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How To Design Hydraulic Cylinder

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Estimated reading time: 37 minutes

The combination of a hydraulic cylinder and mechanism

For the situation that the motion form of the main machine working mechanism is more complicated, the hydraulic actuator can be organically matched with other mechanisms to form a hydraulic mechanical working mechanism to meet the activity requirements. For hydraulic motors and swing hydraulic motors, the working mechanism can be driven to move through the gear mechanism and the screw nut mechanism. For hydraulic cylinders, the working mechanism can be driven in horizontal, vertical, and inclined directions. The schematic diagram and characteristics are shown in 2-1.

Combination working structure of common hydraulic cylinders

  1. Direct drive mechanism

Feature: The piston rod of hydraulic cylinder 1 is directly connected with moving object 2, and the hydraulic cylinder drives the moving object to make horizontal reciprocating linear.

Surface grinder table, combined machine tool power sliding table reciprocating motion, etc.

Feature: The piston rod of hydraulic cylinder 1 is directly connected with the moving object 2, and the hydraulic cylinder drives the moving object to make a vertical reciprocating linear motion.

Press slider and ejector device, hydraulic elevator lifting device, harvester header lifting, etc.

Feature: The piston rod of  hydraulic cylinder 1 is directly connected with the moving object 2, and the hydraulic cylinder drives the moving object to make a reciprocating linear motion in an inclined direction.

Mining and metallurgical machinery, etc.

2. Force-increasing clamping mechanism

Feature: The piston rod of the vertical hydraulic cylinder 1 is connected with the connecting rod mechanism 2, and the workpiece 4 is clamped in the horizontal direction by  clamp 3, and a hydraulic cylinder with a smaller thrust is used to achieve a larger clamping force, and the clamping force varies with the clamped workpiece Changes in size.

Machine tool fixtures, manipulators, etc.

3. Telescopic extension mechanism

Feature: The piston rod of the horizontal hydraulic cylinder 1 is connected with the connecting rod mechanism 2, and the horizontal movement of the hydraulic cylinder is converted into vertical lifting movement of platform 3, which can expand the range and increase speed.

Lifting stage, large-stroke scissor telescopic frame, automobile maintenance lifting platform, etc.

4. Pulley lifting mechanism

Feature: Hydraulic cylinder 1 is installed obliquely, and its piston rod is connected with the steel cable 2 wound on pulley 3 to realize the lifting movement of lift 4.

Hoist, blast furnace feeding device.

5. Swing mechanism

Feature: The piston rod of  hydraulic cylinder 1 is connected with the rocker mechanism 2 which converts the telescopic motion of the hydraulic cylinder into the swing of the rocker.

Construction machinery, construction machinery working mechanism.

6. Rack-gear mechanism

Feature: The piston rod of hydraulic cylinder 1 is connected with rack 2 to convert the reciprocating linear motion of the hydraulic cylinder into the rotary motion of gear 3.

Intermittent feeding mechanism, feeding mechanism.

7. Sector gear-rack mechanism

Feature: The piston rod of hydraulic cylinder 1 connected at the end is connected with the sector gear 2 to convert the horizontal reciprocating linear motion of the hydraulic cylinder into the vertical reciprocating linear motion of rack 3.

Short-stroke working device.

8. Linear motion mechanism

Feature: The piston rod of hydraulic cylinder 1 is connected with rod system 2, which converts the vertical movement of the hydraulic cylinder into positive and negative adjustable bidirectional horizontal reciprocating linear movement.

Periodic motion mechanisms in various types of machinery.

9. Moving cam mechanism

Feature: The piston rod of  hydraulic cylinder 1 is connected to the moving cam 2, which can make  follower 3 perform vertical reciprocating motion according to the predetermined motion law under the cam drive. The structure is simple and compact, and the required cam profile can be easily obtained by CNC machine tools.

Automatic feeding device, etc.

10. Tension and compression clamping mechanism

Feature: The piston rod of hydraulic cylinder 1 is connected with the tapered jacket 2 and the clamping is realized by stretching movement of the piston rod.

Machine tool fixtures, etc.

11. Double-cylinder rigid synchronization mechanism

Feature: The oil circuits of the two hydraulic cylinders 1 are connected in parallel, and their piston rods establish a rigid connection through the rigid member 2 to achieve displacement synchronization.

Press, blast furnace feeder, planter fertilizer box lifting device, harvester header, etc.

