Special Hydraulic Cylinder or ISO Cylinder

Anyone designing a hydraulic system or replacing an actuator will sooner or later face a choice that appears simple but is anything but: is it better to opt for an ISO hydraulic cylinder, available from catalogue with guaranteed lead times, or to invest in a special engineered hydraulic cylinder, designed precisely for that specific application?

The answer depends on a technical and operational analysis that goes well beyond the unit price of the component. It depends on the working environment, the space available within the machine, the fluids used, the load cycles, and the future availability of spare parts but above all it depends on an aspect that is very often underestimated: how much customisation is already accessible within a configurable ISO cylinder, without sacrificing the benefits of standardisation.

What Is Meant by an "ISO Standard" Cylinder and Why It Is Not Synonymous with Inflexibility

In industrial hydraulics, the term "standard" refers to cylinders conforming to ISO standards that define envelope dimensions, mounting interfaces, and operating pressures. The two main reference standards for industrial cylinders are ISO 6020/2, which covers tie-rod and front-flange cylinders up to 160–210 bar with bores from 25 to 200 mm, and ISO 6022, which defines cylinders for heavy-duty applications up to 250 bar with bores from 50 to 200 mm and strokes up to 6,000 mm.

Standardisation guarantees interchangeability: an ISO cylinder can be replaced by one from another manufacturer conforming to the same standard, without modifying the machine. This is a far from negligible advantage for those managing diversified machine fleets or needing to minimise downtime in the event of an emergency replacement.

What many designers fail to fully exploit, however, is the breadth of configurable options that already exist within the ISO framework, without departing from standardisation. Taking ISO 6020/2 tie-rod cylinders as a reference, the variables that can be freely combined include the choice of rod material (standard chromed CK45, but also chromed stainless steel, chromed heat-treated steel, Nikrom, or chromed induction-hardened steel), the type of seals (standard high-sealing, low-friction, Viton® for high temperatures, or water-glycol), the presence of adjustable end of stroke cushioning, the mounting type from all those provided for by the standard, and the addition of magnetic sensors or position transducers.

The result is that in a large number of cases, what the user calls a "custom cylinder" is in reality an ISO cylinder configured with the right options. And a configurable ISO cylinder offers an advantage that a special execution cannot: spare parts (seals, bushings, pistons) are always in stock, standardised, and readily available.

When ISO Configuration Is Sufficient

In the majority of standard industrial applications, the required customisation fits perfectly within the scope of a configurable ISO cylinder. The most frequent cases where this is the correct choice are outlined below.

Aggressive or corrosive environment. If the cylinder operates in conditions of high humidity, salt spray, acid detergent washing, or chemically aggressive environments, the answer is not necessarily a cylinder built entirely from stainless steel, which carries significantly higher costs and production lead times.

In many cases it is sufficient to adopt a chromed stainless steel rod (option RRX) combined with electroless nickel plating on the cylinder body, with a 20 µm thickness and certified resistance up to 1,200 hours in salt spray at rating 10. A high level of protection, achievable on an ISO tie-rod cylinder without modifying the structure of the component.

Presence of dust or abrasive particles. In environments with metal swarf, sand dust, or fine particulate typical of foundries, cutting installations, or stone-processing operations the standard wiper may not be sufficient to protect the rod on retraction. The solution is the metallic wiper seal (option RM), which guarantees perfect rod contact and removes even small particles that an elastomeric wiper allows through.

High-speed cycles or strokes exceeding 2,000 mm. When the stroke exceeds 2,000 mm or operating speeds are high, a build-up of fluid can occur between the wiper and the guide bushing seal. The solution is bushing drain (option SD), which allows excess fluid to be removed and returned to the tank, avoiding unwanted pressure in that area. For very long strokes, the choice also favours ISO 6020/2 front-flange cylinders, which eliminate tie-rod elongation under load and ensure greater geometric stability.

Position control requirement. When the application requires knowing where the piston is along the stroke at discrete points or continuously the answer is the integration of magnetic sensors (MD series) or a magnetostrictive linear transducer (TD/TK series for tie-rod cylinders, TH/TX for front-flange cylinders). These are options available on standard ISO cylinders, with native integration and no structural compromises.

Significant radial loads. If the application geometry does not guarantee perfect alignment between the cylinder axis and the load direction, radial loads on the bushings and seals can become critical. In this case, the correct choice is to select heavy-duty ISO 6022 DP series cylinders, equipped with double bronze piston guidance and a long bronze rod bushing, specifically designed to handle unforeseen radial loads without compromising cylinder service life.

When the Special Engineered Cylinder Becomes the Right Choice

There are situations in which no combination of options on an ISO cylinder fully meets the requirement. In these cases, the special engineered cylinder defined as special execution (SX) is the correct approach, even if it requires a greater investment in terms of time and cost.

Non-standard geometries. If the available space within the machine imposes envelope dimensions different from those prescribed by the standard for example, intermediate bores, custom mounting centre distances, and oil port locations in non-conventional positions the ISO cylinder cannot adapt. In these cases it is necessary to design the cylinder starting from the actual machine dimensions.

Operating pressures outside the ISO range. ISO 6022 cylinders reach a nominal pressure of 250 bar, tested at 375 bar in accordance with ISO 10100. Applications requiring higher nominal pressures call for specific engineering to ensure wall thicknesses, materials, and seals appropriate to those conditions.

Non-conventional special fluids. ISO cylinders support mineral hydraulic oil, phosphate esters, and water-glycol in versions with Viton® or water-glycol seals. For even more unusual fluids — such as high water-content emulsions, very high temperature fluids, or fluids with unusual chemical characteristics it may be necessary to select specific materials and seals that fall outside the standard range.

In all these cases, the advantage of working with a manufacturer that also produces special executions based on the same ISO structure is significant: the standardised components end caps, bushings, tie rods, seals remain the same as those in the catalogue, with all the resulting benefits in terms of guaranteed quality and future spare parts availability.

The Time Factor: An Often Decisive Variable

In evaluating the choice between an ISO cylinder and a special engineered cylinder, delivery lead time is a variable that weighs as much as price, if not more. In the event of an unexpected machine breakdown, each day of lost production has a real cost that can quickly exceed that of any component.

A configurable ISO cylinder with seal options, rod material, and accessories already defined can be produced and delivered in very short lead times thanks to the availability of all components in stock. In urgent cases, lead times can drop to 48 hours. A special execution, on the other hand, requires a design phase, possible drawing approval, and the production of non-stock components, with inevitably longer lead times.

This difference suggests a clear strategy: use the configurable ISO cylinder for applications where the geometry allows, reserve the special cylinder for cases where no alternative exists, and plan in advance for the availability of a spare cylinder in cases where rapid replacement is critical to production continuity. Conforti's online configurator allows the cylinder configuration to be defined and validated independently, 2D and 3D models to be downloaded, and delivery times to be obtained in real time, greatly simplifying this planning phase.

A Technical Choice, Not Just an Economic One

The distinction between an ISO cylinder and a special engineered cylinder is not merely a budget question, but above all a matter of correct technical analysis. Choosing a configurable ISO cylinder when the application allows it means benefiting from interchangeability, guaranteed spare parts, certain lead times, and a lower total cost of ownership over time.

In both cases, the starting point is the same question: must this component perform exactly this task, in this environment, with these loads and these fluids, for how many years and with what acceptable maintenance frequency? Answering this question precisely is the fastest way to arrive at the right choice of cylinder.

For an analysis of your specific requirements, the Conforti technical team is available to support the selection of the most suitable cylinder.