Tempo di lettura stimato: 23 minuti

Hydraulic motor

Question 1: The commonality of hydraulic motors and hydraulic pumps

A motore idraulico is a device that converts the pressure energy of a liquid into mechanical energy. In principle, a hydraulic pump can be used as a hydraulic motor, and a hydraulic motor can also be used as a hydraulic pump. But, although the same type of hydraulic pump and motor are similar in structure, because of the different working conditions of the two, there are some differences in the structure of the two.

• Hydraulic motors generally need forward and reverse rotations, so they should have symmetry in their internal structure, while hydraulic pumps generally rotate in one direction, so there is no such requirement.

• To reduce the oil suction resistance and the radial force, the oil suction port of the hydraulic pump is generally larger than the oil outlet. The pressure in the low-pressure chamber of the hydraulic motor is slightly higher than the atmospheric pressure, so there is no such requirement.

• Hydraulic motors are required to work normally in a wide range of speeds, so sliding bearings or hydrostatic bearings should be used. Because when the motor speed is very low, it is not easy to form a lubricating film if a dynamic pressure bearing is used.

• The vane pump relies on the centrifugal force generated by the high-speed rotation of the vane and the rotor to make the vane always close to the inner surface of the stator to seal the oil and form a working volume. If it is used as a motor, spring must be installed on the root of the blade of the hydraulic motor to ensure that the blade is always close to the inner surface of the stator so that the motor can start normally.

• The hydraulic pump needs to ensure the self-priming ability in structure, but the hydraulic motor does not have this requirement.

• The hydraulic motor must have a large starting torque. The so-called starting torque means that when the motor is started from a stationary state, the torque that can be output on the motor shaft is usually greater than the torque in the running state at the same working pressure difference. Therefore, to make the starting torque as close as possible to the torque in the working state, It is required that the pulsation of the motor torque is small and the internal friction is small.

• Hydraulic pumps that use check valves to distribute oil cannot be used as hydraulic motors at all, as shown in Figure 1-1.

• Due to the above-mentioned different characteristics of motors and hydraulic pumps, many types of hydraulic motors and hydraulic pumps cannot be used in reverse.

Question 2: Selection of hydraulic motor type

1. Rated speed selection

Motors are classified into high-speed and low-speed categories according to their rated speed. Those with a rated speed higher than 500r/min are high-speed motors, and those with a rated speed lower than 500r/min are low-speed hydraulic motors.

The basic forms of high-speed hydraulic motors include gear type, screw type, vane type, and axial plunger-type. Their main features are high speed, low moment of inertia, easy start and braking, and high sensitivity of speed regulation and commutation. Generally, the output torque of a high-speed hydraulic motor is not large (only tens of N·m to a few hundred N·m), so it is also called a high-speed small-torque hydraulic motor.

The basic form of a low-speed hydraulic motor is a radial plunger type, such as a single-acting crankshaft connecting rod type, a hydraulic balanced type, and a multi-acting inner curve type. In addition, there are also low-speed structures in the axial plunger type, vane type, and gear type. The main low-speed hydraulic motor

The main features are large displacement, large volume, and low speed (sometimes up to a few revolutions per minute or even a few tenths of a revolution), so it can be directly connected to the working mechanism without a deceleration device, which greatly simplifies the transmission mechanism. Generally, low-speed hydraulic motors have a relatively large output torque (up to several thousand N·m to tens of thousands of N·m), so they are also called low-speed high-torque hydraulic motors.

2. Structure type selection

Hydraulic motors can also be divided according to their structure types, which can be divided into gear type, vane type, plunger type, and other forms.

• Vane motor. The blade motor has a small volume and a small moment of inertia, so it is sensitive and has a high adaptable commutation frequency. However, the leakage is relatively large, and it cannot work at a very low speed. Therefore, vane motors are generally used on occasions with high speed, low torque, and sensitive actions.
• Axial plunger motor. The structure of the axial piston motor is basically the same as that of the axial piston pump, so its type is the same as that of the axial piston pump, and it is also divided into two types: the straight shaft axial piston motor and the inclined shaft axial piston motor. . Generally speaking, axial piston motors are high-speed motors with small output torque. Therefore, the working mechanism must be driven by a reducer. If the displacement of the hydraulic motor can be significantly increased, the axial piston motor can be made into a low-speed and high-torque motor.
• Swing motor. The working pressure of the single-vane swing motor is less than 10MPa, and the swing angle is less than 280°. Due to the unbalanced radial force, it is difficult to seal between the blade and the housing, the blade, and the stopper, which limits the further increase of its working pressure, which also limits the output. The torque is further improved. For a single blade swing motor, the total efficiency of the blade swing motor = 70% to 95%.
• Double-blade swing motors, under the same conditions of radial size and working pressure, are respectively twice the output torque of single-blade swing motors, but the rotation angle should be reduced accordingly. The swing angle of double-blade swing motors is generally less than 120 °.

