The selection of a hydraulic cylinder is a crucial step in the design of any industrial hydraulic system. A mistake at this stage can compromise the efficiency of the entire application, cause premature failures and generate unexpected maintenance and downtime costs. According to industry statistics, the most frequent cause of cylinder malfunctions is the choice of an unsuitable product, rather than manufacturing defects. In this article, we analyse the five most common mistakes designers make when choosing hydraulic cylinders, providing concrete solutions to avoid them.

ERROR 1: UNDERSIZING THE ROD DIAMETER

The undersizing of the rod is the most critical error in the selection of hydraulic cylinders. Many designers focus exclusively on calculating the required force, neglecting to check the peak load on the rod.

When working in thrust with long strokes, the rod behaves like a column subject to axial compression. According to Euler's theory, once a certain critical length in relation to the diameter is exceeded, the rod tends to bend sideways. This phenomenon causes damage to the guide bushings, premature failure of the seals, accelerated wear of the tube and, in the most serious cases, sudden failure of the rod.

The formula for calculating the critical load takes into account the effective length, moment of inertia, modulus of elasticity of the material and coefficient of constraint at the ends. ISO 6020/2 standards define standard ratios between bore and rod diameter to ensure adequate strength under typical operating conditions.

How to avoid this error: always use the sizing tables in our catalogue, which take into account the correct ratios between stroke and rod diameter. In case of doubt, consult technical support, providing all the application data.

ERROR 2: CALCULATING FORCES WITHOUT AN ADEQUATE SAFETY MARGIN

The second common mistake is to size the cylinder based on the theoretical calculation of force (F = P × A) without considering all the factors that influence the actual load.

Designers often overlook fundamental elements:

• System friction: guides, bearings and mechanical resistance in kinematic mechanisms, which absorb 10 to 30% of the available force during the life of the application.
• Required acceleration: starting a mass quickly requires much more force than constant movement.
• Load losses: valves, pipes and fittings generate pressure losses that reduce the effective force.
• Safety factor: variable operating conditions require margins between 1.5 and 2.5 times the nominal load.

How to avoid this error: always apply an appropriate safety factor. For standard applications, a coefficient of 1.5 is adequate; for severe conditions, increase to 2.5. Verify that the available pressure is effective, taking into account all pressure drops.

ERROR 3: CHOOSING SEALS THAT ARE UNSUITABLE FOR THE OPERATING CONDITIONS

The choice of seals is often underestimated but critical for durability and reliability. Many designers specify ‘standard seals’ without analysing the specific operating conditions, leading to premature sealing problems and leaks.

Seals must be selected based on:

• Fluid type: mineral oils are compatible with most materials, while HFC fluids or glycol water require specific solutions.
• Temperature range: NBR and PU seals work between -20°C and +80°C, while applications up to +150°C require special Viton O-rings.
• Rod speed: speeds above 0.5 m/s require low-friction PTFE seals.

How to avoid this error: always collect complete information on the fluid used, temperatures, pressures and operating speeds. Request seals certified for the specific fluid. Consider ordering replacement kits in advance to ensure rapid maintenance. 

ERROR 4: NOT TAKING INTO ACCOUNT THE OCCURRENCE OF HARMFUL RADIAL LOADS DURING USE

Under operating conditions, the cylinder may be subjected to radial forces that lead to premature wear of the rod surface, guide bushing and seals.

• Highly constrained mounting: the absence of adequate degrees of freedom during movement can give rise to unexpected constraint reactions of a magnitude that is difficult to estimate.
• Misalignment of the cylinder on the machine caused by lack of precision or deformation of the support structure.
• Influence of the cylinder's weight force in the case of horizontal mounting, particularly with long strokes, when the centre of gravity shifts significantly between the fully retracted and fully extended positions.

In some cases, to reduce the radial load on the guide bushing, spacers can be inserted to lengthen the internal lever arm between the bushing and the piston when the rod is fully extended. 

The use of bronze rod guide bushings helps to preserve the surface finish of the rod even in the presence of unexpected radial loads.

How to avoid this error: study the constraints to which the cylinder and load are subject based on the mounting, kinematics and forces involved, including the weight of the cylinder itself; do not overlook unexpected conditions that may arise during assembly and operation. 

ERROR 5: NEGLECTING ENVIRONMENTAL AND OPERATING CONDITIONS

Selecting standard cylinders without considering specific environmental conditions causes rapid failure even with correct sizing.

• Ambient temperature: NBR seals operate between -20°C and +80°C. Cold rooms or alpine areas require special fluids and low-temperature seals. Very hot environments require high-temperature seals.
• Aggressive atmospheres: marine, chemical or food environments require special treatments such as chemical nickel plating or stainless steel.
• Contamination: sandblasting machines, shot blasting machines and foundries expose cylinders to abrasive dusts that require protective bellows and metal scrapers, surface-hardened rods.
• Work cycles: high-frequency applications generate oil heating in the cylinder and therefore require the choice of suitable seals.

How to avoid this mistake: document minimum and maximum temperatures, corrosive agents, contamination, cycle duration and frequency. Request specific recommendations for the environment. Provide additional protection and preventive maintenance proportionate to the severity of operation.

FINAL THOUGHTS: THE IMPORTANCE OF CAREFUL SELECTION

The correct selection of hydraulic cylinders requires in-depth technical expertise and systematic analysis of all application parameters. The five errors analysed represent the most frequent causes of malfunctions and operational inefficiencies.

A methodical approach that considers kinematics, dynamics, materials and environmental conditions ensures the success of the application and minimises life cycle costs. The difference between a cylinder that works for decades and one that requires frequent servicing lies in the design choices.

Specialised technical assistance allows all variables to be correctly evaluated and the optimal solution to be identified. Conforti Oleodinamica offers comprehensive support from selection to after-sales service, ensuring reliable solutions for every industrial sector.