Technology and Equipment
By Larry Castleman and Bill Bishop
Seals play a critical role in ensuring properly functioning rotary systems, such as gearboxes, transmissions, pumps, motors, and other rotating equipment. Many factors can affect seal performance, but mitigating common problems can help ensure seals function at their maximum potential.
Below are what the authors’ experience has shown to be six of the most common reasons seals fail in rotary pumps, as well as mitigation that can likely reduce risk.
Misunderstanding proper installation
Modern seals, be they constructed using rubber or polytetrafluoroethylene (PTFE), are sophisticated in design. They are more optimized (i.e., matching the spring with the seal material), they have more features (e.g., hydrodynamic pumping aids), and the range of standard designs is wider, due to the wide range of application conditions. Each seal lip is made to work in conjunction with either self-contained or applied lubrication and has as its end goals low friction and wear, without leaking.
During installation, a seal lip may be damaged or even turned inside out. Also, small seal lip size allows for a high potential the seal could be installed backwards, which would push fluid in the wrong direction.
- Ensure seals do not go into the system at an angle (a cocked seal); and
- Insert a step in the installation process during which the installer can answer: Is this the right seal lip for this system? Is it facing the correct direction to seal the media? Is it being installed so that the shaft rotation is in agreement with any seal directional pumping features?
Rotary systems commonly 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 damage the seal.
Changing the system fluid usually does not completely alleviate this problem because bearings act as a deterrent to fully flushing the contaminant, which tends to settle near the seal.
- Make sure all system elements are fullycleaned before assembly. Conduct frequent checks on system oil to determine contamination levels and thoroughly flush the system if necessary; and
- Whenever replacing a bearing in a rotary pump, replace the seal as well.
Friction within the system
Rotating seals always track on the same contact area and heat within a rotary system does not dissipate similar to a linear system. Linear systems dissipate heat using convection. Rotary seals have to use conduction as the primary means to transfer heat, as the shaft/seal contact is always at the same location, making convection a much smaller element. Both the shaft surface and the area around the seal tend to experience higher temperatures. This can lead to fluid breakdown, damaged shaft material and degraded and/or brittle seal lips.
The solution may lie in changing some or all of the materials involved (e.g., a situation that has 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 temperature, as can a heat-conductive housing designed to get heat out of the seal area as quickly as possible.
Improper mating surfaces
The shaft finishing process within a rotary system is essential to proper function. First, no burrs or sharp edges should remain, which could damage seals during installation. Second, take precautions to ensure an installer does not embed a microscopic screw pattern on the shaft finish, creating a micro-pump – an imperfection on the shaft that directs fluid outside intended parameters.
For shaft integrity, use plunge grinding rather than traditional grinding during shaft surface manufacturing and ensure there is no angle to the machining marks. In addition, the shaft’s surface finish should be matched to both the seal type and its material. Too rough of a finish will cause the seal to wear more quickly than its design parameters indicate, while too fine a finish will not allow the lip underside to retain lubricant, resulting in higher friction.
Any anomaly within the shaft’s movement can be hazardous to seal function (i.e., 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 other sections. The compressed section will show high wear while the uncompressed section will be prone to leakage.
Axes that do not line up can cause shaft wobble as one portion of the seal will compress with every rotation, resulting in high levels of material fatigue.
Precise machining and properly positioning the bearing within the shaft during installation are important to prevent seals from failing due to uneven compression.
Unchecked fluid conditions
Seal position in a rotary system (tucked behind a bearing) means there is a minimal amount of fluid interchange. Fluid being sheared off of the seal lip to prevent leaks compounds the problem.
Under ideal conditions, 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.
Regularly check fluid conditions too include the following:
- 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; and
- The oil level (seal failure due to low oil levels is quite common).
Larry Castleman is director of product development and Bill Bishop is rotary segment manager for Trelleborg Sealing Solutions.