White paper: is it better to build or buy linear actuators?

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

Various linear motion devices are available today for single-axis movement, whether it be for an assembly operation, testing, packaging or simple motion control. This white paper seeks to define the various types and the options to be considered when deciding whether to build or buy a single-axis actuator.

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 utilised linear motion devices nearly 100 years ago, 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, with his invention of the crankshaft, as the first to apply rotary motion to produce linear, reciprocating motion - which is the foundation of what we address in this paper.

Whether controlled by hydraulics, electro-mechanics, electro-magnetics, vacuum or other means, the principle is the same. Paramount is the need for precise yet fast motion control, while remaining as friction-free as possible, with the minimum of mechanical wear and highly repeatable positioning. Dozens of manufacturers offer such products, in a variety of sizes, styles and materials, for today's machine designers. In this paper 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 versus when to buy a single-axis actuator to suit the particular application at hand.

Several of the most common types of motorised linear actuators are:

• Ball screw-driven and ball-guided units - best suited for high-thrust, heavy-load, precision positioning and stiffness
• Ball screw-driven and single-guided units - lower in cost and suitable for harsh operating environments where contaminants are presents
• Belt-driven and ball-guided units - offer a long use life and smooth operation, plus higher speeds, acceleration and accommodation of medium payloads
• 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, materials, performance and ambient noise conditions to be selected. Furthermore, the cost factors are driven by the availability of standard or easily customised 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. In addition, 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 clean-room 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 semiconductor, where particulate migration is a key consideration.

Purchasing considerations

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 saving is realised in the evaluation process. Additionally, the engineer not only saves time on the design, but also finds 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 catalogue 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 enables selectable modifications to be made, often without surcharge, to the standard catalogued items. Thus, the engineer realises the triple benefit of cost containment, substantial time savings and the elimination of outside machining or in-house shop utilisation 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 or buy takes this critical factor into the calculation, but in different ways.

When pricing the build of a linear motion unit, there is 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 of approximately 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 aluminium £440
• Design time (£40/hr for 21 hrs) £840
• Assembly time £40
• Inspection time £40

TOTAL = £2050

In contrast, buying a unit, especially one configured online, requires only the selection of the most cost-effective system. 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) provides easier budgeting, there are minimal design costs for the unit (once the specifications are entered into the configurator, the CAD file can be downloaded) and, on a practical but important level for the procurement process, there is just one vendor, one price, one purchase order, one part number and one order to follow. If the unit selected is appropriate for the application, this scenario is clearly desirable.

Delivery times

In terms of the time required to execute a build versus buy, the build route clearly requires multiple lead times from multiple vendors. Additionally, there is 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 therefore delay the completion of the machine, test station, pick-and-place system or robotic handling line.

In the end, the decision to build or 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 linear motion requirements. Those pyramid builders were not inclined to outsource, it is true, but their projects did take lifetimes and longer to complete, a scenario that is somewhat impractical today, to say the least.

Visit Misumi's website at www.misumi-europe.com for more information and to request catalogues.