When determining motor control total cost of ownership, consider utility costs, maintenance costs and product yield for the service life of each.
By Aaron Dietrich, director of marketing, Tolomatic, Inc.
Editor's Note: This article is adapted from a comprehensive white paper, "Electric actuators vs. pneumatic cylinders: A comparison based on total cost of ownership." Download the free, full white paper with additional information on determining efficiency and electric utility costs and the effects of leaks, maintenance and replacement and product quality. The paper also includes some application examples.
Pneumatic cylinder actuators, known for their low initial cost and durability, have been a staple in factory automation equipment for decades. They are easy to use and maintain and provide reasonable control over machine movements in industrial plants. However, since the development of more flexible, precise and reliable electric actuators, a debate over which technology offers the best overall solution for industrial plant optimization has been ongoing.
The case for switching to electric actuators has focused on their ability to achieve more precise motion control (position, speed, acceleration and force), along with improved accuracy and repeatability. Electric actuators perform well, but have a higher initial cost. However, other factors contribute to making it a more economical option than air cylinders over the life of the device or machine. They include efficiency, electric utility costs, maintenance, product replacement, product quality, changeover time and cycle times.
Total cost of ownership (TCO) is defined here as:
initial purchase cost + years of service x (yearly replacement costs + yearly maintenance costs + yearly electric utility costs + yearly product scrap + yearly lost production due to changeover time and cycle time).
Determining Efficiency and Electric Utility Costs
While studies and reports focusing on ways to increase pneumatic system efficiency are not lacking, they make little mention of improving a plant’s overall efficiency — electric utility consumption — by considering nonpneumatic solutions that offer lower operating costs and production-boosting performance. It’s no secret that compressed air accounts for much of a plant’s energy usage. A typical compressed-air system’s overall efficiency can be as low as 10% to 15%.
For most applications requiring linear motion, efficiency differences between an electric and a pneumatic system can result in different and varying electric utility costs over the device’s lifetime.
Let’s assume that every pneumatic system has an efficiency of 20%, and every electric system has an efficiency of 80%. With pneumatic systems, efficiency can vary from 10% to 30% depending on air quality, seal quality and wear, leaks in the system infrastructure and a variety of other considerations.
All of these factors require constant attention and maintenance, or system efficiency will suffer. By comparison, electric actuator efficiency does’t change drastically over time. Consider the following pneumatic cylinder applications for a 1-in. (25-mm), 3-in. (80-mm) and 5-in. (125-mm) bore cylinder.
By reducing a sample application’s power costs to some simple formulas, you can get a good estimate of the electric utility cost associated with a single axis of motion.
- Power-OUT (kW) = Velocity (m/sec.) x Force (kN)
- Power-IN (kW) = Power-OUT(kW) ÷ Efficiency (%)
- Electric Utility Cost of Application = (Power-IN) x (Hours/year) x (Electricity
- Cost per kW-hr)
- Assuming $0.08 (8 cents) per kW-hr.
- Application #1: 1-in. bore or 25-mm bore equivalent @ 80 psi
- Force: 0.33 kN (or ~62 lbf)
- Speed: 0.3 m/sec. (or ~12 in./sec.)
- Power-OUT (kW) = 0.1 kW
- Application #2: 3-in. bore or 80-mm bore equivalent @ 80 psi
- Force: 2.5 kN (or ~565 lbf.)
- Speed: 0.2 m/sec. (or ~8 in./sec.)
- Power-OUT = 0.5kW
- Application #3: 5-in. bore or 125-mm bore equivalent @ 80 psi
- Force: 7.0 kN (or ~1,570 lbf.)
- Speed: 0.15 m/sec. (or ~6 in,/sec.)
- Power-OUT = 1.0kW
As with any device consuming electric power, the number of times the device works or is cycled is related directly to the amount of electricity it uses. Therefore, duty cycle (time working ÷ ((time working + time at rest)) plays a large role in calculating electricity costs for a pneumatic cylinder or electric actuator. Note in the tables that, because efficiency is lower in pneumatic systems, energy costs rise more steeply as the duty cycle increases.
With respect to improving efficiency in manufacturing facilities, this table makes it clear why managers need to identify all of the higher duty cycle pneumatic cylinders in the plant and discontinue the practice of basing actuator selection simply on initial cost.
Simplifying Machine Changeover and Setup Time
Applications that require product changeovers and multiple setups often will benefit from conversion to electric actuators. For example, if a process or machine requires changeover or setup to run different sizes or different products in the same machine, then an electric actuator can automate that changeover.
If the application involves adjusting hard-stops for pneumatic cylinder positioning, this too can be automated in an electric system. While a pneumatic system often requires adding rod-lock spacers to the cylinder to gain different or multiple positions, this can be programmed in an electric system.
Consider Cycle Time and Throughput
Another important consideration is cycle time. Compare the profitability of investing in improvements to cycle time and the equipment’s overall throughput and efficiency. That will help in weighing the benefits of replacing pneumatic actuators with electric actuators.
Pneumatic cylinders typically are deployed as two-position devices. If a process has any tooling that has to be moved out of the way for a changeover process or other process reason, then the pneumatic cylinder must be purchased with the full stroke in mind. During runtime, this means that the pneumatic cylinder must cycle back and forth across its full stroke even if it is not required for the runtime process, which increases production time.
Furthermore, if the pneumatic cylinder needs to develop force in this process, additional delays can be introduced because the cylinder must build up air pressure to achieve the desired force.
This typically doesn’t take a lot of time (usually tens or hundreds of milliseconds) but it nonetheless is wasted time in every cycle and is cumulative. Again, an electric actuator can do away with both of these problems. It can stroke the tooling only as much as is needed (not the full stroke) to get it out of the way for the product to move into position, saving valuable cycle time.
Electric actuators also can develop force almost instantaneously because their force is directly equivalent to electrical current through the motor. This helps eliminate any wait time experienced when using a pneumatic cylinder, which has to develop pressure to achieve force.
Tolomatic, Inc., based in Hamel, Minnesota, is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™ program. Tolomatic designs and manufactures electric and pneumatic linear actuators.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.