Calculation of hydraulic cylinder parameters

Question 1: Calculation of motion parameters

  • The movement of the main engine’s actuators can be represented by the displacement cycle diagram (L-t), the velocity cycle diagram (v-t), and the overall machine working cycle diagram, to analyze the movement law.

Displacement cycle diagram

  • Figure 2-1 is a hydraulic cylinder displacement cycle diagram of a hydraulic machine. The ordinate L represents the displacement of the piston, the abscissa t represents the time from the start of the piston to the return to the original position, and the slope of the curve represents the movement speed of the piston. Figure 2-1 clearly shows that the working cycle of a hydraulic press is composed of six stages: fast down, decelerated down, compression, pressure retention, pressure relief, slow return, and fast return.

Speed cycle diagram

Analysis of motion parameters

  • The movement of the main engine’s actuators can be represented by the displacement cycle diagram (L-t), the velocity cycle diagram (v-t), and the overall machine working cycle diagram, to analyze the movement law.

Displacement cycle diagram

Figure 2-1
  • It is a hydraulic cylinder displacement cycle diagram of a hydraulic machine. The ordinate L represents the displacement of the piston, the abscissa t represents the time from the start of the piston to the return to the original position, and the slope of the curve represents the movement speed of the piston. Figure 2-1 clearly shows that the working cycle of a hydraulic press is composed of six stages: fast down, decelerated down, compression, pressure retention, pressure relief slow return, and fast return.

Speed cycle diagram

  • According to the working cycle diagram of the whole machine, the stroke or speed of the actuator, and the acceleration change law, the speed cycle diagram (v-t or v-L) of the actuator can be calculated and drawn. According to the characteristics of the movement form of the hydraulic cylinder in the project, it can be summarized as three types.
Figure 2-2
  • It is the speed cycle diagram of the three types of movement of the hydraulic cylinder.
  • As shown by the solid line 1 in Figure 2-2, the hydraulic cylinder starts to move at a uniform acceleration, then moves at a constant speed, and finally decelerates to the endpoint; as shown by the dotted line 2 in Figure 2-2, the first half of the total stroke of the hydraulic cylinder is done Perform uniform acceleration movement, and finally perform uniform deceleration movement, and the acceleration values are equal; as shown in the double-dotted line 3 in Figure 2-2, the hydraulic cylinder moves at a small acceleration during the majority of the total stroke, and then uniformly decelerates to the end of the stroke. The three-speed curves in the figure not only clearly show the movement laws of the three types of hydraulic cylinders but also show the dynamic characteristics of the three working conditions.

Working cycle diagram of the whole machine

  • In a complex system with multiple hydraulic actuators, the actuators usually work in cycles according to a certain program. Therefore, it is necessary to reasonably arrange the working sequence and working time of each actuator according to the working mode and productivity of the host and draw the working cycle diagram of the whole machine.

Question 2: Calculation of the maximum load force of the hydraulic cylinder

  • For the actuators that drive the working mechanism of the machine tool, the focus is on the time relationship between the load and each working condition; for the actuators that drive the working mechanism of the construction machinery, the focus is on the gravity at each position, and the load diagram uses the position as a variable. The external load of hydraulic actuators includes three types: working load, friction load, and inertial load.
  • Figure 2-1 shows the calculation diagram of the hydraulic cylinder, where Fe is the external working load acting on the piston rod, Fm is the seal of the hydraulic cylinder (between the piston and the inner wall of the cylinder, and between the piston rod and the guide sleeve on the cylinder head The internal sealing resistance of the seal). 

2-1 Hydraulic cylinder calculation diagram


2-2 Flat rail
2-2 Flat rail

2-3 V-rail

Workload Fe

  • The working load has resistance load (the load that is opposite to the direction of movement and prevents movement, also called positive load) and overrun load (the load that promotes movement in the same direction of movement, also called negative load). The common working loads of hydraulic cylinders include gravity, cutting force, squeezing force, and so on. The resistance load is positive, and the overrun load is negative.

