Virtually all gearmotors include rotary shaft seals, most of which eventually leak. Depending on design, application, and intended life, leak-free service life will vary. However, ask maintenance personnel and they will tell you: seal leaks are the most common problem in gearmotors. Evidence of rotary shaft seal failure stains the asphalt of any parking lot. Contacting seals are the most frequently used rotary shaft seals; of those, the most popular are radial lips. An engineered- systems approach to designing gearmotor radial lip sealing improves performance.
Not that simple
A radial lip sealing system is affected by its physically involved components: lip seal, shaft, lubricant, and lubricant delivery system. It is also affected by environmental conditions: temperature, internal pressure, and internal and external contaminants. Installation is another key sealing element.
All these factors can be tuned and refined to optimize different operating conditions. Unfortunately, gearmotor sealing goes frequently under-engineered; lip seals are often expeditiously chosen from catalogs, based only on physical size and cost. By ignoring the rest of the system, this approach results in less-than-optimal seal performance. By identifying opportunities for optimization, seal suppliers, lubricant suppliers, and product designers can work as a design team to engineer sealing systems that exceed user expectations.
Known lip seal operating principles are difficult, if not impossible, to predict analytically because of the number of variables involved. A gearmotor’s lubrication and its interactions with sealing is especially involved. On a properly applied seal, the rotating shaft pumps a separating film of oil into the contact zone between lip and shaft surfaces. Besides reducing lip and surface wear, this cools the contact area; lubrication, in fact, is the main seal-generated-heat removal mechanism.
But oil adheres to the rotating shaft — so why doesn’t it cause oil to leak out of the gearmotor? A properly applied seal will provide a reverse-pumping mechanism, described by the Kuzma-Quian- Muller-Kammuller model. In simple terms, elastic deformation of the lip rubber caused by shaft rotation creates a chevron pattern on lip surface asperities. These vane-like alignments, along with shaft rotation, cause outward and inward oil flow under the seal lip. Inward pumping exceeds outward pumping, and the lubricant remains in the contact band.
The supply of lubricant to seal lips can change dramatically when internal rotating components change directions, so lubrication of the seal during the dynamic operating modes must be assured. It can be properly supplied to the lip by immersion, splash, or lubricant mist environments. However, changes in operating temperatures may also significantly affect the behavior of the lubricant and its ability and availability to lubricate the seal lip; therefore, lubricant supplies should be maintained for all application operating conditions.
Manufacturers should work with seal suppliers to select the most appropriate seal. Similarly, by asking the right questions of gearing suppliers, system designers and users can get superior product performance.
There is no best material for a lip seal. Nitrile Butadiene Rubber (NBR) is the most popular general purpose material. Fluorocarbon (FKM), frequently referred to as Viton, has historically been considered a premium seal material because of its high temperature and material compatibility characteristics. Hydrogenated Nitrile Butadiene Rubber (HNBR) material is now recognized as an excellent overall choice, considering its full spectrum of seal properties. Seal material must be appropriate for application temperatures, pressures, abrasiveness, desired life, and lubricant chemistry.
Lip/shaft contact pressure is controlled by head section geometry, lip material modulus, the lip inside diameter/shaft design interference, and the seal garter-type spring (if employed). General-purpose seals may have higherthan- necessary interference and contact pressure. This pressure is usually specified in ounces per inch of circumferential length. If contact pressure is too light the seal may not follow the dynamic runout of the shaft as it rotates. If pressure is too heavy, wear rates might be excessive. Gearing manufacturers must work closely with a seal supplier to optimize this aspect of the design.
Hydrodynamic pumping aides can be employed in the seal design. One such aide is the helix. The helix is a series of circumferential vanes molded into the seal lip on the airside. Any lubricant that escapes the main contact lip is pumped back under the lip due to the shaft rotation. The helix pumping mechanism can be effective in the early stages of seal life and can provide leak-free performance during break-in. It can also provide protection against leakage due to minor seal or shaft abrasions. This is possible as long as shaft-sliding speeds are not too low.
The wave seal is a variant that utilizes a lip with a two-period semi-sinusoid pattern molded into the design. This design permits oil to slide under the lip in the low angle portion of the circumference, and scrapes oil inward in the steep angle portion of the circumference for increased seal effectiveness.
Shaft surface finish is critical to the conditioning of a seal. If too rough, excessive wear and leakage will occur; too smooth, and lip break-in will not properly occur, also resulting in leakage. A good average finish is 8-18 micro inch AA. Attention must be paid to both average and peak surface asperity heights. Shafts must be plunge-ground after turning operations, to eliminate the resulting invisible lead. If not eliminated, it can pump oil out by the rotating lead thread.
Shaft hardness precludes premature shaft wear, and protects against scratches, nicks, and other handling damage. A hardness of 26 Rockwell C or higher helps prevent the resulting premature leakage.
Environment: gearmotor effects
Long-term seal failure due to hardening and cracking is the most common lip seal failure mechanism. High temperatures accelerate chemical reactions between seal, lubricant, and oil additives. These reactions cause lip hardening, which leads to cracking and ultimately failure. As a guideline, the Arrhenius Rule states that a 20°F reduction in temperature can double the life of a seal operating in the upper end of its material capabilities.
Another factor to consider is internal gearmotor pressure, which can contribute to excessive contact pressure on seal lips. This pressure can increase with temperature in a non-vented gearmotor. If high internal pressure is expected, seals should be designed accordingly.
Internal and external contamination is another threat. Internal contamination is sometimes a result of manufacturing and assembly debris; it also results from operating wear. (Wear debris is common in worm gear reducers.) When contaminants get under the seal lip, they create a direct leak path by either grooving the lip, or lifting it off its shaft. External contamination from dust, dirt, or debris can result in the same problems. By shielding the radial lip seal with a flinger or contacting- face seal, the system can be effectively protected.
The final sealing system element to be considered is installation. Tooling often helps the process. One example: if a seal lip is slid over an unprotected keyway, it will almost always result in seal damage and immediate leakage. Using a tool called a bullet covers the keyway for safe seal mounting onto the shaft seal journal. Tooling that ensures square seal installation is also a good investment to improve installation process reliability. A lip seal must be installed square to the shaft axis, because cocking in relation to the shaft will cause improper lip pumping action, and possibly leakage. All tooling should be kept free of burrs, nicks and contaminants.
Prior to seal installation, the shaft should be checked for damage, and assembly components for contaminants that might lodge under the seal lip. Gearmotor assemblers should be carefully trained to ensure understanding of issues and consequences of improper handling of sealing components and assembly tooling.
Seal system testing
There is no substitute for life testing the sealing system in the actual product in its true operating environment. Unfortunately there is no way to perform accelerated testing of the sealing system. Any change in speed, temperature, or pressure can upset the hydrodynamics and yield invalid results.
This means seal system testing is a long-term process. Valid seal system testing also requires statistically significant sample sizes of the tested product. A test of one or two seal systems is better than none, but it may not ensure long-term reliability across a large-scale production lot. Before investing in a gearing product, it is a good idea to inquire about the degree of seal testing that has been performed by the manufacturer.