The hydraulic motor parameter calculation

1. Displacement

The displacement of a hydraulic motor represents the size of its working chamber, which is an important parameter. Because the torque output of the hydraulic motor during work is determined by the load torque. However, to push the same size of the load, the pressure of a motor with a large working cavity is lower than that of a motor with a small working cavity. Therefore, the size of the working cavity is the main indicator of the working capacity of the hydraulic motor, that is to say, the displacement The size of the hydraulic motor is an important indicator of the working capacity of the hydraulic motor.

2. Mechanical efficiency of hydraulic motor starting

The starting mechanical efficiency of a hydraulic motor is an indicator of its starting performance. Because under the same pressure, the output torque of the hydraulic motor from a standstill to a starting state is greater than the torque in operation, which makes it difficult to start the hydraulic motor with load, so the starting performance is very important for the hydraulic motor Yes, the start-up mechanical efficiency can just reflect the level of its start-up performance.

The reason for the reduction of the starting torque is, on the one hand, that the friction factor is the largest in the static state, and the friction factor is significantly reduced after the relative sliding of the friction surface; on the other hand, it is the most important aspect, because the lubricating oil film in the static state of the hydraulic motor Being squeezed out, it becomes dry friction. Once the motor starts to move, as the lubricating oil film is established, the frictional resistance drops immediately and decreases as the sliding speed increases and the oil film become thicker.

In actual work, it is hoped that the starting performance is better, that is, it is hoped that the starting torque and the starting mechanical efficiency are greater. The multi-acting inner curve motor has the best starting performance, the axial piston motor, the crankshaft connecting rod motor, and the static pressure balance motor are in the middle, the vane motor is inferior, and the gear motor is the worst.

Question 1: Calculation of displacement of hydraulic motor

The displacement of a hydraulic motor represents the size of its working cavity, which is an important parameter because the torque output of the hydraulic motor during work is determined by the load torque. However, to push the same size of the load, the pressure of a motor with a large working cavity is lower than that of a motor with a small working cavity, so the size of the working cavity is the main indicator of the working capacity of the hydraulic motor, that is, the size of the displacement It is an important symbol of the working ability of hydraulic motors.

V_m=2πT_max/(Δpη_mm)

△P=P₁一P₂

Hydraulic motor use

Question 1: The hydraulic motor starts

1. The viscosity requirements of the hydraulic medium when the hydraulic motor is started

When starting the hydraulic motor, if the viscosity of the medium is too low or too high, the lubrication performance of the motor will be affected. If the viscosity is too high, some parts will not be effectively lubricated; if the viscosity is too low, the lubrication performance of the entire motor will be poor. Therefore, try to avoid starting the motor when the viscosity of the hydraulic oil is abnormal.

2. When the plunger motor is used for the first time, the housing should be filled with oil

Although hydraulic motors have the characteristics of self-lubricating, it is best to fill the housing with oil in advance when using plunger hydraulic motors for the first time. This is good for protecting the hydraulic motor and ensuring a smooth start. Otherwise, the rotor of the plunger motor may be in a state of dry friction and easily damaged.

3. Be careful not to overload the hydraulic motor that starts with load

In applications requiring full-load starting, attention should be paid to the starting torque of the hydraulic motor. Because the starting torque of hydraulic motors is common

It is shorter than the rated torque, so if the starting torque value is ignored, the working mechanism may not start.

Question 2: System shock

When designing hydraulic systems and selecting hydraulic components, the following two issues should be considered.

