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The differential circuit is a classic design in hydraulic systems. Its purpose is to enable rapid cylinder extension without increasing the hydraulic pump’s flow rate. (Under low-pressure conditions, return oil from the hydraulic cylinder’s rod chamber is directed to the rodless chamber to regenerate flow, fully unleashing motor power.) This improves overall cycle efficiency.
From a cost perspective, to achieve the same rapid extension speed, a non-differential system requires a pump with a higher flow rate. The flow regeneration function of the differential circuit utilizes return oil from the rod chamber to accelerate the rodless chamber, effectively increasing the pump’s output flow rate. Because the pump displacement is smaller than that of a non-differential system, a smaller power motor can be used to achieve the same operating efficiency. Regarding energy consumption, the regenerated flow is not returned to the tank but directly to the rodless chamber to contribute to the work. This process circulates the hydraulic oil internally, minimizing capacity loss.
Of course, differential circuits also have drawbacks. When the hydraulic cylinders are differentially connected, the large and small chambers are connected and pressures are equal. The hydraulic cylinder thrust is determined by the area of the piston rod (as opposed to the area of the piston in a non-differential connection). Therefore, differential circuits are suitable for low-pressure conditions with lower thrust. Furthermore, if the hydraulic cylinder’s differential extension is suddenly switched to normal extension, shock can easily occur.
The following are schematics of several common differential circuits.
This method of using a P-type valve to achieve differential cylinder connection is the simplest. In the neutral position, the large and small cylinder chambers are directly connected and connected to the P port. The pump’s oil output and the return oil from the cylinder small chamber simultaneously enter the large chamber, accelerating the cylinder’s extension. However, this type of directional valve generally has a limited load capacity in the neutral position, resulting in unstable performance and prone to vibration and noise. It is recommended to add a separate two-position, three-way valve to achieve differential connection, as shown in the figure below.
When the 2/3-way solenoid valve is de-energized, the cylinder extends normally, and the oil in the small chamber returns to the tank. When differential extension is required, the 2/3-way valve is energized to connect the large and small chambers of the cylinder, while disconnecting the oil return path from the small chamber. This allows the return oil from the small chamber to enter the rodless chamber to contribute to the work, enabling rapid cylinder extension. Similarly, this 2/3-way valve can be replaced with two 2/2-way solenoid valves.
The following specialized valve manifold achieves differential cylinder connection by automatically controlling the opening and closing of several check valves using pressure.
When talking about differential circuits, we have to talk about the excavator’s “arm retraction and regeneration” technology. For the convenience of analysis, we will analyze the local hydraulic circuit controlled by the multi-valve arm cylinder used in the excavator separately.
The AR valve in the figure below is a differential oil circuit switching valve (set to 10 MPa). The cylinder rod chamber is connected to port Ba1, and HV is a load-holding valve. The rodless chamber is connected to port Aa1. BaR and AaR are overload protection and anti-cavitation valves. P1 is connected to the outlet of pump 1, P2 to the outlet of pump 2, XBa1, XAa1, XBa2, and XAa2 are connected to the pilot control handle in the cab, respectively. R2 is the main return oil channel.
When the handle retracts the boom (extending the cylinder), return oil from the rod chamber flows from Ba1 through the HV load-holding valve to the left return port of the main reversing valve. However, if the boom angle is outside the deadweight angle, gravity acts as a negative load, generating negative pressure on the rodless chamber’s oil inlet. At this point, the rodless chamber’s oil inlet pressure must be below 10 MPa (the AR valve’s control pressure draws oil from the rodless chamber’s oil inlet), preventing the AR valve from opening. Therefore, return oil from the rod chamber must flow through the regenerative check valve in the left position of the main reversing valve spool to the rodless chamber to perform work (accelerating boom retraction), thus achieving the “boom retraction and regeneration” function. When the grab bucket touches the ground and contacts the load, pressure in the rodless chamber increases. When pressure exceeds 10 MPa, the AR valve opens the rod chamber’s oil return channel, allowing discharge oil from the rod chamber to flow directly through the AR valve into the R2 return channel, enhancing boom retraction performance, as shown in the figure below.
Based on the above analysis, by adjusting the switching pressure of the AR valve, the boom can be quickly retracted before contact with the ground, improving operating efficiency. After contact with the ground, the cylinder switches to normal extension to accommodate the increased load. In addition, this “boom retraction regeneration” design can also compensate for cavitation caused by the boom’s own weight accelerating the oil passage in the rodless chamber of the cylinder during the boom’s descent, achieving two goals at once.