Mechanical friction load FF

  • Friction load refers to the mechanical friction resistance load to be overcome when the hydraulic actuator drives the working mechanism. There are two types of static friction load and dynamic friction load.
  • Mechanical friction load Ff; for machine tools, it is the frictional resistance of the guide rail. Flat guide rail. The frictional resistance of the flat guide rail varies with the way the guide rail is placed.
  • Flat rails placed horizontally (see Figure 2-2)
  • Static friction resistance
  • Ffs-us(G+Fn)
  • Dynamic friction resistance
  • Ffd=ud(G + Fn)
  • Flat guide rails placed obliquely (see Figure 2-2)
  • Static friction resistance
  • Ffs—us (Gcosβ+Fn)
  • Dynamic friction resistance
  • Ffd=pd(Gcosβ+Fn)
  • V-shaped guide rail (see Figure 2-3).
  • Static friction resistance
  • Ffs-us(G + Fn)/sin(a/2)
  • Dynamic friction resistance
  • Fd=ud(G+Fn )/ sin(a /2)
  • In the formula, G—gravity of moving parts, N;
  • Fn——The vertical component of the working load on the guide rail, N;
  • β——Inclination angle of the plane guide rail, (°);
  • α——The included angle of the V-shaped guide rail, (°);
  • Us, Ud-static and dynamic friction factors, selected according to the material and properties of the friction surface. Generally, for sliding guide rails, us=0.1~0.2, Ud=0.05~0.12 (large value at low speed, and small value at high speed); for rolling guide rail, Ud=0.003~0.02 [cast iron to ball (column) takes the larger value, Steel to roller takes the small value]; for cast iron hydrostatic guide rail, Ud=0.005.

Work cycle diagram

Load diagram

2-4 The working cycle diagram and load diagram of the main hydraulic cylinder of a machine tool
 

Question 3: Checking the effective area of the hydraulic cylinder

  • The effective working area of the hydraulic cylinder affects the thrust and speed of the hydraulic system. Therefore, the effective working area calculated according to the load must be checked according to the speed to verify that it can meet the requirements of the minimum stable flow of the throttle or speed control valve. The minimum operating speed of the system is required. After the effective working area meets the minimum working speed requirements of the hydraulic system, it needs to be rounded to ensure that standard sealing elements can be used.
  • For example, for hydraulic cylinders with a very low working speed, such as the feed cylinder of a precision boring combined machine tool, after calculating the size of the cylinder according to the load force.
  • Minimum working speed check cylinder size,
  • A≥q min/v min
  • Where A is the effective working area of the hydraulic cylinder, m2; V min’;
  • The minimum working speed of the hydraulic cylinder, m/s;q min;
  • The minimum stable flow of the system, m3/s, the throttle speed control system depends on the minimum stable flow of the flow control valve;
  • The volumetric speed control system depends on the minimum stable flow rate of the variable pump, and the sample is checked according to the component selection.
  • If the effective area after verification cannot meet the minimum working speed requirement, the cylinder’s size must be determined at the minimum stable speed.

Question 4: Compile working condition diagrams of hydraulic cylinders or hydraulic motors

  • The working condition diagram includes a pressure cycle diagram (p-t diagram or p-L diagram), a flow cycle diagram (q-t diagram or q-L diagram), and a power cycle diagram. Diagram (P-t diagram or P-L diagram), which reflects the pressure, flow, and power requirements and changes of a hydraulic system in a cycle.
  • The situation and the location of the peak are the basis for drawing up hydraulic systems, comparing schemes, adjusting or modifying design parameters for equalizing power distribution, and selecting and designing hydraulic components. p-t diagram (or p-L diagram).
  • According to the final size of the actuator structure, according to the actual load, it can be calculated that the hydraulic actuator is in its action.
  • The working pressure at each stage of the cycle is shown in Figure 2-5 and then drawn into a p-t diagram (or p-L diagram).

