Crossed Roller Bearings used in Robotics

What are Crossed Roller Bearings

Cross Roller Bearings are special mechanical bearings designed to provide high precision and load-carrying capacity in a compact space. These bearings consist of two rings, an inner ring and an outer ring, which contain a series of cylindrical rollers. The rollers are arranged in a crossed pattern with each roller perpendicular to the adjacent roller.

INA Crossed roller bearings SX are bearings for high precision applications, whose main dimensions correspond to the ISO dimension series 18 with very small section height in accordance with DIN 616. They comprise outer rings, inner rings, rolling elements and plastic spacers. The outer ring is split in the circumferential direction and is held together by three sheet metal retaining rings.The cylindrical rollers correspond to DIN 5402 and are in an X arrangement with each other on the raceways. The bearings are very rigid, have high running accuracy and are supplied with normal clearance, low clearance or preload. Bearings with preload have the suffix VSP, while bearings with low clearance have the suffix RL0 . The bearing outer rings are easily fixed to the adjacent construction using clamping rings.

Also available in a corrosion-resistant design

For applications requiring high corrosion protection, the bearings are also available in a corrosion-resistant design with the special coating Corrotect.  Contact Pacific International Bearing for SX compact cross roller bearings for your robotics application. Contact us at [email protected]

Crossed roller bearings

Crossed roller bearings  SX :

• are suitable for robotic applications due to their high running accuracy. SX bearings are perfect for other high precision applications including  machine tools, handling systems, precision mechanical and medical devices and vehicle components
• correspond in their main dimensions to the ISO dimension series 18 with very small section height
• can support axial forces in both directions, radial loads, tilting moments and any combination of loads 
• usually allow designs with two bearing positions to be replaced by one bearing position 
• are very rigid (they can be supplied with normal clearance, clearance‑free or preloaded)
• are suitable for supported and suspended loads
• are always a good choice for a technically and economically leading bearing solution if compact and easy-to-fit rolling bearings with high tilting moment carrying capacity and rigidity, with uniform running free from stick-slip, low rotational resistance as well as high axial and radial runout accuracy are required in only one bearing position.

Bearing design

Crossed roller bearings SX are compact locating bearings with high axial rigidity

Crossed roller bearings SX are bearings for high precision applications, whose main dimensions correspond to the ISO dimension series 18 with very small section height in accordance with DIN 616. They comprise outer rings, inner rings, rolling elements and plastic spacers. The outer ring is split in the circumferential direction and is held together by three sheet metal retaining rings . The cylindrical rollers correspond to DIN 5402 and are in an X arrangement with each other on the raceways. The bearings are very rigid, have high running accuracy and are supplied with normal clearance, low clearance or preload. Bearings with preload have the suffix VSP, while bearings with low clearance have the suffix RL0. The bearing outer rings are easily fixed to the adjacent construction using clamping rings.

The possible circumferential velocity is dependent on the bearing (normal clearance or preloaded) and on the lubrication (grease or oil)

DM = rolling element pitch circle diameter

Load carrying capacity

Suitable for axial loads in both directions, radial loads and tilting moment loads.

Due to the X arrangement of the cylindrical rollers, the bearings can support axial forces in both directions, radial loads, tilting moment loads and any combination of loads by means of a single bearing position. As a result, it is generally possible to reduce conventional bearing arrangements comprising two bearing positions (bearing arrangement with one radial and one axial bearing) to one bearing position This reduces the work required and the costs for the design of the adjacent construction (only one bearing position is processed) and considerably reduces the mounting of the bearings (there is no requirement for the matching of two bearings to each other).

Angular adjustment facility

Crossed roller bearings SX cannot be used for the compensation of misalignments. These bearings are precision bearings for high precision applications. In order to ensure their correct function, it is essential that the specifications for design of the adjacent construction are observed. Skewing of the bearing rings increases the running noise, places increased strain on the plastic spacers, has a negative effect on the running accuracy and a highly detrimental influence on the operating life of the bearings.

Lubrication

Grease or oil lubrication is possible

The bearings are greased as standard but can alternatively be lubricated with oil. The decisive factors in determining the type of lubrication and the requisite lubricant quantity are:

• the size of the bearing
• the design of the bearing environment
• the lubricant feeds
• the operating conditions.

