Multi-Cylinder Hydraulic Circuits

Sequential Action Circuits

In multi-cylinder hydraulic systems, it is often necessary to actuate cylinders in a specific sequence. Examples include the longitudinal and transverse movements of tool rests in automatic lathes, and the positioning and clamping of workpieces in clamping mechanisms. These sequential operations rely on precise control of hydro transmission fluid flow and pressure to ensure proper functionality.

Sequential action circuits can be categorized into three types based on their control methods: pressure control, stroke control, and time control, with the first two being more commonly used in industrial applications. Each method offers distinct advantages depending on the specific application requirements, but all rely on the proper management of hydro transmission fluid to achieve desired performance.

1. Pressure-Controlled Sequential Action Circuits

Pressure control utilizes changes in the hydraulic circuit's pressure to control the sequence of cylinder operations. This method primarily employs pressure relays and sequence valves as control elements to manage the action order. The precise regulation of hydro transmission fluid pressure is critical in these systems to ensure reliable operation.

Figure 7-37 illustrates a pressure-controlled sequential action circuit using two one-way sequence valves. The one-way sequence valve 6 controls the sequence of advance movements for the two hydraulic cylinders, while the one-way sequence valve 3 controls their retraction sequence. When the directional control valve 2 is in the left position, hydraulic fluid—specifically high-quality hydro transmission fluid—enters the left chamber of hydraulic cylinder 4, with fluid from the right chamber returning through the check valve in one-way sequence valve 3. At this point, due to the relatively low pressure, one-way sequence valve 6 remains closed, causing the piston of hydraulic cylinder 4 to move first.

When the piston of hydraulic cylinder 4 reaches the end of its stroke, the oil pressure increases. When it reaches the preset pressure of one-way sequence valve 6, the valve opens, allowing pressurized hydro transmission fluid to enter the left chamber of cylinder 5, with fluid from the right chamber returning directly to the reservoir. This causes the piston of hydraulic cylinder 5 to move to the right. After cylinder 5's piston reaches its rightmost position, the directional control valve shifts to the right position.

At this point, hydro transmission fluid enters the right chamber of cylinder 5, with fluid from the left chamber returning through the check valve in one-way sequence valve 6, causing cylinder 5's piston to retract to the left. When it reaches the end of its stroke, the oil pressure rises again, opening one-way sequence valve 3 and allowing cylinder 4's piston to retract. The reliability of this sequential action circuit largely depends on the performance of the sequence valves and their pressure settings, as well as the quality and properties of the hydro transmission fluid used.

2. Stroke-Controlled Sequential Action Circuits

Stroke-controlled sequential action circuits use signals generated when a working component reaches a specific position to control the sequence of cylinder operations. This can be achieved using limit switches, stroke valves, or similar position-sensing devices, working in conjunction with precise control of hydro transmission fluid flow.

Figure 7-38 shows a sequential action circuit controlled by limit switches. The operating sequence begins when the start button is pressed, energizing solenoid 1YA, causing the piston of hydraulic cylinder 2 to move to the right. When a cam activates limit switch 4, 1YA de-energizes and 3YA energizes, extending the piston of hydraulic cylinder 5. As cylinder 5's piston reaches its extended limit, it activates limit switch 7, de-energizing 3YA and energizing 2YA, causing cylinder 2's piston to retract.

When cylinder 2's piston returns to the left end, it activates limit switch 3, de-energizing 2YA and energizing 4YA, causing cylinder 5's piston to retract. Upon full retraction, it activates limit switch 6, de-energizing 4YA. This completes the full automatic cycle of sequential actions for the two cylinders. Throughout this cycle, the hydro transmission fluid must maintain consistent viscosity and lubricating properties to ensure smooth operation of all moving parts.

Sequential circuits controlled by electrical limit switches offer significant advantages in terms of convenience when adjusting stroke lengths and modifying action sequences. Additionally, electrical interlocks can be implemented to ensure reliable operation of the sequence. These systems can be easily integrated with modern control systems, allowing for complex sequences while maintaining efficient hydro transmission fluid usage.

Synchronization Circuits

Circuits that maintain identical displacement or speed among two or more hydraulic cylinders during movement are known as synchronization circuits. In systems with a single pump supplying multiple cylinders, even if the effective working areas of the cylinders are equal, several factors can hinder synchronous operation. These include unequal loads, varying frictional resistance, different leakage rates, and manufacturing tolerances. The role of synchronization circuits is to overcome these influences and compensate for flow variations, often through careful management of hydro transmission fluid distribution.

