Ultimate Guide to Cylinder Selection for Mechanical Engineers | MachineMFG

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Ultimate Guide to Cylinder Selection for Mechanical Engineers

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1. Selection of cylinder

About Cylinders

1. Selection of types

  • The correct type of cylinder should be selected based on the working requirements and conditions.
  • For applications where the cylinder needs to reach the end of the stroke without impact phenomenon and impact noise, a cushion cylinder should be selected.
  • For light-weight applications, a light cylinder is required, whereas for narrow installation space and short stroke, a thin cylinder is needed.
  • When there is a lateral load, it is advisable to use a cylinder with a guide rod.
  • For applications requiring high braking accuracy, a locking cylinder should be selected.
  • For applications where the piston rod must not rotate, a cylinder with a non-rotating rod function can be chosen.
  • In harsh environments such as dusty environments, it is necessary to install a dust cover on the extended end of the piston rod.
  • When oil-free lubrication or oil supply is not required, a no-oil supply or oil-free lubrication cylinder should be selected.

2. Installation form

  • The selection of a cylinder should depend on factors such as installation location and usage purpose.
  • Generally, a fixed cylinder is used.
  • However, if it needs to rotate continuously with the working mechanism (such as a lathe or grinder), then a rotary cylinder should be selected.
  • If the piston rod is required to swing in a circular arc in addition to linear motion, then a pin-type cylinder is the appropriate choice.
  • If there are special requirements, the corresponding special cylinder should be selected accordingly.

3. The magnitude of the force

The choice of cylinder diameter is an important decision.

To determine the cylinder output thrust and pull, one must consider the size of the load force.

In general, one should select different load rates based on different speeds, in order to achieve theoretical balance conditions for external loads, while also leaving a small margin for cylinder output force.

If the cylinder diameter is too small, the output force may not be sufficient. However, if the cylinder diameter is too large, it may lead to bulky equipment, increased costs, higher gas consumption, and wasted energy.

When designing fixtures, force amplifiers should be used wherever possible to reduce the size of the cylinder.

4. Piston stroke

The stroke of the mechanism is typically determined by its intended use and operating conditions, and is generally not designed for full stroke to prevent the piston from colliding with the cylinder head.

In the case of a clamping mechanism, an additional allowance of 10-20 mm should be added to the calculated stroke.

5. The speed of the piston

The compressed air flow into the cylinder, as well as the cylinder inlet and outlet size and pipe diameter, primarily determine its performance.

For high-speed operations, it is necessary to use a cylinder with a high-speed capability.

The cylinder’s speed typically ranges from 50-800 mm/s.

For high-speed cylinders, it is advisable to use intake pipes with larger inner diameters.

To maintain a steady speed even when the load changes, a throttling device or a gas-liquid damping cylinder can be used to control the speed more easily.

Attention should be paid to the selection of throttle valve to control cylinder speed:

  • When pushing a load with a horizontal cylinder, it is recommended to use the exhaust throttle.
  • When lifting a load with a vertically installed cylinder, it is recommended to use the intake throttle to adjust the speed.
  • If a smooth movement at the end of the stroke is required to avoid impact, a cylinder with a buffer device should be selected.

2. Selection of cylinder

1. Select the type of cylinder according to the operation mode

There are three ways of cylinder operation: double acting, single acting spring pressing in and single acting spring pressing out.

2. Select other parameters

  • The diameter of the selected cylinder is determined according to the relevant load, air pressure and action direction
  • Select cylinder stroke and workpiece moving distance
  • Selected cylinder series
  • Selected cylinder installation type: different series have different installation methods, mainly including basic type, foot seat type, flange type, U-shaped hook and lug type
  • Selected buffer: no buffer, rubber buffer, air buffer, hydraulic shock absorber
  • Selected magnetic induction switch: it is mainly used for position detection, and the magnetic ring is required to be built in the cylinder
  • Selected cylinder accessories: including related joints

3. Selection of direction valve

The scope of application of the selected valve should be consistent with the site conditions.

To ensure reliable performance, it’s important to choose the appropriate valve based on various factors such as gas source pressure, power supply conditions (AC/DC, voltage, and fluctuation range), medium temperature, environmental temperature and humidity, dust, vibration, and so on.

The function and control mode of the selected valve shall meet the working requirements of the system.

To clarify, when selecting a valve for a pneumatic system, it’s important to consider the required position number, path number, memory, on-off state, and control mode of the components. The valve chosen should be one that satisfies the necessary function and control mode.

The flow capacity of the selected valve should meet the working requirements of the system

According to the pneumatic system’s requirements for the instantaneous maximum flow of components, the valve diameter should be calculated based on an average air velocity of 15-25m/s. Additionally, it’s important to determine the flow capacity C value (or kV), CV value, pressure drop under rated flow, standard rated flow, and s value of the required valve. By doing so, it’s possible to select a valve that meets the system’s flow capacity requirements.

The valve selected must meet the working requirements of the system. This implies that the chosen valve must adhere to the necessary performance index based on several factors, including the minimum working pressure or minimum control pressure, dynamic performance, maximum working frequency, continuous power-on capacity, power consumption, service life, and reliability of the pneumatic system.

The selection of valve installation should be based on the valve quality level, system space requirements and easy maintenance

Given the extensive usage of directional valves in our country, the plate type installation method is commonly adopted. Its advantage lies in its ease of assembly, disassembly, and maintenance. The ISO standard also endorses the plate installation method and has developed the container plate installation method.

Therefore, it is recommended to prioritize the plate type installation method. However, due to the ongoing enhancement of component quality and reliability, the tubular installation valve, which occupies less space and can also be assembled, has also found application.

Hence, the selection should be based on the actual situation at hand.

Try to choose standardized products

As the standardized products adopt the means of mass production, the quality is stable and reliable, the degree of generalization is high, and the price is cheap.

The price of the selected valve should be compatible with the system level and reliability requirements

That is, the price of valve should be considered according to the advanced degree and reliability requirements of pneumatic system.

On the premise of ensuring the advanced, reliable and convenient use of the system, we should strive for reasonable price and not pursue low cost regardless of quality.

In the design of large-scale control system, the bus control type of integrated valve and signal should be used as far as possible, and the selection of directional control valve should be considered

1)Selection of Directional Control Valve Series

Different function series of valves should be selected according to the different actuators.

2) Selection of Direction Control Valve Specification

The flow capacity of the selected valve should meet the system’s working requirements. Therefore, the valve’s diameter should be calculated based on the pneumatic system’s requirements for the instantaneous maximum flow of the components.

3) Selection of Control Mode

The appropriate control mode for the directional valve should be selected based on the working requirements and the cylinder’s action mode.

4) Selection of Applied Voltage

The pressure reducing valve should be selected based on the maximum working pressure of the pneumatic control system. The air source pressure should be 0.1 MPa higher than the maximum working pressure of the pressure reducing valve.

If the outlet pressure fluctuation of the pressure reducing valve is small, a precision pressure reducing valve should be selected if the fluctuation of the outlet pressure is not greater than ±0.5% of the maximum working pressure. If remote control is required or the diameter is over 20 mm, the external pilot pressure reducing valve should be selected as much as possible.

Selection of pressure reducing valve

1) Select the valve specification based on the maximum flow through the pressure reducing valve.

2) Choose the type of valve based on the functional requirements. These requirements may include the range of pressure regulation, the accuracy of pressure stabilization (which may determine whether a precision pressure reducing valve is necessary), whether remote control is required (in which case an external pilot pressure reducing valve should be selected), and whether there are any special functional requirements (which may determine whether a large flow pressure reducing valve or a composite function pressure reducing valve is necessary).

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