13 Common Questions Regarding Rotating Shaft Seals - Garlock

Covering retainer plates, lip faces, rubber materials and more.

By Karyn Stratton, Sr. Applications Engineer - KLOZURE®  •  Click here to read the original article on the Pumps & Systems website.

With competitive price and timely delivery, NNK sincerely hope to be your supplier and partner.

Rotating shaft seals can be found in virtually every facet of daily lives—household appliances, cars and trucks and across industries. Though often treated as a commodity product, their failure can result in costly downtime and lost productivity. Despite their use in countless applications, many still have basic questions about how to use them. Here are some frequently asked questions.

1. What is a retainer plate (cover plate), and why is it required with an all-rubber split seal?

A cover plate is a piece of metal (or other hard material) installed after the oil seal has been placed into the housing bore. The cover plate pushes on the back side of the oil seal and compresses it into the housing. All-rubber split seals require cover plates because this style does not have an outer reinforcement to help retain the seal, so it needs compression inside the housing to ensure the best possible sealing capability.

2. Which direction should the lip face when installing my oil seal?

A single lip seal is unidirectional and will only seal in one direction. The primary sealing lip should always be facing or pointing toward the media that needs to be sealed. The media being sealed is usually a grease or oil lubrication, but it could also be a chemical media that needs to stay inside of a tank. If the seal is to function as an excluder and keep out contamination, the seal should face out towards the contaminant, which could be dirt and dust or chemical washdown. Sometimes seals will either have a secondary lip or dual opposed lips to keep dirt or contamination away from the bearing system and/or the media being sealed.

3. Which rubber material should I select for my oil seal?

Proper material selection is essential for good sealing performance. One of the factors that drives elastomer selection is the application temperature. A common rubber used in many sealing applications with mild operating conditions is nitrile (NBR), which is generally rated for use up to 200 F (continuous). Nitriles offer low cost and good wear resistance but will harden and/or crack if used at elevated application temperatures. At temperatures higher than 200 F, some choose carboxylated or hydrogenated nitrile (HNBR), which are generally rated for use up to 300 F (continuous). Another type of elastomer used at higher temperatures would be a fluoroelastomer (FKM), rated for use up to 400 F (continuous). However, in exchange for the higher temperature capability, the cost of these materials will be higher than the standard nitrile.

Aside from temperature considerations, some applications contain chemical media that might require a more compatible material like fluoroelastomer (FKM) or even polytetrafluoroethylene (PTFE), which can be impervious to many chemicals. It is important to note that PTFE lip material is closer to a plastic than an elastomer and will have different application considerations.

4. Which lip seal material(s) should I use if my application requires FDA compliance?

The food and beverage industry often requires FDA compliant seals. This means that the lip and case materials must meet FDA standards for sanitation. Some available lip materials that are FDA compliant include FDA Silicone, FDA Compliant FKM, and some PTFE formulations, to name a few. If the seal has a metal case, FDA compliance requires the use of 316SS. Additionally, much of the equipment in food and beverage applications needs cleaning or steaming when in use. The seals must withstand the cleaning chemicals and the high temperatures associated with these processes.

5. Which lip seal material or design might be recommended for an application with harsh chemicals?

Depending on the application, some applications exposed to chemical media might require a PTFE lip seal, which will be compatible with the widest range of chemicals. In addition, some PTFE lip seal designs can withstand high pressures and dry running conditions that standard elastomer seals cannot. PTFE seals can be found both with and without a metal case, and can be installed in the same groove as a traditional oil seal.

6. Should I select a seal with a metal case or rubber outer diameter (OD)?

Seals with a metal case generally have the most OD interference, or ability to remain inside the housing. The trade-off for this extremely tight fit is that housing damage might occur when a metal case seal is installed. Some users prefer a rubber OD seal because it will not scratch or damage the housing during the installation process. Changing to a rubber OD seal might also do a better job statically sealing previous housing damage caused by a metal-cased seal. Another consideration is that standard steel metal cases might oxidize or rust over time. This would not be a concern with an all-rubber or metal-encapsulated rubber OD seal.

7. I have removed my leaking oil seal from the equipment. How do I know what size oil seal I need?

When replacing a failed oil seal, do not measure the used seal, as it will give erroneous measurements. Along with the fact that different manufacturers use different design interferences, used and worn-down oil seals often suffer damage during removal. Manufacturers design oil seals to fit the user’s equipment, so all a user needs to do is provide the seal manufacturer with the equipment’s shaft diameter, bore diameter and bore depth. Also, check
the markings on the seal, as some oil seals display the equipment dimensions that it requires.

8. How are oil seals measured?

Under most circumstances, users will not need to measure the oil seal. To size the seal correctly, users only need to know the shaft diameter, bore diameter and bore depth of the equipment.
Users might need to measure the seal, though, if they are trying to verify its manufactured size. The most important dimensions on an oil seal are the inside diameter (ID), outside diameter (OD) and width (height). The ID of the seal is not measured directly, as it is difficult to do without stretching or altering the lip. The OD of the seal measures most accurately with a special piece of equipment called a pi tape. The pi tape can consider any out-of-round condition with the seal that a pair of calipers might not. The seal’s width (or height) measures best with a pair of calipers, taking a few measurements around the whole diameter.

