Single Axis Actuators When to Build, When to Buy

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Whether they are termed single-axis actuators, linear guides, slides or tables, all the products discussed herein provide the same generic function, namely, precision linear motion of a load in single axis, as part of a machine, piece of equipment, assembly line or testing device.

A variety of linear motion devices are available today for performing the function of single-axis movement, whether it be for an assembly operation, testing, packaging or simple motion control. This white paper will seek to define the various types and define the options to be considered when deciding whether to build or buy a single-axis actuator, also known as a linear guide, slide or table in the parlance of particular industries.

Over the years, the technology has improved, the motions have become more precisely controlled and the materials of construction have evolved to suit the particular needs of various industries. Automotive and appliance assembly first utilized linear motion devices nearly 100 years ago now, though the principle dates back to the days when manpower was used to engage the pulleys manipulating the building blocks for pyramids. Many credit al-Jaziri in the 13th century, by his invention of the crankshaft, with having applied rotary motion to produce linear, reciprocating motion, the foundation of what we address here today.

Whether controlled by hydraulics, electromechanics, electromagnetics, vacuum or ropes and logs, the principle was and today is the same. Precision yet fast motion control, while remaining as friction-free as possible, with the minimum of mechanical wear and highly repeatable positioning, are paramount. Dozens of manufacturers make such products available, in a variety of sizes, styles and material construction, for today’s machine designers. Here, we will address the advantages and disadvantages of some of the most popular electric motor-driven styles, then address the main focus of this paper, which is the decision about when to build vs. when to buy a single-axis actuator to suit the particular application at hand.

Several of the most common types of motorized linear actuators are:
  1. Ball screw driven and ball guided units-best suited for high thrust, heavy load, precision positioning and stiffness
  2. Ball screw driven and single guided units-lower in cost and ideal for harsh operating environments, where contaminants are presents
  3. Belt driven and ball guided units-offer a long use life and smooth operation, plus higher speeds, acceleration and accommodation of medium payloads
  4. Belt driven and slide guided units-low maintenance and high speed performance, plus smooth, quiet operation under acceleration

Depending upon the industry of application, various factors will determine the most suitable style to be used. Automotive, appliance, packaging, medical, semiconductor, general assembly/test and factory automation applications all take different factors into account in the decision-making process for the right operating mode, construction material, performance and ambient noise conditions to be selected. Furthermore, the cost factors are driven by the availability of standard or easily customized standard product, the quantities required for the application, the ability of the resident engineering talent to design and build, plus the immediacy of need involved.

For a unit to be built, the engineer must calculate the forces, loads and tolerance build-up for each component used in the unit. This is true, regardless of the operational mode selected, of course. Furthermore, the engineer must calculate the use life of each component, the fit, the function, the durability and the maintenance requirements of each component. Certain industries such as medical and semiconductor, where the unit will function in a cleanroom or other highly controlled environment, will mandate the use of certain grades of construction materials, which may or may not be readily available from the traditional component manufacturers. And, if they are available, the performance of the component may not meet the force, load and mechanical specifications of the application. These would include, but not be limited to, the correct size, speed, accuracy, stroke and load-bearing characteristics, as well as the finish and lubricant compatibility for certain industries such as food, medical and semicon, where particulate migration is a key consideration.

In purchasing a unit, the size, speed, accuracy, positioning repeatability, load, stroke and other feature information are sometimes provided at the unit level, not the component level. Thus, an immediate time savings is realized in the evaluation process. Additionally, the engineer not only saves time on the design, but also finds time savings by eliminating the need to investigate if the standard components available on the market can properly fit together. The engineer must still take into consideration the functional durability of the unit overall, its lifespan in the particular application, its maintenance requirements and its availability. However, these questions can usually be answered very quickly, either by catalog documentation online or, yes, even the traditional phone call to a knowledgeable applications engineer. The engineer must still choose the correct size, speed, accuracy, load, stroke and motor compatibility for the job. When a selection is made, the advanced unit suppliers today will have native CAD files ready for downloading in various formats to suit the engineer’s platform, with the most sophisticated now offering a configurable design option online. This function allows selectable modifications to be made, often without surcharge, to the standard cataloged items. Thus, the engineer realizes the triple benefit of cost containment, substantial time savings and the elimination of outside machining or in-house shop utilization to make modifications before final assembly into the end product or manufacturing operation.

As pricing is always a consideration, whether building semiconductors or screwdrivers, the decision to build vs. buy takes this critical factor into the calculation, but in different ways.

When pricing the build of a linear motion unit, there’s the obvious expense of buying individual components separately from various vendors, with the attendant paperwork. Typically, the following components would be required for a single-axis, motor-driven, ball screw actuator with 340mm base length, 150mm table width and 12mm lead screw diameter, able to handle a vertical load approx. 140kg, with the approximate costs shown for each component, based on published prices online today…

Ball Screw
$150
Guide
$270
Support unit, fixed side
$50
Support unit, motion side
$30
Table
$55
Nut bracket
$40
Motor bracket
$80
Stopper (Bumpers)
$15
Base, machined aluminum
$440
Design time ($40/hr. for 21 hrs.)
$840
Assembly time
$40
Inspection time
$40
- TOTAL
$2050

By contrast, the buying of a unit, especially one configured online, requires only the selection of the most cost-effective solution. Since the maintenance of a purchased unit is generally lower, given the reliable manufacturing of reputable suppliers, the long-term cost reduces. Plus, the predictable published cost (approximately half the above price) allow easier budgeting, there are minimal design costs for the unit, once the specifications are entered into the configurator and the CAD file is found suitable for the job and, on a practical but important level for the procurement process, there is just one vendor, one price, one PO, one part number and one order to follow. If the unit selected is appropriate for the application, this scenario is clearly desirable.

In terms of the time required to execute a build vs. buy, the build route clearly requires multiple lead times from multiple vendors. Additionally, there’s the internal engineering time for the selection of components, an assembly drawing, component drawings, order placement, shipping, assembly into production and inspection. Such a project can typically take several weeks up to several months, depending on the complexity of the components chosen. For the buy route, the predictable delivery schedule for a single finished unit allows greater control over the machine build time management. Be cautious, though, because while a single vendor means just one lead time for the unit, many suppliers will have a back order log that delays the delivery substantially. Plus, many suppliers will build a custom unit to suit your application, but the lead time might be eight to twelve weeks and thus delay the completion of your machine, test station, pick-and-place system or robotic handling line.

In the end, the decision to build vs. buy requires consideration of many factors, as well as an honest assessment of the resident engineering and machine shop resources at your company. In many cases and in most industries, the available products from vendors today will represent a time-saving, cost-saving and far more reliable alternative for your linear motion requirements. Those pyramid builders were not inclined to outsource, it’s true, but their projects did take lifetimes and longer to complete, a scenario that’s somewhat impractical today, to say the least.

Mr.Chris Blaszczyk is the manager of product development for the Automation Components Division of MISUMI USA, Inc., located in Schaumburg, IL, near Chicago. Interested parties may contact Chris with comments or questions at 1-800-681-7475 or 847-843-9105, x253. Email him at cblaszczyk@misumiusa.com.

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