Hydraulic cylinders

Few things represent a fluid power system the way a hydraulic cylinder does. These true workhorses operate in industrial and mobile applications.When compared with pneumatic, mechanical or electric systems, hydraulics can be simpler, more durable and also offer greater power density. For example, a hydraulic cylinderhas about ten times the power density of anelectric linear actuator of similar size.

Selecting the right cylinder for an application is critical to attaining maximum performance and reliability, which means taking into consideration several design and performance parameters. Fortunately, an assortment of cylinder types, mounting methods and “rules of thumb” are available to help select the appropriate cylinder.

CYLINDER TYPES

The three most common types of cylinders are tie-rod, welded and ram, the latter of which is single acting, meaning it is powered in one direction only.

Tie-rod cylinders can be single acting, although they are most often powered in both directions. They have machined, square caps and heads being forced together against the barrel by high-tensile steel tie rods fastened by nuts, making them easy to disassemble and repair in the field. Welded cylinders employ a steel barrel with a cap welded to the bottom and the end treatment subsequently welded to the cap. The rod and piston assembly then has to be assembled around the head — which uses a buttress thread for strength — and is tightened into the barrel. Finally, the singleacting ram is typically just a rod inside a barrel with a single port and requires either a spring or mass to retract.

For all cylinders, the critical measurements include stroke length and bore and rod diameter. Stroke lengths vary from less than an inch to several feet or more, depending on the requirement of the machine. Bore diameters can range from 1 in. up to more than 24 in., and piston rod diameters range from 1-2 in. to more than 20 in. In practice, however, the choice of stroke, bore and rod dimensions may be limited by environmental or design conditions.

Image courtesy of RAM Industries

CYLINDER MOUNTING METHODS

Mounting methods also play an important role in a cylinder’s performance. Generally, fixed mounts on the centerline of the cylinder are best for straight line force transfer, ideal column loading and avoiding excessive wear. Pivoting mounts, such as clevis or trunnion, require care in application, because of their capacity to move as the cylinder is stroked, resulting in a possible bent rod or excessive wear.

Common types of mounting include:

Flange mounts — Strong and rigid, but have little tolerance for misalignment. It is recommended to use cap end mounts for thrust loads and rod end mounts for loads under tension.

Side-mounted cylinders — Easy to install and service, but the mounts can sometimes create a bending moment as the cylinder applies force to a load, increasing wear and tear. To avoid this, specify a stroke at least as long as the bore size for side mount cylinders (heavy loading tends to make short stroke, large bore cylinders unstable). Side mounts, such as side lugs, need to be well aligned and the load supported and guided.

Centerline lug mounts — Absorb forces on the centerline, but require dowel pins to secure the lugs to prevent movement at higher pressures or as a result of shock loads.

Pivot mounts — Absorb force on the cylinder centerline and let the cylinder change alignment in one plane. Common types include clevises, trunnion mounts and spherical bearings. Because these mounts allow a cylinder to pivot, they should be used with load is transferred along the centerline of the cylinder. Non-centerline mounting calls for additional support to avoid misalignment, but these are required when the mechanism must pivot through an arc, so load calculations must be factored accurately.

Push or pull or both?

Any cylinder can be used as single acting, which is powered in one direction only, but it can only push or pull. When a cylinder pushes, protection against rod buckling and bending must be ensured, which can be achieved through oversized rod material or with a stop tube to prevent full extension, taking advantage of the piston’s load-bearing effect. When a cylinder pulls, there is little concern for buckling, but you should ensure your force calculations factored in the smaller rod side of the piston, which experiences reduced force compared to the cap side. A double acting cylinder is powered in both directions to push and pull.

What push or pull tonnage is required?

Always assume peak loads will require additional strength. The rule of thumb is to choose a cylinder with a tonnage rating of 20% more than required for the load; however, this is always application-specific, so it’s best to consult a hydraulic professional before you make tonnage assumptions. Cylinder force (lb) is equal to the area of the piston (in. 3 ) times pressure (psi), or F=AxP.

Image courtesy of Yates

Key specifications:

Operating conditions — Cylinders must meet the requirements of the design specification, such as force, maximum pressure and mounti ng confi gurati on, but considerati on for operating conditi ons must also be heeded. Cylinders must also withstand extreme temperatures, humidity and even salt water for marine hydraulic systems. Also, when ambient temperatures rise to more than 300° F, standard Buna-N nitrile rubber seals may fail and will instead require synthetic rubber seals, such as Viton. When in doubt, err on the safe side and choose a cylinder design capable of more of than you will ask of it.

