What Mechanical Engineers Need to Know About Cylinders?

1. Selection of cylinder

About Cylinders

1. Selection of types

  • The type of cylinder should be selected correctly according to the working requirements and conditions.
  • The cushion cylinder should be selected when the cylinder reaches the end of stroke without impact phenomenon and impact noise;
  • Light cylinder is required for light weight, thin cylinder is required for narrow installation space and short stroke;
  • With lateral load, the cylinder with guide rod can be selected;
  • High braking accuracy is required, and locking cylinder should be selected;
  • The piston rod is not allowed to rotate, and the cylinder with the function of non rotating rod can be selected;
  • In the harsh environment such as dust, it is necessary to install a dust cover on the extended end of the piston rod.
  • When no pollution is required, no oil supply or oil-free lubrication cylinder should be selected.

2. Installation form

  • Decide according to the installation location, use purpose etc.
  • In general, the fixed cylinder is used.
  • When it needs to rotate continuously with the working mechanism (such as lathe, grinder, etc.), the rotary cylinder should be selected.
  • When the piston rod is required to swing in circular arc besides linear motion, the pin type cylinder is selected.
  • When there are special requirements, the corresponding special cylinder should be selected.

3. The magnitude of the force

That is the choice of cylinder diameter.

According to the size of the load force to determine the cylinder output thrust and pull.

Generally, according to the theoretical balance condition of external load, different load rates are selected according to different speeds, so that the output force of the cylinder has a little margin.

If the cylinder diameter is too small, the output force is not enough, but if the cylinder diameter is too large, the equipment is bulky, the cost is increased, the gas consumption is increased, and the energy is wasted.

In the fixture design, force amplifier should be used as far as possible to reduce the size of the cylinder.

4. Piston stroke

It is related to the use occasion and the stroke of the mechanism, but generally not full stroke to prevent the piston from colliding with the cylinder head.

If it is used for clamping mechanism, an allowance of 10-20 mm shall be added according to the calculated stroke.

5. The speed of the piston

Mainly depends on the cylinder input compressed air flow, cylinder inlet and outlet size and pipe diameter size.

It is required that the high speed motion should be taken as a large value.

The speed of the cylinder is generally 50-800 mm / s.

For high-speed cylinder, the intake pipe with large inner diameter should be selected;

When the load changes, in order to get a slow and steady speed, it is easier to control the speed by choosing throttling device or gas-liquid damping cylinder.

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

  • When the horizontal cylinder pushes the load, the exhaust throttle is recommended;
  • When the cylinder installed vertically lifts the load, it is recommended to use the intake throttle to adjust the speed;
  • When the end of stroke is required to move smoothly to avoid impact, the cylinder with 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.

That is to say, according to the gas source pressure, power supply conditions (AC / DC, voltage and fluctuation range), medium temperature, humidity, environmental temperature and humidity, dust, vibration, etc., the valve suitable for reliable use under this condition should be selected.

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

That is to say, according to the requirements of the pneumatic system on the position number, path number, memory, on-off state and control mode of the components, the valve that meets the required function and control mode should be selected.

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

That is, according to the requirements of the pneumatic system for the instantaneous maximum flow of components, the valve diameter should be calculated according to the average air velocity of 15-25m / s, and the flow capacity C value (or kV), CV value, pressure drop under rated flow, standard rated flow and s value of the required valve should be found out, so as to select the valve that meets the requirements of the system flow capacity.

The performance of the selected valve shall meet the working requirements of the system, that is, the valve conforming to the required performance index shall be selected according to 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

Considering the wide application of directional valve in our country, the plate type installation method is adopted.

Its advantage is that it is easy to assemble, disassemble and maintain. ISO standard also adopts plate installation method and develops container plate installation method.

Therefore, it is recommended that the plate type installation method be preferred.

However, due to the continuous improvement of the quality and reliability of the components, the tubular installation valve occupies a small space and can also be assembled, so it has also been applied.

Therefore, the selection should be based on the actual situation.

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

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

2) Selection of direction control valve specification

The flow capacity of the selection valve should meet the working requirements of the system, that is, the diameter of the valve should be calculated according to the requirements of the pneumatic system for the instantaneous maximum flow of the components.

3) Selection of control mode

The appropriate control mode of directional valve should be selected according to the working requirements and the action mode of cylinder.

4) Selection of applied voltage

The pressure reducing valve is selected according to the maximum working pressure of the pneumatic control system, and the air source pressure should be 0.1MPa higher than the maximum working pressure of the pressure reducing valve.

When the outlet pressure fluctuation of the pressure reducing valve is small, if the fluctuation of the outlet pressure is not greater than ± 0.5% of the maximum working pressure, the precision pressure reducing valve shall be selected.

If remote control is required or the diameter is more than 20 mm, the external pilot pressure reducing valve shall be selected as far as possible.

Selection of pressure reducing valve

1) Select the valve specification according to the maximum flow through the pressure reducing valve.

2) According to the functional requirements, select the type of valve. For example, the range of pressure regulation, the accuracy of pressure stabilization (whether precision pressure reducing valve should be selected), whether remote control is required (external pilot pressure reducing valve should be selected for remote control), and whether there are special functional requirements (whether large flow pressure reducing valve or composite function pressure reducing valve should be selected).

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