Machinedesign 2416 Electric Cylinders 0 0

Electric linear actuators: make or buy?

Oct. 1, 2000
Engineering an electric linear actuator (electric cylinder) system in-house is often the most cost and time-efficient route to an automated motion solution. Sometimes you are better off buying.

Who could know better than you exactly what your electric linear actuator needs are? And today you can buy so many ready-made subcomponents you can just assemble “modules” — little parts fabrication required. Engineering your own linear actuator system is often the most cost and time-efficient, wisest route to an automated motion solution. Yet, when you consider the time spent away from other projects, the likelihood that in-house linear-motion talent may be sparse, and the chance that hidden problems and costs will come out of hiding at the worst times, the design-yourown approach loses some luster.

Electric linear motion offers some solutions — and problems — different from other drive components. For example, there is no universally accepted dimensional standardization for linear electric drives as there is with older power-transmission components such as bearings and chain drives. Also, electric linear motion technology is not so old as other drive technologies and therefore hasn’t such a long application history. Moreover, many suppliers in the field are themselves fairly new. In any given situation, such differences can be an advantage or a disadvantage.

A make-vs.-buy test

The following 3-point test may help machine designers resolve the make-vs.- buy dilemma. 1. Time-to-market: Can we complete our machine sooner by buying or by making a linear motion solution?

Accelerating new product introductions is a prescription for survival in today’s competitive economy. Development cycle times must be superior to those of your competitors, so you must constantly hunt ways to improve time to market.

Can an actuator supplier build the system faster than you can design and build it yourself? Or — perhaps better — does a standard actuator exist that meets the need? If you design the actuator from a variety of components, consider whether your project can afford a delay caused by late delivery of any one component. What is the chance that an in-house designed product will work right, the first time? If the equipment the actuator is to serve will be used to manufacture your product, find out what production time with the new machine is worth to your company. If you are building a machine for sale, see if you can increase your customer’s revenue by delivering the machine sooner, or if you’ll pay a penalty for late delivery.

Before purchasing any linear motion system in an effort to reduce development cycle time, find out what lead time and on-time delivery performance of the actuator manufacturer are like — especially if the actuator is in the critical path to project completion. Then ask yourself the same question of your own design group and your own shop.

2. Expertise: Do we have the expertise to design a reliable actuator, and would this be the best use of our expertise?

Linear motion control is often peripheral to a machine’s critical processes. No one knows better than you which components most impact your machine’s overall performance and which, therefore, require much design attention.

Likewise, if actuator design will take time away from other more important projects, you may be better off sourcing this subsystem.

The same lack of standardization that can complicate electric linear actuator selection can also simplify it. Electromechanical actuators come in a variety of configurations from a growing number of suppliers, so chances are good that you can find one with the right combination of components for your application.

Several actuator suppliers offer modular designs that can be readily configured to meet your unique requirements. Design modularity, extensive use of CAD tools, CAM software, and CNC equipment also make efficient customization possible. If you decide to source an actuator, see if the manufacturer is willing to customize the product.

Sometimes, of course, you just can’t buy exactly what you need, even customized.

3. True cost: What is our true cost to design a linear motion system, procure parts, assemble, test, correct if needed, and support the system in the field?

From an engineering perspective, it often appears that you can build it yourself for less. Ask this third question before making a final decision. You need to consider what it costs to specify, procure, and inspect each component, over and above component purchase price. Ask yourself the same question about a purchased linear motion system.

Designing your own

Table 1 summarizes a cost model to show the difference between building and buying a simple electric linear actuator. This cost model is based on a simple linear positioning application — back and forth motion to fixed positions with low repeatability ( 60.005 in.), 800-lb thrust, 12-in. stroke, and with no need for particular environmental or safety precautions. A simple limit-switch control is used. Some applications would cost less, some more, depending on load, stroke, duty cycle, accuracy, throughput, control interface, flexibility, and so forth. This estimate includes no redesign. A typical cost to purchase such a system readymade would be around $1,300.

If you decide to buy

Actuator companies differ greatly in product range and in the industries they serve and understand. Actuators themselves range from a few pounds of thrust to several thousand, and from sub-inch to sub-micron repeatability.

Most actuator companies do not offer controls to complete the solution, which can complicate your job. However, some offer the convenience of buying complete actuator-and-control packages, ready to mount, wire, and run.

Be sure to review whether you have provided all the application information needed for the supplier to recommend a reliable, cost-effective solution.

Because of the broad array of standard options available with modular-designed actuators, the actuators are generally built-to-order. Companies that use JIT inventory, in-house machining and assembly, and flexible cell-based operations, can regularly deliver standard actuatorand- control systems within 2 to 3 weeks of order receipt.

Al Statz is Vice President, Sales & Marketing, Industrial Devices Corp., Novato, Calif. He has a B.S. degree in Electrical Engineering from the University of Wisconsin and has worked in the motion control field for the past 8 years.

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