Linear motion for harsh environments: design tips
10 October 2015
Linear motion systems are exposed to a range of extreme industrial environments. Careful specification and selection of motion system components, as well as a thoughtful engineering review can mitigate risks in harsh industrial conditions. Niklas Sjöström reports.
A critical step in designing any mechanical linear motion system is to understand the environmental conditions under which the system will operate. Key design considerations include: temperature, dust and dirt levels, chemical exposure, washdown processes, vibration and shock load, radiation, as well as any other pertinent environmental factors that may be encountered. Make sure to document these key factors prior to material selection. Capture actual data and study fault codes of prior products to ensure that you are working with facts rather than opinions.
Use what application expertise is available from your supply chain to select suitable linear motion components (in terms of materials, coatings, lubricants and so on). Next, develop a robust test validation plan, including both durability and environmental testing, in order to ensure that the materials you have chosen will provide the life and serviceability you expect. Also consider using highly accelerated life testing (HALT), which provides progressively more severe environmental stresses, building to a level significantly beyond what the equipment will see in service. HALT is usually conducted during the development phase to weed out design problems and marginal components.
A new generation of web-based sizing and selection tools simplifies the task of choosing appropriate component materials for load, speed, life, and so on, as well as specific environmental conditions to be met, including: water/chemical spray/fog; moderate to heavy dust-particulate count; high pressure/temperature wash down; water/chemical splash, clean room. The application will recommend linear slide features, such as chrome plating, stainless-steel components, or polymer plain bearings to address the environmental conditions. Let’s now consider these various operating environmental conditions and the appropriate linear motion solution in each case:
‘Clean’ - This environment is characterised as a machine shop setting. You can expect some airborne particles and some degree of humidity to be present; however, personnel would typically function in this environment without any form of protective gear. In this case, standard ball guide, linear bearing and hardware is acceptable; no cover is required and standard grease lubrication is sufficient. Standard aluminium and steel parts are acceptable in these environments as there is little concern for particulates or corrosion.
Moderate to heavy dust particulate count - this environment is characterised by enough airborne particles to require an operator to wear some form of respiratory protection, such as paper mills and factories with large industrial polishing machines. Again, ball guide, linear bearing, hardware and lubrication are all standard, but a cover is advisable. Here, the concern is preventing an ingress of particulates into the bearing, ball guide tracks and drive mechanism. For large particles, a bellows cover or an aluminium shroud covering may be acceptable.
For finer particles, the linear motion system should have a robust sealing system such as a magnetic-strip seal that consists of stainless-steel magnetic bands extending from one end of the channel in the structural support system to the other. The bands are fixed to the end caps and spring-loaded to maintain tension. They run through a cavity in the carriage so the strip is raised off the magnets just ahead of, and behind the carriage as it traverses the system. Another method is to use plastic cover bands, comprising compliant rubber strips that interlock with the base extrusion, like a zip-top freezer bag. Mating tongue-and-groove profiles create a labyrinth seal that keeps out particulates.
Water/chemical splash - this environment represents a condition where product may be exposed to an intermittent splash of fluid. A large printing application, for example, where reservoirs of ink may be changed periodically resulting in occasional/accidental spillage. The contact of fluid with linear guides in this environment may be a result of mishandling product or out of position components but is typically not part of the normal application process.
For these applications, the ball guides and linear bearings must be corrosion resistant. For round rail bearing technology, this would mean either chrome-plated or 440C stainless steel shafting. Linear bearings should have chrome-plated bearing plates and stainless steel balls (but note softer stainless steel ball bearings and shafting typically results in a 30 percent load reduction, which must be considered at the design stage).
For profile rail bearings, a thin-dense chrome plating, such as Duralloy, is recommended. Plain bearings, often referred to as prism-guides, are an alternative to rolling element bearings. The prism guides are an engineered polymer with high resistance to chemical corrosion. For sealing, bellows may be sufficient depending upon the severity of the chemical splash, but a completely sealed unit is ideal.
Water chemical spray/fog - this condition is a step above the previously described environment in terms of severity. Here a spray or fog is part of the process and the linear guides would be directly exposed to this element. This could also be characterised as a condensing environment similar to what is found in a refrigeration application. Coolant misted on machine parts during processing is also part of this class. In this environment scenario, the bearing and guides will be in contact with fluid if no mechanical protection is offered.
For these applications, a completely sealed unit is a must. Because of the severe nature of the chemical spray, it is also highly recommended to avoid rolling element bearings and use prism-guided bearings instead. For the same reason, a ball screw drive system should be avoided when possible in these environments. A polyurethane belt-drive system will be better suited to handle the corrosive chemical environment.
The ball guide or bearing rail and shaft are often available in standard steel, stainless steel, Armoloy plated, or chrome plated. These linear bearing components provide the load and moment load capability of the system. They suit applications such as standard machine shop, moderate to heavy dust particulate count, and impact press type applications with vibration.
High pressure/temperature/wash down - this environment typically describes a food processing application where equipment may be washed down, under pressure, quite frequently. Slides will not only be in contact with fluid but will also see considerable force from the cleaning process.
The washdown environment will cause standard steel parts to rust. Chrome-plated and corrosion resistant bearings and ball guides should be used along with stainless steel hardware. Prism-guides can also be used, but should be avoided in high-temperature applications. The linear unit should be open (no sealing) and have purge fittings or drain holes to remove washdown fluid. For high-temperature applications, an appropriate grease must be used, while all seals and other plastic elements should be removed from the system.
Clean room - here we are generally referring to an ISO Class 3 (sometimes referred to as a Class 1000) clean room environment, although some linear unit product families can be configured for a Class 100 clean room environment. For such environments, the linear slide must generate minimal particulate resulting from its movement. Metal platings are needed to prevent any kind of rusting. There also needs to be removal of any sliding elements from the linear system. This means no seals/wipers in the bearings guides and no cover on the linear unit. Special clean-room rated grease should be used.
Impact/press application/vibration - Shakers and vibrating tables typically associated with sorting equipment fall under this category. Press applications that would impart higher than normal loads on carriages or saddles due to initial impact dynamics are considered part of this class as well.
The use of polymer style bushings (prism guides) handle high shock load applications better than rolling elements. Under such conditions, rolling elements may crack in half or jam the bearing rolling element tracks due to their point-to-point contacts with the bearing surface. Prism guides, however, allow for equal distribution of load along the plain surface, which is ideal for high vibration applications. The use of prism guides can be combined with a cover, if needed, depending upon the particulate count.
Niklas Sjöström is product line manager, systems at Thomson Industries, Inc.
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