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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. 5