Sparking motion

Electric actuators move to a desired linear position in response to an external power source. To accomplish this, components such as screws and belts must be able to operate at high speeds. Certain design and application factors, however, can limit their consistently quick motion.

To provide additional insight into what factors affect speed during electric actuator operation, we asked industry experts for their input. Here's what they shared.

DESCRIBE A CASE WHERE EXCESSIVE SPEED CAUSED AN ELECTRIC ACTUATOR TO FAIL.

John/Exlar: An operator was running a ballscrew actuator at 3,500 rpm. Due to friction and ballistic conflicts in the screw, its operating temperature rose very high, and the ballscrew failed within a few months. It was replaced with a planetary roller screw of the same lead, which allowed a drop in replacement and no programming changes. Since planetary roller screws accommodate higher rotational speeds, this actuator operated for several years.

Andy/Kerk: In many applications, engineers install end-of-travel limit switches to abort motion. In this arrangement, operators should command the motor to stop with controlled deceleration. Moving the limit switch or slowing down the actuator can prevent crashes.

Troy/Tol-O-Matic: There was a situation where an end user exceeded a machine's designed throughput, increasing peak velocity of the belt-driven actuator. Servomotor acceleration and deceleration rates were also raised. While the actuator functioned at these elevated levels initially, the application quickly failed. The belt surpassed its capability, and after a couple of cogged pulley teeth, the actuator's carriage was greatly misaligned. For this installation, a tooling mass attached to the carriage also contributed to failure. Increased acceleration rates (and resulting dynamic moment loads) exceeded the ball bearing support's capabilities. In the end, the carriage, support mechanisms, belt, and tooling were damaged beyond repair.

WHAT DESIGN/ CONSTRUCTION FEATURES IN ELECTRIC ACTUATORS LIMIT SPEED AND WHY?

John/Exlar: In screw-driven actuators, this breaks down to the lead, critical speed of the screw shaft (limited by length and diameter), and allowable rotational speed of the nut on the screw. Ballscrews offer aggressive leads, and thus high linear speed, but ball bearing interactions limit their rotational speed. Planetary roller screws provide even higher rotational speed. However, both are equally limited by the critical speed of its rotating screw shaft.

Andy/Kerk: A screw's rotational speed limit, or critical speed, can affect speed limits on leadscrew-driven actuators. Those rotating at or above critical speed build up resonance, causing them to whip. Screw diameter and length, as well as the bearing support system, determine critical speed. In short, the longer an actuator, the lower its upper speed limit. To improve speed limits, designers can focus on larger screw diameters, better bearing supports, and longer leads.

Yoshi/THK: Generally, an actuator consists of a drive and linear motion guide. Both can restrict speed. Common driving methods for electric actuators include ballscrews, belts, and linear motors.

Ballscrews can limit an actuator's speed because of their own critical speed and DN values. When a ballscrew's rotational speed nears the screw shaft's natural frequency, it resonates and eventually becomes inoperable. Therefore, designers should employ them below their resonant point (critical speed). Regarding the DN values, D represents the ball's center diameter, and N signifies the permissible rotational speed of the nut. (Changing the nut design can increase the value of N.) As an example, the DN value for ground ballscrews is 70,000 and for rolled screws, 50,000. A ball cage and circulation structure that picks up balls tangential to the raceway can raise these DN values to 160,000.

Better ball arrangements can also increase guide speeds. Motion guides limit belt and linear motor speeds in electric actuators because ball collisions in the block generate heat. Designs eliminating this friction achieve velocities up to 5 m/sec.

Troy/Tol-O-Matic: All screw-driven mechanisms have a critical velocity — the rpm at which a screw begins producing resonant vibrations, known as whip. Multiplying this rpm by the lead yields a screw's maximum linear speed. Screw diameter, length, and support mechanisms all affect critical speed.

The nut is a second limitation. Solid nuts, often used with acme or trapezoidal screws, are threaded and made of materials such as Delrin or bronze. Their friction and high heat generation can prevent screws from approaching critical velocity. In ball nuts, bearings run along rolled or ground tracks between the screw and nut and through recirculation mechanisms. This complicated action can also limit speed.

Belt-driven actuators achieve higher velocities because they do not have rotating components. Here, belt material, pulley profile design, and tension affect maximum speed. Designers should be aware that values applied improperly may cause a belt to slip or cog.

Ball bearing mechanisms supporting the pulleys (or load) limit most belt drives. They are limited by the return mechanism, where balls change direction. The reflected inertia of each ball must be contained through this change of direction, which can be challenging in high-speed applications.

WHAT APPLICATION FACTORS LIMIT SPEED?

John/Exlar: Operating at high speeds requires more power to stop a load. Therefore, remaining power can limit motor acceleration and deceleration, restricting a system's ability to achieve top speed in the time available. Bringing a speeding system to a halt can increase settling times and limit available high-end speed.

Andy/Kerk: Stepmotors often limit speed. Servomotors, on the other hand, can reach speeds greater than 5,000 rpm.

Available torque is another factor. While greater torque translates to higher acceleration, it also increases the time to move an actuator to higher speed. An additional factor is the load's ability to accommodate high accelerations: Delicate loads require slower acceleration, such as S-curves, which reduces the time available for an actuator to get up to speed.

Troy/Tol-O-Matic: Load and associated moments can limit speed. For example, suppose a 100-lb mass rests on an actuator where the center of gravity is 6 in. above the bearing system. Both static and dynamic roll and yaw moments are zero. However, moving the center of gravity 6 in. to one side increases the static roll moment to 600 lb-in. Accelerating or decelerating this off-balanced load causes the actuator to experience additional moments, such as dynamic yaw.

Without external support guides, most application loads are not centered directly on a moving carriage. For this reason, designers must consider the full motion profile (including peak thrust) in accelerating and decelerating the load at a specific rate. Peak acceleration values, as well as a load's off-center of gravity, create many dynamic moment loads. It's commonly overlooked, but when multiple moments are applied, the sum of all moments should be calculated together and averaged.

WHAT ARE THE MAIN ATTRIBUTES OF HIGH-SPEED ELECTRIC ACTUATORS?

John/Exlar: The main attributes of screw-driven actuators are a high DN value — which equates to allowable rotary speed — and leads providing high speed when rotated in allowable ranges. Aggressive leads provide more linear travel per revolution than finer leads.

Andy/Kerk: High-speed leadscrew driven actuators generally include high leads. Nut designs that reduce radial and axial plays and that are properly guided also play a role. In addition, servomotors are often employed, along with direct coupling, so gear and pulley ratios don't reduce available linear speeds.

Troy/Tol-O-Matic: One attribute is a bearing system that accommodates the dynamic moment loads and linear velocity rating. A second is the driving force mechanisms, which must operate safely at the required velocity — and meet peak thrust requirements.

For additional information, email the editor at [email protected].