Abstract:
A damping system for an electromagnetically operable shutter includes a detent for arresting the motion of a pivotally supported shutter actuating drive arm. The detent has damper portions that are fixed to the pivotally supported drive arm and bumper portions that are fixed t positions to receive the impact of the dampers as the arm is pivoted in one direction or another to operate the shutter. The damper portions each has a straight edge and the surface of each bumper portion is curved so as to maintain substantially a point contact between the two throughout the duration of the impact. In addition the damper portions are formed of a damped polyurethane and each bumper portion is formed of an ultra high molecular weight polyethylene, the combination of materials providing energy absorption while avoiding the adherence of damper and bumper portions upon impact.

Description:
TECHNICAL FIELD 
     The present invention relates generally to dampers for lightweight mechanical assemblies and more particularly to a damper for a mechanical shutter such as a photographic shutter. 
     BACKGROUND OF THE INVENTION 
     Mechanical assemblies having moving parts frequently require damping to protect the parts from damaging impacts and to increase the life and reliability of the assemblies. This is particularly true where the mechanical assembly is lightweight and the impact is repetitive and at a high rate of speed. Electrically operated lens shutters used in various types of photographic and laboratory equipment meet these criteria. For example, a shutter can open and close in a fraction of a second. Electronically operated shutters can operate at frequencies of 2 to 400 cycles per second and higher. 
     Lens shutters generally are of two types. In one type, a so called “guillotine” shutter has one or two thin, metal blades or leaves arranged to cover a lens opening. Pivot connections allow each blade to swing between a closed position where the blades cover the lens opening and an open position where the blades are drawn aside from the lens opening. 
     In a second type, a plurality of pivotally mounted blades, usually five, are arranged around the lens. Each blade is connected to a rotatable ring. In the operation of these rotary shutters, the rotation of the ring in one direction causes the blades to swing in unison to an open position. Counter rotation of the ring swings the blades to a closed position over the lens opening after exposure. 
     It is common in the first type of shutter to provide a shock absorber or damper that absorbs the impact as the blades are pivoted between the open and closed positions. In this respect, reference is made to U.S. Pat. No. 3,595,553 and U.S. Pat. No. 3,664,251, the disclosures of which are incorporated herein by reference. As disclosed in these references, the shock absorber operates to stop the shutter blade very rapidly, yet softly without damage and with little or no bounce. To Applicant&#39;s knowledge, a comparable shock absorbing system has not been used in connection with the rotating ring type of shutters. 
     One material known to have damping characteristics suitable for use in the damping system of photographic shutters is a ISOLOSS® high density molded product made by Aearo Company. The ISOLOSS® products are high density damped polyurethane thermoset polymers. Sheets and molded parts of this material have been used in a variety of applications relating to noise, shock and vibration reduction. The material has favorable hysteretic damping properties, good compliance and is able to absorb and store mechanical energy while effectively dissipating it in the form of heat. 
     Dampers made from this polyurethane molded material have been used for some time in combination with other materials such as metals and Teflon coated metal and have provided good results in some applications. However, one disadvantage of the polyurethane molded shock absorbing formulations is that they have a rubbery characteristic and tend to stick to the surfaces that they impact while performing the damping function. In an attempt to remedy this, the impact surface often is coated with Teflon. A Teflon coated surface can reduce or eliminate the sticking when contacting the polyurethane molded damper, however, the Teflon coated surface introduces other problems. Teflon tends to scale and form a powdery residue somewhat like talcum powder in its consistency. In many applications the residue is not harmful but, in some applications, such as optical shutters, the residue may find its way to the surfaces of optical components such as lenses. This powder on the surface of the lens is detrimental to the operation of the lens. 
     Accordingly, it is an object of the present invention to provide an improved damping system for photographic shutters and in particular, high speed rotary shutters. 
     Another object of the present invention is to provide a damping system employing a polyurethane molded damper that eliminates sticking of the damper to the impact surface. 
     A further object is to provide a damping system having an increased operational life. 
     SUMMARY OF THE INVENTION 
     In the present invention a rotary shutter is provided with a damping system wherein the swinging motion of the shutter drive arm through an arcuate path of travel is arrested by a detent positioned to engage a member carried by the drive arm. The member is a beam that is generally triangular in cross section. Fixed to each side of the beam is a damper formed of a highly damped polyurethane thermoset material. Impact of the dampers against the detent arrests the movement of the drive arm. 
     The impact surface of the detents comprises a bumper composed of an ultra high molecular weight polyethylene. This material, together with the shape and disposition of the bumper and damper provide a limited contact area. Having a limited contact area concentrates the impact energy and improves the efficiency of the damping system. When the damper is struck, the material at the point of impact undergoes a transformation from a glass phase to a rubber phase and it is the rubber phase that tends to stick to the impact surface. By limiting the area of contact to a line or more preferably a point contact, the potential “sticky” area is reduced to a minimum. 
     Accordingly, by careful selection of the shape, materials and arrangement of the damper and bumper, sticking is virtually eliminated. Life testing demonstrates that the components have a much greater life than heretofore achieved with other components. Damping characteristics of the combination are close to ideal for electromechanical shutters and likely for other applications in that the design essentially eliminates bounce or rebound after impact. 
