Patent Publication Number: US-8979402-B2

Title: Friction member for brake mechanism and camera shutter using the same

Description:
RELATED APPLICATIONS 
     This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201010576901.7, filed on Dec. 7, 2010 in the China Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to friction members and camera shutters and, in particular, to a friction member used for a brake mechanism and a camera shutter using the same. 
     2. Discussion of Related Art 
     In order to obtain a camera shutter with high shutter speed and high durability, it is important to make shutter blades light and to improve the brake performance of a brake mechanism of the shutter. 
     The braking force of the brake mechanism varies according to the thickness of friction members such as washers, and a set of friction members having proper thickness to calibrate the braking force. Polyethylene terephthalate (PET) is often used as a material of the friction members. However, the friction members made of PET can be damaged by the blade in the braking process with an acceleration of the shutter speed. Thus, the initial braking performance cannot be maintained, and the shutter blade can be damaged in an early stage, or the durability of the shutter may be extremely decreased. 
     What is needed, therefore, is to provide a friction member for brake mechanism and a camera shutter using the same that can overcome the above-described shortcomings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the embodiments can be better understood with references to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  shows a scanning electron microscope (SEM) image of a drawn carbon nanotube film. 
         FIG. 2  shows an SEM image of a pressed carbon nanotube film. 
         FIG. 3  shows an SEM image of a flocculated carbon nanotube film. 
         FIG. 4  shows an SEM image of an untwisted carbon nanotube wire. 
         FIG. 5  shows an SEM image of a twisted carbon nanotube wire. 
         FIG. 6  is a schematic view of one embodiment of a friction member for brake mechanism in a camera shutter. 
         FIG. 7  is a cross-sectional view along a broken line VII-VII of the friction member in  FIG. 6 . 
         FIG. 8  is a top view of one embodiment of a camera shutter using the friction member shown in  FIG. 6 . 
         FIG. 9  is an exploded view of one embodiment of a brake mechanism for the camera shutter shown in  FIG. 8 . 
         FIG. 10  is a cross-sectional view of one embodiment of a friction member for brake mechanism in a camera shutter. 
         FIG. 11  is a cross-sectional view of one embodiment of a friction member for brake mechanism in a camera shutter. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
     A friction member is to provide a stable friction force for a brake lever in the brake mechanism in a camera shutter, thereby braking the brake lever. Therefore, the friction member should have high mechanical strength and wearability. The friction member can be any of number of desired shapes according to demands of space and any other requirements. The friction member can be a washer with a ring-shaped sheet structure. A thickness of the washer can be in a range from about 50 micrometers (μm) to about 500 μm. In one embodiment, the thickness of the washer can be in a range from about 50 μm to about 100 μm. The washer defines an inner diameter and an outer diameter, and the inner diameter and the outer diameter of the washer can be selected as desired. 
     The friction member includes a carbon nanotube polymer composite. The carbon nanotube polymer composite includes a carbon nanotube structure and a polymer. The carbon nanotube structure is about 5% to 80% by weight of the friction member. In one embodiment, the carbon nanotube structure is about 10% to 30% by weight of the friction member. 
     The polymer can be thermoset or thermoplastic, such as epoxy resin, polyolefin, acrylic resin, polyamide (PA), polyurethane (PU), polycarbonate (PC), polyoxymethylene resin (POM), polyethylene terephthalate (PET), polymethyl methacrylate acrylic (PMMA), or silicone. 
     The carbon nanotube structure can be a free-standing structure including a plurality of carbon nanotubes. Adjacent carbon nanotubes tightly combine with each other and define a plurality of micropores. The carbon nanotube structure is located within the polymer. The polymer covers surfaces of the carbon nanotube structure and fills into the micropores. 
     The carbon nanotube structure can include at least one carbon nanotube film comprising a plurality of carbon nanotubes. If the thickness of the carbon nanotube film is thick enough, the carbon nanotube structure in the friction member can be a single carbon nanotube film. If the thickness of the carbon nanotube film is relatively thin, the carbon nanotube structure can include a plurality of carbon nanotube films stacked on each other, and the adjacent carbon nanotube films closely combine with each other. The carbon nanotubes in each of the carbon nanotube film are substantially parallel to a surface of the carbon nanotube film along the carbon nanotubes extending directions. 
