Tempo di lettura stimato: 24 minuti

Selezione della pompa idraulica

Il pompa idraulica è un dispositivo di conversione dell'energia, che converte l'energia meccanica del motore di azionamento nell'energia di pressione dell'olio per soddisfare le esigenze dell'attuatore per guidare il carico esterno.

Domanda 1: prestazioni della pompa idraulica

Le principali prestazioni delle pompe idrauliche di uso comune sono riportate in Tabella.

Domanda 2: Selezione del tipo di pompa idraulica

The principle of selecting the hydraulic pump is: according to the working condition of the main engine, the size of the power and the performance requirements of the system, first determine the type of the hydraulic pump, and then determine its specification and model according to the pressure and flow required by the system. Hydraulic Pump Parameter

• Per sistemi idraulici generici di macchine utensili, paletta a doppio effetto pompe, and pressure-limiting variable vane pumps are often used; while road construction machinery, port machinery, and small construction machinery, gear pumps with strong anti-pollution capabilities are often selected; In occasions, plunger pumps are often chosen. Hydraulic Pump Parameter
• Pay attention to whether the flow of the hydraulic pump is adjustable. Generally speaking, if the hydraulic power is less than 10kW, the working cycle is on-off, the hydraulic pump can be completely unloaded when not in use, and the hydraulic pump is required to output full flow under most working conditions, you can consider using a fixed pump; if the hydraulic power If the flow rate is larger than 10KW, the variable pump can be considered. Hydraulic Pump Parameter
• Pay attention to parallel pumps and series pumps. Gear pumps and vane pumps can also be made into several pumps connected in parallel, and use a double pump or triple pump with the same drive shaft, or can be connected in series to form a multi-stage pump. When the hydraulic system-the flow rate changes greatly within a working cycle, multiple pumps can be used. Multiple pumps usually have one oil suction port and multiple oil outlets. The pressure oil from each oil outlet can supply oil to different actuators of the system, or can be combined to supply a certain actuator. Hydraulic Pump Parameter

Il calcolo dei parametri della pompa idraulica

Domanda 1: Calcola la pressione massima di esercizio della pompa idraulica

La pressione massima di esercizio P_P della pompa idraulica dipende dalla pressione massima di esercizio dell'attuatore (cilindro idraulico o motore idraulico), vale a dire

P_P≥P₁+ p

Domanda 2: Calcola la portata massima della pompa idraulica

La portata massima q_P (m³/s) della pompa idraulica principale dipende dalla portata q_v required by the system. Hydraulic Pump Parameter

Per un sistema in cui più cilindri idraulici o motori idraulici agiscono contemporaneamente, la portata massima della pompa idraulica dovrebbe essere

Q_P≥q_vK( ∑q )max  

Domanda 3: Calcolo della portata della pompa piccola quando si calcola la fornitura di olio della pompa doppia

If the oil supply of the hydraulic pump is selected according to the working conditions (for example, when two pumps are used for oil supply, the small flow pump supplies the flow of the working conditions), the minimum overflow of the overflow valve should also be considered when calculating the oil supply. Hydraulic Pump Parameter

Domanda 4: Selezionare la pressione nominale della pompa idraulica da determinare

The pressure of the hydraulic pump obtained by the above calculation is the static pressure of the system. In the dynamic working process of the system, the dynamic pressure of the system far exceeds the static pressure due to pressure overshoot or periodic pressure pulsation during the transition process. Therefore, when selecting a hydraulic pump, the rated pressure of the hydraulic pump should be 25%~60% higher than the maximum calculated pressure of the system (a high-pressure system takes a small value, medium and the low-pressure system takes the larger value) so that the hydraulic pump has a certain pressure reserve. Hydraulic Pump Parameter

• Generally speaking, in fixed equipment, the rated pressure of the hydraulic pump should be about 25% higher than the normal working pressure, and the rated pressure of the hydraulic pump for vehicles should be about 45% higher than the normal working pressure to ensure the pump has a sufficient life. If the reliability requirements of the hydraulic system are particularly high, the rated working pressure of the hydraulic pump can be selected higher. Hydraulic Pump Parameter
• If the hydraulic system only requires the supply of high-pressure and small-flow working medium in a short period, the maximum working pressure of the hydraulic pump can be selected smaller at this time (when selecting a hydraulic pump, it is not advisable to reduce the specifications of the large-capacity hydraulic pump. It is because when the working pressure and working flow of the hydraulic pump are less than its rated value, the overall efficiency of the hydraulic pump will decrease). Hydraulic Pump Parameter
• The highest working pressure on the sample is the allowable pressure during short-term impact. Do not have impact pressure in every cycle, otherwise, the life of the hydraulic pump will be significantly shortened or even damaged. Hydraulic Pump Parameter

