The moving assemblies of heavy mechanisms that run at limit speeds have huge inertia, the energy of which has to be suppressed in the last segment of the stroke. Otherwise, the high load may loosen the assembly’s fastener and the base frame itself, and when load values are critical, the moving part may just fly out of slots. The breakdown is serious and dangerous.
Impulse suppression at the end of the trajectory is reached via forced cushioning, the mode of which is embedded in the wiring diagram of the system’s hydraulic motor. Herewith, it is important to maintain stroke smoothness, the uniform speed decrease, and quick recovery of the initial state.
The stroke speed can be changed as follows:
- You can change the oil flow in the piston cavity. To do so, a mechanical control is installed to regulate the oil flow in the work main line.
- Movement of the piston couple can be decelerated via a special damping unit included in the hydraulic cylinder structure. In this case, the process is controlled by a hydraulic action.
1. Structural Methods of Achieving the Cushioning Effect
See Fig. 1:
Method 1. A throttle valve is built into cover (3) of the hydraulic cylinder.
Method 2. A gap in a fastening ring of the piston rod’s tapered head to the hydraulic cylinder’s cover is changed smoothly.
See Fig. 2:
Method 3. Plunger cushioning or cushioning with throttling orifices in the cover is used.
See Fig. 3:
Method 4. Radial throttling grooves in the piston rod head are closed one by one.
Method 5. Longitudinal throttling grooves in the piston rod head are closed gradually.
See Fig. 4:
Method 6. A double piston is used.
1. Throttle cushioning (see Fig. 1)
The most common variant of the structure, where the damper is built into cover (3) of the hydraulic cylinder. The drop of the piston speed is achieved by closing smoothly the main oil return line (passages 6 to 8), with subsequent oil removal from the working piston cavity through throttle valve (7). Piston (1) rigidly connected to piston rod (2) quickly returns to its initial position when the power fluid is supplied under pressure to its cavity through a check valve.
1.2. Plunger cushioning (see Fig. 2).
Plunger (5) with a follower is pressed against spring (7) to thrust washer (8). Follower (6) projects beyond the left end face of cover (4) precisely by half of the stroke of piston (1). In has passage (9) to feed oil to the piston tank through a special hole and intermediate chamber (11). At the end of the trajectory, piston rod (2) abuts with its boss (3) against the plunger follower to move it rightwards under a spring force, reducing in part the diametrical cross-section of the chamber and thereby the work flow of the oil fluid at the drain to groove (10).
1.3. Cushioning in the hydraulic system with an extra piston (see Fig. 4)
The structure of piston rod (5) is such that it includes two collars, which create a cushioning effect. An extra piston formed this way can move along the centreline in the hole of main piston (4). The cavity of cylinder tube (1) contains covers and bushings (6) limiting this movement.
The size of the bushings is selected according to the length of the cushioning section. Moving rightwards simultaneously with the piston rod, piston (4) stops against bushing (6), but the piston rod continues to move until stopping with collar (5) against the left edge of piston (4). This significantly reduces the work area for the oil flow and, among other things, decreases the force that pushes the piston rod. As a consequence, the piston couple gradually drops the speed and stops. Unfortunately, this simple structure does not make it possible to control the cushioning intensity.
2. Structural Designs of Hydraulic Cylinders. Description of the Operating Principle of Their Cushioning Systems
Below, we are going to review most common and excellently performing developments that are used to power heavy-duty equipment in a variety of national economy sectors.
2. 1. A standard structure with the cushioning function
See Fig. 5:
A hydraulic cylinder selected for heavily loaded mechanisms typically has the following basic components:
- cylinder tube body (1),
- cover (2),
- piston rod (6) + piston (5) – an unchanged working couple.
The cushioning mode during movement is ensured by additional tapered bushing (4) located on the piston rod. The piston rod head also has a tapered shape. When the piston is moving leftwards, the bushing enters the matching hole on the cover with its tapered portion to create an obstacle on the oil outflow path.
When the piston approaches the end right position, a gap created by the bushing becomes the minimum one, and a portion of oil under pressure starts to flow out from the piston rod cavity through passages to enter the throttling assembly. This suppresses a significant portion of the moving mass’s inertia. Of course, the speed drops, and the piston rod decelerates.
During the right-hand stroke, the tapered portion of the piston rod enters the matching hole on the cover. For initial acceleration of the piston couple, the cover and the flange have a system of valves installed on the oil path through the feed opening to the cylinder’s working zone.
