How to Select the Best Mounting Configuration for Pneumatic Cylinders: Considerations for Optimum Performance

By Vitor Rangel, Product Marketing Specialist, Actuators, from Emerson

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If you have chosen a pneumatic actuator properly sized for your application, now it’s time to decide how you are going to mount it. Selecting the best mounting configuration for your cylinder application is a critical design consideration to ensure optimum performance and maximize cylinder life expectancy.

To facilitate installation and worldwide interchangeability, most cylinder mounts are built to NFPA or ISO standards. Although numerous variations are available, mounting styles will typically fall into one of three main categories: fixed centerline mountings, fixed non-centerline mountings and pivoted centerline mountings.

When selecting the right mount for the application, the primary element to take into consideration is the application itself. In most cases, the intended function of the actuator and the application requirements or constraints will largely determine which mounting type to choose. Some key factors to consider are the cylinder stroke (long versus short), the plane of motion (horizontal, vertical or along a curved path), the size of the load and the load condition (guided versus nonguided), the column strength of the piston rod and the installation space. In some circumstances, the material the mounting is made of could potentially be relevant as well depending on its mechanical properties and the application environment.

It’s worth noting that all of these factors are also applicable to high-pressure hydraulic cylinders for which the operating pressure could also become a decisive factor. Certain cylinder mounts will have reduced pressure ratings to prevent premature failures associated with excessive shear and bending stresses that result from the high forces and operating pressures that can be conceived with such designs.

Like properly adjusting cushioning, if the mounting configuration is not carefully considered it can cause cylinders to fail prematurely. Therefore, we would like to provide you with some guidelines and best practices to help you avoid any problems associated with cylinder mounting.

Understanding Cylinder Mounting Terminology

In general, the codification of cylinder mounting types covered by NFPA and ISO standards will consist of two letters and a number as per the NFPA examples shown below:

Fixed centerline mountings – Mounting types MX1, MX2, MX3, MF1, MF2, MF5, MF6, ME3, ME4, MS3.

Fixed non-centerline mountings – Mounting types MS1, MS2, MS4, MS7

Pivoted centerline mountings – Mounting types MP1, MP2, MP3, MP4, MT1, MT2, MT4, MU3

The first letter indicates if it’s a mount (M) or an accessory (A) and the second letter designates the style:

Note: It’s also possible for mounting codifications to consist of 3 letters. This is a common practice among double rod configurations where the letter (D) is added to the standard designation (e.g. MDF1 – double rod cylinder with front flange mount).  

Having covered the basics of mounting nomenclature, let’s now take a look at the distinct benefits and limitations of each of the main mounting categories.

Fixed centerline mountings

As the name suggests, fixed centerline mountings are best used in applications where the cylinder’s thrust is focused on the centerline of the cylinder rod. With the force focused this way, the cylinder may be attached at the head end or the cap end via square or rectangular flanges, extended tie rods or with centerline lugs (although this method is less commonly used). Fixed centerline mounts are suitable for straight-line force applications and tend to be more stable against sway on the power extension stroke. Flange mounts offer good strength and rigidity but low tolerance for misalignment whereas extended tie rod designs are less rigid but more modular, allowing for greater mounting flexibility.

When choosing a fixed centerline mounting, it’s important to be aware of a few additional things. To prevent misalignment and sagging under loads, one should attach the cylinder mount to a rigid structure (machine frame) and, if possible, ensure that the travel path of the rod end is linear and guided as this could also cause the pneumatic cylinder to jam. In the case of long stroke cylinders, the free end of the cylinder body may also require additional support to avoid some of the issues that were previously mentioned.

Fixed non-centerline mountings

In fixed non-centerline mountings, the thrust of the cylinder is aligned parallel to, but not on, the centerline of the cylinder rod. This mounting style is probably the easiest to mount and allows for simple, straightforward service and replacement. The cylinder may be attached via end lug mount, side lug mount or side tapped mount.

The offset thrust line of this mounting style does require special consideration as it places bending stresses and additional loads on the mounting bolts that can cause the bolts to wear or become loose over time. Therefore, to maximize service life and safety, this mounting must be very well aligned; the load should travel in a precise, linear path and be well supported and guided both horizontally and vertically. Rigid mounted cylinders cannot tolerate a consistent fixed misalignment, especially at full retraction. That being said, a minor misalignment that is zero at full retraction and increases very slightly with stroke is typically acceptable on heavy-duty constructions.

