Abstract:
A spring powered return mechanism for linear movement is disclosed. The mechanism is characterized by a novel spring design that allows for high linear speed and energy efficiency. The design features few parts that are simple to manufacture and assemble. In addition, the design allows variable return forces along the movement axis by modifying the spring geometry.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention is directed in general to linear return mechanisms, and, in particular, to a novel spring design that allows simple construction with high linear speed capability and low energy loss. Application areas of the mechanism include power tools, office machines, and in general other linear acting devices.  
         [0003]     2. Description of the Related Art  
         [0004]     Existing linear return mechanisms fall into three basic categories: springs, and/or plastic members, gas/hydraulic, and rotary to linear conversion mechanisms.  
         [0005]     Known spring return mechanisms consist of a coil spring configured in either a compression or tensile manner. As the body to be returned moves away from the starting point, the spring resists this movement. When the moving force is removed the spring returns the body to the starting point. The force exerted by the spring on the body is proportional to the distance moved according to the spring constant. This is energy lost to the system. In addition, the linear speed is limited by spring physics. For a high strength steel spring, the maximum velocity is around 35 feet per second (10 meters per second) for extended use. This speed limit is related to the material properties such as strength, rigidity and density. Light rigid materials such as titanium and beryllium allow substantially higher speed—as high as 100 feet per second (30 meters per second). However, the cost of titanium limits its use while the toxicity problems of beryllium are well known.  
         [0006]     Devices of this type are taught in U.S. Pat. Nos. 2,585,942 and 4,544,090. U.S. Pat. No. 2,585,942 shows a fastener-applying device uses a helical spring located beneath the piston which returns the staple driving piston to its starting position after a fastener is driven. U.S. Pat No. 4,544,090 shows a driver return assembly for an electromechanical fastener driving tool which uses an elastomeric cord attached to the driver at one end and to an anchor at the other end. The cord passes about at least two pulleys to compensate for stretch in the cord to assure that the driver is returned to its normal, retracted position after each working stroke.  
         [0007]     Another category of linear return mechanism uses either gas or liquid as commonly found in pneumatic or hydraulic cylinders. Here, the body moved is the piston with an attached mass via a connecting rod. The piston is propelled by the pressurized fluid and then returned by reversing the pressurized side of the piston using valve means. These systems require the availability of a source of pressurized fluid such as a pump or compressor and also valve and control means. Speeds for pneumatic systems are limited to around 35 feet per second (10 meters per second). Hydraulic system speeds are slower.  
         [0008]     Devices of this type are taught in U.S. Pat. Nos. 3,040,709 and 3,622,062. U.S. Pat. No. 3,040,709 uses a volume of air entrapped in an air return chamber as the piston assembly drives a fastener to provide an upwardly directed force to return the piston and driver blade to its upper position. U.S. Pat. No. 3,622,062 uses the exterior portion of the driver blade to facilitate complete decompression of the air return chamber and an adjustable seal on the drive piston enabling a small amount of pressurized air to bleed past the piston head during the drive stroke to facilitate pressure buildup in the air return chamber and effect more rapid return of the drive piston to its firing position.  
         [0009]     Another variation of return system uses a gas spring. The gas spring consists of an enclosed cylinder with gas pressurized at a few thousand psi. The spring force acts outwardly from the gas pressure acting on the area of the piston rod since the gas pressure is the same on both sides of the piston. The spring force can be nearly constant if a large gas reservoir and large valving means is provided, but without sufficient reservoir, the force will increase with movement. Speeds are limited by seal design at a maximum of 50 feet per second (15 meter/sec).  
         [0010]     Yet another variation of a gas spring ironically relies on a vacuum on one side of a piston that is contained in a cylinder closed on one end wherein the vacuum (or partial vacuum) is formed as the piston is withdrawn. Here the force is atmospheric pressure (14.7 psi) acting on the piston diameter. Since the air reservoir is the atmosphere, there is no reservoir effect on the force; thus, the spring force remains constant with linear movement. Speeds are limited by seals and generally limited to 50 fps (15 meter/sec).  
         [0011]     A device of this type is taught in U.S. Pat. No. 6,755,336. This patent shows a piston assembly slidably received within a cylinder wherein as the tool progresses through its power cycle, the piston assembly creates a vacuum which draws the piston assembly back towards the sealed end of the cylinder to reset the assembly to its starting position.  
         [0012]     Another category of return mechanism uses rotary motion that is converted to linear lotion by a generally flexible member such as a chain, belt, or cable. A motor or a torsional spring may provide the rotary motion. In the case of motion provided by a spring, the return force increases with the linear movement, whereas a motor with control can provide a constant return force. Speed capability can be very high but the complexity for motor and control limits use.  
