Optimum buckling stability with long hydraulic cylinders
When hydraulic cylinders are under enormous pressure loads, this harbours a major risk that must be demonstrably eliminated during the design of the cylinder: The piston rod can buckle suddenly and without prior warning.
But what exactly causes a hydraulic cylinder to buckle? With hydraulic cylinders that work under pressure, there is a risk that the piston rod will exceed a critical load value (the buckling strength) and buckle.
One practical example is long-stroke cylinders for cranes. These are usually very long, work under pressure and have to withstand extreme loads.
Factors such as the length and diameter of the cylinder, the material properties and the type of load play a decisive role here. Cylinders with a thin piston rod and/or long stroke are particularly susceptible to buckling failure.
Such a buckling failure can lead to a high safety risk with possible personal injury and also means a complete functional failure, which can result in high costs. Preventing such a failure is therefore a top priority.
At ACONA, we provide you with valid proof of the buckling stability of your hydraulic cylinders and guarantee that your cylinders can reliably fulfil their tasks even under extreme conditions.
Hydraulic cylinder crane: An application for long cylinders
Hydraulic cylinders in cranes are a typical area of application for long-stroke cylinders, which must be specially designed for high pressure loads and precision.
When moving heavy loads, these hydraulic cylinders have to absorb an enormous compressive force while remaining stable in order to ensure the operation of the crane.
Hydraulic cylinders in cranes are often operated at an angle or in a horizontal installation position. In these cases, the dead weight of the hydraulic cylinder leads to deflection, which also favours buckling. The inclination changes over the stroke of the cylinder, which means additional complexity.
Conservative methods for verifying the buckling stability of piston rods for hydraulic cylinders
When designing hydraulic cylinders, the conservative Euler method is often used to verify buckling stability.
In addition to the factors taken into account, such as the length, diameter and elasticity of the material, the theory is based on the idealised condition that the area moment of inertia of the piston rod is present over the entire cylinder length.
This does not correspond to reality, as the piston rod often has a significantly lower area moment of inertia than the cylinder barrel.
In practice, the calculation according to Euler therefore often leads to oversized designs and thus to excessive costs.
The DNV standard considers the moments of inertia of the piston rod and the cylinder barrel separately and therefore represents the load case more precisely.
The deflection of the cylinder due to lateral forces or dead weight as well as the tilting of the cylinder via the gap dimensions of the seals are not considered in the DNV and according to Euler for the sake of simplification. A high safety factor must therefore be taken into account.
As you can see, the conservative calculations are simple and quick and have therefore been used for many years for good reason, but are not accurate enough in modern applications.
FEM analysis: How we at ACONA precisely detect buckling
To rule out buckling of hydraulic cylinders and thus ensure safe operation, we at ACONA rely specifically on the finite element method (FEM).
In addition to the exact load cases, the FEM analysis also allows factors such as geometry and deflection as well as non-linearities such as component contacts and gaps to be precisely taken into account. The design is simulated realistically and enables the production of hydraulic cylinders that are resource-saving, geometrically optimised and extremely resilient.
With the help of FEM analysis, we can verify the buckling stability of even long cylinders with large strokes and high operating pressures or optimise the design at critical points. To summarise, the use of an FEM simulation leads to an efficient and safe product.
FEM analysis and conservative methods: the advantages and disadvantages summarised
Conservative methods
Advantages
-
Can be carried out quickly as they are often based on simplified calculations
-
Offer a simple solution without complex simulations
Nachteile
-
No valid proof of safety for specific load cases and conditions
-
Oversizing of the hydraulic cylinder, which leads to higher costs and more material consumption
-
Poorly suited for applications where size and weight are limited
FEM analysis
Advantages
-
Provides valid proof of safety through detailed simulation of real load situations
-
Enables precise dimensioning of the cylinder, which saves material and costs
-
Takes into account the specific application, load case, gaps and possible deflection of the piston rod
Disadvantages
-
High computing effort, especially for detailed models
-
Need for specialised software and expertise, which requires additional resources
How we ensured the buckling stability of a hydraulic cylinder for use in open-cast mining
From our location in Herzogenrath near Aachen to the whole world! In this case, we are supplying the cylinders for a project in South America.
Our customer from the Hamburg area contacted us with an initial design proposal for a hydraulic cylinder. Specifically, it was for a service crane to be used in open-cast mining. The boundary conditions are predefined. Among other things, the hydraulic cylinder should be able to withstand a piston-side pressure of 160 bar.
But instead of going straight into production, we at ACONA rely on our greatest strength - sound engineering. The design department scrutinises the design and uses the customer data to carry out a calculation in accordance with Euler and DNV standards. The result is sobering: the cylinder would buckle under these conditions.
In close cooperation with the customer, the operating conditions under which the cylinder works in compression are analysed in detail. It turns out that the actual compressive force during operation does not correspond to 160 bar, but is still too high to provide proof of buckling stability using conservative methods.
The key question is therefore: What is the maximum permissible compressive force to ensure safe utilisation?
This is where the FEM analysis provides clarity. Using realistic simulations, the result shows a permissible pressure of 55 bar. The solution? We recommend installing a pressure relief valve upstream of the cylinder and setting it accordingly to guarantee the safety of the system. In this way, proof of buckling stability can be provided.
What particularly sets us apart at ACONA: While other providers submit similar offers but do not carry out any recalculations, we stand out with our engineering and great expertise in the industry.
‘At ACONA, we see ourselves as a development partner: We not only manufacture hydraulic cylinders for you, but also support you in optimising your products and systems.’
Tarik El Azzar
DirectorACONA-Hydraulik GmbH & Co. KG
The result: the customer is happy. Thanks to our expertise at ACONA, we not only deliver a safe and functional hydraulic cylinder, but also create the basis for a long-term partnership.
Contact
What can we do for you?Whether hydraulic cylinder enquiry, tube machining, repair or general questions.
We will be happy to advise you.