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A bellows elastic element is a convoluted unit that expands and contracts axially with changes in pressure. The pressure to be measured can be applied to either the outside or the inside of the bellows; in practice, most bellows measuring

Figure 8-5.Helical Bourdon tube.

Figure 8-6.Simple bellows gauge.

devices have the pressure applied to the outside of the bellows (fig. 8-6).

Simple Bellows Elements

Bellows elastic elements are made of brass, phosphor bronze, stainless steel, beryllium-copper, or other metal suitable for the intended service of the gauge. Motion of the element (bellows) is transmitted by suitable linkage and gears to a dial pointer. Most bellows gauges are spring-loadedthat is, a spring opposes the bellows and thus prevents full expansion of the bellows. Limiting the expansion of the bellows in this way protects the bellows and prolongs its life. Because of the elasticity in both the bellows and the spring in a spring-loaded bellows element, the relationship between the applied pressure and bellows movement is linear.

Dual Bellows Indicators

Another type of bellows element is the dual-bellows element. Figure 8-7 is a schematic diagram of this indicator. Dual-bellows element pressure indicators are used throughout the Navy as flow-measuring, level-indicating, or pressure-indicating devices.

Figure 8-7.Differential pressure sensor dual bellows.

Figure 8-8.Dual bellows assembly.

When in operation, the bellows will move in proportion to the difference in pressure applied across the bellows unit assembly. The linear motion of the bellows is picked up by a drive arm and transmitted as a rotary motion through a torque tube assembly (fig. 8-8). The indicating mechanism multiplies rotation of the torque tube through a gear and pinion to the indicating pointer.

Bellows elements are used in various applications where the pressure-sensitive device must be powerful enough to operate not only the indicating pointer but also some type of recording device.


Often when a measured pressure reaches a certain maximum or minimum value, it is desirable to have an alarm sound a warning, a light to give a signal, or an auxiliary control system to energize or de-energize. A pressure switch is the device commonly used for this purpose. One of the simplest pressure switches is the single-pole, single-throw, quick-acting type shown in figure 8-9. This switch is contained in a metal

Figure 8-9.Typical pressure switch.

case that has a removable cover, an electrical connection, and a pressure-sensing connection. The switch contains a seamless metallic bellows located in its housing. Changes in the measured pressure causes the bellows to work against an adjustable spring. This spring determines the pressure required to actuate the switch. Through suitable linkage, the spring causes the contacts to open or close the electrical circuit automatically when the operating pressure falls below or rises above a specified value. A permanent magnet in the switch mechanism provides a positive snap on both the opening and closing of the contacts. The switch is constantly energized. However, it is the closing of the contacts that energizes the entire electrical circuit.

Another pressure switch is an electric-hydraulic assembly that is used for shutting off the pumps motor whenever the system pressure exceeds a pre-determined maximum value (fig. 8-10). The switch is mounted on the pump housing so that the formers low pressure ports drain directly into the pump housing. This pressure switch principally consists of a flange-mounted hydraulic valve to which is fixed a normally closed electrical limit switch. The valve consists of two hydraulically interconnected components, the pilot valve sub-assembly, which bolts on the bottom of the body (l), functions to sense system pressure continuously and initiates pressure switch action whenever this pressure exceeds the adjusted setting of the pilot adjustment. System pressure is directed into the bottom port and is applied against the exposed tip of the pilot piston (5). This piston is held on its seat by compression from the piston spring (6) which is dependent on the position of the adjusting screw (8). Whenever the pressure causes a force sufficiently large enough to raise the pilot piston from its seat, fluid flows through an interconnecting passage to the actuating piston (2) chamber. The accompanying fluid force raises the actuating piston against the force of spring 3 and causes depression of the extended switch plunger. This, in turn, disconnects the contained electrical switch, which may be connected into the pump motors electric supply system.

Pressure switches come in many sizes and configurations depending on how they will be used.

Figure 8-10.Electric-hydraulic pressure switch.

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