Design and calculation of hydraulic cylinder

As the executive component of the hydraulic system, the hydraulic cylinder converts the hydraulic energy into mechanical energy to drive the working mechanism of the main engine to do work. Due to the great differences in the use occasions and conditions of hydraulic cylinders, in addition to the selection from the existing standard product series, they often need to be designed according to the specific use occasions. 3.1 design content The design of hydraulic cylinder is a part of the whole hydraulic system design. It is usually carried out after the working condition analysis of the whole system. The design content is to determine the structural form, size, materials and relevant technical requirements of each component (cylinder barrel and cylinder head, piston and piston rod, sealing device, exhaust device, etc.), and reflect these contents through the drawn hydraulic cylinder assembly drawing and non-standard parts working drawing. 3.2 type and installation mode selection of hydraulic cylinder The input of hydraulic cylinder is liquid flow and pressure, and the output is force and linear speed. The hydraulic cylinder has simple structure and good working reliability. It is widely used in various departments of industrial production. In order to meet the various requirements of different types of machinery, hydraulic cylinders have many different types. Hydraulic cylinder can be widely divided into general structure and special structure. The general-purpose hydraulic cylinder has three typical structural forms: (1) Pull rod type hydraulic cylinder The front and rear end covers are connected with the cylinder barrel with four (square end cover) or six (circular end cover) pull rods, and the front and rear end covers are square, rectangular or circular. The cylinder barrel can be made of high-precision cold drawn pipe provided by the steel pipe factory and cut according to the corresponding size of the stroke length. Generally, the inner surface can meet the use requirements without machining (or only finishing). The main parts such as front and rear end covers and pistons are universal parts. Therefore, the pull rod hydraulic cylinder has the advantages of simple structure, simple disassembly, high universality of parts, low manufacturing cost and is suitable for mass production. However, it is limited by stroke length, cylinder bore diameter and rated pressure. If the stroke length is too long, the length of the pull rod will be too long, which is easy to skew during assembly, resulting in leakage at the end of the cylinder barrel; If the inner diameter of the cylinder barrel is too large and the rated pressure is too high, the large-diameter pull rod shall be selected due to the strength requirements of the pull rod material, but the radial size shall not allow the pull rod diameter to be too large. (2) Welded hydraulic cylinder The cylinder barrel and the rear end cover are connected by welding. The cylinder barrel and the front end cover are connected by internal thread, internal snap ring, external thread, external snap ring, flange, steel wire retaining ring and other forms. The welded hydraulic cylinder is characterized by small overall size and can withstand certain impact load and harsh external conditions. However, due to the restriction of the connection strength between the front end cover and the cylinder barrel, such as threads, snap rings or steel wire retaining rings, the inner diameter of the cylinder barrel shall not be too large and the rated pressure shall not be too high. Welded hydraulic cylinders usually have rated pressure and inner diameter of cylinder barrel, and the maximum stroke is allowed when the processing conditions of piston rod and cylinder barrel permit. (3) Flange type hydraulic cylinder The cylinder barrel is connected with the front and rear end covers by flange, and the flange and cylinder barrel are connected by integral, welding, thread and so on. Flange type hydraulic cylinder is characterized by high rated pressure, large inner diameter of cylinder barrel and large overall dimension. It is suitable for severe impact load and external working conditions, also known as heavy-duty hydraulic cylinder. Flange type hydraulic cylinder usually has rated pressure and inner diameter of cylinder barrel, and the maximum stroke is allowed when the processing conditions of piston rod and cylinder barrel permit. Therefore, the hydraulic cylinder of our designed hydraulic lifting platform truck should choose (2) welded hydraulic cylinder is more appropriate. Of course, the connection of cylinder barrel needs to be analyzed and determined according to specific conditions 3.3 motion and load analysis of hydraulic platform The movement mode of the hydraulic platform is mainly to realize the rise and fall of each hydraulic cylinder, but in the process of rise and fall, there will be a period of acceleration in the lifting stage. When it accelerates to the required speed, the plaAs the executive component of the hydraulic system, the hydraulic cylinder converts the hydraulic energy into mechanical energy to drive the working mechanism of the main engine to do work. Due to the great differences in the use occasions and conditions of hydraulic cylinders, in addition to the selection from the existing standard product series, they often need to be designed according to the specific use occasions. 