     Accordingly, the present invention may be characterized in one aspect thereof by a damping system for arresting motion of a small mass such as a photographic shutter or the like driven by a drive arm mounted for swinging motion through an arcuate path of travel. The damping system includes a detent having opposite ends arranged to arrest motion of the arm at each end of the path of travel. In this respect the opposite ends of the detent carry bumpers formed of a high molecular weight polyethylene that are disposed to impact a damper on the drive arm, the damper being composed of a high density polyurethane thermoset material. Both the bumper and damper are shaped to provide substantially point contact during the duration of the engagement between the damper and the bumper. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front elevation view of a shutter incorporating the damping system of the present invention; 
     FIG. 2 is a perspective view on an enlarged scale of a portion of FIG. 1; 
     FIG. 3 is a plan view of the structure shown in FIG. 2 with portions removed for clarity; 
     FIG. 4 is a view taken generally along lines  4 — 4  of FIG. 3; 
     FIG. 5 is a view on an enlarged scale of a portion of FIG. 4 showing the disposition of components with the shutter in a closed position; 
     FIG. 6 is a view on an enlarged scale of a portion of FIG. 3; and 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, FIG. 1 shows a rotary shutter generally indicated at  10  incorporating the damping system of the present invention. It should be appreciated that various components, brackets and wiring harnesses of the shutter assemble have been omitted for clarity. For purposes of orientation, the shutter includes a base plate  12  having a central aperture  14 . Supported at the underside (not shown) of the base plate are a plurality of shutter blades  16 , portions of which are seen through the aperture. The rotary shutter is conventional. It is sufficient for purposes of the present invention to say that the shutter blades are operatively attached to a driver plate (not shown) that rotates with respect of the base plate. Rotation of the driver plate in one direction moves the shutter blades aside and opens the aperture. Reciprocal rotation of the driver plate moves the shutter blades back to the position shown in FIG. 1 wherein the aperture is closed. 
     The means for operating the driver plate includes an electromagnetic actuator  18  mounted to the base plate. The actuator includes an armature  20  that engages a laterally extending rocker arm  22 . The rocker arm engages a drive arm  24 . The drive arm is flat and formed of very thin sheet metal to minimize mass and is pivotally supported so its end  26  moves in an arcuate path of travel. At its end  26 , the drive arm is connected to a link  28 . The link has a connector  30  that extends through an elongated opening  32  in the base plate for connection to the driver plate. 
     The swinging movement of the drive arm  24  is damped by the damping system of the present invention generally indicated at  34 . As best seen in FIGS. 2 and 3 the damping system includes a pair of bumpers  36  attached to a shaped spring wire  38 . One of the bumpers is an inner bumper  36 A and the outer is an outer bumper  36 B. 
     The spring wire that supports the bumpers preferably is formed of piano wire about 0.020 inches in diameter and is shaped to form a torsion spring. In this respect the spring wire is formed with a central spine  40  that is bifurcated (FIG.  3 ). The two portions  42 ,  44  of the bifurcated spine  40  are joined at one end  46  and are spaced apart and unjoined at an opposite end  48 . Each of the two portions  42 ,  44  in turn is bent to form opposed, generally U-shaped arms  50  extending outward from the central spine  40 . As best seen in FIGS. 4 and 5, the opposite ends  52  of the U-shaped arms are each bent downward from the plane of the central spine  40  and support the bumpers below the plane of the central spine  40 . 
     As formed, the arms  50  of the spring wire are bent downwardly from the plane of the central spine  40 . However, as shown in FIGS. 1 and 2, the spring wire  38  is carried by a stop plate  54  that is disposed beneath the arms and engages the arms to force them to assume an orientation that is closer to the plane of the spine. This pre-loads the arms  50  to provide a bias urging the arms to pivot downwardly from the plane of the central spine  40 . 
     The stop plate in turn is rigidly fixed to a relatively heavy and rigid support wire  56 . The support wire  56 , with a diameter of 0.050 inches, is over twice the diameter of the spring wire  38 . The ends  58 ,  60  of the support wire are fixed as rigidly as possible to the base plate  12  (FIG. 1) so as to minimize as much as possible the flexure of the support wire. 
     As best seen in FIGS. 2,  3  and  6 , each bumper  36  is generally oval in shape. The ends of the oval shape are each provided with a groove  62 . These grooves receive the end  52  of the U-shaped arms for attaching the bumpers to the spring wire  38 . 
     The damping system  34  is further shown in FIGS. 3,  4  and  5  to include components carried by the drive arm  24 , which interact with the bumpers  36  and the spring wire  38 . These components include a beam  64  that is fixed to a surface of the flat drive arm adjacent its end  26 . The beam is generally triangular in cross section and the two upstanding sides or legs of the triangular beam extend upwardly from the drive arm and into the space between the downwardly bent ends  52  of the spring wire. Fixed to and extending outward from each of the upstanding legs is a damper  68 . One of the dampers is an inner damper  68 A and the other is an outer damper  68 B. Each damper is generally triangular in cross section and is fixed to the beam such that a longitudinally extending corner  70  of each damper is disposed to contact one of the bumpers  36  (FIGS.  5  and  6 ). 