Oil into the rodless cavity

Oil into the rod cavity

Differential connection

2-5 Calculation diagram of single piston rod hydraulic cylinder
  • If multiple actuators working at the same time in the system, the q-t diagram (or q-L diagram) of each actuator should be stacked.
  • Plus, draw the total q-t diagram (or q-L diagram) of the system;
  • P-t diagram (or P-L diagram);
  • P-t diagram (or P-L diagram) can be drawn from the p-t;
  • Graph (or p-L graph) and q-t graph (or q-L graph) root;
  • According to the hydraulic power, P=pq plotted.
  • Figure 2-6 is an example of a working condition diagram of a hydraulic cylinder. The working condition diagram of the hydraulic cylinder is to select other elements in the system. Components and hydraulic basic circuits, for the drafting of the hydraulic system; Both the system and the comparison of schemes play an important role.
  • The maximum pressure and maximum flow in the working condition diagram determine the maximum working pressure and the maximum working pressure of hydraulic components such as hydraulic pumps and hydraulic valves. Maximum working flow.
  • The main parameter values of the hydraulic system determined according to the working condition diagram reflect the rationality of the original design parameters and are the main parameters of the system. The revision and finalization provided the basis.
  • By analyzing the change law of pressure and flow in each stage of the working condition diagram, the hydraulic circuit and oil can be selected reasonably. The structural form of the source, a perfect hydraulic system plan is drawn up. When the actuator in the hydraulic system is a hydraulic motor, because the hydraulic motor is a series of design products, this step becomes an option. Determine the working pressure p, displacement Vm, maximum speed max, and maximum flow max of the hydraulic motor, and then perform rounding to determine the specifications of the hydraulic motor. As for drawing the pressure diagram, flow diagram, and power diagram of the hydraulic motor according to the load and speed, it is the same as that of the:

hydraulic cylinder

2-6 Example of working condition diagram of hydraulic cylinder

Question 5: Calculation of cylinder length of hydraulic cylinder

Calculation of the length of hydraulic cylinder

  • The cylinder length L1 of the hydraulic cylinder is determined by the maximum working stroke length plus various structural requirements, namely.
  • L1=L+B+A+M +C
  • In the formula, L is the maximum working stroke of the piston;
  • B-Piston width, generally (0.6~1)D;
  • A-Piston rod guide length, take (0.6~1.5)D;
  • M-The sealing length of the piston rod is determined by the sealing method;
  • C-Other lengths.

Pay attention to the problem

  • Generally, the length of the cylinder should not exceed 20 times the inner diameter. In ensuring that the conditions of movement stroke and load force can be met. Next, the outline size of the hydraulic cylinder should be reduced as much as possible.
  • Special problems of high-pressure long hydraulic cylinders. When the length of the hydraulic cylinder is particularly large and the working pressure is high, a special structure should be adopted to ensure the deformation of the hydraulic cylinder. Not too big. The specific solution is to install a reinforcing hoop in the middle of the cylinder tube of the hydraulic cylinder, as shown in Figure 2-7.

2-7 High pressure long hydraulic cylinder
  • Calculation of minimum guide length of hydraulic cylinder
  • When the piston rod is fully extended, the distance from the midpoint of the piston bearing surface to the midpoint of the sliding surface of the guide sleeve is called the minimum guide length H (Figure 2-8). For general hydraulic cylinders, the minimum guide length should meet the following formula:
  • HL /20+D/2
  • Where L is the maximum working stroke of the hydraulic cylinder, m;
  • D-Inner diameter of cylinder, m.
  • The length A of the sliding surface of the general guide sleeve, when D<80mm, take A=(0.6~1.0)D; when D>80mm, take A=(0.6~1.0)d.
  • The width B of the piston, take B=(0.6~1.0)D. To ensure the minimum guide length, it is not appropriate to increase A and B excessively. It is best to install a spacer K between the guide sleeve and the piston. The spacer width C is determined by the required minimum guide length, namely:
  • The use of spacers can not only ensure the minimum guide length but also improve the versatility of the guide sleeve and the piston.

2-8 Guide length of hydraulic cylinder

Pay attention to the problem

  • The guide length of the piston rod should not be too small. If the guide length is too small, the initial deflection (deflection caused by the clearance) of the hydraulic cylinder will increase, which will affect the stability of the hydraulic cylinder. Therefore, a minimum guide length must be ensured in the design. The length A of the guide sleeve generally varies with the size of the hydraulic cylinder and the type and purpose of the piston rod seal, but the general size should be more than 0.6 times the diameter of the piston rod to ensure sufficient stability of the piston rod, as shown in Figure 2- 9 shown.
  •  The guide sleeve of the high-speed and long-stroke hydraulic cylinder should adopt a special structure. For hydraulic cylinders with a speed greater than 1000mm/s and a stroke greater than 4000mm, local overheating caused by high-speed action will cause significant wear of the guide sleeve and the appearance of metal powder. This not only requires forced lubrication on the surface of the guide sleeve from the structural point of view but also requires special treatment such as high-frequency quenching on the surface of the piston rod. At the same time, hydrostatic bearings may also be considered.

