EXTENDING THE LONGEVITY OF ROLLING CUTTER BITS

Abstract

Analysis of failures in oil and gas equipment indicates that the rolling cutter is the weak link in drill bits. One of the causes of rolling-cutter breakdown is contortion of the balls and roller of the thrust bearings of the bit, which leads to a cle ft in the rolling cutter. A design of rolling-cutter bit, which possesses extended longevity, has been developed at the Sarapul Polytechnic Institute (branch of the Izhevsk State Technical University). Lack of circulation of metal-blocking lubrication in the zone of contact between the hemispherical surfaces of the rolling cutter and shank is a deficiency of present devices. Figure 1 shows a rolling cutter bit with shank 1 and journal 5, the surface of which is built in the form of a hemisphere. Rolling cutter 8 is mounted on the shank using articulated rolling bearing 3. Channels 4 for the feed of lubricant through knurls 9 onto the spherical surface of the journal of reservoir 7 are located in the journal. A layer of copper film 6 is situated between the mating surfaces of the hemispheres of the rolling cutter and journal, and labyrinth seal 2 is installed between the shank and rolling cutter. Rotation from the drill rig is transmitted to the shank and rolling cutter via the housin g of the drill bit. As the rolling cutter turns about its own axis, friction develops between the contact surfaces of the hemisphe res of the rolling cutter and shank, as a result of which the layer of copper film applied to the surface of the hemispheres of the rolling cutter (or shank) is heated. A lubricant (2/3 of glycerin and 1/3 of a hydrochloric acid solution) is fed into the cont act zone through channels 4 and knurls 9. A pressure is created in the lubricant fluid in the contact zone by centrifugal forces, and channels 4 are evacuated, promoting the entry of lubricant from the contact zone to reservoir 7. When there is a difference in surface potentials between the copper alloy (+) and iron alloy (‐), copper particles proceed onto the journal (or rolling cutter), the uncompensated surface structure of the iron alloy (the potential of which increas es with increasing temperature in the contact zone), is attained, and the components existing in the iron alloy proceed into the contact zone, thereafter settling in the knurls. A thin (atomic) layer of copper film, which additionally plays the role of lubri cant, is formed between the contact surfaces. A uniform layer of copper film over the entire contact surface ensures high contact stiffness. The mating surfaces of the cutting roller and journal are fabricated with a high accuracy on machines manufactured with the aid of a computer; this makes it possible to apply a thin layer (0.01‐0.02 mm) of copper (or its alloy); during assembly, moreover, a fabrication error is compensated (if a thin layer of copper is applied) due to rotation of the rolling cutter around its own axis and the axis of the journal (a superfluous layer of copper settles in the knurls of the journal, and thereafter proceeds into the recesses of the rolling-cutter cleft of the articulated rolling bearing, compensating for the clearance in the latter). Thus, the fabrication and assembly of the rolling cutter and journal is simplified many times over, the productivity and accuracy of fabrication is improved, and the longevity of the bit is extended. In the design, use of a simplified labyrinth seal between the rolling cutter and shank (SV-synthesis coating) appreciably reduces the ingress of soil to the contact zone; this also extends the longevity of the bit.