Pivoted centerline mountings

Pivoted centerline mountings are generally used in applications where the path of the load is curved or misalignment is an issue and cannot be avoided. With this mounting style, the centerline of the cylinder can swing in one or more directions with the major movement usually occurring in one plane. Clevis, eye and trunnion mounts are all examples of pivoted centerline mountings.

The clevis mount is one of the most versatile and widely used mounts, and it is best suited for short-stroke, medium or smaller bore cylinder applications. A trunnion mount, on the other hand, can be used in applications where the clevis mount would normally be used but would cause the overall length of the cylinder to be excessive.

For long stroke and/or heavy cylinders, the center or intermediate trunnion mount is best. In general, this mount supports the weight of the cylinder and should be located near the balance point of the cylinder at the time of maximum thrust.

The spherical mount is also a common and particular style of pivoted centerline mountings. Comprised of a spherical bearing, this mount allows for some angular movement (usually 5 to 10 degrees of motion) in a plane perpendicular to the major plane of pivot movement and should be used in conjunction with a grease fitting for maximum effectiveness and serviceability.

Last but not least, when considering pivoted centerline mounts, it is important to keep in mind that the rod end cylinder attachment should also be allowed to pivot, and it is best to use the cylinder manufacturer’s accessory brackets and close tolerance pivot pins to guarantee a proper fit and operation.

Additional considerations to optimize cylinder performance

As it has been alluded to throughout this article, to preserve the life of your cylinder and ensure top performance, it is very important to keep the cylinder thrust as close to the centerline of the piston rod as possible and free from misalignment or side thrust. Off-center thrust and side loads significantly shorten the anticipated rod bearing and rod seal service life and should be avoided at all costs.

Contact us to discuss your requirements of Aventics Cylinder. Our experienced sales team can help you identify the options that best suit your needs.

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In addition to the mounting style, there are several other aspects that should not be overlooked when installing a cylinder. Care should be taken to avoid damaging the exposed portion of the piston rod. Threaded pieces should be torqued to specification and pulled tight against thread shoulders to minimize bending and reduce fatigue stress. Excessive rotation of the piston rod within the cylinder should be avoided to prevent possible scoring of the cylinder tube and damage to the piston seals. Lastly, additional support or incorporation of an intermediate mount should be considered to prevent damaging sag on long stroke cylinders.

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When programming and executing high-speed automation, accuracy in motion sequences—particularly endpoint accuracy—is crucial to manufacturing speed, throughput, and quality. Every movement within a motion sequence must be precise; not only in its positioning, but also in its timing.

Electromechanical positioning is the most common way to accomplish this task in automation systems. In electromechanical systems, linear devices (e.g., profiled rails, ball screws, and linear modules driven by electric motors) lift, move, and place components. These systems can provide excellent positioning accuracy. However, some inherent drawbacks may, in time, increase overall manufacturing costs and affect productivity.

An alternative to electromechanical positioning that’s gaining popularity is electropneumatic positioning. Electropneumatic systems use electrically operated valves to control air flow or pressure to move an actuator to the required location.

In many cases, numerous advantages are associated with pneumatic positioning, including greater productivity, better repairability, and lower total cost of ownership. Pneumatic actuation systems also provide engineers with a number of implementation options, letting them select the best design for a given task.

Electromechanical vs. pneumatic

Accurate positioning refers not only to an actuator’s endpoint accuracy, but also to the accuracy of its position at any selected point along a path of motion. Although endpoint accuracy is important, positioning control throughout a motion sequence is often the desired goal. A positioning system should be able to find and maintain the required position anywhere along its stroke length.

With the advent of high-speed production systems across many industries, automation engineers now focus more than ever on finding cost-effective, energy-efficient positioning-control technologies that improve accuracy. Electromechanical positioning can meet these requirements. However, over the long term, certain aspects of electromechanical technology can lead to higher costs and cause production problems.

For example, electromechanical-positioning systems can heat up and undergo temperature-related changes as well as wear and tear. That, in turn, requires significant downtime to cool. In fact, lower-cost systems often are only capable of 50% duty cycles, spending as much time off as they spend working. Pneumatic-positioning systems, on the other hand, do not include mechanical parts that can overheat. This permits a 100% duty cycle with components that stay cool and can work around the clock.

Also, pneumatic-positioning systems are often more energy-efficient. For instance, electromechanical systems consume electricity simply to hold a position. And without power, electromechanical systems reset themselves, losing any previous work leading up to a power outage. Productivity suffers as a result.

Pneumatics doesn’t have this problem. When designs use pneumatic flow control, a valve in the center position blocks airflow in either direction and the cylinder piston automatically stays put. In the event of power loss to a production system or plant, it’s much easier to restart the operation with pneumatic-positioning systems. Proportional pressure control and normally closed valves provide the same benefits when power is lost.

