Linear actuators with belt drives, ballscrews, or leadscrews for their rotary-to-linear mechanical transmission most commonly pair with servomotors, stepper motors, or brushless motors. Belt-driven actuators frequently employ servomotors with planetary gearboxes for speed and precision. In contrast, ballscrew and leadscrew actuators often integrate servo or stepper motors that are coupled directly or via gear reducers to the linear mechanical drive.
No matter permutation, motors and their gearing connect to linear actuators via couplings such as zero-backlash bellows or elastomeric jaw couplings. Then the motor frame or gearbox housing mounts to the actuator body via machined aluminum or steel brackets. These are either standardized National Electrical Manufacturers Association (NEMA) mounts, metric flange mounts, or custom adapters to align the motor output shaft, coupling, and actuator input. Bracket flanges, pilot bores, tapped bolt patterns, or slotted bases enable fine-tuned alignment.
Gearboxes and motors may be separately specified for customization, but over the last two decades it’s become increasingly common for design engineers to specify motor-gearbox assemblies pre-integrated by the supplier for easy installation with minimal misalignment or backlash. Pre-integrated units also optimize performance by reducing actuator tolerance stack-up and boosting actuator-assembly stiffness.
For speed reduction and to suit application requirements, linear actuators employ planetary, spur, worm, and bevel gearing (as well as right-angle variations). For example, spur gearing offers simplicity and compactness with moderate torque and speed reduction for mid-range precision applications. Spiral and hypoid bevel gearing offers compact right-angle transitions and high torque density for space-constrained designs or linear actuators to mount on and operate vertical axes.
Most precision linear actuators with critical accuracy and repeatability requirements use ac motors (including servomotors) to get dynamic performance, torque density, and position control via closed-loop feedback and integration with a motion controller. However, another third or so of industrial linear actuators use dc motor variants such as brushless or stepper motors. Small linear actuators (especially those with leadscrews) employ brushed dc motors as well, though not usually for precision automation. Rather, these actuators often work in consumer products, powered elements in automotive interiors, and medical equipment.
Mount geometry for linear actuators
Most NEMA and metric motor mounts attach to the actuator drive end. Variations that accept right-angle or bevel gear drives feature side mounts; these can reduce actuator-assembly length in space-constrained machinery.
Mount brackets are made of machined aluminum or steel. Typical geometries include a square or circular flange with a center pilot bore (a precisely machined circular boss) and mounting holes in a standard array; four-bolt arrangements are common for average-sized motors. Mounting holes can be slightly elongated to facilitate slight adjustments.
The most common NEMA mount sizes are those for NEMA 17, 23, 34, and 42 motors. The numbers indicate the approximate dimensions of the motor faceplate. For example, a NEMA-23 motor has a 2.3 x 2.3-in. mount flange, and a NEMA-34 has a 3.4 x 3.4-in. flange. Even the bolt-circle diameters and tapped-hole patterns are standardized to allow interchangeability across motor brands and models.
Metric motor mounts follow International Electrotechnical Commission (IEC) standards or manufacturer-specific metric dimensions defined by flange diameter — 60 mm, 80 mm, 90 mm, or 115 mm, for example. The circular flange has a center pilot bore encircled by symmetrically spaced bolt holes or slots. Common sizes are IEC56, IEC71, IEC80, IEC90; again, these names denote specific flange diameters and bolt-circle patterns.
The special case of 12-volt linear actuators
Linear actuators are typically characterized by their drive mechanism — belt drive, ball or leadscrew drive, pneumatic drive, and so forth. 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 such as 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-volt linear actuator includes a 12-volt dc motor integrated into or tightly coupled with the actuator body. 12-volt actuators are driven almost exclusively by one of two mechanisms — a ballscrew or a leadscrew. 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. However, most 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-volt 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-volt 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.
Performance and selection: 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-volt 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 body size of the actuator. Next is stroke length, since a small actuator may meet the thrust requirements, but may not be able to achieve the necessary stroke length.
Like slider type linear actuators, rod-style actuators driven by a ballscrew or leadscrew can back drive. When the application requires vertical operation, remember to check that the vertical load doesn’t exceed the back driving torque of the screw.
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 needed duty cycle, or on time — because motor heating can be a limiting factor for the actuator’s performance.
Suitable applications: Virtually any time a load must 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-volt actuators are often used for ergonomic positioning of worktables or patient beds.
In conveying operations, these actuators are commonly used to stop or divert product along the conveyor, depending on the process requirements. And because they’re fully enclosed and available in IP-rated or hygienic designs, rod-style actuators are suited for the pharmaceutical and food and beverage markets, where purely thrust operations (such as inserting, labeling, or stamping) are typical.
Leave a Reply
You must be logged in to post a comment.