This invention relates to the progressive cavity apparatus, and more particularly to drive trains for progressive cavity devices and to progressive cavity driving, drilling, and pumping apparatus.
Progressive cavity devices are well known in the prior art, both as pumps and as driving motors. These devices are also known as single-screw rotary pumps and single-screw rotary motors. These devices have a single shaft in the shape of a helix contained within the cavity of a flexible lining of a housing. The generating axis of the helix constitutes the true center of the shaft. This true center of the shaft coincides with its lathe or machine center. The lined cavity is in the shape of a double threaded helix with twice the pitch length of the shaft helix. One of the shaft or the housing is secured to prevent rotation; the part remaining unsecured rolls with respect to the secured part. As used in herein, rolling means the normal motion of unsecured part of progressive cavity devices. In so rolling, the shaft and housing form a series of sealed cavities which are 180.degree. apart. As one cavity increases in volume, its counterpart cavity decreases in volume at exactly the same rate. The sum of the two volumes is therefore a constant.
When used as a pump, the unsecured part, whether shaft or housing, is rotated by external forces so as to roll with respect to the secured part. Fluids entering the housing are pumped through it by the progressing cavities. When used as a motor, the unsecured part, whether shaft or housing, rolls with respect to the secured part in response to fluids flowing through the housing. Whether the progressive cavity device is used as a motor or a pump, the part that is unsecured and free to rotate is known generally as the rotor and the secured part is known generally as the stator.
When used as a motor, the unsecured part or rotor produces a rotor driving motion. The driving motion of the rotor is quite complex in that it is simultaneously rotating and moving transversely with respect to the stator. One complete rotation of the rotor will result in a movement of the rotor from one side of the stator to the other side and back. The true center of the rotor will of course rotate with the rotor. However, the rotation of the true center of the rotor traces a circle progressing in the opposite direction to the rotation of the rotor, but with the same speed. Thus, one complete rotation of the rotor will result in one complete rotation of the true center of the rotor in the opposite direction. Thus, the rotor driving motion is simultaneously a rotation, an oscillation, and a reverse orbit.
Examples of progressive cavity motor and pump devices are well known in the art. The construction and operation of such devices may be readily seen in U.S. Pat. Nos. 3,627,453 to Clark (1971); 2,028,407 to Moineau (1936); and 1,892,217 to Moineau (1932).
Despite the simple construction of progressive cavity devices, use of the devices as motors in driving and drilling apparatus has proven difficult. This difficulty stems in part from the complex rotor driving motion described above. Attempts have been made to convert this complex motion into rotational motion for driving or drilling. The most successful device in the past for conversion of this motion has been a universal joint attached to the driving end of the rotor and connected to a universal joint attached to the object to be driven or drill to be rotated. This approach suffers from several disadvantages. First, the universal joint tend to fail quickly if run in abrasive environments. The fluids used in progressive cavity drilling apparatus often are or quickly become abrasive. A further problem encountered with the prior art conversion devices is that the object to be driven or the drill to be operated are driven at the same speed as the rotor. There are many applications where a speed reduction is quite desirable. For example, in drilling oil or gas wells, the rotors of the progressive cavity driving apparatus presently used rotate at speeds approaching 325 revolutions per minute. At this speed, the oil and gas well drill bits being driven tend to wear out far too quickly since they are designed to run at speeds of around 100 to 150 revolutions per minute. This excessive wear on the drill bits also causes difficulty in drilling directional oil and gas wells. Directional wells drilled with progressive cavity drilling devices equipped with prior art conversion devices are slanted at sharp angles since the drill bit can be used for only a limited time. These sharp angles cause problems in drilling and in producing such wells.
Conversely, there are many applications in using progressive cavity devices as pumps where an increase in the speed of the driven rotor over that of the external driving force is desirable. Double universal joints do not provide such an increase in speed.