails have a lattice framework over which a sailcloth is spread. These were the earliest type of sails in northern European windmills.
  Spring Sails, invented in 1772 by Andrew Meikle, have shutters adjusted by a spring. Each sail is adjusted individually and, as with Common Sails the mill has to be stopped to enable an adjustment to be made.
  Roller Reefing Sails, invented in 1789 by Stephen Hooper, use a canvas strip wound around a roller in the place of shutters. The mill does not have to be stopped in order to adjust the sails.
  Patent Sails, invented in 1819 by William Cubitt, combine the shutters of the Spring Sail with the automatic adjustment of the Roller Reefing Sail. Single Patents have shutters on the trailing side of the sail, Double Patents have shutters on both sides of the sail for its whole length.
  Samson Head
  An iron collar and plate bearing that fits over the pintle of a post-mill's post, that supports the weight of the crown tree, around which the buck of the mill is constructed. An example is visible at High Salvington windmill.
  Stock
  The beam that passes through the canister of the windshaft, which the sails are bolted onto.
  Stone Nut
  The Stone Nut is a small gear driven by the Great Spur Wheel, Head Wheel, or Tail Wheel. It drives the Runner Stone either from above (Overdrift) or below (Underdrift).
  Tail Wheel
  The Tail Wheel is carried on the Windshaft in a Post Mill and drives a Stone Nut. Millstones driven by the Tail Wheel are always Overdrift stones.
  Trestle
  The Trestle is the substructure of a Post Mill, usually enclosed in a protective structure called a roundhouse, which also serves as a storage facility. Post mills without a roundhouse are called Open Trestle Post Mills.
  Underdrift
  Millstones driven from beneath are known as Underdrift stones.
  Upright Shaft
  The Upright Shaft is the main vertical shaft found in Smock and Tower mills. It is also found in some Post mills. It carries the Wallower at its top end, and a Great Spur Wheel at the bottom end. The Great Spur Wheel drives two or more Stone nuts.
  Wallower
  The Wallower is a driven gear at the top of the Upright Shaft in Smock, Tower and some Post mills. It is driven by the Brake Wheel
  Windshaft
  The Windshaft carries the Sails and al
caption: Tower Mill with Roller Reefing Sails
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