Bosch Rexroth precision module

Five Key Factors for Selecting Precision Linear Modules

March 3, 2021
Understanding design and performance characteristics can help machine and system designers select the best products to satisfy requirements of the machines they are building.

At a Glance:

  • Semiconductor manufacturing, laser cutting machine tools, electronics assembly and lab automation systems require precision linear modules that need to operate with tight endpoint accuracy and smooth travel with minimal vibration.
  • The module’s housing design and materials of construction are critical factors that can govern long-term accuracy and repeatability.
  • No matter how well designed and engineered, linear modules need proper lubrication throughout their life cycle to maintain precise and stable motion.

Ready-to-install linear modules are used across many different industries to move materials, products and production tooling in a wide variety of machines.

Machine designers have multiple options to choose from when selecting linear modules, based on specific production and performance requirements. But there are some industries and system applications where precise and accurate motion is the most critical requirement.

In particular, applications such as semiconductor manufacturing, laser cutting machine tools, electronics assembly and lab automation systems require precision linear modules that need to operate with extremely tight endpoint accuracy and smooth travel with minimal vibration throughout the motion cycle.

Understanding several key design and performance characteristics that distinguish precision linear modules can help machine and system designers select the best products to satisfy the requirements of the machines they are building.

The Need for Precise Motion

Ultra-precise and delicate motion for laser cutting, automated sampling systems in medical testing equipment or moving semiconductor wafers through a fabrication tool requires extremely stable, nearly vibration-free motion during travel. Reaching the targeted endpoint with the utmost accuracy is the primary goal.

Stable motion is often critical to protect extremely fragile materials from damage or degradation caused by linear travel. Semiconductor wafers are a perfect example: They are extremely delicate, and a finished wafer may contain chips potentially worth millions of dollars, depending on the size.

Each wafer must be transported through hundreds of process steps, and each time it’s moved from one step to another, vibration in the linear module risks damaging the in-process wafer, reducing its final value. The less vibration, the lower the risk.

Endpoint accuracy is equally critical for productivity purposes. If a tray of electronic parts is moving through a high-speed automated assembly process, maximum throughput productivity is achieved when the linear module delivers the tray into the assembly tool with micron-level accuracy.

It’s also important to note that this stable motion and endpoint accuracy must be repeatable through thousands of motion cycles every day. If it takes several milliseconds to fine-tune the positioning of the parts, those milliseconds add up to hours of additional production time, reducing throughput and potentially inflating costs and impacting delivery schedules.

To achieve these goals, be sure to consider the key design, materials, construction and functionality found in high-performance precision linear modules.

Materials of Construction

The module’s housing design and materials of construction are critical factors that can govern long-term accuracy and repeatability.

In the world of linear modules, aluminum or steel is most commonly used to create the housings, or “profiles.” Aluminum housings are commonly used in more standard linear modules since they can be cost-effectively extruded to satisfy a wider range of module size and length.

However, it’s important to consider precision linear modules manufactured with steel machined housings. These housings typically exhibit much less modular elasticity and deviation from the desired travel path, compared to aluminum-based modules (which are also very stable but simply not to the level steel housings can support).

Modular elasticity causes the module to take the shape of the machine frame it’s mounted to. With an extruded aluminum housing, if there is some deviation—like a twist or bow where the module is attached—it can reflect that deviation.

Because precision linear modules have machined steel housings, that kind of deviation is prevented, ensuring very high flatness or straightness of travel. This contributes to vibration reduction, endpoint accuracy and locational repeatability. In addition, look for modules that have a machined reference edge with integrated guide tracks on the housing. Some companies like Bosch Rexroth even allow the user to specify which side has the machine ground reference edge for faster mounting and easy alignment.

Proper Sizing for the Right Application

When very precise operation is required, be sure to select the right size components to handle the load. Axial or torsional loading, for example, may require wider or heavier-duty components than simple radial loading. In addition, for many advanced applications in machine tools, semiconductor production and electronics fabrication, the production systems are relatively small, requiring compact precision modules that can easily fit into tight machine spaces. Many suppliers offer different sizes.

