For safe and efficient operation, fluid power systems are designed to operate at a specific pressure and/or temperature, or within a pressure and/or temperature range. You have learned that the lubricating power of hydraulic fluids varies with temperature and that excessively high temperatures reduce the life of hydraulic fluids. Additionally, you have learned that the materials, dimensions, and method of fabrication of fluid power components limit the pressure and temperature at which a system operates. You have also learned of means of automatically controlling pressure in both hydraulic and pneumatic systems. Most fluid power systems are provided with pressure gauges and thermometers for measuring and indicating the pressure and/or the temperature in the system. Additionally, various temperature and pressure switches are used to warn of an adverse pressure or temperature condition. Some switches will even shut the system off when an adverse condition occurs. These devices will be discussed in this chapter.

Many pressure-measuring instruments are called gauges. However, this section will be restricted to two mechanical instruments that contain elastic elements that respond to pressures found in fluid power systemsthe Bourdon-tube and bellows gauges.

The majority of pressure gauges in use have a Bourdon-tube as a measuring element. (The gauge is named for its inventor, Eugene Bourdon, a French engineer.) The Bourdon tube is a device that senses pressure and converts the pressure to displacement. Since the Bourdon-tube displacement is a function of the pressure applied, it may be mechanically amplified and indicated by a pointer. Thus, the pointer position indirectly indicates pressure.

The Bourdon-tube gauge is available in various tube shapes: curved or C-shaped, helical, and spiral. The size, shape, and material of the tube depend on the pressure range and the type of gauge desired. Low-pressure Bourdon tubes (pressures up to 2000 psi) are often made of phosphor bronze. High-pressure Bourdon tubes (pressures above 2000 psi) are made of stainless steel or other high-strength materials. High-pressure Bourdon tubes tend to have more circular cross sections than their lower-range counterparts, which tend to have oval cross sections. The Bourdon tube most commonly used is the C-shaped metal tube that is sealed at one end and open at the other (fig. 8-1).

The C-shaped Bourdon tube has a hollow, elliptical cross section. It is closed at one end and is connected to the fluid pressure at the other end. When pressure is applied, its cross section becomes more circular, causing the tube to straighten out, like a garden hose when the water is first turned on, until the force of the fluid pressure is balanced by the elastic resistance of the tube material. Since the open end of the tube is anchored in a fixed position, changes in pressure move the closed end. A pointer is attached to the closed end of the tube through a linkage arm and a gear and pinion assembly, which rotates the pointer around a graduated scale.

Bourdon-tube pressure gauges are often classified as simplex or duplex, depending upon whether they measure one pressure or two pressures. A simplex gauge has only one Bourdon tube and measures only one pressure. The pressure gauge shown in figure 8-1 is a simplex gauge. A red hand is available on some gauges. This hand is manually positioned at the maximum operating pressure of the system or portion of the system in which the gauge is installed.

When two Bourdon tubes are mounted in a single case, with each mechanism acting independently but with the two pointers mounted on a common dial, the assembly is called a duplex gauge. Figure 8-2 shows a duplex gauge with views of the dial and the operating mechanism. Note that each Bourdon tube has its own pressure connection and its own pointer. Duplex gauges are used to give a simultaneous indication of the pressure from two different locations. For example, it may be used to measure the inlet and outlet pressures of a strainer to obtain the differential pressure across it.

Differential pressure may also be measured with Bourdon-tube gauges. One kind of Bourdon-tube differential pressure gauge is shown in figure 8-3. This gauge has two Bourdon tubes but only one pointer. The Bourdon tubes are connected in such a way that they indicate the pressure difference, rather than either of two actual pressures.

As mentioned earlier, Bourdon-tube pressure gauges are used in many hydraulic systems. In this application they are usually referred to as hydraulic gauges. Bourdon-tube hydraulic gauges are not particularly different from other types of Bourdon-tube gauges in how they operate; however, they do sometimes have special design features because of the extremely high system pressures to which they may be exposed. For

example, some hydraulic gauges have a special type of spring-loaded linkage that is capable of taking overpressure and underpressure without damage to the movement and that keeps the pointer from slamming back to zero when the pressure is suddenly changed. A hydraulic gauge that does not have such a device must be protected by a suitable check valve. Some hydraulic gauges may also have special dials that indicate both the pressure (in psi) and the corresponding total force being applied, for example tons of force produced by a hydraulic press.

Spiral and helical Bourdon tubes (figs. 8-4 and 8-5) are made from tubing with a flattened cross

section. Both were designed to provide more travel of the tube tip, primarily for moving the recording pen of pressure recorders.