Improved grip self tightening quick-change chucks and couplings
Application Note 21
Grip is such a basic function in daily life that we take it for granted - without it we could not stand and nor could we do very much else! and so it is also within engineering. Grip is the consequence of friction, thus an increase in friction is useful in improving grip.
The following comments apply specifically to grip between metals in rubbing contact as encountered in virtually every machine tool, power tool or hand tool plus most other machines.
The characteristic of dry friction is shown in the green curve below, which shows a higher initial or 'static' level. This means that once slip starts friction (and strength, grip etc.) falls. Thus in conventional gripping tools like a shaft coupling, collar or chuck the only way to arrest slip once it starts is to reduce the load.
In complete contrast a Trib treated surface behaves in the reverse and as slip starts its friction rises and this arrests slip. This behaviour is shown in grey below.
The above characteristic is exploited in the designs for quick release chucks shown below. These are taken from a recent patent application. They are all self gripping chucks designed to automatically tighten as torque is applied and to relax when torque is removed.
Fig 1A shows a tool shank 1, a drill or router, held between three circular jaws 2. The jaws are made of sintered hard steel impregnated with a friction enhancing agent. The jaws roll in arcs 3 within an outer cam shape 4. The drill is loaded by simply pushing between between the rollers as shown. As torque is applied the rollers try to rotate and jam, the presence of the friction enhancing agent ensures a very high level of grip
Fig 1B shows a refinement in the design that allows different size tool shanks to be griped by changing the size of the outer ring 10.
Fig 1C employs a tapered split collet 11 running in a matching outer tapered ring 12 that allows the jaws to be adjusted similar to a conventional three jaw chuck, however the chuck does not need to be tightened, rather it simply needs to be set at the size of the tool shank and it automatically grips and release as in Fig. 1A.
Fig. 1 D uses similar principles where an inner shaft or shank is gripped by six roller grips 14 running in an outer cam ring 16.
Fig. 1E shows an internal chuck or coupling in which three rollers 17 run in a cam form 18 that pushes the roller outwards to grip the ring 19.
Fig. 1F shows an assembly of caged rollers 27 running in a shaped cam ring 25 so that applying torque in either direction causes the rolls to simultaneously move inwards or outwards and lock onto the O/D or I/D of shafts, thereby to affect a coupling as shown in Fig 1G.
Fig. 1H shows an extension of the ides of using rollers placed either side of a sinusoidal ring cam. The rollers are held loosely within cages. The assembly is pressed between a shaft and outer ring and as torque is applied the rollers roll and take up a gripping position, the grip enhanced by the presence of chemical friction enhancing agents.
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