All About U-Seals

You have probably heard a couple terms out there if you have ever searched for replacement U-Seals; U-cups, Polypaks, Un-Loaded & Loaded, Single Lip, Double Lip and so forth. If you are in need of this particular part, and aren’t familiar with these types of seals, it can be quite confusing. But no need to worry, because in this article we are going to cover the common types and terms for U-Seals. Hopefully after reading this, replacing and working with them will be much easier and allow you a more clear understanding of what to look for.

Let first get started with looking at what an actual U-Seal is used for and then break it down further into the common types. U-Seals are a type of rod or piston cylinder seal (hydraulic/pneumatic) that has a unique “U” shaped profile. The outside is a static lip seal that functions against mating surfaces that have no relative motion between each other. On the other hand, the inside of the U-Seal is a dynamic lip seal that is used for when there is motion between surfaces. Lastly before we move on, let's look at loaded and unloaded.

Loaded or Unloaded?

Loaded U-seals/U-Cups or Polypaks, are loaded with an o-ring or spring, between the two lips of some seals and help increase the sealing force at low pressures while blocking or separating fluid in reciprocating motion applications. The loaded U-Cup is that when you add a "loader or energizer" to a lip seal, you turn it into a squeeze seal. This provides high sealing force at low pressure. As the system pressure increases, the lip loading also increases, automatically compensating for the higher pressure while maintaining a positive seal. With an unloaded seal, the seal is simply not loaded with these components.

Types of U-Seals
Standard U-Seal
Standard- The standard U-seal profile is square in cross section and can be used for both rod and piston applications.The name "standard" means that the height and the cross-section are the same, and the lips are not beveled. Essentially this means the outside of the lip and inside are the same. This design is commonly used to replace existing hydraulic packing and o-rings without changing gland design. Remember this is not to be confused with “standard sealing” which is the way in which it actually seals within the application, not the seal itself.

Deep U-Seal
Deep- The deep o-ring loaded U-seal design is a modified version of the standard's square shape. The deep U-Cup has a height greater than the cross section to increase the stability of higher application. The rectangular configuration gives them greater “down stream” stability while their greater lip interference provides for better low pressure sealing.Generally, you would have to make a modification to the gland for this seal when replacing o-rings, but this design is ideal for replacing packing.

Type B U-Seal
Type B- "B-Type" simply means that the lips are beveled. But just like our previous example, they are also deep. Again, this just means the height is larger than the cross-section.This provides greater film breaking and increased unit loading at the sealing surface. By adding the back bevel on the lips, it aligns the seal interface near the centerline of the o-ring energizer to provide increased sealing force which adds to the low pressure sealing capability while also increasing the high pressure sealing ability. This type of seal is generally found in Rod Cylinders.

Type Square B U-Seal
Square B- The square B offers additional sealing ability due to the back bevel lip design, but still allows you to replace o-rings without changing the groove. They are also designed to provide better low pressure performance and a longer seal life due to its unique film breaking capabilities. 



When replacing U-Seals on cylinders, it is important to remember what to look for. Also keep in mind, that a lot of the part within the various housings that you need through to reach the seals may need to be replaced as well. While your components are already taken apart, take some extra time to analyze what will be the next parts that are showing signs of wear and tear. It is always best to replace everything that you can all at once so it saves you time and money in the long run. 

Remember, we are here to help with any questions that you may have. While we a not certified hydraulic engineers, we will do our best to answer any questions that you may have. Reach out via our contact page, social media, phone, or email us at and make sure to check out our U-seals.


Fluid powerHydraulicsPiston sealsPneumaticsPolypaksRod sealsSealsU-cupU-seals