     Referring to  FIG. 1 , the carbon nanotube film can be a drawn carbon nanotube film formed by drawing a film from a carbon nanotube array. Examples of the drawn carbon nanotube film are taught by U.S. Pat. No. 7,045,108 to Jiang et al. The thickness of the drawn carbon nanotube film can be in a range from about 0.5 nanometers (nm) to about 100 μm. 
     The drawn carbon nanotube film includes a plurality of carbon nanotubes that are arranged substantially parallel to a surface of the drawn carbon nanotube film. A large number of the carbon nanotubes in the drawn carbon nanotube film can be oriented along a preferred orientation, meaning that a large number of the carbon nanotubes in the drawn carbon nanotube film are arranged substantially along the same direction. An end of one carbon nanotube is joined to another end of an adjacent carbon nanotube arranged substantially along the same direction by van der Waals attractive force. A small number of the carbon nanotubes are randomly arranged in the drawn carbon nanotube film, and has a small if not negligible effect on the larger number of the carbon nanotubes in the drawn carbon nanotube film arranged substantially along the same direction. It can be appreciated that some variation can occur in the orientation of the carbon nanotubes in the drawn carbon nanotube film. Microscopically, the carbon nanotubes oriented substantially along the same direction may not be perfectly aligned in a straight line, and some curve portions may exist. It can be understood that contact between some carbon nanotubes located substantially side by side and oriented along the same direction cannot be totally excluded. 
     More specifically, the drawn carbon nanotube film can include a plurality of successively oriented carbon nanotube segments joined end-to-end by van der Waals attractive force therebetween. Each carbon nanotube segment includes a plurality of carbon nanotubes substantially parallel to each other, and joined by van der Waals attractive force therebetween. The carbon nanotube segments can vary in width, thickness, uniformity, and shape. The carbon nanotubes in the drawn carbon nanotube film are also substantially oriented along a preferred orientation. The width of the drawn carbon nanotube film relates to the carbon nanotube array from which the drawn carbon nanotube film is drawn. 
     The carbon nanotube structure can include a plurality of drawn carbon nanotube film. An angle can exist between the orientation of the carbon nanotubes in adjacent films, stacked, and/or coplanar. Adjacent carbon nanotube films can be combined by only the van der Waals attractive force therebetween without the need of an additional adhesive. An angle between the aligned directions of the carbon nanotubes in two adjacent drawn carbon nanotube films can range from about 0 degrees to about 90 degrees. Micropores are defined between two adjacent carbon nanotubes in the drawn carbon nanotube film. If the angle between the aligned directions of the carbon nanotubes in adjacent drawn carbon nanotube films is greater than 0 degrees, the micropores can be defined by the crossed carbon nanotubes in adjacent drawn carbon nanotube films. 
     Referring to  FIG. 2 , the carbon nanotube film can also be a pressed carbon nanotube film formed by pressing a carbon nanotube array down on the substrate. The carbon nanotubes in the pressed carbon nanotube array are arranged along a same direction or along different directions. The carbon nanotubes in the pressed carbon nanotube array can rest upon each other. Adjacent carbon nanotubes are attracted to each other and combined by van der Waals attractive force. An angle between a primary alignment direction of the carbon nanotubes and a surface of the pressed carbon nanotube array is about 0 degrees to approximately 15 degrees. The greater the pressure applied, the smaller the angle obtained. If the carbon nanotubes in the pressed carbon nanotube array are arranged along different directions, the carbon nanotube structure can be isotropic. Here, “isotropic” means the carbon nanotube film has properties identical in all directions substantially parallel to a surface of the carbon nanotube film. The thickness of the pressed carbon nanotube array can range from about 0.5 nm to about 1 mm. The length of the carbon nanotubes can be longer than 50 μm. Clearances can exist in the carbon nanotube array. Therefore, micropores can exist in the pressed carbon nanotube array and be defined by the adjacent carbon nanotubes. Examples of the pressed carbon nanotube film are taught by US PGPub. 20080299031A1 to Liu et al. 
