Design by Objective: Rough neighborhoods

Feb. 1, 2004
What's considered an "extreme" environment today? In what sorts of applications are extreme environments most likely to appear, and why

What should engineers know about component interactions, good or bad, and how they play into extreme environments?

John/Exlar: Extreme environments for motors and actuators are washdown environments, from various degrees of soaking to total submersion, including caustic and non-caustic chemicals, high-temperature areas, vacuum-duty settings, explosionduty environs, and other places contaminated with abrasives, corrosive chemicals, and other materials.
Patrick/Bosch Rexroth:
The textbook definition of extreme, used by engineers, has historically been related to issues such as contamination, temperatures, and invasive media in the environment, such as radiation. But with the increasing demand for productivity and efficiency, the definition has come to include additional factors such as speed, acceleration, lubrication life expectancy, and load carrying capacity.
Karl/Bosch Rexroth: Things to watch out for:
• Chemical composition of liquids can affect seals.
• Temperature changes may cause condensation and vacuum effects.
• Equipment may need to be derated or resized for higher ambient temperatures.
• Installation of exposed equipment must be easy to access if regular maintenance is required.
Patrick/Bosch Rexroth:
Motion system components interact extensively in virtually all applications. Guide rails affect the ballscrew, and vice versa, in positioning control, speed, and life expectancy. Motors and encoders rely on motion components to execute the accuracy needed. The engineer must be aware of the potential weakest link in the equation and design so that it is as close in performance as possible to the rest of the system.

What sorts of applications are likely to cause extreme environments?

John/Exlar: Many food-processing environments are considered extreme due to their washdown requirements; cleaning chemicals often sprayed at high pressures can deteriorate many metals, while drastic temperature changes (as in the case of a cold food process washed down with steam) can also damage motion systems.
• Undersea applications involve exposure to salt water as well as the obvious submerging. Actually, we manufacture one servomotor-based integrated actuator that operates undersea to control the release of cables from submarines. The actuator can operate for 90 days filled with seawater rather than lubricant.

Materials are the most challenging part of designs such as these; the entire housing for our underwater actuator is made of Monel, a high nickel-content alloy that resists corrosion from seawater. All the internal components, including the servomotor components are manufactured so that they can exist in a seawater environment.
• Grinding and polishing applications can expose products to harsh abrasives.
• Glass and plastic manufacturing, power generation, and process control can all be high-temperature applications due to the elevated temperatures required.
Karl/Bosch Rexroth:
Single-axis applications in machining systems are tough, because the low servo axis content doesn’t warrant the cost of a station enclosure. However, an IP65 drive makes the installation cost-effective. Another extreme application: fully enclosed machine tools for metal cutting with high-pressure cutting coolants when motors are located inside the machining areas.

We’ve actually applied many servomotors in totally enclosed rotary transfer machines using air pressure kits to prevent moisture penetration. Our latest IP65-rated drive, the DuraDrive, incorporates features that allow preventative maintenance and blower exchange during operation.
Patrick/Bosch Rexroth: Extreme environments commonly occur in machine tool and woodworking applications for reasons like sealing and lubrication. Metal chips and wood dust get into every crack and crevice of a machine, therefore sealing is extremely important. Recently, very aggressive coolant formulas have been introduced for machine tool productivity. A side effect of its use is the breakdown of lubrication at an accelerated rate. Robotics and automation achieve linear speeds of 10 m/sec and accelerations of 5, 10, or even 20 gs. These limits will destroy any component product incapable of this type of performance.

What is the biggest limiting factor in a motion system when it comes to extreme environments?

John/Exlar: For a motor or actuator product the two biggest limiting factors are materials and sealing. The materials from which the product is constructed must withstand the elements of its environment, while sealing must keep elements of the environment out of the product.

Dissimilar metals will corrode rapidly in wet environments. Product materials should be considered carefully. Very high or very low temperatures can cause thermal expansion or contraction significantly changing the fit of adjoining parts. This can affect several aspects of product performance or ability to deal with an extreme environment.
Karl/Bosch Rexroth:
The biggest limiting factors are heat losses and sealing. Heat losses can be overcome by implementing an easy-to-maintain cooling system. Blowers can be replaced during axis motion (or machine operation) if servo drives are located for outside access (they normally are). In applications where motors are located inside machining areas with coolants present, the following actions should be taken:

• Install motor splash guarding.
• Do not use motors that are fan cooled — if cooling is needed, use liquid cooled. (Note that liquid-cooled motors are the coldest element in the machine and show moisture condensation. Therefore they should have pressure kits to prevent moisture from penetrating seals.)
• Prevent water pooling by use of permitted motor mounting locations. Mount motor with connectors facing down so that liquid will naturally run away. Form a “drip loop” in the cable (below the motor) to allow any liquid to drip off the cable rather than collect on the connector. • Install an air pressure kit on the motor to prevent vacuum effects that suck moisture into the motor when the motor cools down.
Patrick/Bosch Rexroth:
The protection from both solid and liquid contamination is hard to achieve. The seal designs must be robust enough to deflect particles. Equally important though, is a full-contact capability that excludes unwanted fluids, and ensures the active lubrication remains protected and contained where needed. Optimizing the material choices and seal profile design addresses these concerns.

How do engineers better deal with extreme environments in their design?

John/Exlar: Look at the big picture first, and decide what is an important requirement of a product’s performance in an extreme environment. Other aspects of performance may be alterable for the best combination of environment tolerance and product performance. Many times not every desire made of a product can be provided in an extreme environment. For example, sealing against very high pressure may mean using seals that don’t allow maximum rotary shaft speeds on a motor. Or, explosionproof versions of a product may be larger than the non-explosionproof version.
Karl/Bosch Rexroth:
Check supplier recommendations, ask for installation references, and look for specific maintenance instructions and recommendations. Also:

• IPxx ratings are based on water - check specifics on coolant and oil exposures.
• Check chemical composition for equipment paint and material interaction. Some chemicals can dissolve paint or attack aluminum on motor housings and seals.
• Avoid blower-cooled motors, and use larger or liquid-cooled motors.
• Installing sheet-metal guarding can minimize exposure for extended trouble-free service.
Patrick/Bosch Rexroth:
Consider the performance requirement of the application when choosing machine components. If any of the items discussed so far are changed or new, it is very possible the ‘default’ component chosen on the previous machine may not be adequate. To assume that a 10% increase in speed, for example, doesn’t affect the motion components could be potential disaster.

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