Copyright Goodheart-Willcox Co., Inc. 182 Fluid Power system to the exterior walls of the tank to promote rapid heat transfer. Fluid movement is slow in the reservoir when compared to the fl uid velocity through system compo- nents and lines. This reduction in the rate of movement allows some of the heavier particles being carried by the fl uid to settle to the bottom of the reservoir. This process helps clean the fl uid, but it should not be con- sidered a reliable fl uid fi ltering method. Larger, heavier metal or dirt particles will settle out, but many of the smaller particles will not. This is especially true when the fl uid has a high viscosity. Regular fi lters properly placed in the system provide much more reliable fl uid cleaning. Most reservoirs are vented to the atmosphere. Venting exposes the surface of the fl uid in the reservoir to normal atmospheric pressure. This is important to the operation of a hydraulic system as it allows air that has become trapped in the fl uid to escape. Typically, hydraulic fl uid contains small quantities of air when it is being used in a system. The air may actually be dissolved in the fl uid without a visible indication of its presence. This type of situation normally does not cause operational problems. However, when the fl uid begins to appear cloudy, it is an indication that large quantities of small air bubbles have become entrained in the fl uid. This can cause foaming, spongy system operation, and increased operating temperatures. A reservoir that is adequately sized and containing proper baffl es will allow entrained air to form larger bubbles. These larger bubbles rise to the surface of the liquid and escape to the atmosphere, thus reducing or eliminating the problems associated with entrained air. Water that may have become mixed with petro- leum-based hydraulic fl uids also has an opportunity to separate as the mixture passes through the reservoir. Water is heavier than the hydraulic fl uid. This allows water to settle to the bottom of the reservoir. It can then be drained off, if a suffi cient quantity exists. Separation of the two liquids is relatively slow, so it is important to have a reservoir of adequate size to allow the fl uid suf- fi cient time to separate as it moves through the tank. If the water and oil form an emulsion, this separation process is not possible. 8.1.2 Construction of a Reservoir The construction details of a reservoir may vary consid- erably, depending on the application of the hydraulic system. In most manufacturing situations, the reservoir is a tank located in close proximity to the system pump. It is easily recognized as a tank. In other situations, the cavities in structural members of a machine may serve as the reservoir. However, each design must provide fl uid storage and help condition system fl uid by assist- ing in the removal of heat and contaminants. 8.1 Reservoir The reservoir is the component that holds the system fl uid not currently in use in the system pump, control components, actuators, and lines. This reserve fl uid supply is necessary to make up fl uid lost in leakage during system operation. It is also needed to compen- sate for the varying volume between extension and retraction positions of cylinders. The availability of this reserve fl uid supply is basic to the operation of any hydraulic system, even though the reserve quanti- ties required will vary depending on circuit design and operation. 8.1.1 Functions of the Reservoir The primary role of the reservoir is to provide storage for the hydraulic fl uid required for system operation. However, a well-designed and well-constructed reser- voir provides a number of important additional func- tions, as listed below. See Figure 8-1. • Removal of heat. • Separation of solid particles. • Release of trapped air. • Separation of water. In even the least complex designs, the reservoir dissipates system heat to the atmosphere through radiation. This radiation primarily occurs through the reservoir walls. The amount of heat transferred may be increased by incorporating simple devices, such as cooling fi ns placed on reservoir walls. Baffl es in the interior of the reservoir may also be used to increase cooling by directing warm fl uid returning from the Remove dirt particles Remove excess heat Remove entrained air Store system fluid Goodheart-Willcox Publisher Figure 8-1. Besides storage, the reservoir helps cool and clean the hydraulic fluid. This helps to prolong the life of both the system fluid and components.