If you have ever browsed a parts catalog and noticed letter codes after a bearing model number, you may have wondered what they mean. These suffixes are far from decorative. They carry precise technical meaning and tell engineers and technicians exactly how a bearing is constructed, sealed, and intended to be used. Among the most commonly encountered suffix combinations is "UU," a designation that appears on both radial ball bearings and linear motion bearings alike.
Bearings
Breaking Down Bearing Suffix Codes
Bearing nomenclature follows internationally recognized conventions, most notably those established by ISO, JIS, and individual manufacturers like NSK, SKF, INA, and THK. A bearing model number is essentially a condensed specification sheet. The numeric portion typically describes the bore diameter, the series (light, medium, or heavy duty), and the outer dimensions. The letter suffixes that follow indicate features related to sealing, shielding, clearance, material treatment, or special configurations.
The letter "U" in bearing nomenclature generally refers to a contact rubber seal. A rubber seal, unlike a metal shield, physically touches the inner ring of the bearing during operation. This contact creates a more effective barrier against contaminants like dust, water, and fine particles. It also helps retain the grease packed inside the bearing during manufacture, extending the intervals between lubrication maintenance.
When you see a single "U" in a suffix, the bearing has one rubber seal on one side. When you see "UU," the bearing carries rubber seals on both sides. This double-seal arrangement is a full enclosure, meaning the internal rolling elements and lubricant are protected from the operating environment on both faces of the bearing simultaneously. This configuration is especially valued in dirty, humid, or chemically active environments where one-sided protection would leave the bearing vulnerable.
UU in Radial Ball Bearings
In standard deep groove ball bearings, such as those in the 6000 or 6200 series, a "2RS" suffix is often used as well to describe two rubber seals. The "UU" suffix serves the same fundamental purpose but is more common in certain Japanese manufacturer conventions and in linear motion products. The practical difference between RS and UU is mainly in the lip geometry and contact pressure, which varies by manufacturer and affects the torque drag the seal introduces to the rotating assembly.
Engineers selecting sealed bearings for high-speed applications must factor in this drag. A bearing spinning at thousands of RPM will generate more frictional heat with tight contact seals than with non-contact shields, and the seal material itself, typically nitrile rubber (NBR) or hydrogenated nitrile butadiene rubber (HNBR), has operating temperature limits that must not be exceeded.
UU in Linear Motion Bearings: The LM20UU
Linear bearings represent a different class of motion component entirely. Rather than rotating around an axis, they slide along a shaft, translating rotary power into linear movement. The suffix "UU" on a linear bearing carries a slightly different physical meaning. Here it refers to two contact seals, typically made from felt or rubber compound, fitted at both open ends of the bearing housing. These end seals wipe the shaft clean as the bearing travels, preventing grit from being dragged into the ball recirculation zone.
The LM20UU is one of the most widely specified standard linear ball bearings in the motion control and automation industry. The designation breaks down cleanly: "LM" identifies it as a linear motion bearing in the standard series, "20" indicates a shaft bore diameter of 20 mm, and "UU" confirms the double end-seal configuration. The LM20UU is part of the ISO metric standard for linear bearings, which means components from different manufacturers are largely interchangeable in terms of outer diameter (32 mm), length (42 mm), and mounting interface.
This bearing is found in CNC router gantries, 3D printer linear axes, automated dispensing machines, medical imaging gantries, and semiconductor handling equipment. It performs best on hardened and ground steel shafts, typically 20 mm chrome steel with a surface hardness of HRC 58 to 64 and a surface roughness below Ra 0.4 microns. Running an LM20UU on an unhardened shaft is a common installation error that dramatically shortens service life, as the ball elements will deform the shaft surface under load.
The dynamic load rating of a standard LM20UU is approximately 690 N, with a static load rating of around 1,010 N. These figures assume clean conditions, proper shaft alignment, and adequate lubrication maintained through the grease nipple or felt seal saturation at regular intervals.
The Open-Type Variant: LM20UUop
Some application layouts require the bearing housing to have a lengthwise slot cut into it, allowing the bearing to be mounted on a shaft that passes through a structural wall or carriage plate without threading the bearing onto the shaft end. This is where the LM20UUop becomes relevant. The "op" suffix stands for "open," describing a longitudinal gap in the outer sleeve. The LM20UUop retains the same double contact seals on both ends as its closed counterpart but sacrifices a portion of its load-carrying circumference due to the slot. Engineers typically derate the allowable load by roughly 20 to 30 percent when using open-type bearings, and they position the bearing so the slot faces the direction of minimum load during operation.
SQ LM20UUOP
Why Seal Selection Matters
Choosing the right seal type is not simply about contamination control. It directly affects the bearing's operating life, maintenance schedule, and even the mechanical efficiency of the machine it serves. In environments with water jet exposure or chemical mist, a double-contact rubber seal is non-negotiable. In cleanroom semiconductor environments, however, the concern shifts away from contamination protection and toward outgassing, which requires specially formulated lubricants and seal compounds that do not release volatile organic compounds into the controlled atmosphere.
Understanding what "UU" means is, ultimately, understanding a very small piece of a much larger engineering language. Every digit and every letter in a bearing part number represents a decision made by the designer, a constraint imposed by the application, and a commitment to a specific performance envelope. Reading that language fluently is part of what separates a well-specified machine from one that fails prematurely in the field.