Domanda 5: selezionare la portata nominale della pompa idraulica da determinare

Per quanto riguarda la portata della pompa idraulica, in fase di selezione è possibile selezionare la portata massima richiesta dall'impianto, che non deve superare troppo per evitare eccessive perdite di potenza. Dopo aver redatto la struttura della pompa idraulica, le specifiche e i modelli della pompa idraulica possono essere determinati facendo riferimento ai relativi manuali tecnici o campioni di prodotto in base alla pressione massima di esercizio e alla portata massima di uscita della pompa idraulica.

Nella selezione effettiva, poiché la cilindrata della pompa, l'intervallo di velocità e il flusso di uscita a pressioni diverse a velocità tipica sono generalmente indicati sul campione del prodotto. Pertanto, quando la portata richiesta q_v dell'impianto è nota la portata della pompa q_P (L /min), velocità n (r/min) e spostamento V (mL/r) dovrebbero essere considerati in modo completo. Infatti, poiché il flusso in uscita della pompa è

    Q_PVnη_v×10^-3

• The output flow of the hydraulic pump should include the flow required by the actuator (the total flow can be obtained from the time chart when there are multiple actuators), the minimum overflow flow of the overflow valve, the sum of the leakage of each component, and the motor reverse rotation (usually 1r/s or so), the flow reduction caused by the reduction in efficiency of the hydraulic pump after long-term use (usually 5% to 7%). Hydraulic Pump Parameter
• The flow rate of the hydraulic pump is related to the working condition, and the maximum flow rate of the hydraulic pump is determined by the maximum flow rate in the system working condition diagram. When selecting the flow rate of the hydraulic pump, it should not be less than or equal to the maximum flow rate when the hydraulic system is working. Hydraulic Pump Parameter
• The flow rate of the hydraulic pump is related to the speed of the prime mover. When the prime mover and the hydraulic pump are connected by a coupling, the speed of the prime mover should be within the working speed range of the hydraulic pump, otherwise, a speed increasing or decelerating device should be installed between the prime mover and the hydraulic pump. Hydraulic Pump Parameter

Domanda 6: Per quanto riguarda la velocità e l'efficienza della pompa

• La pressione massima e la velocità massima della pompa idraulica devono essere utilizzate contemporaneamente per prolungare la durata della pompa idraulica. Il manuale del prodotto fornisce una tabella guida più dettagliata dei parametri della pompa idraulica. Quando si seleziona, è necessario seguire rigorosamente le normative nel manuale del prodotto.
• Maggiore è la pressione e minore è la velocità, minore è l'efficienza volumetrica della pompa idraulica e l'efficienza volumetrica della pompa variabile diminuirà quando viene regolata la cilindrata della pompa variabile. Quando la velocità è costante, l'efficienza totale della pompa idraulica è massima a una certa pressione e l'efficienza totale della pompa variabile è massima a una certa cilindrata ea una certa pressione. L'efficienza totale della pompa idraulica ha una grande influenza sull'efficienza del sistema idraulico. Pertanto, è tabù scegliere una pompa idraulica a bassa efficienza. Dovresti scegliere una pompa idraulica ad alta efficienza e cercare di far funzionare la pompa idraulica in un'area di condizioni di lavoro efficiente.
• Consider the problem from the perspective of improving energy conversion efficiency. Don’t just consider the rated working conditions, but should make the efficient working range as wide as possible. When choosing a hydraulic pump, do not reduce the specification of a large-capacity pump. This is because when the pump’s working pressure and working flow are less than its rated value, the overall efficiency of the pump will decrease. Generally speaking, the volumetric efficiency of the hydraulic pump is more important than the mechanical efficiency. Because the hydraulic pump can obtain power from the prime mover to compensate for the mechanical friction loss, but the leakage loss in the hydraulic pump cannot be compensated by increasing the power input. Of course, the rotational speed of the hydraulic pump should not be too high or too low, otherwise, its mechanical efficiency will decrease. It is generally believed that when the hydraulic pump works in the range of 1000~1800r/min, it has little effect on the overall efficiency. Hydraulic Pump Parameter
• In the case of using an engine to drive the hydraulic pump, low speed is prohibited when the oil temperature is low, otherwise, it will be difficult to absorb oil, and there will be a risk of seizing failure due to poor lubrication; at high speeds, cavitation, vibration, abnormal wear, and flow must be considered The possibility of instability and other phenomena, to avoid drastic changes in the speed of the hydraulic pump from having a huge impact on the strength of the internal parts of the hydraulic pump. Hydraulic Pump Parameter

