The present invention relates to an improvement on a nail gun described in Japanese Patent Application Kokai No. HEI5-16077 filed by the present assignee. More particularly, the present invention improves safety of a pneumatically operated nail gun for striking with a bit a head of a nail inserted into and supported by a nail guide of the nail gun so that the nail protrudes from the nail guide when supported therein.
Japanese Patent Application Kokai No. HEI5-16077 describes a light-weight nail gun for driving in nails long enough to protrude from the nail guide when supported therein with two to five strikes. The above-described nail gun is designed to be used in the manner of a hand-held hammer which drives in nails of the same size with a similar number of strokes.
The following text will provide an explanation of the nail gun according to Japanese Patent Application Kokai No. HEI5-16077 with reference to FIG. 1. Expressions such as "upward," "downward," "above," "below," "upper surface," and "lower tip" are used in explanations of conventional art and the present invention to define the various parts when a nail gun is disposed in an orientation for driving a nail downward into a workpiece.
FIG. 1 shows the nail gun in a non-firing mode. A drive air chamber 4 provided in a grip 51 of the nail gun is filled with compressed air from a compressor C. The compressed air enters the drive air chamber 4 through an air entrance port 3. An air passage 57 (to be described later) is also formed in the drive air chamber 4.
A trigger valve chamber 5 is formed with a trigger valve hole 52 and an atmospheric communication hole 53. A trigger valve plunger 12 is supported in the trigger valve chamber 5 so as to be movable in the vertical direction, that is, from in the trigger valve hole 52 towards and into the atmospheric communication hole 53 and vice versa. A trigger valve spring 17 urges the trigger valve plunger 12 to plug the atmospheric communication hole 53. A trigger lever 11 abuts the trigger valve plunger 12 so that when the trigger lever 11 is pulled, the trigger valve plunger 12 pulls out of the atmospheric communication hole 53 against the urging of the trigger valve spring 17 and plugs the trigger valve hole 52. When the trigger lever 11 is released, it returns to its initial position by the urging of the trigger valve spring 17. The trigger valve chamber 5 is in fluid communication with a head valve chamber 7 via an air passage 6. In the non-firing mode shown in FIG. 1, the drive air chamber 4 is in fluid communication with the trigger valve chamber 7 via the trigger valve hole 52. Therefore, during the non-firing mode, compressed air from the drive air chamber 4 fills the head valve chamber 7 by passing through the trigger valve hole 52, the trigger valve chamber 5, and the air passage 6.
The lower wall of the head valve chamber 7 is formed by a surface of a head valve 8. Below the head valve 8 is provided a cylinder 9. A piston 10 is supported in the chamber of the cylinder 9 so as to be movable in the vertical direction. The piston 10 hermetically separates the cylinder chamber into an upper cylinder space 58 and a lower cylinder space 59. A through hole 54 is formed through the head valve 8. A spring 55 provided in the through hole 54 urges the head valve 8 downward. In the non-firing mode, the combination of the urging of the spring 55 and the compressed air in the head valve chamber 7 presses the head valve 8 against the upper surfaces of the cylinder 9 and the piston 10, forming a seal between the head valve 8 and the cylinder 9. This seal prevents compressed air in the drive chamber 4 from entering the upper cylinder space 58 through the air passage 57. In the non-firing mode, the upper cylinder space 58 is in fluid communication with the atmosphere via the through hole 54 and an exhaust hole 18.
Next, a firing mode of the nail gun will be explained. To start the firing mode, an operator pulls the trigger lever 11. The trigger lever 11 pushes the trigger valve plunger 12 out of the atmospheric communication hole 53 and into the trigger valve hole 102. This blocks communication between the drive air chamber 4 and the trigger valve chamber 5, and brings the trigger valve chamber 5 into communication with the atmosphere via the atmospheric communication hole 53. As a result, compressed air in the head valve chamber 7 escapes to the atmosphere through the air passage 6, the trigger valve chamber 5 and the atmospheric communication hole 53.
