Patent Application
20070114260
1. Field of the Invention
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.
2. Description of the Related Art
Existing linear return mechanisms fall into three basic categories: springs, and/or plastic members, gas/hydraulic, and rotary to linear conversion mechanisms.
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.
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.
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.
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.
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).
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).
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.
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.
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.
Consequently, a need exists for a spring powered return mechanism as a replacement for traditional coil spring, gas/hydraulic, or rotary conversion mechanisms.
It is an object of the present invention to provide a device having a high linear velocity capability.
It is further an object of the present invention to provide a device having a high-energy efficiency when compared with most existing mechanisms.
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.
These and other objects of the present invention will be more readily apparent from the description and drawings below.
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:
FIGS. 5A-D is a series of drawings illustrating alternative spring designs;
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.
Referring now to the drawings,
In
In
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.
Alternative spring 11 designs are shown in FIGS. 5A-D. There are several design possibilities that can be deployed successfully. In
Alternative design arrangements are shown in FIGS. 6A-B. In
In
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.
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.
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.
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 11a of spring 11 act in conjunction with rollers 12 to return piston 13 against lower surface 18 of frame 17.
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.
