THK Link Balls – RBI and BL Series

THK Link Balls are compact spherical sliding joints designed to connect rods, shafts, cylinders, and other moving components while allowing smooth oscillating movement with minimal clearance. THK offers Link Balls in different configurations to accommodate different mounting arrangements, load directions, thread types, and application requirements. The two main THK Link Ball families are the RBI Series and BL Series. Choosing the correct series depends mainly on the direction of the applied load, mounting configuration, required thread, permissible tilting angle, and operating conditions.

THK RBI Series Link Balls

The THK RBI Series is a straight-type Link Ball designed primarily for applications where the load is applied along the axis of the ball shank. The holder and mounting arrangement are aligned with the ball shank, making the RBI Series suitable for axial push and pull loads. THK's technical documentation identifies the RBI model for axial loading, while the BL model is intended for radial loading. The RBI design also incorporates grease and a protective boot on applicable models to support smooth movement and wear resistance.

THK RBI-D Series

The RBI-D Series is a standard RBI configuration with a threaded connection. It is commonly selected when a compact female-threaded connection is required along the axis of the Link Ball. Typical applications include:

Pneumatic and hydraulic cylinder linkages
Linear actuator connections
Machine mechanisms
Industrial automation equipment
Rod and linkage assemblies
Positioning and adjustment mechanisms
Light-duty suspension or support mechanisms

The RBI-D Series is particularly useful where the connected components need to move through a limited oscillating angle while transmitting axial pushing or pulling forces.

THK RBI-BD Series

The RBI-BD Series provides a different thread configuration from the standard RBI-D models. The BD designation is commonly used where a different thread pitch or connection arrangement is required for the corresponding size. When selecting an RBI-BD model, the complete model number should be checked carefully because thread dimensions and specifications vary by size. RBI-BD Link Balls can be used in applications such as:

Important Considerations for THK RBI Link Balls

1. Load Direction

RBI Link Balls are primarily designed for axial loading. The applied force should be aligned with the intended load direction. THK specifically states that the Link Ball should not be used in a different load direction from the one specified for the model.

2. Push and Pull Loads

Both pushing and pulling loads may occur in linkage applications. The actual load should be compared with the permissible load capacity of the selected model.

3. Permissible Tilting Angle

The Link Ball must remain within its specified permissible tilting angle. Exceeding the allowable angle can damage the holder or protective boot.

4. Thread Compatibility

Always confirm the thread diameter, pitch, and thread configuration before ordering. Similar-looking models can have different thread specifications.

5. Operating Environment

Temperature, dust, water, chemicals, vibration, and other environmental conditions should be considered when selecting the appropriate Link Ball and lubrication arrangement.

THK BL Series Link Balls

The THK BL Series is a perpendicular-type Link Ball designed primarily for applications where the load is applied in the radial direction relative to the ball shank. Unlike the RBI Series, which is designed for axial loading, the BL Series is designed for radial loading. THK's selection documentation identifies BL as the radial-load model and RBI as the axial-load model. The BL Series combines a spherical sliding bearing with an integrated ball shank, providing a compact connection for oscillating mechanisms and moving machine components.

THK BL-D Series

The BL-D Series is a standard threaded BL Link Ball configuration. It is suitable for applications where a threaded ball shank is required for connection with a rod, bracket, or other mechanical component. Typical applications include:

The BL-D Series is especially useful where the Link Ball is installed perpendicular to the primary load direction and radial forces are applied to the joint.

THK BL-BD Series

The BL-BD Series is another threaded configuration within the BL family. Depending on the size, the BD version provides a different thread specification from the corresponding D model. BL-BD models are commonly considered when the application requires a particular thread pitch or connection arrangement. Typical applications include:

When replacing an existing Link Ball, the complete model number should be matched rather than selecting a model based only on the nominal size.

THK BL-DA Series

The BL-DA Series is an aluminum-holder version of the BL family. The lighter holder construction can be useful where reducing component weight is important. Typical applications may include:

Lightweight automation mechanisms
Compact mechanical linkages
Precision equipment
Motion-control mechanisms
Applications where reduced moving mass is important

For BL-DA models, both the mechanical load and the weight-saving requirement should be considered when selecting the appropriate size.

