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Tài liệu đào tạo DrawWorks Brake System Training Course (Part I)

Thảo luận trong 'Các vấn đề về thủy lực & ứng dụng' bắt đầu bởi forever_young, 26/1/10.

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    Sunday, May 24, 2009
    DW Brake System
    Training Course

    1- Course Objectives
    2- Introduction
    3- Hydraulic Pumps and Pressure Regulation
    4- Pneumatics
    5- Types of control valve
    a. Poppet valves
    b. Spool valves
    c. Pilot-operated valves
    d. Check valves
    e. Shuttle a valves
    f. Proportional valves
    6 - The DW Brake System construction
    7 - The Brake System construction
    8 - The Brake System Operation
    9 - The Brake System Maintenance and Troubleshooting

    1. Course Objectives
    To understand:
    • The operation of the valves, pumps and hydraulic components of the DW Brake System.
    • The Hydraulic concepts behind the DW Brake System.
    • The DW Brake System Construction
    • The DW Brake System Operation
    • The DW Brake System Maintenance and Troubleshooting
    • Complete one written test & achieve an overall pass mark of 80%

    2. Introduction
    Most industrial processes require objects or substances to be moved from one location to another or a force to be applied to hold, shape or compress a product. Such activities are performed by Prime Movers; the workhorses of manufacturing industries. In many locations all prime movers are electrical. Rotary motions can be provided by simple motors, and linear motion can be obtained from rotary motion by devices such as screw jacks or rack and pinions. Where a pure force or a short linear stroke is required a solenoid may be used (although there are limits to the force that can be obtained by this means).

    Electrical devices are not; however, the only means of providing prime movers. Enclosed fluids (both liquids and gases) can also be used to convey energy from one location to another and, consequently, to produce rotary or linear motion or apply a force. Fluid-based systems using liquids as transmission media are called hydraulic systems (from the Greek words hydra for water and aulos for a pipe; descriptions which imply fluids are water although oils are more commonly used). Gas-based systems are called Pneumatic systems (from the Greek pneumn for wind or breath). The most common gas is simply compressed air. although nitrogen is occasionally used.

    2.1. Hydraulic Fundamental Principles (Pascal`s law)

    Figure (1) Forces and pressure in closed tanks
    · Pressure in an enclosed fluid can be considered uniform throughout a practical system.
    · This equality of pressure is known as Pascal's law, and is illustrated in Figure (1) where a force of 5 kgf is applied to a piston of area 2 cm2

    . This produces a pressure of 2.5 kgf cm-2 at every point within the fluid, which acts with equal force per unit area on the walls of the system.

    Figure (2) Mechanical advantage

    · This expression shows an enclosed fluid may be used to magnify a force.
    · In Figure (2) a load of 2000 kg is sitting on a piston of area 500 cm2 (about 12 cm radius). The smaller piston has an area of 2 cm2.
    · An applied force f given by;

    will cause the 2000 kg load to rise.
    · This is called to be a mechanical advantage of 250. Energy must, however, be conserved.

    · To illustrate this, suppose the left hand piston moves down by 100 cm (one meter).
    · Because we have assumed the fluid is incompressible, a volume of liquid 200 cm2 is transferred from the left hand cylinder to the fight hand cylinder, causing the load to rise by just 0.4 cm.
    · So, although we have a force magnification of 250, we have a movement reduction of the same factor.
    · Because work is given by the product of force and the distance moved, the force is magnified and the distance moved reduced by the same factor, giving conservation of energy.

    3. Hydraulic Pumps and Pressure Regulation
    · A hydraulic pump (Fig. 3) takes oil from a tank and delivers it to the rest of the hydraulic circuit. In doing so it raises oil pressure to the required level. The operation of such a pump is illustrated in Figure 3.a.
    · On hydraulic circuit diagrams a pump is represented by the symbol of Figure 3.b, with the arrowhead showing the direction of flow.
    · Hydraulic pumps are generally driven at constant speed by a three phase AC induction motor rotating at 1500 rpm in the UK (with a 50 Hz supply) and at 1200 or 1800 rpm in the USA (with a 60 Hz supply).
    · Often pump and motor are supplied as one combined unit. As an AC motor requires some form of starter, the complete arrangement illustrated in Figure 3. c is needed.
    3.1. Pump Types
    There are two types of pump illustrated in Figure 4.

