Creating a control for a platform with pneumatic brakes

Thread Starter

SS23

Joined Dec 12, 2019
24
Hey folks! I would like to get some feedback from you all about creating a pneumatic system for a movable platform.

I need to control the movement of this platform on rails by APPLYING/RELEASING pneumatic brakes.
The platform can move when compressed air is supplied to the clamping elements and is BRAKED when no air is supplied.

Air pressure 6-8 bars.
Piping diameter: Approx. 20 mm

For now I have thought of a solenoid valve that I could place in between the compressor wall outlet and the piping of the pneumatic brakes of the platform. The issue I am facing is that the pressure in the pipes after the solenoid valve closes is not released and hence, that wouldn't completely apply the brakes. I thought of a valve with one input (From compressor) and two outputs (01 to atmosphere and 01 brakes). That would close the input in BRAKED condition and connect the 2 Outputs to release the pressurized air in the circuit after the solenoid.

I would like to control this all by a wall mounted switch that BRAKES (meaning stops Airflow) when pressed and RELEASES BRAKES (opens solenoid valve) when not pressed.

Please suggest some ideas on it. Or maybe if there could be a simpler way to implement this. Thanks!

Picture1.jpg
 

Sensacell

Joined Jun 19, 2012
2,850
If I understand correctly? the valve fails to release the pressure when the power is removed?

You need a 3-way valve that VENTS when the power is off
 

shortbus

Joined Sep 30, 2009
8,696
You still need a 3 way valve.
No you don't. Look again at what is shown in the link, and how it works. When the pressure is applied to the brakes or cylinder or what ever is being powered on it makes what they call a "bat wing" move to seal up the exhaust port. When the control vale goes to the off, no pressure position, the bat wing move back to cover the inlet port and let the air or hydraulic fluid out of the cylinder. Have you ever heard the little whoosh of air when a semi sets the park brake and lets off the service brake? That is the dump valve at work. You would hear it going down the road too, every time the driver lets off the brakes when slowing down.
 
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You basically need a 3-way valve. You can control the output of air if you need too. i.e. control the bleeding of air. You would also want a small tank. Depending on the compreseor, it might need an unloader and check valve which vents air so the compressor can start without a load.

Finally, you might need an auto air vent, to remove water.
 

GetDeviceInfo

Joined Jun 7, 2009
1,866
No you don't. Look again at what is shown in the link, and how it works. When the pressure is applied to the brakes or cylinder or what ever is being powered on it makes what they call a "bat wing" move to seal up the exhaust port. When the control vale goes to the off, no pressure position, the bat wing move back to cover the inlet port and let the air or hydraulic fluid out of the cylinder. Have you ever heard the little whoosh of air when a semi sets the park brake and lets off the service brake? That is the dump valve at work. You would hear it going down the road too, every time the driver lets off the brakes when slowing down.
Quick exhaust valves are common throughout industry. According to the posted schematic, the directional valve has blocked ports in one position. Although this position means no more source pressure, it does not mean a removal of existing pressure on the load port. That pressure can remain until leakdown.
I did go back and read your link , and it does say that the control valve must exhaust.
 
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Sensacell

Joined Jun 19, 2012
2,850
No you don't. Look again at what is shown in the link, and how it works. When the pressure is applied to the brakes or cylinder or what ever is being powered on it makes what they call a "bat wing" move to seal up the exhaust port. When the control vale goes to the off, no pressure position, the bat wing move back to cover the inlet port and let the air or hydraulic fluid out of the cylinder. Have you ever heard the little whoosh of air when a semi sets the park brake and lets off the service brake? That is the dump valve at work. You would hear it going down the road too, every time the driver lets off the brakes when slowing down.
It's essential for any valve, 'dump', or not, to see a drop in inlet pressure to activate, that's why you must have a 3-way valve.

Otherwise- how does the dump valve know to open? if a 2-way valve is used, the only way the outlet pressure drops is from leakage on the downstream side- an hour later?
 

shortbus

Joined Sep 30, 2009
8,696
Although this position means no more source pressure, it does not mean a removal of existing pressure on the load port
You are missing the part where this is for a brake . That is only a single line plumbed to the actuator/cylinder, there is no return line to the actuating valve. Brake cylinders have a spring or for disc brakes rely on the slight back force of the caliper seals to move the piston out and remove the clamp action. Look at any brake system, air , air over hydraulic or just hydraulic like in your car, there are no return lines to a three way valve.


It's essential for any valve, 'dump', or not, to see a drop in inlet pressure to activate, that's why you must have a 3-way valve
Like above, this is a brake system, they have no return line from the brake cylinder, none. Brake cylinders or calipers are single acting cylinders not double acting, which would use a three way valve. I worked part time in a truck repair shop for a couple of years, never once saw a three way valve or return lines to a three way valve.
 

shortbus

Joined Sep 30, 2009
8,696
I did go back and read your link , and it does say that the control valve must exhaust.
Where exactly?

Quote, " When the control valve exhausts air from the exhaust valve inlet port, the nitrile poppet shifts to seal the inlet port and open the exhaust port to the cylinder. The pressurized air is then allowed to exhaust directly through the exhaust valve to atmosphere. Normally the air must travel back through the long air line to the control valve to exhaust. However, by mounting the exhaust valve directly on the cylinder, the piston retracts quickly since the distance to atmosphere is very short and unrestricted." From my link https://clippard.com/cms/wiki/how-does-quick-exhaust-valve-work No where did I read anything about a three way valve, so please help me see the light.
 