     When the carbon nanotube structure includes a plurality of pressed carbon nanotube films including the carbon nanotubes substantially arranged along a same direction, an angle between the aligned directions of the carbon nanotubes in two adjacent pressed carbon nanotube films can range from about 0 degrees to about 90 degrees. 
     Referring to  FIG. 3 , the carbon nanotube film can be a flocculated carbon nanotube film formed by a flocculating method. The flocculated carbon nanotube film can include a plurality of long, curved, disordered carbon nanotubes entangled with each other. A length of the carbon nanotubes can be greater than 10 centimeters. In one embodiment, the length of the carbon nanotubes is in a range from about 200 μm to about 900 μm. Furthermore, the flocculated carbon nanotube film can be isotropic. The carbon nanotubes can be substantially uniformly distributed in the carbon nanotube film. The adjacent carbon nanotubes are acted upon by the van der Waals attractive force therebetween, thereby forming an entangled structure with micropores defined therein. The thickness of the flocculated carbon nanotube film can range from about 1 μm to about 1 millimeter (mm). In one embodiment, the thickness of the flocculated carbon nanotube film is about 100 μm. 
     The carbon nanotube structure can also include a plurality of stacked carbon nanotube layers. Each of the carbon nanotube layers includes a plurality of pure carbon nanotube wires substantially parallel to each other. Adjacent carbon nanotube wires define a plurality of interspaces. The polymer in the friction member also fills the interspaces. Each carbon nanotube wire includes a plurality of carbon nanotubes. Adjacent carbon nanotube layers are closely combined with each other by van der Waals attractive force. An angle between the carbon nanotube wires in at least two carbon nanotube layers can range from about 0 degrees to about 90 degrees. In one embodiment, an angle defined by the carbon nanotube wires in adjacent two carbon nanotube layers ranges from 0 degrees to about 90 degrees. The carbon nanotube wires in each carbon nanotube layer can be close arranged side by side or separated from each other with a determined distance. 
     The carbon nanotube wire can be untwisted or twisted. Referring to  FIG. 4 , treating the drawn carbon nanotube film with a volatile organic solvent can obtain the untwisted carbon nanotube wire. In one embodiment, the organic solvent is applied to soak the entire surface of the drawn carbon nanotube film. During the soaking, adjacent substantially parallel carbon nanotubes in the drawn carbon nanotube film will bundle together, due to the surface tension of the organic solvent as it volatilizes. Thus the drawn carbon nanotube film will be shrunken into an untwisted carbon nanotube wire. The untwisted carbon nanotube wire includes a plurality of carbon nanotubes substantially oriented along a same direction (i.e., a direction along the length direction of the untwisted carbon nanotube wire). The carbon nanotubes are substantially parallel to the axis of the untwisted carbon nanotube wire. In one embodiment, the untwisted carbon nanotube wire includes a plurality of successive carbon nanotubes joined end to end by van der Waals attractive force therebetween. The length of the untwisted carbon nanotube wire can be arbitrarily set as desired. A diameter of the untwisted carbon nanotube wire ranges from about 0.5 nm to about 100 μm. Examples of the untwisted carbon nanotube wire are taught by US Patent Application Publication US 2007/0166223 to Jiang et al. 
     Referring to  FIG. 5 , the twisted carbon nanotube wire can be obtained by twisting a drawn carbon nanotube film using a mechanical force to turn the two ends of the drawn carbon nanotube film in opposite directions. The twisted carbon nanotube wire includes a plurality of carbon nanotubes helically oriented around an axial direction of the twisted carbon nanotube wire. In one embodiment, the twisted carbon nanotube wire includes a plurality of successive carbon nanotubes joined end to end by van der Waals attractive force therebetween. The length of the carbon nanotube wire can be set as desired. A diameter of the twisted carbon nanotube wire can be from about 0.5 nm to about 100 μm. 