    1- Hydrodynamic Pump (Figure 4.a, )
    Fluid is drawn into the axis of the pump, and flung out to the periphery by centrifugal force. Flow of fluid into the load maintains pressure at the pump exit. Should the pump stop, however, there is a direct route from outlet back to inlet and the pressure rapidly decays away. Fluid leakage will also occur past

    Figure (3) The hydraulic pump

    Figure (4) Types of Hydraulic Pumps

    the vanes, so pump delivery will vary according to outlet pressure.
    Hydrodynamic pumps (Fig. 4.a), are primarily used to shift fluid from one location to another at relatively low pressures.

    2- Positive Displacement (hydrostatic Pump (Figure 4.b)
    As the piston is driven down, the inlet valve opens and a volume of fluid (determined by the cross section area of the piston and the length of stroke) is drawn into the cylinder.
    Next, the piston is driven up with the inlet valve closed and the outlet valve open, driving the same volume of fluid to the pump outlet.
    Should the pump stop, one of the two valves will always be closed, so there is no route for fluid to leak back. Exit pressure is therefore maintained (assuming there are no downstream return routes).
    More important, though, is the fact that the pump delivers a fixed volume of fluid from inlet to outlet each cycle regardless of pressure at the outlet port. Unlike the hydrodynamic pump described earlier, a piston pump has no inherent maximum pressure determined by pump leakage: if it drives into a dead end load with no return route (as can easily occur in an inactive hydraulic system with all valves closed) the pressure rises continuously with each pump stroke until either piping or the pump itself fails.

    3.2. Pump Power

    Fig. (5)Derivation of pump power

    The motor power required to drive a pump is determined by the pump capacity and working pressure.

    In Figure 5, a pump forces fluid along a pipe of area A against a pressure P, moving fluid a distance d in time T. The force is PA, which, when substituted into above Eq.


    Figure (6) Filter Position
    Dirt in a hydraulic system causes sticking valves, failure of seals and premature wear. Even particles of dirt as small as 20 microns can cause damage.
    Filters are used to prevent dirt entering the vulnerable parts of the system, and are generally specified in microns or meshes per linear inch (sieve number).
    See the three filter positions shown in Fig. 6

    4. Pneumatics
    4.1. Stages of air treatment
    Air in a pneumatic system must be clean and dry to reduce wear and extend maintenance periods. Atmospheric air contains many harmful impurities (smoke, dust, water vapour) and needs treatment before it can be used.
    In general, this treatment falls into three distinct stages, shown in Figure (7).
    First, inlet filtering removes particles which can damage the air compressor.
    Next, there is the need to dry the air to reduce humidity . This is normally performed between the compressor and the receiver and is termed primary air treatment.
    Finally; the treatment is performed local to the duties to be performed, and consists of further steps to remove moisture and dirt and the introduction of a fine oil mist to aid lubrication.

    Fig. 7 Three stages of air treatment
    4.2. Air Dryers

    Figure (8) Air filter and water trap
    Air flow through the unit undergoes a sudden reversal of direction and a deflector cone swirls the air (Figure 8-b). Both of these cause heavier water particles to be flung out to the walls of the separator and to collect in the trap bottom from where they can be drained.
    Water traps are usually represented on circuit diagrams by the symbol of Figure 8-c.

    Figure (9) Refrigerated Dryer
    Dew point can be lowered further with a refrigerated dryer, the layout of which is illustrated in Figure 9. This chills the air to just above 0~ condensing almost all the water out and collecting the condensate in the separator. Efficiency of the unit is improved with a second heat exchanger in which cold dry air leaving the dryer pre-chills incoming air. Air leaving the dryer has a dew point similar to the temperature in the main heat exchanger.

