Foreword
The design, debugging, and maintenance of machine tool lubrication systems play a vital role in improving machining accuracy and extending machine tool lifespan. However, the following problems still exist in the electrical control of lubrication systems: First, the monitoring of the lubrication system's operating status. CNC machine tool control systems generally only monitor the oil level in the tank to prevent insufficient oil supply, but cannot promptly respond to common lubrication system issues such as oil leaks and oil circuit blockages. Second, the simplistic lubrication cycles and oil supply times lead to waste. The required lubrication dosage varies depending on the CNC machine tool's operating state; for example, less lubricating oil is needed during machine pauses compared to machining phases.
The lubrication system plays a crucial role in the overall machine tool system. Its design, debugging, and maintenance are vital for improving machining accuracy and extending the machine tool's service life. Modern machine tool guideways, lead screws, and other sliding pairs are primarily lubricated using centralized lubrication systems. A centralized lubrication system uses a hydraulic pump to supply lubricating oil at a specific volume and pressure to distributors in all main and secondary oil circuits. The distributors then distribute the oil to each lubrication point according to the required amount. Simultaneously, a controller monitors lubrication time and frequency, issues fault alarms, and initiates machine shutdown functions to achieve automatic lubrication. The characteristics of a centralized lubrication system are timed, quantitative, accurate, and efficient operation; it is convenient and reliable, contributing to increased machine life and guaranteed performance.
1. Introduction to the Lubrication System
Centralized lubrication systems can be classified into damping lubrication systems, progressive lubrication systems, and volumetric lubrication systems based on the lubrication elements used.
1.1 Single-line damping lubrication system
This system is suitable for machine tool applications where the lubrication points require relatively low oil volumes and periodic oil supply. It utilizes a damping distributor to distribute the pumped oil to the lubrication points in a specific ratio. It is generally used in circulating systems but can also be used in open systems. The oil volume at each lubrication point can be controlled by time. This lubrication system is very flexible, allowing for the addition or removal of one lubrication point, and can be installed by the user. Furthermore, if one point becomes blocked, it does not affect the operation of other points, making it widely applicable.
1.2 Progressive Lubrication System
The progressive lubrication system mainly consists of a pump station and a progressive plate distributor, and can be equipped with a control device for monitoring. Its features include the ability to alarm and terminate operation upon blockage at any lubrication point to protect the equipment; accurate metering and high pressure; and suitability for both thin oil and grease lubrication. It can handle up to 100 lubrication points and pressures up to 21 MPa.
A progressive lubrication system consists of a base plate, an end plate, and at least three intermediate plates. A single valve set can have up to eight intermediate plates, lubricating 18 points. Its working principle involves plungers in the intermediate plates sequentially actuating from a certain position to supply oil. If a blockage occurs at any point, the next outlet will not actuate, thus stopping the entire lubrication system. A blockage indicator shows the location of the blockage, facilitating maintenance. Figure 1 illustrates the progressive lubrication system.
Figure 1 Progressive lubrication system
1.3 Volumetric Lubrication System
This system uses a metering valve as a distributor to supply oil to lubrication points. A pressure relay is included in the system so that when the system oil pressure reaches a predetermined value, a signal is sent to delay and stop the motor. Lubricating oil is then supplied from the metering distributor. The system is unloaded through a directional valve and maintains a minimum pressure, allowing the metering valve distributor to replenish lubricating oil. The motor restarts, and this process repeats until the specified lubrication time is reached. This system typically operates at pressures below 50 MPa and can handle hundreds of lubrication points. It has a wide range of applications and reliable performance, but it cannot be used as a continuous lubrication system.
The structural principle of the metering valve is as follows: it consists of two oil chambers, an upper and a lower one. Under high pressure in the system, the oil is pumped to the lubrication point. Under low pressure, the oil stored in the lower chamber is forced into the oil discharge chamber located in the upper chamber by its own spring reset and cup-shaped seal. The discharge volume is 0.1~1.6mL and can be combined according to actual needs.
2. Control principle of lubrication system
The control of the machine tool lubrication system is divided into two parts: electrical control and PLC automatic control.
2.1 Electrical Control Principles
The electrical control diagram of the lubrication system is shown in Figure 2. The main power supply of the lubrication motor is controlled by the AC contactor KM1. Automatic control is achieved through PLC control.
