1. Field of the Invention
This invention relates to impact drilling tools and more particularly to pneumatically actuated impact drilling tools for earth drilling. The tool utilizes a reciprocally movable hammer which strikes an anvil to create an impact force on the drill bit.
2. Description of the Prior Art
Prior to the development of the present invention there have been numerous types of pneumatically operated impact drilling tools. Bassinger U.S. Pat. No. 3,616,868 discloses an earlier form of pneumatic fluid actuated impact tool. Another form of pneumatically operated impact tool is shown in Bassinger U.S. Pat. No. 3,826,316. There are many types of pneumatically operated devices which have been suggested to provide a repeated impact on a drill bit. Normally, the drill bit is attached to or a part of an anvil that is hit by a reciprocating hammer.
In a pneumatically operated reciprocating hammer there are several valve functions which must be performed. A check valve is required to prevent back flow of air within the drilling tool which might draw cuttings within the moving parts of the tool. A valve function should also be provided to cause compressed air to exert force on the lower end of the hammer to raise the same relative to the anvil. Other valve functions are required to exhaust the pressurized air which raises the hammer and to supply compressed air to the upper end of the hammer to drive the same against the anvil. On each reversal of movement of the hammer the pressurized air on the opposite end must be exhausted before the hammer is moved.
In pneumatically operated rotary drilling tools, the drill rate of a standard drill bit using standard air pressure becomes the key to the success of the percussion tool. However, increased drilling rate cannot be accomplished at the expense of destroying the drill bit. It has been found in the past that a standard hold down force can be applied to rotary drill bits with an impact force being superimposed thereon to greatly increase the rate of drilling. It has also been found that if the impact force is increased and the hold down force decreased the drill rate can be increased without damage to the drilling equipment. Since the pressure of the pneumatic fluid is normally fixed, the downward impact of the hammer is dependent upon the upper surface area subjected to the pneumatic pressure, the stroke length of the hammer and the time required for pressurization and exhaust.
A typical pneumatically operated impact drilling tool sold commercially is illustrated in U.S. Pat. No. 3,503,459. The drilling tool shown in this patent has certain limitations including weak structural walls of the casing, expensive to manufacture, smaller surface area in the hammer and slow pressurization and exhaust. Any undercut or passage through the casing of an impact drilling tool seriously weakens the lateral strength of the tool, especially for small diameter tools. The pneumatic tool of the aforementioned patent is particularly weak in the outer casing which makes it subject to damage during operation.
Various types of percussion drilling devices have been designed and patented where the entire upper diameter of the hammer is acted upon by the pressurized fluid to drive the hammer downward against the anvil. However, to perform the necessary valving functions each of these devices requires undercuts in the casing with cross bores, slots, undercuts and/or vertical feeds being necessary within the hammer element itself. To insure against structural damage of the hammer element, each of these bores, cross slots, undercuts, etc., must end in a rounded surface. All of these problems result in decreased strength of the hammer, increased expense of manufacture and decreased lateral strength of the drilling tool.
In Bassinger U.S. Pat. No. 3,964,551, there is shown a pneumatically operated percussion type rotary drilling tool having a hammer element which reciprocates along the axis of the drilling tool to strike an anvil which is integral with a bit. The hammer is repeatedly raised and driven downward by pneumatic fluid and valving functions are controlled from the center of the hammer element. No undercuts or feeds extend through the casing. The pressurization and exhaust times are minimum. That pneumatic impact tool, however, is somewhat more complex than is desirable, particularly in the design of the feeder tube and the necessity for special cooperative tubing and valving for exhausting compressed air from the tool.