The first hydraulic press may have been invented in the 3rd Century BC, but the fluid power universe has become a little more complicated since then. Today’s hydraulic cylinders, which essentially convert fluid pressure and flow into force and linear movement, are complex devices incorporating a wide range of individual components available in a multitude of dimensions, configurations, and materials.
Although pneumatic systems are in some respects simpler, they are generally incapable of achieving the transfer of higher loads and forces. Hydraulic cylinders also have the advantage of smoother, more controllable movement as they are devoid of the spring-like action associated with the release of gaseous fluid media. As an added benefit, hydraulic systems can perform ancillary functions such as lubricating and cooling.
However, since the availability of power and media is a non-negotiable factor in fluid power system design, it should be noted that a properly designed and sized pneumatic system can achieve higher performance where a compact footprint is not required.
Specifying hydraulic cylinders is essentially a balancing act as each design factor influences one or more of the many other design considerations.
Although NFPA standards and ISO-compliant guidelines are a great starting point for hydraulic system design, many industries have guidelines of their own. Working with an engineering manufacturer experienced with all these standards can expedite the design process.
Cylinder manufacturers can offer a range of options capable of achieving the widest scope of performance requirements that increase the likelihood that standard components will meet the design criteria of an application. The major factors to consider when specifying hydraulic cylinders include:
Capacity: Medium-duty systems account for most of industrial applications and are typically at 1000 PSI. Standard heavy-duty hydraulic cylinders are capable of handling pressures as high as 3000 PSI, which are typically required for hydraulic presses, automotive applications and other related industrial applications.
Stroking distance requirements: Although custom stroke distances above 10 feet (3.05m) are possible. Pressure rating can be a concern. Rod diameter needs to be determined to handle the load.
Speed: Standard hydraulic cylinder seals can easily handle speeds up to 3.28 feet (1 meter) per second. The tolerance threshold for standard cushions is roughly two thirds (2/3) of that speed. Frequently, a standard low-friction seal is the better choice for higher speed applications.
Temperature: Hydraulic cylinder systems using standard components can be designed to meet application temperatures as hot as 500°F (260°C) and as cold as -65°F (-54°C). Applications requiring temperature extremes at either or both ends of the temperature spectrum require extensive knowledge of the interdependency of individual components to achieve the best balance of short- and long-term performance expectations.
Mounting styles: There are three types: a) Fixed mounts that absorb force along the centerline of a cylinder, b) Fixed mounts that do not absorb force along the centerline, and c) Pivot mounts that drive a load in a curved path.
Cylinder bore size: Bore size is related to operating pressure. It is the amount of push or pulls force required that determines the bore size needed.
Piston rod size: OEM design engineers probably request customization of piston rod sizes more frequently than any other hydraulic cylinder component. Remember to always consider that push or pull is never independent of stroke length when determining rod size.
Cylinder configurations: For applications requiring equal force pressure on both sides of the piston, a standard double-acting cylinder configuration using pressure to extend and retract the cylinder, combined with a four-way directional control valve to direct pressure to either the head or the end cap, is almost always preferable to more customized solutions.
Rod ends/rod threading: Standard threads can be made in inch or metric format, customization is rarely needed or warranted due to delays, expense, and the inability to readily mate with accessory components.
Cylinder body tube: Standard cylinder bodies are plain steel or chrome-plated and will be able to handle a majority of applications. Using alloy steels, stainless steel or brass materials are prevalent in special applications like a water type environment.
Stop tubing: Stop tubing is generally used to lengthen the distance between the rod bearing and the piston bearing in order to reduce bearing load on push-stroke cylinders when the cylinder is fully extended. Stop tubing is especially critical for horizontally mounted cylinders where it helps to restrict the extended position of the rod.
Seals: Experienced hydraulic system manufacturers will offer seals to meet a complete range of temperatures and fluid types and can help guide an engineer’s specification to meet precise application requirements.
Every industrial application is unique, and there are many ancillary components involved in hydraulic cylinder specifications. Energy-absorbing cushions, pillow blocks, regenerative circuits, over- or under-sizing ports — all these and more contribute to optimizing the performance of hydraulic systems, depending on each application’s specific performance requirements.
As with the specification of more fundamental components, selecting appropriate ancillary components can present a specification challenges. For example, cushions are intended to retard the force of motion, but OEM engineers sometimes overlook the fact that fluids are typically not moving very fast anyway and may not require such redundancy in certain types of systems. An engineer may be tempted to take a “belt and suspenders” approach to design push/pull systems by using cushions with spring cylinder systems, overlooking the fact that the oil needs to work its way through the cap, hoses, valves and so on. In such cases, specifying standard single-action cylinders with cushions may be wiser than attempting to insert cushions into spring cylinders.
There are certainly applications for which specifying the right cylinder for the right duty requires some customization either in component size, material type, or configuration. However, far more often than not, partnering with an experienced hydraulic system solution manufacturer early in the design process will save the OEM engineering team time and money while ensuring the system does its assigned duties as efficiently as possible for as long as possible.
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Article contributed by Jim Hauser, senior engineer, Parker Hannifin Corporation's Cylinder Division