Rethinking Engineering - A Holistic Approach to Linear Motion Design

By Al Ng, Director, Engineering – Rails, Guides & Components, Thomson Industries, Inc.

For the best linear motion system designs; avoid canned solutions, develop a robust product development process, use modified or configured parts where practical, select competent and capable suppliers, and use online selection and configuration tools. This approach will improve product performance while reducing time to market and engineering costs.

The best linear motion designs have the right balance of function, performance, durability, and energy consumption among other attributes, to enable the system to outperform the competition. The best way to achieve this goal is to take a 360 degree view of all reasonable alternatives while locking down the few truly critical parameters and keeping other parameters undefined for as long as reasonably possible. For this 360 degree view,

—Maintain a fluid process with multiple options as contingencies.
—Develop a robust product development process marked by milestones and formal design reviews.
—Consider the use of modified or configured parts and assemblies that offer the exact features you need at a cost that is at or close to standard components.
—Find the right development partner that offers a broad portfolio of products and strong engineering expertise and support.
—Take advantage of online selection and configuration tools to design, size, and select your linear motion system.

The net result will be higher product performance, reduced installed cost, and shorter development cycles.

Design habits can be hard to break, but not doing so has costs. For example, specifying a profile rail system for a new design because it is familiar and was used in your last few designs may seem like a way to save time but it could easily result in a less than optimal design.

Consider all your options
Whether to guide, support, locate, or accurately move machinery components and products, linear motion systems deliver low friction, smooth and accurate motion for nearly any moment or normal loading condition. The range of basic building blocks includes: lead screw drives and ball guides, lead screw drives and slide guides, ball screw drives and ball guides, ball screw drives and slide guides, belt drives and ball guides, belt drives and slide guides, and belt drives and wheel guides. Selecting the system that best fits the application is critical but it is only one part of the process of designing a linear system.

Don’t rely solely on previous experience; it can limit creativity because it predisposes you to follow a particular path too early in the design stages. Substantial improvements can often be achieved with a systematic selection process at the stage when requirements are being fully defined. Keep all options in mind while anticipating possible specification changes and scope creep from unforeseen environmental, physical, monetary, or other conditions. It may be helpful to create a matrix/table of product options by features, dimensions, performance specifications, and so on. Pareto them if necessary. This approach ensures that the linear motion technology is selected to match the requirements of the application rather than the other way around.

Specifying, for example, a profile rail system for a new design because it is familiar and was used in your last few designs may seem like a way to save time but it could easily result in a less than optimal design. Profile rail or square rail systems are one of the two major types of linear guides; the other is the round rail system. Profile rail systems are popular because they generally offer higher accuracy, higher rigidity, higher load-life capacity, and are compact. The ball track groove is only slightly larger in radii than that of the balls themselves. The geometry cradles the balls as they infinitesimally flatten under load, slightly expanding the contact area between the balls and the races. As a result, profile-rail bearings are roughly ten times stiffer than a traditional round rail assembly with ball and shaft surfaces that are convex.

It’s important to weigh the pluses and minuses of each type of linear guide. The mounting surfaces for profile rail systems must be precise, which makes them difficult to install. Round rail ball bushing bearing systems, on the other hand, accommodate torsional misalignment caused by inaccuracies in carriage or base machining or machine deflection.

But the mounting surfaces must be precise for profile rail bearings, which makes them difficult to install. Profile rail designs are especially sensitive to flatness errors that can cause binding. Surfaces must be carefully prepared or the parts may need to be shimmed and adjusted during installation.

Round rail ball bushing bearing systems, on the other hand, accommodate torsional misalignment caused by inaccuracies in carriage or base machining or machine deflection with little increase in stress to the bearing components. This self-aligning-in-all-directions design is forgiving of poor parallelism and variations in rail height. These bearings allow for smooth travel when mounted to wider-tolerance prepared surfaces.

So it’s important to weigh the pluses and minuses of each type of linear guide. If high accuracy and high load capacity are critical, then profile rail systems are probably your best choice. If the accuracy and load requirements of this particular application are not that great, then it may make sense to go with a round rail system because their lower sensitivity to alignment makes them more robust.

Robust design process
Such decisions are often critical to product performance. For this reason, gate mechanisms are often used to provide technical and general management with oversight and control over go or no-go decisions. Schedule design reviews at defined points in the process where the product or process design is assessed before the project continues. The review team should consist of members who are knowledgeable about the technology involved but not directly involved in the project in order to provide objective viewpoints. Reviews should not be limited to simply the design of the product but should also address product lifecycle requirements such as product portfolio consolidation or extensions, as well as program requirements such as cost, schedule, and risk.

