Linear actuators: Make versus buy

Parker Hannifin’s Hybrid Actuator Systems are high-force self-contained linear actuation. They offer te benefits of electrical actuators and the power density and failsafe characteristics of traditional hydraulics.

With the range of linear actuators on the market, it’sbecoming easier for machine builders and end users to find astandard or “customized standard” product that meets even themost unique application requirements. But there are still timeswhen it makes sense, from a financial or technical standpoint, todesign and build an actuator in-house.

However, the make vs. buy decision isn’t as clear-cut as simply adding up the costs of do-it-yourself components and comparing that to an off-the-shelf solution. Choosing between a homemade actuator and a pre-made one means considering many factors, some of which are difficult to quantify in terms of time or dollars. In addition to the technical details of design and assembly, there’s the basic business evaluation of whether it can (and should) be done in-house, and the often-overlooked factor of after-sale support.

LINEAR-ACTUATOR BUSINESS EVALUATION

The first question a company should ask before they embark on making their own linear actuator is: do we have the expertise to do this in-house? Keep in mind that this includes not only the mechanical design, but also sizing and selecting the motor and controls and integrating the system. In many cases, one department or functional area has experience in mechanical design, while another department has the electrical expertise. In these cases, capture both knowledge sets by setting up a crossfunctional team for the design and engineering phase.

If the answer to the expertise question is “yes” the next thing to consider is whether you have the capacity and resources to do it in-house. This is a question that is often glossed over, because managers assume that if a project is done in-house, the company has control over quality, cost, and timeline. But building an actuator means relying on multiple outside suppliers and being at the mercy of their delivery and quality. If the project is delayed, customers could be affected or profits could be impacted.

KEY QUESTIONS: Do we have the linear motion and controls expertise to build the actuator in-house? What are the consequences if the project is delayed?

Some actuators are application specific. Case in point: Tolomatic’s new RSX press model (RSX096P) electric linear actuator increases the force range of the existing RSX product family to 40,000 lbf (178 kN) and is for force in the extend direction. The RSX press-model electric actuator works for metal-forming and assembly pressing, punching, stamping, riveting, fastening, and joining.

MANUFACTURING AND ASSEMBLY REQUIREMENTS

Building an actuator in-house means that you have the freedom to design it for your specific needs. The main components of any actuator are the base, the guideway, and the drive unit. One of the simplest types of actuators to build in-house is a design based on an aluminum extrusion. The benefit of using an extrusion is that they are readily available in a broad range of cross-sections and lengths. They also require little or no modification for mounting the guides and the drive mechanism. The drawback to using an extrusion is that, when high travel accuracy is required, it’s difficult to obtain a precise mounting surface.

The next option is a machined base plate. Relatively inexpensive and easy to machine, aluminum plate is a good solution for many in-house built actuators. If very high travel accuracy and rigidity are needed, machined steel plate is typically the best choice. Whether aluminum or steel, if a machined base plate is used, be sure to assess whether the machining can be done in-house. This also applies to other machined parts, such as motor and gearbox mounts, carriage plates, and protective covers. These items can easily be overlooked when evaluating the time and capacity needed for in-house production.

The type of guide to be used is often decided hand-in-hand with the decision regarding the actuator’s base structure. When an aluminum extrusion is used as the base, the best choice is typically a single guide based on cam rollers, wheels, or round shaft and linear ball bushings. For higher rigidity, profiled rails and carriages can be used. But unless the extrusion can be machined for a precise mounting reference, the benefit of higher travel accuracy that profiled rails normally provide will be forfeited by inaccuracies in the extrusion.

Applications that have high moment loads typically require two guides mounted in parallel, which necessitates the use of either a very wide extrusion, or a base plate to accommodate the side-by-side guides. The drive mechanism, regardless of whether belt, screw, rack & pinion, etc., should be mounted between the two guides in order to reduce the effects of binding. In dualguide designs, it’s important to align the two guides properly to avoid binding, which can cause additional forces on the bearings and reduce their service life.

Drive mechanisms are usually belts or screws. Belt drives are relatively easy to integrate because they don’t require precise alignment. However, they must be properly tensioned to remove “slack” from the belt and allow it to meet its force rating. Tensioning during assembly is usually not an issue but think about how the end-user will check the belt tension and re-tension it if necessary. This should be a relatively simple maintenance process and not require significant disassembly and reassembly time for the user.

