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Platform Scale: Four Knife-Edge Hinges and Two Levers to a Steelyard (Fairbanks, 1831)
A scale for wagons and carts, reading the same wherever the load stands. Erastus and Thaddeus Fairbanks of St. Johnsbury, Vermont, patented it in 1831. A plank platform nine by twelve feet rests on four knife-edge hinges, one at each corner, and two levers gather the load from all four onto one rod. The rod pulls on an ordinary steelyard, which then reads only a tenth or a twentieth of the load.
Result: a platform scale reading ten times (or twenty times) what the steelyard shows, the same wherever the load stands on the platform.
Source: E. & T. Fairbanks, US Patent No. 118 (1837), restating their patent of 13 June 1831. Unknown: the material and sizes of the hinges, knife-edges, fulcra and levers. Picture: patent drawing.
Advanced
1 week
Instructions
1
1
Build the frame
Build the frame
Lay two timber bed-sills, about 10 inches square and 16 feet long, parallel and about 7 feet apart. Bolt two end timbers, 8 by 14 inches and 12 feet long, across them near their ends. Just inside the end timbers, lay the two rockers, 8 inches square and 9 feet long, with their tops flush with the end timbers. Ours: carriage bolts.
Materials for this step:
Squared Timber6 2 sills, 2 end timbers, 2 rockers
Carriage Bolt1 setTools needed:
Crosscut Saw
Auger
Hammer2
2
Fit the four knife-edge hinges
Fit the four knife-edge hinges
Bolt a hinge firmly under each end of each rocker, one at every corner of the machine. Each hinge carries two knife-edges. One rests on a fulcrum standing on the bed-sill; the other reaches out to carry the platform. Set every knife-edge parallel to the others: knife-edges at an angle cramp and bind. Not given: the material and size of the hinges, knife-edges and fulcra.
Materials for this step:
Unknown Material1 4 hinges, 8 knife-edges, 4 fulcraCarriage Bolt1 setTools needed:
Auger
Unknown Dimensions3
3
Join the levers
Join the levers
Run a horizontal lever from each rocker. The longer lever reaches across nearly to the opposite rocker. The shorter one ends at a point equally far from the axes of both, and hangs there from the longer lever by a chain, rod or stirrup, so the two act as one. From the end of the longer lever a rod rises straight up to a steelyard hung from a fixed support. With the two knife-edges of each hinge 6 inches apart and the rod hung 60 inches from the lever's axis, the load is ten times the steelyard reading; at 120 inches it is twenty times. Not given: the lever material. Ours: an iron chain and a wrought iron rod.
Materials for this step:
Unknown Material1 2 levers
Iron Chain1 lever link
Wrought Iron Rod1 rod to the steelyard
Steelyard1 pieceTools needed:
Unknown Dimensions4
4
Lay the platform
Lay the platform
Lay two timbers, short enough to pass freely between the rockers, parallel over the bed-sills. Spike stout planks 9 feet long across them, making a platform of about 9 by 12 feet. Under these timbers fasten iron plates that rest on the inner knife-edges of the hinges. Ours: nails as the spikes.
Materials for this step:
Squared Timber2 platform timbers
Timber Plank1 for 9 x 12 ft
Iron Plate4 pieces
Nails1 as neededTools needed:
Crosscut SawHammerMaterials
9- 2 platform timbersPlaceholder
- 2 setsPlaceholder
- 1 2 leversPlaceholder
- 1 lever linkPlaceholder
- 1 rod to the steelyardPlaceholder
- 1 piecePlaceholder
- 1 for 9 x 12 ftPlaceholder
- 4 piecesPlaceholder
- 1 as neededPlaceholder
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