HYDRODYNAMIC COUPLINGS IN BELT CONVEYOR DRIVES FOR BULK MATERIALS

Transcription

1 Voith urbo (ty) Ltd 6 Saligna Street Hughes Business ark Witfield, Boksburg 459 el Fax HYDRODYNAMIC COULINGS IN BEL CONVEYOR DRIVES FOR BULK MAERIALS he design of reliable belt conveyors, particularly for steady operating conditions, is governed by numerous standards and regulations. he design engineer has to pay special attention to the starting and braking sequences with a view to the service life and the capital cost of the belt. hese operating conditions can be influenced positively by judicious use of hydrodynamic couplings in belt conveyor drives. Hydrodynamic power transmission In the conveying and materials handling industry, hydro dynamic couplings are generally installed between motor and gear unit. As a result of their operating characteristics, the interaction between driving motor and belt conveyor can be influenced as follows: Separation of motor start-up and belt conveyor start-up. Delayed and/or controlled build-up of belt tension. orque limitation (steplessly adjustable). Switch-on delay and load sharing for multi-motor drives. Damping of torsional vibrations. Creep speed (depending on coupling design). Conveyor at standstill while the motor is still running (depending on coupling design). Hydrodynamic couplings based on the Föttinger principle operate as described in VDI standard no. 53. An elementary coupling consists of two bladed wheels (centrifugal pump and turbine). hese blades are surrounded by a shell and form a working space in which the operating fluid circulates (Fig. ). Mechanical power is transmitted wear-free as the circulating fluid flows continuously between pump and turbine wheel. orque is created by a change of kinetic energy of the fluid when passing from the pump to the turbine wheel. Commensurate with physical laws, hydrodynamic couplings (turbo couplings) are classified as fluid machines. hese machines are charac terised by torque being transmitted proportionally to the square of the input speed. If portrayed graphically, this is designated as primary behaviour or slip parabola. he operating behaviour (secondary behaviour) can be represented by the characteristic relationship of the performance figure ( = f(o) (characteristic curve)) (Fig. ). Usually, the characteristics of a turbo coupling are described as a function of the torque depending on the speed ratio = n /n of various fillings (secondary performance chart). Depending on coupling type and filling volume, the magnitude and shape of relevant characteristic curves may vary considerably. In order to achieve a mass flow which transmits power, a difference in speed between pump and turbine is required even during nominal operation. his difference is mainly quoted as slip s = - o. Fig. : View of characteristic relations, typical performance charts of two coupling types Characteristic relation ~m = f( ) Characteristic curve ~m = f(,f) erformance chart A = Starting point N = Rated operating point Fig. : hysical regularities A max. filling A max. filling p $ ~ ra p rj ṁ $ ~ V K, min N N D ump urbine : p a ua j ui Euler s turbine equation = = m$ ^r $ C - r $ C h Hydrodynamic model equation = = m$ t $ ~ $ D p 5 min. filling min. filling nt nt o = n o = p np Constant-fill coupling Fill-controlled coupling For start-up and overload couplings, the characteristic curve occurring during continuous operation should be as steep as possible, in order to keep the slip during nominal operation at a minimum. In the high slip range up to the break-away point A, the curve should run horizontal to achieve torque limitation. Absolute levels of torque can be transmitted by varying the fluid level.

