Machinery's Handbook, 31st Edition
2794 SHOCK ABSORBERS AND HYDRAULIC DAMPERS Gas springs can be either push or pull (tension or traction) type, depending on where the gas chamber is relative to the piston rod. Gas springs usually have a reservoir of oil near the rod outlet of the cylinder that is used to lubricate the rod seal. They are designed to be stored and installed with the rod pointing down, at an angle of 60° or less from vertical to ensure that the seal is lubricated. The oil also serves to dampen the end of stroke. When they cannot be installed rod down, special gas springs with provisions for lubrication are required. Due to their long, narrow form factor, side loading should be avoided. Sizing gas springs is done with consideration of the required stroke, compressed length, diameter, compressed force, and extended force. Extended force is normally given at a point short of full extension because they should never be fully extended in operation to minimize the chances of buckling. Hysteresis is the difference between the actual and rated output force of a spring caused by friction. K factor, or progression, is the spring rate of a gas spring, and is calculated based on the following equation: K = --- where F 2 is force at the compressed position, and F 1 is the uncompressed force. Progression occurs because as the rod enters the chamber during compression, it re- duces the available volume for the gas to occupy. This causes a pressure increase. Hydraulic Dampers (Hydraulic Shock Absorbers): These devices are used to safely control the speed of applied loads. Dampers consist of a piston in a pressure tube con- taining a hydraulic fluid. When force is applied to the piston rod to move the piston, the damping fluid must pass through orifices in the piston. As a result, the damping force is dependent on the piston speed. As the fluid is forced through the orifices, kinetic energy is converted into thermal energy. Using valving on the orifices, it is possible to regulate the damping forces in each direction independently. Hydraulic dampers are usually self- contained, sealed units. They can be single or double acting. In some designs, the damping speed is adjustable over a range by turning the rod while fully extended. Hydraulic damp- ers are sized according to diameter, stroke length, and required damping characteristics. When extreme speeds and forces are required, cavitation/foaming of the oil can introduce gas bubbles into the chamber and cause a drop in damping capacity. Hydraulics F 2 F 1 Hydraulic Fluids.— Hydraulic systems use liquids as their medium. Water is a common hydraulic medium in general cases. When actuators are involved, petroleum based oils are the most common type of hydraulic fluid, but silicones, synthetics, and water based fluids are also common. Hydraulic fluids can be broken down in these major categories: hydraulic fluids based on mineral oils and related hydrocarbons (ISO 11158), environ - mentally acceptable hydraulic fluids (ISO 15380), fire- resistant, water-free hydraulic flu - ids (ISO 12922), and fire-resistant, water-containing hydraulic fluids (ISO 12922). When selecting a hydraulic fluid, one must consider viscosity, viscosity index, lubricity, compressibility, flammability, foaming resistance, resistance to rust/oxidation, resistance to bacterial growth, pour point, flash point, chemical compatibility with system seals/ components, cleanliness, cost, and environmental impact. The manufacturer of the hy- draulic components should be consulted as well. Viscosity at the required temperatures is the most important selection factor. Many petroleum products are graded according to the ISO Viscosity Classification System, approved by the international standards organization (ISO). Each ISO viscosity grade number corresponds to the mid-point of a viscosity range expressed in centistokes (cSt) at 40°C. A chart of viscosity related to the various systems of grading fluids is given in Fig. 7. Fluid Conditioning and Stabilization.— To provide useful work, fluid must have energy added in the form of pressure. Pumps are usually used to pressurize fluid, unless gravity can perform the needed energy. Strainers and filters are needed to keep contaminants out
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