Mechanical power is transmitted through the reciprocating and rotating parts such as shaft, gear, sprocket, couplings, motor, etc. whereas the fluid power is transmitted by the pressurized fluid which can be the air, gas, or oil. In general, systems with air as a medium are known as Pneumatic system,s and with oil are known as Hydraulic systems.
Hydraulic or Pneumatic systems have many components like pumps, compressors, hoses, valves, and cylinders. In this article, we will specifically talk about the Cylinder’s sealing system. The design process of the cylinder includes the side wall & back wall thickness calculations, selecting the manufacturing process, designing the sealing system, selecting the right tolerances, surface finish, and other aspects. However, we will discuss more on the design guidelines for designing the Sealing System of cylinders.
As the fluid power transmission is based on the pressure exerted by the fluid (mainly oil) on the piston, so there should be no drop in pressure due to leakage of fluid (here the leakage may be within the cylinder as well as outside the cylinder). For example, high-pressure fluid (air or oil) may leak from the ends of the cylinder which is an external leakage that will result in the pressure drop). Also, fluid may leak from clearance (small gap) between the cylinder bore and piston which is internal leakage and that will result in the pressure drop. Due to both of these types of leakages, the hydraulic system may not work at full efficiency or exert the desired force. As the external leakage happens from the stationary parts of the cylinder, so the seals used to prevent that leak are Static Seals such as O-ring. Whereas, internal leakage happens from the moving parts like pistons so the seals used to prevent that leakage are Dynamic Seals such as piston seals and rod seals.
Classification of Cylinder Based on Pressure and Application conditions
Light duty – operating pressure up to 160 bar, temperature up to 70 °C with moderate fluctuation in temperatures, rare pressure peak, rare eccentric or side load, less or almost no contamination
Application examples of light-duty hydraulic cylinders – Indoor lifting machines, clamping devices, small capacity hydraulic press in shop floor or similar stationary machines
Medium Duty – operating pressure up to 350 bar, temperature up to 90 °C with some fluctuation in temperatures, moderate pressure peaks, moderate eccentric or side load, presence of external contamination or moisture
Application examples of medium-duty hydraulic cylinders – cylinders of agriculture equipment.
Heavy Duty Cylinders – operating pressure of 400 bars, temperature up to 90 °C or occasionally going up to 110 °C with temperature variations, frequent pressure peaks due to the sudden load on the system, Heavy side loads with harsh contamination and moisture conditions
Application Examples of Heavy-duty cylinders – Off-highway and military equipment
Sealing Arrangement, Understanding the types & functioning mechanism of seals
As we know the purpose of a hydraulic cylinder is to exert force in the form of Push and/or pull. So now there are the following key conditions for its design.
- 1. The cylinder should be able to exert the desired force at a certain pressure
2. Calculating cylinder wall thickness based on cylindrical stress, end cover thickness, and other elements
3. For maximum efficiency of a hydraulic system, there should be no external or internal leakage
4. The Piston of the cylinder should be able to take the eccentric or side loading (if any), which means the piston should be properly guided inside the cylinder so that I do not collide with the cylinder walls inside surface
5. Keep the contamination like dust or other particles out from entering the cylinder
Required actions for designing:
For condition 1 – Calculating the cylinder and piston diameter for the required force
For condition 2 – this topic will not cover cylinder stress calculation, refer article https://theengineeringblog.com/calculator-for-stress-in-cyclinder/
For condition 3 – the designer has to design a proper sealing system – Piston seal, Road Seal, and O-ring
For Condition 4 – design the proper piston guiding system – Guide Rings
For condition 4 – place a wiper seal to prevent the contamination from going inside
Calculation of Cylinder and Piston Diameter
First, we will calculate the cylinder diameter based on inputs such as pressure, Pushing and pulling force required, and space constraints. Refer to the online calculator program below for the same.
Selection of Seals
Piston Seal – this is one of the most important parts of a hydraulic cylinder, piston seal helps to create sealing between the cylinder bore and the piston, thus preventing internal leakage or pressure drop. A piston seal is installed on the piston groove and the sealing surface contacts with the cylinder bore.
Rod Seal – The Rod seal is fitted into the grooves of the cylinder or its gland. It prevents the leakage from cylinder (bore) surface. As the Rod seals maintain sealing contact in sliding motion between the cylinder head (or gland) and the piston rod it also helps the formation of lubrication film on the piston rod- which is also important to prevent corrosion. Rod sealing systems for heavy-duty applications such as Pressure 400 bar typically consist of a combination of rod seal and buffer seal. Buffer seal protects from sudden pressure peaks in the system.
Wiper Seal – As the name suggests, the wiper seal “wipes out” unwanted material that tries to enter the cylinder such as dust particles, moisture, and other contaminations. For example, agricultural equipment running in the field is exposed to heavy mud or dirt. Wiper seal plays a critical role in maintaining the cleanliness of the oil. Similarly, the hydraulic shock absorber of an automobile, which contains a hydraulic cylinder for damping the vibrations, is exposed to external contaminations
Guide Rings- They accommodate radial loads of forces acting on the cylinder assembly and guide the rod in the cylinder head as well as the piston in the cylinder bore. Guides are made of polymer materials and prevent metal-to-metal contact between moving parts in a working hydraulic cylinder.
