Abstract:
A subjective ophthalmic refractor of a type comprising a ring-shaped cylinder power scale having an internal gear actuated by a scale drive gear connected to a cylinder power adjustment knob of the refractor is improved by providing a bearing insert within the cylinder power scale. The bearing insert includes radially and axially facing annular bearing surfaces extending about a substantial portion of the scale, and a cut-out region allowing space necessary for the scale drive gear.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to ophthalmic instruments, and more particularly to subjective ophthalmic refractors for evaluating refractive characteristics of a patient&#39;s eye. 
     BACKGROUND OF THE INVENTION 
     A subjective ophthalmic refractor typically comprises left-eye and right-eye batteries each having a defined viewing path along which an operator may selectively introduce combinations of testing lenses having known refractive properties. During examination, the patient is positioned in a darkened room with his or her eyes aligned to view a projected target chart along the viewing paths defined by the left-eye and right-eye batteries. The operator then performs well-known refracting procedures, including refraction using astigmatic charts and the Jackson cross-cylinder test. A goal of the examination procedure is to determine the sphere power, cylinder power, and cylinder axis of each eye so that a suitable pair of corrective lenses may be prescribed. 
     U.S. Pat. No. 2,968,213 describes an ophthalmic refractor of the prior art. FIG. 5 of the &#39;213 patent is an exploded view illustrating the internal components of a left eye lens battery, and serves to illustrate a lonstanding arrangement for mechanically coupling a pair of rotatable cylinder lens carriers 8 and 9 and an associated ring-shaped cylinder power scale 50 of the battery to an adjustment knob 20 used by the operator to set a chosen cylinder power in the viewing path of the battery. As can be seen at FIG. 1 of the &#39;213 patent, the indicia on scale 50 can be viewed by the operator through an opening 52 in the battery housing. Typically, the indicia are numerical cylinder power values from 0 to 6.00 diopters in quarter-diopter increments, and are angularly spaced at regular angular increments about a central axis of the ring-shaped scale. The mechanical interconnections from knob 20, through cylinder power scale 50, to the lens carriers 8 and 9 are designed such that rotation of knob 20 positions two lenses (or a lens and an empty lens cell), one from lens carrier 8 and one from lens carrier 9, in series in the viewing path to produce a resultant cylinder power. Lens carrier 8 is a “weak” cylinder lens carrier having, for example, a blank lens cell (zero power) and four cylinder lenses ranging in power from 0.25 diopters to 1.00 diopters at quarter diopter increments. Meanwhile, lens carrier 9 is a “strong” cylinder lens carrier having, for example, a blank lens cell (zero power) and four cylinder lenses ranging in power from 1.25 diopters to 5.00 diopters at 1.25-diopter increments. Consequently, by indexing the weak cylinder lens carrier 8 five times for every one index movement of strong cylinder lens carrier 9, a cylinder power range of 0.00 diopters to 6.00 diopters at quarter-diopter increments is possible in agreement with the indicia on scale 50. 
     With continued reference to U.S. Pat. No. 2,968,213, it will be seen that adjustment knob 20 drives a shaft 21 having at its opposite end a dual gear comprising a small front gear 23 and a larger rear gear 25. Larger gear 25 meshes with a gear 31 fixedly connected to the hub of a driver plate 27, whereby rotation of knob 20 and larger gear 25 produces counter-rotation of driver plate 27. As driver plate 27 rotates about its axis, four short pegs 39a-39d on the driver plate successively engage weak cylinder lens carrier 8 to index only the weak lens carrier, and a fifth longer peg 40 engages both the weak and strong lens carriers 8 and 9 to index both carriers, in the manner of a Geneva mechanism. A spring-biased roller 48′ cooperates with five circumferential detents 53′ in driver plate 27 to allow the operator to feel each index position at adjustment knob 20. 