    5. Types of Control Valves
    Generally; the load is connected to ports labeled A, B and the pressure supply (from pump or compressor) to port P. In the hydraulic valve, fluid is returned to the tank from port T. In the pneumatic valve return air is vented from port R. See Figure 10.

    Figure (10) Valves in a pneumatic and hydraulic system
    Figure 11 shows internal operation of valves. To extend the ram, ports P and B are connected to deliver fluid and ports A and T connected to return fluid. To retract the ram, ports P and A are connected to deliver fluid and ports B and T to return fluid.

    Figure (11) Internal valve operation
    Another consideration is the number of control positions. Figure 12 shows two possible control schemes. In Figure 12-a, the ram is controlled by a lever with two positions; extend or retract. This valve has two control positions (and the ram simply drives to one end or other of its stroke).
    The valve in Figure 12-b has three positions; extend, off, retract. Not surprisingly the valve in Figure 12-a is called a two position valve, while that in Figure 12-b is a three position valve.

    Figure (12) Valve control positions
    A complete valve description needs;
    1- Number of Ports
    2- Number of positions and
    3- Action
    Figure 13 shows one possible action for the 4/3 valve (Port/Position).This unload the pump back to the tank (without need of a separate loading valve), while leaving the ram locked in position.

    Figure (13) One possible valve action for a 4/3 valve
    Other possible arrangements may block all four ports in the off position (to maintain pressure), or connect ports A, B and T (to leave the ram free in the off position).
    5.1 Valve Symbols
    Designations given to ports are normally as shown:

    In Figure 14-a, for example fluid is delivered from port P to port A and returned from port B to port T when the valve is in its normal state a. In state b, flow is reversed.

    Shut off positions are represented by T, as shown by the central position of the valve in Figure 14-b.

    The internal flow paths can be represented as shown in Figure 14-c. This latter valve, incidentally, vents the load in the off position.

    In pneumatic systems, lines commonly vent to atmosphere directly at the valve, as shown by port R in Figure 14-d.

    Figure (14) Valve symbols
    Figure 15-a shows symbols for the various ways in which valves can be operated. Figure 15-b thus represents a 4/2 valve operated by a pushbutton. With the pushbutton depressed the ram extends. With the pushbutton released, the spring pushes the valve to position a and the ram retracts. Actuation symbols can be combined. Figure 15-c represents a solenoid-operated 4/3 valve, with spring return to centre

    5.2 Poppet valves

    5.2 Spool valves

    5.3 Pilot-operated Valves

    With large capacity pneumatic valves (particularly poppet valves) and most hydraulic valves, the operating force required to move the valve can be large. If the required force is too large for a solenoid or manual operation, a two-stage process called pilot operation is used.
    5.4 Check Valves
    Check valves only allow flow in one direction. The simplest construction is the ball and seat arrangement of the valve in Figure, commonly used in pneumatic systems. Free flow direction is normally marked with an arrow on the valve casing.​
    5.5 Shuttle Valves
    A shuttle valve, also known as a double check valve, allows pressure in a line to be obtained from alternative sources.
    It is primarily a pneumatic device and is rarely found in hydraulic circuits.
    Construction is very simple and consists of a ball inside a cylinder, as shown in the Figure. If pressure is applied to port X, the ball is blown to the fight blocking port Y and linking ports X and A.
    Similarly, pressure to port Y alone connects ports Y and A and blocks port X. The symbol of a shuttle valve is given in Figure.​
    5.5 Proportional Valves
    The solenoid valves described so far act, to some extent, like an electrical switch, i.e. they can be On or Off. In many applications it is required to remotely control speed, pressure or force via an electrical signal. This function is provided by proportional valves.
    A typical two position solenoid is only required to move the spool between 0 and 100% stroke against the restoring force of a spring. To ensure predictable movement between the end positions the solenoid must also increase its force as the spool moves to ensure the solenoid force is larger than the increasing opposing spring force at all positions.​

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