If a ball or lead screw is the drive mechanism, determine what type of end support is needed. The fixed-simple arrangement (an angular contact thrust bearing on the driven end and a single ball bearing on the non-driven end) is most common, but if significantly high speeds or buckling loads will occur, a fixed-fixed bearing arrangement may be appropriate.

KEY QUESTIONS: Can the necessary machining, alignment, and assembly be done in-house? Does manufacturing have the capacity to meet the required production?

CUSTOMER-SUPPORT NEEDS FOR ACTUATOR END USER

Most actuators perform a critical function in the machine or process, and downtime can be catastrophic to production output. This makes customer support an essential consideration in the make vs. buy decision. Do you have staff who can support the actuator if troubleshooting or service are needed? Are you able to carry the spare parts required for quick repairs? These considerations are key whether the actuator is part of a production machine sold to external customers, or part of an in-house machine — with one of your own departments as the customer.

Also remember that components with rolling or recirculating elements will likely need relubricating during the life of the machine. This is especially true for ball screws, linear ball bearing guides, and profiled rail guides. Even if components are “lubed for life” application conditions may warrant re-lubrication during the machine’s useful lifespan. Taking userfacilitated maintenance into account during the design phase will ease service requirements.

KEY QUESTIONS: Do we have the structure and resources (staff and inventory) to support end users if troubleshooting and repairs are needed? How easy is it for the user to perform basic maintenance?

Just as buying a pre-made actuator should include an assessment of all the costs involved, making an actuator in-house requires considering the engineering, manufacturing and service requirements, as well as the cost of components.

WHAT IS A 12-VOLT LINEAR ACTUATOR?

Linear actuators are typically characterized by their drive mechanism — belt drive, ball or lead screw drive, or pneumatic drive. But it’s not unusual for rod style electric actuators to be classified by the input voltage — commonly 12 or 24 volts — of their integrated motors. These actuators provide thrust force, much like a pneumatic or hydraulic cylinder. In fact, rod style electric actuators are widely used to replace pneumatic or hydraulic cylinders, due to their simplicity and the potential cost savings that can be realized by switching from fluid power to electrically driven motion.

As the name implies, a 12-V linear actuator includes a 12-V dc motor, integrated into or tightly coupled with the actuator body. 12-V actuators are driven almost exclusively by one of two mechanisms – a ball screw or a lead screw. And most designs incorporate gearing or use a gear motor to optimize the thrust and speed characteristics of the actuator. The most basic design includes a limit switch at each end of the stroke, meaning that the actuator fully extends and retracts, with no intermediate positioning. But manufacturers offer programmable limit switches as an option for intermediate positioning capabilities.

Because these electric rod-style actuators are often used to replace hydraulic or pneumatic cylinders, some of their basic design features follow the precedents set by the other technologies. Mounting is a good example. A 12-V linear actuator is typically mounted in the same manner as a pneumatic or hydraulic cylinder, with most having both clevis and trunnion mounting options. In some 12-V linear actuator product lines, you’ll find body sizes and mounting options that meet ISO, NFPA, and other standards, which makes the conversion from a pneumatic or hydraulic actuator to an electrical actuator much simpler in existing applications.

One of the most crucial differences between rod style and slider type actuators is that rod style actuators provide only thrust force. Their primary use is for pushing or pulling a load, via a tube or rod that extends and retracts from the actuator. While a plain bushing guides the rod, there are no linear guides to support and carry the load. In most applications, support and guiding for the load is provided by tracks or rails independent of the actuator. This operating principle explains why these actuators have several different monikers, including electric cylinders, thrust type actuators, and rod style actuators.

Sizing and selection of a 12-V linear actuator is fairly straightforward, because the motor is preselected and integrated into the actuator. The first parameter to be considered is typically thrust, as it will often dictate the overall actuator size. Next is stroke length, since a small actuator may meet the thrust requirements, but may not deliver the necessary stroke length.

Note that like slider type linear actuators, rod-style actuators driven by a ball screw or lead screw can back drive. When the application requires vertical operation, remember to check that the vertical load doesn’t exceed the screw’s back driving torque.

With an initial actuator selection based on thrust force and stroke, the speed and duty cycle requirements can then be checked. The allowable force and speed combinations are typically provided by the manufacturer, in the form of a performance curve or chart. Once it’s confirmed that all other parameters are within the actuator’s capabilities, it’s important to check the required duty cycle, or “on time” because motor heating can be a limiting factor for the actuator’s performance.

