This invention relates generally to downhole pneumatic rock drills (DHD), and more particularly to drills that do not require oil lubrication for sliding surfaces in contact within the drill.
Downhole drills, such as those described by Kurt in U.S. Pat. No. 4,084,646 and by Fu et al in U.S. Pat. No. 5,085,284, are well known in the art. These devices all require the use of special purpose, petroleum oil lubrication to reduce wear of the relatively sliding parts and to prevent friction welding (galling), and subsequent failure of those parts. This lubricant is introduced as a mist in the operating air stream and exhausted into the bore hole (and ultimately the atmosphere) with the air exhausted from the drill. Since the used oil is not recoverable, the operator of the drill must bear considerable expense in providing lubricant for the drill. The open lubrication system may also create environmental problems by introducing oil into the air, ground, and in the some cases, groundwater. This has resulted in DHDs being prohibited in certain applications, groundwater monitoring wells, for example. It is therefore advantageous to produce a DHD which does not require oil lubrication.
DHDs made according to the prior art effect sealing of the operating chambers of the drill by means of a close fit between sliding contact surfaces of the major components of the drill. As normal wear progresses, performance of the drill deteriorates. Ultimately, some or all of the major components of the drill must be replaced to restore drill performance. Unless all worn parts are replaced, performance cannot be restored to new condition. Since the wearing parts are major components of the drill, considerable expense is incurred by such restoration. Due to the close sliding fits required in the prior art, lubrication failure or contamination introduced into the DHD frequently results in catastrophic failure of one or more major components of the drill. Such failure results in lost production, repair expense, and in warranty costs for the manufacturer. It is therefore advantageous to produce a DHD with replaceable seal and bearing elements that prevent catastrophic failure of major drill components and that can restore drill performance following normal wear.
Conventional modern valveless or semi-valveless DHDs typically supply air to the operating chambers via a system of grooves, slots and/or undercuts in the hammer casing ID, piston, or in a "control rod" disposed in the center of the DHD and slidably engaged with the piston. In these DHDs, valving of the air flow is accomplished by the interaction of the termini of these features during the progression of the piston stroke. The grooves, etc. are usually relatively wide to provide adequate flow area for supply air. The termini of these ports are relatively square to precisely define the valving sequence, known in the art as "timing points." If replaceable bearings and seals are introduced to such an arrangement, the seals and bearings will enter the groove or slot. When the seal and/or bearing encounter the terminus of a port, considerable damage to, or catastrophic failure of the seal and bearing element(s) results. It is therefore advantageous that a DHD including replaceable, self-lubricating bearings and seals be provided with a porting arrangement that prevents damage to the bearings and seals.
The foregoing illustrates limitations known to exist in present DHD's. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.