• For some functions with greater impact, such as the vibration function of a road roller, a hydraulic motor is used. The vibration force requirements, the relatively large moment of inertia of the vibration eccentric shaft, coupled with the viscosity of the lubricating oil in the vibration chamber, so the vibration motor’s starting shock is particularly large, and in the vibration transmission system, the most vulnerable component is the vibration motor Spindle.
• For the hydraulic plunger pump motor itself, if its displacement is relatively large, the weight of its rotating body is relatively large. The moment of inertia is relatively large, and the starting speed of the engine is generally 600~700r/min. For example, the spline pitch diameter of the spindle is 50mm, the linear velocity at which the spindle is impacted is 1.57~1.83m/s. The rotating body is also affected by the valve plate. Because of the friction force of the thrust plate, the main shaft of a large displacement plunger pump has the problems of easy gear removal and shaft breakage. Because of the above two situations, for some impactful functions, pay attention to increasing safety when selecting hydraulic motors. Factor, increase safety measures, the recommended measures are as follows.
• Increase the elastic coupling. Because the elastic coupling will absorb a certain impact, prolong the impact time and reduce the impact on the shaft. The impact force (strength) can effectively reduce the probability of shaft wear and fatigue fracture, and the cost of the elastic coupling is higher than that of the motor. The main shaft is much lower, and the man-hour to replace the coupling is much less than that of the motor main shaft, which can greatly reduce the loss of lost work.
• The design uses a spline sleeve connection. If the elastic coupling is not designed, use the spline sleeve connection according to the conventional design. The hardness and rigidity of the motor spindle must be higher than that of the spline sleeve so that the low-cost spline sleeve can be used as a vulnerable part. When mandatory scrapping is required
• It is limited and replaced regularly, which can ensure the trouble-free operation of the hydraulic motor for a long time, save the cost, and do not delay the construction time.
• In addition, the matching accuracy of the design spline sleeve and the spindle must be strictly regulated, and the coordination between the spline sleeve and the gear pair of the spindle must be strictly regulated.
• The smaller the gap, the better. The smaller the fit clearance, the smaller the idle stroke of the motor before the force is applied, the smaller the impact of the motor spindle on the tooth surface of the spline sleeve, and the longer the life of the two tooth surfaces; on the contrary, if the fit-gap is large, after each time The impact of starting the motor will continuously increase the gap between the shaft and the sleeve, which will increase the idling stroke of the spindle, and the linear velocity when reaching the tooth surface of the spline sleeve will become larger and larger. Phenomenon such as teeth.
• According to experience, the phenomenon of spline sleeve de-toothing has occurred, but the cost of replacing the spindle is relatively high. The cost difference is as follows.

1. The price of the main shaft of the imported hydraulic motor is very high, while the price of the spline sleeve is lower.

2. The replacement of imported hydraulic motors requires professionals, and the maintenance cost is high; while replacing the spline sleeves, most of the users themselves. It can be completed and the cost is low.

3. The city where the construction site is located does not–there must be a professional maintenance company, transportation, and travel expenses also increase the cost; and replacement costs. Most of the key sets can be completed by themselves, and this part of the cost is not required.

4. It takes a long time to replace the main shaft, and the main machine loses a lot of lost work, and the replacement of the spline sleeve is shorter.

• Use a larger motor. If the power of the engine and the production cost of the main engine allows, a larger specification can be used. Motors with larger specifications have larger design torques and greater overall strength, so the safety factor is increased, and the probability of damage is reduced.
• Swing hydraulic motors should consider the problem of absorption of hydraulic shock. When the direction of movement of the hydraulically driven swing load changes sharply, high pressure will be generated in the two cavities of the motor’s inlet and outlet ports, although. The design of the swing motor has considered this pressure, but when the impact pressure is too large, a high-sensitivity relief valve must be installed near the oil inlet and outlet ports of the swing motor to avoid damage to the swing motor, as shown in Figure 1-2.
• It is not suitable for air to enter the hydraulic motor. The gas in the hydraulic system is generally left in the system during the installation of the system, and the failure of the seal is also the reason for the air entering the system. When the hydraulic system is initially working, it will inevitably contain air in the pipeline of the system. An important part of system debugging is to exhaust the air in the system, which is especially important for hydraulic motors. The hydraulic medium in the motor has a sudden change from high pressure to low pressure. The frequency of this process is very high, about 10 times per revolution. When the hydraulic oil entering the motor contains air, it will locally produce steam at the sudden pressure change. Undesirable consequences such as corrosion and impact will quickly damage the motor. Residual gas in the servo control system will seriously degrade the dynamic characteristics of the system and even cause the system to lose stability. Therefore, after the system is installed, the residual air should be completely removed before the formal work. If necessary, an exhaust valve can be installed in the system.

Question 3: Hydraulic motor speed limit

1. Minimum stable speed limit

The minimum stable speed refers to the hydraulic motor in the rated load, which does not appear to crawl the phenomenon of the minimum speed. The so-called crawling phenomenon is when the hydraulic motor working speed is too low, often can not maintain a uniform speed, into an unstable state when moving and stopping.