     The twisted carbon nanotube wire can be treated with a volatile organic solvent, before or after being twisted. After being soaked by the organic solvent, the adjacent substantially parallel carbon nanotubes in the twisted carbon nanotube wire will bundle together, due to the surface tension of the organic solvent when the organic solvent volatilizes. The specific surface area of the twisted carbon nanotube wire will decrease, and the density and strength of the twisted carbon nanotube wire will be increased. 
     Referring to  FIG. 6  and  FIG. 7 , one embodiment of a friction member  40  for a brake mechanism in a camera shutter is provided. The friction member  40  is a washer  40 . The washer  40  is ring-shaped and defines an inner surface and an outer surface. In one embodiment according to  FIG. 6 , the washer  40  defines an inner diameter and an outer diameter. The washer  40  is a carbon nanotube polymer composite including a pure carbon nanotube structure  41  and a PET  44 . The pure carbon nanotube structure  41  includes a plurality of stacked drawn carbon nanotube films  42 , and the angles between the aligned directions of the carbon nanotubes in adjacent drawn carbon nanotube films  42  can be about 90 degrees. The drawn carbon nanotube film  42  defines a plurality of micropores. The PET  44  is coated on surfaces of the carbon nanotube structure  41  and filled into the micropores. The thickness of the washer  40  is about 50 μm. The carbon nanotube films  42  are about 20% by weight of the washer  40 . 
     The washer  40  is made of the carbon nanotubes and PET. The carbon nanotubes have good mechanical properties. The tensile strength of the carbon nanotubes is about 100 times greater than the tensile strength of steel, and the elastic modulus of the carbon nanotubes is substantially equal to that of diamond. Therefore, the washer  40  is wearable and durable. The angles defined by the carbon nanotubes in adjacent drawn carbon nanotube films  42  are about 90 degrees, therefore, the washer  40  is prevented from cracking along the aligned directions of the carbon nanotubes, and is strong along any direction. 
     A method for making the washer  40  includes steps of: 
     providing the plurality of drawn carbon nanotube films  42 ; 
     forming the carbon nanotube structure  41  by stacking on the drawn carbon nanotube films  42 , and angles defined by axial extending directions of the carbon nanotubes in each two adjacent drawn carbon nanotube films  42  being about 90 degrees; 
     applying PET to the carbon nanotube structure  41  to form a carbon nanotube PET composite; this may be done by dipping the carbon nanotube structure  41  into a liquid PET or coating the liquid PET on the carbon nanotube structure  41 ; and 
     stamping the carbon nanotube PET composite to form the washer  40 . In one embodiment, the liquid PET can be a PET solution or a melted PET. 
     Referring to  FIG. 8  and  FIG. 9 , one embodiment of a camera shutter  100  using the washer  40  is provided. The camera shutter  100  includes a shutter substrate  10 , a drive mechanism  20 , two brake mechanisms  30 , and a blade structure (not shown). 
     The shutter substrate  10  is configured to support the drive mechanism  20 , the brake mechanism  30 , and the blade structure. The shutter substrate  10  includes a body  12  and defines an aperture  14 . When the camera shutter  100  is in use, an amount of light irradiates a photosensitive element through the aperture  14 . When the camera shutter  100  is not in use, the blade structure covers the aperture  14  to prevent the light from irradiating the photosensitive element. 
     The drive mechanism  20  connects with the blade structure and drives the blade structure to open or close, thus, the aperture  14  can be opened or closed. The drive mechanism  20  includes two shafts  26 , a front blade driving lever  22  and a back blade driving lever  24 . The front and back blade driving levers  22 ,  24  are respectively connected with the shutter substrate  10  by the two shafts  26 . The front and back blade driving levers  22 ,  24  are located on a same side of the shutter substrate  10 . The front and back blade driving levers  22 ,  24  can be respectively rotated around the shafts  26  clockwise or anti-clockwise along two moving paths to drive the blade structure opening or closing, thereby opening or closing the aperture  14 . 