Sensacell

Joined Jun 19, 2012
2,850
Like above, this is a brake system, they have no return line from the brake cylinder, none. Brake cylinders or calipers are single acting cylinders not double acting, which would use a three way valve. I worked part time in a truck repair shop for a couple of years, never once saw a three way valve or return lines to a three way valve.
Yes- the brake is supplied by a single airline, no return. But this single line must be depressurized to change the state of the brake.
If you supplied the air using a two-port, open or closed valve, the downstream side would never vent to the atmosphere and would remain pressurized.
 

GetDeviceInfo

Joined Jun 7, 2009
1,866
Where exactly?

Quote, " When the control valve exhausts air from the exhaust valve inlet port, the nitrile poppet shifts to seal the inlet port and open the exhaust port to the cylinder. The pressurized air is then allowed to exhaust directly through the exhaust valve to atmosphere. Normally the air must travel back through the long air line to the control valve to exhaust. However, by mounting the exhaust valve directly on the cylinder, the piston retracts quickly since the distance to atmosphere is very short and unrestricted." From my link https://clippard.com/cms/wiki/how-does-quick-exhaust-valve-work No where did I read anything about a three way valve, so please help me see the light.
‘Where’ is in plain sight. ‘When’ is the trigger.
Let’s clarify the term ‘3 way’. A 2 way valve typically has 2 ports, in and out. 2 positions of which one allows flow, the other blocks flow. A 3 way has an additional port, typically an exhaust. In a 2 position valve, one position directs the out port to source pressure, the other position directs the out port to atmosphere, generally termed, exhaust.
Revisiting the quick exhaust valve, we see that exhaust isn’t initiated by the valve itself, but by the depressurization of the supply line. This can only happen by ‘exhausting’ the supply line. Beyond that, you are correct, the quick exhaust valve, once sensing an exhausted source, transitions to an atmospheric dump.
 

shortbus

Joined Sep 30, 2009
8,696
You guys are using a different way of calling out a control valve than anything I've ever heard. In the world I came from spool valves were two position or three some times four, a two position has 3 ports, in the "off" position vents either to the tank with hydraulics or the atmosphere with air. That is how the dump valve can work like it should. A 3 way valve has 4 ports and 3 positions
 

GetDeviceInfo

Joined Jun 7, 2009
1,866
You guys are using a different way of calling out a control valve than anything I've ever heard. In the world I came from spool valves were two position or three some times four, a two position has 3 ports, in the "off" position vents either to the tank with hydraulics or the atmosphere with air. That is how the dump valve can work like it should. A 3 way valve has 4 ports and 3 positions
That was becoming apparent.
Let’s clarify for the sake of the OP, as their posted schematic will not work, and the introduction of a dump valve alone, or with the indicated valve, will not work.
To be honest, after 40 years of Millwrighting throughout industry, I too am confused about the terminology. It does appear though, that industry standard counts ‘ways’ as ports, and positions as potential spool positions. I see a / appearing, and have run into foreign manufactures using different terminologies, but NA usage has been standard, in my experience.
With that being said, I recommend that the OP consider replacing the indicated, 2 way, 2 position valve, with a 3 way, 2 position valve. If a quick dump action is desired, include that as well.
 

Sensacell

Joined Jun 19, 2012
2,850
Yes- very confusing indeed.

Attached is an image- and schematic of a 2 port valve.

It's either capped-off, or open for flow through- but does NOT vent the output side to the atmosphere.
This type of valve will not work with a single-acting actuator, the pressure cannot be relieved
.Figure-5B-2-way-2-position-normally-closed-direct-acting-solenoid-valve-spring-return.jpg
 

Thread Starter

SS23

Joined Dec 12, 2019
24
If I understand correctly? the valve fails to release the pressure when the power is removed?

You need a 3-way valve that VENTS when the power is off
Sorry for the delay in following the thread. Had a covid scare, all good now. Yes, I am trying to look for a 3-way valve (normally closed type) that VENTS as you suggested. And do you have any suggestions for connecting the valve to a simple wall mounted on-off switch?
 

Thread Starter

SS23

Joined Dec 12, 2019
24
You basically need a 3-way valve. You can control the output of air if you need too. i.e. control the bleeding of air. You would also want a small tank. Depending on the compreseor, it might need an unloader and check valve which vents air so the compressor can start without a load.

Finally, you might need an auto air vent, to remove water.
The compressor is located in the basement of the site and is also used for other equipment. And is always in a working state every morning to the evening when we use the equipments. There is an input line supplied in the wall from the compressor. During my observations, the pressure in the line stays between 6-8 bars. So I guess a simple 3-way valve should be sufficient, isn't it?
 

Thread Starter

SS23

Joined Dec 12, 2019
24
Quick exhaust valves are common throughout industry. According to the posted schematic, the directional valve has blocked ports in one position. Although this position means no more source pressure, it does not mean a removal of existing pressure on the load port. That pressure can remain until leakdown.
I did go back and read your link , and it does say that the control valve must exhaust.
Yeah I should have corrected the schematics. Mainly the source pressure must be blocked and then the pressure on the other side of the valve could be vented to the third empty port in the atmosphere.
 
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