Non-contact seals are almost always used in high-speed precision applications. Their effectiveness depends, in principle, on the sealing action of the narrow gap between the shaft and housing. Because there is no contact, these seals generate almost no friction and do not, in practice, limit speeds, making them an excellent solution for machine tool applications.
Seal variants range from simple gap-type seals to multi-stage labyrinth seals (fig. 1). Compared to gap-type seals, multi-stage labyrinth seals are considerably more effective as their series of axially and radially intersecting components make it more difficult for contaminants and cutting fluid to enter the bearing.
In highly contaminated environments, a complex labyrinth seal design is often required. Labyrinth seals can have three or more stages to keep lubricant in and contaminants out of the bearing arrangement. The principle of a highly effective labyrinth seal, outlined in (fig. 3), consists of three stages:
- the primary stage
- the secondary stage
- the final stage
This design, with drainage chambers and collecting provisions, is derived from studies done by the Technical University of Stuttgart, Germany.
The primary stage consists of a splash guard (1), a housing cover (2) and the shaft to form a labyrinth. The splash guard uses centrifugal force to direct contaminants away from the cover, while the housing cover prevents contaminants from entering the labyrinth directly. A radial gap (3) between the housing cover and the shaft should be between 0,1 and 0,2 mm.
The secondary stage is designed to collect any fluid that manages to pass the primary barrier and drain it away. Starting with annular groove(s) in the shaft (4), the main design features of this stage include a large drainage chamber (5) and an outlet hole (6). Annular groove(s) deter fluid from travelling along the shaft under non-rotating conditions, causing it to drip into the drainage chamber instead. When the shaft is rotating, fluid is flung from it and collected in the drainage chamber and drained through the outlet hole. Large drainage holes (~ 250 mm2) in the collection area limit the amount of fluid that collects in the chamber.
Features used in the previous stages are incorporated again in the final stage. This section consists of labyrinth rings (7) with radial gaps between 0,2 and 0,3 mm, a fluid retardation chamber (8), a collector (9) to guide the fluid toward the drainage area and an outlet hole (10) with a drainage area of ~ 150 mm2. An additional chamber, collector and a ~ 50 mm2 drainage hole (11) can be incorporated if space permits. A final radial labyrinth gap (12) of ~ 1 mm avoids capillary action.
When designing these types of sealing arrangements, the following should be taken into consideration:
- In order to avoid inward pumping effects, the labyrinth components should progressively decrease in diameter from the outside.
- Machine lead on rotating components can move fluids in either axial direction very effectively depending on the hand of the lead and the direction of rotation. This can, in uni-directional applications, be exploited to reinforce the effectiveness of gap or labyrinth seals if carefully incorporated into the design. Machine lead on rotating components of gap and labyrinth seals should be avoided when the application rotates in both directions or for uni-directional applications where its action would work against the effectiveness of the seal.
- Under severe operating conditions, an air barrier can be created by applying air, under pressure, between the labyrinth gaps or inside the spindle itself. The air flow must however be balanced so that the dominant flow is always outward.
- A sealing system that takes up considerable axial space is favourable, as this enables large drainage areas and collectors to be incorporated into the system. In these cases, however, the spindle is less rigid as a result of the long overhang from the front bearings (and cutting force position).
Contact seals (fig. 4) are generally very reliable. Their effectiveness, however, depends on a number of factors including:
- the seal design the seal material
- the contact pressure
- the surface finish of the seal counterface
- the condition of the seal lip
the presence of lubricant between the seal lip and counterface
Friction between the seal lip and counterface can generate a significant amount of heat at higher speeds (A ≥ 200 000 mm/min). As a result, these seals can only be used in lower speed spindles and/or in applications where the additional heat does not significantly affect spindle performance.