July/August 2019

HYDRAULICS The demand for increased operating pressures within a compact design is perhaps the single most important factor driving the uptake of new steel materials for hydraulic cylinders. For example, 180 bar was a typical requirement for construction machinery in the 1970s. Over time, this has increased to 210 bar and up to 280 bar. And in the near future we are looking at up to 35 MPa (350 bar). Piston rod design This doubling of operating pressure means that the hydraulic cylinder must now handle double the load. To achieve this, the main focus is on the chrome- plated and nickel-chrome-plated bars and tubes used primarily as piston rods, where the principal parameters (see Figure 1) are: 1) Dimensioning against fatigue, buckling and impact failure. 2) Tailoring the surface for low seal wear and limiting the risk of surface damage or corrosion. Cromax steels The main cost factor for the hydraulic rod is the bar diameter. Material, handling and cutting cost increase with the weight of the bar, while the cost of surface operations (machining, grinding, chrome plating etc.) increase with the surface area. As a result, the cost of a rod increases exponentially with the diameter. As a general rule of thumb, the cost of a piston rod decreases by 15 percent if the diameter is reduced by 5 mm. Other effects are lower weight, higher force in the minus direction and less fluctuation in the oil level in the tank, which could translate into lower energy consumption and possibilities for alternative designs. Ovako has developed Cromax steels specifically as ‘piston-rod’ steels that offer the ideal combination of performance, surfaces and services to minimise the total cost of the piston rod (see Figure 2). Slenderness ratio is crucial Hydraulic piston rods must be dimensioned against buckling, fatigue and impact failure. The risk of buckling depends on the yield strength, the elastic modulus of the material and the geometry of the rod - defined as the slenderness ratio. Generally speaking, the buckling stress at small slenderness ratios corresponds to the yield stress of the material. It then decreases with an increasing slenderness ratio and eventually becomes independent of yield strength. In practice, 70 to 80% of all hydraulic cylinders are designed with low or moderate slenderness ratios (below 100). This means that yield stress has an important influence in defining the required diameter of the rod for cylinders operating in conditions where there is a risk of buckling. Fatigue limit In many applications, the piston rod operates in a dynamic environment with push-pull forces that make it susceptible to fatigue. Each rod has a unique fatigue limit and dimensioning is made on the basis of experience or through fatigue testing of finished piston rods or cylinders. As a general rule, fatigue strength increases with the tensile strength of the rod material. Guaranteed impact toughness might also be an additional considered for safety critical applications. Cost reduction Since rod diameter is the main cost factor, the strength of the rod material is the main lever to reduce cost. Compared with the most commonly used material for hydraulic cylinder rods (C45E), the cost reduction potential by using stronger materials is of the order 20 to 30%. However, not all high-strength steels are suitable for piston rods. The main problems derive from the inherent impact of slag inclusions which intersect the surface and segregation at the centre of 32 HYDRAULICS & PNEUMATICS July/August 2019 www.hpmag.co.uk A material advantage for hydraulic piston rods Mattias Awad, head of marketing and technology, OVAKO Cromax AB, offers a guide to material selection as hydraulic cylinders are required to deliver higher performance at lower cost. Figure 1: The principal parameters in hydraulic piston rod design. 1800 1600 1400 1200 1000 800 600 400 200 0 0 20 40 60 80 100 120 140 Cost, index Diameter, mm Cost of a piston rod Figure 2: Impact of rod diameter on piston rod cost.