2 Voith urbo (ty) Ltd 6 Saligna Street Hughes Business ark Witfield, Boksburg 459 el Fax Fig. 3: Basis of coupling design rimary performance chart Secondary performance chart s=00% s=60% s=4% ~ Characteristic Curve ~ ~ = o = he primary and secondary characteristics of a coupling are best illustrated in a three-dimensional diagram (Fig. 3). When looking at the system, such a graph is useful if the torque being built up during motor run-up is sufficiently high to break away the conveyor. Fig. 3 shows all relevant hydrodynamic correlations. As a result of the reciprocal action between primary and secondary function, coupling curves can be achieved that take into account criteria such as time, speed, slip and filling volume. Coupling types and designs Hydrodynamic couplings are manufactured in a great variety of sizes and designs for the entire performance and speed range required by the materials handling industry. Some of the features of hydrodynamic couplings which are relevant for specific applications are part of their design or type. Apart from the steady-state characteristics of a conveyor, its startup and retardation behaviour must also be taken into consideration when a coupling is selected. Fluid couplings suitable for belt conveyors can be classified as shown in Fig. 4. he classification depicts two basic types with individual designs. For fill-controlled couplings, the curve for any filling should fall continuously while the speed is increasing. In this way, controlled start-up phases with narrow torque limitation are possible. If the interception between coupling curve and load curve is clear-cut, it is possible to set stable operating points below the nominal speed, e. g. for inspection of the unloaded conveyor. Developing a curve which is perfect for the application solely in accordance with hydraulic laws is still very difficult. Coupling curves are therefore mainly determined during tests. Fig. 4: ypes and designs of hydrodynamic couplings for belt conveyors he constant-fill coupling is a popular type used for bulk materials handling, due to its uncomplicated design, minimum maintenance and very competitive cost. his coupling type is filled with operating fluid prior to commissioning, and there is no external oil supply. For designs and 3 (Fig. 4), the operating fluid for the coupling is distributed differently, particularly for operating conditions other than continuous operation (e. g. starting and stopping). his coupling type is mainly chosen to enable a load-relieved start of the motor, torque limitation and to influence the torsional vibration behaviour. heir basic characteristic curve corresponds to the one shown in Fig. for start-up and overload couplings. Non-delay fill coupling Constant-fill coupling Filling set at standstill With delay fill chambers Hydrodynamic coupling assive filling adjustment With delay fill chamber and annular reservoir 3 Intervention in the circuit flow Change in mass flow / spinning Regulating blade / sleeve valve Fill-controlled coupling Filling setting Scoop trim coupling with stationary reservoir Variable filling during operation Fill control / drain coupling with stationary reservoir 4 Inflow / outflow control Scoop control coupling with rotating reservoir

3 Voith urbo (ty) Ltd 6 Saligna Street Hughes Business ark Witfield, Boksburg 459 el Fax Fill-controlled couplings are used on belt conveyors with special requirements relating to the build-up of tensile force, torque limitation and operating behaviour. Fill-controlled couplings which allow altering the fluid level in the working chamber are available in different designs. hese couplings are equipped with an additional external fluid circuit which can be used for varying the fluid level as well as cooling. With design 4, the fluid level is determined by matching the supply flow rate with the discharge rate of fluid at the spray nozzles. ilot or control valves are used as actuators in the supply flow. Design 4 distinguishes itself by its compact design and good control behaviour. Selection criteria No-load start of motor For belt conveyor drives, asynchronous motors are now widely used. he advantages of these motor types, i.e. minimum maintenance and simple direct on-line (DOL) starting, are not ideal for belt conveyor start-ups. Also, the starting behaviour, limited thermal load capacity and high current are unattractive. With DOL, the motor builds up its break-away torque in milliseconds and generates its design-inherent torque during start-up. his torque vs. speed curve is a characteristic feature of the individual motor and its characteristic does not depend on the load torque. Dependent on its type and design, the hydrodynamic coupling can aid the asynchronous motor in several ways. he load on the motor during start-up resulting from load torque and the mass to be accelerated is solely determined by the coupling; the belt conveyor is virtually separated from the motor. he coupling torque is built up from zero with the square of the motor speed. For identical nominal operating conditions, the amount of startup load can be selected from a wide range. Depending on their design, constant-fill fluid couplings can build up a considerable amount of torque during motor start-up (Characteristic curves a to c). Empty fill-controlled couplings, on the other hand, just generate a slip torque which can be regarded as insignificant (Characteristic curve d). Even the constant-fill coupling of design without delay chamber already provides noticeably softer motor start in the peak current range. It allows torque to build-up in seconds rather than milliseconds and its operating fluid provides additional thermal storage capacity on for start-up. he graph in Fig. 7 also shows how this type has developed in recent years from to VV and VVS couplings with delay chamber and annular chamber. he application of fill-controlled couplings eliminates the disadvantageous features of asynchronous motors almost completely. Belt stress during start-up In large conveyor systems, the belt is in most cases the most expensive component and therefore determines the investment cost and the economy of the entire plant. lant engineers and operators therefore ask for the development of an ever more improved design for the optimum conveyor belt selection. Fig. 5: Build-up of coupling torque in relation to motor speed with output (turbine wheel) at standstill Fig. 6: Belt stress during start-up with various constant-fill couplings orque ratio / N (Motor) 00 % U N Motor 80 % U N a b c Constant-fill coupling: a = coupling b = VV coupling c = VVS coupling d = Fill-controlled coupling U N : Nominal voltage orque / N.6.4 M Run-up Driven machine K L; J L; J 5 s 40 s ime d Required: t A 5 t U t U = L / c c = 0.6 km/s to c =.0 km/s t A relative to / N = 0 Motor speed n/n Syn ype ype V ype VV ype VVS loaded conveyor ype VVS empty conveyor M : Motor torque L : Load torque K : Coupling torque N : Rated torque J: Moment of inertia L, J : Empty system t A Start-up time Shock wave travelling speed t U retensioning of belt: ype : t A ~ s ype V / VV: t A ~ 0.5 s ype VVS: t A ~. 3 s L 3

