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We were discussing the working principle and its internal construction of various valves used in hydraulic units such as pressure relief valve, Pilot operated pressure relief valve, Pressure reducing valve and Pressure sequence valve.

Today we will see here the working principle and internal construction of counterbalance valve used in hydraulic units.

Counterbalance valve is also a type of normally closed valve which permits the flow of fluid in one direction but will offer resistance in fluid flow in other direction until pressure reaches up to a level at which valve opens.

Let we have one hydraulic cylinder which is mounted vertically and supporting a weight. Now during lowering the weight and gravity force acting over the heavy load, motion will be uneven and we can use a flow control valve at the outlet of cylinder to control the flow of fluid leaving the cylinder but it could only be applicable if cylinder speed is very slow.

Let if we need to lower the vertically loaded hydraulic cylinder with higher speed, we could not use flow control valve here and we will have to go for counterbalance valve here. If hydraulic cylinder falls and stops suddenly at the end of motion then it may damage the load, cylinder and also it may harm the surrounding people.

Counterbalance hydraulic valve will provide the hydraulic resistance to the hydraulic cylinder.  Counterbalance valve will be used here, between directional control valve and outlet of cylinder, in order to avoid the falling of weight due to gravity.

Let us see the internal construction and after that we will see here the working concept of counterbalance valve.

Internal construction of hydraulic counterbalance valve

Figure shows the internal construction of counterbalance valve. “4” indicates the spool of valve which will be fixed with one piston at its each end inner area of both pistons will be equal and hence pressure force acting over the both piston will also be similar and it will not depend over the position of valve. “5” indicates the primary port of counterbalance valve and “8”indicates the secondary port of valve. Primary port of valve will be connected with the rod end port of cylinder and secondary port of valve will be connected with the directional control valve.
Hydraulic counterbalance valve: internal construction

“1”indicates the pressure adjusting screw which is used to set the pressure of valve at slightly higher than the required pressure to avoid the falling of cylinder or load due to gravity. There will be one small pilot piston which will be attached with the bottom of spool of valve. “6”indicates the pilot passage. “7”indicates the check valve which will permit the flow of fluid during lifting the load but will restrict the flow of fluid during lowering of load. “3” indicates the spring of valve.

There will be slightly leakage of hydraulic oil at the top piston of spool and this oil will be accumulated around the spring and this accumulated hydraulic oil will cause the hydraulic lock as oil could not be compressed. Therefore there will be one internal drain line indicated by “2” will provide the path for accumulated hydraulic oil to go to secondary port i.e. “8”.

Let us see the working......

During lowering of the weight or extension of cylinder

Figure shows the hydraulic circuit for handling the load in vertical direction with the help of vertically mounted cylinder. Counterbalance valve is attached here between rod end port of cylinder and directional control valve. During lowering of the weight or extension of cylinder, hydraulic oil leaving the cylinder will enter to the primary port of counterbalance valve i.e. “5” but as valve will be normally in closed position and secondary port of valve i.e. “8” will be blocked by top piston of spool, oil could not flow to the valve secondary port.

Oil will flow through the pilot passage i.e. “6”and will exert the pressure force over the bottom piston of spool of valve. When pressure increases and overcome the setting pressure of valve, spool will move in upward direction and will permit the flow of hydraulic oil towards secondary port to directional control valve to reservoir.
During lifting of the weight or retraction of cylinder

During retraction of hydraulic cylinder, hydraulic oil will enter to the valve through the secondary port i.e.”8”. Spool of valve will come in closed position under the action of spring force of valve. Hydraulic oil will flow to the rod end port of cylinder through the primary port of valve i.e.”5” via the check valve “7”.

We will discuss another type of hydraulic valve in our next post.

Do you have any suggestions? Please write in comment box.

Image courtesy: Google

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