Important Considerations for THK BL Link Balls

1. Radial Load Direction

The BL Series is primarily intended for radial loading. It should not be selected simply because the thread size matches the application. The actual load direction must also be suitable for the BL design.

2. Permissible Tilting Angle

The operating angle of the Link Ball must remain within the specified permissible tilting angle. Excessive angular movement can cause damage to the holder or boot.

3. Thread Type

Check the exact thread diameter and pitch. For example, two models with the same nominal size may use different thread specifications.

4. Load Capacity

The selected BL model should have sufficient load capacity for the actual radial load, including static loads, dynamic loads, vibration, and shock loads where applicable.

5. Installation Alignment

Although Link Balls allow oscillating movement, excessive misalignment or forced installation should be avoided. The connected components should allow the Link Ball to operate within its specified angular range.

THK RBI vs BL Link Balls

The simplest way to distinguish the two main THK Link Ball families is by their intended load direction.

Feature	THK RBI Series	THK BL Series
Basic type	Straight Link Ball	Perpendicular Link Ball
Main load direction	Axial	Radial
Typical connection	Rod / cylinder / axial linkage	Rod / bracket / perpendicular linkage
Main use	Push and pull linkage	Radial-load oscillating linkage
Models	RBI-D, RBI-BD	BL-D, BL-BD, BL-DA
Key selection point	Axial load and thread	Radial load and thread

THK officially identifies BL/RBI as its Link Ball product family and distinguishes the models according to their intended load direction.

How to Select the Correct THK Link Ball

When selecting a THK Link Ball, do not select the model only by thread size. The following information should be checked:

Load Direction

Determine whether the primary load is axial or radial.

Axial load → consider RBI
Radial load → consider BL

Thread Size and Pitch

Confirm the complete thread specification, including diameter and pitch.

Load Capacity

Compare the actual operating load with the rated or permissible load of the selected model.

Tilting Angle

Check the required operating angle and make sure it remains within the permissible range specified for the model.

Installation Space

Check the overall length, holder dimensions, ball diameter, shank dimensions, and available installation space.

Operating Environment

For applications exposed to water, dust, chemicals, high temperatures, vibration, or other demanding conditions, the operating environment should be considered carefully before selecting the model.

THK Link Ball Applications

THK Link Balls are used in a wide range of mechanical and industrial applications where components need to be connected while allowing controlled oscillating movement. Common application areas include:

Industrial Automation

Link Balls can be used in actuator mechanisms, automated machinery, positioning systems, and mechanical linkages.

Pneumatic and Hydraulic Equipment

RBI Link Balls are suitable for many cylinder and rod connection applications where axial push and pull forces are involved.

Machine Tools

BL and RBI Link Balls can be incorporated into compact mechanical mechanisms requiring controlled angular movement and low clearance.

Conveying and Material Handling Equipment

Link Balls can be used in mechanical control and linkage systems where components move relative to each other.

Mechanical Control Systems

Their compact spherical sliding structure makes Link Balls useful for connecting rods, levers, actuators, and other moving components.

THK Link Ball Model Identification

When ordering a THK Link Ball, always provide the complete model number. For example:

THK RBI-D
THK RBI-BD
THK BL-D
THK BL-BD
THK BL-DA

The complete model number is important because thread specifications, dimensions, materials, and other characteristics can vary between different models and sizes.

Why Choose the Correct THK Link Ball Model?

A Link Ball is a relatively small mechanical component, but its load direction and installation geometry directly affect its service performance. Selecting the wrong model can result in excessive load on the holder, abnormal wear, or damage. For this reason, the application should be evaluated based on load direction, load capacity, thread specification, tilting angle, installation dimensions, and operating environment rather than by size alone. If you need help identifying a THK Link Ball model, provide the complete part number, application, quantity, and any available drawing or dimensions. We can help check the model and provide the corresponding product information.