Hydraulic temperature switch

A temperature switch is an instrument that automatically senses a change in temperature and opens or closes an electrical switching element when a predetermined temperature level is reached. Figure 6.47 is an illustration of a common type of temperature switch which has an accuracy of ±1 F maximum.

temperature-switch

This temperature switch is provided with an adjustment screw at the top end in order to change the actuation point. In order to facilitate its mounting on the hydraulic system whose temperature is to be measured, the bottom end of the switch is provided with threads. As in the case of pressure switches, temperature switches can also be wired either normally open or normally closed.

Hydraulic sequencing valve application

Let us consider a hydraulic circuit in which two cylinders are used to execute two separate operations as shown in Figure 6.24.

sequencing-valve-application

Now, let us assume that cylinder A is required to extend completely before cylinder B extends. This can be accomplished by placing a sequencing valve just before cylinder B as shown. The pressure value of the valve is set to a predetermined value say 28 kg/cm2 (400 psi). This ensures that the operation involving cylinder B will occur after the operation involving cylinder A or in other words, cylinder B will not extend before a pressure of 28 kg/cm2 (400 psi) is reached on cylinder A.

Practical application of a pressure reducing valve in a hydraulic system

Let us consider a hydraulic circuit where one cylinder is required to apply a lesser force than the other as shown in Figure 6.20. Here cylinder B is required to apply a lesser force than cylinder A. This is accomplished as follows.

A pressure-reducing valve is placed just before cylinder B in the hydraulic circuit as shown. This arrangement allows flow to the cylinder, until the set pressure value on the valve is reached. At this point where the set pressure is reached, the valve shuts off, thereby preventing any further buildup of pressure. The fluid is bled to the tank through the drain valve passage resulting in the easing-off of the pressure, as a result of which the valve opens again. Finally a reduced modulated pressure equal to the valve results.

pressure-reducing-valve

Rotary four-way direction control valves

Although most direction control valves are of spool type design, other types are also used. One such design is the rotary four-way valve, which consists of a rotor closely fitted in the valve body.

The passages in the rotor connect or block-off the ports in the valve body to provide the four flow paths. The design shown above is a three-position valve in which the centered position has all the four ports blocked. Rotary valves are usually actuated either manually or mechanically.The operation of this valve is illustrated below (Figure 6.13).

rotary-control-valve

This design contains lapped metal-to-metal sealing surfaces which form a virtually leak proof seal. The gradual overlapping of the round flow passages produce a smooth shearing action which results in lesser load on the handle during operation and absence of sudden surges. Also there is no external leakage because of the presence of a static seal on the rotating shaft (non-reciprocating and non-pressurized). The high-pressure regions are confined to flow passages. This type of valve can take higher velocities and more flow than a spool valve of the same size.

These valves are available in a variety of three-way and four-way and two- and three position flow path configuration.

Electro-hydraulic stepping motors

An electro-hydraulic stepper motor (EHSM) is a device, which uses a small electrical stepper motor to control the huge power available from a hydraulic motor (Figure 4.10).

It consists of three components:
1. Electrical stepper motor
2. Hydraulic servo valve
3. Hydraulic motor.

These three independent components when integrated in a particular fashion provide a higher torque output, which is several hundred times greater than that of an electrical stepper motor.

The electric stepper motor undergoes a precise, fixed amount of rotation for each electrical pulse received. This motor is directly coupled to the rotary liner translator of the servo valve. The output torque of the electric motor must be capable of overcoming the flow forces in the servo valve. The flow forces in the servo valve are directly proportional to the rate of flow through the valve. The torque required to operate the rotary linear translator against this axial force is dependent on the flow gain in the servo valve.

The hydraulic motor is the most important component of the EHSM system. The performance characteristics of the hydraulic motor determine the performance of the EHSM. These are typically used for precision control of position and speed. These motors are available with displacements ranging from 0.4 cubic in. (6.5 cm3) to 7 cubic in. (roughly 115 cm3). Their horsepower capabilities range between 3.5 hp (2.6 kW) and 35 hp (26 kW). Typical applications include textile drives, paper mills, roll feeds, automatic storage systems, machine tools, conveyor drives, hoists and elevators.

electro-hydraulic-stepping-motor

Limited rotation hydraulic motor

A limited rotation hydraulic motor provides a rotary output motion over a finite angle. This device produces a high instantaneous torque in either direction and requires only a small amount of space and simple mountings.

Rotary motors consist of a chamber or chambers containing the working fluid and a movable surface against which the fluid acts. The movable surface is connected to an output shaft to produce the output motion.

Figure 4.1 shows a direct acting vane-type actuator. In this type, fluid under pressure is directed to one side of the moving vane, causing it to rotate. This type of motor provides about 280° rotation.

limited-rotation-hydraulic-motor

Rotary actuators are available with working pressures up to 350 kg/cm3 (4978 psi). They are typically foot mounted, flanged or end mounted. Most designs provide cushioning devices. In a double vane design similar to the one depicted in the figure above, the maximum angle of rotation is reduced to about 100°. However in this case, the torque-carrying capacity is twice that obtained by a single vane design.