Domanda 7: Calcola la potenza motrice della pompa idraulica

Se la pressione e la portata della pompa sono relativamente costanti durante il ciclo di lavoro (ovvero, la curva pt e la curva qt sul diagramma delle condizioni di lavoro cambiano in modo più fluido), la potenza di azionamento della pompa idraulica P_P Should be calculated as follows Hydraulic Pump Parameter

P_PP_PQ_P/η_P

Domanda 8: La scelta del motore

• Nella scelta del motore elettrico della pompa idraulica si deve considerare anche il fattore di influenza della velocità. La velocità sincrona del motore elettrico non può essere superiore alla velocità nominale della pompa idraulica.
• Hydraulic pumps of fixed equipment are usually driven by electric motors. According to the power calculated above and the rotational speed of the hydraulic pump and its use environment, select its model specifications from the product catalog or manual (rated power, rotational speed, power supply, structure (vertical, horizontal, open, closed, etc.) ], and calculate the overload capacity to ensure that the peak overload of the motor is less than 25%~50% in each working stage. Hydraulic Pump Parameter
• Since the hydraulic pump usually starts under no load, there is no excessively high requirement on the starting torque of the motor, the load changes relatively smoothly, and the number of starts is small, so a cage-type three-phase asynchronous motor can be used. But if the power of the hydraulic system is large and the grid capacity is not large, a wound rotor motor can be used. For hydraulic pumps that adopt variable frequency flow adjustment schemes, AC asynchronous motors controlled by variable frequency speed regulation or electromagnetic speed regulation should be used to drive the hydraulic pump. Hydraulic Pump Parameter
• Motors of the same type with the same capacity (power) usually have different speeds for selection. Low-speed motors have a large number of magnetic poles, large dimensions and weights, high prices, and require pumps with a large displacement (under a certain flow rate), while high-speed motors are the opposite. Therefore, the speed of the motor should be considered comprehensively with the flow and displacement of the pump. Hydraulic Pump Parameter
• The working environment of the hydraulic pump is different, and the protection form of its drive motor is required to be different: an open motor (protection mark is IP11) should be used in a clean and dry environment; a protective motor should be used in a cleaner and clean environment (protection mark is IP22 and IP23) drive; humid, dusty, high temperature, corrosive or weather-prone environments should be driven by enclosed motors (protection mark IP44); explosion-proof motors should be used in explosive environments. Hydraulic Pump Parameter

Domanda 9: La scelta del motore a combustione interna

La maggior parte dei sistemi idraulici delle macchine mobili utilizza motori a combustione interna per azionare le pompe idrauliche.

When the hydraulic pump is driven by an internal combustion engine, in one case, the hydraulic pump is only a part of the internal combustion engine’s drive load; in another case, the entire power of the internal combustion engine is used to drive the hydraulic pump. In the former, the power of the internal combustion engine is large, which can always meet the power required by the hydraulic pump. The speed of the internal combustion engine should match the optimal speed of the hydraulic pump.

High-speed internal combustion engines usually have a deceleration device to make the hydraulic pump work within the optimal speed range. In the latter, the system in which the entire power of the internal combustion engine is used to drive the hydraulic pump is called a full hydraulic drive system. The full hydraulic drive system usually adopts a variable pump or variable motor volumetric speed control system to meet the requirements of large changes in the speed of mobile machinery.

