Chuck assembly for dental handpieces

An improved positive-grip chuck assembly, particularly suitable for use with high-speed air-driven dental handpieces, in which a tubular chuck has a radially-flexible bur-clamping section provided with an external shoulder, and the bore in which the chuck is received has an internal shoulder engagable with the external shoulder of the bur clamping section of the chuck to flex the bur-clamping section inwardly and to hold the same in a constricted condition for securely gripping the mid-portion of the shank of a dental bur. Piloting contact is made between the chuck and bur along circumferential zones spaced axially from the bur-clamping zone, and the rotor surrounds and braces the chuck portion defining the lower pilot zone to prevent distorting forces applied to the tip of the bur from being propagated to the bur-clamping zone.

BACKGROUND 
The chucks used for releasably holding burs in high-speed dental handpieces 
have chucking actions that can be generally categorized as falling within 
two main groups. The first group includes collet chucks which are normally 
wrench operated and which are threadedly mounted within the rotary members 
(usually bur tubes) of the handpieces so that the terminal gripping 
fingers of the chucks are forced to close or allowed to open depending on 
whether the wrenches are rotated in one direction or the other. The second 
group includes spring-gripped chucks which depend on the resilience or 
spring characteristics of the chucks for bur retention. While efforts have 
been made to develop a fully satisfactory chuck which overcomes certain 
disadvantages that have come to be associated with chucks of these types, 
such efforts have not been entirely successful. 
Wrench operated chucks, whether of the pull-to-tighten or push-to-tighten 
variety, tend to undergo bur "walkout" or rejection from high-speed 
turbine-driven dental handpieces during conditions of extremely heavy 
cutting, particularly in tenacious types of material such as gold alloys 
and some silver amalgams under which the cutting dynamics are severe. The 
problem is characteristically associated with high-speed handpieces--that 
is, handpieces which develop rotational speeds in excess of 100,000 
revolutions per minute and, more particularly, speeds of the order of 
200,000 to 500,000 revolutions per minute. Handpieces capable of such high 
speed operation are invariably of the air turbine type, in contrast to 
electrically-powered handpieces. Tests have shown that axial walkout may 
occur even without accompanying torsional slippage of the bur. Although 
the axial walkout problem is a serious one which has existed for a number 
of years, the precise reasons why the problem occurs do not appear to have 
been fully known, or at least have not been fully analyzed and reported in 
any known technical literature. 
A variation of the push-to-tighten chuck is a double-ended type, that is, a 
chuck with spring fingers at opposite ends, cooperatively associated with 
a separate nut. While advantages in eliminating or reducing bur walkout 
can generally be anticipated with this type of chuck, there are 
disadvantages which include the possibility of the nut loosening during 
cutting, thereby presenting the danger of the chuck, bur, and/or 
tightening nut releasing during handpiece operation. U.S. Patents 
illustrative of double-ended chucks are Nos. 3,619,904 and 3,488,850. 
While spring grip type chucks are known to be more resistant to bur 
walkout, they are commonly associated with other troublesome 
disadvantages. Since spring action, in contrast to a positive clamping 
action, is responsible for bur retention, problems of durability, 
dependability, wear and fatigue are often associated with spring grip 
chucks. Also, problems are often encountered in attaching or removing 
burs. In the use of a spring grip chuck, the bur is normally removed by 
utilizing a small diameter push rod which the operator must direct with a 
force strong enough to overcome the frictional forces exerted by the 
spring. Conversely, in order to insert a bur, the bur must generally be 
placed against a soft brass or plastic slug so that it can be pushed into 
place without damage to the cutting surface of the bur. Major 
disadvantages of spring grip chucks have resided in the fact that it is 
often difficult and dangerous for operators to apply sufficient force to 
overcome the springs for bur insertion and removal without damaging the 
burs, the handpieces, or both. U.S. Patents illustrative of spring grip 
chucks are Nos. 2,963,519, 3,088,745, 3,255,527, 3,321,209, 3,426,429, 
4,021,919 and 4,012,841. 
Other U.S. patents showing wrench-operated collet chucks are Nos. 
3,325,899, 3,496,638, 3,813,782 and 3,888,008. 
SUMMARY 
This invention is concerned with a chuck assembly which provides the 
advantages of both spring-grip and wrench-operated chucks without the 
aforementioned disadvantages of prior chuck constructions. More 
specifically, it is an object to provide an improved chuck assembly which 
is relatively simple in construction, is wrench-operated for positive 
gripping action, is capable of withstanding the substantial torsional 
forces generated when such a chuck is tightened to grip a dental bur, and 
is remarkably free of bur walkout problems even when the high-speed 
handpiece is used to cut through gold alloys, amalgams, and other 
tenacious materials. A further object is to provide a chuck assembly which 
is durable, does not wear readily or break easily and is field 
replaceable. In addition to its other advantages, the chuck assembly is 
easily operated to release and secure a bur and, in contrast to at least 
some conventional wrench-operated chuck assemblies, does not tend to 
score, gouge, or otherwise damage the surface of a bur shank during normal 
use. 
