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The Nuts & Bolts of Selecting the Correct Linear Drive Train

Tue, 05/20/2014 - 3:35pm
Jim Monnich, Engineering Manager, Parker Hannifin

Whether you are designing your own motion solution, selecting components from a supplier, or working with a system solutions provider, understanding drive train selection is a key to a successful and reliable machine. In this endeavor, it’s important to understand the performance characteristics of each drive train technology. 

Balancing an Elephant on a Beach Ball

Selecting the right drive train for motion in your machine is kind of like balancing an elephant on a beach ball. Multitudes of key operating characteristics are both required and desired for machine design. Each of the different potential drive trains that can be applied to solve your motion problem has a unique performance in each of the characteristics relative to the other drivetrain technologies. Therefore, making the right selection is not always about maximizing a single characteristic — instead it is about defining an optimal level on a number of different fronts. In other words, a good selection results from reaching a point of equilibrium by balancing the most important of the key performance characteristics.

 The list of potential performance characteristics that might be of interest is significant. To focus on the selection process, all the options are classified into four major categories, including precision, expected life, throughput, and special considerations (PETS).

The Five Common Drive Trains

The five most commonly used drive trains in linear motion are ball screw, lead screw, timing belt, rack & pinion, and linear motor. When other technologies or form factors might work for an application, the basic constructs and prioritization methods should still apply.

Ball screw drive trains include a threaded rod and matched ball nut with recirculating ball bearings between the nut and screw interface surfaces. The rolling ball bearings provide for high efficiency and duty cycle.

Ball screws are ideal for applications requiring high precision, repeatability, and force density. Ball screws can achieve moderate speeds, but are limited based upon screw whip. It can be relatively difficult to properly align the screw to the line of travel to ensure maximum life and minimize the maintenance required. In addition, a ball screw can be quite noisy depending upon the grade of the screw.      

Lead screw drive trains include a threaded rod and matching threaded nut sliding interface. In some cases, the nut is preloaded against the screw to reduce backlash.

Lead screws work well for low duty cycle applications, or applications requiring small adjustments. Lead screws are typically about half as efficient as a ball screw, so they require about twice the torque to achieve the same thrust output of the screw. One advantage of the technology’s low efficiency is its inherent resistance to back driving, which can be ideal for vertical applications that would otherwise require a brake to hold the payload under loss of power.

Timing belt drive trains include two cogged pulleys, typically one driven and one idler, tied together with a timing belt that has a carriage attached. This technology is the most simple and common drive train in linear motion.

Timing belts are a robust mechanism for high-speed applications requiring long life and maintenance-free operation where precision greater than 100 microns is required. Design hurdles associated with belt drives include the need for a reliable belt tensioning system as well as rugged pulley bearings to handle thrust loads.

Rack & Pinion drive trains include a machined linear gear (rack) and round mating toothed gear where the round gear is typically mobile and the rack stationary. Rack and pinions are suited for very long travels requiring high speed; however, they are not a precision technology because there is no good way of removing system backlash from the drive train. In addition, rack and pinion drives are often quite noisy.

Linear motors include a row of magnets that interfaces with an electromagnetic carriage to move a load in a linear direction. Linear motors are direct drives without the mechanical advantages and disadvantages of the other common drive trains.

Linear motors provide high speed, acceleration, and precision with minimal backlash following error and settling times. This is often one of the most costly technologies, largely because of the cost of magnets and linear feedback devices needed. Force density can be difficult because of the lack of a mechanical drive train, which also makes using a linear motor in a vertical application difficult.

Focus on Precision

Recall that the key performance characteristics for precision are resolution, repeatability, accuracy, and velocity control. Always start with an understanding of needs relative to resolution. Starting with the proper minimum incremental step is the foundation for all other measurements regarding precision.

For accuracy, higher levels are a system cost driver both from a material and implementation standpoint. First, determine if you really need accuracy. Techniques to avoid requiring a system with true accuracy are either teaching or mapping system positions.

Velocity control is typically only required for scanning applications with high resolutions or high-speed "on-the-fly" dispensing applications. This is another characteristic that will drive up the cost of the system.

When it comes to precision, linear motors and ball screws are typically far superior. The majority of motion applications do not require high levels of precision, which is why the timing belt remains the commonly applied technology.               

Focus on Expected Life

The key performance characteristics for expected life are mechanical efficiency, mechanical wear resistance, contamination resistance, and maintenance. Mechanical efficiency is the most important characteristic to consider unless the situation involves a dirty or otherwise harsh environment. High efficiency of your drive train device is going to equal long life and lower energy consumption, meaning that it will require less maintenance and will allow you to minimize the size of upstream components like motors or drives.

Wear resistance is typically a function of the type of friction involved in the mechanism. The lower the friction of the device, the lower the wear.  The one exception is for the linear motor. Although this drive train is typically considered "frictionless," there are moving cables in many configurations that create a wear point and may limit system life if not properly managed.

Contamination resistance or dirt resistance must be a high priority in harsh environments.  However, for most environments, it has a low priority.

Maintenance is closely related to the mechanical efficiency and wear resistance.  The ideal scenario is that no field maintenance is required for the intended life of the machine. That said, all mechanical devices have a life limitation.  In design, it is critical to understand how wear or relaxation will impact the longer-term "precision" characteristics of the system.

When it comes to expected life, timing belts are the most suitable technology because of their high efficiency and limited maintenance requirements.

Focus on Throughput

The key performance characteristics for throughput are speed capacity, maximum acceleration, frequency response and duty cycle. For speed capacity, depending on the length of travel required, either speed or accel/decel is most important. For applications with a longer travel where more of the cycle time is spent at the top velocity, speed is most critical. On the other hand, for applications with shorter moves where more time is spent accelerating and decelerating, accel/decel is most important.

Frequency response is a key attribute for applications with tight position requirements (high levels of precision) that are time sensitive. Most pick-and-place applications don’t require high levels of frequency response. The attribute can be a cost driver.

Duty cycle is typically a characteristic

that should be checked and validated to ensure the selected design will work in the application.

When it comes to throughput, linear motors are the best choice because they can provide high speeds and high accelerations. That they have no mechanical compliance means they can also provide a high frequency response.

Special Considerations

The key performance characteristics for special considerations are force density, material cost, implementation cost, and travel length. Force density is an issue of packaging. As machine designs continue to miniaturize, the characteristic becomes more important, especially when you are specifying end effectors or tooling mounted to an axis.

Both material and implementation costs are always key considerations because it is important to get the right combination of other characteristics at the minimum cost.

Lastly, for most applications, travel length restrictions are not an issue.

The Upshot

No doubt, selecting the best drive train is a balancing act. When you optimize on one key performance characteristic, you are likely to give up a benefit relative to another performance characteristic. Of utmost importance is to understand the performance requirements of your machine. First, understand your requirements relative to precision, expected life, throughput, and special considerations. The rest is a matter of picking a few attributes from each category and leveraging the guidelines provided.

For more information, visit www.parkermotion.com.

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