How to Choose rotating hydraulic cylinder?

Whether wheel loaders in road construction, dumpers on construction sites or hydraulic clamping devices in machining centers - they all have one thing in common: They use hydraulic cylinders to efficiently transfer high forces.
But how do you find the right hydraulic cylinder for an application? In this article, we show you which factors are decisive in the selection process and what to look out for.

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Selection criteria for hydraulic cylinders

In our selection criteria, we concentrate on the key parameters of a hydraulic cylinder.
Your cylinder manufacturer will clarify further technical details - such as the connection thread or the operating temperature - with you at the enquiry stage.

Single-acting or double-acting hydraulic cylinder?

With hydraulic and pneumatic cylinders, a distinction is made between single-acting and double-acting cylinders.

• Single-acting cylinders are only actively moved in one direction by the pressure medium, while the return movement is usually caused by external forces such as the device's own weight.
• Double-acting cylinders, on the other hand, are actively controlled by the pressure medium both during extension and retraction.

But what does this mean in practice?

Example: Single-acting cylinder (tipper)
A classic application example for a single-acting cylinder is a tipper. This transports heavy loads (e.g. gravel) and unloads them by tilting the loading area.

• When the loading surface is raised, pressure is applied to the piston side of the cylinder - the cylinder extends and the loading surface tilts.
• No additional pressure is required when lowering. The dead weight of the cargo bed is sufficient to allow the cylinder to retract again after the lowering valve has opened.

Example: Double-acting cylinder (steering axle of a wheel loader)
Double-acting cylinders are used when active movement in both directions is required. One example is the steering axle of a wheel loader:

• Let us assume that the wheel of the wheel loader turns to the right when pressure is applied to the piston rod.
• As there is no external force for a counter movement, the ring side of the cylinder must also be pressurized in order to move the wheel to the left again.

Required force

A key parameter of a hydraulic cylinder is the force to be applied. Depending on the application, the force required for the active movement must be determined.

Together with the maximum buckling force and the permissible operating pressure, the required piston diameter can be calculated based on the required force. 
More on this in the following practical example.

Required effective stroke

The effective stroke is the difference between the retracted and extended position of a hydraulic cylinder. Depending on the application, it can vary from a few centimetres to several meters. It is determined by the geometric conditions of the design.

The installation length of the cylinder depends largely on the effective stroke and is made up of:

• Zero length of the cylinder
• Effective stroke of the hydraulic cylinder

The zero length is the length of the cylinder without effective stroke. It includes all necessary components such as guide bands, connections, etc. This information is usually provided by the cylinder manufacturer, depending on the piston diameter, connection type and design of the cylinder.

Calculation of the installation length:

Installation length = zero length + effective stroke

Practical example: Simple scissor lift table

For our practical example, we are looking at a scissor lift table - a classic area of application for hydraulic cylinders. 
To keep it as simple as possible, we choose a standard lift table without additional functions.

This means:

• The lift table only has a lifting and lowering function.
• There are no other extras such as a tilting device or a powered rotating platform.

The cylinder type: Single-acting hydraulic cylinder

Our scissor lift platforms use single-acting hydraulic cylinders. Due to the solid construction and the associated dead weight of the platform and the scissors, the cylinder retracts automatically when the lowering button is pressed.

How does it work?

• The hydraulic unit is only active during the lifting process.
• For the downward movement, it is sufficient to open the lowering valve - the dead weight compresses the cylinder again.

You can find more details on how it works on our landing page for hydraulic lift tables.

What effective stroke is required?

As already mentioned, the required effective stroke is determined by the geometric conditions at the place of use. The aim of every lift table manufacturer is to achieve a pressure curve that is as constant as possible.

The pressure curve is significantly influenced by the ratio of the lever arms between the load and the hydraulic cylinders. Taking into account the installation length of the cylinder and the lever arm ratios, we have determined the following values for cylinder selection:

Technical data of the cylinder

• Retracted length: 516 mm
• Extended length: 767 mm
• Effective stroke: 251 mm

Operating pressure vs. buckling force: What determines the piston diameter?

The geometric conditions not only influence the required effective stroke, but also the required force of the hydraulic cylinder.

With a scissor lift table, the load acts on the hydraulic cylinders via the scissor jaws and the cylinder grippers. The lever arm ratio determines the force required to extend the cylinders and lift the load.

Once the required force is known, the piston diameter can be calculated. Two factors play a key role here:

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1. The operating pressure
2. The permissible buckling force

In our example, we would like to carry out the calculation for both factors, but for the moment let us note the following generally valid relationships:

Operating pressure and piston diameter:

The operating pressure is inversely proportional to the piston diameter. This means that the larger the piston diameter, the lower the required operating pressure.

Permissible buckling force and piston diameter:

The permissible buckling force increases with increasing piston diameter. This means that the thicker the cylinder, the more stable it is against buckling.

Material costs and piston diameter:

The material costs increase with increasing piston diameter.

Maximum operating pressure

Manufacturers of hydraulic components specify a maximum permissible operating pressure for their components. The under-oil units we use, for example, allow a maximum continuous operating pressure of 250 bar. The hoses we use are also designed for this operating pressure.

To ensure the durability of our lift tables, the hydraulic cylinders are designed so that a maximum operating pressure of 200 bar is not exceeded.

