Good day, I recently built a pneumatic vise for my bandsaw and I'm having some trouble with air pressure. Its a regulated line but for some reason when I first connect the air line or if it sits connected unused for a long period of time. The valve is stiff and seems to build pressure. If I actuate the valve the air cylinder will actuate at a very fast rate. Even though the exhaust ports are regulated as well. I was thinking an automatic relief valve that could blow off any excess pressure but I would need it to be adjustable and I haven't been able to find any product that fits that description. If anyone has any advice on this matter I would greatly appreciate the input.

Key components are:
Bimba Double acting 2" bore 5" stroke cylinder
5 port 3 position sprung manual level

 

The air cylinder actuates at a very fast rate because there is no pressure in the cylinder side that is being exhausted, so there is nothing to regulate on the exhaust ports. Post a pneumatic schematic of your system if you need more help, extra components or different piping arrangement could help.

 

The supply line is a rubber 1/4" with standard quick connect fittings
Internal lines from valve to cylinder are nylon 1/4" lines with push to connect fittings.

 

When the valve is in the centre are the A and B valve ports blocked or connected to exhaust? If they are connected to exhaust that would be the cause of pressure loss inside the cylinder. If they are blocked any small leak in the lines, cylinder or valve would be the cause of pressure loss. Replacing the three position spring valve with a two position toggle valve would probably be the simplest solution. To test this always activate the valve in the opposite direction to what you want first for a second or two, then in the correct direction.

 

I'm not sure what you mean by connected? But yes, if I actuate in the opposite direction first then there is enough pressure loss for the cylinder to operate correctly. Unfortunately the nature of the vise wont allow for a two position toggle valve. Unless my understanding of the two position is inaccurate? With this design, I need to be able to 'bump' the valve so that the cylinder only moves very slightly as to barely crack open the vise. Its my understanding that with a two position valve the cylinder will stroke fully in either direction regardless of how much you actuate the valve lever.

But it seems like you're implying that the problem is between the cylinder and the valve and not necessarily the air pressure form the supply line?

 

When you actuate in the opposite direction you are not causing pressure loss, you are pressurising the side of the cylinder that will be exhausted when you actuate in the correct direction, the pressure is needed as you are currently controlling flow from your exhaust ports.
When the valve is in the centre position the cylinder ports can be closed, open or connected to pressure, depending on the valve type, see image below.
I presume you have closed centres, there should be an image on your valve. I didn’t realise you needed to stop your vice in mid positions. Spray all your fittings on your valve and cylinder with soapy water then look for bubbles to see if you have any small leaks that can be fixed.
If this doesn’t fix it then there must be small leaks inside the valve or cylinder. Remove flow controls on exhaust ports and replace with silencers. Add inline flow controls in the lines that connect the valve outlets to the cylinder, these have a check valve inside as well so need to install in correct direction, check outlet side towards valve, image below. This controls the flow into the cylinder rather than the flow out.

 

When you actuate in the opposite direction you are not causing pressure loss, you are pressurising the side of the cylinder that will be exhausted when you actuate in the correct direction, the pressure is needed as you are currently controlling flow from your exhaust ports.
When the valve is in the centre position the cylinder ports can be closed, open or connected to pressure, depending on the valve type, see image below.
I presume you have closed centres, there should be an image on your valve. I didn’t realise you needed to stop your vice in mid positions. Spray all your fittings on your valve and cylinder with soapy water then look for bubbles to see if you have any small leaks that can be fixed.
If this doesn’t fix it then there must be small leaks inside the valve or cylinder. Remove flow controls on exhaust ports and replace with silencers. Add inline flow controls in the lines that connect the valve outlets to the cylinder, these have a check valve inside as well so need to install in correct direction, check outlet side towards valve, image below. This controls the flow into the cylinder rather than the flow out.

Wouldn't a check valve keep the cylinder from being able to exhaust?

 

The check valve is a parallel path inside the inline flow control, zoom in on image in previous post, schematic on side.

 

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