3.1 design content The design of hydraulic cylinder is a part of the whole hydraulic system design. It is usually carried out after the working condition analysis of the whole system. The design content is to determine the structural form, size, materials and relevant technical requirements of each component (cylinder barrel and cylinder head, piston and piston rod, sealing device, exhaust device, etc.), and reflect these contents through the drawn hydraulic cylinder assembly drawing and non-standard parts working drawing. 3.2 type and installation mode selection of hydraulic cylinder The input of hydraulic cylinder is liquid flow and pressure, and the output is force and linear speed. The hydraulic cylinder has simple structure and good working reliability. It is widely used in various departments of industrial production. In order to meet the various requirements of different types of machinery, hydraulic cylinders have many different types. Hydraulic cylinder can be widely divided into general structure and special structure. The general-purpose hydraulic cylinder has three typical structural forms: (1) Pull rod type hydraulic cylinder The front and rear end covers are connected with the cylinder barrel with four (square end cover) or six (circular end cover) pull rods, and the front and rear end covers are square, rectangular or circular. The cylinder barrel can be made of high-precision cold drawn pipe provided by the steel pipe factory and cut according to the corresponding size of the stroke length. Generally, the inner surface can meet the use requirements without machining (or only finishing). The main parts such as front and rear end covers and pistons are universal parts. Therefore, the pull rod hydraulic cylinder has the advantages of simple structure, simple disassembly, high universality of parts, low manufacturing cost and is suitable for mass production. However, it is limited by stroke length, cylinder bore diameter and rated pressure. If the stroke length is too long, the length of the pull rod will be too long, which is easy to skew during assembly, resulting in leakage at the end of the cylinder barrel; If the inner diameter of the cylinder barrel is too large and the rated pressure is too high, the large-diameter pull rod shall be selected due to the strength requirements of the pull rod material, but the radial size shall not allow the pull rod diameter to be too large. (2) Welded hydraulic cylinder The cylinder barrel and the rear end cover are connected by welding. The cylinder barrel and the front end cover are connected by internal thread, internal snap ring, external thread, external snap ring, flange, steel wire retaining ring and other forms. The welded hydraulic cylinder is characterized by small overall size and can withstand certain impact load and harsh external conditions. However, due to the restriction of the connection strength between the front end cover and the cylinder barrel, such as threads, snap rings or steel wire retaining rings, the inner diameter of the cylinder barrel shall not be too large and the rated pressure shall not be too high. Welded hydraulic cylinders usually have rated pressure and inner diameter of cylinder barrel, and the maximum stroke is allowed when the processing conditions of piston rod and cylinder barrel permit. (3) Flange type hydraulic cylinder The cylinder barrel is connected with the front and rear end covers by flange, and the flange and cylinder barrel are connected by integral, welding, thread and so on. Flange type hydraulic cylinder is characterized by high rated pressure, large inner diameter of cylinder barrel and large overall dimension. It is suitable for severe impact load and external working conditions, also known as heavy-duty hydraulic cylinder. Flange type hydraulic cylinder usually has rated pressure and inner diameter of cylinder barrel, and the maximum stroke is allowed when the processing conditions of piston rod and cylinder barrel permit. Therefore, the hydraulic cylinder of our designed hydraulic lifting platform truck should choose (2) welded hydraulic cylinder is more appropriate. Of course, the connection of cylinder barrel needs to be analyzed and determined according to specific conditions 3.3 motion and load analysis of hydraulic platform The movement mode of the hydraulic platform is mainly to realize the rise and fall of each hydraulic cylinder, but in the process of rise and fall, there will be a period of acceleration in the lifting stage. When it accelerates to the required speed, the platform will realize a period of uniform motion, and then slow down and stop. In the whole movement process, the external load of the hydraulic lifting platform is mainly composed of the gravity of the vehicle and the self weight of the platform itself. Therefore, the size of external load is relatively stable. 3.4 design calculation and selection of hydraulic cylinder Vehicle quality Maximum lifting mass of single cylinder of hydraulic cylinder in four hydraulic columns Maximum lifting height of lifting platform: The rising speed is equal to the falling speed: Maximum load of hydraulic platform under rising condition Mechanical efficiency of hydraulic cylinder: The working pressure of hydraulic cylinder can be seen from table 3-1 Table 3-1 working pressure under different load conditions Load f / kn <5 5~10 10~20 20~30 30~50 >50 Working pressure P / MPa <0.8~1 1.5~2 2.5~3 3~4 4~5 ≥5 3.4.1 design and calculation of hydraulic cylinder barrel 1. Calculation of inner diameter D of hydraulic cylinder By formula （；；；） Yes. According to table 3-2, after rounding into standard value, the inner diameter of hydraulic cylinder is d = 80mm. Table 3-2 inner diameter dimension series of hydraulic cylinder mm (GB / T 2348-93) eight ten twelve sixteen twenty twenty-five thirty-two forty fifty sixty-three eighty （90） one hundred （110） one hundred and