If there is any uncertainty as to whether the lubricant or type of lubrication is suitable for a particular application, please consult an authorized Schaeffler Distributor (Pacific International Bearing Sales Inc) or the lubricant manufacturer respectively.

Grease lubrication

If the bearing is to be lubricated with grease, a high quality lithium soap grease to DIN 51825–KP2N–20 is suitable, for example Arcanol Load150 or Load220.

Influences on the lubrication interval

The lubrication intervals are essentially dependent on:

• the operating conditions
• the environmental influences such as contamination, water, etc.
• the type of bearings.

Precise lubrication intervals can only be determined by means of tests under the specific application conditions. The observation period selected must be sufficiently long and the condition of the grease must be checked at regular intervals.

Grease operating life

If relubrication is not possible, the grease operating life becomes the decisive factor. Based on experience, the guide value for the grease operating life in the majority of applications is higher by a factor of 2 than the guide value for the lubrication interval. At operating temperatures above +70 °C, the lubrication interval and therefore the grease operating life are reduced. In order to ensure operational reliability, the grease operating life should not exceed 3 years.

Oil lubrication

Selection of the oil

A lubricant film which is capable of supporting loads must form in the contact zones between the rolling elements and the raceway. Depending on the operating speed, the oil at operating temperature must have at least the nominal viscosity ν1. The guide value for ν1 is dependent on the mean bearing diameter dM and the speed.

Influence of temperature on viscosity

As the temperature increases, the viscosity of the oil decreases. When selecting the viscosity, the lower operating temperature must also be taken into consideration. With increasing viscosity, the flowability of the lubricant is reduced. As a result, the level of power losses will increase.

Suitable oils

For oil lubrication, suitable oils are type CLP to DIN 1517 or HLP to DIN 51524 of the viscosity classes ISO VG 10 to 100.

With oil lubrication, oil change intervals must be observed

At higher temperatures, aged oil and additives in the oil can impair the operating life of the plastic used for the spacers. Stipulated oil change intervals must therefore be observed.

Sealing

Provide seals in the adjacent construction

Crossed roller bearings SX are not sealed. As a result, sealing of the bearing position must be carried out in the adjacent construction. This must reliably prevent:

• moisture and contaminants from entering the bearing
• the egress of lubricant from the bearing.

Schaeffler seal profiles

Material by the meter for radial and/or axial sealing of the bearing position

For sealing of the bearing position in the adjacent construction, Schaeffler supplies various seal profiles by the metre. These profiles are intended for axial and/or radial sealing and – depending on the seal profile – fulfill a wide range of requirements (for example: under normal requirements for sealing, under heavy contamination, for low frictional torque, if only limited space is available, for sealing of fluids, at low speeds or under swivel operation). In addition to the seal profiles with a radial or axial sealing effect respectively, double direction profiles (with both axial and radial sealing effect) are also available. Mounting drawings can be requested for the individual seal profiles.

The seal profiles are not suitable for applications that require leakage-free operation; this applies not only to oil but also to grease lubrication. If leakage losses are unacceptable, measures such as rotary shaft seals can be used. The area around the bearing seal must be designed such that the seal profiles are not damaged during operation.

Seal profile material

The standard material for the profiles is the synthetic elastomer NBR 70. This material has good resistance to oil and grease as well as good wear resistance. Operating temperature of seal profiles 

Speeds

Limiting speeds in the product tables

Rolling bearings cannot rotate at unspecified high speeds, but are generally restricted by the operating temperature that is permissible in relation to the lubricant and the material of the bearing parts ➤ section. The product tables give the kinematic limiting speeds nG oil and nG grease for the bearings.

The limiting speeds nG oil and nG grease are the kinematically permissible speeds for a bearing and apply to oil and grease lubrication respectively. Even under favourable mounting and operating conditions, these speeds must not be exceeded without prior consultation with Schaeffler.

Noise

The Schaeffler Noise Index (SGI) is not yet available for this bearing type. The data for these bearing series will be introduced and updated in stages.

