Increasing the Operating Safety of the Machine-Tools by Using Hydro-pneumatic Accumulators. Modeling - Simulation

Transcription

1 ISBN xxx-x Proceeding of 011 International Conference on Otimization of the Robot and Maniulator (OPTIROB 011) Sinaia, Romania, 6-8 Mai, 011,. xxx-xxx Increaing the Oerating Safety of the Machine-Tool by Uing Hydro-neumatic Accumulator. Modeling - Simulation Dan Prodan 1+, Anca Bucureteanu 1 and Emilia Balan 1 1 Machine and Production Sytem Deartment, Univerity Politehnica of Bucharet Abtract. Thi aerwork reent the reult of the theoretical and exerimental reearch that ha had a object of tudy the hydraulic intallation with hydro-neumatic accumulator. It reent imulation for determining the erformance of ome hydraulic afety ytem deigned for actuating brake, for enuring bearing lubrication when the circuit incororating the um fail and for lifting load in emergency ituation. A hydraulic intallation deigned for a grinding machine i reented at the end. Keyword: machine-tool, accumulator, afe oeration. 1. Calculation of Safe Sytem Incororating Accumulator The accumulator are hydraulic comonent that allow the receiving, torage and tranmiion of the hydrotatic ower a volume of reured liquid. The reduced comreibility factor of the liquid make difficult the torage of ower in mall volume, but allow the tranmiion of high effort. Unlike liquid, ga ha higher oibility regarding the comreibility and that allow toring higher energy in maller volume. Aociating liquid and ga in ecial contruction ha led to the manufacturing of the hydroneumatic accumulator [1]. It i given the oeration cheme in Figure 1 []. Fig. 1: The oeration cheme of a hydro-neumatic accumulator The accumulator Ac oerate within reure min and. It ueful volume i V 0, and it initial charging with nitrogen i made at the reure 0. Uually, thi reure verifie the following relation: 0 k min k (1) The charging of the accumulator with oil i made through the check valve Cv, in the tranitory hae min. The dicharge i made through the throttle Tv during the hae correonding to the reure lowering, min. If coniderring the charging and the dicharging of the accumulator with oil are made under iothermal condition (T=ct.), then: = [ ] 0 V0 = Vmin = min V () + Correonding author. Tel.: ; fax: addre: rodand004@yahoo.com.

2 where: V min minimum volume occuied by nitrogen, V imum volume occuied by nitrogen. In the dicharging hae of the accumulator, the imum volume of the available oil i: 1 1 ΔV = V = Vmin V (3) o 0 min Thi volume i dicharged through the throttle Tv. The throttle characteritic i conidered a being: Q = C S Δ (4) ρ where: Q throttle flow, C throttle contant, S ueful urface of the throttle, ρ denity of the oil, Δ reure dro on the throttle. If conidering the accumulator a being in the hae of dicharge, a in Figure, then two ituation can be ditinguihed: a. the dicharge toward a negligible reure circuit, = 0; b. the dicharge toward a ytem with reure, negligible value From the relation above it reult: a. b. Fig. : The hae of dicharge of the accumulator dv 0 V0 d = = Q = C S ( S) (5) dt dt ρ If conidering the dicharge diagram in Figure a, by integrating the relation above it i obtained: min d C tu S = dt 0 V0 ρ (6) 0 For the charging diagram in Figure b, by integrating the relation (5) it reult: min d C tu S = S 0 V0 ρ 0 dt (7) For determining the time the accumulator can act a a ource of energy for the ytem, it i neceary to calculate the integral of the relation above. When = 0 for examle the afety ytem of the hydrotatic lubrication intallation it i obtained: 0 V tu0 = (8) 3 min min C S ρ If the accumulator erve a circuit with the reure, which ha a value that cannot be neglected for examle the locking and claming ytem - it i obtained:

3 t u = C S V ρ 0 0 min 1 min + arctg arctg A a ractical examle: given the value V 0 = 4 l, = 80 bar, min = 40 bar, 0 = 30 bar, = 5 bar, then t u0 = 46 and t u = 65 are obtained. For both cae, the time can be determined by uing imulation oftware [3].. Simulation of Braking Sytem Incororating an Accumulator The imulation take into conideration the diagram in Figure 3 [4]. The um P driven by the electric motor EM uck u the oil from the tank through the abortion filter SF. The reure valve PV allow the adjutment of the imum reure. The directional control valve D1(S1) enure the charge of the circuit, ecific to the re-controlled intallation []. The reure filter F filter the oil. The circuit rovided excluively for braking in emergency ituation contain: the accumulator Ac, the directional control valve D(S), the check valve CV. The reure witch PS confirm the reure required for brake actuation. min (9) Fig. 3: SF abortion filter, EM - electric motor, P - um, M1, M manometer, PV reure valve, D1, D directional control valve, F - filter, S1,S - electromagnet, CV check valve, Ac - accumulator, PS reure witch, B hydraulic brake, OC other hydraulic circuit. The manometer M1 and M dilay the reure in the general circuit and the reure in the brake circuit. Normally, the intallation alo erve other circuit OC, not hown in the diagram. In cae of emergency um failure, main ower failure, ie break, etc. the directional control valve D(S) i actuated. The accumulator, which ha been charged until that moment, dicharge and actuate the brake B. After dimenioning the required accumulator, a imulation i recommended for checking the oerating mode of the brake B. Figure 4 reent the reult of the imulation for the evolution of the reure when activating (S+) and deactivating (S-) the brake. Fig. 4: The reult of the imulation

