hydraulic system

[keyur trivedi]

hello everybody,
I have just made a hydralaulic system for clamping and boring. As the pressure reaches the pressure switch operates the valves to clamp. But after some times it again do the same thing. So this on/off cycle goes on and it disturbs my boring operation. Is it because of internal leakage or faulty press. switch. And during my boring operation in manual testing as i press push button in feed the tool jumps and as i leave the push botton it again jumps while rest of the time it works smoothly. What should be the possible reasons for that. I will be thankful if u can guide me through this.

Regards,

keyur v. trivedi

[Doug Hanson]

Hi keyur, The typical way to do a clamping/machining circuit is to use a sequence valve which basically gives a pressure priority to the clamping circuit, once clamping pressure is achieved, then flow is allowed to the feed circuit. This valve will maintain clamping pressure at all times during the cycle.
The surging your experiencing depends on how your circuit is arranged. When the your clamping valve shifts to re-clamp, there may be a hesitation in feed, or if both circuits are valved, when the clamp circuit pressures up and then feed valve is then shifted, the increase in pressure (from the clamp having pressured up) will cause a surge in your feed circuit. The momentarily higher pressure drop across your feed valve results in a briefly higher flow rate, hence the surge. If you are using a load sensing pump with shuttle valves to achieve the signal pressure, the higher signal pressure from the clamp achieving clamping pressure will keep the pump outlet pressure approx. 200 psi higher than the clamping pressure, this increase pressure drop across the feed control valve will result in a higher feed rate, until your clamping valve shifts back to nuetral.
Do you have some more information as far as the components and circuitry used?

[keyur trivedi]

hello mr. doug,

Since attachment is not being attached in the forum i mailed u an attached copy on han.hydr@telus.net. the same post is also there. Well pressure switch problem has been solved. But surging is still there. that may cause accidents. The following characteristics are of my problem.

1). Surging occurs only in manual opeartion only, while in auto cycle system works smoothly.

2). Surging occurs during switching and relieving of push button only, while rest of the time system runs smoothly.

3). In drilling the jump of 10 to 15 is there while in boring it's 8 to 10 mm.



I hope the above reasons and my circuit will give you satisfactory information regarding my problem. And yes all valves are solenoid operated and spring return. And when single coil valve is on the fluid goes for clamping and as press. switch cuts off it goes to drill and bore.



Thank you, with regards,



keyur v. trivedi

[Doug Hanson]

Keyur, here is your drawing reposted as a PDF:

[Doug Hanson]

The reason your clamping cylinder valve keeps shifting to reclamp over and over again, is that there is some leakge in the control valve, it only takes a few drops of leakage to get the pressure switch contacts to close, causing the control valve to shift to re-apply pressure to the clamping cylinder. There are a few ways to improve on this, One way is to replace the simple pressure switch with a double make, double break pressure switch which has seperate rising and falling pressure settings, so you can set a minimum and maximum clamping pressure. Bassically it gives you a larger dead band so the pressure switch is not constantly opening and closing. A better way is to also use an accumulator. However an even better arrangement is to use a sequence valve, and eliminate the pressure switch all together.

See the attached drawing for a rough idea. What I don't like about my drawing is that there is no seperate adjustment for feed rate or cutter speed. This would involve adding proportional flow controls, there are some 'auger/spreader' valves that are well equiped for this purpose.
Also when the cylinder strokes out, the motor will then stop because of back pressure so there should be a case drain on the motor, unless it is a Charlynn or equivelant operated at pressures that allow for there useage without case drain lines (less than 1500 psi I believe).
Notice the counter balance valve on the feed cylinder, to prevent lunging if the cut should become light, or a cutter breaks. The counter balance valve will keep the cylinder moving fairly smoothly regardless of load. It will generate a little extra heat though.
Another way to arrange motors and feed cylinders are to use the motor inlet line to back pressure the feed cylinder, so as the motor load increases, the feedrate automatically decreases.

A few questions on your drawing:
What are the blocks under the control valves to the far left and far right.
What is the valve at the far right for?
Is the 'boring' actuator a motor and the drilling actuator a cylinder?

[keyur trivedi]

hello mr. doug,
Regarding your questions, The block given under clamping directional valve is 2 way pilot operated nrv. In order to lock the substance for long time. And another block is 2-way double throttle double check valve. Control valve provided far right is to provide different feed for boring and drilling. When it's p-t there is no feed only rapid motion is available. When it's p-a and b-t, it's operated parallel to control valve of drilling. Hence feed for drill is available. While when it's p-b and a-t at that time feed for boring operation is available. It's done because whole the stroke of cylinder is not driven in feed only some part during operation is under feed. And yes as you can see meter-out feed control is there for system. Thanks for your guidance, and circuit. The next step i will put is to replace the pressure switch.

Thanks, Regards.

Keyur v. trivedi