Seals are robust components for closing off the area around rotating shafts. Since sealing zone conditions are complex (involving relatively high pressures and temperatures, shear loads, oxygen access, and changing interactions between seal material and lubricant) selection is not simple. Seal material must be compatible with the lubricant, and seal geometry must be suitable for a range of requirements. For optimum sealing, it also requires an appropriate riding surface on the shaft. Because it is virtually impossible to manufacture surfaces without a spiraling lead, seals must feed back into the sealed chamber any leakage arising from the shaft surface. It's the proper shaft surface that makes this tiny leakage manageable.
Every ground surface has a preferred direction, so any radial shaft seal leakage depends on the direction of rotation. A seal compensates for this by acting as a microscopic pump. When designed improperly, this “micropump” can transport liquids and gases underneath the contact. (For example, a radial shaft seal can continuously pump air from the surrounding environment into a gearbox.) Conversely, when a shaft surface and seal are in optimal condition, the micropump returns microscopic amounts of leakage back into the chamber being sealed. That is how different kinds of disturbances (irregularities in shaft topography, shaft eccentricities, housing offset, skewed installation in the casing, or axial runout, and vibrations) can to some extent be compensated.
But let's return to the shaft's role. Its surface must be relatively free of lead and within prescribed surface roughness parameters. (DIN 3760 or 3761 and ISO 6194 are recommended for ground shafts: Rmax = 6.3 µm, Rz = 1 to 5 µm, Ra = 0.2 to 0.8 µm.) In addition, shafts should withstand abrasive wear; they should be made resistant to any possible internal materials (casting sand, oil residues, metal particles, varnishes) and external (water, dust, and sludge) contamination.
In short, the surface should still maintain the film of lubricant under load (e.g. pressure) and the machining process should be made as economical as possible — increasingly important these days. To achieve the last two requirements, there is a wide range of possible surface machining processes for shafts: grinding, turning, roller burnishing, peening, honing, polishing, and machine milling. We'll now review suitable processes.
The requirement for smoother shaft surfaces has made plunge grinding of sealing shaft surfaces the most common machining method. Decades of experience have shown that plunge grinding reliably produces functional surfaces for radial shaft seals. However, when defined process parameters (such as the rotational speed ratio) go unchecked, the dressing feed, cut depth, and cycle instructions for the grinding disk are not precisely held. Especially important: If the sparking-out time (the time at which sparks stop after allowing the shaft to grind down to specification) is insufficient, premature seal failure results. External influences such as machine vibrations and bearing play can further exert negative influence on surface structures. Avoiding these problems is an art; results of changes in these process parameters are difficult to measure in a laboratory and so cannot be spotted by quality control.
An individual helix or thread (independent of pitch) is not normally harmful to radial shaft seals, due to its small cross sectional area. Instead, problems occur because the ground surface typically has several of these threads. To illustrate, if the ratio between the grinding disk and shaft is 10:1, it can result in a 10-start thread structure on the shaft surface — the pitch of which corresponds precisely to the feed of the dressing tool. If the fluid travelling though these threads (corresponding to the direction of rotation) can exceed the natural pumping capacity of the seal, it leaks.
Note that no ground surface will ever be entirely homogenous; abrasive grit will always press the surface peaks to one side or will tear out whole areas. But the greater this damage on the axial plane's shaft, the lower the resistance to fluid flow — resulting in increased leakage. The factor that best helps avoid leaking is a sufficient grinding sparking-out time: 30 seconds should be regarded as a minimum. If followed, radial shaft seals can compensate for the minor imperfections remaining in the surface texture.
The continuous drive towards reduced shaft texture has led to the neglect of machining processes other than grinding. However, alternative machining methods can return significant cost savings. Because of increased cost pressures, roller burnishing and turning are now being investigated. Compared to grinding, setup cost can be reduced by up to 95%, processing times by up to 40%, and machinery costs by up to 50%. Plus, over the last few years hard turning of shafts has been continuously improved so that now it's incorporated into many production processes.
When a turned surface has a defined technical lead, it lends itself to systems where the shaft rotates predominantly in one direction: motor, gearbox input and output, and even differential input. The helical lead on the shaft surface can thus help the radial shaft seal to pump sealed lubricant back into the system. Tests show that over the widest range of operating conditions, radial shaft seals can operate without problems when used in the appropriate direction of rotation. Some systems manufacturers now use turned counter-surfaces for seals.
In many applications, however, the direction of rotation is unknown, or rotation can take place in both directions. For such applications, seals with a bidirectional pumping feature (or seals complying with DIN 3760) are best.
Roller burnishing promotes a strengthening of the shaft surface. Since this finishing process is often used on shafts (for example, to increase notch impact strength for vibrating loads, especially step-diameter changes) it is convenient to work the seal counter surface at the same time. In addition to straightforward surface strengthening, this process has the advantage of neutralizing the lead in the turned base structure. Due to the high specific pressure on the surface, peaks are pressed down into valleys. This can drastically reduce the surface roughness value, increasing the load bearing proportion of the profile.