The maximum speed of the internal combustion engine should meet the maximum flow required by the system and not exceed the maximum allowable speed of the hydraulic pump. If the engine speed is too high, a deceleration device should be installed. The maximum power of the internal combustion engine should be slightly greater than the maximum power required by the hydraulic system. Hydraulic Pump Parameter

Progettazione del circuito della pompa idraulica

Domanda 1: Progettazione del sistema chiuso

A closed hydraulic system that uses a manual servo variable pump to control the commutation of the actuator should avoid self-priming and oil replenishment. As shown in Figure 1-1, in the process of using a manual servo variable pump to control the commutation of the actuator, the variable mechanism of the servo variable pump must pass the zero position (that is, the position when the variable pump has no flow output). The variable mechanism of the pump is set to zero.

From the variable principle of the servo variable pump, it is known that pressure oil must be available to realize the variable. However, when the variable mechanism is in the zero position, no pressure oil can be generated when the self-priming oil is used. Hydraulic Pump Parameter

Note that corresponding measures should be taken to form a closed hydraulic system with a variable pump and a single-rod hydraulic cylinder. In Figure 1-2, the variable pump and the single-rod hydraulic cylinder form a closed hydraulic system. When the piston rod is extended, there will be no problem. When the piston rod is retracted, the hydraulic cylinder discharges more oil than it enters. The amount is much larger. If no measures are taken, the hydraulic cylinder cannot move at this time. Hydraulic Pump Parameter

When the hydraulic motor is operating as a hydraulic pump, avoid the main pump from being subjected to working pressure difference. Figure 1-3 In a closed system composed of a variable pump and a hydraulic motor, when the variable mechanism of the variable pump is set to zero, the motor operates as a meter under the action of external power. Adjusting the overflow valve can control the motor to operate as a pump.

The working pressure difference also acts on the main pump, but at this time, the variable mechanism of the main pump is set to zero as a stop valve. However, the high pressure cavity of the main pump has to bear high-pressure, and the relevant parts of the high-pressure cavity are equivalent to bear static high pressure, which is very disadvantageous. In addition, when the variable mechanism of the main pump deviates from zero due to vibration and other reasons, the system pressure will suddenly drop and cause an accident. Hydraulic Pump Parameter

When designing the pump-motor closed system, pay attention to the situation of heavy objects falling at the moment of driving. As shown in Figure 1-3, the heavy lifting closed system is composed of a hydraulic pump and hydraulic motor. When the brake of the motor is released, the heavy object forms an excess torque to drive the hydraulic motor to rotate, so that the hydraulic motor is in working condition of the hydraulic pump.

With the increase of the falling speed of the weight, once the torque of the pressure oil at the motor outlet is not enough to balance with the load torque of the weight acting on the hydraulic motor, the falling speed of the weight will be out of control, causing a crash accident.

Because the pressure built up by the heavy object causes oil compression and oil leakage, which are the main reasons for the sliding of the heavy object at the moment of driving, a one-way valve can be installed in the high-pressure pipeline of the system close to the hydraulic motor to connect the hydraulic motor with other The components are separated,

and the pressure signal of the heavy object can also be used to control the variable mechanism of the hydraulic pump so that the outlet pressure of the hydraulic pump automatically tracks the pressure established by the weight at the motor outlet so that the sliding speed of the heavy object can be significantly reduced at the moment of driving.

The sliding distance is greatly reduced, and the working reliability of the hydraulic system is also improved. Hydraulic Pump Parameter

The variable pump determines the amount of oil supplement for the closed system of the main pump. In a closed system where the variable pump is the main pump, it is inevitable to supplement the oil. However, only the actual flow rate of the variable pump in the system is used as the basis for determining the replenishment, which sometimes results in insufficient replenishment. If the rated speed of the variable pump is 1500r/min, and the speed of the variable pump in the system is only 1000r/min,

or the actual maximum flow of the system variable pump is only 90% of the rated flow of the pump, etc., insufficient oil supplement will occur. Hidden dangers. Of course, when determining the replenishment amount of a closed hydraulic system, the leakage caused by pipelines, valves, and actuators still needs to be considered. Hydraulic Pump Parameter

The oil in the replenishment circuit should not be returned to the oil without cooling. If the oil in the replenishing circuit uses uncooled return oil, the temperature of the system oil will rise, which will adversely affect the system. If the leakage in the components and the system increases, the sealing device rapidly ages and deteriorates, loses the sealing performance, accelerates the oxidation of the oil, causes the oil to deteriorate, and reduces the service life of the oil. Therefore, the oil in the replenishment circuit should not be used. Cooled oil return. Hydraulic Pump Parameter

