It’s no secret that seals play a critical role in ensuring proper function of rotary systems, but designers may not be aware of just how many factors can affect seal performance. The good news is that every factor can be mitigated to a significant degree, greatly reducing the risk of seal failure.
By fully understanding the importance of installation, the potential for contamination, the amount of friction within the system, the mating surfaces involved, the shaft dynamics and the fluid conditions, designers can select a seal that will function for the maximum time possible. We’ll examine each factor in detail.
Installation
A seal’s lip is key to making sure seal wear is low. The right amount of lubricant under the lip keeps friction/wear low without resulting in a leak. Today’s sophisticated seal lips are designed to do just that, but this functionality adds a degree of complication to installation.
First, seal lips (whether rubber or polytetrafluoroethylene [PTFE]) tend to be fragile.
If the geometry of the seal’s installation is not ideal, the seal lip may be damaged or even turned inside out during installation.
Second, the seal lips’ small size means they are relatively easy to install backward, pushing fluid in the wrong direction.
Fortunately, both problems can be solved by paying close attention to the following during installation:
Ensure that seals do not go into the system at an angle (a cocked seal). Ensure that ideal conditions exist during seal installation to avoid damaging the seal lip.
Insert a step in the installation process during which the seal is checked: Is this the right seal lip for this system? Is it facing the correct direction for the media to be sealed? Is it being installed so that the shaft rotation is in agreement with any directional pumping features of the seal?
Contamination
In rotary systems, it’s common to have a seal near a ball bearing or other rotary bearing. Debris in the system usually migrates toward the bearing, causing further debris to come off the bearing and damaging the seal.
Changing the system fluid usually doesn’t fully alleviate this problem because the bearing acts as a deterrent to full flushing of the contaminant. This tends to settle near the seal.
To reduce the chance of seal failure due to contamination, consider the following factors:
Make sure all system elements are fully cleaned before assembly. Pay special attention to small metal debris from the machining process.
Conduct frequent checks on the oil in the system to determine the level of contamination. If the level is high, perform a thorough flushing of the system.
Whenever replacing a bearing in a rotary pump, replace the seal as well. It may have been exposed to contaminants and could be prone to failure.
Friction
Because heat within a rotary system does not dissipate in the same way as in a linear system, both the shaft surface and the area around the seal have a tendency to experience higher temperatures.
This can lead to fluid breakdown, damaged shaft material, and degraded or brittle seal lips.
The solution may lie in changing some or all of the materials involved. In other words, a situation with a significant amount of friction may call for changing from a rubber seal to a higher temperature rubber or PTFE material, hardening the shaft or changing to a higher viscosity fluid.
Designers may also want to consider fluid flow.
More lubricant between the seal and the shaft can reduce the temperature, as can a heat-conductive housing designed to get heat out of the seal area as quickly as possible.
Mating Surfaces
The finishing process on the shaft within a rotary system can be essential to proper function. First, there should be no burrs or sharp edges that could damage the seals during installation. Second, unless precautions are taken, a microscopic screw pattern can be embedded in the shaft finish, creating a micro-pump that inadvertently directs fluid. The solution is to use plunge grinding rather than traditional grinding and ensure that there is no angle to the machining marks.
In addition, surface finish of the shaft should be matched to the seal type and material.
A finish that is too rough will cause the seal to wear quickly, while too fine a finish will not allow lubricant to be retained under the lip, resulting in higher friction.
Shaft Dynamics
Any anomaly within the shaft’s movement can be hazardous to seal function. For example, a less-than-ideal bearing allows the shaft to shift, putting undue wear on the seal. Likewise, if a shaft is not properly aligned when assembled, one section of the seal will be more compressed than the other sections. The compressed section will show high wear while the uncompressed section will be prone to leakage.
Axes that don’t line up can cause a problem as well, since shaft wobble causes one portion of the seal to be compressed with every rotation, resulting in high levels of material fatigue.
The answer lies in the design first and foremost: ensuring that everything in the finished product will line up exactly.
Precise machining and proper positioning of the bearing within the shaft are also important to prevent seals from failing due to uneven compression.
Condition of Fluid
The position of the seal in a rotary system (tucked behind a bearing) means there is a minimal amount of fluid interchange. Fluid being sheared off the seal lip to prevent leaks just compounds the problem. Under ideal conditions, the fluid is thin enough to lubricate the system without becoming so thin that it leaks.
This is a difficult balance, given that the lack of fluid interchange means fluids tend to break down over time. Thus, it’s important to check fluid conditions regularly for a number of factors:
- excessive air in the system, which can cause air bubbles around the seal
- high moisture content in the air around the pump, which can cause water content in the fluid to increase beyond normal values
- oil level (Seal failure due to low oil levels is common.)
When we talk about fluid condition, we come full circle in terms of reasons for seal failure. When fluid conditions aren’t optimal, the bearings may wear prematurely. This causes the bearings to become unaligned with the shaft, which in turn causes debris that can damage the seal.