Hydraulic cylinder, also known as oil cylinder or actuator, is an energy conversion device used to convert the pressure energy of oil into mechanical energy

A hydraulic actuator that drives the working mechanism to perform linear reciprocating motion or reciprocating swing, with a simple structure, large force transmission, and easy operation

Small dynamic inertia, easy to achieve reciprocating motion control, convenient layout and installation, and other advantages, compared to lever, connecting rod, gear rack

The combination of ratchet, pawl, cam and other mechanisms can achieve various mechanical movements and has been widely used in hydraulic systems.

4.1.1 Types and working principles of hydraulic cylinders

The physical essence of hydraulic cylinder operation is to use oil pressure to overcome loads, and to use oil flow rate to maintain movement speed

The working parameters of the input hydraulic cylinder are the pressure and flow rate of the oil, that is, the input hydraulic power.

There are various types of hydraulic cylinders, which can be divided into reciprocating linear motion hydraulic cylinders and reciprocating swing hydraulic cylinders according to their motion modes; By action

The method is divided into single acting hydraulic cylinder and double acting hydraulic cylinder; According to structural characteristics, it can be divided into piston type, plunger type, swing type, and extension type

Among them, piston type hydraulic cylinders are the most commonly used.

1. Piston hydraulic cylinder

The piston type hydraulic cylinder can be divided into two structures: double rod and single rod, and its fixation methods include cylinder block fixation and piston rod fixation

Two types.

(1) Double rod piston cylinder

The structure and graphical symbols of the double rod piston cylinder are shown in Figure 4-1, which consists of the cylinder barrel, piston, piston rod, and cylinder head.

According to different installation methods, it can be divided into two types: fixed cylinder barrel and fixed piston rod. Figure 4-2 (a) shows the cylinder barrel

A fixed double rod piston cylinder with oil inlet and outlet ports arranged at both ends of the cylinder barrel. The piston drives the workbench to move through the piston rod,

When the effective stroke of the piston is L, the motion range of the entire workbench is 3L, which occupies a larger area. As shown in Figure 4-2 (b)

The shown is a double rod piston cylinder with a fixed piston rod, the cylinder body is connected to the workbench, and the movement range of the entire workbench is a hydraulic cylinder

Two times the effective travel L (2L), resulting in a small footprint.

Due to the fact that the piston rod diameters at both ends of a double rod piston cylinder are usually equal, the effective areas of its left and right chambers are also equal. When inputting the same pressure and flow rate of oil into the left and right chambers respectively, the thrust in the left and right directions of the hydraulic cylinder

(2) Single rod piston cylinder

A single rod piston cylinder only has a piston rod in one chamber of the hydraulic cylinder, which can be divided into single acting and double acting. Figure 4-3

Shown as a double acting piston cylinder with a steel ball lock, its reciprocating motion is achieved by the hydraulic pressure acting on the piston, at its extreme position

Limited position, in order to improve the reliability of piston rod fixation, a steel ball lock is installed. Single rod piston cylinders also have cylinder barrel fixing and piston rod fixing

There are two installation forms, regardless of which one, the range of motion of the workbench is equal to twice the effective stroke of its piston.

Due to the fact that there is only one piston rod in a single rod piston cylinder, the effective area of action at both ends of the piston is not equal. When facing both cylinders separately

When the cavity is supplied with oil and the supply pressure and flow rate are the same, the thrust and speed generated by the piston (or cylinder block) in both directions are not consistent

Etc. The thrust generated on the piston is proportional to the effective area of the oil inlet chamber, that is, the thrust generated by the rodless chamber is greater than that of the rodded chamber; and

The speed of piston movement is inversely proportional to the effective area of the oil inlet chamber, that is, when the oil enters the rodless chamber, the effective area is larger and the speed is slower,

When entering the rod cavity, the effective area is small and the speed is fast.

When there is no rod cavity (effective working area A) for oil inlet and a rod cavity (effective working area A) for oil return, the piston thrust F

1 2 1

And movement speed

υ Respectively

one

π

F1=(p1 A1 − p2 A2)=[D p1 − p2 (D − d)] (4.3)

2 2 2

four

Q 4Q

υ = = (4.4)

1 2 1

A π D

When there is oil entering the rod chamber and no oil returning from the rod chamber, the piston thrust F and movement speed

υ Respectively 2 2

π

F p A p A D d p D p

2=(1 2 − 2 1)=[(−) 1 − 2] (4.5)

2 2 2

four

Q 4Q

υ = = (4.6)

2 2 2 2

A π (D − d)

Compared to the above formula, it can be seen that,

υ <υ 、 F>F, i.e. no rod chamber inlet pressure oil working

1 2 1 2

When the thrust is high, the speed is low; When there is pressure oil in the rod chamber, the thrust is small and the speed is high. Therefore, single rod piston cylinders are commonly used in equipment with a large load in one direction but a low operating speed, and a fast return without load in the other direction.

Output speed of single rod piston cylinder in two directions

υ and υ The ratio of is called the velocity ratio

2 1

φ ， It is an important parameter for the dimensional characteristics of a single rod double acting piston hydraulic cylinder. Figure 4-4 Differential cylinder

If pressure oil is simultaneously applied to the left and right chambers of a single rod piston cylinder, as shown in Figure 4-4

The so-called differential connection, a single rod piston cylinder with differential connection, is called a differential hydraulic cylinder. During operation, due to the fact that the effective area of the rodless chamber is greater than that of the rod-shaped chamber, the piston moves to the right while causing the oil in the rod-shaped chamber to flow at a rate of Δ Q) Inflow into the rodless cavity increases the flow rate into the rodless cavity (Q+ Δ Q) This also accelerates the piston movement speed.

Hydraulic cylinder piston thrust

F and motion speed υ Respectively

3 3

π

F3=p (A1 − A2)=pd2 (4.7)

four

υ = Q+ Δ Q=4Q (4.8)

(4.8)

A π d

3 2

one

According to equations (4.7) and (4.8), the thrust of the hydraulic cylinder in differential connection is smaller than that in non differential connection,