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Pressure and Time Switch

Pressure and Time Switch

Pressure and Time Switch

(OP)
Dear Experts

I have a machine that has one double effect cylinder and I need the cylinder to go forward. Then the cylinder will reach a load that has to be compacted. So I need the cylinder to maintain certain value of pressure for 5 seconds and then retract
My question is: Is there a pressure switch that can be installed on the forward pipe of the cylinder that can detect once the desire pressure is reached and also has a timer function?

Thanks in advance

RE: Pressure and Time Switch

I do not know of a commercial off-the-shelf pressure switch (with an electrical output) that includes a built-in delay timer.

Typically a pressure switch's electrical contacts would be used to trigger an electrical off-delay timer module and when the timer times-out at a setting of 5 seconds, its output would change state and provide that change-of-state electrical signal to your control system.

RE: Pressure and Time Switch

In pneumatic cylinders the speed is controlled by controlling the exhaust flow. The pressure on both ends of the cylinder will be at supply air pressure until the rod hits a stop, or load, and then the exhaust side pressure drops to zero.

You can set the compaction load by regulating the supply pressure. You start your time delay when the exhaust side pressure reaches close to zero, so you use a pressure switch set at close to zero pressure. Pneumatic timers are simply a chamber with a needle valve that restricts flow in or out. Pneumatic timers are commercially available, or you can make your own. You can do what you want entirely pneumatically.

The supply pressure and timer adjustments do interact somewhat in this scheme.

RE: Pressure and Time Switch

(OP)
Thanks for your reply

I forgot to specified that the cylinder is an hydraulic cylinder, compact pressure should be around 700 psi. The machine already has an adjustable pressure switch at the high pressure side so I am thinking that using an on-delay as suggested by danw2 is the best way to do it

RE: Pressure and Time Switch

A mistake a lot of people make is they assume that controlling pressure is good enough. When the pressure get to 700 psi on the cap side of the piston, what is the opposing pressure on the rod side?
Normally you want to know the differential force.
Another problem is response time. The pressures/force can increase rapidly when contact is made. It is easy to overshoot the desired force by a significant amount. Gauges are not fast enough.

I had a customer that used only pressure on the cap side to use a as a process variable for their press. They couldn't understand why they had so many defects. It took me about 4 hours of explaining and demonstrating that the pressure on the rod side was not 0 as they had assumed. It was much higher so the amount of force they were applying was much less than they thought.

Peter Nachtwey
Delta Computer Systems
http://www.deltamotion.com
http://forum.deltamotion.com/
IFPE Hall of Fame Member

RE: Pressure and Time Switch

Is there an electronic proportional pressure regulator in hydraulic?. Sure there is one in pneumatic. It makes such tasks easier. All you have to do to set up a closed loop in your PLC using 4-20mA signals (for set pressure and output pressure feedback) and program the basic loop in ladder using an inbuilt timer and you are good to go. No need of external pressure switches and timers.

RE: Pressure and Time Switch

(OP)
I do not need a lot of precision in pressure value


RE: Pressure and Time Switch

I do not know of a pressure switch with built in timer.

I suppose the cylinder is controlled with a regular 4/3 directional control valve.

I would problably use a pressure reducing valve set on the desired pressure (700psi) on the "pressing" side of the cylinder and just actuate the directional control valve for as long as you like, maybe with a pressure switch and a programmed timer if the time is important. The cylinder will then stop when a load pressure of 700psi is reached and you will hold this pressure for a certain amount of time.

However you will problably need a small PLC to achieve this task.

RE: Pressure and Time Switch

...and make sure you do not have a meter-out flow control or counterbalance valve or any other such restriction on the other side of the cylinder that will interfere with what you are doing. If the cylinder needs a counterbalance valve, account for its setting.

RE: Pressure and Time Switch

It seems to me like this sort of hydraulic logic element must have existed at some time - a valve that unseats at a certain pressure and then allows leakage through a needle valve to build pressure to sequence another valve that sends the piston the other way with an internal spring that resets the logic valve. Set the needle in the orifice to set the time.

Likewise there must be a pressure controlled switch that trips at the right pressure and starts a timed relay to run the hydraulic valves.

Anymore and I think people toss a PLC / microcontroller on it with the pressure switch and maybe some other safety interrupts and call it a day.

RE: Pressure and Time Switch

Any hydraulic cylinder with a speed control valve can be turned into a timer, several different ways.

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