     In a start position with the shutter closed, the corner  70  of the inner damper  68 A is in contact with the inner bumper  36 A (FIGS.  4  and  6 ). To operate the shutter, the electromagnet  18  is energized and the armature  20 , operating through the rocker arm  22 , pivots the drive arm  24  so its end  26  is swung upwardly as viewed in FIG.  1 . This pulls the connecting link  28  and rotates a driver plate (not shown) to open the shutter. Pivoting the drive arm carries the outer damper  68 B forward and into contact with the outer bumper  36 B wherein the engagement of one against the other arrests the motion of the drive arm. When the electromagnet is de-energized, a spring  72  returns the drive arm back to the start position to close the shutter. The return motion of the drive arm then is arrested by the engagement of the inner damper  68 A against the inner bumper  36 A. 
     In a shutter having an aperture of 45 mm the operating speed of the shutter can be up to 5 Hertz and the speed increases as the aperture size decreases. Operating speeds of up to 400 hertz or more are possible in shutters having an aperture of 2 mm. Accordingly, it is important to arrest the forward and reverse motion of the drive arm quickly, smoothly and with little or no bounce. Several aspects contribute to the successful operation of the damping system of the present invention. For example, one of the damper and bumper is formed of a highly damped polyurethane thermosetting resin that provides high performance damping, isolation, and shock and motion control with a loss factor in excess of 1.0. A preferred material is a urethane solid having the following properties: 
     Normal Hardness (ASTM D2240) Shore A Durometer Impact at 73° F. (23° C.) of about 58, 
     Glass transition temperature (ASTM D575) of about 18° F. (−8° C.), 
     Maximum loss factor at 10 Hz and 54° F. (12° C.) of about 0.94, 
     Rebound (ASTM D2632) Bashore Resilience 
     Rebound, 1st impact @ 20° C. of about 4.5% 
     Rebound, 2 nd  impact @ 20° C. of about 0.0%, 
     Compression Load Deflection (ASTM D575) 
     10% Deflection about 82 psi (565 kPa) 
     20% Deflection about 180 psi (1241 kPa) 
     30% Deflection about 305 psi (2103 kPa) 
     Compression Modulus about 845 psi (5826 kPa). 
     A suitable material is ISOLOSS® HD made by Aearo Company. 
     The other of the damper and bumper is formed of an ultra high molecular weight polyethylene (UHMWPE) having a molecular weight in the range of 3 to 6 million. Such a material typically has a density of at least 0.930 g/cm 3  and up to 0.965 g/cm 3 . Surprisingly, such a high molecular weight polyethylene provides effective damping when used together with the polyurethane despite the hardness of the polyethylene, which is considered a detriment to effective damping. 
     In a preferred arrangement the bumper is made of the UHMWPE whereas the damper is formed of the polyurethane. The polyethylene is extrudable to a desired shape of the bumper, as described hereinbelow, whereas the urethane material is not extrudable and must be molded to shape. Also, the urethane is readily attachable to the beam with an adhesive whereas an adhesive attachment of the polyethylene to the beam does not hold up over time and under the stress of repeated impacts. 
     The present invention also has the components of the damping system arranged to provide substantially point contact between bumper  36  and the damper  70  during the entire time interval of contact between the two. This is accomplished by providing the bumper with an oval shape as shown and the damper with a longitudinally extending corner  70 . The arrangement of the straight corner  70  of the damper striking against the curved surface of the oval bumper (FIG. 6) maintains substantially point contact for the duration of the contact time. Limiting the engagement to substantially point contact improves the efficiency of the damping system. 
     A further contribution to the impact absorbing characteristics of the damping system results from the loading of the arms  50 . As described above, the arms are loaded so they are biased in a downward direction. Accordingly an impact of a damper  68  against a bumper  36  that tends to rotate the arm  50  upwardly about the central spine is resisted by the downward bias of the arm. Furthermore, as shown in FIG. 5, each end  52  of the U-shaped arm  50  is bent downward at an angle of about 77° or within about 13° of a line perpendicular to the plane of the arm  50 . This allows the impact of the damper to be partly absorbed by both an upward flexing in unison of the arm  50  and of the bifurcate central spine  40  of the spring wire. The angle is maintained to insure that the corner  70  of the damper strikes the approximate center of the bumper  36 A so the two do not disengage as the bumper deflects upwardly against the resistance offered by the urging of the downward bias of the arm. The angle also prevents the adhesion of the two materials during the contact compression phase when the beam is moved causing the damper corner to impact the bumper. 
     While all the components as described cooperate to provide an effective damping system, it was found that forming the damper and bumper of polyurethane and high molecular weight polyethylene respectively contributed significantly to the effectiveness of the damper. In particular, these materials served to increase the operational life of the shutter by six times from one to six million operations while virtually eliminating the adhesion of the damper and bumper materials without the use of either anti-stick materials such as Teflon or lubricants.