A decade of linear motion evolution

Bob Love, Technical Manager for THK UK, chronicles the evolution of the linear motion guide mechanism and his company's Caged Ball technology.

Ten years after the introduction of THK's innovative Caged Ball technology, the company's linear guides are continuing to deliver excellent levels of efficiency and performance.

Introduced over ten years ago, the technology provided a major breakthrough for a wide variety of industry sectors, significantly extending the service life of linear motion guides by lowering levels of noise, friction and required maintenance.

Exceptional levels of speed, positional accuracy and repeatability became achievable through a technological improvement, developed in response to a number of intrinsic features of earlier linear motion guide devices.

In order to explain why Caged Ball technology was, and still remains revolutionary, it is important to understand the history behind linear motion systems.

Based on the early bush type linear guides of the 1940s profile, linear motion guides have been used in a wide range of positioning and handling applications since their development in the early 1970s.

The very first linear system, known as the linear bush type, comprises a cylindrical shaft, onto which a bush or sleeve is mounted that travels along the length of the shaft.

This linear movement is aided by ball bearings, which circulate around the internal body cavity of the sleeve, rolling against the surface of the shaft as the sleeve moves.

This simple construction can, however, suffer from relative low capacity and wear under heavy loads.

To overcome this, the ball spline shaft was introduced, with longitudinal splines giving greater contact geometry and increasing load capacity.

This simple construction was subsequently refined further by the development of a rectangular profiled rail and carriage or block, with the block moving by means of recirculating balls set in the block and running in two point circular arc contact raceways along the length of the rail.

These earlier bush, spline and profile rail devices are still in use today.

However, although they can be used in a wide range of applications, they do suffer from a number of mechanical limitations including relatively low levels of positional accuracy and repeatability, lower speeds, and limited load bearing capacities.

These limitations are largely the result of the rudimentary design of the re-circulating ball system, which suffers from friction caused by collisions between ball bearings when the carriage body is in motion.

As a result, the ball bearings are subject to greater frictional variation, generated noise and particle emissions and regular maintenance.

Caged Ball technology was originally developed to address these limitations and has since evolved to become the de facto standard in many applications where high levels of precision, speed and low maintenance are required.

The Caged Ball principle is based on proven rotary bearing technology combined with improved contact geometry and a patented cage and ball retention mechanism.

The Caged Ball mechanism consists of a specially developed ball cage, which encases each ball within a lubricated compartment and prevents them from rubbing against each other when in motion.

Since the caged ball bearings move uniformly and are kept at a constant distance apart, they do not collide when they travel around the carriage body and along the rail, thereby eliminating the problems of collision, noise, heat and friction, resulting in quieter performance and lower maintenance requirements.

Perhaps as importantly, this system allows exceptional speeds of up 5m/s with positional accuracy and pinpoint repeatability better than 1um.

The result is smooth movement, low maintenance costs and an extended service life, factually up to 3.2x higher than most standard guides.

Caged Ball technology eliminates collision and metal-to-metal friction which, aided by permanent internal lubrication, allows the ball bearings to last longer and run truer to the rail profile.

Perhaps as importantly, the ball bearings run in precision ground close conformity two point circular arc raceways with a 45-degree contact angle, maximising capacity, reducing differential slip and providing equal load support in radial, reverse and lateral directions, while maintaining positional accuracy at high speeds.

To conclude, the key benefits of THK's Caged Ball technology are that high positional accuracy, speed, load bearing capacity and quiet operation can now be integrated into the latest linear motion guide systems, serving as a virtually maintenance free cost-effective approach to present-day and future linear motion system endeavours.

By comparison with traditional linear motion systems, Caged Ball linear motion guides extend service life and decrease costly downtime considerably by reducing the collision between ball bearings, thereby providing a smoother performance and improved contact between the rail and carriage.

The technology resolves previous mechanical restrictions while providing added benefits such as high positional accuracy with greater repeatability at higher speeds than conventional models.

In the last decade, THK's Caged Ball linear motion guides have made linear motion systems more reliable and more precise than was previously possible.

With THK's Caged Ball becoming ever more prevalent and efficient, the technology seems set to offer further benefits for decades to come.

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