Hydraulic Training Series - Chapter 6 - Flow Control Valves

Summary

This video on hydraulic flow and control valves explores key concepts in regulating the speed of hydraulic cylinders and motors. Paul Cook explains how flow control valves, including meter-in, meter-out, and bleed-off circuits, manage flow rate to control speed. The video highlights different valve types, such as needle valves and pressure-compensated flow controls, and their applications in varying workload conditions. It also discusses the impact of temperature on oil viscosity and flow control performance. Practical examples demonstrate how to adjust flow and manage overload protection in hydraulic systems, providing a comprehensive understanding of fluid dynamics in hydraulics.

Takeaways

• 😀 Flow control valves regulate the speed of hydraulic cylinders or motors by controlling the flow rate.
• 😀 A flow control valve is essentially a variable orifice that can adjust the flow of oil into or out of a cylinder, directly impacting its speed.
• 😀 Metering the oil into the cylinder (meter-in circuit) works for opposing loads but cannot prevent runaway loads.
• 😀 Metering the oil out of the cylinder (meter-out circuit) helps prevent runaway loads and offers more reliable speed control.
• 😀 A bleed-off circuit allows excess flow from the pump to be diverted to the tank, saving energy, but is less precise in speed control.
• 😀 Flow control circuits, whether meter-in, meter-out, or bleed-off, require the pump to operate at the relief valve setting.
• 😀 Pressure changes in the workload affect the flow rate through the orifice, potentially reducing speed if not compensated for.
• 😀 A pressure-compensated flow control valve maintains consistent flow rate and speed despite changes in workload pressure by adjusting the pressure difference across the orifice.
• 😀 Bypass-type flow control valves, with a built-in relief valve, are energy efficient because they avoid excess flow going over the relief valve setting.
• 😀 Temperature-compensated flow controls adjust for changes in oil viscosity, maintaining consistent speed despite variations in temperature.
• 😀 Pressure-compensated flow control valves are limited to meter-in applications and cannot function in bleed-off circuits or with overrunning loads.

Q & A

What is the primary purpose of a flow control valve in a hydraulic system?

The primary purpose of a flow control valve in a hydraulic system is to regulate the rate of flow, which controls the speed of hydraulic cylinders or motors.

How does the size of the cylinder affect its speed in a hydraulic system?

The size of the cylinder affects its speed because a larger diameter cylinder holds more oil and takes longer to complete its stroke, while a smaller cylinder moves faster for the same flow rate.

What is the function of a relief valve in a hydraulic system?

A relief valve provides overload protection by directing excess flow to the tank when the system pressure exceeds a preset limit, preventing damage to the system.

What is a meter-in circuit, and when is it typically used?

A meter-in circuit is one where the flow control valve is placed before the cylinder, controlling the flow of oil into the cylinder. It is used for applications with opposing loads, where the load does not cause runaway motion.

Why is a meter-out circuit preferred over a meter-in circuit for certain hydraulic applications?

A meter-out circuit is preferred for applications where the load could cause runaway motion. It controls the exhaust flow from the cylinder and prevents the load from pulling the cylinder faster than the oil supply.

What is the main advantage of a bleed-off circuit?

The main advantage of a bleed-off circuit is energy efficiency. Unlike meter-in or meter-out circuits, it does not require excess oil to flow over the relief valve, but instead bleeds off the necessary amount of flow to control the cylinder speed.

What are the limitations of using a bleed-off circuit?

The limitations of a bleed-off circuit include reduced accuracy in controlling cylinder speed, as it is influenced by changes in pump delivery and leakage rates. It is also less effective where workload pressure fluctuates significantly.

How does a pressure-compensated flow control valve maintain consistent cylinder speed despite workload changes?

A pressure-compensated flow control valve adjusts to changes in workload pressure by maintaining a constant pressure difference across the orifice. It uses a spring-loaded poppet or a sensing line to keep the flow rate consistent, thus ensuring constant speed.

What is the role of a bypass type flow control valve, and how does it differ from other flow control types?

A bypass type flow control valve is used in meter-in circuits and helps save energy by bypassing excess flow directly to the tank, rather than letting it pass through the relief valve. It is designed to maintain a constant pressure difference, ensuring consistent speed regardless of workload pressure changes.

What is the function of the temperature compensating throttle in a pressure-compensated flow control valve?

The temperature compensating throttle adjusts to changes in oil temperature, compensating for changes in viscosity. As the oil temperature increases and the oil becomes thinner, the throttle automatically adjusts to maintain consistent cylinder speed.