Basic Hydraulics - Accessory Components
CHAPTER 10 - Accessory Components
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| Figure 10.1 - a piston and bladder type accumulator |
Accumulators
Accumulators are devices that store energy in the form of fluid under pressure. Because of their ability to store excess energy and release it when needed, accumulators are useful tools for improving hydraulic efficiency. Industrial hydraulic accumulators are typically classified as hydropneumatic. This type of accumulator applies a force to a liquid by using compressed gas. The two most common types of hydropneumatic accumulators are the bladder type accumulator and the piston accumulator. The name of each type indicates the device separating gas from liquid. A hydropneumatic accumulator has a fluid compartment and a gas compartment, with a gas type element such as a bladder separating the two. The bladder is charged through a gas valve at the top of the accumulator, while a poppet valve at the bottom prevents the bladder from extruding into the pressure line. The poppet valve is sized so that the maximum volume metric flow cannot be exceeded.
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| Figure 10.2 - a charged bladder type accumulator |
To operate, the bladder is pre-charged with nitrogen to a pressure specified by the manufacturer according to the operating conditions. When the system pressure exceeds the gas pre-charged pressure, the poppet valve opens and hydraulic fluid enters the accumulator. The changing gas volume in the bladder determines the useable volume or useful fluid capacity.
Accumulators store energy that can be used during power failure or when additional energy is needed. In certain situations additional flow may be needed. An accumulator can be used to supplement the flow rate of a pump.
Pressure Switches
There are two types of pressure switches: the bourdon tube switch and the piston switch, shown in Figure 10.3. This pressure switch consists of a micro switch, a spring, a mechanical stop, a push rod, and a piston actuator. External lights are often used to indicate that the switch has been activated.
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| Figure 10.3 - a piston type pressure switch |
When pressure builds in the system, it enters the device, applying force to the piston actuator. This energy is transferred to the mechanical stop, compressing the spring, and driving the push rod up until it activates the micro switch. Pressure switches are used to open or close an electrical circuit when a predetermined pressure has been reached.
Pressure Gauges
Bourdon tube pressure gauges measure the pressure in a system and display it on a calibrated dial. The units of calibration are displayed in psi, bar, and psia.
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| Figure 10.4 - a bourdon tube pressure gauge |
The bourdon tube is a coiled metal tube. It is connected to system pressure. Any increase in pressure within the system causes the tube to straighten out. The end of the tube is connected to a mechanical linkage, which turns a gear. This gear in turn meshes with another gear, driving the pointer needle.
Flow Meters
The purpose of a flow meter is to measure flow. It is usually not bi-directional and acts as a check valve blocking flow in the reverse direction. The main components consist of a metering cone and a magnetic piston, which is held in the no-flow position by a tempered spring. Fluid first enters the device, flowing around the metering cone, putting pressure on the magnetic piston and spring. As flow increases in the system, the magnetic piston begins to compress the spring, indicating the flow rate on the graduated scale.
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| Figure 10.5 - a standard flow meter |
Manifolds
As the number of connections in a hydraulic system increase, so does the possibility of leaky fittings. Hydraulic manifolds drastically reduce the number of external connections required.
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| Figure 10.6 - a manifold with valves stacked on top |
Manifolds used for modular valve stacking incorporate a common pressure and return port with individual A and B work ports for each valve station. At each station additional control valves may be added by sandwiching or stacking the valves vertically. This is accomplished without any external connections. Manifolds are specified according to system pressure, total flow, number of workstations, valve size or pattern.
SUMMARY
Accumulators are devices that store energy in the form of fluid under pressure.
Pressure switches are used to open or close an electrical circuit when a predetermined pressure has been reached.
Bourdon tube pressure gauges measure the pressure in a system and display it on a calibrated dial.
Hydraulic manifolds drastically reduce the number of external connections required.
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