1. Synchronization Circuits with Series Cylinders

Figure 7-39 illustrates a synchronization circuit using series-connected cylinders. In this configuration, the fluid discharged from the return chamber of the first cylinder is directed into the inlet chamber of the second cylinder. If the effective areas of the series-connected cylinder chambers are equal, synchronous movement can be achieved. This circuit allows the two cylinders to handle different loads, but the pump supply pressure must exceed the sum of the working pressures of both cylinders to ensure proper flow of hydro transmission fluid through both cylinders.

Leakage and manufacturing tolerances can affect the synchronization accuracy of series-connected cylinders. After multiple reciprocating cycles, significant desynchronization may occur, requiring compensation measures. To achieve synchronous movement, the effective areas of cylinder 5 and cylinder 7 must be equal. During the downward stroke, if the piston of cylinder 5 reaches the bottom first, it activates limit switch 4, energizing solenoid 1YA. This allows hydro transmission fluid to flow through directional control valve 3 and pilot-operated check valve 6 to replenish the upper chamber of cylinder 7, allowing its piston to continue to the bottom.

If the piston of cylinder 7 reaches the bottom first, it activates limit switch 8, energizing solenoid 2YA. This directs hydro transmission fluid through directional control valve 3 to the control port of the pilot-operated check valve 6, causing it to open in the reverse direction. This allows cylinder 5 to return fluid through the pilot-operated check valve 6 and directional control valve 3, enabling its piston to continue to the bottom. These compensation mechanisms help address desynchronization issues that may arise despite using high-quality hydro transmission fluid.

Non-Interfering Circuits for Multi-Cylinder Systems

In single-pump, multi-cylinder hydraulic systems, the rapid movement of one cylinder can cause a pressure drop in the system, affecting the stability of working feeds in other cylinders. Therefore, in multi-cylinder hydraulic systems requiring stable working feeds, non-interfering circuits for fast and slow speeds must be employed. These circuits ensure that pressure fluctuations in one part of the system do not affect others, maintaining consistent hydro transmission fluid supply to all cylinders.

The circuit shown in Figure 7-42 is designed so that each hydraulic cylinder can perform automatic cycles of rapid advance, working feed, and rapid retraction. The circuit uses a dual-pump supply system where hydraulic pump 1 is a high-pressure, low-flow pump that supplies hydraulic fluid for the working feed of each cylinder. Hydraulic pump 12 is a low-pressure, high-flow pump that delivers low-pressure hydro transmission fluid for the rapid advance or retraction of each cylinder. Their pressures are regulated by relief valves 2 and 11, respectively.

This dual-pump configuration offers significant advantages in multi-cylinder systems. During rapid traverse movements, both pumps can supply fluid to the cylinders simultaneously, providing the high flow rate needed for fast movement while maintaining relatively low pressure. When a cylinder transitions to working feed, it connects only to the high-pressure, low-flow pump, while the low-pressure pump is unloaded through a relief valve or unloading valve. This separation ensures that the rapid movement of one cylinder does not affect the working pressure of others, as each circuit receives appropriately conditioned hydro transmission fluid.

The key to successful operation of these non-interfering circuits lies in proper valve sizing and pressure setting to match the requirements of each cylinder's operation. Additionally, using high-quality hydro transmission fluid with consistent viscosity characteristics across the operating temperature range helps maintain the desired performance. Regular maintenance, including fluid analysis and filtration, is essential to prevent contamination that could affect valve performance and circuit responsiveness.

In conclusion, the design and implementation of multi-cylinder hydraulic circuits require careful consideration of sequence control, synchronization, and non-interference between cylinders. Pressure-controlled and stroke-controlled methods each offer distinct advantages for sequential operations, while synchronization circuits overcome inherent system variations to maintain coordinated movement. Non-interfering circuits ensure stable operation across multiple cylinders with varying speed requirements. Throughout all these systems, the proper selection, maintenance, and management of hydro transmission fluid play a critical role in achieving reliable, efficient, and precise operation.

Engineers must carefully analyze system requirements, including load conditions, speed variations, positioning accuracy, and environmental factors, when selecting the appropriate circuit configuration. Regular maintenance, including monitoring hydro transmission fluid condition, filter replacement, and leak detection, is essential to ensure long-term performance and reliability of multi-cylinder hydraulic systems.