The minimum pressure surface for the hydraulic cylinder results from the maximum permissible operating pressure:

The minimum piston diameter can be determined from this:

If the operating pressure were the sole criterion, the piston diameter of the cylinder would have to be at least 68.9 mm.

Permissible buckling force

Particularly with longer hydraulic cylinders, the operating pressure is no longer the decisive factor, but the permissible buckling force.

To ensure a high level of safety against buckling, a safety factor of at least 3 is applied. This means that the maximum force on the hydraulic cylinder would have to be three times as high for there to be a risk of the cylinder failing due to buckling.

As the second Euler case applies in our example, the critical buckling force can be calculated using the following formula:

The area moment of inertia of a circle is determined using the following formula:

Taking the safety factor into account, we obtain the formula for calculating the minimum piston diameter by conversion:

1Type of cylinders

First element to take into account is the type of cylinder. HPS International manufactures and sells 6 types of cylinders for various applications :

• Block cylinders (compact) for plastic injections, molding, cosmetics and packaging.
• Tie rods for plastic injection, molding and aluminum injection applications.
• Round cylinders (ISO, DIN, CNOMO) for plastic injection and molding.
• Special cylinders (made to measure, double bore) for plastic injection, molding and die casting.
• Auto-locking cylinders for plastic injections, molding, cosmetics and packaging.
• Cylinders for foundries for aluminum injection.

• Example : Block cylinder
• High Speed : Go for the RCVN
• Compact : VBL, VBLS, VCR, VXP
• With mechanical sensor : VCE
• With inductive sensor : VDI
• With magnetic sensor : VBM
• Cooling : VRE

2Bore

The first essential information to take into account when choosing the size of the bore is the operating pressure.

Knowing the operating pressure you want to use will help you choose the size of the cylinder bore.

The second key information needed to choose the size of the bore is the amount of force required for the application. Here is a quick formula to find the approximate size of the bore in the direction of cylinder extension :

• F = required force (N)
• P = operating pressure (bar)
• A = Surface of the bore (mm²)
• D = bore diameter (mm)
• D = √4.F / √π.√p

As a general rule, the resulting bore diameter is rounded to the next standard bore size.

3Rod

Now that you know the minimum size of the hydraulic cylinder bore, you must now choose the ideal rod size. Most HPS cylinders come with several rod options.

The correct rod is chosen based on the necessary stroke length, which has an effect on the buckling resistance of the rod.

When selecting the rods, the smallest rod in the bore should only be used for reduced stroke thrust loads or in reduced pressure applications. The largest rod should be used in cases that depend on its reliability and maximum strength.

If the desired rod diameter is greater than the largest diameter in the bore size of the selected cylinder, it is time to re-examine the cylinder design parameters. It is usually in these situations that the application requires a custom hydraulic cylinder.

4Mounting

The hydraulic cylinders are divided into two types, depending on their mounting style, either a pivot mounting or a straight line mounting. Pivot mounted cylinders are used when a load is to be moved in an arc and includes supports such as a clevis and a trunnion.

Cylinders mounted in a straight line are used when the load is to be moved in a linear direction. It includes a flange support and a back or front foot.

5Seals

A hydraulic cylinder must have the right seals, as they play a vital role in preventing any oil leaks. This, in turn, guarantees a continuous and flawless use of the cylinder. The seal should be selected based on the cylinder's applications, operating environment and the maximum operating pressure it can withstand.
HPS International offers 4 types of seals :

• Standard N Nitrile seals, for temperatures from -20 ° to + 80 ° C, an operating speed of 0.5 m / s. Fluids: mineral oil, filtration ISO 19/17/14.
• Viton V FPM seals, for temperatures from -20 ° to + 200 ° C, an operating speed of 0.5 m / s. Non-combustible fluids based on Ester Phosphate (HFD-R), ISO filtration 19/17/14.
• Glycol G nitrile seals, for temperatures from -20 ° to + 90 ° C, an operating speed of 0.5 m / s. Fluids: Water-Glycol (HFC), ISO filtration 19/17/14.
• PTFE P seals in viton / PTFE, for temperatures from -20 ° ... + 240 ° C, an operating speed of 0.5 m / s. Non-combustible fluid based on Ester Phosphate (HFD-R), ISO filtration 19/17/14.

Other elements to consider when choosing a cylinder such as power, operating mode and switches.

6Sensors

HPS International offers 4 types of hydraulic cylinder sensors. The choice depends on the needs and demands of the clients.

For more information, please visit China Collet Chuck.

• Mechanical : Adjustable, reliable, easy and fast assembly, compact (HPS).
• Inductive : Non-adjustable, reliable, easy and compact.
• Magnetic : Adjustable, popular, compact.
• Linear : Precise, compact (inside the stem).

7Hydraulic concepts:
Pressure, force, flow

• Pressure (bar) : Power creation
  P = F/S
• Forces developed by pushing (daN) :
  F = P x S1
• Forces developed by pulling (daN) :
  F = P x S2
• Flow (l / min) : Speed Creation
  V = Q / S

• With unit :
• P : bar Q flow: cm3/min
• F: daN S: π x r² cm² Vspeed: cm/min
• N = 100 daN = 1KN = 0.1 Ton