Every type of hydraulic pump has a certain degree of delivery irregularity, causing pressure ripple in a system. Unfortunately, the fluid-borne pressure ripple can travel throughout a circuit and generate unwanted vibration and noise far from the pump. That can lead to problems ranging from excessively loud operation that is annoying to nearby workers to overload and premature structural fatigue failure in components, said Mario Antonio Morselli, research director at Stem-Numerical Engineering, Modena, Italy. For instance, when ripple frequency matches a natural frequency in the application components, then fatigue failures can occur if the nominal ripple is much lower than the pressure of the network, he explained.
Fluid power engineers have, for decades, sought out solutions that lower vibration and pulsation in a hydraulic system. Early attempts include gear pumps with a high number of teeth, which are simple and produce less mechanical noise, but have a low specific delivery and low efficiency.
Another idea is the “duo” pump that consists of two gear pumps, one a half-tooth out of phase with the other. Because of the added complexity, high cost and lower efficiency, it is not widely used.
More common today are double-contact gear pumps with little or no backlash. They offer a compact design and quieter operations, but face demanding, high-precision production capabilities to ensure consistent performance.
Newer contributions to low-noise operation include Settima’s Continuum pump and Danfoss’s Shhark pump. The Continuum features what is termed zero kinematic delivery ripple. It is a helical gear pump characterized by a continuous meshing of teeth with a special profile and a helical ratio near 1:1. It is well known and is typically used when there are strict requirements for low noise emission. The Shhark is an external gear pump with highly asymmetrical tooth profiles that allow a high number of teeth (almost double) in the same volume of a standard pump. It lowers the delivery ripple with the same efficiency and delivery rate compared to standard designs.
Another option for machine designers to reduce pressure ripple is to damp it after its generation by pumps. Variations include:
• A Helmholtz resonator — a volume connected to the supply line or pipe with a “neck.”
• A concentric hole-cavity resonator, where a number of holes around the diameter of the pipe wall are surrounded by a contained volume. It performs much like a Helmholtz device.
• A quarter-wave resonator, a closed pipe connected perpendicular to the main pipe; and the similar extended inlet (or outlet) resonator.
And then there are pressurized-gas devices. Here, a standard gas/oil bladder accumulator attached to the main pipe helps dampen pulsations. In some cases, a perforated pipe inside the main line positioned near the accumulator inlet improves performance.
Properly designed and installed, all these designs can lower noise and vibration to a certain degree. However, they must be properly engineered and sized to a specific system, and add to cost and complexity as well.
Now, Morselli is working on another alternative for fluid power engineers. Loosely termed the “V-Box,” it is a straightforward, compact device that has no moving parts and contains no gas or bladders. It is reportedly effective from low to very high frequencies, connects in-line or in a T-configuration, causes minimal flow restriction and is cost-effective due to simple construction.
The unit has only four elements: three basic spiral elements connected in series and a separating plate. A through-duct in each element ensures negligible pressure drop. Each component has the same spiral shape, which simplifies production, say for die casting.
At first glance, the device looks much like a compact, quarter-wave resonator, but shaped as a spiral connected with the through-duct by a rectangular window. But actually, this is not a simple quarter-wave filter because of the flexibility of the internal structure, said Morselli. An element with thin spiral walls acts like a Helmholtz resonator; while spiral elements with thick walls and high wall stiffness will act like a quarter-wave filter resonator. Thus, the design and construction can be such that performance lies anywhere between the two modes, and offers system designers a high degree of freedom in targeted frequencies and is effective over a wide band of frequencies. A typical unit might measure about 7 in. long.
Prototypes have been subjected to numerous lab and field tests, with excellent results, said Morselli. For instance, tests on Turolla/Danfoss gear pumps (55 cc/rev) showed a sizeable reduction in peak-to-peak ripple and ambient noise at 180 bar (2,600 psi) and 1,200 to 1,800 rpm. And in tests with a 45 cc Rexroth A10 piston pump at Purdue’s Maha Lab, running at speeds from 1,800 to 2,800 rpm, pressures from 140 to 210 bar (2,030 to 3,045 psi), and displacements ranging from 75 to 90%, hydraulic RMS pressure-ripple reductions averaged 80%. In addition, a range of field tests showed peak-to-peak attenuation ranging from about 80 to 95%, regardless of the frequencies involved.
The cost-effective device damps pressure ripple in hydraulic networks, said Morselli. It uses components that are simple to manufacture, and it is reliable because it does not contain any moving components, bladders, gas or valves.
The basis of this device is a new kind of resonating element that combines the Helmholtz and quarter-wave resonator characteristics using the deformability of the internal structure, making it effective on a wide range of frequencies. The concept has been validated by numerous tests, both on bench devices and in actual hydraulic applications.
Filed Under: News, Pumps & Motors