9. Does oil seal thickness matter?

As long as the seal correctly fits the shaft and the bore diameters, there is not any sealing advantage to different seal widths. If the seal is larger in width than the actual bore depth, the only potential issue might be interference with the surrounding equipment. Conversely, if the seal width is smaller than the actual bore depth, this is usually not a problem unless the seal is an all-rubber style being compressed by a retainer or cover plate. In this case, the best practice is to take up the extra space inside the bore housing with gasket spacers to ensure correct compression on the seal by the cover plate.

10. What is the purpose of the spring used in oil seals?

The spring provides loading around the sealing circumference and keeps the sealing lip tight against the shaft surface. Some seal designs use a garter spring design, which applies pressure everywhere around the shaft. Other seal styles employ a finger spring design, which provides an independent finger action only in areas where needed. Spring dumping can happen to garter springs, as they typically rest inside a molded groove disconnected from the sealing lip. If the spring dislodges during installation, for example, it can enter the equipment at startup, often causing serious damage to expensive bearings and other equipment. Finger spring designs, however, are usually either molded in or mechanically connected to the seal so the spring will not fall off or damage the equipment.

11. How long does an oil seal last?

Seal life is difficult to determine on equipment in the field because operating conditions are unknown, and they vary considerably. Furthermore, the applications are usually not ideal (defined as clean lab-type environments, fully lubricated, low speed, ambient temps, no pressure, no misalignment or runout, no exposure to chemicals or solids, correctly sized components, etc.), and the reality is that these factors change seal life considerably.    

12. I replaced my oil seal and found the shaft grooved underneath the lip. What causes this, and what should I do?

When grooving occurs at the shaft, this is typically a result of shaft hardness and/or finish not meeting industrial recommendations of a minimum 30 Rockwell C and 10 to 20 microinches Ra. The lip materials themselves will not cause shaft grooving, however, if there is dirt or contamination in the application. It might be possible for some particles to stick underneath the lip and groove of the shaft if it is too soft. It is not recommended to install a new oil seal over shaft grooving, as this could likely result in leakage. Cover any shaft grooving with a shaft sleeve, or offset the new sealing lip to contact a surface free of grooves or scratches.

13. Should I glue the ends of my split seal back together during installation?

If the split seal needs a garter spring, there will generally be a hook or screw attachment that will pull the two split ends together. Some other split seal designs, with a finger spring, hold tightly together by the interference (or press fit) between the OD of the seal and the machined bore housing. In either case, it is generally not recommended to glue the split ends of the seals back together, as it is difficult to get the lip alignment correct. Also, the chemical compatibility of the adhesives and lip elastomer requires testing.

Anyone in the maintenance field who has repaired a pump or gearbox knows the one component that always seems to get replaced during a rebuild is the lip seal. It usually is damaged when being extracted or during disassembly. Perhaps the lip seal is the reason the piece of equipment was pulled from service due to leakage. Regardless, the fact remains that lip seals are vital machine components. They keep oil or grease in and help keep contaminants out. Lip seals seem to be on almost every piece of plant equipment, so why not take the time to learn how to correctly select and install them.

Purpose of Lip Seals

The primary purpose of a lip seal is to exclude contaminants while retaining lubricants. By nature, lip seals function by maintaining friction. They can be used in a variety of applications from slow-moving equipment to high-speed rotation and in temperatures from below freezing to more than 500 degrees F.

To be effective, a lip seal must maintain proper contact with its rotating counterpart. This will be influenced by proper selection, installation and care of the seal once it is installed. I’ve often seen a new lip seal start to leak as soon as it was put into service. This typically is due to improper installation. Other seals will leak initially but then stop leaking once the seal material has seated to the shaft.

Selecting a Lip Seal

Maintaining functional lip seals begins with the selection process. When choosing a material, you must consider the operating temperature, the lubricant in use and the application. The most common lip seal material is nitrile (Buna-N). This material functions well at temperatures of minus 40 degrees F up to 275 degrees F. Nitrile lip seals cover most applications in industry from new equipment to replacement seals. They have excellent resistance to petroleum oils, water and hydraulic oils, but what really sets these seals apart is their low cost.

Want more information on high speed rotary shaft seals? Feel free to contact us.

Another affordable option is Viton. Its temperature range is minus 40 degrees F to 400 degrees F, depending on the specific compound. Viton seals provide good resistance to petroleum oils and can be employed with gasoline and transmission fluid.

Other seal materials that can be used with petroleum oils include Aflas, Simiriz, carboxilated nitrile, fluorosilicone, highly saturated nitrile (HSN), polyurethane, polyacrylate, FEP and silicone. All of these materials have specific applications and precise temperature ranges. Be sure to take into account the process and environment before selecting a seal material or making a switch, as the correct material can prevent a costly failure.