In addition, it’s important to consider other basic linear motion design criteria, such as the environment in which the system is operating, the angle at which the load is mounted, the speed required, travel distance and the required duty cycle. This is known in the industry as LOSTPED (load, orientation, speed, travel, precision, environment and duty cycle).

Motion Components

Precise and stable motion is also a product of the motion components that drive the linear module. For precision linear modules, ball screw drives provide the optimum solution.

Ball screw drives are ultra-efficient in converting rotary motion into linear motion. As mechanical drive elements, they can be installed in X-Y-Z orientations and perform movements with the precision and repeatability needed.

Ball screws with full contact seals offer a unique combination of high rigidity, high precision and respectable speed, making them useful in a wide variety of precision motion applications. Specifically, their ability to handle substantial axial loading often makes them a better choice than linear motors, particularly in metal-, wood- and stone-cutting applications.

Equally important is the design of the linear guides in precision modules. Accuracy in linear guides depends on many factors: the trueness of the rail on which the runner block or bearing travels, the raceways inside the bearing through which the balls or rollers travel and the flatness of the rail mounting surface.

One of the most important areas to assess is the smoothness of ball recirculation inside the runner block as it travels along the rail. Applications at the very high end of the accuracy spectrum can be adversely affected by even minute movement of the balls in the recirculation chamber, or by just a slight pivoting of the rail system about its axis.

Any deflection or clearance at all reduces accuracy, and any roughness in the recirculation of the balls can cause inaccuracy. To address this issue, leading linear module suppliers incorporate guides that optimize the recirculation at key transition points, providing extremely smooth motion consistently as the balls circulate in the bearing raceways.

Precision linear modules that feature both ball screw assemblies and optimized linear guides combined with steel housings provide many of the key characteristics required in high-precision, high-speed automated systems.

Lubrication and Sealing

No matter how well designed and engineered, linear modules need proper lubrication throughout their life cycle to maintain precise and stable motion. One way to ensure that lubrication is effectively integrated into overall system maintenance practices is to select precision modules that make it easier and convenient to lubricate the modules on an ongoing basis.

Most precision modules are available with conventional industrial lubricant for initial greasing. Other modules have choices for more advanced lubrication offerings—for example, to address clean room or electronics industry requirements.

Bosch Rexroth recently upgraded its line of precision modules to include more advanced LSS standard and LSC clean room lubricants. There’s also an option to connect to central lubrication systems using liquid grease. Automated re-lubrication increases operational reliability, ruling out human error when manual lubrication is performed.

Linear modules require lubrication because they have moving parts, but moving parts can generate tiny airborne particles if the module is not properly sealed (lubricants themselves can also disperse into the air). It’s important to assess ahead of time the sealing options provided by precision linear module suppliers, particularly for clean room environments or lab automation systems with sensitive biological samples.

Look for a precision ball screw nut and linear carriage assembly that’s sealed from both sides with ball screw assembly seals. This design significantly reduces the risk of lubricants escaping outside.

Configuration and Technical Support

One final element to consider when selecting precision linear modules is the level of tech support available from the supplier to help machine builders select, specify, configure and order the modules they need.

Choosing a linear module supplier with easy-to-use, step-by-step online sizing and configuration tools can help machine designers quickly configure and order the right modules when needed. Some companies also enable selection and sizing for the mechanics, motor and drive combination with a single tool.

When direct help is required, it also makes sense to work with linear module suppliers with extensive experience in linear motion technologies. These companies provide technical support from linear motion engineering experts via phone, email or real-time online chat tools. In many cases, when machine builders are unsure about specific sizing and performance requirements of their applications, these experts will have solved those kinds of challenges in the past.

For today’s most advanced automation systems, achieving high levels of throughput shouldn’t require sacrificing quality for speed. Especially for industries and applications that need ultra-precise linear motion, vibration-free transport and extremely tight endpoint accuracy, choosing the right precision linear module can play a critical role in manufacturing productivity, efficiency and quality.

Justin Lackey is a product manager, systems at Bosch Rexroth Corporation.

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