Sorting Out Linear Actuators

Jan. 15, 1999
Motors, gearing, drive screws and controls all play a critical role in determining actuator performance.

Tim Levindofske
Timothy Sulski
Enidine Inc.
Orchard Park, N.Y.

Electromechanical actuators have become the preferred means of generating motion in countless applications ranging from material handling and entertainment ride simulators to microchip handling and antenna positioning. There are a number of reasons why.

For one, the actuators are rugged but lightweight, making them well suited for industrial use. They are also built for tough environments. Construction often includes noncorrosive aluminum and stainless-steel hardware with sealed and lubricated-for-life ball bearings. Wiper seals on the translating tube protect internal mechanisms from foreign particles. The designs typically operate from –20 to 150°F, with lower or higher temperature ranges available.

Another plus is outstanding performance with little maintenance. Low-backlash belt or gear drives ensure precise positioning, and ball or Acme screws are built for long life.

Finally, the systems are highly flexible. Manufacturers can deliver a complete actuator system or mate individual subsystems with the customer’s motor or controls. In either case, specifying motor, drive screw, gearing, and other critical components lets designers precisely tailor an electromechanical actuator to an application.

Motor options
Electromechanical actuators provide linear motion through a series of components working hand-in-hand. An electric motor turns a coupling, gear, or pulley that rotates the drive screw. This forces a drive nut to travel linearly along the screw which, in turn, provides linear thrust and motion to the external rod.

Precisely matching an actuator to an application, therefore, requires selecting components to operate well together. The first order of business is to define design requirements. Three main considerations are usually thrust, speed, and maximum stroke.

Then look at the type of motor and controls that provide the needed motion profile, accuracy, and fall within the allotted budget. For instance, requirements are obviously different for simple extension and retraction versus a complex profile with multiple speeds and stops. So first determine the right type of motor, then size it to provide the required thrust.

Experts typically recommend reviewing the capabilities of four different motor designs that are commonly used on linear actuators and positioners. The first, a permanent-magnet, brush-type dc design accelerates quickly to high speeds, offers high torque in a small frame, and provides maximum torque at stall. The motors require no starting capacitors, allow for speed control, and typically carry the lowest cost.

Permanent-magnet dc servomotors offer all of the benefits of permanent-magnet dc brush motors but have better brushes and are more precise. This also makes them a bit more expensive.

Step motors, a third option, feature long life because they have no brushes to wear out. They are moderately priced, produce high torque, are rated for continuous duty, and offer high precision.

Brushless servomotors are the fourth type. As the name implies they also have no brushes and can generate high speed and torque in a small frame. Other features include fast acceleration and deceleration, compatibility with incremental or absolute encoders, and a capability for speed and positioning control. They are rated for continuous duty and generally provide long life.

Because there can be considerable overlap in the capabilities of different motors, always keep in mind overall costs. For instance, it is a waste of money to apply a stepper to a basic linear-motion job when a less-expensive brush-type motor will do.

Opting for a servosystem before a review of other options produces another common design problem. Close-loop control means gathering feedback data from an encoder or position sensor and relaying it to the controller. The controller, in turn, directs the stepper or servomotor to adjust for system error. The problem is, servosystems can be expensive. An open-loop system often provides suitable accuracy and repeatability at a much lower cost.

Mechanical considerations
Another key component is the drive screw, and this usually means choosing between Acme and ball screws. The choice is often clear cut. An Acme screw and nut assembly — at about one-third the price of a comparable ball-screw assembly — is typically the more-economical option in limited-duty applications. They are constructed of cold-rolled 1018 steel or a stainless steel. Efficiency typically ranges from about 30 to 65%, but they offer self-locking capabilities. Acme screws also generally run quieter than ball screws so are preferred in applications where noise is a consideration.

Advantages ball screws hold over Acme-thread screws include higher efficiency, speed capability, load capacity, and duty cycle, as well as less friction and longer life. One typical construction of 1018 steel is hardened to Rc 65.

Ball screws use high-quality steel nuts with recirculating steel balls to handle heavy loads and ensure long life. Normal backlash is about 0.004 to 0.015 in. On Acme screws, Delrin drive nuts (a combination of Delrin and Teflon) offer quiet, low-friction operation. Manufacturers recommend bronze nuts over Delrin when higher load capacity is required.

Operating life of linear actuators depends on many factors, such as velocity, thrust, load, moment, environment, duty cycle, and the type of screw and motor. But the choice of screw is often straightforward: when the duty cycle exceeds 60%, use a ball screw. For instance, in a continuous duty application experts recommend a ball screw and nut along with a motor rated for continuous (100%) duty. Acme screws (with leads less than
0.5 in.) and nuts produce relatively high friction with efficiencies of less than 60%, and generate heat at high linear velocities and high duty cycles. Ball screw and nut combinations have efficiencies of 90% or higher and are designed for high loads and speeds. Proper selection lets actuator life reach 50 million cycles under normal operating conditions.

After selecting a screw type, look at the design details as they relate to actuator speed and accuracy. That means narrowing the design in terms of lead, pitch, efficiency, and diameter.

The screw lead is the linear distance the drive nut moves for each screw revolution. For example, Dynact actuators and positioners use precision rolled screws with a lead accuracy of ±0.004 to ±0.015 in./ft of screw length. Ground screws are available with lead error of around ±0.0005 in./ft of length, nonaccumulative, but at a considerably higher price. The higher the lead of the screw, the less effort required to backdrive either the screw or the nut. As a rule, the screw lead should be more than one-third the screw diameter to satisfactorily backdrive. Typical values are shown in the table.

Pitch designates the number of threads per inch of screw length. Screw efficiency is a measure of the torque required to rotate the screw, drive the nut, and move a known load,

Load-carrying capacity of an actuator is directly related to the screw diameter, although the torque capacity of the motor is more often the limiting factor in a design.

The screw size — both length and diameter — is also a consideration when it comes to system stability. All screws have limits to the rotational speed at which they can turn. That is because every screw has a particular speed — called the critical velocity — at which the screw will resonate, whip, vibrate, and render the system unstable and unusable. Rotational speed, screw diameter, and length determine critical velocity. To prevent this condition, consult with critical-velocity tables or the manufacturer for the particular operating conditions under consideration.

Another design consideration is the gear reduction between motor and drive screw. This is often determined by thrust requirements and space constraints. Three common types of gear reduction are synchronous (timing) belts and pulleys, worm-gear drives, and inline drives with no reduction.

No-slip synchronous belts have certain advantages over worm gears. Among them are long life, low backlash, and they are unaffected by minor misalignments. In addition, they require no lubrication, run quiet, and have power-transmission efficiencies of approximately 90%.

Worm-gear drives have the advantage of a high reduction ratio. Typical gear ratios range from 10:1 to 40:1. Timing belts generally have much lower ratios, on the order of 2:1. Worm gears also take up little space and have high load capacity, but efficiency is only about 50%. And they are limited to low-speed applications, generally about 2 ips.

Direct drives have no gear reduction. The motor shaft connects to the drive screw with a mechanical coupling. Advantages include zero backlash, long life, no gear noise, and the highest efficiency, around 99%. However, due to lack of reduction gearing, thrust capabilities in direct-drive actuators are limited by motor capacity. And because the motor mounts to the end of the actuator, they are not a good choice where overall length is a concern.

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