Lip Seal Construction

Once the seal material has been selected, the next step is to consider the seal’s construction. In the past, a simple lip seal consisted of a leather strap on a wheel axle. Today’s lip seals have multiple parts that affect how the seal performs. There are various contacting patterns as well as non-spring and spring-loaded seals. A non-spring seal generally will be less expensive and be able to retain viscous materials like grease at slow shaft speeds. Typical applications include conveyors, vehicle wheels and greased components. Spring-loaded seals are typically used with oils and can be found on a wide range of equipment.

Installation

After the seal material and construction have been chosen, the lip seal must be installed correctly for it to perform. There are a number of products on the market dedicated to this single task. Most look like a socket set that will position the seal squarely into its bore. These tools can work well if selected carefully, but most off-the-shelf versions aren’t as effective, especially when a shaft is already installed.

For these instances, I prefer to use a pipe that is large enough to slip over the shaft and makes good contact with the lip seal’s outer shell. If you can find something that catches the outer casing, you can prevent damaging the inner metal ring that attaches to the lip seal material. Just be sure to set the seal in straight and at the correct depth. Failure to properly set the seal perpendicular to the shaft can cause immediate leakage.

If you have a used shaft, it likely has developed a wear ring where the old lip seal was placed. You should never set the contact patch to a previous contact point. If this is unavoidable, there are products that you can slip over the shaft to help with the damaged surfaces. This is usually quicker and more cost effective than replacing a shaft. Keep in mind that the lip seal will need to be resized for the added sleeve.

When installing lip seals, make certain the job is done correctly. I’ve seen guys install seals with a punch so they wouldn’t have to take the extra time to locate the right tool. One wrong thump from a hammer can tear sealing material, puncture the seal casing or push the seal through the housing.

Always take time when installing lip seals, and lubricate the shaft and seal adequately to prevent tearing or sticking. Also, verify that the lip seal is the correct size. There must be an interference fit in the bore and on the shaft. Not having the proper size can result in the seal rotating on the shaft or becoming dislodged from the equipment.

Maintaining Lip Seals

To help your lip seals stay as healthy as possible, you must keep your oil clean, cool and dry. Any contamination in the oil will get into the contact patch and deteriorate the shaft and elastomer. Likewise, the hotter the oil becomes, the more wear and tear will be exerted on the seal. Lip seals should also be kept as clean as possible. Painting over a seal or having dirt built up around it can create excessive heat and quickly degrade the elastomer.

Failure Causes

If you’ve ever pulled out a lip seal and seen a groove cut into your shaft, this is probably due to particle contamination. Without good breathers, all the dust and dirt that gets into your equipment can destroy not only your bearings and gears but also your shafts and lip seals. Of course, it is always better to exclude contaminants than to try to remove them. Too tight of a fit between the lip seal and the shaft can also create a groove.

Excessive temperature is the leading cause of seal failures. As temperatures rise, the lubricating film becomes thinner, resulting in dry running conditions. Elevated temperatures can also lead to the elastomer cracking or blistering. Nitrile seal life decreases by a factor of two for every increase in temperature of 57 degrees F.

The oil level can be another factor in a lip seal’s lifespan if it is too low. In this situation, the seal will eventually become hard and not be able to follow the shaft, resulting in leakage.

Cold temperatures can cause seal embrittlement. Selecting the proper lubricant and seal material can help in these colder environments.

Seals may also fail due to shaft runout. This can be caused by misalignment, shaft imbalance and manufacturing inaccuracies, to name a few. Different elastomers will tolerate varying amounts of runout. Adding a garter spring will help with any runout that can be measured.

Excessive pressure is another potential cause of lip seal failure. If you’ve ever walked past a pump or gearbox and noticed oil leaking out the seal, the sump has somehow overpressurized and leaked to the point of least resistance. This can be attributed to clogged breathers or non-vented sumps. Applications with higher pressures should employ a special seal design.

How to Inspect a Lip Seal

Among the things to look for when inspecting lip seals include deteriorated or cracked elastomer. This is a telltale sign that heat is an issue. Also, verify that the lip seal is still in place. I’ve seen a few pumps that had the wrong seal installed. Upon startup, the vibration and movement caused the seal to come out of the bore and spin on the shaft.

Any oil leaks around the seal should be a red flag to investigate further. Worn seals can cause leakage, plugged breathers or damaged bearings that allow radial motion.

When analyzing any lip seal failure, look at the seal, shaft and bore. While inspecting the shaft, you typically will see a contact or wear zone where the lip seal was riding. This will appear as a black scuffmark where the elastomer has transferred to the shaft.

Remember, in order to keep your lip seals in good operating order, you must maintain the sump. Cover all seals before painting, keep the oil at the proper level, make sure oil coolers are functioning correctly, and select the right seal design and material. If you are proactive in your approach to rebuilding and installing equipment, you can give your lip seals and machinery a fighting chance at survival.

If you want to learn more, please visit our website Oil Seal Kit.