Temperature range

The operating temperature of the bearings is limited by:

• the dimensional stability of the bearing rings and cylindrical rollers
• the material of the plastic spacers
• the lubricant
• the seal material in the adjacent construction.

Possible operating temperatures of the bearings ➤ Table.

Operating temperature	Crossed roller bearings	Schaeffler seal profiles
	–30 °C to +100 °C	–40 °C to +80 °C

In the event of anticipated temperatures which lie outside the stated values, please contact Pacific International Bearing Sales Inc.

Cages

The rollers are guided by plastic spacers

In the crossed roller bearings SX, the rolling elements are separated from each other and guided not by typical rolling bearing cages but by plastic spacers ➤ Figure. The plastic selected and the design of the running surfaces for the cylindrical rollers give low-friction running of the bearings.

Internal clearance

The crossed roller bearings are available:

• with normal clearance (radial and axial clearance)
• with low clearance (radial clearance/preload)
• with preload VSP (preload min. and max.).

Dimensions, tolerances

Dimension standards

The main dimensions of crossed roller bearings correspond to dimension series 18 in accordance with DIN 616.

Tolerances

The dimensional and running tolerances are based on DIN 620-2 and DIN 620-3 and are in the range P6 and P5.

Suffixes

Suffixes describe the design and features of a bearing in more detail.

The unique feature of crossed roller bearings is the use of spacers between the rollers. These spacers serve to prevent direct contact between the rollers, thereby reducing mutual friction. This design minimizes the torque resistance required for rotation, which greatly improves the efficiency and smoothness of the bearing’s operation. The crossed configuration allows the bearings to support high axial and radial loads simultaneously and to handle tilting moments. This makes crossed roller bearings particularly suitable for applications requiring high rigidity and accuracy, such as rotary tables, robot arms, and precision machinery.

Pacific International Bearing additionally  distributes for IKO, a premier bearing company who designs and manufactures a variety of crossed roller bearings. IKO crossed roller bearings  can handle radial, thrust and moment loads at the same time. IKO CRB bearings feature rollers crossed at right angles between inner and outer rings.

IKO crossed roller bearings are used in machine tools, industrial robots and medical equipment due to their high rotational accuracy.

IKO Roller guide options include a cage or separator as well as a full complement design. IKO full cage and separator styles have a lower coefficient of friction for high-speed rotation.IKO Full complement models handle heavier loads at lower speeds.

IKO offersSealed and open designs . As with most manufacturers IKO Sealed bearings incorporate synthetic rubber seals to protect against dust, dirt and grease.

Crossed Roller Bearings in Robotics

Crossed roller bearings are designed to offer high rigidity and precise rotational accuracy, which is essential for robotic applications that require multi-directional load support. This type of bearing has rollers that alternate crosswise, improving its ability to handle complex loads compared to traditional ball bearings.

Crossed roller bearings have cylindrical rollers arranged at 90 degrees to adjacent rollers in a single row. This design allows the bearing to support axial, radial, and moment loads simultaneously, distributing the load evenly across a larger surface. AISI 52100 chrome steel is a typical material used for these bearings due to its durability and resistance to deformation under heavy loads. Stainless steel types, including AISI 440C, are used for applications that require corrosion resistance.

Crossed roller bearings are graded by their manufacturing precision, which is indicated by P5, P4, and P2 in accordance with ISO 492. These grades reflect the bearings’ dimensional and running accuracy, which is critical in high-precision robotic applications.

Below is a table detailing specific crossed roller bearings used in robotic systems, highlighting their part numbers, load capacities, and critical dimensions:

Each bearing is specifically chosen for its precision, suitable for robotic applications such as rotary tables, positioning stages, and arm joints in automation equipment.

Crossed roller bearings are differentiated by their ability to provide high static and dynamic load ratings. These ratings are essential for selecting bearings capable of enduring the operational stresses found in robotic applications. For instance, the XSU080168 from INA offers a high dynamic load rating suitable for applications where high rotational speeds and precision are necessary.