That said, liquid gives relatively poor wetting on a surface that is too smooth. Thus, under certain loads it is difficult for a lubricant film to form or to be sustained. (Depending on operating conditions, this can result in thermal overload at the sealing edge of the shaft seal.) For this reason, roller burnished finishes are suitable as rider surfaces for seals, but it is important to adhere to the production parameters. A prerequisite is that the surface roughness values should be Ra = 0.1 to 0.8 µm, Rz = 0.8 to 5 µm, and Rmax less than 7 µm.
Allowable limits with regard to operating conditions have not yet been completely determined — but seals operating in the field with peripheral speeds of 20 m/s at oil sump temperatures to 130°C cause no problems.
Peening of shafts is also used for strengthening many components (for example, turbine blades.) In this process, a surface is blasted with steel, glass, or ceramic beads; depending on the blast energy this causes a certain amount of surface strengthening. An additional benefit of sealing surfaces is that lubricants adhere excellently to the crater-like surface structure and wet them effectively. This enables a permanent exchange of lubricant under the sealing contact.
In fact, frictional torque and power loss of radial shaft seals on peened surfaces is 10 to 30% less than on ground shafts. The sealing edge temperature is correspondingly lower, which in turn increases the service life of seals, especially at high load (where high peripheral speeds and oil-sump temperatures exist.) This effect produces a marked reduction in harmful oil carbonization. And although no increase in hardness can be measured by means of normal hardness measurement methods, localized wear on the shaft in the area of the sealing lip is also significantly decreased.
For peened structures to be used as counter surfaces, process parameters must be defined. Peening shot (the nature and diameter of beads) is important. If axial movement of the shaft is likely to occur, it is advisable to polish the peened surface for a slight rounding of the crater peaks; this reduces wear on the sealing edge of the radial shaft seal. Just as with turned surfaces, the pump action of the seal must be large enough to compensate for the lubrication state (which can actually become too effective in the case of peening) or to transfer back any leakage into the unit being sealed.
The peening process is simple, cost-effective, and can be used to cover-up minor surface defects (up to 50 µm). However, note that here the designer should consult with manufacturers when stipulating peening parameters.
A crisscrossed surface texture is produced by honing, also called super-finishing. The advantage of this surface is that it readily binds itself to lubricants and ensures that adequate lubrication is achieved, even under extreme conditions. Such a structure is achieved by rotating the shaft and translating the tool. This gives rise to a crisscross structure, albeit one that appears neutral at first glance.
Honed or super-finished surfaces are only conditionally suitable for use as counter surfaces. Although lubrication conditions are excellent and the wear on the mating components is low, such structures are nonetheless unsuitable for use as counter surfaces for radial shaft seals. Why? When the surface is smooth it doesn't allow for lubrication under the lip. However, with sealing grease this is not a problem.
Other alternatives to tried-and-tested (but expensive) plunge grinding are available and have been investigated.
Polishing of surfaces. In the past it was quite common to polish counter surfaces for radial shaft seals. For repairs in particular, polishing is still a widespread method for eliminating minor damage or removing dirt. In addition, polishing can repair expensive components that have been incorrectly ground. One caveat: polishing has the same disadvantage as grinding in that it can induce a helical lead on the surface. Where polishing is used as a processing step, the same surface roughness parameters must apply as for grinding.
External reaming. External reaming results in a structure similar to honed surfaces; it is not possible to achieve a reliable seal on these crisscrossed structures.
Plunge turning. A “neutral” surface texture is produced on the shaft surface with plunge turning. This is generally suitable as a surface for seals.
Four-axis machining and quick-point grinding. Only isolated empirical values are available for processes such as four-axis machining and quick-point grinding, so it is not possible to make any statements as to their suitability for finishing sealing surfaces at the present time.
Deep-drawn sheet. Deep-drawn wear sheets are frequently employed for repairs. The sealing area is cleaned and, if necessary, lightly repolished. Next, a deep-drawn sheet is pressed on, providing a new surface for the seal to ride. Depending on the operating conditions, it is possible to seal such surfaces as reliably as ground surfaces. However, the prerequisite is that they should exhibit no damage: free of pores, blow holes, scratches, ridges, or imperfections in the material. To guarantee this, only materials of the correct quality should be used. Typically these materials are relatively soft. How do sheets fight abrasion then? The forming process often induces adequate abrasion resistance.
Have string, will travel
There is no reliable method to measure the exact characteristics of a texture's lead. Nonetheless, representative results can be achieved by means of a widely-used thread method. Although this method doesn't give quantitative measurements of the helical pitch, it is helpful and (with slight variations) is used worldwide. Here's how it works: A special thread is wetted with oil, and laid around the shaft to be checked. A weight on the thread (of about 50 grams) provides consistent wraparound contact with the shaft. When the shaft rotates, the thread begins to move axially if a lead is present. This method has its weaknesses; it has been demonstrated that the thread does not react with very small or large helix angles. Nevertheless, in many cases this simple measurement method has made it possible to demonstrate that a surface structure has had a detrimental effect on the seal.
In the past, efforts to develop an alternative measurement method have failed. The principle that a lead structure could be mathematically described, after measurement of the surface texture, would appear promising. However, measurement and analysis times are still so lengthy that it's financially unviable for production at the moment. If the required hardware and software can be redeveloped, this measurement method would be useful in solving several problems and for understanding the influence of various process parameters on surface quality.