fm hydraulics

8/12/2019 FM Hydraulics

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Con. 4409-frthlf-2013 JS-5083

SEM IV FLUID MECHANICS AND FLUID POWER PROD

(REVISED COURSE) (2008-2013) Model Answer for some questions which are not

available in recommemded books.

Q. Explain cracking pressure and pressure override wrt Pressure Relief valves?

(M.U. 6 times- FM&FP) : 5marks.

Cracking pressure and pressure overrideThe pressure at which a relief valve first

opens to allow fluid to flow through is known as cracking pressure. When the valve is

bypassing its full rated flow, it is in a state offull-flow pressure.The difference between full-

flow and cracking pressure is sometimes known aspressure differential, also known

aspressure override.

In some cases, this pressure override is not objectionable. However, it can be a disadvantage

if it wastes power (because of the fluid lost through the valve before reaching the maximum

setting). This can further permit maximum system pressure to exceed the ratings of other

components. (To minimize override, use a pilot-operated relief valve.)

Relief valves are either direct-acting or pilot-operated.

Q. Explain different types of valves.. : (2 sure Questions ) : (M.U. all papers): (5-20marks)

Pressure control valves used to control force in a hydraulic system.

For accurate control of force, six types of pressure control valves have been develop.

Relief valve


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I will begin presenting information about one of the common pressure control valves namelypressure relief valve.It is one of the most usedhydraulic valves.

A relief valve is a pressure operated valve which by-passes pump delivery to the tank,limiting system pressure to a predetermined maximum value.

The main purpose of apressure relief valve is to protect the hydraulic system fromexcessive pressure.

The valve should not be used to direct excess pressure to the tank, as this may cause thesystem to overheat.

This valve limits the maximum system pressure. It is a normally closed valve whosefunction is to limit the pressure to a specified maximum value by diverting the pump flowback to the tank.

When no flow is available in the hydraulic system due to safety reason, the pressure relief

valve let go of the fluid back to tank to maintain the desired system pressure in the

hydraulic circuit.

It provides protection against any actuators overloads in the hydraulic system.

One important function of a pressure relief valve is limiting the force or torqueproduced by the hydraulic cylinders and motors.

Pressure Reducing Valves

Pressure reducing valves are normally open pressure valves and their functions are toreduce pressure in one or more legs of a hydraulic circuit.

Reduced pressure results in a reduced force being generated. Pressure reducing valves arethe only pressure control valves that are normally open.

A normally open pressure control valve has primary and secondary passages connected.Pressure at the bottom of the spool is sensed from the pilot line which is connected to thesecondary port.

The reducing valves will limit the pressure to the motor, therefore limiting the output torqueof the motor.

Keep in mind that the pressure reducing valves are normally open.
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Counterbalance valve

A counterbalance valve is a normally closed pressure valve and is used in cylinders action to

counter a weight or overrunning load. In a simple circuit of a cylinder application, without a

counterbalance valve the load would fall uncontrolled or overrun, and pump flow would notbe able to keep up.

To avoid the uncontrolled operation, we placea counterbalance valve just after the cylinder.

Counterbalance valves are used to aid a cylinder in lowering a load at a controlled rate.

Counterbalance valves control the gradual buildup of pressure in a hydraulic system.

Brake valve

Brake valve is a normally closed pressure control valve with internal and remote pilotpassages which are connected simultaneously for its operation.

During running of the hydraulic system remote piloting is using system pressure to kept

valve open. Any back pressure on the hydraulic motor that might arise because ofdownstream resistance and subsequent load on the hydraulic motor is eliminated.

Assume the spring of the valve is set at 950psi/65bar that keep the valve spool seated.

When the pressure exceeds 950psi/65 bar the valve actuates and the spool moving off hisseat. The spool area is usually 8 times the area of the piston. With the remote pilotconnected to the opposite hydraulic motor line a pressure of 120psi/8 bars is required toopen the valve.

The brake valve being a normally closed pressure valve does not allow fluid to pass through

in the reverse direction. Thereforea check valveis used to give this reverse flow andbypass the valve.

Sequence valve
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A sequence valve is a normally closed pressure control valve used for ensuring a sequential

operation in a hydraulic circuit, based on pressure. In other words allows one operation totake place before another.

When the pressure set is reached the valve opens.

If properly adjusted the sequence valve assures that the cylinder will extend before motorstarts.

The sequence valve has a drain line from the spring chamber as the outlet is not connectedto the tank because is under pressure.

Unloading valve

Unloading valves are remotely piloted, normally closed pressure control valves, used to

direct flow to the tank when pressure at a particular location in a hydraulic circuit reaches apredetermined value.

Unlike the simple spool type relief valve the internal passage that connected the main lineto the spool is now externally operated.

When the system pressure reaches the unloading valve setting, the valve opens divertingflow back to the tank at minimum pressure.

_______________________________________________________________

For Private Circulation only. Chief Moderator : MU-PROD-FM&FP-R.R.Ghaisas

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