The hydraulic motor in the low-speed crawling phenomenon for the following reasons.

• The size of the friction force is not stable. The usual friction is increased with the speed, and the stationary and low-speed region of work. The internal frictional resistance of the motor, when the work speed increases instead of increasing, but reduces, the formation of the so-called “negative characteristics”. The so-called “negative characteristic” of resistance. On the other hand, the hydraulic motor and the load is driven by the hydraulic oil are compressed and the pressure rises.
• Leakage size is not stable. The leakage volume of the hydraulic motor is not the same for each instance, it also fluctuates periodically with the rotor rotation of the phase angle change. Due to the low speed in the motor flow is small, the proportion of leakage increased, the instability of the leakage volume will affect the work of the motor involved in the flow value, thus causing fluctuations in speed. When the motor is running at low speed, its rotating part, and the load it carries show less inertia, the above influence is more obvious, thus the phenomenon of crawling. Therefore can not make the hydraulic motor running speed is too low, the hydraulic motor label on a generally marked with the motor’s minimum stable speed should try to use higher than the speed.

2.The highest speed limit

The maximum speed of the hydraulic motor is mainly subject to the service life and mechanical efficiency of the limit, speed increase, the wear and tear of the motion of the vice intensify, service life is reduced, high speed, the hydraulic motor needs to be fed into the flow is large, so the overflow part of the flow rate increases accordingly, the pressure loss also increases, thus reducing the mechanical efficiency. For some hydraulic motors, the speed is also limited by the backpressure. For example, the crankshaft linkage hydraulic motor, speed increase, the return oil back pressure must be significantly increased to ensure that the connecting rod will not hit the surface of the crankshaft, thus avoiding the impact phenomenon. As the speed increases, the backpressure required in the oil return chamber should also be increased. However, an excessive increase in back pressure will cause the efficiency of the hydraulic motor to drop significantly. To make the motor’s efficiency not too low, the motor’s speed should not be too high.

3. In high-power hydraulic motor should not use throttle speed regulation

The efficiency of the throttling speed is very low when the power of the hydraulic motor is large, if the throttling speed, the system efficiency will be very low, the heat generated will be very large, the system temperature rises faster, is not conducive to the normal work of the hydraulic system. Therefore, the high-power hydraulic motor system if there are speed regulation requirements, should use the volume speed regulation.

4. Walking or lifting drive with a hydraulic motor must be added to the speed limit valve

Drive walking machinery or lifting machinery with the hydraulic motor in the hydraulic circuit must be set up to play a role in limiting the speed of the hydraulic components, to avoid walking machinery downhill or lifting equipment lifting the weight of the rapid fall of the speed out of control, over speed, thus causing serious accidents.

Question 4: Hydraulic motor connection

1. Drain port connection

• Although from the general concept, all the return pressure is not high (close to atmospheric pressure), but many hydraulic systems in the return oil still have a certain pressure, and the hydraulic motor drainage cavity does not allow pressure (hydraulic motor drainage mouth of the internal is connected to the shell cavity, the motor shaft seal only play a sealing role, not pressure resistance. If this port is connected with other return lines, it is easy to cause damage to the motor shaft seal, resulting in oil leakage). Therefore, it is not allowed to connect the drain port of the hydraulic motor and other oil return lines, as shown in Figure 1-3.

• The installation orientation of the motor drain pipe should be upwards. The hydraulic motor drain should be separated back to the oil tank, as far as possible installed in the highest part of the shell, the highest point of the drain pipe installation should be higher than the shell, so that the inside of the shell is full of oil, to ensure that the main bearing and internal movement mechanism to obtain good lubrication, drain pipe leakage oil back to the oil tank should be smooth, as shown in Figure 1-4.

2. Return port connection

The return oil of the crank linkage hydraulic motor should not be directed back to the oil tank. Crank linkage hydraulic motor in the high speed, the rod will sometimes be close to the crankshaft surface, and sometimes out of the crankshaft surface, the phenomenon of impact. The multi-action internal curve hydraulic motor to do the return movement of the plunger and roller is also due to the role of inertia force from the surface of the rail. To ensure that no collision and the phenomenon of debouching, it is necessary to make the return oil of the hydraulic motor have certain backpressure. Therefore, the return oil of this kind of motor should not be directed back to the oil tank.