The chuck assembly consists essentially of two main elements: a rotor 
having a bore, and a chuck received within that bore. The term "rotor" is 
used herein to refer to the rotatably mounted member of a high-speed 
turbine-driven dental handpiece. Ordinarily, the rotor will either be the 
turbine itself or a bur tube secured within that turbine. While the 
invention might be utilized with dental handpieces other than 
turbine-driven high-speed handpieces, the problem of bur walkout is not 
encountered with low-speed handpieces and, therefore, a major benefit of 
the invention would not be realized. 
The chuck is formed in one piece, is tubular, and defines a bore for 
receiving the shank of a dental bur. The chuck includes an integral 
bur-clamping or bur-gripping section disposed intermediate the length of 
the chuck (and of the rotor's bore), such section being defined by an 
external shoulder or bearing surface and being capable of flexing radially 
inwardly to clamp the mid-portion of the shank of a dental bur extending 
therethrough. 
The external shoulder of the bur-gripping section is engagable with an 
internal shoulder within the bore of the rotor so that as the shoulders 
are urged axially into forceful engagement the bur-gripping section of the 
chuck will flex radially inwardly to engage and grip the mid-portion of 
the bur shank. 
Above and below the bur-gripping section--that is, spaced from opposite 
ends of that section--are elongated pilot passages which snugly but 
slidably receive portions of the bur shank in zones spaced axially from 
the bur-gripping section. The rotor extends substantially the full length 
of the chuck and includes a lower end portion which surrounds and braces 
the lower pilot section of the chuck to prevent flexing forces directed 
against the bur during operation from being propagated beyond the lower 
pilot zone to the bur-gripping zone. The result is a construction which is 
positive in operation and remarkably free of bur walkout or ejection in 
use. 
In the disclosed embodiment the pilot sections and bur-clamping or 
bur-gripping sections of the chuck are integrally formed. The clamping 
section is disposed intermediate the length of the chuck and its radial 
deformability is achieved by providing a plurality of openings or slots in 
circumferentially-spaced positions about the chuck. In the best mode 
presently known for practicing the invention, the chuck is provided with 
just two such slots arranged in diametric opposition. Not only does the 
two-slot construction perform at least as effectively as constructions 
having greater numbers of slots, but the two-slot construction achieves 
such results while providing superior torsional rigidity. Such torsional 
rigidity is particularly important because of the threaded interconnection 
between the chuck and rotor and the fact that the chuck is tightened by 
rotating it within the rotor until the parts are in such forceful 
engagement that the intermediate slotted section of the chuck is deformed 
or flexed radially inwardly to grip the bur. Such forceful engagement 
between the chuck and rotor imposes considerable torsional strain on the 
longitudinally-elongated and arcuate web portions spaced by the slots of 
the chuck. In addition to its superior strength and performance 
characteristics, the two-slot chuck construction has the advantages of 
being easier and less-costly to manufacture. 
Other features of the structure and its operation, and other advantages and 
objects of the invention, will become apparent from the specification and 
drawings.

DETAILED DESCRIPTION 
Referring to FIGS. 1 and 2, the numeral 10 generally designates a dental 
handpiece having a housing 11 and a cap 12 threadedly secured to the 
housing at the upper end thereof. The handpiece selected for illustration 
is an air-driven handpiece having a pair of ball bearing assemblies 13 and 
14 supporting rotor 15 for rotation within the chamber defined by the 
housing. The rotor includes driving means in the form of turbine 15a and 
bur tube 15b. The turbine depicted in FIG. 1 is an axial-flow turbine 
which is driven by air discharged from stator 16, the stator in turn 
receiving drive air supplied by drive air passage 17. The bulk of the air 
discharged from the turbine is exhausted through discharge passage 18. 
The rotor tube or bur tube 15b is provided with internal threads 19 at the 
upper portion of its axial bore 20. A sloping internal shoulder 21 is 
located intermediate the upper and lower ends of the bore. Shoulder 21 
defines the boundary between two sections of the bore 20 of different 
diameter, the lower section 20a adjacent the bur-receiving end of the 
handpiece having smaller diameter than upper section 20b. 
As shown most clearly in FIGS. 3 and 4, a tubular chuck 22 is disposed 
within the rotor tube 15b. The chuck has a bore 23 extending therethrough, 
the lower cylindrical portion 24 of the bore defining a socket for 
receiving the shank of a bur, and the upper portion 25 being of 
non-circular cross section (preferably square) to define a socket for 
receiving a suitable wrench (not shown). 
