Hydraulic and pneumatic systems measurements by bmd18385

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									             Hydraulic and pneumatic systems measurements

                                       o
                                      H˝s Csaba, csaba.hos@hds.bme.hu



                                             December 5, 2005


                                                    Abstract

    This document describes the laboratory measurements of the subject ’Hydraulic and Pneumatic Systems’
for third-year BSc students. There are two measurements in hydraulics and one in pneumatics; however,
only two occasions are devoted to the laboratory (on the two last lecture of the term). Thus, every student
misses one of the hydraulic measurements. During the measurements, students work in groups of four or
three (depending on the actual number of the whole class) and hand in a joint report by the end of the term.
The report includes a brief but accurate and traceable summary of the (a) aim(s) of the measurement, (b)
the description of the test rig, (c) the steps of the measurement, (d) the raw data and (e) the results of the
work. Hand-written reports are also accepted until the lecturer is able to read the writing.


Contents
1 General notes                                                                                                  1

2 Efficiency of an open hydraulic circuit                                                                          2
  2.1 General description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    2
  2.2 The measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      3

3 Direct operated pressure relief valve                                                                          3
  3.1 The test rig . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
  3.2 The measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .      4

4 Pneumatic control                                                                                              5


1     General notes
    • This measurement description has to be read and understood. Students not having a clue what to do
      in the lab or failing to explain the experimental set up, the steps of the measurement, etc. are not
      allowed to attend.
    • Millimetre paper, pocket calculator, pencil and rubber (a big one!) are needed.
    • Reports are to be submitted for the hydraulic measurement. It can be prepared by hand or by computer
      as long as it is readable, clean and the figures and tables look nice. The sections of the reports are:
        –   Introduction and aims of the measurements.
        –   Description of the system (with figures) and steps of the measurement.
        –   Measured ’raw’ data.
        –   Calculations (mostly the equations used for data processing) and results, graphs, etc.
        –   A brief summary.




                                                        1
2 Efficiency of an open hydraulic circuit

      2       Efficiency of an open hydraulic circuit
          The aims of the measurement are is to determine the efficiency of an open hydraulic circuit in the case
      of two control techniques;
          • to measure the ’outer’ characteristic curve of the system, i.e. Mm (nm ), the torque of the hydraulic
            motor as a function of the revolutionary speed,
          • Hungarian students only: to determine the efficiency of controlling with a restriction valve connected
            in series and
          •    Hungarian students only: to determine the efficiency of controlling with varying the revolutionary
              speed of the pump.

      2.1       General description
          The sketch of the experimental rig is presented in Figure 1. The electric motor ’1’ drives the pump
      denoted by ’3’ through the clutch ’2’. The input power of the pump is Pp,in = Mp 2πnp and the output
      power is Pp,out = (pp − p0 )Q. Mp can be measured with the help of the balance motor torque equation:
      M = mgk, where m is the mass needed to equilibrate the arm of the motor, g = 9.81m/s2 and k is the length
      of the arm, to be measured ’on-site’. The flow rate of the pump can be measured with a flow meter and
      two pressure gauges are mounted to determine the pressure before (pp ) and after (pm ) the restriction valve.
      The pressure drop through the restriction valve is ∆prv = pp − pm , Thus the power loss is P ′ = ∆p Q. The
      input power of the hydraulic motor ’6’ is Pin,motor = pm Q and the output power is Pout,motor = Mm 2πnm .

                                   pp                 Q                          pm

                  1        2             Qp            Qrv Qm       5                 2        7   2       8



                                            3             4               6


                                 np , M p                                           nm , M m

Figure 1: Sketch of the experimental rig. 1: electric motor, 2: clutch, 3: pump, 4: pressure relief valve, 5: restriction
valve, 6: hydraulic motor, 7: torque meter, 8: generator. Qp stands for pump flow rate, Qrv represents relief valve flow
rate and the flow rate of the hydraulic motor is denoted by Qm . The pump output pressure is pp and the pressure after
the restriction valve is pm .



                                    ∆p
                                                  pressure relief valve         pump at np = . . . [rpm]


                                                restriction valve
                                                                                      ∆prv



                                                                                      ∆pm


                                                                                          Q
                                                       Qm                 Qrv

                  Figure 2: Controlling the open circuit with restriction valve connected in series.

         As it is known from the lectures, the torque demand on the load side defines the pressure level on which
      the system operates: Mm ∝ ∆pm . On the other hand, the flow rate defines the revolutionary speed of the
      hydraulic motor: nm ∝ Q. The hydraulic aggregate (pump + relief valve) is characterised by its performance


                                                                2
3 Direct operated pressure relief valve

      curve (see Figure 2). The intersection of the load demand ∆pm and the characteristic curve of the aggregate
      defines the flow rate Q. However, if the resulting flow rate is not the desired one (which is usually the case),
      it can be set with the restriction valve, see Figure 1.

