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What’s The Double Acting cylinder

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Double Acting Cylinder

Double Acting Cylinder is the most commonly standard cylinder in industrial automation system. They are high performing, long lasting and with the requested viton seal are able to operate in high temperatures.

What is the double acting cylinder?

It’s a cylinder whichever direction of the piston move terwords, they both need the air or hydraulic liquid to actuate. That means all direction movement needs the pressurized fluid to push the piston, including extend and retract action.

It’s name which is relative to the single acting cylinder.

The following figure show a typical double acting cylinder’s working process.

Extend Process

The above figure shows the inner structure of the typical double acting cylinder. When we inject compress air to the A Chamber from back cap port ⑥, the air push the piston ④ to move forward. And the meaning time, the air in the B Chamber is vented from front cap port ①.

Retract Process

When we inject compress air to the B Chamber from front cap port ①, the air push the piston ④ to move backward. And the meaning time, the air in the A Chamber will escape from front cap port ⑥.

Types of the double acting cylinder

Compared with the single acting cylnder, we used much more double acting cylinders in the industry system. Almost all of the cylinders could be double acting design.

1. Standard roundline cylinder: we adopt lots of this kind of cylinder in our automation machine.

2. Tie-rod cylinder and profile cylinder. Could be with the big bore size, and huge force output.

3. Compace cylinder: Be with the compact desging, it’s ideal cylinder for the application where the space is limited.

4. Double rod cylinder and guided cylinder: To avoid the piston rod rotate in the linear movement process.

5. Rodless cylinder: Good choice for the long stroke movement, run faster, save space.

What is the single acting cylinder?

A single-acting cylinder only creates one direction force, push or pull a subject. That’s mean we can only add load in one direction movement. And when the piston rely on the spring force to move to opposite direction. The following figures show how a profile single acting cylinder structure!

Auto shrink Model

The above figure shows the inner structure of the typical Auto shrink single acting cylinder. When we inject compress air to the A Chamber from back cap port ⑥, the air push the piston ④ and therefore compress the spring. The piston move forward. The same time, air in the B Chamber is vented from front cap port ①.

When we release the air in chamber A, the piston will retract under the press of the spring.

Auto extend Model

The above figure shows the inner structure of the typical Auto extend single acting cylinder. When we inject compress air to the B Chamber from FRONT cap port ①, the air push the piston ④ and therefore compress the spring. The piston move backward. The same time, air in the A Chamber is vented from back cap port ⑥.

When we release the air in chamber B, the piston will retract under the press of the spring.

When should we use the double acting cylinder?

1. When you need load to add in double direction. That mean when the cylinder rod extend out, it undertakes a load; and when the cylinder retract, there is still a load added.

2. Even you only add one direction load, but due to the safety or reliability consideration, the cylinder must fully recover the original status under lquid press other than the spring force. As the high-frequency reciprocating motion may lead to the spring fatigue. Thus had no enough force to push the piston back to the zero position.

3. In most of heave duty working situation, the double acting cylinder is best choice.

4. Almost all of the big bore cylinders are the double acting cylinder.

5. In most cases, even the load is enough lightweight and only be added in one driection, but due to low frequency action, the designer still prefer to the double acting cylinder.

The above five situations occupy high percent of the normal application. So we adopt more the double acting cylinders in many industries than single acting cylinders.

When should we use the single acting cylinder?

One Direction Load: We adopt single acting cylinders mainly in industrial applications where only need one direction, for example, ejecting parts/items off conveyor belts.

Safety Consideration: For the single acting cylinder, the designer may have other idea. That’s the safety consideration. Once any power supply emergency happen, the cylinder could recover to the original state due to the spring force.

Energy Saving: As in some situation, only need to add one direction lightweight load, and need low frequency work. In order to save energy, the single acting cylinder is a good choice. They also need less valve and piping cost compared to double acting cylinders

Disadvantage:

1. Be with same stroke length, the single acting cylinder’s dimension is longer than the double acting cylinder. As Spring takes too much space and limits working area. Therefore, It will take up more installation space.

2. Long time keeping at the working state may lead to the spring fatigue. Thus will lead to need more time to recover original state.

How to work out the output force?

The units of measurement.

The most important is the relationship of the pressure, piston surface area, and the output force.

A commonly used unit of pressure:

1Mpa=1N/mm²;

1Bar=0.1Nmm²;

1Kpa=0.001N/mm²;

1Psi=0.00689N/mm²

Cylinder force F = A × P

In which: F = force, in Newtons (N).

A = piston area in square centimeters (mm²).
P = differential pressure in MPa(1N/mm²)

For Instance : How to calculate the force of a 32mm bore size cylinder when the pressure is 1Mpa ?

A=兀r²=3.14*16²(The radius :r=Bore size/2=16)(兀≈3.14)

P=1Mpa=1N/mm²

F=3.14*16²*1=803.84N

The force chart for the standard bore size versus air pressure.

How to choose the mounting accessories?

The following criteria affect the selection of mounting accessories:
Load travel displacement: The path that the load must travel will define whether the cylinder will be fixed to a mounting surface or free to rotate about a pivot point.
Required cylinder force: The required force will determine if the cylinder will be providing a push or pull force on the load. Selecting suitable mounting attachments for the required force is necessary for proper cylinder function. Components such as mounting bolts or mounting surface can prematurely fail if not sized with mounting attachments.
Mounting structure and space: The structure in which the cylinder is mounted to must be able to withstand the shear stress during actuation. Side mount cylinders will expose the mounting surface to additional shear stress during actuation.
Load alignment: The alignment of the cylinder to the load will be critical to prevent side loading and excessive wear on the cylinder components. Centerline loading of the cylinder is ideal as it distributes force evenly along the cylinder. If a side mount attachment is required, the mounting surface and bolt should be sized correctly. If misalignment is possible, an attachment, which will compensate for the misalignment will be required.
Stroke length: Cylinders with a long stroke must have a properly sized piston rod diameter to prevent rod buckling. Guide the load when possible to prevent misalignment.
Pivot Angle: Pivot-mounted cylinders must consider the actuation angle with respect to the load, as the effective force on the load will be decreased depending on the angle.
Operating Environment: The dimensions or alignment of mounting attachments or accessories may change under extreme environmental conditions such as temperature or vibration. To prevent misalignment or cylinder failure operating conditions must be taken into consideration.

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