Hydraulic motors and pumps are used in a variety of environments, that can include dusty and dirty conditions
These environments can be damaging to shaft seals due to dirt particles eventually penetrating the shaft sealing area. This will begin to erode the elastomer, altering the shaft surface finish in the sealing area and seal failure will eventually occur.
Replacing the seal requires removing the pump / motor, disassembling and repairing it, and reinstallation. The result? Significant system downtime increasing operating expense.
Seals are critical because they seal-in the necessary case-flow that is used to lubricate the internal components and carry heat away from the pump or motor but are the ‘Achilles heel’ of a hydraulic system.
Often taken for granted, these seals need protection from undue stress and wear because they can make the difference between smooth functioning, trouble-free operation and failure. Failure costs time and time costs money.
When seals are subjected to small amounts of wear, they may begin to slowly weep. This can be subtle and not easy to detect. When weeping begins, it may be tiny and undetectable. Yet, even a small leak tends to attract dirt and can lead to more wear.
Once significant wear occurs, the pump or motor seal will need replacement. Replacing damaged shaft seals is labourious and costly and may require multiple hydraulic components to be disconnected to get at the seal failure. Once the seal is replaced, multiple components in the system have to be reconnected before the system can be restarted.
The time to protect from seal damage is early in the system design, before problems occur. A good quality seal that can make the difference between long lifetime of a hydraulic system and expensive failure and downtime.
The qualities of the substance Teflon® or polytetrafluroethylene (PTFE), are ideally suited to the use of shaft seals.
» Resistant to many chemicals; This includes ozone, chlorine, acetic acid, ammonia, sulfuric acid and hydrochloric acid. The only chemicals known to affect these coatings are molten alkali metals and highly reactive fluorinating agents.
» Non stick; Very few solid substances will permanently adhere to a Teflon. While tacky materials may show some adhesion, almost all substances release easily.
» Outstanding performance at extreme temperatures; In fact it can temporarily withstand temperatures of 260°C and cryogenic temperatures of -240°C and still havethe same chemical properties. It has an initial melting point of 342°C (+/- 10°C) and a secondary meltingpoint of 327°C (+/- 10°C).
» Low coefficient of friction; Is the ratio of the force required to make two surfaces slide over each other. A low number equals low resistance and smooth operation. This indicates the difficulty in sliding one surface against another. The coefficient of friction is generally in the range of 0.05 to 0.20, depending on the load, sliding speed, and type of Teflon coating used.
» Non wetting; Teflon finishes are both hydrophobic and oleophobic, cleanup is easier and more thorough.
» Weather and UV resistance.
All of these benefits contributed to teflon shaft seals being used in a range of gear pumps and motors available from jbj Techniques Limited.
Marzocchi Pompe’s finite element analysis (FEA) software was used to determine the best characteristics of the components of the new radial seal ring which hasa much stiffer structure than normal rings. The Teflon (P450) material adapts evenly to the texture of the machining on the shaft to guarantee a perfect seal.
The properties of the P450 material make it possible to bear high-pressure loads and temperatures between -30 and +200ºC. To guarantee a long operating life, it was necessary to perform in-depth research on the methods for grinding the area where the sliding of the seal ring’s lip takes place.
This gear pump high pressure, teflon shaft seal can bear over-pressures of over 210 bar without resulting in damage. This radial sealing is particularly useful in single direction hydraulic motors when, in certain conditions of use, high over-pressures will be generated at the motor output.
With bi-directional motors, the area adjacent to the seal ring of the drive-shaft is maintained at atmospheric pressure by the drainage circuit.
In single-direction motors, this area can be directly connected to the output (by running it backwards) 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 a consequent leakage of fluid.
The Teflon (P450) material adapts evenly to the texture of the machining on the shaft to guarantee a perfect seal.
For further details please see www.jbj.co.uk/MarzocchiTeflonSeal.html
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