In this article, we’ll look at the fillers employed in PTFE resins.
In its virgin form, PTFE resin isn’t the best sealing material for dynamic shaft applications. Therefore, different fillers are added to achieve the desired results. The most common fillers are fiberglass, graphite, carbon, coke flour and molybdenum, although any filler can be added to virgin PTFE resin as long as the material can withstand maximum sintering temperature of 710-730 degrees F.
In order to develop lip seal products properly from PTFE resins, it is vital that the engineer understands the favorable and unfavorable characteristics of the resin.
Several characteristics and properties of PTFE resin must be considered to realize a reliable product. They are:
Today we’ll continue our look at PTFE rotary seals by focusing on three areas: housing/bore considerations, pressure and shaft velocity and shaft misalignment and runout.
Housing/Bore Considerations
Typical PTFE rotary lip seals are pressed into the bore to assure proper OD sealing and seal retention in the housing. Most seal and housings are made from steel and cast iron. Take care when softer materials – aluminum, bronze, plastic – are used for the housing. Aluminum has a thermal expansion rate almost double that of steel. Metal case designs can lose the required press fit in an aluminum housing when they go through thermal cycles due to the higher rate of thermal expansion of aluminum.
A finish range of 32 to 63 μin Ra (0.8 to 1.6 μin Ra) is recommended for service pressures up to 3 psi (0.20 bar). For thicker fluids such as grease, a 125 μin Ra (3.17 μin Ra) finish would be acceptable with no system pressure.
A lead in chamfer is strongly recommended for all seal housings. The chamfer aligns the seal during installation and helps keep the seal from cocking. Both corners of the chamfer should be free of burrs or sharp edges. For pressurized rotary applications, take additional precautions to ensure the seal isn’t pushed from the housing.
Over the past few weeks, we’ve gone into a lot of detail about how PTFE rotary lip seals work.
Today we’ll offer up a short glossary of some of the terms used when discussing these seals. We'll also break down some of the factors affecting PTFE rotary lip seal design.
Over the past few weeks, we’ve been discussing the basics of PTFE rotary seals. In today’s entry, we’ll take a look at PTFE radial lip seal design principles.
PTFE radial lip seals generally incorporate a uniformly thin element cross section, made to compensate for the high flexural modulus of PTFE, especially in cases of severe shaft run-out. The thin sections also minimize thermal expansion and compressive “creep” and their effects on maintaining a controlled contact pattern on the shaft surface.
Most PTFE seal constructions have the “body” portion of the element clamped between the two metal cases. To maintain proper retention pressure on the element, a thin element keeps compression set and “creep” at a minimum.
In our last blog post, we talked about some of the benefits and uses of PTFE rotary lip seals.
But how do PTFE rotary seals work? In this post, we’ll try to answer that question in more detail.
Rotary shaft seals work by squeezing and maintaining lubricant in a slim layer between the lip and the shaft. Sealing is aided by the hydrodynamic action caused by the rotating shaft, which creates a slight pump action.
PTFE resin was discovered in 1938, but it wasn’t until the 1950s that it gained notice as a possible rotary lip seal material. However, PTFE (polytetrafluoroethylene) seals fell out of favor in the 1950s and 1960s, as they were shown to be unreliable performers in a number of applications.
In more recent decades, there has been significant progress in the areas of PTFE lip seal design and material processing.