Hydraulic cylinders

- Apr 06, 2020-

A hydraulic cylinder is a hydraulic actuator that converts hydraulic energy into mechanical energy and performs linear reciprocating motion (or swinging motion). It has simple structure and reliable work. When it is used to realize reciprocating motion, the deceleration device can be eliminated, and there is no transmission gap, and the movement is stable, so it is widely used in the hydraulic system of various machines. The output force of the hydraulic cylinder is proportional to the effective area of the piston and the pressure difference between the two sides; the hydraulic cylinder is basically composed of the cylinder barrel and the cylinder head, the piston and the piston rod, the sealing device, the buffer device and the exhaust device. The buffer device and exhaust device depend on the specific application, other devices are essential.





1. Classification of hydraulic cylinders


The structure of hydraulic cylinders is various, and there are many classification methods: according to the movement mode, it can be divided into linear reciprocating and rotary swing; according to the effect of the hydraulic pressure, it can be divided into single-acting and double-acting; according to the structure The form can be divided into piston type, plunger type, multi-stage telescopic sleeve type, rack and pinion type, etc .; according to the installation form can be divided into tie rods, earrings, feet, hinge shaft, etc .; according to the pressure level can be divided into 16Mpa, 25Mpa , 31.5Mpa, etc.


1. Piston


A single piston rod hydraulic cylinder has a piston rod at only one end. As shown in the figure is a single-piston hydraulic cylinder. Both the inlet and outlet ports A and B at both ends can pass pressure oil or return oil to achieve two-way movement, so it is called double-acting cylinder.




The piston can only move in one direction, and the movement in the opposite direction needs to be completed by external force. But its stroke is generally larger than that of piston hydraulic cylinders.


Piston type hydraulic cylinder can be divided into two types of single rod type and double rod type. The fixing method is fixed by the cylinder body and the piston rod. There are single acting type and double acting type according to the hydraulic pressure. In a single-acting hydraulic cylinder, the pressure oil is only supplied to one cavity of the hydraulic cylinder. The hydraulic pressure makes the cylinder move in one direction, and the movement in the opposite direction is realized by external forces (such as spring force, dead weight, or external load, etc.); The movement of the hydraulic cylinder piston in two directions is completed by the two chambers alternately feeding oil, and is completed by the action of hydraulic pressure.


The figure shows a schematic diagram of a single-rod double-acting piston hydraulic cylinder. It only has a piston rod on one side of the piston, so the effective area of the two chambers is different. When the oil supply amount is the same, different chambers feed oil, and the piston moves at different speeds; when the load force to be overcome is the same, different chambers feed oil, the required oil supply pressure is different, or after the system pressure is set, the sanitation The load force that the garbage truck hydraulic cylinder can overcome in two directions is different.



2. Plunger


(1) The plunger hydraulic cylinder is a single-acting hydraulic cylinder, which can only achieve movement in one direction by hydraulic pressure, and the return stroke of the plunger depends on other external forces or the weight of the plunger;


(2) The plunger is only supported by the cylinder liner and not in contact with the cylinder liner, so that the cylinder liner is easy to process, so it is suitable for long-stroke hydraulic cylinders;


(3) The plunger is always compressed during work, so it must have sufficient rigidity;


(4) The weight of the plunger is often large, and it is easy to sag due to its own weight when placed horizontally, causing the seal and guide to wear on one side, so its vertical use is more advantageous.


Telescopic


Telescopic hydraulic cylinders have two-stage or multi-stage pistons. The sequence of extending pistons in telescopic hydraulic cylinders is from large to small, while the sequence of no-load retraction is generally from small to large. The telescopic cylinder can achieve a longer stroke, and when retracted, the length is shorter and the structure is more compact. This type of hydraulic cylinder is often used in construction machinery and agricultural machinery. There are multiple pistons in one movement. When each piston moves sequentially, its output speed and output force change.




Double acting single piston rod type hydraulic cylinder


3. Swing


Oscillating hydraulic cylinder is an actuator that outputs torque and realizes reciprocating motion. There are several forms such as single blade, double blade, and spiral swing. Blade type: The stator block is fixed on the cylinder, and the blade and rotor are connected together. According to the oil inlet direction, the blade will drive the rotor to swing back and forth. The spiral swing type is divided into single spiral swing and double spiral. Now double spiral is more commonly used. The linear motion of the piston in the two spiral vice descending hydraulic cylinders is transformed into a compound motion of linear motion and rotation motion to achieve swing motion.


2. Working principle of hydraulic oil cylinder


A hydraulic cylinder is an actuator in a hydraulic transmission system. It is an energy conversion device that converts hydraulic energy into mechanical energy. The hydraulic motor realizes continuous rotary motion, while the hydraulic cylinder realizes reciprocating motion. There are three types of hydraulic cylinders: piston cylinder, plunger cylinder, and swing cylinder. The piston cylinder and plunger cylinder realize reciprocating linear motion, output speed and thrust, and the swing cylinder realizes reciprocating swing, outputting angular velocity (rotation speed) and torque. In addition to being used individually, hydraulic cylinders can also be used in combination of two or more or in combination with other mechanisms. In order to complete special functions. The hydraulic cylinder has a simple structure and reliable operation, and is widely used in the hydraulic system of the machine tool.


Three, hydraulic cylinder dynamic diagram