Although the chuck of FIGS. 1-5 is formed of a single piece of material, it 
may conveniently be considered as being composed of a plurality of 
sections or portions, namely, a lower end portion 22a, an upper end 
portion 22b, and an intermediate portion 22c. The wall of the intermediate 
portion 22c is provided with an external shoulder adapted to engage the 
internal shoulder 21 of the rotor tube when the parts are assembled as 
illustrated in FIG. 3. The upper portion 22b is externally threaded, the 
threads of that portion mating with the internal threads of the rotor 
tube. Because of the downwardly and inwardly sloping configuration of the 
shoulders, the intermediate portion 22c of the chuck is pressed radially 
inwardly when the chuck is screwed downwardly to urge shoulders 21 and 27 
into forceful engagement with each other. Since the dimensional difference 
between the outside diameter of a bur shank and the inside diameter of 
bore portion 24 is small, and since such parts are manufactured to close 
tolerance, only slight inward flexing or deformation of the wall of the 
intermediate bur-clamping portion or section 22c is required in order to 
grip the bur. To promote effective flexure without the need for applying 
excessive twisting force to rotate the chuck 22 within the rotor tube 15b 
after the sloping shoulders have made contact with each other, openings or 
slots 28 are formed in the wall of the intermediate bur-clamping portion 
22c. The openings or slots are circumferentially spaced and, in the 
embodiment depicted in FIGS. 1-5, are longitudinally elongated, running 
the full length of the intermediate bur-clamping portion 22c. 
Where autoclavability is required or desired, the material of choice for 
the chuck as well as other parts of the handpiece is believed to be 
stainless steel. The slotted chuck construction shown in the drawings is 
intended to be fabricated out of stainless steel although it is believed 
apparent that other materials having the necessary properties of 
flexibility, strength, and durability might be used. 
It should be observed that above and below the bur-clamping section 22c of 
the chuck are upper and lower portions 22b and 22a, respectively, which 
define upper and lower pilot guide passages 29 and 30. In the illustration 
given, such passages are portions of the cylindrical section 24 of the 
bore 23. As previously indicated, bore portion 24, and especially pilot 
passage sections 29 and 30 of that portion of the bore, are only slightly 
larger in diameter than the shank of the bur to be received therein. The 
relationship has been referred to as a snug but slidable fit; in technical 
terms, the spacing should be as small as manufacturing tolerances permit, 
hopefully of the order of magnitude of 0.0005 of an inch. By reason of the 
close but slidable fit between the shank of a bur and the upper and lower 
pilot passages at each end of the chuck's bur-gripping section, and the 
bracing effect of the surrounding rotor tube 15, particularly the bracing 
effect of that rotor tube on the lower piloting portion of the chuck, 
concentricity between the bur and rotor may be achieved and maintained. 
The gripping of the shank of the chuck in an intermediate zone disposed 
between and spaced from the upper and lower pilot zones, and the bracing 
of those pilot zones, are believed to be responsible for the absence of 
bur walkout during operation of handpiece 10. 
The wrench used for adjusting chuck 22 is not shown because it forms no 
direct part of the present invention and because such wrenches are widely 
used and well known. Reference may be had to U.S. Pat. Nos. 3,235,899 and 
3,888,008 for wrenches adaptable for use with the chuck assembly disclosed 
herein. The square socket 25 has already been described for the purpose of 
coupling the wrench to chuck 22. Notches 31 (FIG. 4) are formed in the 
upper end of rotor tube 15b for the purpose of coupling the wrench to that 
tube, it being understood that in the operation of either loosening or 
tightening the chuck, one part (the chuck or the rotor tube) must be 
rotated while the other part (the rotor tube or the chuck) must be held 
stationary. 
FIG. 5 depicts in enlarged and greatly exaggerated fashion the cooperative 
relationship between the parts during operation of the chuck assembly. As 
the chuck 22 is screwed downwardly to force external shoulder 27 into 
camming engagement with internal shoulder 21 of rotor tube 15b, the 
deformable intermediate portion 22c of the chuck is flexed or deformed 
radially inwardly into tight gripping engagement with the shank 32 of a 
dental bur. It will be observed that the wall of the intermediate portion 
is flexed radially into a curved configuration and that clamping contact 
between the chuck and the bur occurs along a zone of substantial 
longitudinal extent. The length of that zone contrasts sharply with the 
line of contact or biting action characteristic of conventional collet 
chucks. The longitudinal curvature of the deformed intermediate portion 
22c not only eliminates any appreciable scoring or marring of the surface 
of shank 22 but, at the same time, produces an extraordinarily effective 
gripping action which is highly resistant to bur walkout and slippage. 