         [0013]     A device of this type is taught in U.S. Pat. No. 5,320,270. This patent shows a tool having a conically shaped flywheel which cooperates with a drum to cause a driver coupled to the drum by a cable to be pulled through a working stroke. A torsion spring causes the drum to rotate in the opposite direction, unwinding the cable and forcing the driver to return to its normal unactuated position.  
       SUMMARY OF THE PRESENT INVENTION  
       [0014]     Consequently, a need exists for a spring powered return mechanism as a replacement for traditional coil spring, gas/hydraulic, or rotary conversion mechanisms.  
         [0015]     It is an object of the present invention to provide a device having a high linear velocity capability.  
         [0016]     It is further an object of the present invention to provide a device having a high-energy efficiency when compared with most existing mechanisms.  
         [0017]     A still further object of the present invention is to provide a simple self-powered mechanism consisting of a few parts that are simple to manufacture.  
         [0018]     These and other objects of the present invention will be more readily apparent from the description and drawings below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The accompanying drawings, incorporated in and forming a part of the specification, illustrate several aspects of the present invention, and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0020]      FIG. 1  is a sectional view of the present invention in the initial position;  
         [0021]      FIG. 2  is a sectional view of the present invention in fully extended position;  
         [0022]      FIG. 3  is a sketch illustrating the effect of ramp angle, geometry and friction;  
         [0023]      FIG. 4  is a graph showing the relationship of ramp angle versus friction coefficient for a given design;  
         [0024]     FIGS.  5 A-D is a series of drawings illustrating alternative spring designs;  
         [0025]      FIGS. 6A  and B are drawings illustrating alternative embodiments;  
         [0026]      FIG. 7  illustrates a design alternative on the guide rollers;  
         [0027]      FIG. 7A  is a sectional view from  FIG. 7  along line  7 A- 7 A;  
         [0028]      FIG. 8  depicts means to retain contact between rollers and springs;  
         [0029]      FIG. 8A  is a sectional view from  FIG. 8  along line  8 A- 8 A;  
         [0030]      FIG. 9  is a plan view, partly in cross section, of a guidance means for the present invention;  
         [0031]      FIG. 10  is a plan view, partly in cross section, of an alternative guidance means for the present invention;  
         [0032]      FIG. 11  is a sectional view taken along line  11 - 11  of  FIG. 10 ; and  
         [0033]      FIG. 12  is a fragmentary plan view of a fastener driving tool which includes the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     Reference will now be made in detail to the present preferred embodiments of the invention, an example of which is illustrated in the accompanying drawings, wherein like numerals indicate the same elements throughout the views.  
         [0035]     Referring now to the drawings,  FIG. 1  is a sectional view of a mechanism employing the principles of the present invention. The mechanism, generally designated at  10 , illustrates the invention in the initial (returned) position. The design includes torsional spring  11  having a pari of arms  11   a  engaged with rollers  12  mounted for rotation on a moving body  13  via pins  14 . Body  13  is guided to move along a vertical axis  15  by a set of rails  16  depending from a frame  17 . Spring arms  11   a  are preloaded inwardly on rollers  12  thus forcing the body upwardly against a lower surface  18  of frame  17 .  
         [0036]     In  FIG. 2 , the mechanism  10  of  FIG. 1  is shown in the fully activated or extended position. Body  13  has been propelled along axis  15  by a propulsion force  19  acting upon body  13 . When propulsion force  19  is removed, body  13  is returned to the initial position by spring  11  acting on rollers  12 .  
         [0037]     In  FIG. 3 , a schematic shows the relationship of ramp angle Θ versus roller  12  diameter and pin  14  diameter. Spring arm  11   a  exerts force  20  on roller  12 . If the ramp and friction values are favorable, the net roller force component overcomes the friction at pin  14 . The ramp angle Θ is directly proportional to friction—with more friction, a larger ramp angle is needed to propel the body upwardly along in a direction  21 .  
         [0038]      FIG. 4  is a graph showing the relationship of ramp angle Θ versus friction coefficient μ. The relationship between the minimum ramp angle and friction is determined by the following equation:  
         μ   ⁡     (   Θ   )       :=       D     d   ⁢               ·     sin   ⁡     (     Θ   ·   deg     )               
 where D is the roller diameter and d is the pin diameter. 
 
 For a given friction coefficient μ the ramp angle Θ be equal to or greater than the value shown on the graph in order for the body to be returned to its initial position. On the graph, a friction coefficient of 0.15 corresponds to a minimum angle of 2.2 degrees. In practice, a somewhat larger angle would be selected. 
