MagneGear Linear and Rotary Magnetic Gears Technical Paper - Dexter Magnetic Technologies - #1

Technical Paper

The Problem

Geared devices are a necessity in many types of downhole tools. The primary purpose of gearing is to provide either a speed change or a force change. Limitations in diameter on most downhole tools necessitate the use of a mechanical advantage in order to perform a particular operation. The use of reduction gearing has long been used as a simple way to gain that mechanical advantage.

Utilizing a planetary gearbox in a downhole environment presents many challenges. The operational temperature rating on a gearbox for downhole operation is the same as for other components, typically -40°C to 175°C. Typical differential pressure experienced downhole is 15,000 - 20,000 psi. High Pressure High Temperature (HPHT) conditions can approach 250°C and >30,000 psi.

Planetary gearboxes typically consist of a housing, an input shaft, output shaft, one or more ring gears, one or more spur gears, and one or more sets of planetary gears. A typical single stage planetary gearbox will have no less than six simultaneous gear meshes, three planetary gears each meshing with the ring gear on the outer diameter and the spur gear on the inner diameter. The gears require lubrication, and the input and output shafts must exit the gearbox housing. Those parameters require the use of dynamic seals to keep the lubricant inside the gearbox as well as keeping anything outside the gearbox from leaking into the gears. Most often some type of pressure compensation device is required within the gearbox. While sealing a low speed shaft with a differential pressure of 20,000 psi is challenging, sealing a high speed shaft at the same pressure can border on the impossible. High speed dynamic seals are the nemesis of every downhole tool designer.

The Solution

The advent of high energy rare earth magnetic materials coupled with recent innovations in magnetic design has led to the development of non-contacting magnetic gear systems. The result is a magnetic drive system, consisting of three concentric and non-contacting elements, whose operation can best be described as equivalent to a planetary gearbox. Any similarities between the subject magnetic gear system and a mechanical planetary gear, however, end with the description of like result. It should also be mentioned that the high energy rare earth magnetic materials that provide the force that propels the gear have been in use for a number of years in downhole operation. They are used routinely in downhole devices such as surface controlled subsurface safety valves, nuclear magnetic resonance formation logging tools, mud pulse telemetry tools, as well as many other applications.

Mechanical vs. Magnetic

The first dissimilarity between magnetic and mechanical is the non-contacting element. As mentioned earlier, mechanical planetary gearboxes have at least six mechanical interfaces. The magnetic gear system has none. There is no friction between gear members, therefore the second difference is that there is no lubrication required.

Further differentiation between magnetic gear systems and mechanical gearboxes is the ability to expose any or all of the components to drilling or production fluids. The concentric rings that form the magnetic gear system are full volume structurally sound solid bodies, packaged in an application suitable housing material such as Inconel. Therefore yet another benefit to the magnetic gear then becomes that it is not necessary to pressure compensate any part of the system.

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