4 Voith urbo (ty) Ltd 6 Saligna Street Hughes Business ark Witfield, Boksburg 459 el Fax As a consequence it is vital that drive systems meet these criteria. Non-steady operating conditions such as starting and stopping with varying loads make high demands on drive systems. he drive system is expected to provide a smooth build-up of torque (initial belt pull), low torque limitation and adaptation of the starting torque to the load condition. All fill-controlled couplings with suitable solenoid valve system allow that torque build-up and close adaptation to load conditions can take place within the narrowest of limits. Depending on their design, constant-fill couplings fulfil the expectations to varying degrees (Fig. 6). Based upon investigations on the dynamic stress in belt conveyor systems, the torque build-up time t A is introduced to compare and evaluate the belt pull. he belt can always be assumed to experience quasistatic stress, if the torque build-up time t A is five times larger than the shock wave travelling speed t U on the return belt. he relevant synergies are illustrated in Fig. 6, where c represents the shock wave travelling velocity, dependent on the belt design and the free belt length L. he three constant-fill coupling designs have the following features: Design ( coupling): Suitable for small belt conveyors with a possible torque limitation of up to.8 times the nominal torque at good nominal slip. he torque is not adapted to the load condition of the conveyor. Applications are usually found in combination with gear motors. Design (V and VV coupling): Suitable for medium-size belt conveyors with a possible torque limitation of up to.6 times the nominal torque for V and.4 times the nominal torque for VV couplings. Due to the system, the starting curve adapts itself by a limited amount to the load condition. Standard designs are available in combination with motor and gear unit. Design 3 (VVS coupling): As a result of its smooth build-up of torque, this type is suitable also for longer belt conveyors with starting times up to 50 s. orque limitations of up to.4 times the nominal torque are possible at good nominal slip. Excellent adaptation of the starting torque to the load condition of the conveyor. For starts without load, the starting torques are below the nominal torque, hence protecting the belt. his coupling design is the result of long-term co-operation with belt conveyor manufacturers and operators. he long experience in manufacturing turbo couplings is illustrated on the table for constant-fill couplings (Fig. 7). Fig. 7: Constant-fill coupling series V / VV VVS Selection criteria depending on application Apart from the selection criteria given by motor and belt, other application-inherent factors have to be borne in mind. Heat increases in proportion to the slip which is the operating principle of a hydrodynamic coupling. During steady operation or starting, this slip heat can be dissipated via the surface (constant-fill couplings) or via an external cooling circuit with heat exchanger (fill-controlled couplings e.g. type KL). herefore, the number of starts per operating period and the installation and environmental conditions should be taken into consideration, as they may impair the dissipation of heat. With multi-motor drives, load sharing of the motors occurs automatically due to the coupling slip, which can be further enhanced by changing the oil fill. Unequal loads might be the result of differing belt tension at the drive drums. In addition, differences may occur even with new conveyors as a consequence of the drum diameter tolerances and the rated motor slip (according to VDE /- 0 %). hese effects can be reinforced by wear and replacement of drive components. he slip adaptation is carried out by changing the fluid level while the conveyor is at standstill, or, with fill-controlled couplings, by controlling the fluid level. urbo couplings can influence the stopping of the belt conveyor directly (with fill-controlled couplings; by interrupting the power flow as a result of draining) or less so indirectly (with constantfill couplings; by separating the rotating masses). Due to their operating principle, turbo couplings are suitable both for special operating conditions such as creep speed (fill-controlled couplings) and regenerative braking. hese operating conditions have to be discussed in more detail with the coupling manufacturer. 4

