Twelve Tips for Selecting a Hydraulic Cylinder

Hydraulic Cylinders use fluid pressure and flow to generate linear motion and force. They work well in industrial machines like presses and plastic molding machines, as well as mobile equipment like excavators and mining trucks. Compared to pneumatic, mechanical, or electric linear motion systems, hydraulic systems can be simpler, more durable, and offer higher power density.

Hydraulic cylinders are available in a variety of types and sizes to meet a wide range of application needs. Choosing the right cylinder is crucial for achieving optimal performance and reliability. Here are 12 practical tips for selecting, specifying, and using the best cylinder for the job.

1. Choosing the Right Cylinder Type

The two basic Hydraulic Cylinder Designs for industrial applications are tie-rod cylinders and welded cylinders.

Tie-rod cylinders use high-strength threaded steel rods on the outside of the cylinder housing for added strength and stability. In the United States, this is the most common cylinder type. They are used in most general industrial applications, such as plastics machinery and Machine Tools, although they are often limited to a maximum working pressure of 3,000 psi. These cylinders are manufactured to NFPA standards, making them interchangeable in size and pressure rating with any other cylinder built to that standard.

Welded or round cylinders have a heavy-duty outer shell with the barrel welded directly or bolted to the end caps, eliminating the need for tie rods. They are designed for higher pressures of 5,000 psi or more and are often preferred in more rugged applications, such as presses, steel mills, and offshore environments with harsh environments and wide temperature fluctuations.

Unlike American OEMs, European manufacturers typically use round cylinders for nearly all general industrial applications. (They also use tie rod cylinders, but typically for pressures up to 160 bar (2,350 psi). However, due to their design, tie-rod cylinders are less expensive than mill-type cylinders—another reason for their widespread use in the United States.

In addition, cylinders are often custom-made. NFPA cylinder standards specify dimensions, pressure ratings, mounting types, and more—they are standard catalog products. However, engineers designing custom machinery often need to deviate from the standards through special mounting, port sizes, or configurations to accommodate specific applications. Approximately 60% of cylinders sold in the United States are catalog products, while 40% are modified products with unique requirements.

2. Select the Appropriate Mounting Hardware

The mounting method also plays a significant role in cylinder performance. The cylinder mounting method primarily depends on whether the cylinder barrel is stationary or rotatable.

For fixed cylinders, a fixed mount on the cylinder centerline is generally best for linear force transmission. and minimize wear. Among the different variations, flange mounts are generally preferred. The load is centered on the cylinder, and opposing forces are evenly balanced across the rectangular or Circular Flange. They are strong and rigid, but have little tolerance for misalignment. Experts recommend using cap-end mounts for thrust loads and rod-end mounts for tension loads.

Centerline trunnion mounts also absorb centerline forces but require dowel pins to secure the trunnion to prevent movement under high pressure or shock conditions.

Side- or foot-mounted cylinders are relatively easy to install and service, but they generate offset loads. When the cylinder applies force to the load, the bracket experiences bending moments, which can increase wear. Heavy loads often make long-stroke, small-bore cylinders unstable.

Side- and foot-mounted mounts need to be well aligned and located on the same plane, supporting and guiding the load. Otherwise, misalignment can cause damage. Side loads can cause cylinder wear and seal leaks. Engineers must also be aware of shear forces on the bolts. Add pins or shear pins and keyways to the rear legs to prevent forces that could shear the mounting bolts. If additional support is needed, add another set of legs in the middle of the cylinder, in addition to the legs at the head and cover ends.

3. Choose the Correct Pivot Mount When the Cylinder Body Moves

Pivot mounts absorb forces on the cylinder's centerline and allow the cylinder to change alignment in a single plane. Common types include clevis, trunnion, and spherical bearing mounts.

Clevis mounts can be used in any orientation and are generally recommended for short-stroke and small- to medium-bore cylinders. Cylinder engineers prefer clevis mounts with spherical bearings to plain bearings because they allow more misalignment and are therefore more forgiving. However, if the rear clevis is In addition to spherical bearings, they also recommend using a rotatable rod-end attachment—such as a spherical rod spherical bearing. This combination helps compensate for any side loads or potential misalignment.

Trunnion mounts are available in head, center, and rear mount versions. The center trunnion design is perhaps the most common because it offers designers greater flexibility. They can be precisely specified in most locations, either in the center or at the front or rear of the cylinder, depending on the application requirements. However, once specified, the mount is not adjustable.

For all cylinder types, important parameters include stroke, bore, rod diameter, and pressure rating.

4. Piston Rod Diameter is Critical

Perhaps the most common mistake in hydraulic design is underspecifying the piston rod, leaving the cylinder more susceptible to stress, wear, and failure. Piston rod diameters range from 0.5 inches to 2. 0 inches and above, but they must be sized appropriately for the available load. In push applications, it's important to correctly size the piston rod diameter according to Euler calculations to avoid bending or flexing of the piston rod.

When designing a cylinder to produce the required force, piston rod size is always the first consideration. From there, work backward and determine the bore diameter for the available pressure, and so on.

5. Prevent Piston Rod Bend

In long-stroke cylinders, a fully extended piston rod can bend under its own weight. Excessive bending can cause wear and damage to seals and bearings. It can even cause the piston to cock in the bore, scoring and damaging the cylinder's inner surface. Piston rod deflection should not exceed 1 to 2 mm.

Piston rods at risk of bending or misalignment require additional support. Depending on the stroke length, a stopper with increased bearing area may be necessary. Tubing (increasing the load-bearing area of ​​the cylinder) may be necessary, but it may be necessary to prevent excessive wear and scoring. Engineers may also consider using a larger diameter piston rod for added strength. However, this also adds weight and can be counterproductive, so careful calculations should be made. In extreme cases, users may also need to add external mechanical support to the piston rod, such as a saddle-type bearing.