     The two brake mechanisms  30  are respectively located at the clockwise terminations of the two moving paths to brake the drive mechanism  20 . Each brake mechanism  30  includes a support shaft  34 , two washers  40 , a brake lever  32  between the two washers  40 , and a Belleville spring  36 . One of the washers  40 , the brake lever  32 , the other washer  40 , and the Belleville spring  36  are harnessed on the support shaft  34  in sequence. The brake lever  32  is located at the clock wise terminations of the moving path of the front blade driving lever  22  or the back blade driving lever  24 , to brake the front blade driving lever  22  or the back blade driving lever  24 . The support shaft  34  supports the two washers  40 , the brake lever  32  and the Belleville spring  36 . The two washers  40  are urged against the brake lever  32  by the Belleville spring  36 . 
     When the front blade driving lever  22  is rotated clockwise to reach the termination of the moving path, the front blade driving lever  22  blocks off the brake lever  32  and makes the brake lever  32  rotate around the support shaft  34 . When the brake lever  32  is rotated, a frictional force is generated between the two washers  40  and the brake lever  32  by the urging force of the Belleville spring  36 . The rotating energy of the front blade driving lever  22  is absorbed by the frictional force. Thus, the front blade driving lever  22  and the blade structure are further braked. The back blade driving lever  24  operates in the same manner as that of the front blade driving lever  22 . 
     It can be understood that the friction member is not limited to the washer  40 , it also can be other types which can provide a stable friction force for a brake lever in the brake mechanism in a camera shutter. 
     Referring to  FIG. 10 , one embodiment of a friction member  50  for a brake mechanism in a camera shutter is provided. The friction member  50  is a ring sheet-shaped washer with a thickness of about 50 μm. The friction member  50  includes a carbon nanotube structure  51  including a plurality of stacking carbon nanotube layers  53  and an epoxy resin  54 . 
     In one embodiment, the carbon nanotube layers  53  are about 25% by weight of the friction member  50 . Each carbon nanotube layer  53  includes a plurality of twisted carbon nanotube wires  52 . Each of the twisted carbon nanotube wires  52  includes a plurality of carbon nanotubes defining a plurality of micropores. In one embodiment, a plurality of interspaces is defined by the adjacent twisted carbon nanotube wires  52 . The epoxy resin surrounds the surface of the carbon nanotube structure  51  and infiltrates the micropores. In one embodiment, the twisted carbon nanotube wires  52  are located side by side without spaces and are parallel to each other. The angles defined by the twisted carbon nanotube wires  52  in each adjacent two carbon nanotube layers are about 90 degrees. Therefore, the friction member  50  can avoid cracking along any directions, and has proper strength in directions parallel to the surface of the friction member  50 . 
     A method for making the friction member  50  can include the following steps of: 
     providing the twisted carbon nanotube wires  52 ; 
     forming a first carbon nanotube layer and a second carbon nanotube layer; wherein forming the first carbon nanotube layer comprises arranging twisted carbon nanotube wires  52  side by side along a first direction, and forming the second carbon nanotube layer comprises placing twisted carbon nanotube wires  52  side by side along a second direction that is substantially perpendicular to the first direction; 
     layering a plurality of first carbon nanotube layers and second carbon nanotube layers alternatively to form the carbon nanotube structure  51 ; 
     applying a liquid epoxy resin to the carbon nanotube structure  51  to form a carbon nanotube epoxy resin composite; this may be done by dipping the carbon nanotube structure  51  into a liquid epoxy resin or coating the liquid epoxy resin on the carbon nanotube structure  51 ; and 
     stamping the carbon nanotube epoxy resin composite to form the friction member  50 . 
     Referring to  FIG. 11 , one embodiment of a friction member  60  for a brake mechanism in a camera shutter is provided. The friction member  60  includes at least two carbon nanotube composite layers  62  stacked on each other. Each carbon nanotube composite layer  62  includes the carbon nanotube structure  61  including a plurality of carbon nanotubes and a polymer  624 . The carbon nanotube structure  61  can include the at least one carbon nanotube film or the at least one carbon nanotube wire. The carbon nanotube structure  61  is located in the polymer  624 . That is to say, the polymer  624  is coated on surfaces of the carbon nanotube structure  61  and fills the micropores defined by the carbon nanotubes and/or the carbon nanotube wires. 