Domanda 2: tabù sulla progettazione del circuito della pompa idraulica

The built-in overflow valve of the hydraulic pump is not suitable for system pressure adjustment, but should only be used as a safety valve. A simple, direct-acting relief valve is installed in some types of hydraulic pumps, and the valve has a small diameter. It is generally not suitable for system pressure adjustment, but only as a safety valve. The structure of the relief valve and the safety valve are similar,

but the diameters of the two types of valves are different, and the purpose of the use is also different. The relief valve used for system pressure regulation requires a large valve diameter, which is mainly used to stabilize the system pressure, and the flow rate varies. The safety valve is mainly used to protect the system pressure from overloading.

Usually, the diameter is small and the flow rate is small, and the system pressure cannot be returned to normal. Therefore, the built-in relief valve of the hydraulic pump should not be used for system pressure adjustment, but should only be used as a safety valve. Hydraulic Pump Parameter

The constant power speed control circuit should not be used alone. In Figure 1-5, the constant power pump and the variable motor form a so-called constant power speed control loop. Generally, the speed control loop is required to have greater speed rigidity. But the speed rigidity of this circuit is proportional to the square of the motor displacement, but when the displacement of the variable motor is reduced, the speed rigidity will drop sharply, and the torque that can be output is also reduced, so there are few such circuits Use alone. Hydraulic Pump Parameter

Excessive pressure loss in the pipeline causes the output flow of the constant power variable pump to fail to meet the expected requirements. As shown in Figure 1-6, the outlet of the constant power variable pump is connected to the hydraulic cylinder through a one-way valve and a reversing valve, and the return oil of the hydraulic cylinder also needs to flow to the oil tank through the corresponding valve. Because some valve specifications are selected too small, the pressure loss is too large. As a result, the output flow of the pump is much smaller than expected, which fails to meet the requirements of the hydraulic cylinder to move quickly. Hydraulic Pump Parameter

When multiple actuators adapt throttling speed regulation, avoid using only one hydraulic pump for oil supply. When there are multiple actuators in the system, each agency cannot always maintain the coordination of flow and pressure requirements. However, the use of a throttling speed control circuit inevitably has overflow loss and throttling loss. At this time, if only one pump is used to supply oil, the pump must be selected based on the highest pressure and flow required by the system, which cannot adapt to the low-pressure small flow and high-pressure large flow conditions, resulting in low efficiency of the system, as shown in Figure 1- 7 shown. Hydraulic Pump Parameter

The diameter of the stop valve at the suction port of the hydraulic pump should not be too small. To facilitate maintenance, a shut-off valve is generally installed at the inlet of the hydraulic pump, which will increase the flow resistance of the pump. Therefore, the diameter of the shut-off valve must be large enough to prevent the vacuum of the hydraulic pump from being too high. The shut-off valve should have sufficient pressure resistance. When selecting the shut-off valve on the high-pressure pipeline, in addition to the maximum working pressure of the system, the instantaneous pressure change of the system should also be estimated to prevent damage. Hydraulic Pump Parameter

The speed of the hydraulic pump should not be too high. The hydraulic pump has a rated speed, and it is not allowed to rotate too high during use, to avoid problems such as insufficient oil suction, increased wear, and reduced life of the hydraulic pump. Generally, the maximum speed of gear pumps and vane pumps is 2000r/min, while plunger pumps should work at a speed of 1000r/min, which should not exceed 1500r/min. Hydraulic Pump Parameter

Domanda 3: problema di autoadescamento

When used in an open hydraulic circuit, the hydraulic pump needs to have a certain self-priming capacity. The occurrence of cavitation may not only damage the hydraulic pump, but also cause vibration and noise, which will make the control valve and executive components operate poorly, and have a bad influence on the entire hydraulic system. Hydraulic Pump Parameter