5 Voith urbo (ty) Ltd 6 Saligna Street Hughes Business ark Witfield, Boksburg 459 el Fax Examples of application In the materials handling industry, an individual belt conveyor is often just a link in a close chain of materials handling machinery. In synergy with other machines and equipment, the conveyor should guarantee a continuous flow of materials. he start-up control and operation monitoring system of individual conveyors and their drive systems should be suitable for integration into a central control system, in order to ensure that the entire plant can be operated automatically. Arduous starting conditions can be caused by weak electrical systems and/or the build-up of dirt and dust, which can be endemic in mines, etc. For such complex installations, fill-controlled couplings are the preferred choice. Drives with fill-controlled couplings allow the motors to be started in sequence while the coupling is drained, which minimises the strain on the electrical system. he belt conveyor can only be started after receiving green light from the general operation monitoring system (voltage, speed, lubrication pressure, etc.). With integrated systems it is particularly important that the starting sequences and starting times of the individual conveyors are adapted with each other. herefore, start-up times should be adjustable, independent of the load condition (Fig. 8). After the release signal from the monitoring system, the conveyor is started by a micro-processor-controlled start-up system. he control system is designed as a multi-stage control cascade with different parameters. he torque build-up time is used as the main control variable until the conveyor has broken away, while constant acceleration is used as a control variable after conveyor break-away. Subordinate control circuits can be used in order to monitor whether the torque limits, the operating fluid temperature and, with multi-motor system, the load sharing are correct. Specific start-up problems can be solved either by using a drive unit complete with start-up controller and an interface that is compatible with the central control system, or a compact coupling unit that can be integrated into the customer s own system. In practice, companies often use different coupling types for various belt conveyors, in order to achieve optimum operating conditions. he constant-fill coupling type VVS has been developed specifically for the soft start of belt conveyors. In addition to its internal delay chamber it also features an external annular chamber. he following paragraph describes the distribution of the operating fluid and its effect on the operation of the coupling for the three operating conditions standstill, 00 % slip and normal operation. When the conveyor is at standstill, the operating fluid is distributed to the three chambers (delay chamber, working chamber and annular chamber). During motor start-up and 00 % slip the operating fluid level in the delay chamber remains almost constant, while the external annular chamber is filled with fluid from the working chamber as a result of centrifugal effect during the initial motor revolutions (see Fig. 9). he remaining fluid in the working chamber builds up a very low torque from standstill. he working chamber is then filled (timedependent via internal nozzles) with the fluid from the delay chamber. In this way it is possible to achieve a low starting torque during motor run-up with ensuing smooth build-up of torque and low slip in normal operation (see Fig. 6). he smooth torque build-up and the adaptation to existing load conditions have been proven on the test stand. he fill-controlled coupling type KL is suitable for high-powered belt conveyors with start-up times of up to several minutes. he starting torque is perfectly adapted to the load conditions of the belt conveyor (Fig. 8). he following paragraph describes the operating conditions of the fill-controlled coupling type KL. Fig. 8: Starting a belt conveyor with a fill-controlled coupling N 0 t 0 : Motor run-up t t : ull build-up (torque build-up) t t 3 : Conveyor start-up (a = constant) n min : Inspection run n n N j j N j max t max Coupling Full load n artial load n min t 0 0 t 0 t t t t 3 t 3 n j 5

6 Voith urbo (ty) Ltd 6 Saligna Street Hughes Business ark Witfield, Boksburg 459 el Fax At standstill of the belt conveyor, the operating fluid of the coupling is situated in the oil tank. With an empty coupling the motor starts under no load conditions. When the motor has come up to speed the coupling is filled via the filling valve. orque increases smoothly by filling the coupling. he fill process continues and the belt is tensioned gradually until the belt conveyor breaks away. hrough the controlled filling of the coupling, the belt is able to accelerate in the course of several minutes independent from the load state of the conveyor. At nominal speed the coupling is filled. he hydrodynamic principle provides natural load sharing between different drives. For inspection speed the coupling is only partially filled. he empty conveyor belt then moves at 5 0 % of its nominal speed. Due to the standstill cooling function of this coupling type, frequent start-ups in succession are possible (Fig. 0). Fig. 9: VVS constant-fill coupling operation Standstill Start-up Fig. 0: KL fill-controlled coupling operation Nominal Operation Summary he application of hydrodynamic couplings on belt conveyors and the operating principles of these couplings are governed by the criteria on which individual coupling types or designs are selected. he aforementioned examples can only be regarded as an extract of the numerous applications for hydrodynamic couplings on belt conveyors for bulk material conveying. Bibliography VDI regulation 53: Hydrodynamic ower ransmission, April 994 VDI Regulation 360 Belt conveyors for bulk materials, part, draft 996 Funke, H.: Hydrodynamic couplings in conveyor systems, Lecture at Esslingen echnische Akademie, April 984 Voith cr394en: Hydrodynamic Couplings rinciples, Features, Benefits Standstill cooling Motor start Machine start Inspection speed 6