When the pressure in the head valve chamber 7 drops to atmospheric pressure, the pressure of the compressed air in the air passage 57 is sufficient to overcome the downward urging of the spring 55, thereby raising the head valve 8. This blocks fluid communication between the atmosphere and the upper cylinder space 58 while at the same time bringing the upper cylinder space 58 into fluid communication with the drive air chamber 4 via the air passage 57, since the lower surface of the head valve 8 is moved away from the upper surface of the cylinder 9. Compressed air enters the upper cylinder space 58, thereby forcing the piston 10 downward. As the piston 10 moves downward, pressure of the air in the lower cylinder space 59 increases, forcing the air through middle holes 14 and lower holes 19 formed in the cylinder 9. The air forced through the middle holes 14 and lower holes 19 enters a return air chamber 16 formed around the cylinder 9. When the piston 10 passes below the middle holes 14, rubber rings 15 formed to the middle holes 14 prevent air from passing from the return air chamber 16 into the upper cylinder space 58. Pressure in the return air chamber 16 continues to increase until the piston 10 hits the damper 13 provided at the base of the cylinder chamber for cushioning the impact of the piston 10.
At this point the piston 10 abuts the damper 13. While the trigger lever 11 is being pulled, the piston 10 is forced into this dead center position by compressed air from the drive air chamber 4. However, when the trigger lever 11 is released, the head valve chamber 7 is again brought into communication with the compressed air in the drive air chamber 4. The head valve 8 is forced downward against the cylinder 9, thereby bringing the upper cylinder space 58 into communication with the atmosphere via the through hole 54 and blocking communication between the upper cylinder space 58 and the drive air chamber 4. The compressed air in the upper cylinder space 58 is exhausted through the exhaust hole 18 and the high pressure of the air in the return chamber 16 forces the piston 10 upward into its initial position.
A bit 20 integrally formed to the underside of the piston 10 is inserted into a bit guide 21 bolted onto the housing 2 below the cylinder 9. The bit guide 21 is formed with a hole for receiving the bit 20. A cylindrical nail guide 24 is supported on the tip of the bit guide 21 by a nail guide holder 27 so as to be movable in the vertical direction. A first spring 23 is provided in the nail guide holder 27 between the bit guide 21 and an upper peripheral plunger of the nail guide 24 so as to urge the nail guide 24 downward.
The following text will describe a nail driving operation. A nail 30 to be driven is inserted into the nail guide 24. In this case, the nail 30 is partially projected out of the nail guide 24. The nail gun is pressed against a workpiece 29 made of, for example, wood, through the nail 30. An operator pulls the trigger lever 11, thereby causing compressed air to force the piston 10 downward as described above. The bit 20 moves downward with the movement of the piston 10 as guided by the hole in the bit guide 21. The bit 20 strikes the head of the nail 30 supported in the nail guide 24. The operator releases the trigger lever 11, whereupon the piston 10, and also the bit 20, returns to its initial position as described above. Consequently, the nail 30 can be repeatedly struck by repeatedly pulling and releasing the trigger lever 11.
When the nail 30 is driven into the workpiece 29 to the extent where the nail guide 24 abuts the workpiece 29, the pressure exerted on the nail gun by the operator forces the nail guide 24 into the nail guide holder 27 against the urging of the first spring 23. In this way, an appropriate distance can be continually maintained between the bit 20 and the head of the nail until the nail 30 is pounded level with the surface of the workpiece 29. When the nail 30 is completely driven into the workpiece 29, the trigger lever 11 is released so it returns to its original position pictured in FIG. 1.
However, in the nail gun disclosed in the JP5-16077 publication, several problems are acknowledged.
If an operator places his finger on the trigger lever 11 when carrying the nail gun or when inserting a nail 30, he can accidentally pull the trigger lever 11, causing the piston 10 to plunge down and push the bit 20 rapidly out of the nail guide 24. The bit 20 can drive into and damage the workpiece or articles other than the workpiece. There is also the danger of the operator himself being injured by the bit 20. Also, operating the piston 10 when no nail 30 is inserted in the nail guide 24, that is, when the nail gun is empty, can damage the piston 10 or the damper 13.
Another conventional nail gun is provided with a safety system, where a safety arm is vertically movably provided near the firing portion which guides the nail. By the upper tip of the safety arm is provided a safety device for preventing operation of the trigger lever so that the trigger lever can not be operated unless the firing portion is pressed against the workpiece. However, in this type of conventional nail gun, the nail is completely hidden within the nail guide, so that the safety arm can be provided near the firing portion. On the other hand, in the nail gun described in the JP 5-16077 publication, since the applied nail 30 has a long length which protrudes out of the lower tip end of the nail guide 24, the protruding nail 30 prevents the nail guide 24 from being directly pressed against the workpiece 29. Therefore, the above-described safety device cannot be utilized.