Hydraulics vs. pneumatics
Pressurized fluids act in a certain manner in most situations. However, there are instances where a gas-type fluid does not perform as its liquid counterpart does. As mentioned earlier in this chapter, a pneumatic actuator is incapable of holding a position against increasing external forces because the air can be compressed more. Other situations such as flow-control circuits, return-line backpressure, energy-transfer considerations, and more are covered and explained in the text.
Conventions used in this manual
All schematic symbols and drawings are in accordance with the International Standards Organization (ISO) format. These symbols and representative parts are laid out in Chapter 4 either in whole or in part. Some symbols are made up of several standard parts and are not shown in their entirety in Chapter 4.
When a symbol is not shown it is good practice to use the symbol shown in the suppliers catalog. If no symbol is given there then use standard symbol parts to make a representation of the new item.
As in all cases of drawings using schematic symbols, the circuit designer may use his or her experience or opinion to interpret some parts. This usually does not make the schematic harder to read, just different. If a part representation is not clear, refer to the material list and check the supplier's catalog for an explanation of the valve's function.
To better understand how a part or circuit works, consider using color coding for the lines and components. Color coding is instituted by the instructor, designer, or engineer and is according to his or her interpretation, so it might not be consistent in each case. Most training manuals and manufacturers use the following color code.
- Red: Working fluid flow lines, usually from the pump to a device. This line is always solid. It can represent plastic tubing as small as 5/32-in. OD for air or any size pipe or tubing for hydraulics.
- Blue: Return lines from valves and other devices for hydraulic circuits. This line always is solid, and can represent any size pipe or tubing.
- Yellow: Metered or flow-controlled fluid that is at a reduced speed in relation to the same line without a restriction. This line could be solid or a series of long dashes if pilot flow must be metered.
- Orange: A reduced-pressure line, such as a pilot-pressure line or one carrying accumulator precharge gas. This line could be a solid after a reducing valve or a long-dashed line for pilot flow.
- Green: Pump inlet lines (suction lines) or drain lines. These lines would be solid for the pump inlet and a series of short dashes for drains. Two types of lines with the same color are not confusing -- even when in close proximity to each other.
- Purple or indigo: These colors usually indicate working fluid that has been pressure-intensified by area differences or load-induced conditions. These pressures are usually greater than the setting of the main relief valve or reducing valve that feeds the circuit.
- Lines without color are considered non-working or to have no flow at present.