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Reciprocating Hydraulic Seals Used For Water-Based Fluids
A major use of water based fluids is in longwall mining roof supports, where HFA (5 / 95) fluids are generally used. These have the advantage over mineral oil of much greater fire resistance and lower cost and over plain water in that the 5% component provides corrosion protection, and a bactericide, which means that stainless steel components are not required, and that maintenance of the fluid is easier. Their disadvantage is poor lubricity. In recent years there have been major developments in the sealing technology used in roof support applications.
Seals for Roof Support Cylinders
The mining roof support has hydraulic cylinders for two separate purposes. The legs, which are often double telescopic cylinders, support the roof of the mine. The ram is generally a conventional double acting cylinder for positioning purposes. The legs are semi-static when loaded by the roof, and the full bore pressures are governed by a yield valve which will release fluid, allowing the cylinder to slowly converge at pressures between 400 and 500 bar. In the case of a double telescopic cylinder the pressure in the inner stage is trapped, and will be greater than that in the outer stage by the area ratio of the bores. 1000 bar pressures are not uncommon in these instances.
In a converging leg friction and pressure forces on the piston seal are additive, unlike a conventional double acting cylinder where the forces are opposed. This obviously increases the extrusion forces on the piston seal during convergence. Maximum pump pressures, which are fed to both the annuli of the legs for lowering the roof support chock, and to the positioning rams are now around 315 bar. It is fortunate that the operation of the roof support cylinders is intermittent in view of the poor lubricity of the HFA fluids.
Over the years sealing applications have evolved from the traditional rubber and rubber-fabric seal constructions to a design which has a face of hard wearing thermoplastic polyester elastomer in contact with the bore, whereas the older design relies on a nitrile rubber dynamic sealing element.
The initial objective of the development which led to the polyester faced design was a seal which could run on a greater range of bore finishes than the rubber-fabric design.
During the 1980's, the fluids used in roof support equipment were generally mineral oil based emulsions, HFAE. These formed a soapy scum which helped to provide boundary lubrication for the seals and bearings in cylinders, but these tended to block valves and filters. For these reasons cleaner fluids, both micro emulsions and synthetic HFAE's were developed. There has also been a growing requirement for environmentally acceptable fluids to be used in these applications. As such newer fluids did not in many cases provide the lubricity of the older HFAE's which in turn saw problems arise due to the destruction of seals with rubber dynamic elements. As the piston seal in the roof support leg is critical in supporting the roof, the change from the old design to the new has been rapid.
With regard to gland sealing, these are, of course less safety critical, but visible leaking is becoming increasingly unacceptable. Plastic bearings have long been used on the pistons to avoid the possibility of metal-to-metal pick up; however s.g. iron bearings were commonly used on both chrome-plated and phosphor-bronze plated rod finishes. This inevitably led to rod and gland scoring with increasingly arduous duty, this problem has been overcome by the use of fabric-reinforced polyester bearing strips. Dirt entry and corrosion problems have led to improvements in wiper designs with the introduction of an external flap on the wiping lip to prevent the entry of dirt via the outer diameter of the wiper. The gland seal design continues to include a plastic anti-extrusion ring, but is now of a larger section to withstand the increased metal-to-metal gaps which are the result of using bearing strip, the sealing material is now a responsive hydrolysis-resistant thermoplastic polyurethane, energized by a nitrile element.