Figure 2 Electrical control schematic diagram of the lubrication system
2.2 Automatic Control Principle
Figure 3 shows the automatic control flowchart of the lubrication system. Once the system is ready, the CNC sends a signal to start the lubrication system. After the initial lubrication lasts for 15 seconds, the motor stops working.
Figure 3. Flowchart of automatic control for the lubrication system
When pressure switch SP2 is activated due to pressure reduction, a 25-minute timer begins. After the timer completes, pressure switch SP2 deactivates, and the lubrication motor resumes operation for 15 seconds, cycling through the circuit. QF4 is the lubrication motor overload protection switch, and SL is the lubrication oil detection switch. When the motor is overloaded or the lubrication oil is insufficient, the system will issue an alarm signal.
3. PLC control of CNC machine tool lubrication system
In CNC systems, most automatic control is achieved through PLCs, and the control of lubrication systems is no exception, with the motor's operation controlled by a PLC program. This section uses the FX-24M as an example to illustrate the PLC control of a lubrication system.
3.1 Lubrication PLC Control Principle
As shown in Figure 3, the lubrication system first uses two interacting time relays (T) to achieve intermittent operation during lubrication. Then, the PLC prepares a signal, setting M8001 to 1 to initiate the initial lubrication. After the initial lubrication, when the SP switch closes, intermediate relay M1 activates, restarting the lubrication system. After 15 seconds, time relay T2 activates to 1. When the SP switch opens, intermediate relay M0 activates, causing time relay T3 to operate. After a 25-minute delay, time relay T3 activates to 1, intermediate relay M1 stops operating, and all timers reset. When the SP switch closes again, intermediate relay M1 activates again, thus achieving the cyclic operation of the lubrication system. Alarm functions are then implemented using switches SL and QF4. Manual control of the lubrication system is achieved using X3.
Figure 4. Ladder diagram of PLC control for lubrication system
3.2 Lubrication System I/O Address Allocation
As shown in Figure 5, M8001 is the PLC ready signal, X1 is the SL lubricating oil insufficient detection signal, X2 is the QF4 lubricating motor overload detection signal, and X3 is the manual control jog button. Y1 is the motor overload alarm signal, Y2 is the lubricating oil insufficient alarm signal, Y3 is the lubricating oil pump operation control signal, and Y4 is the lubricating oil operation indicator signal.
Figure 5 External wiring diagram of the lubrication system PLC
4. Processing of lubrication alarm signals
4.1 Abnormal Pressure
The lubrication system in CNC machine tools operates on an intermittent oil supply mode. Therefore, the pressure in the lubrication system is checked periodically, specifically after each use of the lubrication pump. If a malfunction occurs, such as oil leakage, pump failure, or oil circuit blockage, the pressure in the lubrication system will suddenly drop or rise. In this case, the machine tool should be stopped immediately for inspection to prevent the situation from escalating.
4.2 Oil level too low
The conventional approach was to group the "low oil level" signal with the "abnormal pressure" alarm signal as an emergency stop signal. Once the PLC system received this signal, the machine tool would immediately enter an emergency stop state, simultaneously cutting off power to the servo system. However, unlike the "abnormal pressure" situation caused by oil circuit blockage or leakage in the lubrication system, insufficient oil in the lubrication pump tank will not affect the machine tool's performance in the short term, and there is no need to immediately stop the machine tool. However, if this occurs, the control system should promptly display relevant information to remind the operator to add lubricating oil. If the operator fails to add lubricating oil within the specified time, the system will immediately put the machine tool into a pause state. Only after timely replenishment of lubricating oil will the operator be allowed to run the machine tool and resume interrupted work. This approach to the "low oil level" signal can avoid unnecessary downtime and reduce auxiliary processing time, especially when machining large molds. In the design, we categorized the "low oil level" signal as a "feed pause" signal in the electrical control system, adopting an alarm handling method of "reminder – warning – pause, prohibition of automatic operation". If the oil level in the tank is too low, a red indicator light will appear on the control panel, and a warning message will also be displayed on the screen to alert the operator. If the signal does not disappear within a specified time, the machine tool will quickly enter a feed pause state, suspending all automatic operations. After adding sufficient lubricating oil to the tank, the operator can simply press the "cycle start" button to cancel this state and allow the machine tool to resume the machining operation it was in before the pause.
5. Conclusion
In the electrical control design of CNC machine tools, neglecting the lubrication system is crucial for users, as it ensures that all machine components receive regular and measured lubrication. Continuous improvement and refinement of product design are essential to reduce machine tool malfunctions and enhance product reliability.
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