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Modelling Gears

From DT Online

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Description

Working model Gears can be made by arranging short lengths of material (e.g. Dowel, Stripwood, Lolly Sticks, Matchsticks, Cocktail Sticks) around discs of Card, Ply or MDF.


Gears provide an alternative to Pulleys when transfering power between revolving shafts or axles. Gears have to be positioned more carefully than Pulleys if they are to Mesh successfully. Two Meshing Gears will always rotate in opposite directions whereas Pulleys will only do so if the Drive Belt is given a half twist.


'Real' Gears provide a much more positive drive than Pulleys and can transfer much more power but this is not usually of concern when making models. The construction of Stripwood model Gears, however, can lend some realism to models such as Windmills and Watermills.


Construction
Construction Simple Gears for working models can be made by arranging a number of Spokes around a disc of Card, Ply or MDF.


The Spokes may be made from Dowel, Stripwood, Lolly Sticks, Matchsticks, or Cocktail Sticks and glued with PVA (providing only bare, unpainted wood and card is used).

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Templates and Jigs A degree of accuracy is required when spacing the Spokes around the discs.


This can be done by placing the MDF or Ply disc on top of a paper or card Template which has been marked out with Spoke positions as shown. Short lengths of material can then carefully be glued in place.


Note: Cover the Template with tracing paper to stop glue sticking to it.


If required, an assembly Jig can be made by gluing the Template on to a piece of Softwood or Insulation Board and fixing small Nails or Panel Pins in place to locate the Spokes.

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Lantern Gear
Lantern Gear Also known as Cage Gears and so named because of their resemblance to those items. They are simply made by gluing short lengths of Dowel between two discs of Card, Ply or MDF.


Care must be taken to drill the location holes accurately and a Template can be used to assist.

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Peg (or Pin) Gear These Mesh with each other or with a Lantern Gear and can be regarded as another Lantern Gear with one disc removed.


Peg Gears resemble Contrate Gears (aka Crown Gears) and can be used in similar applications (i.e. transferring power to shafts at right angles). Lantern Gears in Mesh with Peg Gears can be seen in old Windmills, Watermills and some clock mechanisms.


Note: In old Windmills, the large Brake Wheel which meshes with the smaller Wallower, often had wooden teeth (i.e. made from Hornbeam or Fruitwood). This made for quiet running and easy gear tooth replacement when worn . . . but wear was minimised by having an extra gear tooth on the larger gear such that the same two teeth did not keep coming into contact with each other. This extra gear tooth was known by Millwrights as the 'Hunting Cog'.

(See Wilton Windmill and the Teacher’s Guide – Technology in the Education Resources section)


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Spur Gears
Spur Gear The simplest form of Gear is a rotating disc with straight teeth projecting from its rim.


These can be made by arranging a number of Spokes around a disc of Card, Ply or MDF.


If two gears are to Mesh together on parallel shafts as shown, it is easier to set up if the Spokes are made from one of the wider sections of material such as Lolly Sticks set on edge.

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Meshing Gears
Gear Geometry Different size Gears can be used to speed up or slow down an axle they are connected to but if they are to Mesh at all, the gear teeth must all be the same size.


Gears can be imagined to Mesh as if they were plain circles (i.e. with no teeth at all). These imaginary circles are known as the Pitch Circles and it is the length of each tooth along the circumference of the Pitch Circles which matters. This distance is known as the Circular Pitch and must stay the same for any two Meshing gears.


To achieve this, the diameter of the Pitch Circles have to change with the number of teeth required - a 6 tooth gear therefore would have to be only half the diameter of a 12 tooth gear.


Note: As with Pulleys, a 6 tooth Driver gear would cause a 12 tooth Driven Gear to rotate at half speed - and vice versa.


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Animations
Pin Gear Drive
Worm Gear Drive - spring and pins give alternate ways for producing the worm and worm-wheel.
Pin Gear Driving a Rack
Pin Rack (try making a Pin Rack which is driven by a Pin Gear?)


DT Online Buyers' Guide
MDF wheel 32mm disc pkt 100 100 Pack of 5mm Hardwood Wooden Dowels - 60cm Long Elliott 36-Piece Medium Wooden Hardwood Clothes Pegs Natural Matchsticks (Pack of 2000) 100 Plain Wooden Standard Lolly Sticks Giant Lollipop Sticks Natural Wood (Pack of 100) Playbox 25mm Unpainted Round Wooden Beads (50 Pieces) 5 x Richard Burbidge Decorative Wood Stripwood - Pine - 900 x 8 x 8mm
MDF wheel 32mm disc pkt 100 100 Pack of 5mm Hardwood Wooden Dowels - 60cm Long Elliott 36-Piece Medium Wooden Hardwood Clothes Pegs Natural Matchsticks (Pack of 2000) 100 Plain Wooden Standard Lolly Sticks Giant Lollipop Sticks Natural Wood (Pack of 100) Playbox 25mm Unpainted Round Wooden Beads (50 Pieces) 5 x Richard Burbidge Decorative Wood Stripwood - Pine - 900 x 8 x 8mm