Consider modified or configured parts and assemblies
In many cases, the importance of the technology selection decisions is matched by the importance of modifications and customizations that more closely match the requirements of the application. The future of mechanical motion control won’t necessarily be marked by major advances in the technologies themselves, but rather by the specification of cost-effective components that precisely match a machine’s performance requirements.

Increasingly, components suppliers offer modified or configured parts and assemblies with the exact features, performance, and form factor you are looking for, and at a price and delivery time that is at or close to the cost of standard components. Another approach involves configuring a standard linear actuator to optimize its performance in a specific application. In some cases, the components supplier can provide field test equipment that measures the loads and stresses on the actuator. The supplier then configures an existing linear actuator to efficiently fit that specific performance profile by modifying its length, mounting options, feedback options, cabling and connectors, operating speeds, and other options.

Technological advances also increase your ability to obtain a configuration that suits your product without investing in a complete custom solution. An example is combining electric actuators with electronic controls to add cutting edge features that improve performance, ergonomics, safety, and cost. In this case, you provide these features by writing software.

This trend towards configured or custom components will help you reach your design goals without the expense of a custom solution. As a result, we see customers increasingly seeking out the expertise of motion control vendors to deliver configured solutions rather than just components to reduce total landed cost and time to market.

Select a supplier with broad capabilities
Linear motion suppliers with a broad range of field proven products and superior technical expertise and support are best positioned to meet these requirements.

“Gate mechanisms” can help technical and general management with oversight and control over go or no-go decisions. Design reviews at defined points in the process, for example, assess the product or process design before a project continues. Reviews should not be limited to the design of the product; they should also address product lifecycle requirements such as product portfolio consolidation or extensions, as well as program requirements such as cost, schedule, and risk.

Online configuration tools
The wide range of standard, modified, or configured parts and assemblies can be designed, sized, and selected through online selection and configuration tools. For example, a new approach to linear systems design lets you configure custom assemblies that are matched specifically to the requirements of your application based on economical and readily available standard components. You enter the key parameters of the application including mounting configuration, positioning requirements, environmental conditions, loading conditions and motion requirements. These requirements are filtered through a comprehensive set of calculations such as linear bearing load/life, ballscrew load/life and ballscrew critical speed. The application then presents a list of products that meet the basic requirements. You can easily evaluate the features, performance, cost and durability of the configurations and pick the one that optimizes the final product. The design tool then provides outputs that include 3D models, pricing, delivery times and ordering information.

These configuration tools reduce the time and cost involved in designing and sourcing a linear system. Additionally, they let you focus scarce engineering resources on core competencies while taking advantage of the linear systems supplier’s extensive experience.

Configuration tools

Step 1: Establish the system orientation. Pick the application’s orientation: inverted, vertical, horizontal side, or horizontal. Then select the mounting configuration – fully supported, end supported or intermittently supported.

Step 2: Enter the positioning requirements and the stroke length, which is defined from hard stop to hard stop. Select whether the positioning requirement will be defined in terms of accuracy, repeatability, or maximum allowable backlash. Then pick a value for the selected choice.

Step 3: Define critical environmental conditions to determine the correct material selection, cover strategy, and lubrication scheme. Select a condition from the following choices: clean, water/chemical spray/fog, impact/press application/vibration, moderate to heavy dust particulate count, high pressure/temperature washdown, water/chemical splash and clean room. Based on the environment selection, the application will recommend linear slide options such as chrome plated ball guide, stainless steel ball guide, Raydent surface ball guide, CR linear bearings, polymer plain bearings, and so on. You can change these options.

Step 4: Enter the load and applied force. The load is the weight that the carrier or saddle supports, including the payload, fixturing, and tooling. Locate the center of gravity of the load with respect to the center of the carriage/saddle by entering x, y and z values. Enter the applied or external force. This process-related force is assumed to be exerted at the center of gravity of the load.

Step 5: Enter the move profile requirements; move distance, move time, and dwell time. The program determines the appropriate acceleration rates for each individual system that meets the requirements of the applications. Select one of the systems in the solution set. The application presents several move profiles. The green move profile is based on maximum acceleration and the red move profile is based on minimum acceleration. Determine a recommended move profile between the maximum and minimum and provide the desired acceleration rate.

Based on the recommended move profile, the application calculates the bearing and drive loads, and ballscrew critical speed. You can also enter the acceleration rate. When you do so and press the update button, the application presents the selected move profile and updates the safety factors based on the new move profile.

Finally, you have the opportunity to select options such as motor mounts, cover type, brake, limit switches, and gearhead. The application presents the total price of the system, and dimensions. You can download a 3D CAD model of the solution in the native format of 20+ major CAD software packages or a neutral file format. You then can view and print the specifications, save the application or request a quote.

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