     Meanwhile, small front gear 23 meshes with internal gear teeth on ring-shaped scale 50 to rotate the scale in coordination with the indexing of cylinder lens carriers 8 and 9, whereby an appropriate cylinder power value marked on the scale appears through opening 52. Scale 50 is constrained both radially and axially by three bearings 56 located at respective positions about the circumference of scale 50. Each bearing 56 includes a low friction circular base slidably contacted by the marked side of scale 50, and a stepped cylindrical retainer post fastened coaxially along with the base to the refractor housing to define a gap through which the circumferential outer edge of scale 50 passes. Due to space limitations within the refractor housing, bearings 56 are not spaced at regular 120° angular intervals about the rotational axis of scale 50. This reliable arrangement for rotatably supporting and positioning scale 50 has heretofore remained unimproved, despite the existence of certain drawbacks with respect thereto. In particular, bearings 56 are difficult to adjust during assembly, the tolerances of the retainer posts and scale 50 cause misalignment of indicia in view opening 52, and the localized support at irregular angular intervals creates a poor fit. Consequently, the rotation of adjustment knob 20 does not feel as smooth and uniform to the user as would be desired. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide an improved arrangement for mounting a rotatable ring-shaped scale in an ophthalmic refractor that allows for easy assembly. 
     It is another object of the present invention to provide an improved arrangement for mounting a rotatable ring-shaped scale in an ophthalmic refractor that achieves a smoother adjustment knob feel to the user without introducing more demanding tolerances in critical components. 
     It is a further object of the present invention to provide an improved arrangement for mounting a rotatable ring-shaped scale in an ophthalmic refractor that reduces the overall number of bearing components. 
     In furtherance of these and other objects, an ophthalmic refractor of a type comprising a ring-shaped cylinder power scale having an internal gear, a rotatable cylinder power adjustment knob, and a scale drive gear fixed for rotation with the adjustment knob and meshing with the internal gear of the cylinder power scale is improved with respect to the manner in which the cylinder power scale is supported and located. In particular, a prior art arrangement of three bearing posts irregularly-spaced at points on the outer circumference of the cylinder power scale is replaced by a generally circular bearing insert located in the interior of the scale and mounted on an existing axle stem fixed to the an eye battery housing of the refractor. The cylinder power scale is confined against radially directed movement by a first retaining surface of the bearing insert that faces in a radially outward direction for engaging the crests of the scale&#39;s internal gear teeth. The cylinder power scale is confined against axially directed movement by a second retaining surface defined by a circumferential lip on the bearing insert, and by a low-friction bearing plate fixed to the eye battery housing. A U-shaped cut-out region in the bearing insert allows space for the scale drive gear. The arrangement of the present invention provides more uniformly and extensively distributed bearing surfaces for guiding the gear-driven rotation of the cylinder power scale in an improved manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying drawing figures, in which: 
     FIG. 1 is a front elevational view of an ophthalmic refractor formed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is an exploded perspective view of an eye battery of the ophthalmic refractor shown in FIG. 1; 
     FIG. 3 is an enlarged cross-sectional view of a portion of the eye battery shown in FIG. 2; and 
     FIG. 4 is an enlarged perspective view showing a portion of a cylinder power scale of the eye battery and a bearing insert therefor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a subjective ophthalmic refractor  10  formed in accordance with a preferred embodiment of the present invention. Ophthalmic refractor  10  is of a type well-known in the art of ophthalmic instruments in that it generally comprises a right eye battery  11 A and a left eye battery  11 B that are mirror images of each other. Eye batteries  11 A and  11 B comprise respective housings  12 A and  12 B and respective viewing paths  14 A and  14 B along which a patient facing a back side of the instrument gazes during examination. The construction and operation of ophthalmic refractor  10  are generally and substantially as taught in U.S. Pat. Nos. 2,968,213 and 2,995,065, both these patents being incorporated herein by reference. 
