Marzocchi Pompe has developed a radial seal configuration for pump shafts that can tolerate enormous internal over-pressure without damage. This sealing system is particularly useful in single-direction hydraulic motors when, under certain conditions, high over-pressure is generated at the motor output.
With bi-directional motors, the area adjacent to the drive shaft seal ring is maintained at atmospheric pressure by a drainage circuit. In single-direction motors, this area is directly connected to the output so any over-pressure impacts directly on the seal ring, causing the lip to turn over or the ring to be expelled from its seat with consequent leakage of fluid.
The new configuration from Marzocchi Pompe comprises a single-lip, radial sealing ring manufactured from a Teflon composite material together with a reinforcement ring. This external reinforcement ring fully follows the form of the sealing ring, and is sized so as to bear the axial force generated by any internal over-pressure.
Importantly, the motors are supplied with an internal drainage circuit, so they always drain inside with a lower pressure. In combination with the high-performance sealing ring, this internal drainage substantially increases product reliability as well as aiding installation where space is limited as it avoids the need for additional pipework.
The new radial sealing arrangement was subject to exhaustive testing prior to released, but before performing the laboratory tests, the entire system that fastens the ring onto the flange had to be strengthened, so that it could support all the stresses imposed on the ring.
As far as the individual components were concerned, optimum characteristics were determined using computer-based finite element analysis tools. As a result, the new radial seal ring has a much stiffer structure than normal rings, which meant that special assembly procedures had to be adopted.
The Teflon (P450) material chosen for this application conforms evenly to the texture of the machining on the shaft to guarantee a perfect seal, and the internal pressure ensures that the lip of the seal ring is constantly compressed against the shaft. An increase in pressure is accompanied by an increase in friction, with heat being generated in the sliding zone as a result.
Teflon P450 tolerates high loads and high friction over an operating temperature range of between -30 and +200ºC. These properties, plus close attention to the machining of the area where the sliding of the seal ring’s lip takes place, guarantee long service life for this arrangement.
Defining the limits
Validation tests have revealed that the system is able to work on a continuous basis at a constant pressure of 30bar without fluid leakage and with no discernible wear to the ring and the shaft. Tests performed at higher pressures have also demonstrated the possibility of the system functioning for short periods of time (several minutes) with over-pressures of up to 140bar.
However, if over-pressure (80-100bar) persists for 30 minutes or more, the temperature in the area of ring-to-shaft contact can reach 300°C, compromising the integrity of the P450 material and the ground shaft - and thus the sealing function.
Notwithstanding these extremes, pressure peaks of up to 210bar caused by errors of manoeuvre, an incorrect timing of the valves in the hydraulic circuit, or shaft immobility, can be accommodated by this sealing system without danger of fluid loss or permanent deformation of the components.
The robust and reliable characteristics of this product make it particularly suitable for applications that function as sub-systems of machines where the precise circuit characteristics are unknown.
Marzocchi Pompe products are supported exclusively in the UK by JBJ Techniques.