Selecting mechanical components for duty in harsh environments
13 March 2019
As electromechanical actuators and other mechanical motion components expand in capability for heavy duty applications, so does their resistance to harsh conditions.
Extreme temperatures, high particulate levels, chemical exposure, high-pressure washdowns, vibration and shock, radiation, and electromagnetic interference are among the present threats to reliable motion control. System designers can mitigate these threats when selecting motion control components through careful evaluation of the following five design elements that contribute to robust operation: material selection, coatings, sealing strategy, vibration/shock resistance and maintainability.
Factor One: Material Selection
New alloys and hybrid materials can withstand temperature extremes, exposure to chemicals or abrasive materials, frequent washdown, and other environmental forces. Stainless steel, hard-anodised aluminium and resistant polymers are among the most popularly deployed materials used to ruggedise motion control components.
Poor material selection will result in premature failures, expensive in-field service and repair, and lost productivity. Maximum protection comes when all motion control components – from the actuator housing to the smallest bearing – are selected with worst case operating scenarios in mind.
Factor Two: Coatings
Motion control components used in extremely corrosive environments should also be coated with high-performance materials. This improves operation and reduces downtime, especially in environments where corrosive chemicals, salt spray or submersion will require additional protection. The right coating and material combination can extend component life 200-fold.
Testing chemical resistance
The most widely applied standard for chemical resistance is ISO 15003, which provides design requirements and guidance for the manufacturers of electrical and electronic equipment used in mobile agricultural machinery, forestry machinery, landscaping and gardening machinery. It provides test protocols for specific environmental conditions and defines severity levels for extreme environmental conditions that might occur during typical operations. Testing for chemical resistance, for example, requires the ability to withstand three days of operation amidst the following chemical concentrations:
• Diesel Fuel – 100%
• Hydraulic Oil – 100%
• Brake Oil – 100%
• Ethylene Glycol – 50% aqueous solution
• Urea Nitrogen – saturated solution
• NPK Fertiliser*9 (7.5% each N, P, K) – saturated solution
Other standards applied to corrosion resistance, particularly Diesel Exhaust Fluid (DEF) are DIN 70070-05, AUS 32 and ISO 22241-1.
Factor Three: Sealing strategies
Unless a component is adequately sealed, chemicals and particulates can get into internal component mechanisms, causing damage, build-up and clogging, creating fertile ground for microbial growth. Deploying inadequately sealed motion control technology in harsh environments will require additional cost of enclosures and maintenance, and potentially more frequent maintenance.
Every component should be sealed, including motor mounts. Wipers, seals and gaskets are integral to success. The wiper brushes contaminants from the extension tube during operation and prevents water intrusion. The seals back up the wipers. Gaskets provide sealing between housings, cover tubes, motors and rear mounting components.
One of the best indicators of a system’s sealing effectiveness is International Protection Marking, or IP Code. Its ingress protection (IP) rating is the global standard for measuring protection from the ingress of solid objects and liquids. The IP rating specifies the degree of environmental protection an enclosure provides against foreign materials that could impact performance.
The IP rating is composed of two numbers (Table 1). The first represents resistance to solids and the second to liquids. An IP rating of 66, for example, means protection from particulates the size of dust and from high-pressure water jets from any direction, which is considered the minimum for use in harsh environments today.
Factor Four: Vibration and Shock Resistance
Operating in harsh conditions often requires ability to handle heavy loads or withstand sustained forces such as intense vibration. Constant torque and movement weaken a component over time, cause failure and may often require accommodations by product manufacturers.
Planetary gear designs provide more shock resistance than parallel gearing. Cutting one internal gear directly into the output housing improves stiffness. Leveraging a single-piece housing and ring gear, in this way, eliminates gear slippage inside the housing, offering higher peak torque. Avoiding bushings with rolling elements can also help systems handle constant stress, as can depth/through case hardening of shafting. Proper mating of all components is also critical to shock protection.
Testing shock resistance
Compliance with industry standards can guide in assessing shock protection. Table 2 compares the three most common standards applied to motion control equipment: MIL- S -901, EN60068-2-27 and EN60068-2-32.
Testing vibration resistance
To ensure that safe completion of common vibration profiles, OEMs usually run protocols on multiple components and planes, testing housings, structural bolts, and any included printed circuit assemblies/sensors. They test with the unit not powered or operating and mounted on a vibration machine guided by the power spectral density levels shown in Table 3.
Factor Five: Maintainability
Components that require minimal maintenance improve employee safety and minimise potential for workplace accidents, while reducing costs. Lubrication is a key maintenance driver for motion control systems. Components that pre-lubricate with grease or self-lubricate can greatly reduce or eliminate ongoing maintenance demands. However, products that use oil for lubrication, however, require more maintenance and present greater risk to human safety.
Thomson solutions for harsh environments
Motion control equipment manufacturer Thomson Industries offers a wide range of products designed for operation in harsh and hazardous environments.
Its web-based sizing and selection tools simplify the task of choosing appropriate component materials for load, speed, life and so on, as well as for specific environmental conditions to be met, including water/chemical spray/fog; moderate to heavy dust-particulate count; high pressure/temperature wash down; and water/chemical splash, clean room. The applications recommend linear slide features, such as chrome plating, stainless steel components or polymer plain bearings to address the environmental conditions. Thomson engineers will also design custom components to meet customer requirements.
The right stuff
As long as harsh and hazardous environments require extended presence by humans, many opportunities to improve productivity will remain untapped. Motion control technology can reduce the need for human exposure, while at the same time, enabling performance of operations beyond human capability.
But mechanical components require protection too, and careful consideration of materials, coatings, sealing strategies, vibration and shock resistance, and maintainability can help system designers ensure they will get maximum performance and life out of the systems they intend to deploy in challenging environments.
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