2-9 Guide length of hydraulic rod

Question 6: Determining the tolerance of cylinder shape and position

  • The cylinder must ensure the necessary shape and position tolerances. To ensure that the hydraulic cylinder has a lower starting pressure and does not occur “unusual” during movement, sufficient attention should be paid to the shape and position tolerance of the cylinder.
  • Under normal circumstances, the roundness, and cylindricity error of the cylinder diameter cannot be greater than half of the cylinder diameter tolerance.
  • The circular runout of the cylinder end facing the cylinder axis is not more than 0.04mm per 100mm.
  • The position error of the earring hole of the earring type hydraulic cylinder to the axis of the cylinder barrel is not more than 0.03mm.
  • The axis position tolerance of the pin type hydraulic cylinder pin is not more than 0.1mm, and the perpendicularity error is not more than 0.1mm on the length of 100mm.
  • The straightness error of the cylinder axis is not more than 0.03mm per 500mm length.

Question 7: Calculation of piston parameters

Piston length calculation

  • Although the piston is not expected to bear radial force from the perspective of design thinking, due to the limitation of the actual structure, the piston of the hydraulic cylinder.
  • It is inevitable to bear a considerable part of the radial external force; for this reason, the length of the piston of the hydraulic cylinder must be appropriate, generally, 0.6 to 1.0 times the outer diameter of the piston, to increase the guiding effect of the piston and increase the supporting surface area, to reduce wear, The purpose of improving the service life of the hydraulic cylinder, as shown in Figure 2-10.

The shape and position tolerance of the piston is determined.

  • The quality of the hydraulic cylinder largely depends on the quality of the piston, so the machining of the piston should be required to have sufficient precision geometric tolerances. Generally speaking, the error of the outer diameter of the piston, the roundness of the inner hole, and the cylindricity cannot be greater than half of its dimensional tolerance; the tolerance of the concentricity of the outer diameter of the piston to the inner hole and the sealing groove should be within 0.02mm.

Determination of the tolerance of the piston rod shape and position

  • Generally, the straightness tolerance of the piston rod is required to be less than 0.02mm/100mm; the roundness and other geometric accuracy tolerances are not greater than the outside.
  • 1/2 of the diameter tolerance; the concentricity tolerance between the shaft diameter and the outer circle matching the inner hole of the piston is not more than 0.01~0.02mm; the piston is installed.
  • The perpendicularity tolerance between the shaft shoulder and the axis of the piston rod is not more than 0.04mm/100mm.

2-10 Plunger length calculation

Hydraulic Cylinder Structure Design

Question 1: Problem with the connection structure of the cylinder end 

1. Flange connection

Flange connection at the end of the cylinder, as shown in Figure 2-11. The cylinder block of Figure 2-11(a) and Figure 2-11(b) is a steel pipe and the end welding method; Figure 2-11(c) the cylinder body is a steel tube with thick end titanium; Figure 2-11(d) the cylinder body is forging or casting, this structure is the most widely used, and its advantage is that the structure is relatively simple; Processing; easy to load and unload; high strength, able to withstand high pressure.


Figure 2-11 Flange connection at the end of the cylinder

Pay attention to the problem

The weight is larger than the threaded connection but smaller than the tie rod connection; the outer diameter is larger.

2. External thread connection

The external thread connection of the cylinder end is shown in Figure 2-12.


Figure 2-12 External thread connection of cylinder end

3. Internal thread connection

The internal thread connection at the end of the cylinder is shown in Figure 2-13. The advantage is that the weight is small and the outer diameter is small.


Figure 2-13 Internal thread connection of cylinder end

Pay attention to the problem

  • Special tools should be used when the end structure is complicated. When screwing the end, it is possible to twist the sealing ring, as shown in Figure 2-13(a).
  • When the end cover and the cylinder barrel are connected by a thread (internal thread of the cylinder, external thread of the end cover), special care should be taken not to damage the sealing ring during the assembly process. This requires the diameter of the inner thread tip on the cylinder barrel to be larger than the outer diameter of the sealing ring placed on the end cover. Otherwise, the sealing ring will be damaged in the process of assembling the end cover, as shown in Figure 2-14(a).
  • Since the initial pressure point of the seal ring after entering the cylinder barrel cannot be grasped, a proper guiding transition zone should be machined at the end of the internal thread on the cylinder barrel to avoid damage to the seal ring during the assembly process, as shown in Figure 2-14(b) Shown.