      2.2      The measurement
           The actual measurement consists of the following steps:
          1. Determine the ’outer’ characteristic curve Mm (nm ) at np = 1300[1/min] by increasing the load of
             the generator with fully opened restriction valve. Meanwhile, calibrate the torque meter (record the
             signal of the electric torque meter and the balancing weights) and the tachometer (electric device and
             Jacquet’s indicator). Your data sheet will look like this:
                                 No.     nm          nm       Mm            m        Q        pp          pm
                                       [units]     [rpm]     [units]       [kg]    [units]   [bar]       [bar]



          2. BSc students only. Determine the ’outer’ characteristic curve Mm (nm ) at np = 1300[1/min] again by
             setting a constant load load on the generator and closing restriction valve step by step. As your torque
             meter and tachometer is already calibrated, it is sufficient to record the electric devices. Now your
             data sheet will look like this:
                                 No.      nm        Mm         Q            pp       pm      valve setting
                                        [units]    [units]   [units]       [bar]    [bar]        [unit]



          3. Hungarian students only. Choose a power level and find the P = const. curve with varying the
             restriction valve setting. Mp and np are to be measured to allow the calculation of efficiency.
          4. Hungarian students only. Choose a power level and find the P = const. curve with varying the pump
             revolutions. Again, Mp and np are to be measured to allow the calculation of efficiency.


      3      Direct operated pressure relief valve
      3.1      The test rig
          The aim of the measurement is to determine the flow-through characteristics of a direct operated pressure
      relief valve, i.e. Q2 (pp ) in Figure 3. The flow through the relief valve ’6’ can be set by the restriction valve
      (throttle) ’5’. If ’5’ is completely closed, all the flow rate from the pump flows through the relief valve. If ’5’
      is fully opened, nothing flows through the relief valve. In between the pump’s flow rate is divided between
      ’5’ and ’6’. Once the throttle ’5’ is completely closed, the flow can be further increased by increasing the
      revolutionary speed of the pump. To determine the flow rate


                                         Q1       pp   Tp
                                                                       3


                           Qp          Qrv Q1                                                    Q2
                                                                                     Qth                     7



                                                                                                     6           8
                             1           2                    4                       5


Figure 3: Sketch of the experimental rig. 1: pump, 2: pressure relief valve, 3: flow control valve, 4: supplementary
restriction valve, 5: main restriction valve, 6: pressure relief to be measured. Qp stands for pump flow rate, Qrv represents
relief valve flow rate, the flow rate through the main restriction valve is Qth and the flow rate of the relief valve to be
measured is denoted by Q2 . The pump output pressure is pp , the temperature of the fluid is Tp .


         It is known that the characteristic curve of a direct operated pressure relief valve (PRV) consists of two
      parts; (a) while it is partially opened (normal operation) and (b) fully opened part (overload), see Figure 4.



                                                                  3
3 Direct operated pressure relief valve

     If the PRV is partially opened (x < xmax ), the flow rate is given by

                                                                               D2 π
                                  r
                                    2
                       Q1 = A(x)      ∆p, where A(x) = xDπ and s(x + x0 ) = ∆p      ,                             (1)
                                    ρ                                           4

     where s[N/m] is the spring constant, x0 [mm] is the pre-compression of the spring and ∆p = pin − pout. The
     first equation is simply the flow rate - pressure drop relationship, the second gives the flow-through area and
     the third one is a force balance on the piston. If the PRV is overloaded (x = xmax ), we have
                                                                r
                                                                  2
                                                 Q2 = xmax Dπ       ∆p.                                        (2)
                                                                  ρ
     Thus we see that                 8 “            ”   q
                                      < ∆p D 2 π − x0 Dπ 2 ∆p            if   x < xmax
                                            4s             ρ
                              Q(∆p) =                q                                   .                        (3)
                                                       2
                                      :    xmax Dπ ρ ∆p                  if   x < xmax


                                                             ∆p     characteristic curve

                                                                                              Q1


                pout                               xmax
                              x       pin                                          Q2
                                  D                pin

                                                                                                   Q[l/min]

         Figure 4: (Left) Schematic view of the relief valve. (Right) Characteristic curve of the relief valve.



     3.2    The measurement
       1. By closing restriction valve ’5’ step by step, walk through the characteristic curve. At every point,
          record the input pressure pp and the flow rate. The flow rate can be determined with the help of th
          metering tank ’7’, where the oil level elevation during some period of time is measured. If ∆t[s] denotes
          the time period of the measurement and ∆h[m] is the level elevation, we have Q = α ∆h/δt, where
          α[m2 ] denotes the surface of the tank. (Note that the usual dimension of flow rate [l/min] while the
          previous equation gives [m3 /s].) Your data sheet will look like this:

                                             No.      pp     ∆t    ∆t       Q
                                                     [bar]   [s]   [s]   [l/min]



       2. Hungarian students only. During the measurement of the characteristic curve, measure the vibration
          spectra on the PRV, at least in three points on each segment (pre-opening, normal operation and
          overload). Also record pressure histories with electric pressure meters (in steady state) and compare
          the spectra of the pressure signal to the mechanical ones.
       3. Hungarian students only. Transient measurement: record three pressure histories while switching on
          and off the PRV with the help of element ’3’. Compare the amplitude and the frequency.




                                                             4
4 Pneumatic control

    4     Pneumatic control
        Build a system which is capable to follow the tracking diagram below. Design the circuit at home and
    bring your sketch with you to the lab.

                                     +                                        D2

                                C1
                                     -                                        D1

                                     +                                        D3

                                C2
                                     -                                        D4

                                                  one cycle

     Figure 5: Pneumatic control tracking diagram. C1,2 denote cylinders and D1...4 stand for limit switches.




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