3. Output shaft and load connection

• The hydraulic motor shaft end can not bear the radial force, due to the hydraulic motor support bearing resistance to radial force is very weak, so the hydraulic motor output shaft can only bear the torque. If the motor output bearing is subjected to radial force in use, the bearing of the hydraulic motor may be damaged in a short time, resulting in the scrapping of the whole hydraulic motor. For cases where radial forces are required, it is recommended that a support bearing be used, as shown in Figure 1-5.

• The oscillating hydraulic motor can not bear axial and radial loads. Ordinary oscillating hydraulic motors are not allowed to carry axial or radial loads when used at rated pressure. In lower than the rated pressure when using, can be to a certain extent allowed to add axial load. But in principle, whether the axial load or radial load, should be other bearings to bear, as shown in Figure 1-6.
• Hydraulic motor output shaft and the load must ensure. The hydraulic motor in and load connection must ensure that there is sufficient, in general, the deviation can not be greater than 0.1mm. if the deviation is too large, will make the hydraulic motor bearing by the resulting periodic radial load, the bearing will soon fail, causing motor failure. The motor should be installed in the frame with sufficient rigidity. Install the motor bracket, the seat is required to have sufficient rigidity, to withstand the motor output torque when the role to its counterforce. Such as the installation of motor frame stiffness is not enough, will produce vibration or deformation or even accidents, can not guarantee the drive machine and motor shaft connection between the concentricity of control in 0.1mm requirements.
• The hydraulic motor in the drive large inertia load should be set up when the safety valve. The hydraulic motor in the drive of large inertia load, can not simply use the method of closing the reversing valve to make it stop. When the hydraulic motor is driven by the inertia load rotation, the hydraulic motor working state from the hydraulic motor into a hydraulic pump. If the method of closing the reversing valve to stop, will lead to a sudden and substantial increase in pressure on the original return line, and serious cases may be the weak link in the pipe impact damage or make the hydraulic motor parts fracture failure. For this reason, a suitable safety valve should be set on the motor’s return line to ensure the normal operation of the hydraulic system.

Question 5: Multi-hydraulic motor circuit design

Two sets of closed hydraulic systems in parallel, due to improper design leads to two actuators’ power difference is too large.

As shown in Figure 1-7(a), the hydraulic system for the mixer drive. The system uses two sets of the parallel closed hydraulic circuit, there are two hydraulic motors from both sides to drive the rigid rotor. Due to improper design, the left and right side of the motor drive power is too large, and even appear to do a negative work phenomenon. The reason for this phenomenon is the poor design of the circuit and the unreasonable configuration of the piping structure, coupled with the inconsistent displacement and leakage coefficient of the left and right motors, resulting in asymmetrical pressure and asymmetrical and uneven power flow on both sides. Therefore, the original machine on the two connector blocks will be canceled, design a double “X” shaped connecting connector block, as shown in Figure 1-7 (b), you can solve the above problem.

The output shafts of several hydraulic motors are mechanically connected to achieve synchronization with a charging circuit.

The output shafts of several hydraulic motors are mechanically connected to achieve drive. Generally, it is to drive several hydraulic cylinders to work synchronously, in this kind of synchronous circuit, each motor must be set up with a suitable charge circuit. However, it is not possible to ensure that the volumetric efficiency of each cylinder is identical, and this will inevitably lead to an unsynchronised position after a few cycles. In addition, the volumetric efficiency of the hydraulic motors cannot be identical either. It is, therefore, necessary to set up an end-of-cycle replenishment circuit, as shown in Figure 1-8.

Question 6: Leakage of hydraulic motor

A brake must be used for hydraulic motors that remain braking for a long time

Since hydraulic motors always leak, it is not reliable to close the inlet and outlet ports of a hydraulic motor to achieve a braking condition. A hydraulic motor with the inlet and outlet ports closed will still slip slightly, so when the braking state needs to be maintained for a long time, a separate brake must be set to prevent rotation, as shown in Figure 1-9.

The flow in the closed motor circuit must be reasonable

Theoretically speaking, there is no oil circuit in the closed motor circuit, the pump, and motor flow as long as equal, as shown in Figure 1-10, but in practice, the flow required by the motor must not be greater than or equal to the pump flow. The reason for this is that the pump and motor in a hydraulic system work with varying volumes, and leakage is inevitable in this process, and will become more and more serious as the working time goes on. Therefore, if the flow rates of the pump and motor in a closed motor circuit are equal, the motor can’t obtain the required output power.