Hydraulic fluid types

Fluids may be classified as follows:
• Ideal fluid
• Real fluid
• Newtonian fluid
• Non-Newtonian fluid.

Ideal fluid
An ideal fluid is one, which is incompressible and has no viscosity. Such a fluid is only imaginary, as all existing fluids possess some viscosity.

Real fluid
Simply speaking, a fluid which possesses viscosity is known as a real fluid. All fluids, in actual practice are real fluids.

Newtonian fluid
During the course of our discussion on viscosity, we have seen that shear stress is proportional to the velocity gradient i.e. ra dv/dy. A real fluid in which the shear stress is proportional to the velocity gradient is known as a Newtonian fluid.

Non-Newtonian fluid
A real fluid in which the shear stress is not proportional to the velocity gradient is known as a non-Newtonian fluid.

Orifice plate-type hydraulic flowmeter

Another method by which flow rate can be determined involves the use of an orifice plate-type flowmeter in which an orifice is installed in the pipeline as shown in Figure 2.14.

orifice-flow-meter

The figure also shows the presence of two pressure gages, one each on either side of the orifice. This arrangement enables us to determine the pressure drop (AP) across the orifice when the fluid flows through the pipe and given by AP = Pi- P2- The higher the flow rate, greater will be the pressure drop.

The actual flow rate can be determined by the following equation:

equ-2

Where
Q is the flow rate in m3/s
C is the flow coefficient (C = 0.80 for a sharp-edged orifice and 0.60 for a square-edged orifice)
A is the area of the orifice opening in m^
S is the specific gravity of the flowing fluid and
AP = P1-P2 is the pressure drop across the orifice in psi or kPa.

Rotameter hydraulic flow measurement

The Rotameter also known as variable area flowmeter is the most common among all flow measurement devices. Figure 2.12 shows the operation of a Rotameter. It basically consists of a tapered glass tube calibrated with a metering float that can move vertically up and down in the glass tube. Two stoppers one at the top and the other at the bottom of the tube prevent the float from leaving the glass tube. The fluid enters the tube through the inlet provided at the bottom. When no fluid is entering the tube, the float rests at the bottom of the tapered tube with one end of the float making contact with the lower stopper. The diameter of the float is selected in such a way that under conditions where there is no fluid entry into the tube, the float will block the small end of the tube completely.

rotameter

When the fluid starts entering the tube through the inlet provided at the bottom, it forces the float to move upwards. This upward movement of the float will continue, until an equihbrium position is reached at which point the weight of the float is balanced by the upward force exerted by the fluid on the float. Greater the flow rate, higher is the float rise in the tube. The graduated tube allows direct reading of the flow rate.

Types of fluid flow

Fluid flow can be classified as follows:
• Steady and unsteady flows
• Uniform and non-uniform flows
• Laminar and turbulent flows
• Rotational and non-rotational flows.

Steady flow
Fluid flow is said to be steady if at any point in the flowing fluid, important characteristics such as pressure, density, velocity, temperature, etc. that are used to describe the behavior of a fluid, do not change with time. In other words, the rate of flow through any crosssection of a pipe in a steady flow is constant.

Unsteady flow
Fluid flow is said to be unsteady if at any point in the flowing fluid any one or all the characteristics describing the behavior of a fluid such as pressure, density, velocity and temperature change with time. Unsteady flow is that type of flow, in which the fluid characteristics change with respect to time or in other words, the rate of flow through any cross-section of a pipe is not constant.

Uniform flow
Flow is said to be uniform, when the velocity of flow does not change either in magnitude or in direction at any point in a flowing fluid, for a given time. For example, the flow of liquids under pressure through long pipelines with a constant diameter is called uniform flow.

Non-uniform flow
Flow is said to be non-uniform, when there is a change in velocity of the flow at different points in a flowing fluid, for a given time. For example, the flow of liquids under pressure through long pipelines of varying diameter is referred to as non-uniform flow. All these type of flows can exist independently of each other. So there can be any of the four combinations of flows possible:
1. Steady uniform flow
2. Steady non-uniform flow
3. Unsteady uniform flow
4. Unsteady non-uniform flow.

Laminar flow
A flow is said to be laminar if the fluid particles move in layers such that one layer of the fluid slides smoothly over an adjacent layer. The viscosity property of the fluid plays a significant role in the development of a laminar flow. The flow pattern exhibited by a highly viscous fluid may in general be treated as laminar flow.

Turbulent flow
If the velocity of flow increases beyond a certain value, the flow becomes turbulent. The movement of fluid particles in a turbulent flow will be random. This mixing action of the colliding fluid particles generates turbulence, thereby resulting in more resistance to fluid flow and hence greater energy losses as compared to laminar flow.