     The present invention relates to an improvement involving a cylinder power adjustment system found in each eye battery,  11 A and  11 B. For sake of simplicity, the invention is described with respect to right eye battery  11 A only, it being understood that left eye battery  11 B is a mirror image of right eye battery  11 A. Battery  11 A comprises a cylinder power adjustment knob  16 , a cylinder power scale  18  having numerical indicia  20  indicative of a cylinder power introduced in viewing path  14 A, and an opening  22  in battery housing  12 A for allowing an appropriate power value on scale  18  to be viewed by an ophthalmic practitioner. 
     Referring also now to FIGS. 2 through 4 of the drawings, adjustment knob  16  is fixed on a shaft  24  having at its opposite end a dual gear comprising a small front gear  26  and a larger rear gear  28 . Larger gear  28  transmits torque to a Geneva mechanism for indexing a pair of cylinder lens carriers (not shown) as described in referenced U.S. Pat. No. 2,968,213. Small gear  26  meshes with an internal gear  30  of cylinder power scale  18  to rotate the cylinder power scale simultaneously and in concert with the rotational indexing of the cylinder lens carriers. Cylinder power scale  18  includes a frontward-facing first side  18 . 1  on which indicia  20  are marked or applied, and a rearward-facing second side  18 . 2 . 
     In accordance with a preferred embodiment of the present invention, eye battery  11 A comprises a bearing insert  32  located internally with respect to ring-shaped cylinder power scale  18 . Bearing insert  32  is generally circular in shape and includes a U-shaped cut-out region  34  providing space to accommodate small gear  26  and larger gear  28 , a central aperture  36  allowing the insert to be mounted on an existing stem  37  defining a rotational axis for a pegged driver plate (not shown) of the Geneva mechanism, and a recessed region  39  surrounding central aperture  36  and adjacent to cut-out region  34  for accommodating a gear (not shown) fixed to the pegged driver plate. As best seen in FIG. 4, bearing insert  32  comprises a first retaining surface  40  facing in a radially outward direction for engaging the crests of gear teeth of internal gear  30 , thereby confining scale  18  against radially directed movement. FIG. 4 also shows a circumferential lip  42  of bearing insert  32  that serves to define a forwardly-facing second retaining surface  44  for confining scale  18  against axially directed movement. The radial reach of lip  42  is chosen such that second retaining surface  44  is spaced from, but remains close to, internal gear  30 . Bearing insert also includes a pair of fastener holes  45 . 
     Eye battery  11 A further comprises a bearing plate  46  fixed relative to housing  12 A and arranged in surface-to-surface contact with a radially inner portion of first side  18 . 1  of cylinder power scale  18 . Bearing plate  46  is preferably formed of ultra high molecular weight (UHMW) plastic or other low friction material so as to minimize a coefficient of friction between the surface of bearing plate  46  and first side  18 . 1  of scale  18 . The pair of fastener holes  45  through bearing insert  32  are alignable with a pair of fastener holes  47  provided through bearing plate  46 , whereby the bearing insert  32  and bearing plate  46  can be fixed to housing  12 A by a pair of fasteners (not shown). 
     As will be appreciated from the foregoing description, the irregular three-point bearing arrangement on the outside of the cylinder power scale according to the prior art is replaced by an internal bearing arrangement providing first and second retaining surfaces  40  and  44  distributed uniformly about a rotational axis of the scale through an included angle limited by the width of cut-out region  34  in bearing insert  32 . In accordance with the present invention, the included angle of first and second retaining surfaces  40  and  44  about a central axis of bearing insert  32  is greater than 180° and preferably as large as possible. In a current commercial embodiment, the included angle through which first and second retaining surfaces  40  and  44  continuously extend is approximately 300°. The improved bearing arrangement of the present invention eliminates binding, uneven frictional resistance, and assembly problems associated with the prior art to provide repeatably accurate rotation of cylinder power scale  18  throughout the lifetime of the instrument.