This is a good tutorial for people who would like to know more about hydraulics but have not been able to find a good resource. If you would like to know an excellent resource for information on hydraulics then purchase the Fluid Power Designer Lightning Reference Handbook by Paul Munroe Hydraulics, it is highly recommended.
All hydraulic systems must have some form of a reservoir to hold the fluid in the system. Most systems have vented tanks, however aircraft are one application where a closed tank is appropriate. The symbol shown here is a vented tank, a box with the line in the center would indicate a closed system. The line could also not go to the bottom of the tank, that would mean that the line stops above the fluid level in the tank and the fluid falls in. It is better to stop the line below the fluid level, otherwise the falling fluid may cause bubbles in the fluid.
A pump displaces fluid which creates flow. There are fixed displacement pumps and variable displacement pumps. The pump symbol is very similar to a hydraulic motor symbol, the difference is that the pump has the small triangle pointing out and a motor has the small triangle pointing in to the center. An angled arrow typically indicates that a device is variable, thus this is a variable volume pump. Fixed displacement pumps provide the same output volume with the same input RPM. Variable displacement pumps can change the output volume while maintaining the same input RPM. Hydraulic pumps are precision components and have very close tolerances, they must be treated with care.
Hydraulic lines carry the fluid from the pump throughout the system. There are two basic types, rigid and flexible. Rigid lines are used to connect items that will not move in relation to each other. Manifolds connected with rigid lines are the most reliable transfer method. The dots at the end of the line show a connection point, if two lines cross and this dot isn't shown then the lines are not connected.
A flexible line is used to carry fluid to items that have a lot of vibration or movement in relation to each other. Some examples where flexible lines are used, the pump unit (vibration) or blades on a tractor, due to the movement.
Hydraulic fluid is virtually non compressible, if the fluid can't go anywhere the pump will stall, and damage to the pump and motor can result. All hydraulic systems must have a pressure relief valve in line with the pump. The pressure relief will drain into the tank. The dashed line indicates a pilot line, this is a small line that only flows enough fluid to control other valves. The pressure of this pilot line acts against the spring on the other side of this valve. When the pilot pressure exceeds the spring force then the valve spool shifts over and opens the valve, this allows flow to the tank. This causes a drop in the pressure on the pump side, which also reduces the pilot pressure. When the pilot pressure is less than the spring force the spring closes the valve. The relief valve in the position described above will control the maximum pressure in the hydraulic system.
A directional valve will control which device the fluid will flow to. These valves are the primary devices used to sequence the motion of equipment. There are many different types of directional control valves. The valve is generally specified by number of positions and number of ways (ports). The valve is made up of two parts, the body and the spool. When valves shift the spool is moved in relation to the body, this opens and closes passages that the fluid flows through. Remember that the valve actuator always pushes the spool, this will help you read the drawings. You read the operation of a valve in a circuit in the following manner. The box(s) with arrows in it show the flow of fluid when the valve is shifted. The box without arrows and/or away from the actuator shows the flow, if any, in the neutral position. This is also the box you use to count the number of ports the valve has.
This valve has two positions (2 boxes) and 2 ways (ports); thus 2 position, 2 way. It is shown with a manual actuator (on the right) and has a spring return to neutral. This valve is called normally closed because both ports are blocked when in neutral. It could be used on a safety device like a safety gate, if the gate isn't closed, actuating the valve, then the flow will be stopped, preventing movement of the connected device.
This valve has three positions (3 boxes) and 4 ways (ports); thus 3 position, 4 way. It is shown with a closed center, when the valve is neutral all ports are blocked. The small boxes on each end with diagonal lines through them, C1 and C2, are electrical coils, this is an electrically actuated valve. The port marked P is Pressure and the port marked T drains to tank. The ports marked A and B connect to an external device, like a cylinder. When C1 is energized the valve will shift, putting pressure to the B port and draining the A port to the tank. Likewise when C2 is energized the pressure port connects to the A port and the B port drains to the tank.
This valve has three positions (3 boxes) and 4 ways (ports); thus 3 position, 4 way. It also is electrically actuated. The jagged lines next to the coil indicates springs, when the coil is de-energized the opposite spring will force the spool back to the center position. This valve also drains to tank when in neutral, this is a standard valve on molding machines. They drain to tank when de-energized for safety.
A cylinder is one of the devices that creates movement. When pressure is applied to a port it causes that side of the cylinder to fill with fluid. If the fluid pressure and area of the cylinder are greater than the load that is attached then the load will move. Cylinders are generally specified by bore and stroke, they can also have options like cushions installed. Cushions slow down the cylinder at the end of the stroke to prevent slamming. If the pressure remains constant a larger diameter cylinder will provide more force because it has more surface area for the pressure to act on.
If you use formulas occasionally a handy trick is to set up a spreadsheet that has the formulas built in, then all you need to do is enter the numbers.
The outlet flow of a pump in GPM is:
Flow (GPM) = RPM * Pump Displacement(Cu.in./Rev)
Both the force and speed of a cylinder are dependent on knowing the area of the cylinder. Remember that the area on the rod end of a cylinder is different than that of the non rod end. You must subtract the area of the rod itself from the overall area of the cylinder. The same formula can be used to determine the area of the rod. As you will see when pressure is applied to the rod end of the cylinder (pump pressure and volume constant), it will move faster and have less force.
Area (Sq.In.) = Pi * Diameter^2
Area (rod end) = Area - area of the rod itself
A cylinder usually has two forces, the force when applying pressure to side with the rod, and the side without the rod. An exception can be a double ended cylinder, it has a rod end sticking out of each cylinder end. This can also be applied to pneumatics.
Force = Pressure (PSI) * Net Area (Sq. In.)
Note - Inaccuracies have been discovered in the following equation and are being evaluated.
The speed of a cylinder is dependent on the flow rate to the cylinder and the area of the cylinder. This formula assumes no loss of fluid over a relief valve.
Velocity (Ft./Sec.) = 231 * Flow Rate (GPM)
12 * 60 * Net Area (Sq.In.)
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