4 When activating the brake the reure rie to bar in 0. aroximately, thi correond to a braking force Ff over 5800 dan. The deactivation of the brake i accomlihed a fat a the activation. For higher force, the reure or the braking urface hould be increaed. 3. Simulation of Safety Sytem Incororating an Accumulator [4], [5] It i conidered the diagram of the actuation of a cylinder on vertical direction, a given in Figure 5. The intallation i made baed on the three hydraulic module: MOD1 re-control module, MOD u and down actuation module and MOD3 afety module ued for the lifting u of the load without requiring the oeration of the um. The ilot oerated valve i rovided to revent accidental failure of the load [6]. The um driven by the electric motor EM, uck u the oil from the tank through the abortion filter SF. The reure valve PV allow the adjutment of the imum reure. The reure filter F filter the oil. The circuit intended excluively for emergency cae contain: the accumulator Ac, the directional control valve D(S) and the check valve CV1 and CV. The reure witch PS confirm the reure required for the actuation. The manometer M1 and M dilay the reure in the general circuit and the accumulator circuit. Normally, the intallation erve the lifting and decending circuit module MOD. Fig. 5: SF abortion filter, EM electric motor, P - um, M1, M - manometer, PV reure valve, D1, D, D3 directional control valve, F - filter, S1, S, S3, S4 - electromagnet, CV1, CV check valve, Ac - accumulator, PS reure witch, C hydraulic cylinder, M mobile ma, PCV ilot controlled valve, MOD1, MOD, MOD3 hydraulic motor. In cae of emergency um failure, main ower failure, ie break, etc. the directional control valve D(S) i actuated. The accumulator that ha been charged until that moment dicharge and actuate the iton cylinder C on the lower urface. The oil accumulated in the accumulator get relaxed and lift u the load M. Figure 6 reent the evolution of the reure on the lower urface of the iton and the oition of the rod (it withdrawal) from the moment the commend of emergency lifting u i given (S+). Fig. 6: The evolution of the reure It i noticed that the lifting u of the mobile aembly i accomlihed in le than 3 econd. Actually, it can lat longer, deending on everal factor that have not taken into conideration: tightening tye of the cylinder, friction force on the guideway or on the iton rod, etc. However, ome of modification uually made after building u a rototye can be avoided by imulation and that mean aving time and money.

5 4. Exerimental Reearche Carried out For the traight cu wheel grinding machine, in cae of accidental toage of the magnetic table uly, the workiece can be thrown away. That lead to workiece damage and rereent a eriou threat for the oerational afety [6]. The olution reented further (Figure 7) allow lifting u the grinding wheel almot intantaneouly, o a the workiece are no longer driven when the ower uly of the magnetic table i interruted. If the ower uly of the magnetic table fail, the circuit incororating the accumulator actuate two linear hydraulic motor that lift u the wheel. Fig. 7 For imulation, it ha been conidered the uage of an accumulator with V0 =.5 l, loaded to 0 = 80 bar. The working head with the holding device and the wheel weight 000 kg. In accordance with the catalogue of hydraulic equiment manufacturer, the time required to witch on the directional control valve i 10-5 m, for D.C. driving. When imulating, it i noticed that after 0.08 the head lift u by 7 mm, which i a dimenion ufficient for the rotection of the workiece on the magnetic table. The directional control valve witching time i added. Therefore, it can be conidered the time of 0.1 i ufficient for a lifting u higher than 5 mm. After building-u the real intallation, during tet, it ha been noticed the lifting u i done in a time hort enough to eliminate the rik of throwing away the workiece from the magnetic table. 5. Concluion The hydro-neumatic accumulator can be ued to increae the oerational afety of variou hydraulic intallation of the machine-tool. The election of the accumulator hould be done for each cae by taking into conideration the real working condition. The modeling rereent a erie of equation or ytem of equation, algebraic or differential, linear or not, decribing the oeration of the tudied element or ytem. The inut arameter uoed to be known and the outut arameter to be determined hould be clearly defined for thee model. Defining thoe arameter i ometime difficult for the hydraulic intallation of the machine-tool. A ubjective factor intervene: the exertie of the hydraulic deigner. Comuter-aided Deign cannot be done jut uing the oftware and the comuter. A erie of factor and coefficient i neceary and neither the manufacturer of the hydraulic comonet nor the ecialied book rovide them. They have to be exerimentally determined in laboratorie endowed with ecial equiment. 6. Reference [1] D. Prodan, M. Duca, A. Bucureteanu and T. Dobrecu, Hydrotatic Actuating. Organologie, AGIR Publiher Houe, ISBN X, Bucharet, 005. [] D. Prodan, Heavy Machine Tool. Mechanical and Hydraulic Sytem, Printech Publiher Houe, ISBN , Bucharet, 010. [3] D. Prodan, Machine-Tool. Modeling and Simulation of the Hydrotatic Element and Sytem, Printech Publiher Houe, ISBN , Bucharet, 006. [4] H. Exner, R. Freitag, H. Gei, R. Lang, J. Oozer, P. Schwab, E. Sumf and U. Otendorff, The Hydraulic Trainer Volume 1 Rexroth, ISBN , [5] T. Deaconecu, Hydraulic iving Sytem, Tranilvania Univerity Publiher Houe, ISBN ,

6 Braov, 007. [6] D. Prodan, A. Bucureteanu and E. Bălan, Hydraulic Sytem to Secure the Magnetic Table of a Grinding Machine, Proceeding of HERVEX, INOE 000-IHP Pre, , ISSN , Ramnicu Valcea, 007.