DS-JUN22-PG24_Layout 1 20/06/2022 11:36 Page 1
FEATURE HYDRAULICS & PNEUMATICS New desigN
opportuNities for hydraulic systems Hydraulic cylinders are used in
critical roles in construction and agricultural vehicles, wind turbine
pitch control and lifting equipment. Here Mattias Awad, head of
marketing and technology for Ovako Cromax AB, explains how selecting clean steel can increase resistance to the common failure mechanisms of buckling and fatigue
B
uckling affects only hydraulic cylinders that act in compression. It happens when an excessive force causes sudden and
unpredictable crumpling, and represents risk for both machine operators and people nearby. To protect equipment against buckling of
slender rods, engineers use Euler theory to keep compressive forces below a critical buckling threshold. This works particularly well for long slender structural columns of buildings, where it has been integrated into design codes and methodologies. However, when it comes to less slender
rods such as hydraulic cylinders, Euler theory overestimates buckling resistance. Both the American Institute for Steel and the European Convention for Constructional Steel offer methods to assess buckling strength for calculations in the non-elastic area. The design standard for cranes is another helpful resource for engineers working with cylinders. It includes a process to assess the effective length of piston rods, which vary depending on whether a rod is connected just at either end or has an additional mechanical support at the gland in the middle.
fatigue in Double-acting cylinDers
For double-acting hydraulic cylinders that experience both compressive and tensile forces over many thousands of cycles, designers should be aware of fatigue failure. The loading cycles act on areas of elevated stress around tiny non-metallic particles called inclusions in the steel, which leads to micro-cracks initiating and ultimately propagating and leading to failure. Fatigue cracking is most likely to occur in high-stress areas where cross section is reduced,
2 DESIGN SOLUTIONS JUNE 2022 4
for example a thread root or a fillet. Alternatively, they can initiate at defects in welded joints.
HigH-performance material for ligHtweigHting
It’s normal practice to upsize a design’s mechanical dimensions as a factor of safety to reduce risk of failure from buckling or fatigue. However, it’s also possible to use high performance clean steel to provide this factor of safety – and as a result, designers can reduce the size of a piston rod, and therefore an entire hydraulic system. It has high yield strength, which reduces
risk of buckling, as well as high fatigue strength that protects against fatigue failure. Yield and tensile strength are influenced
by the production process and the alloying elements in a steel. Other factors alike, fatigue strength increases with the tensile strength of the rod material. However, fatigue strength can be improved by taking special measures during steel production to reduce the number and size of the non-metallic inclusions. The result is clean steel and, by reducing harmful inclusions, it reduces the potential for microscopic areas in the steel affected by elevated stress. Special types of steel have been developed
to meet the needs of hydraulic systems. For example, Ovako’s Cromax 280X has yield strength of 520 N/mm2 more than 305 N/mm2
, which is significantly for C45E, a steel grade
that is often used in hydraulics. By upgrading to a high-performance material, a designer could choose to reduce the piston rod’s diameter or transmit greater loads.
Design for manufacture
The other important aspect of material selection is to consider its impact on machinability and production efficiency. For example, it is particularly important that material for hydraulic cylinders can be processed efficiently during turning and threading. Also important is suitability for welding
processes, particularly for friction welding, which is often applied to hydraulic cylinders. It is therefore important to find a balance between the alloys selected. This avoids the phenomenon of centre segregation, where brittle constituents form in the heat affected zone (HAZ) following welding. Matching alloys carefully maintains the steel’s microstructure and properties, thus making them more amenable to welding. To support this, we carry out stringent testing
on our special Cromax grades for hydraulic systems to ensure they are amenable to machining and that they maintain performance after welding. We’ve also developed a piston rod predictor
tool in our steel navigator tool. This takes account the methodologies in the AISC and ECCS standards so that designers of hydraulic systems can compare and contrast different materials and their capability to reduce weight or boost load-carrying performance without impacting on the design factor of safety.
Ovako Cromax
www.ovako.com/en
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66
