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The Flexible Shaft: Turning a Tool Round a Corner
A dentist's drill must reach any tooth at any angle while it spins. A rigid shaft cannot follow the hand; universal joints give a few fixed bends. James B. Morrison of St. Louis tried two answers. His dental engine of 1871 carried the drive by a belt over pivoted, sprung arms. A year earlier his US 106,498, *Improvement in Flexible Shafts*, dated 16 August 1870, carried it through "a wire spiral or helix … enveloped in a short piece of india-rubber pipe, which allows the free flexure of the helix", in sleeves that act as bearings, with an adjustable head holding the bur and a parallel bracket for the instrument tray.
The flexible shaft outlived the dental use: it drives rotary tools, speedometers and hedge trimmers today. This rung works out why only thin wires can bend and keep turning, and makes a flexible shaft from wound spring wire.
Débutant
About 3 hours
Consignes
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Bending stress, fatigue and torque
Bending stress, fatigue and torque
Chargement du notebook Jupyter…
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The sibling: belt over sprung arms
The sibling: belt over sprung arms
Morrison's other answer to the same problem carries the drive by a belt over pivoted arms. The embedded blueprint builds it; compare the two ways of reaching round a corner.
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Read the 1870 flexible power-conveyer
Read the 1870 flexible power-conveyer
Fig. 1 of US 106,498 shows the apparatus on a stand: a pulley shaft B, joined by a wire helix C in a rubber pipe c to a shaft section D, turning in a swivel-eye E; a double hinged parallel bracket H, I carrying the instrument tray L, which stays level however the bracket swings; and further helix sections R and t′ leading through a sleeve to the socket-head holding the burr U.
The patent's three claims, in order: the flexible shaft with its enveloping sleeve, which gives journal-bearing to the enclosed shaft and the tool; an adjustable head joined to the sleeve by a hinge, so the tool can be set at any angle; and the combination with a parallel bracket that swings, rises and extends.
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Wind a flexible shaft and drive it
Wind a flexible shaft and drive it
Clamp a 3 mm steel rod in a cordless drill as a mandrel. Clamp one end of a length of 0.8 mm spring steel wire to the rod and, turning the drill slowly, wind a close helix about 400 mm long, every turn touching the last. Slide it off: that is the core.
Push a short length of steel rod into each end and secure it with a dab of solder or a crimped sleeve. Chuck one end in the drill; put a small drill bit in a pin chuck on the other end, held in a length of aluminium tube as the sleeve-bearing.
Bend the core round a 100 mm radius and run the drill in the direction that tightens the coil: the bit turns and will drill soft wood. Reverse the drill: the coil opens, winds up and barely turns the bit. Compare with a commercial rotary-tool flex shaft.
Matériaux pour cette étape :
Fil d'acier à ressort1 pièce
Barre d'acier brut1 pièce
Tube d'aluminium1 pièce
Soudure étain-plomb1 pièce
Arbre flexible pour outil rotatif1 pièceOutils nécessaires :
Perceuse sans fil
Jeu de forets
Étau d'établi
Fer à souder
Gants de travail en cuir
Lunettes de sécurité transparentes5
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A flexible shaft that fails
A flexible shaft that fails
Flexible shaft faults.
Flow
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History and honest limits
History and honest limits
**James B. Morrison** of St. Louis was granted **US 106,498**, dated 16 August 1870, for his flexible power-conveyer "for dental and other purposes"; his treadle dental engine with sprung arms followed in 1871 (US 111,667). Flexible shafts with multi-layer wire cores now drive rotary tools, instrument cables and garden tools.
**Honest limits.** The bending-stress figures are the simple beam formula for one wire; a real core's stresses depend on its winding. A hand-wound single helix is a demonstration, not a tool shaft — it will not last.
Matériaux
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Outils requis
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