     In one embodiment, the carbon nanotube structure  61  includes at least two drawn carbon nanotube films, the carbon nanotubes in the carbon nanotube structure  61  are substantially oriented along a same direction, thus, the carbon nanotubes in each carbon nanotube composite layer are substantially oriented along a same direction; the carbon nanotubes in adjacent two carbon nanotube composite layers form an angle along the carbon nanotubes oriented directions. The angle can range from about 0 degrees to about 90 degrees. In one embodiment, the carbon nanotube structure  61  includes a plurality of carbon nanotube wires substantially parallel to each other, thus, the carbon nanotube wires in the carbon nanotube structure  61  substantially extend along a same direction. The carbon nanotube wires in two adjacent carbon nanotube composite layers form an angle along extending directions of the carbon nanotube wires. The angle can range from about 0 degrees to about 90 degrees. 
     In one embodiment, the friction member  60  includes two layers of sheet-shaped carbon nanotube composite layers  62 . Each carbon nanotube composite layer  62  includes a plurality of drawn carbon nanotube films  42  substantially arranged along a same direction. Namely, the carbon nanotubes are substantially arranged along the same direction in the each carbon nanotube composite layer  62 . The angle defined by the extending directions of the carbon nanotubes arranged in the two carbon nanotube composite layers  62  can be from about 0 degrees to about 90 degrees. In one embodiment, the angle is about 90 degrees. The polymer  624  is PET. The polymer  624  wraps around the surfaces of the drawn carbon nanotube films  42  and fills in the micropores defined by the carbon nanotubes in the drawn carbon nanotube films  42 . The thickness of the friction member  60  is about 50 μm. The drawn carbon nanotube films  42  are about 30% by weight of the friction member  60 . 
     A method for making the friction member  60  can include the steps of: 
     providing at least two carbon nanotube composite layers  62 , wherein the at least two carbon nanotube composite layers  62  includes the drawn carbon nanotube films  42  and PET; 
     stacking at least two carbon nanotube composite layers  62  on each other, and angle defined by the extending directions of the carbon nanotubes arranged in adjacent two of the carbon nanotube composite layers  62  is about 90 degrees; 
     hot-pressing the stacked carbon nanotube composite layers  62 ; and 
     stamping the hot pressed carbon nanotube composite layers  62  to form the friction member  60 . 
     Each carbon nanotube composite layer  62  is made by stacking the drawn carbon nanotube films  42  one by one to form the carbon nanotube structure  61  including the carbon nanotubes substantially arranged along a same direction; and dipping the carbon nanotube structure  61  into a liquid PET, or coating the liquid PET onto the carbon nanotube structure. 
     It can be understood that the friction members  50 , or  60  can be substituted for the friction member  40  to be used in the camera shutter  100 . 
     According to the above descriptions, the friction members for the brake mechanisms in camera shutters and the camera shutter using the friction members of the present disclosure have the following advantages. 
     First, the friction members are made of the carbon nanotubes and the carbon nanotubes have good mechanical properties as mentioned above. Therefore, the friction members are wearable and durable. The speeds and durability of the camera shutter using the friction members can be improved. 
     Second, the carbon nanotubes in the friction members are intercrossed with each other, therefore, the friction member can avoid cracking along the aligned directions of the carbon nanotubes, and is very strong along any direction. 
     Third, the friction members are formed by a predetermined proportion of the carbon nanotubes and the polymer, and the carbon nanotubes and the polymer work well together to provide good desirable mechanical properties to the friction members, especially when the weight of the carbon nanotubes in the friction members is relatively low. 
     It is to be understood that the above-described embodiment is intended to illustrate rather than limit the disclosure. Variations may be made to the embodiment without departing from the spirit of the disclosure as claimed. The above-described embodiments are intended to illustrate the scope of the disclosure and not restricted to the scope of the disclosure. 
     Depending on the embodiment, certain steps or methods described may be removed, others may be added, and the sequence of steps may be altered. It is also to be understood that the description and the claims drawn relating to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not taken as a suggestion as to an order for the steps.