twenty-five （140） one hundred and sixty （180） two hundred （220） two hundred and fifty （280） three hundred and twenty （360） four hundred （450） five hundred 2. Calculation of cylinder wall thickness and outer diameter The inner diameter D of this design is 80mm. According to the hydraulic design manual, the outer diameter D1 of the hydraulic cylinder is 95mm and the thickness of the cylinder wall is 7.5mm. Generally, the wall thickness of the hydraulic cylinder selected according to the formal method can meet its strength, but we have to check it for safety. Since d = 80mm and outer diameter D1 = 95mm, it can be calculated according to the first strength theory, that is, according to the pitch diameter formula of thin-walled cylinder Where --- cylinder wall thickness; 　 ---Inner diameter of cylinder barrel; 　　 ---Cylinder test pressure, at rated pressure of hydraulic cylinder, at rated pressure; ---Allowable stress of material. Is the tensile strength of the material, n is the safety factor, taken here. No. 45 steel is selected and quenched and tempered. According to Liu Jingxiang in engineering mechanics, the tensile strength of No. 45 steel is taken, so: Due to the working pressure of the hydraulic cylinder, it is taken as therefore Because 7.5mm > 0.8mm, the strength is sufficient. 3.4.2 design and calculation of piston rod Piston rod is the main part of hydraulic cylinder courier power. It must have sufficient strength and stiffness to bear various effects such as tension, pressure, bending force, vibration and impact. 1. Calculation of piston rod diameter It is determined according to the stress condition of the piston rod. Generally, when it is under tension, d = 0.3 ~ 0.5d. Under pressure: When p < 5MPa, d = 0.5 ~ 0.55d When 5MPa ＜ P ＜ 7MPa, d = 0.6 ~ 0.7d P> D = 0.7d at 7MPa Because P = 1.5MPa, d = 0.066858mm, d = 0.036771mm According to the following table, the diameter of piston rod d = 40mm Table 3-3 diameter series of piston rod mm (GB / T 2348-93) four five six eight ten twelve fourteen sixteen eighteen twenty twenty-two twenty-five twenty-eight thirty-two thirty-six forty forty-five fifty fifty-six sixty-three seventy eighty ninety one hundred one hundred and ten one hundred and twenty-five one hundred and forty one hundred and sixty one hundred and eighty two hundred two hundred and twenty two hundred and fifty two hundred and eighty three hundred and twenty three hundred and sixty four hundred Strength check of piston rod (1) Check according to strength conditions By formula Where d --- diameter of piston rod; F --- force on piston rod; ---The allowable stress of piston rod material,, is the tensile strength of the material and is the safety factor, generally taken as. From the allowable stress of 45 steel, Therefore, the strength of the piston rod meets the requirements. Check according to bending stability When the piston rod is fully extended, the distance between the outer end of the piston rod and the support point of the hydraulic cylinder shall be checked for stability. According to the theory of material mechanics, when the axial load of a straight compression rod exceeds the critical compression load, it can lose the balance of the original linear state, which is called instability, and its stability condition is Where --- maximum thrust of hydraulic cylinder; ---Critical pressure load of hydraulic cylinder; ---Stability safety factor, generally taken as. The critical pressure load of hydraulic cylinder is related to the material, length, stiffness and support at both ends of piston rod and cylinder block. The relevant calculations are as follows: By formula Where --- calculated length of piston rod; ---The installation form coefficient of the end point is fixed at both ends, so; E --- modulus of elasticity of material, modulus of elasticity of steel; J --- moment of inertia of cross section of piston rod, of solid rod. And,, So, And (when 4 is taken), Therefore, the bending stability of piston rod meets the requirements. 3.4.3 determination of minimum guide length H When the piston rod is fully extended, the distance from the midpoint of the piston support surface to the midpoint of the sliding surface of the guide sleeve is called the minimum guide length. If the guide length is too short. It will increase the initial deflection of hydraulic cylinder caused by clearance and affect the working performance and stability of hydraulic cylinder. Therefore, the hydraulic cylinder must be designed to have a certain minimum guide length. For general hydraulic cylinders, the minimum guide length shall meet the requirements of the following formula: Where l - maximum working stroke; D --- inner diameter of cylinder barrel. Determination of working stroke of hydraulic cylinder: The maximum lifting height of the lifting hydraulic cylinder is 1.5m. The working stroke of the hydraulic cylinder selected according to table 3-4 is 800mm. Table 3-4 hydraulic cylinder piston stroke parameter series mm (GB / T 2348-80) twenty-five fifty eighty one hundred one hundred and twenty-five one hundred and sixty two hundred two hundred and fifty three hundred and twenty four hundred five hundred six hundred and thirty eight hundred one thousand one thousand two hundred and fifty one thousand and six hundred two thousand two thousand and five hundred three thousand and two hundred four thousand Therefore, l = 0.8m and d = 0.08m are substituted into the formula Get: Calculation of piston width B: Take. The length a of the sliding surface of the guide sleeve is determined by the formula It can be seen from the previous data, so it is taken Take. Length C of intermediate spacer K: From the formula, we get:.