In hydraulics, seals are “kept alive” by a lubricating film.
Standard scrapers from the catalog are usually made of a wide variety of elastomers, such as NBR, EPDM, or PU. If rods are moved through them, they must be lubricated, as running them dry would lead to stick-slip, noise, high friction, and/or excessive wear. For pistons and rod seals that extend from a “lubricated environment” into a “contaminated” area, it is necessary to wipe contaminants off the piston rod as it retracts. For hydraulic applications, this is generally not a problem, since the hydraulic oil, in turn, lubricates the lip of the rubber seals. (Due to the geometry of the lips, a microfilm usually remains, which is sufficient to prevent direct contact with the dry rod. See an excerpt from the lecture notes of the ➥University of Stuttgart.)
After reading the article, the following conclusion can be drawn: The hydrodynamic adjustment of the lips is, in and of itself, extremely difficult. The angles of the groove rings must be selected appropriately in each case so that the oil film is maintained during both retraction and extension and excessive drag does not occur in either direction.
What to Do If the Engine Runs Dry?
However, even slight changes in lubrication conditions can turn the task into a real nightmare. For example, if no lubricant is permitted or if it is constantly being washed away, the lip comes into direct contact with the mating surface. This can be the case in clean environments such as the food industry, beverage technology, the medical and pharmaceutical sectors, as well as in semiconductor production.
This is when the greatest weakness of rubber scrapers becomes apparent: the high coefficient of friction µ, which causes the driving forces to increase many times over. The result: the rubber lip squeaks, jerks, and wears out. This compromises the sealing function, and the guide may wear out. In the worst-case scenario, the entire system may fail due to excessive play or seizing/jamming.
The Solution: Custom-Made Scrapers Made of UHMW-PE
Plastic scrapers, e.g., Enduflon P1 (UHMW-PE) are vastly superior to rubber scrapers. For one thing, due to their low coefficient of friction, they wipe a piston rod or shaft dry and clean. For another, the material is considered extremely wear-resistant even in its unfilled state. This ensures that even abrasive, very fine, or caked-on dust is reliably scraped away. This requires a mating surface that is sufficiently smooth (<, approx. Rz 1 µm) and hard (>, approx. 58 HRC). A distinctive feature of the design is the ability to adjust for wear. Due to the manufacturing process, the lip tends to shrink in diameter because of its memory effect, thereby remaining airtight throughout its service life. In addition, sliding surfaces can be attached to these scrapers, so that a single (!) component both seals and guides.
The fact that these seals are machined also allows them to be adapted to existing installation spaces, thereby enabling custom solutions.
What has been your experience? I look forward to receiving lots of feedback and comments!
Stefan Bock
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