INTERESTING FACTS FROM A TO Z
If the cylinder wall expands too fast during operation, the piston seal’s self bias cannot adapt to this change accordingly. Liquids leak past and the seal blows by. This constructional fault is not recognizable during test runs and has been one of the factors responsible for the devastating Challenger disaster. Indentations on both sides of the seal can swiftly intercept the expansion of the cylinder.
When starting up a system from standstill, break-away friction can be high. Free sulphur causes contact corrosion in elastomers. High surface pressure results in high adhesion forces and the seal may be considerably deformed. In the case of rotary seals, the seal can be dragged along with the rotary shaft.
Cavitation – micro diesel effect
Air bubbles can cause considerable damage, especially in hydraulic systems. They are mainly caused by air dissolved in oil, which is released at a pressure slightly below the atmospheric pressure (as opposed to water). If hydraulic liquid is dragged out, the pressure within the chamber decreases, air bubbles are released and accumulate at the seals. Within these air bubbles, an ignitable mixture of oil and water can develop. An explosion of this mixture causes local pressure waves with high pressure peaks and an extreme increase in temperature, and leads to a destruction of the seal material. In water, air bubbles damage the seals because of the compression temperatures.
If the hydraulic fluid flows into the clearance, it must be transported back against the flow (otherwise there is leakage). The stronger the drag flow, the stronger the drag pressure.
If a part of the hydraulic or pneumatic appliance is exposed to the environment (for example, the piston rod), foreign matter (dirt, sand, water, ice, etc.) may adhere to the exposed part’s surfaces. These foreign substances must not enter the system, because they can damage seals, guides, valves and pumps. Therefore, even before installing seals, the entire system must be kept free of residue, chips, dirt and other foreign substances.
Leakage when cold starting
When starting hydraulic systems, low temperatures cause viscidity, which increases drag flow pressure. The dragged out liquid is not carried back, and leakage occurs.
If the operating speed is too high, the lubricating film can lift the seal from the sealing surface. For elastomers, a speed of 0.5 m/sec is the maximum.
The operating temperature is made up of the pressure medium’s temperature and of the seal’s frictional heat which, in the worst case, can reach 100°C. The optional media temperature is between 0°C and +50°C. Higher temperatures can result in the destruction of the material, while lower temperatures change the material’s hardness and elasticity.
Pressure peaks, which may result from the method of operation and from imprecise operating mechanisms, are dangerous because they can strongly exceed nominal pressure and thus shorten the lifespan of a seal.
Plastic memory effect:
Plastic materials have the tendency to return to their original shape when heated. (They “remember” their unformed shape).
Attention must be paid to their compatibility with the seal material. Particularly in hydraulic systems in the pharmaceutical industry, seals are exposed to highly aggressive pressure media.
Repair of cylinders
We cooperate with companies which specialize in the construction and repair of cylinders.
The sliding friction of a hydraulic seal is comparatively low while for pneumatic seals, it must be kept low through lubrication. The sliding friction of a seal changes during its lifespan (new or broken in) and strongly depends on the materials used.
Stroke length influences the stress on the seal. If the stroke length is larger than the seal diameter, stress duration increases. At a stroke length of 10 times the seal diameter, a critical limit is reached. If the stroke length is shorter than the seal length, the lubrication film cannot be renewed and a dry run occurs.
After long stoppage times there is the danger of dry runs, and break-away friction is increased.
Stick-Slip (Sliding back)
The Stick-Slip effect prevents a uniform movement of the rod or piston. In case of insufficient lubrication the seal is not evenly supported and thus it can be axially displaced through the movement. As soon as the static friction decreases, the seal snaps back. The so-called sliding back occurs.