12-08-2013, 04:59 PM
DESIGN AND MANUFACTURING OF HYDRAULIC CYLINDERS
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Design of a Hydraulic Cylinder
Importance of safe design
Hydraulic cylinder is actuated by hydraulic fluid at very high pressure. Any failure of cylinder may cause an explosion or ejection of a high velocity jet of oil, which may cause extensive damage to property and fatal injury to human being. Hence most precaution should be taken while designing a hydraulic cylinder.
Design of Cylinder Tube
In designing cylinder tube, we basically calculate it's inside diameter and wall thickness. Inside diameter and wall thickness depends upon capacity of cylinder and working pressure. In following paragraphs we will study how to determine these parameters.
Capacity of Hydraulic Cylinder
Capacity of a hydraulic cylinder is decided by the purpose for which cylinder is going to be used. Hence before designing a cylinder it's capacity is determined, considering the operation it has to perform.
Working Pressure
Working pressure of a system can vary from minimum possible to 700 Bar or more. It is selected on two factors:
1. Size Constraint of cylinder:
Some presses have larger press tables mounted directly on piston rod. To support press-table firmly piston-rod should be of sufficiently large diameter. For large piston-rod diameter, cylinder Inside Diameter should be more. Keeping same pressing capacity if Inside Diameter of cylinder has to be increased then pressure has to be reduced. So to support large size of table, working pressure of system reduced.
Similarly in most of the cases the space and size restriction in equipment where cylinder has to be fitted, influence the selection of working pressure
2. Type of valves and pump to be used in system.
If there is no restriction on size of cylinder, then pressure is decided by type of hydraulic valves and pump to be used in system. Type of valve and pump to be used is decided by:
a) Monetary budget.
b) Automation of system.
When budget is too low for manufacturing a hydraulic system, then gear or vane pump is used. These pumps give optimum performance and continue pressure below 175 Bar. Hence for such pumps, system pressure is kept 175 bar or less than that. For common budget when piston pump is to be used then pressure could be kept up to 350Bar.
When system is to be used along with control panel. Then solenoid operated direction control valves have to be used. Such valves are designed for 315 Bar to 350 Bar. But they give optimum performance below 300 Bar. Hence when solenoid operated direction control valve is to be used then pressure is selected bellow 300 Bar.
First we will discuss various theories and formulas for theoretically calculating the required dimension of cylinder tube. Then we will discuss the practical part of it. That is how to manipulate the theoretically calculated size so that practically it is possible, feasible and economical to manufacture the hydraulic cylinder tube.
Theoretical Design of Main Shell OR Cylinder Tube
Cylinder tubes are divided in to two categories:
1) Thin Cylinder Tube
2) Thick Cylinder Tube
When ratio of cylinder bore to wall thickness is more than 10 then it is called thin cylinder. And when it is equal to or less than 10 then it is called thick cylinder. When carrying out calculations concerning the design of hydraulic press, one important assumption that needs to be made is that the cylinder is a thick type owing to high pressures involved.
Design Of Thick Cylinder
Equations to calculate wall thickness, and assumption made are different. Maximum radial stresses are generally equal to the internal pressure, and it is maximum at inner surface of cylinder.
σ r (max) = -p
For calculating tangential stress following four equations are used:
1) Lame's equation
Lame's equation is used for designing cylinder of brittle material and it depends on maximum-stress theory of failure, and could be used for open as well as closed cylinder.
Introduction
These notes relate to the stresses and strains existing in thick walled cylinders when subjected to internal and external pressures. The notes include the analysis of two or more cylindrical parts, assembled by press fitting or shrinking, resulting in an interference fit between the parts. The equations resulting enable estimates of the forces need to assemble and separate the parts and the maximum torque which can be transmitted by the assembly