Dealing with decompression
The manifold system also contains a Digga-designed and patented flow-reversal bypass valve, dubbed a “swoosh” valve, to control fluid decompression. Fluid decompression is inherent to screw anchoring and similar operations and occurs when torque load on the rotating tool drops sharply and suddenly. The hydraulic motor normally encounters high resistance to rotation from the load, so fluid pressure rises. Hose and tubing expands in response to the high fluid pressure, and the fluid itself may compress. (Hydraulic fluid is often assumed incompressible, but it is not. A fluid’s bulk modulus serves as a measure of a fluid’s resistance to compressibility.)
If a bit breaks through or fractures a rock, the torque load will suddenly drop to near zero. The compressed fluid will then release its stored energy, causing a violent surge in motor velocity. The swoosh valve compensates for this fluid decompression to smooth out flow surges.
The Digga I-Drive was recognized with an “Australian International Design Award” in 2009 as one of the year’s “best examples of Australian design and innovation, and the high quality of design expertise available to manufacturers in Australia and internationally.”
The I-Drive is available in four standard sizes for handling maximum flow of 375, 500, 750, and 1000 lpm, and maximum torque rating of 300,000 Nm. They come in single-, dual, and three-speed configurations to deliver high speed/low torque, mid-speed/mid torque, or low speed/high torque operation.
For information on the I-Drive, visit www.digga.com or email firstname.lastname@example.org.
An alternative to piston motors
Gerotor and Geroler hydraulic motors are low-speed, high-torque (LSHT) devices often used in direct-drive applications. Deepak Ganapathy, of Eaton’s Hydraulics Group, Eden Prairie, Minn., said they are available in sizes small enough to compete with electric servomotors to moderately large ones that would compete with small and mid-size piston-type motors.
Ganapathy explained that Eaton’s Char-Lynn gerotor and Geroler motors have an inner rotor, called the “star,” and a non-concentric outer rotor. The star and the outer rotor have an unequal number of teeth. The volume of the spaces between the teeth of the two rotors changes continuously during each complete cycle. Pressurized fluid flows into a small cavity that expands into a larger space, creating torque on the shaft attached to the inner rotor as the rotors turn.
Eaton’s Char-Lynn motors are based on what its developer, Lynn Charlson, called the orbit principle — which gives them their high power density, modularity and economical design. The orbit principle describes the motion of the star in both gerotor and Geroler motors as it rotates inside the stationary outer rotor.
Geroler motors use a roller in the spaces between the rotors, whereas the two rotors contact each other in gerotor motors. Geroler motors are more efficient because the rolling friction between the rotors is less than the sliding friction between the rotors in a gerotor motor.
Ganapathy said that the choice between a gerotor and Geroler motor is based on duty cycle and cost for most applications. The Geroler design is more efficient, has a longer service life, and is more expensive. The gerotor design is, in many cases, “good enough” and less costly.
Another factor to consider is operating pressure, which determines the type of valve for controlling flow through the motor. The choices are:
Ganapathy explained that the higher efficiency of Geroler motors is important in applications like the Digga I-Drive, where space and power-density are critical factors. “The 6000 medium-pressure disc valve motors are very compact for their torque output, as well as being considerably less costly than a radial-piston motor with the same capacity. Geroler motors deliver very smooth low-speed performance, which is also important in applications like the Digga I-Drive. These motors have a low no-load pressure drop due to their relatively large fluid passages, which causes them to generate less heat.”
Whether gerotor, Geroler, or radial piston, low-speed, high-torque motors typically do not require a gear box to reduce speed and increase torque. Ganapathy said that based on power density, cost, and reliability, this makes them a better choice than an axial-piston motor with gear box in space-constrained applications like the Digga I-Drive.