6. Be Aware of Shock Loads

Stroke length, the distance required to push or pull a load, can vary from less than an inch to several feet or more. However, when the cylinder extends or retracts, ensure that the piston does not bottom out and generate shock loads at the end of the stroke. Engineers have several options: add an internal cushion to decelerate the load at the end of the stroke; add an external mechanical stop to prevent the cylinder from bottoming out; or use proportional valve technology to accurately measure flow and safely decelerate the load.

7. Weighing the Aperture Relationship to Working Pressure

To produce a given force, engineers can specify a larger cylinder that operates at a lower pressure, and vice versa. Generally speaking, a system operating at a higher pressure but with a smaller cylinder is more cost-effective. Furthermore, the benefits are multiplicative. A smaller cylinder requires less flow, which in turn requires smaller pumps, lines, valves, etc. Many installations reduce overall costs by switching to higher pressures.

That is, both the rated (standard) pressure and the test pressure of the cylinder account for variations. The system should never exceed the nominal rated design pressure of the cylinder.

8. Add a Safety Factor

While design calculations are essential, actual operation will differ from theoretical results. Always assume that the peak load will require additional force. A rule of thumb is to select a cylinder with a rated tonnage 20% higher than the load requirement. This compensates for load friction, hydraulic system Losses include efficiency loss, actual pressure below rated system pressure, and adhesion on cylinder seals and bearings.

9. Match the Seal to the Job

Seals are perhaps the most vulnerable aspect of a hydraulic system. The right seal reduces friction and wear and extends service life, while the wrong seal can lead to downtime and maintenance headaches. This may go without saying, but make sure the seal material is compatible with the fluid. Most hydraulic systems use a mineral oil, and standard nitrile rubber seals tend to work well. However, applications involving synthetic fluids (such as phosphate esters) require fluoroelastomer seals. Polyurethane is also incompatible with highly water-based fluids such as water glycol.

Regardless of the fluid used, keep it clean. Contamination and dirt in the fluid can damage the seals. It can also scratch the inside of the barrel and eventually ruin the cylinder.

Standard nitrile rubber seals may fail if the operating temperature exceeds 300°F. Fluoroelastomer seals, such as synthetic rubber seals, can typically handle temperatures up to 400°F. F, and fluorocarbon seals can withstand even higher temperatures. When in doubt, assume the situation will be worse than it initially appears.

10. Add a Cylinder Head Drain Port

Perhaps 90% of cylinder failures are due to seals. This is true even if engineers specify the appropriate seals for the fluid, pressure, environment, and application, as seals wear over time and require replacement. Most experts recommend regular seal maintenance rather than waiting for failure at an inopportune time.

If the cylinder is located in an inaccessible location, making maintenance difficult, or if a leak would damage the product or cause costly downtime, order a cylinder with a "cylinder head drain port." This is A special port, called a "head drain port," is machined into the cylinder head between the primary and secondary seals, or between the primary seal and the wiper. If the primary rod seal begins to fail and leak, oil will bypass the seal and flow out of the head drain port—usually through a tube into a collection bottle. If oil collects in the normally empty bottle, it's a visual indicator that the seal is wearing and will soon need to be replaced.

Cylinders often have a secondary rod seal or double-lip rod wiper to temporarily prevent oil from leaking from the rod end, giving maintenance personnel time to schedule repairs.

11. Observe the Materials

The type of metal used for the cylinder head, base, and bearings can have a significant impact. Most cylinders use SAE Using 660 bronze for the piston rod bearing and medium-grade carbon steel for the cylinder head and base is sufficient for many applications. However, stronger materials, such as 65-45-12 ductile iron for the piston rod bearing, can offer considerable performance advantages for demanding industrial tasks.

Also, consider temperature extremes. Typical carbon steels used for cylinder components are generally suitable for operating temperatures ranging from -5°F to approximately 200°F. For example, in Arctic conditions far below 0°F, standard steel may become brittle, and an alternative material may be required.

12. Protect the Piston Rod

Due to the contact between the piston rod and the external environment, it must resist the erosion of harmful substances such as water, salt air, and corrosive substances. Generally speaking, chrome plated carbon steel is the norm in industrial applications. But in humid or high humidity environments, such as marine hydraulic systems, most piston rods are made of chrome plated 17-4PH stainless steel. Some oil cylinder manufacturers offer special protective coatings.

Engineers love and hate protective piston rod dust covers for dirty and worn conditions. Installing a protective sleeve on the piston rod can prevent dust, metal shavings, and other external pollutants from entering, otherwise it will damage the piston rod and ultimately damage the seal. However, if the boots are punctured or torn, dirt will be sucked in and may not be able to be expelled, which is worse than having no boots at all. Maintenance personnel must regularly inspect boots for wear or tear, as these dust covers may accelerate cylinder damage.

Summary

Selecting the optimal hydraulic cylinder is a multifaceted process that requires careful consideration of cylinder type, mounting style, and operating environment to ensure durability and performance. Key factors include choosing between tie-rod and welded designs based on pressure requirements, properly sizing the piston rod to prevent bending, specifying appropriate seals and materials for fluid and temperature compatibility, and incorporating safety features like head drain ports and cushioning systems. By accounting for shock loads, adding a safety margin, and protecting components from contamination and misalignment, engineers can significantly enhance reliability, reduce maintenance costs, and extend the service life of hydraulic systems across diverse applications.