Tabù per garantire la capacità di autoadescamento

• Pur confermando la capacità di autoadescamento della pompa idraulica utilizzata, è necessario determinare e progettare la posizione di installazione della pompa idraulica rispetto al livello del serbatoio dell'olio in base al calcolo della resistenza della linea di aspirazione in considerazione delle condizioni di temperatura di esercizio di il dispositivo idraulico e la viscosità dell'olio idraulico. Linea di aspirazione. Quando si calcola la capacità di autoadescamento della pompa idraulica, è necessario lasciare un margine sufficiente.
• The distance from the center of the hydraulic pump to the oil surface is not more than 500mm, and the suction pressure should be within -125mmHg, otherwise cavitation is prone to occur, causing damage to parts, noise, vibration, and other failures. Hydraulic Pump Parameter
• Installare un filtro di aspirazione dell'olio da 150 mesh sulla linea di aspirazione. Alcuni produttori di pompe a stantuffo stabiliscono che non è consentito installare un filtro dell'olio sulla linea di aspirazione, ma ciò dovrebbe avere requisiti rigorosi sulla pulizia dell'olio nel serbatoio. Installare il filtro dell'olio sulla pompa idraulica, ma questo dovrebbe avere requisiti rigorosi sulla pulizia dell'olio nel serbatoio dell'olio. Installare un filtro dell'olio della tubazione con una precisione di filtraggio di 25 mesh sul lato di uscita della pompa idraulica.
• Notare che la velocità della pompa idraulica non può essere maggiore della velocità nominale.
• Fare attenzione che il diametro del tubo di aspirazione non sia inferiore al valore consigliato (vedi catalogo prodotti delle dimensioni di installazione), e il tubo di aspirazione sia al massimo un raccordo a gomito.
• If the oil distribution plate needs to reduce the deflection angle of the swash plate when starting, it cannot guarantee self-priming. If the user needs a small flow, he should use the variable mechanism to change the flow after the hydraulic pump is started at full deflection. Hydraulic Pump Parameter
• For hydraulic pumps with poor self-priming, avoid installing filters on their suction pipes. As shown in Figure 1-8(a), if a hydraulic pump with poor self-priming is equipped with a filter at its suction port, as the pressure drop of the filter increases, the minimum suction pressure of the hydraulic pump will not be guaranteed. As a result, the hydraulic pump is insufficient to absorb oil, the volumetric efficiency also drops sharply, and vibration and noise occur until the hydraulic pump is damaged. It should be changed to the structure shown in Figure 1-8 (b). Hydraulic Pump Parameter

Il problema del riflusso e dell'autoadescamento

• Per pompe con portata maggiore di 160L/min, si consiglia di adottare autoadescanti a riflusso.
• Il diametro del tubo di aspirazione non deve essere inferiore al valore consigliato e il diametro della valvola di intercettazione deve essere il doppio del diametro del tubo di aspirazione.
• La lunghezza del tubo di aspirazione della pompa dell'olio è L < 2500 mm, i gomiti del tubo non devono essere più di due, la distanza dal tubo di aspirazione alla parete del serbatoio è H1> 3D, la distanza dal tubo di aspirazione al fondo del serbatoio è H≥3D e D è il diametro del tubo dell'olio.
• Quando il livello dell'olio più basso del serbatoio dell'olio è 300 mm più alto del centro dell'ingresso dell'olio della pompa dell'olio, la pompa idraulica può iniziare ad autoadescarsi con un piccolo angolo di deflessione.

Il problema dell'autoadescamento della pompa dell'olio ad installazione verticale

• La distanza dalla bocca di aspirazione della pompa dell'olio al livello dell'olio più basso non è superiore a 500 mm.
• Il giunto di riempimento dell'olio sul tubo di ritorno dell'olio deve essere più alto della linea di lubrificazione dei cuscinetti della pompa dell'olio (la superficie frontale del coperchio della flangia dell'estremità dell'albero).

Domanda 4: problema di raffreddamento

Versare olio nella pompa idraulica e nell'alloggiamento del motore per il raffreddamento per evitare l'alta pressione nell'alloggiamento. A volte è necessario sfiatare l'olio nella carcassa della pompa idraulica e del motore per raffreddare la pompa idraulica e il motore. Questo è necessario, ma ricorda di non portare la pressione dell'olio nel carter troppo alta, altrimenti causerà la tenuta della pompa idraulica e dell'albero motore Allo stesso tempo, alcune pompe a pistoni aumenteranno la resistenza del pistone in aspirazione camera di estendersi verso l'esterno.

Quando la pompa idraulica funziona spesso a eccentricità zero o la pressione di esercizio del sistema è inferiore a 8 MPa in modo che la perdita della pompa idraulica sia troppo piccola e il corpo della pompa si surriscalda, il circuito dell'olio può essere forzato a circolare e raffreddarsi.