Fluid type — Most hydraulic systems use a form of mineral oil, but applications using toxic synthetic fluids — such as phosphate esters — require Viton seals, which will not break down or swell in the fluid. Once again, Buna-N seals may not be adequate to handle some synthetic hydraulic fluid, although the gentler synthetics, such as PAO-based stock, will be fine. Hydraulic systems using high water-based fluids may require stainless-steel construction, as well as PTFE (Teflon) seals, especially if no glycol is used in the fluid.

Seals — Seals are the most vulnerable component of a hydraulic system. Properly applied seals can reduce friction and wear, lengthening service life, but incorrect types can lead to downtime and maintenance headaches as a result of failures. Every manufacturer likes to use a different style, so it is important to replace them with a similar type and material when rebuilding.

Cylinder materials — The type of metal used for cylinder head, cap and bearing can make a big difference in performance and reliability. Most cylinders use bronze for rod bearings and medium-grade carbon steel for heads and bases. But stronger materials, such as 65-45-12 ductile iron for rod bearings, can provide a sizable performance advantage for tough industrial tasks. The type of piston rod material can be important in wet or high-humidity environments (like marine hydraulics) where stainless steel may be more durable than the standard case-hardened carbon steel with chrome plating used for most piston rods. A new option for rod surface treatment is nitriding, which is an oxidation process to increase the surface hardness of metals and corrosion resistance.

WHEN SHOULD YOU USE STAINLESS STEEL CYLINDERS?

Image courtesy of Higginson Equipment

IMAGE COURTESY OF HIGGINSON EQUIPMENT

Stainless steel air and hydraulic cylinders are used in applications where corrosion resistance is the utmost priority. Standard cylinders are made from combinations of alloy steel, such as 1018, 1045 and 4140, which are all susceptible to oxidation and rust in humid or wet applications. Even when epoxy painted, carbon steel cylinders exposed to surface abrasion, dents or harsh chemicals can wear away any paint, leaving exposed steel to corrode.

Marine environments often require the usage of stainless steel cylinders, both onshore and offshore. Saltwater is especially damaging to standard alloy steel, which will rust rapidly when exposed to saline. Maritime cylinders used for cranes, boat lifts, davits or other machinery do well when made from stainless steel alloys.

Offshore oil rigs are especially prone to aqueous corrosion, and although special construction and coatings can mitigate some oxidation, the nature of pneumatic and hydraulic cylinders makes them more prone to corrosion. Manufacturing cylinders in 316-grade stainless steel produces an especially resistant raw material for offshore applications.

Stationary marine applications are only half of the equation when it pertains to the effectiveness of stainless steel. Shipbuilders use stainless actuators in locations most prone to corrosion, such as trim actuators, steering components or life craft hoists. An onshore crane failure will just be annoying and time-consuming to repair, but the failure of a steering cylinder could be deadly, so all precautions must be taken to encourage reliability. Stainless actuators are often the best answer.

Environments containing caustic or corrosive chemicals will make do with stainless steel cylinders as well. Although poor for marine applications, 304 stainless steel works well in the metals industry where chemicals like sodium hydroxide are used in the cleaning process. Another tough environment requiring the use of stainless cylinders are pulp and paper mills. The black liquor by-product is extremely corrosive to steel, so 304 stainless comes to the rescue once again, extending the life of linear actuators in demanding applications. That being said, even stainless cylinders will eventually corrode when exposed to black liquor, showing you how demanding the environment is.

Sometimes a cylinder must not only resist the damaging effects of its ambient environment, but it must also aid in the protection and safety of that environment as well. Food grade applications often require stainless steel air and hydraulic cylinders. Indeed, they excel here because food applications are often damp or altogether wet. A bottling plant, for example, uses stainless steel extensively, not just for extended life of the machinery, but to prevent corroding or rusting machinery from contaminating the product.

Because stainless steel is less likely to rust, its surface finish remains true for longer periods of humid exposure. A clean, smooth surface prevents the adhesion and accumulation of food particles and bacteria, which is clearly the top concern for food and beverage production.

Food grade cylinders are manufactured to different standards compared to “off-the-shelf” cylinders, with features added to reduce the accumulation of bacteria. For example, a quality stainless air cylinder used in food and beverage might have a polished surfaced to prevent bacterial adhesion, rounded corners to reduce burrs and welded or threaded construction to eliminate cavities or pockets. Typical tie-rod cylinders, for example, provide hiding spaces under the tie rod locations, making washdown less effective.

Food grade applications can be so extreme as to eliminate externally adjustable cushions, which provide a pocket for food and bacteria to reside. Cushions may still exist but will be the nonadjustable type without any exterior cross drillings. Even the rod wiper may be manufactured from food grade polymers to prevent cross-contamination. Although stainless steel cylinders are more costly than standard steel alloys, some applications absolutely require their superior corrosion resistance properties.