When using linear actuators, what applications present the most challenges in terms of machine productivity and why?

John/Exlar: Applications that require high force and speed are challenging because high speed requires low mechanical advantage, and high force is best achieved with high mechanical advantage. Applications with extremely high acceleration are also difficult in rotary screw design actuators. Actuators that lend themselves to very high acceleration are also typically limited in force capability.
Dave/Kollmorgen: Any application requiring high speed, high throughput, high accuracy, and/or low heat generation, such as electronics assembly pick and place or PCB drilling, or semiconductor die bonding and metrology, presents challenges.
John/Norco: Applications which require custom engineering, exotic materials, or special manufacturing will consume the most time and manpower. These applications can be found within many industries, from medical to industrial and military.
Andrew/Rollon: Existing machine or device modifications often create the largest challenges. Unlike fresh applications, modifications and mid-production upgrades force engineers to fit ideas and concepts into physical packages that were probably not designed for them. If not thought out correctly, the solutions may work, but may not add real value to the system.
Danielle/Bosch Rexroth: Processes that require downtime to change out tools or to perform maintenance and repairs are the biggest deterrent to optimum productivity. Harsh environments, such as woodworking, metal cutting, and water jet applications, create special challenges to increasing productivity because chips or other contaminants can enter the linear bearing systems and cause failure. Protective sealing strips help prevent contamination and reduce downtime.

What are the worst cases of improper design and implementation you’ve seen? Describe what went wrong and how these and similar problems can be prevented.

Danielle/Bosch Rexroth: The worst situations occur when the application or machine is under-designed at the outset or later changed because the application was not well understood during the design phase. I recall one application in which a customer had to purchase all new linear actuators due to a poor understanding of the real application requirements. Once they had built the machine, they discovered it needed to move twice as fast and carry double the load that it was designed for.

The following acronym can serve as a reminder of the major factors in system design: LOSTPED stands for Load, Orientation, Speed, Travel, Precision, Environment, and Duty Cycle. Each factor needs to be considered as early as possible during the design phase to ensure proper actuator specification.
John/Baldor: The main cause of problems in linear motion systems is incomplete specifications. In one of the worst situations we have seen, the designer acquired numerous single-axis stages designed with a tight bend radius for design compactness. He then designed his machine around a performance level that exceeded cable and velocity limitations, resulting in many hours of rework and redesign. This could have been avoided if proper information and full specs were available up front.
John/Exlar: Application implementations that do not prevent high-speed end crashes are regularly seen. When a large mass is stopped in a crash situation with close to no deceleration time, the energy absorbed by the actuator is extremely high. Eventually, repeatedly stopping an actuator operating near its rated load in a crash situation will usually result in actuator damage.
Al/Danaher LMS: Bent actuators are one of the worst cases of improper design. The cover/extension tube can get bent by offset load or by extending the actuator around a pivot point. To avoid bending the tube, keep the load along the extension tube axis, keep the mounting pins parallel, and make sure there is clearance when the actuator is fully extended and retracted.
Andrew/Rollon: I have seen quite a few improper designs. I tend to separate them into two types: when the design is functional but not the best way to accomplish the designer’s intent; the second is when the design is a poor concept that is poorly engineered. The second type is much worse and is often times downright dangerous. Many improper designs are simply caused by the engineer living in “magical micron tolerance” land while the assemblers work in the real world. These designs have unbelievably tight, unnecessary tolerances in every aspect of the project — tolerances that are not needed for the project and only add time and expense during assembly.
Dave/Kollmorgen: Some examples of real-life failures: a machine had a very compliant structure and caused control issues; failure to follow the manufacturer’s thermal protection recommendation resulted in a fried motor; and drives not matched to motors caused overheating or underperformance.
John/Norco: Proper specification and implementation all comes back to one, all-important thing — knowing the application.

Describe best practices in designing with linear actuators. Can you offer any application-specific tips to other engineers?

Dave/Kollmorgen: Take the time to adequately define the machine structure and motor performance based on mass, force, and motion profiles. Realize that the machine (especially in terms of rigidity) may not be able to handle the improved motor/system performance. Use proper shielding and grounding techniques. Gain an understanding of all system components’ new technology and how to use it.
John/Norco: Knowing the application involves a number of factors, including load, weight of the object being moved, the speed required for movement, acceleration and deceleration rates, time from cycle start to stop, the environment, and rate of use (continuous versus cyclical). The actuator does not operate alone; the whole system should affect actuator selection.
Andrew/Rollon: To properly design in a linear actuator, you must have realistic, clear goals for the linear system. How heavy is the moving package? What are its dimensions? How fast do you need to move? Are there accelerations? What sort of lifetime is required? All of these questions, and others, affect the actuator choice. Also, we suggest running the calculations with the real loads, then, based on the other factors, choosing the desired safety factor and then the correct system.
John/Baldor: When designing with linear actuators, there are a few simple tips. The system is only as accurate as the bearings and supporting structure. Typically the maximum acceleration or velocity of the system will be limited by the bearings (5 g), encoders (<1 m/sec at 0.1 μm), cables (bend radius 7 x diameter), or available bus voltages.

When designing a system to achieve improved performance through higher accelerations, remember that F = ma. For any given acceleration you command, the reaction forces into your mechanical structure are proportional. Machinery has been known to “walk across the floor” after redesigning the motion system but not the supporting structure. This also applies to mechanical resonance set up by reducing cycle times to milliseconds rather than tenths of seconds.
Al/Danaher LMS: It’s critical that the actual load on the actuator – both in terms of weight and direction – be determined. Mechanical disadvantages and spring loading can often increase loading on the actuator. Just use the actuator to position the load and other bearings to support the load and keep it parallel to the actuator axis.
Danielle/Bosch Rexroth: The machine’s final location, oddly enough, is sometimes not even considered until machine installation. Designers and users need to consider the end application and the requirements that will be placed on the machine daily.
John/Exlar: Pay special attention to atypical operating modes. For example, what happens when the operator triggers an E-stop with the actuator moving at high speed? Oftentimes this will result in a freewheeling mode and crash if not dealt with in the machine operation logic.

What can linear actuator manufacturers do to improve productivity?

John/Norco: Manufacturers should never assume that the customer has chosen precisely the right actuator, even if they order by part number. In some cases, though, time or confidentiality prohibits customers from being open and specific about an application. John/Exlar: Assuming that the actuator is sized to properly perform the application as requested by the customer at the desired rates, expected productivity will naturally result.

Maximum up time means maximum productivity. Reviewing every damaged actuator allows design changes that prevent damage caused by the atypical operation, even if that operation is outside the actuator specifications.
Danielle/Bosch Rexroth: Productivity depends on the machine design, its full use, and downtime minimization. So, linear actuator manufacturers should provide quality modules that are easy to install, maintain, and service. By reducing actuator maintenance, you increase the machine’s overall productivity. And, when a manufacturer produces the linear motion components inside the actuators, the same design principles should be applied to guide rails, screws, bushings, and shafts.
Dave/Kollmorgen: Manufacturers should aim to provide more force per motor size with lower heat generation.
Andrew/Rollon: Users do not want to lubricate and maintain the units. Manufacturers should provide maintenance-free products when possible and design units that last longer but still cost less.
John/Baldor: To improve productivity, throughput is not only enhanced by increasing the acceleration, but also by improving peak velocities and settling times. By switching to a linear servosystem, a customer could see potential improvements in settling times from 100 msec to less than 10 msec. Linear motors can achieve peak velocities well in excess of 4 m/sec easily (faster than ball screw speeds), while improving the positioning repeatability and mean time between failures of the system as well.

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