Performance under pressure

Many industrial segments have stringent demands that drive the development of application- specific steel grades, such as bearing steel or spring steel. The same is true for materials for hydraulic components. Under the Cromax brand name, Ovako has than more than 40 years of experience in developing steel grades tailor-made for hydraulic cylinder applications. Ovako Cromax is the pioneer and technology leader in the field of long chrome plated bars used as piston rod material. Our strong position in this segment, with in-house steel production, has made it possible to design application-specific steel grades focused on delivering the best possible performance for piston rods. The Cromax piston rod steel grades 180X, 280X and 482 have been optimized in close cooperation with major OEMs and independent cylinder manufacturers to meet the growing demand for more efficient and reliable hydraulic systems. A key factor is the plating process with special consideration of surface properties and corrosion resistance. But equally important are critical mechanical properties, such as resistance to buckling and fatigue performance in threads and grooves. Any optimization must of course also consider the manufacturing processes including how well the components can be machined and welded.

Ovako Cromax can utilize our three major Ovako metallurgies and an extended R&D network to control the chemical analysis, production process and level of inclusions. The ambition is nothing short of establishing a track record for delivering steel grades with unmatched consistency and properties that outperform the industry standard for hydraulic components. Performance under pressure indeed!

Buckling of hydraulic cylinders

The nature of a hydraulic cylinder requires a fairly long and slender design. That makes the risk of buckling an important dimensioning factor. Buckling is instability in push-mode, i.e. when the rod is subjected to a compressive load. An overload will result in a sudden and large lateral deformation which causes the piston rod to collapse. Hence, there is a risk of dangerous consequences if buckling occurs. The risk of buckling depends on the yield strength, the elastic modulus of the material and the geometry of the rod, as defined by the slenderness ratio – the ratio between the diameter and the effective length. Generally speaking, the buckling stress at small slenderness ratios corresponds to the yield stress of the material. It decreases with an increasing slenderness ratio to the extreme case, where it becomes independent of yield strength (known as Euler buckling in the elastic area).

strength facilitates down-sizing of the piston rod which gives advantages in material cost, easier machining and handling, transportation costs and CO2 footprint. Furthermore, the possibility to downsize opens up new opportunities in the hydraulic system design, such as obtaining a higher force in the cylinder minus (compressive) direction and less volume difference between full and zero cylinder stroke.

The Piston Rod Predictor

To assist our customers in visualizing the possibilities of down-sizing, Ovako has introduced the Piston Rod Predictor as a module in the Steel Navigator digital service platform. This tool helps you to compare the resistance of different steel grades to buckling in piston rod applications. In the predictor, you can input the diameter of an existing rod to calculate what diameter would be required with an alternative steel grade to achieve the same safety factor. Alternatively, you can use your maximum load and desired safety factor to calculate the minimum rod diameter for different steel grades. The database includes Cromax steel grades and commonly used standard grades for piston rod applications. If the material you would like to compare is not available in the list, you can input the relevant mechanical properties manually. The methods used to calculate the buckling stress are the European Convention

guidelines can help optimize the set up for machining operations. In our second webinar, Patrik Ölund will explain the quantification of clean steel and it’s dramatic influence on the fatigue life of critical components. In our third webinar Katarina Kangert and Göran Nyström will talk about our way being Carbon Neutral Now. Date will be announced shortly.

The first webinar will be held January 13, 2022, 15.00-16.00 CET. Please click below to accept your invitation.

All the best,

Joakim Hultgren, President and Managing Director, Ovako Cromax
Mattias Awad, Head of Marketing and Technology, Ovako Cromax