Figure 2-14 The end cover and the cylinder barrel are connected by a thread 1-end cover; 2-sealing ring; 3-cylinder tube

4. External snap ring connection

The outer snap ring connection at the end of the cylinder, as shown in Figure 2-15, has the advantages of smaller weight than tie rod connection, compact structure, and small size.


Figure 2-15 External snap ring connection at the cylinder end

Pay attention to the problem

The outer diameter of the cylinder body needs to be machined, and the half-ring groove weakens the cylinder body, and the cylinder body wall thickness must be thickened accordingly.

5. Internal snap ring connection

The snap ring connection at the end of the cylinder body, as shown in Figure 2-16, has the advantages of compact structure and lightweight.


Figure 2-16 The inner snap ring connection at the end of the cylinder block 1-spring ring; 2-shaft sleeve; 3-half ring

Pay attention to the problem

When installing, the end is deeper into the cylinder, and the sealing ring may be scratched by the edge of the oil inlet.

6. Tie rod connection

The tie rod connection at the end of the cylinder is shown in Figure 2-17. This mechanism is widely used. The advantages are that the cylinder body is easy to process, easy to install and unload, and the structure is versatile.


Figure 2-17 Connection of tie rods at the end of the cylinder

Pay attention to the problem

  • The weight is relatively large and the overall size is relatively large.
  • Tie rod structure should be avoided for long hydraulic cylinders. When the length of the hydraulic cylinder is greater than 1500~2000mm, the tie rod structures should not be adopted. Although the tie rod hydraulic cylinder has the characteristics of good manufacturability and maintenance performance, due to the action of hydraulic pressure, it is easy to elongate the tie rod and cause leakage, as shown in Figure 2-18.
  • Hydraulic cylinders with tie rod structures should not be used in harsh working environments. In the occasions where the use conditions are too bad and the seal ring and guide sleeve of the piston rod needs to be replaced frequently, and the equipment using the hydraulic cylinder does not allow the hydraulic cylinder to be removed, the hydraulic cylinder should not adopt a tie-rod structure. The reason is that when replacing the seal ring and guide sleeve of the piston rod of the tie rod hydraulic cylinder, the hydraulic cylinder must be removed first, and then the four tie rods can be removed before the replacement, which is very inconvenient. In this case, use The core barrel structure is better.

Figure 2-18 Long hydraulic cylinder should avoid the use of tie rod structure

7. Welding

The end of the cylinder is welded as shown in Figure 2-19. Its advantages are simple structure and small size.


Figure 2-19 Welding of the end of the cylinder

Pay attention to the problem

  • The cylinder body may be deformed; the inner diameter of the cylinder after welding is not suitable for reprocessing, and it is locally hardened.
  • The end welding seam should be at a certain distance from the working surface of the hydraulic cylinder. When using the hydraulic cylinder with end welding, the distance between the welding part and the working surface of the hydraulic cylinder should not be less than 20mm, as shown in Figures 2-20. This is because the inner surface of the hydraulic cylinder barrel after the end is welded, that is, the working surface is no longer processed after welding, and the welding process will cause the cylinder barrel to deform to a certain extent. If the distance is too close, the friction force of the hydraulic cylinder will increase or even get stuck when it moves near the end.

Figure 2-20 The distance between the end welding seam and the working surface of the hydraulic cylinder

8. Steel wire connection

The steel wire connection at the end of the cylinder body, as shown in Figure 2-21, has the advantages of a simple mechanism, lightweight and small size.


Figure 2-21 Wire connection at the end of the cylinder

Pay attention to the problem

  • It is inconvenient to assemble and disassemble, and the wire is difficult to assemble and disassemble.
  • When there are no special requirements on the size and weight of the fixed machinery, it is recommended to adopt the form of a flange or tie rod connection mechanism.
  • When there are special requirements for the size and weight of the movable machinery, it is recommended to adopt the form of a flange, external thread, or external snap ring connection mechanism.

Question 2: Cylinder body material selection

Cylinder body materials are commonly used seamless steel pipes of 20 steel, 35 steals, and 45 steals. The cylinder body of the hydraulic cylinder whose working temperature is lower than -50℃ must be made of 35 steel and 45 steal, and it must be quenched and tempered. The cylinder body welded to the end is made of 35 steel, which is mechanically pre-processed and then quenched and tempered. The cylinder body that is not welded to other parts uses quenched and tempered 45 steel. Cylinder body materials are also useful for forged steel, cast steel, and aluminum alloy.

