Principle of Throttle Speed Control. The throttle speed control circuit adjusts the flow rate by changing the flow area of the flow control valve to regulate the flow to the actuator, thus adjusting the speed of movement.
If only a throttle valve is present in the control circuit, all the hydraulic oil output by the hydraulic pump flows into the hydraulic cylinder through the throttle valve. Changing the size of the throttle valve orifice only affects the speed at which the oil flows through the throttle valve, while the total flow rate remains constant. In this scenario, the throttle valve cannot regulate the flow rate, and the speed of the hydraulic cylinder will not change.
1) Inlet Throttle Speed Control Circuit
The inlet throttle speed control circuit places the throttle valve in the inlet line of the actuator.
A. Characteristics of the Circuit
Since it is a constant flow pump supplying oil, with a constant flow rate and the relief valve set at pressure pt, and the pump's oil supply pressure at p0, the flow rate q1 entering the hydraulic cylinder is determined by the adjustment of the throttle valve opening area a. The pressure acts on piston A1, overcoming the load F, and driving the piston to move to the right at a speed v=q1/A1. Since the pump supplies oil at a constant rate, q1 is less than qB, so p0=the relief valve set oil supply pressure pt=const.
Equation for piston force balance:
p1 A1= F+p2 A2
Flow rate entering the cylinder:
q1=Ka▽pm
▽p= pb-F/A1
q1=Ka(pb-F/A1)m
B. The speed-load characteristic equation of the inlet throttle speed control circuit is:
where k is the coefficient related to the throttle valve's form, flow state, oil properties, etc.; a is the flow area of the throttle valve; m is the throttle valve orifice exponent (for thin-walled small holes, m=0.5). When F increases and a remains constant, the speed v decreases.
C. Speed-load characteristic curve of the inlet throttle speed control circuit
D. Advantages of the inlet throttle speed control circuit: The pressure in the return chamber and return pipe of the hydraulic cylinder is lower. When using a single-rod piston hydraulic cylinder, allowing the oil to enter the rodless chamber with a larger effective working area, larger thrust and lower speeds can be achieved. This circuit is commonly used in hydraulic systems that require low impact and minimal load variation.
E. Circuit efficiency
η=FV/qBp0
qBp0= p0q1+p0qY
= p1q1+▽p q1+p0qY
p1q1= FV useful power
▽p q1 throttle loss
pbqY—relief loss
Therefore, the efficiency is around 20%.