2.2. The system of cushioning by changing the main line resistance
See Fig. 6:
With this scheme, a cushioning assembly is built into cover (1), where plunger (3) – which has step passage on the side of left face (13) and is supported by spring (14) – is located inside bore hole (2). On the same side, there are radial groove (17) and groove (5) on the outer surface of the plunger, and there is check valve (15) inside face passage (4). On the other plunger face (8), there is cavity (19), which is connected through the hole in the plunger and through the bore hole in the cover to fluid passage (12).
During the right-hand stroke of piston (11), the fluid from chamber (10), between the cover, the piston, and cylinder tube (20), goes to cavities (18) and (19) through face passage (9). From there, it goes through hole (7) in wall (6) of the plunger and bore hole (16) in the fluid passage to drainage. Abutting against the plunger, the piston moves it rightwards, shifting hole (7) outside the bore hole, and at the same time compresses the spring. Therefore, oil is drained only through groove (5). Herewith, the piston is gradually being cushioned due to a change in the groove cross-section.
At the end of the stroke, the bore hole of the fluid passage aligns with groove (17), ensuring the pressure supply through the check valve to the piston chamber. This is how the left movement of the piston starts. Under the spring’s action, the plunger arrives at its initial position, disconnecting the groove and the fluid passage. The fluid pressure now is supplied through hole (7) to the right portion of the cover and from there via passage (9) to the piston’s working chamber to ensure the required speed of its leftward movement.
2.3. A hydraulic cylinder with a built-in cushion without the plunger couple
See Fig. 7:
This scheme is simplified due to plunger exclusion. Here, cover (2) is fastened with studs to cylinder tube (1). The cover is equipped with special boss (9), cavity (10) of which is connected via a hole with fluid passage (14) and via a union with the main fluid line. Throttle valve (12) is fastened using locknut (17) on the opposite side of the fluid passage. It is connected via the groove to piston chamber (15). Cup (6) with a poppet valve locked by ring (16) is installed in piston cavity (5). The cup is supported by the spring and fixed with stopper (8).
Though speed adjustments via cushioning are limited by the throttle valve and though the damping distance is determined by the structural combination of the relevant bosses on the cover, the piston, and the piston cup, this system operates in a rather reliable manner due to its simplicity.
During the vertical travel, the oil from the piston chamber easily enters the fluid passage through hole (14) in cavity (10) of the cover boss. While the piston is moving further, the cover boss is closed by poppet valve (6), and the power fluid has only one path left – through groove (11) with the throttle valve. This artificial obstacle causes an increase in resistance for oil flow and results in piston cushioning. When the fluid is fed from the main line, its pressure presses spring (7) out and releases the poppet valve. Access to the piston chamber opens, and the return stroke cycle begins.
2.4. A double-stroke cushion for heavy loads
See Fig. 8:
When piston rod deceleration is needed, during the both forward and return stroke, you can do the following. Cylinder tube (1) is equipped with covers (2) and (3) on both sides. A forward-stroke cushion is mounted in cover (3) in the form of a throttle assembly with holes located in the bushing, to boss (11) of which the spring presses cup (8) with seal (13).
Hole (5) is connected to the piston chamber of the hydraulic cylinder via longitudinal and transverse grooves. Return-stroke cushioning is ensured by the second throttle mechanism installed on piston rod (6). The cup is pressed to retaining ring (19) on the piston rod and bushing (17) by spring (20) and is sealed with seals (23) and (21). Throttling orifices (18) are made in the bushing to connect supply hole (4) with the rod space.
At the end of the forward stroke, piston (7) presses on the cup and – by compressing spring (12) – moves it into bushing (9), thereby closing holes (10). This creates resistance to oil squeeze-out from the cup cavity and leads to cushioning. When the pressure is supplied through hole (5), the oil enters the piston area via grooves (14) and (15). The piston, in turn, starts to move back. Herewith, the cup under the action of the spring returns to its initial position while opening the throttling orifices through which its cavity is filled with oil again.
During the return stroke, cup (16) abuts against a box, and the bushing continues its movement to close the throttling orifices. The rising resistance to oil squeeze-out from the cup cavity leads to cushioning. While the fluid is pumped into hole (4), it enters the piston rod space through the relevant grooves and starts the forward-stroke phase. Herewith, the cup becomes empty under the action of the spring while opening the throttle valves and letting the oil enter its cavity. The cycle is over.
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