Interesting facts from A to Z

Read about interesting facts about seals and gaskets from A to Z.

Working speed

If the speed is too high, the lubricating film can lift the seal off the sealing surface. Velocities of 0.5 m/sec (3.28 ft/s) are the upper limit for elastomers.

Operating pressure

Pressure peaks, which are partly caused by the mode of operation as well as by unclean control processes, are so dangerous because they can reach a multiple of the nominal pressure and thus significantly shorten the service life of a seal.

Print media and substances

Here, care must be taken to ensure compatibility with the sealing material. Seals are exposed to highly aggressive pressure media and substance, especially in hydraulic systems in the pharmaceutical industry.

Foreign body

If a part of the hydraulic or pneumatic device is exposed to the ambient conditions (e.g. piston rod), any adhering foreign substances (dirt, sand, water, ice, etc.) must not be introduced into the system. These foreign substances can damage seals, guides, valves and pumps. For this reason, even before installing seals, care must be taken to ensure that the entire system is free of machining residues, chips, dirt and other foreign particles.

Breakaway friction

After the system has come to a standstill, high breakaway friction may occur during start-up. In the case of elastomers, free sulphur leads to contact corrosion. With a high surface pressure, high adhesion forces occur, and the sealing body can be considerably deformed. In the case of rotor seals, the seal may be entrained by the rotation of the shaft.

Plastic Memory Effect

Plastic materials tend to revert to their unformed state when heated (they "remember" their original shape).

Drag pressure

If the hydraulic fluid flows into the sealing gap, it must be returned against the flow (otherwise there is a leak). The stronger the drag flow, the higher the drag pressure.

Gap Extrusion

If the seal is pressed into the sealing gap by the high pressure or high temperatures, this is referred to as gap extrusion. The seal is severely deformed and damaged.

Cylinder repair

We cooperate with companies that specialize in the construction and repair of cylinders.

Working temperature

This is composed of the heat of the pressure medium and the frictional heat at the seal, which in the unfavourable case can reach 100°C (212°F). The optional media temperature is between 0°C (32°F) and +50°C (122°F). Higher temperatures can lead to the destruction of the material, too low temperatures result in a change in hardness and elasticity.

Blow-By

If the cylinder wall widens very quickly during operation, this cannot compensate for the self-preloading of the piston seal. Liquid penetrates into the resulting gap and the seal blows through. This design error cannot be detected by trial runs and has led, among other things, to the devastating Challenger disaster. The expansion of the cylinder can be quickly absorbed by notches on both sides of the seal.

Sliding friction

The sliding friction of a hydraulic seal is relatively low; for pneumatic seals, it must be kept low through proper lubrication. The sliding friction varies with the operational life of the seal (whether new or worn) and is particularly dependent on the material used.

Stroke length

The stroke length influences the load on the seal. If the stroke length is greater than the seal diameter, the duration of the load increases. With a 10x longer stroke length, a critical range is reached. For stroke lengths shorter than the length of the seal, the sliding film cannot be renewed – dry running occurs.

Cold start leakage

Low temperatures when starting up hydraulic systems increase the drag pressure due to the temperature-related viscous fluid. The entrained liquid is not returned and a leak occurs.

Air separation – micro-diesel effect

Air bubbles cause significant damage, particularly in hydraulic systems. This is mainly due to the air dissolved in the oil, which gets released at a pressure just below atmospheric pressure (unlike in water). When hydraulic fluid leaks, the pressure in the chamber decreases. 

This releases air bubbles that accumulate around the seals. These air bubbles can create an ignitable oil-air mixture, whose explosion results in local pressure waves with high peaks and an extreme increase in temperature, leading to the destruction of the sealant. In water, the compression temperatures cause the air bubbles to damage the seals.

Downtime

For extended periods of idleness, there is a risk of dry running. Additionally, the breakaway friction is increased.

Stick-Slip (Back Sliding)

Stick-slip disrupts the smooth movement of the rod or piston. With an insufficient lubrication film, the seal does not sit evenly and can thus be axially displaced by the movement. When the static friction reduces, the seal snaps back. This leads to what is known as jerk-back movement.