Lip seals and mechanical face seals--performance criteria.

Argesanu, Veronica ; Jula, Mihaela ; Luchin, Milenco 等

1. INTRODUCTION

Labyrinths, stuffing boxes, lip seals, bushings, spiral-groove seals, and mechanical seals made out of a very large number of materials, are the systems that are commonly used for sealing rotating shafts. The behavior of a seal is determined by the complex interaction of a number of factors. Advantages are usually attained at the price of disadvantages in the order directions. For example if the roughness is constant, an increase of the contact pressure reduces leakage, but the wear and frictional heat increase. As against this, increasing leakage losses can reduce the friction and the heat production, but the effectiveness of the unit as a seal is reduced. Again, a high friction may not only lead to increased wear but also, due a thermal distortion, to considerable leakage losses, or it may cause the seal to break down because of a thermal stress cracks.

Depending on application, sealing rubber should be strong, heat resistant, cold resistant, or resistant to chemical attack. The characteristics must often be combined; some of them are mutually incompatible. Anyhow, all sealing rubber applications require good friction properties: high wear resistance and low coefficient of friction.

2 FRICTION AND WEAR IN DYNAMIC CONTACT SEALS

The performance of seals is characterized by the degree of tightness, service life, power losses, by the extent of damage to the contacting surface in operation, etc. the degree of tightness, wear life [t.sub.w], and performance factor I are the most important characteristics of seal performance.(Argesanu, Madaras, 2003) In addition to the above factors, temperature, whose level is determined by their joints action, also affects the performance of dynamic seals. Whereas temperature has the major influence on the frictional effects in the contact area, the leakage is caused by reduction in the contact area pressure and distortions in the geometry of the rubbing surface due to wear, increased thermal deformations, etc. In some instances, these factors are interdependent. The service conditions of sliding contact seals in machinery, determined by combinations of the above factors, are very diverse. In a face seal (fig2), an axial force pressed a rotating floating ring 5 against a fixed counterface 6. The axial leakage path between the floating ring and the shaft is closed by a static seal such as an O-ring 7.(Mayer, 1987)

[FIGURE 1.a OMITTED]

[FIGURE 1.b OMITTED]

[FIGURE 2 OMITTED]

The static and sliding surface of the traditional stuffing box are effectively interchanged, with the advantage than the geometry if the sliding sealing surface can now be produced more accurately and less expansively and there is no longer any wear on the shaft or shaft sleeve. (Gheorghiu, Argesanu, 1996). To compensate for any lack of alignment of the seal faces and for longitudinal thermal expansion of machine and seal, as well as wear of the seal faces, the face seal must contain at least one flexible member such as diaphragm, bellows, elastometric seal, or springs 1,3.(fig2).(Argesanu, Madaras, 2003) In selecting sliding materials, consideration should be given to operating conditions, ease of manufacture and material costs. The chemical activities as well as the physical and mechanical properties have to be considered. By selecting materials with appropriate thermal conductivity coefficients, by additional cooling, lubrication and load "balancing". The sealing medium also has a considerable influence on the life of a mechanical seal. Mounting seals on elastomeric rings has a very beneficial effect on wear because of damping actions of the elastomer. Often the durability of a seal is determined not by the wear of the seal alone but by the resistance of ageing of any elastomers used. Intermittent operation as well as increases of contact pressure, friction coefficient, sliding speed and temperature will reduce the life. Since the effects of adhesive wear, abrasive wear, corrosive wear and erosive wear, let alone vibration, temperature and material effects, can be cumulative. If it defines the intensity of the power lost by friction in the area of contact as the ratio of the power lost by friction and land sliding (Gheorghiu, Argesanu, 1996):

[P.sub.fr]/[A.sub.al] = ([mu] * [p.sub.d] * v).sub.a] (1)

where: [mu]--friction coefficient in the contact area;

[p.sub.d]--pressure in the area of contact;

v--relative speed.

This may give an appreciation of the value limit of operating of the sealing. If we consider for example [mu] = 0.1 ... 0.3 for PTFE lip seal(fig.1.b.) or [mu] = 0.005 ... 0.1 for the elastomer lip seal(fig.1.a.), the pressures of work of 3MPa sliding speeds of 12m/s: [P.sub.fr]/[A.sub.al] = 3.6 W/[mm.sup.2] for PTFE and [P.sub.fr]/[A.sub.al] = 7.2W/[mm.sup.2] for elastomers.

