Marzocchi Pompe’s solution is made up of a single-lip radial seal ring in a Teflon composite material and reinforcement ring, both of which are the result of a special design and construction process. The external reinforcement ring fully follows the form of the seal ring, and has been sized to bear the axial force generated by internal over-pressures. Before performing laboratory tests, it was necessary to strengthen the entire system that fastens the ring onto the flange, so that it could bear all the stresses submitted to the ring.
To ensure the efficiency of the seal ring under any operating condition, the surface finish or 'roughness' has to fall within a narrow set of values: to avoid burning, the course surface finish should not be too low (Ra >0.2ìm), while to avoid wearing down the lip of the seal ring too quickly, it should not exceed Ra
To optimise the sliding contact between the seal ring and the surface of the shaft, a special study was undertaken using a three-dimensional machine to control the REMOG surface. By means of a special software, it was possible to precisely identify the generating lines that the profile takes on according to the applied finish-grinding process.
In addition to the roughness value, its shape is extremely important too. During the finish-grinding process, particular profiles are produced that may also have a helical direction on the surface of the shaft and could prevent the lip of the seal ring from retaining the fluid.
These profiles are initially generated while the grinding wheels are being sharpened. During the machining process, the impressions created on the grinding wheel as a result of sharpening are transferred to the surface of the piece being worked, resulting in a specific profile with preferential structure and relative roughness value.
The characteristics of these profiles therefore depend on the parameters used for sharpening the grinding wheel and for the machining process, such as the cutting speed, feed speed, type of lubricant and relative temperature.
By making a three-dimensional analysis of the development of the ground surface, several important periodic parameters of the profile can be ascertained:
The number of threads: this depends on the ratio between the number of revolutions of the grinding wheel and that of the piece being worked (for example: if the grinding wheel speed is 1,200rpm and the speed of the piece is 400rpm, the number of threads will be 3).
The pitch: the axial distance between two corresponding peaks;
Groove depth: the vertical distance between the bottom and the peak of the roughness;
The transversal area of passage: the area between two consecutive peaks;
Helix angle: the angle formed between the direction of the helix and the radial direction at the axis of the shaft being worked.
An efficient sealing system requires the profile to be placed in such a way that the transversal areas of passage never communicate with the outside. This is possible if two conditions are met: the course surface finish or 'roughness' value must be between 0.2-0.3ìm, and the profile produced by grinding must have a very narrow helix angle. As a result, there must be complete control of the production process, which, in addition to the type of material used, depends on the characteristics of machining, sharpening methods and final control of the conditions.
Defining the limits
At Marzocchi’s R&D laboratories, validation tests were conducted to verify the limits of the sealing system. These revealed that the system is able to work on a continuous basis at a constant pressure of 40 bar (580psi) without spilling of fluid and without the ring and the shaft noting wear.
Tests performed at higher pressures also demonstrated the possibility of its functioning for short periods of time (several minutes) with over-pressures up to 140 bar (2,030psi).
If the operating conditions must be maintained with very high over-pressure values (80-100 bar/1,160-1,450psi) for prolonged periods of time (over 30 minutes), the heat generated by the friction between the ring and the shaft can result in temperatures of over 300°C (570°F).
This inevitably plays a part in deteriorating the structure of the P450 material used in the seal ring as well as compromising the ground profile on the shaft, thereby reducing its sealing function.
In static conditions, with the shaft immobile, this sealing system can bear overpressures of over 200 bar (2,900psi) without leading to any loss of fluid or deformation of the components (a condition that can be verified on several pieces of machinery, where erroneously, the inlet and outlet pipes are inverted).
Pumps and gear motors are positive displacement components widely used in the design of hydraulic systems. Compact in size, they have a simple construction and are reliable, offering an excellent ratio between cost and performance.
Many applications suit the use of the single-direction motor, which is of a construction similar to the pump, but has very low starting torque, high transmitted torque, a wide range of operating speeds, very high efficiency and can be installed without the use of the drainage pipe.
The use of P450 in the radial seal of single-direction motors enables these parts to be safely used in heavy-duty applications, effectively protecting the sealing system on the shaft and preventing any fluid leakage.
Marzocchi Pompe’s many years of experience with these types of products and its continuous development, design and testing, research into specific materials and sophisticated production techniques, assures the production of high quality components. Working together with jbj Techniques for over 30 years . . . an excellence in engineering!