SUITABLE APPLICATIONS FOR 12-V ACTUATORS

Virtually any time a load needs to be pushed or pulled, without being guided or carried, a rod style actuator is a good choice. This includes opening and closing sliding doors in applications such as rail cars and machining centers. In the medical industry, 12-V actuators are often used for ergonomic positioning of work tables or patient beds.

In conveying operations, these actuators can stop or divert product. Because they’re fully enclosed and available in IP-rated or hygienic designs, rod style actuators also excel in pharmaceutical and food and beverage designs needing purely thrust operations for inserting, labeling, or stamping.

HIGH-SPEED LINEAR ACTUATORS: WHAT QUALIFIES THEM AS SUCH?

Like many terms used in the linear motion industry, such as heavy duty, miniature, and corrosion-resistant, to name a few, there is no industry standard that specifies what constitutes a high-speed linear actuator.

Nevertheless, there are some general guidelines that manufacturers follow when classifying and marketing their actuators as high speed. These guidelines are typically based on the drive mechanism, actuator type, and even primary use or industry. Understanding these distinctions can help you make an informed decision when your application calls for a high-speed linear actuator.

These PA programmable actuators from Nook Industries come as complete systems (including motor, drive, controller, and power supply) and are rated for continuous duty. Acme-screw actuators are for lower duty cycles where self-locking is needed; toothed belt drives are for parallel motor mounting. Strokes reach 24 in. with ±0.004 to ±0.001 in. repeatability; the actuators also have standard motor and gearhead flanges. There are IP54 or IP65 versions depending on the motor.

SPEED IS PRIMARILY DEPENDENT ON THE DRIVE MECHANISM

The limiting factor of a linear actuator’s speed capability is typically the drive mechanism.

Because lead screw designs are based on sliding contact and generate high heat due to friction, they often have lower maximum speeds than ball screws of a similar size. So of the screw technologies, actuators based on ball screw drives are more likely to be deemed high speed than those based on lead screw drives.

Actuators based on belt drives or rack and pinion assemblies are typically able to reach higher speeds than ball screws, provided they are properly tensioned (for belt drive versions) or preloaded (for rack and pinion versions). Actuators with steel reinforced belts can achieve speeds of 10 m/sec or higher, while rack and pinion driven actuators can commonly reach speeds of 5 m/sec.

ACTUATOR TYPE ALSO PLAYS A ROLE IN MAXIMUM SPEED

Another factor comes into play when discussing high speed linear actuators: the type of actuator. The “high speed designation is most often applied to thrust-rod type actuators (also called electric cylinders) because their primary applications involve pushing-pulling and inserting operations, which typically require very short extension and retraction times. These actuators can be either ball screw or lead screw driven, with speeds ranging from 0.1 m/sec to more than 1 m/sec.

A few manufacturers even offer belt-driven rod-style actuators that can reach speeds to 2.5 m/sec.

Slider or carriage-type actuators (also called rodless actuators) can achieve even higher speeds than rod-type actuators in many cases. But because their primary uses are for positioning and transport, typically with high loads, they are less often marketed as high speed. Rodless or slider-type actuators have a wide range of drive options, including lead screw, ball screw, rack and pinion, belt, and linear motor.

Linear motors inherently provide the highest speed capabilities, with no mechanical parts to limit speed or create friction and heat. But when incorporated into a linear actuator, linear motor drives are limited by the speed of the guide mechanism. Similarly, steel reinforced belt drives can achieve speeds greater than 12 m/sec, but like linear motors, are limited by the maximum speed of the guide. The most common guide systems used with linear motors and belt drives are recirculating profiled rail bearings, whose maximum speeds typically reach up to 5 m/sec. limiting the overall speed of the actuator to 5 m/sec or less.

However, higher speeds can be achieved when belt drives are paired with cam roller guides instead of recirculating profiled rail bearings. With preloaded cam roller guides and a properly tensioned, steel-reinforced belt drive, these high speed linear actuators win the race, with travel speeds up to 10 m/sec.

SPECIAL CASE OF MINIATURE LINEAR ACTUATION

For micro and nano-positioning applications, actuators of choice are often based on voice coil or piezo technologies. Ultrasonic piezo actuators can reach speeds of 0.5 m/sec or greater, but typically have maximum strokes of 100 mm or less. Voice-coil actuators run to 0.30 m/sec with strokes to 150 mm.

While these specifications may not fit the general definition of high-speed linear actuators, considering the rapid acceleration that is required to reach these speeds in very short stroke lengths, piezo and voice coil designs can easily be classified as high-acceleration actuators.