fig1 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 fig2 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 fig4 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 . fig4 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 .
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 .