The one-piece chuck may be easily removed by simply unthreading it from the 
rotor tube 15b and extracting it through the enlarged opening 33 in the 
cap of the hand-piece. Therefore, should cleaning of the chuck be desired, 
or should replacement become necessary, such operations may be readily 
performed by the dentist or his assistant. Since the lower section 20a of 
the bore of the rotor tube is smaller in diameter than the upper section 
20b, and since the lower portion 22a of the chuck is likewise smaller than 
the upper portion 22b, there is no possibility that during handpiece 
operation the chuck might somehow become loosened and released from the 
lower end of the rotor tube. 
In the embodiment of FIGS. 1-5, the unitary chuck 22 is provided with four 
circumferentially-spaced slots 28. Chuck 22' of FIGS. 6-8 is substantially 
identical to chuck 22 except that only two diametrically-opposing slots 
28' are provided. The rotor and handpiece in which the chuck 22 is mounted 
may be identical to the parts already described; therefore, in FIGS. 6-8 
the same numerals are used to designate elements that do not differ from 
those previously described. 
The effectiveness of the embodiment of FIGS 6-8 appears to confirm what 
applicant has theorized to be the mechanics of bur walkout in high speed 
(200,000 to 500,000 or more rpm) dental handpieces. In a conventional 
wrench-operated chuck, spring fingers at the chuck's lower end are forced 
into tight engagement with the shank of a dental bur. Slight flexure of 
the exposed shank of the bur below the jaws in response to laterally 
directed forces against the tip of the bur tends to cause a biting and 
pivoting action to take place along an arc of movement opposite from the 
point of applied force, accompanied by a slight sliding or slipping action 
along that arc of movement on the same side as the applied force. With 
each revolution of the flexed bur, any given point on the surface of that 
bur engaged by a jaw of a conventional chuck alternately pivots against 
that jaw and then, 180.degree. later, when the point of contact generally 
faces the point of applied force, may slip slightly downwardly from the 
jaw. For reasons not fully understood, walkout has not been found to be a 
significant problem with low or medium speed (i.e., under 200,000 rpm) 
handpieces. However, with high-speed handpieces, such alternate biting, 
pivoting, and sliding action may result in rapid walkout at some risk to 
dentist, patient, or both. 
The characteristic of both of the embodiments disclosed herein is that 
piloting of the bur at the lower end of the chuck, braced by the bur tube, 
occurs a substantial axial distance from the area in which the bur is 
gripped and held. In FIG. 6, the numeral 40 generally designates the zone 
or area of piloting action at the lower end of chuck 22', whereas numeral 
41 indicates the zone or area of forceful gripping contact between the 
chuck and a bur 32. The two zones 40 and 41 are spaced axially apart a 
substantial distance. When flexing movement of the lower portion of a bur 
takes place (such movement is shown in exaggerated form by broken lines 42 
in FIG. 6), contact between the chuck 22' and bur 32 in pilot zone 40, 
coupled with the reinforcement of the chuck's lower end provided by the 
surrounding bur tube 15b, prevent the flexing action of the bur tube from 
being transmitted or propagated upwardly to the bur-clamping zone 41. A 
primary reason why bur walkout is so effectively prevented is believed to 
reside in the spacing of the two zones and the external bracing of the 
chuck's lower end by the rotor tube 15b, thereby isolating the gripping 
zone 41 from flexing action of the bur occasioned by lateral forces 
generated when the tip of the bur is forced against a work object. 
Spacings and deformations are exaggerated in FIGS. 6 and 8 for illustrative 
purposes. When the intermediate bur-gripping portion of the chuck is 
cammed inwardly into tight gripping contact with a bur through forceful 
engagement between external and internal shoulders 27 and 21, 
respectively, the side walls or web portions of the chuck on opposite 
sides of slots 28' are urged inwardly as indicated by broken lines in FIG. 
8. The bur is therefore tightly gripped along diametrically-opposing 
surface portions. It has been found that effective gripping action takes 
place even though the areas of forceful contact are diametrically-disposed 
rather than being spaced at three or more points about the circumference 
of the bur, and it is believed that such effectiveness arises to a 
considerable extent because of the braced piloting of the bur at the lower 
end of the chuck in a zone 40 spaced axially from gripping zone 41. 
Since only two slots 28' are provided, chuck 22' has outstanding torsional 
rigidity and is better able than chucks having a greater number of slots 
to withstand the substantial twisting forces that develop when such a 
chuck is tightened or loosened by a wrench. Also, to the extent that 
formation of a pair of slots requires less machining, and simpler 
machining procedures, than the formation of a greater number of slots, the 
chuck of FIGS. 6-8 is advantageous in terms of ease and lower costs of 
manufacture. 
While in the foregoing, embodiments of the invention have been disclosed in 
considerable detail for purposes of illustration, it will be understood by 
those skilled in the art that many of these details may be varied without 
departing from the spirit and scope of the invention.