In addition, pneumatic components are relatively easy and economical to repair. When pneumatic cylinders break or wear out, they can be repaired or replaced at a low cost. On the other hand, when an electromechanical system has issues, it typically requires an expensive new electric drive.

These advantages, combined with the inherent low-maintenance nature of pneumatic devices, create a significantly lower true cost of ownership versus an electromechanical setup. In fact, the cabling alone on an electromechanical device can cost more than an entire pneumatic system.

Pneumatic positioning

Pneumatic-powered actuation provides a proven method for accurate and reliable positioning. Most pneumatic-positioning systems, which combine control valves, cylinders, and simple sensors, rely on directional flow control to control positioning. Such a system monitors piston position feedback and decides the direction air needs to flow to achieve the desired position. Shifting a valve and allowing compressed air flow into the proper cylinder port extends the actuator to the required position. Reversing the valve retracts the cylinder in the opposite direction.

This type of system provides a broad range of directional flow control, translating to precise, incremental positioning control. The valves for such a system are selected based on the bore and stroke of the pneumatic actuator. In general, control of air flow can range from 0 to lpm.

Still, some drawbacks hinder the use of pneumatic flow control for positioning. The most common concern is associated with a basic limitation of pneumatics: Such systems cannot be perfectly sealed, and an incremental or intermittent loss of air and pressure is unavoidable.

With directional flow control, minor leaks cause the pneumatic cylinder piston to move slightly. The positioning system’s controller receives motion feedback from the sensor, and the controller responds by slightly adjusting flow to compensate for piston movement. This slight but constant back-and-forth motion, known as dithering, negatively affects system accuracy and functionality.

Proportional pressure positioning

Responding to concerns about positioning accuracy and dithering, engineers at AVENTICS developed a system that relies on direct-acting proportional control of air pressure—rather than directional flow control—to improve the precision of pneumatic-positioning systems.

The cost-effective design integrates control electronics with a range of standard pneumatic valves, cylinders, and actuators. But it differs from directional flow-control setups in that it uses direct-acting proportional pressure regulators to constantly monitor and adjust pneumatic pressure. That, in turn, permits precise control of cylinder positioning.

The electropneumatic positioning system (EPPS) uses two proportional pressure regulators mounted on the pneumatic cylinder, one on either side of the cylinder piston. One side maintains a constant pressure using the first proportional pressure regulator (E/P valve), while the second E/P valve controls pressure on the opposite side.

The unit’s electropneumatic positioning controller (EPPC) receives data from, and sends analog pressure command signals to, the proportional pressure regulators. The controller compares the pressure ratio of one side to the other. A proportional-integral-derivative (PID) loop delivers an analog output to the second proportional pressure regulator to adjust the pressure as needed. Greater pressure on the second side means the piston will move forward, and vice versa.

The main advantage of this method is enhanced accuracy. Proportional pressure regulators constantly and precisely control the pressure, eliminating effects of dithering. The direct-acting pneumatic pressure regulators can control pressure within 0.10 psi, and the EPPS system can use any of the company’s pneumatic actuators. As a result, system designers have much more flexibility.

Note that every actuator has different physical characteristics. Therefore, it’s difficult to make a broad statement regarding positional accuracy. However, with a properly selected actuator and optimized tuning of the controller, ±1-mm positioning accuracy is certainly possible. This is better than traditional flow-controlled pneumatic positioning.

Economical precision

While electromechanical and flow-based electropneumatic systems offer effective positioning capabilities, direct proportional pneumatic-control systems such as the EPPS maintain a number of inherent advantages.

The EPPS is designed to economically deliver the benefits of pneumatic positioning to a broad range of automation applications. The system uses standard, off-the-shelf components that are easy to integrate, and they can be quickly replaced when needed. The EPPS integrates an AVENTICS SM6-AL (Hall Effect) sensor, and does not require the cylinder piston rod to be gundrilled as in traditional pneumatic systems. By eliminating gundrilling, standard cylinders can be used, thus reducing cost and delivery time.

Although it relies on electronic controls, the EPPS does not require complex programming. A user simply inputs an analog position command, saving time and resources. Standard analog command for stroke set-point is 0 to 10 V dc or 4 to 20 mA.

In general, electropneumatic systems are more durable than electromechanical systems because they do not require electric motors or drives at the actuator. As a result, they can withstand harsh conditions such as temperature extremes, dusty conditions, and wet and dirty environments.

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