 
         [0039]     Alternative spring  11  designs are shown in FIGS.  5 A-D. There are several design possibilities that can be deployed successfully. In  FIG. 5A , a torsional spring  11 ′ with one or more concentric loops is illustrated. A pair of dual offset loops  22   a  are shown in  FIG. 5B . Dual individual loops  22   b  are depicted in  FIG. 5C , which loops are fixed on either side of a support  23 . In  FIG. 5D , the curvature of the spring arms  11   b  of spring  11 ′ is shown. By employing curvature, the ramp angle can be adjusted through the stroke of the mechanism and, thus, the return force can also be adjusted as desired. This approach can be used to minimize energy loss in the return mechanism.  
         [0040]     Alternative design arrangements are shown in FIGS.  6 A-B. In  FIG.6A , the spring  11  is shown in a lower position. The mechanism works in a similar manner- that is, spring arms  11   a  push against rollers  12  and force body  13  upwardly against surface  18  of frame  17 .  
         [0041]     In  FIG. 6B , another alternative design arrangement is presented. In this embodiment, the torsional spring arms have been replaced by a set of compression springs  24  and corresponding pivoting links  24   a , which are fixed for rotation on either side of a support  26  by a set of pins  27 . The mechanism works similar to the previous embodiment in that springs  23  push inwardly on rollers  12  as links  24   a  pivot about pins  27 . The resultant force moves body  13  in an upward direction. While compression springs  24  have been shown in this arrangement, clearly other types of springs can be readily adapted to the disclosed mechanism. The alternative spring designs include tensile, leaf, cantilever, and combinations of these spring designs.  
         [0042]     The spring material can be metallic, plastic, or composite. The spring material must have rigidity, yet allow flexibility. In addition, durability is required for high-speed applications; for this reason, foraminous materials are unsuitable due their lack of toughness.  
         [0043]     The cross section of the torsional springs may be circular, elliptical, or rectangular (including square). Since no orientation is required, the circular cross section eases manufacturing. However, since torsional springs have primarily bending stress, the circular cross section has high stress points at the outer edges. Meanwhile, a rectangular or square cross section distributes stresses more evenly across an edge. However, square or rectangular cross sections are more difficult to fabricate due to the need for orientation.  
         [0044]      FIGS. 7 and 7 A illustrate a means of guiding the spring arms  11   a  on roller  12 . A groove  28  is provided within each roller  12  for accommodating spring arm  11   a , thus guiding the arm. For a circular section of spring arm  11   a , a similar semicircular groove on roller  12  would be provided.  
         [0045]      FIGS. 8 and 8 A depict means to keep spring arms  11   a  in contact with rollers  12 A retaining member  29  is attached to shaft  14  of roller  12 , thus entrapping spring arm  11   a  within groove  28  of roller  12 . Again, a square section is depicted, but clearly a similar device could be designed for circular, elliptical or rectangular sections of spring arm  11   a.    
         [0046]      FIGS. 9, 10  and  11  show a design enhancement in restricting and guiding the movement of spring arms  11   a . A pair of guide members  30  each having a cutaway portion  32 , provide guidance to spring arms  11   a  and also limit their movement. Note that, to absorb impact of arms  11   a , guide members  30  could be made of an elastomeric material.  
         [0047]     All the designs depicted in the figures utilize a pair of spring arms in a planar arrangement. However, designs have been envisioned that use  3 ,  4  or more spring arms in multiple planes. The number can be varied according to the requirements of the return system. A design with one spring arm is also feasible.  FIG. 12  shows an exemplar fastener driving tool which is suitable for the use of the present invention for returning the drive piston. Referring now to  FIG. 12 , there is shown a fastener driving tool, generally designated at  50 . Tool  50  is preferably of the type described in U.S. Pat. No. 6,830,173, which patent is assigned to the assignee of the present invention, and is incorporated by reference herein. Tool  50  contains a housing  51 , a magazine  52  for containing a strip of fasteners  54 , means  56  for connecting tool  50  to a suitable power source, and a trigger switch  58  for activating a firing cycle for tool  50 . Tool  50  also contains a guide body  60  and a cylinder sleeve  62  within housing  51 . A return assembly  64 , similar to the mechanism  10  shown in  FIG. 1 , is positioned within sleeve  62 , and a driver blade  65  is affixed to the bottom of assembly  64 .  
         [0048]     In operation, when trigger switch  58  is activated, moving body or piston  13  is propelled downwardly by force supplied by the power source, causing driver blade  65  to travel within cylinder sleeve  62  to strike a fastener from strip  54  witin magazine  52  in guide body  60 , driving the fastener into a workpiece. When the cycle is completed, the force of spring arms  11   a  of spring  11  act in conjunction with rollers  12  to return piston  13  against lower surface  18  of frame  17 .  
         [0049]     In the above description, and in the claims which follow, the use of such words as “clockwise”, “counterclockwise”, “distal”, “proximal”, “forward”, “outward”, “rearward”, “vertical”, “horizontal”, and the like is in conjunction with the drawings for purposes of clarity. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or limit the invention to the precise form disclosed, and many modifications and variations for the device are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.