Engineers must also consider the mounting structure of these bearings. Crossed Roller Bearings typically come with mounting holes pre-drilled in the inner or outer ring to facilitate direct integration into the system. This feature simplifies the mounting process and reduces the potential for installation errors that can affect performance and life.

To ensure optimum performance and longevity, crossed roller bearings require regular maintenance. This includes periodic lubrication to minimize friction and wear. For robotic applications, synthetic greases or specialty oils are recommended due to their superior performance under fluctuating temperatures and loads.

Challenges and Solutions

Friction and heat dissipation

One of the primary concerns in the operation of crossed roller bearings is the management of friction and associated heat dissipation. These issues are critical because the crossed arrangement of the rollers increases the contact area and friction compared to conventional bearings.

Lubrication solutions. For crossed roller bearings, viscosity grades typically range from 20 to 30 cSt at 40°C. Lubricants are chosen based on their ability to maintain film strength at high temperatures and their thermal degradation resistance.

Surface treatments. Techniques such as ion implantation and cryogenic treatment are employed to enhance the surface hardness and roughness, thereby reducing the friction coefficient. Typical surface roughness values after such treatments are improved to Ra 0.05 micrometers or better.

Precision installation and alignment

Proper installation and precise alignment are imperative for the optimal performance of crossed roller bearings. Misalignment can lead to uneven load distribution and increased wear.

Installation accuracy. Installation procedures specify a maximum allowable misalignment of 10 arc seconds to ensure uniform load distribution across the rollers.

Alignment technology. The use of laser alignment tools during installation is recommended to achieve the necessary precision. These tools can help align within tolerances as tight as 5 micrometers radially.

Crossed Roller Bearings in Action

Articulated Robot Arms

In articulated robot arms, crossed roller bearings are primarily used in the joints and rotating interfaces. These bearings support the arm components as they perform complex and multi-axis motions that are common in assembly and welding tasks. The orthogonal arrangement of the rollers in these bearings allows them to support loads in all directions (radial, axial and moment loads) simultaneously, which is critical to maintaining the accuracy and stability of the robot arm during operation.

Operational detail. When an articulated robot arm moves to position a component or perform a task, each joint must rotate smoothly and stop precisely. Crossed Roller Bearings, such as IKO’s series, ensure minimal clearance and high rigidity in these joints. For example, the CRBF3515AT model, with its tight dimensional tolerances and high load capacity, provides the necessary control and stability to allow the robot arm to perform precise movements even under high load conditions.

SCARA Robots

Selective Compliance Assembly Robot Arm (SCARA) robots are designed for high-speed pick-and-place applications. Crossed Roller Bearings in SCARA robots are critical at the pivot points and the base rotary shaft, where high-speed rotation and positional accuracy are required. The bearings facilitate fast and precise motion, minimize rotational resistance and maintain accuracy over long periods of operation.

Operational detail. Since SCARA robots perform repetitive tasks, crossed-roller bearings enable the arm to move quickly and stop abruptly with high precision. Bearings such as THK, known for their low rotational torque and high-speed capability, are often used. The RB5013 model, for example, supports the robot’s need for fast, precise movements without sacrificing stability or increasing wear, thus extending the robot’s service life.

Delta Robots

Delta robots are used for tasks that require very high accuracy and speed, such as sorting or packaging in production lines. These robots use crossed roller bearings in their parallel arms and in the base where the arms join, allowing smooth and precise multidirectional motion. The bearings are designed to handle the dynamic loads that occur during high-speed operations.

Operational detail. The fast, synchronized movements of the delta robot arms place high demands on the bearings’ load-carrying capacity and fatigue resistance. The high precision of crossed roller bearings, such as those in INA’s SX series, ensures that each movement is executed with minimum error and maximum repeatability. In particular, model SX011814 provides the necessary robustness and precision to ensure the high performance and reliability of the delta robot.

Conclusion

For those looking to integrate or upgrade the bearings in their robotic systems, PIB Sales offers a comprehensive online catalog tailored to meet diverse application needs. We invite you to explore our selection to find the perfect bearing solutions for your robotics projects. Should you have any questions or need assistance in selecting the right bearings, please do not hesitate to contact us at [email protected]. We are here to support your engineering needs with expert advice and premium products.