Question 3: Technical requirements for cylinder block design

The technical conditions of cylinder design are shown in Figure 2-22
  • The cylinder inner diameter D adopts an H9 fit.
  • Heat treatment: quenching and tempering, hardness 241~285HB.
  • The conicity and ovality of diameter D are not more than half of the diameter tolerance.
  • The curvature of the axis is not more than 0.03mm in the length of 500mm.
  • The non-perpendicularity of the end face T is not more than 0.04mm on the diameter of 100mm.
  • When the cylinder body and the end are connected by thread, the thread adopts a 2a precision metric thread.
  • When the tail of the cylinder body is earring shape or the cylinder body is shaft pin type:
  • The deviation of the axis of the hole d1 from the cylinder diameter D is not more than 0.03mm;
  • The non-perpendicularity of the axis of the hole d1 to the cylinder diameter D is not more than 0.1mm in the length of 100mm;
  • The disjoint degree of the axis of the shaft diameter d: to the cylinder diameter D is not more than 0.1mm;
  • The non-perpendicularity of the shaft diameter d2 to the cylinder diameter D is not more than 0.1mm in the length of 100mm.

Figure 2-23 The end cap should not be too thin

Question 4: End cap design

  • The end cap should not be too thin. The end cover of the hydraulic cylinder bears the hydraulic pressure is relatively large. If the end cover is too thin and the bolt spacing is too large, local gaps will easily occur under the action of hydraulic pressure, which will cause oil leakage, as shown in Figures 2-23.
  • The geometric tolerance of the end face of the hydraulic cylinder. General requirements the perpendicularity tolerance of the end of the hydraulic cylinder facing the axis of the hydraulic cylinder should be less than 0.04mm/100mm.
  • When there is a severe impact during work, the cylinder tube and end cover of the hydraulic cylinder cannot be made of brittle materials, such as cast iron.

Question 5: The connection between the piston and the piston rod is formed

  • Under normal working conditions of the hydraulic cylinder, the piston, and the piston rod are connected by thread, as shown in Figure 2-24.
  • When the working pressure of the hydraulic cylinder is large and the working machinery is vibrating, the half-ring connection is adopted, as shown in Figure 2-25. According to the specific situation, the piston and the piston rod are also made into a whole.

Figure 2-24 Piston and piston rod are connected by a thread

Figure 2-25 Piston and piston rod are connected by a half-ring

Question 6: Selection of piston material

  • Wear-resistant cast iron; gray cast iron (HT150~HT200); steel (some wear-resistant rings with outer diameter sleeve nylon 66 or nylon 1010); aluminum alloy.
  • The friction pair in the hydraulic cylinder should avoid using the same material. The friction coefficient of the friction pair of the same material is relatively large, and all the two surfaces that need to be lubricated in relative motion should be avoided. The same is true for hydraulic cylinders, pistons, and cylinders Piston rod and guide sleeve Avoid using the same material between them to facilitate lubrication and reduce friction.

Question 7: Technical requirements for piston design

The non-perpendicularity of the end face shall not be greater than 0.04mm on the diameter of 100mm; the ovality and conicity of the outer diameter d shall not be greater than half of the diameter tolerance.

Question 8: Piston rod structure design

  • Avoid stress concentration at the transition of the piston rod shaft diameter 

When using a long-stroke hydraulic cylinder, it is necessary to comprehensively consider selecting a piston rod with sufficient rigidity and installing a spacer ring.

  • Consider the selection of piston rod and spacer

When using a long-stroke hydraulic cylinder, it is necessary to comprehensively consider selecting a piston rod with sufficient rigidity and installing a spacer ring.

  • Consider using a piston rod protective sleeve

When the working environment is seriously polluted and there are more impurities such as dust, sand, moisture, etc., a piston rod protective sleeve is required.


Figure 2-26 The problem of the shaft diameter transition of the piston rod
forming01

Grace Guo

In the last ten years, Grace has sold a third of our company’s hydraulic presses. She knows hydraulic presses as well as our engineers, and our customers trust her as much as we trust our own machines. If you need hydraulic presses and related products, you won’t be disappointed with Grace’s recommendations.

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COMMENT

This Post Has 2 Comments

  1. Abed

    This paper is really good, I will keep it, thanks.

  2. Jorge

    Do you have a hydraulic press machine?

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