3 FRICTION CONTACT PROBLEMS BY FEM

The simulation consists in solving the moving equations of the floating ring associated with the interaction between the two rings due to the determination of the functional factors of a face seal. When friction is considered, the tangential displacement in the interface implies energy dissipation. The problem is solved by incremental computation.(Knothe, Wells, 1992) Portions of the structure can have areas of gaps which can open and close or slide in relation to each other. Similarly, boundary conditions can change dooring a nonlinear analysis. The distribution configuration of the equivalent stresses under the form insosurfaces (fig.3) exprimed in [Mpa] that reveals the most solicitated zones of the mechanical face seal. The configuration of those quantities is an expected one with a maximum to the inner radius of the rings of the mechanical seal. The FEM analysis is the only way to reestablish, by mathematical means, the rubbing contact pressure distribution from a face seal interface. This allows the fizico-mechanical and functional influence factors evaluation on the sealing performance of a seal.

4 PERFORMANCE CRITERIA

A comparative situation between the face seals and lip seals based on technological, operational and cost is presented in tab.1. When comparing the values it must be borne in mind that the sealed pressure p1 for lip seals are lower that the corresponding mechanical seals. Despite this, it is evident that on account of smaller leakages of the buffer fluid and sealed product, and greater operational safety and reduced maintenance, mechanical seals are much superior to lip seals. Whereas the initial prices of the lip seals are lower than those of the face seals, the position is reverse upon installation and putting into operation. The labor-intensive maintenance costs are about 20 times higher with leap seals. Despite the more expensive spare parts for mechanical seals their total costs comes lower than for lip seals, on account of the long life of the former.(Mayer, 1987, (Muller, Waschle, 1990)

It also is taken into account that due to labor savings, smaller leakage losses, greater operational safety, and reduced-down time, face seals are even more economical than would appear. By means of a metallic support jacket in the carbon ring which is certainly deformed much more than the tungsten carbide seal under high pressure loads, because of its low modulus of elasticity, the previous very good operating characteristics of conventional thermohydrodynamic seals could be further improved at higher pressure. In seals, with the stabilized seal gap, the hydrodynamics of the circulations grooves are a better design. Seals gap are much less sensitive to pressure changes and have even longer service lives.

5. CONCLUSION

The behavior of a seal is determined by the complex interaction of a number of factors. Advantages are usually attained at the price of disadvantages in the order directions.

If we compare specific losses through friction: in the usual pressure for lip seals "uncharged" or classical "charged" with

[theta] = [F.sub.cr]/[pi] * d (2)

face seals we can notice the net advantage of he lip seals, these having as a plus size and lower costs. However, in the field of higher speed and pressure and for hard working environments, the face seals are still irreplaceable. (fig. 4)

[FIGURE 3 OMITTED]

6. REFERENCES

Argesanu, V; Madaras, L,(2003). Leakage, wear and friction in the mechanical face analyzed by FEM, ROTRIP Galati, Romania

Gheorghiu, N; Argesanu,V,(1996). Comparison between the performances of the lip and face seals, Arad

Knothe, K.; Welles, H, (1992).Finite Element, Introduction for Engineers, Springer-Verlag, Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong, Barcelona, Budapesta

Mayer, E, (1987). Mechanical Seals, Hewnes--Butterworth, London, Boston

Muller, H-K; Waschle, P,(1990). EWDR-A new seal type for pressured shafts, Anbetriebstechnik 29 Nr.10

Tab. 1. Performance parameters of seals

Specification Seal tipe
parameters Face seal Lip seal
/functional
conditions

Materials and High maintenance, Easy to maintain,
technology involves polishing for the sealing
operation the active surfaces edge, E class of
 of rings precision

Space assembly At d = idem
 maximum Minimum

Condition correcting with advance
procession of
axle radial + axial Only radial

can compensate very good medium/good
for irregularities
shape and
position of axle

Axle wear zero existing

Power lost reduced loaded-big
due to friction unloaded-reduced

maintenance expenses reduced

Lifetime expenses comparable

Initial expenses high low

Lubricatiors necessary
and cooling
measures

Type of fluid no restriction limited--based on
 compatibility with
 the lip material