[Chris Fultz]

This is still an excellent series of posts and the schematic described here will work. BUT, if the system is not properly controlled, part of the pneumatic system is operating above 60 psi if your system regulator is set at 60 psi.

Below I will try to explain some of the basics of the pneumatic system and why I say that.

WHY 1?

To hold a cylinder mid-position, the forces on either side of the actuator must be equal. This force is a function of the pneumatic pressure, but the pressures will be different.
The pressures do not ‘equalize’, the forces on either side of the moveable inner plate equalize.

WHY 2?

One end of the cylinder has a larger surface area (working area) than the other. The rod end has a smaller surface area because of the rod, and this reduces the working area of that end. If you read the pneumatics table in the book, you will notice an “extended” and a “retracted” force. The working area is why they are different.

WHY 3?

Lets use a 2” cylinder as an example.

On this cylinder, the major diameter is 2”. The rod diameter is 0.625”.

Major end: 2” dia = 1” radius, the area is pi x (radius squared). 3.14 x 1 squared
= 3.14 square inches. With 60 psi (pounds per square inch), the force of the cylinder is
3.14 x 60 = 188 pounds (as listed in the pneumatics manual).

Minor end: The major area is the same – 3.14 square inches. However, the extension rod reduces the working area. The rod diameter is 0.625”, so the radius is .3125”. Using the same formula for area, the area of the rod is 0.307 square inches. To get the working area of the minor end, you have to subtract the rod area from the major area. So, 3.14 square inches – 0.307 square inches = 2.83 square inches. With 60 psi, the force of the cylinder on this end is 2.83 x 60 = 170 pounds (as listed in the pneumatics manual).

This is why the cylinders we use will have more force in one direction than the other.
And this is why part of the pneumatic system can be higher than 60 psi (the legal limit) when you use the solenoids to do partial stops.

WHY 4?

Assume the system regulator is set at 60 psi. and the cylinder is (somehow) - half way extended. On the major end, the force applied is 188 pounds (60 psi acting on 3.14 square inches). When using the cylinders normally, with the solenoid vents opened, this force will push the air from the minor end of the cylinder and the rod will fully extend. But in this situation, the vents are closed, so the air cannot escape. The rod will continue to move until the FORCES acting on the internal plate surface are equalized.

This only happens when the air pressure on the minor end of the cylinder has increased to counteract the smaller working area. The movement of the major end of the cylinder will compress the air in the minor end – remember the minor end is a closed area because the solenoid vents are capped. The Major End will continue to extend until the pressure in the minor end reaches 66.5 psi.

WHY 5?

The minor end of the cylinder has a working area of 2.83 square inches. For this end to produce 188 pounds of force, it will require 66.5 psi (188 divided by 2.83 = 66.5 psi). This pressure is generated by the compressing action of the major end of the cylinder moving and reducing the area in the minor side of the cylinder.

By closing the vents, you have created a system that will operate above the maximum allowable pressure (60 psi) by 10%.

SOLUTION:

You can still utilize the setup described, but you must reduce the main operating pressure in the system to do so. If you want to be able to stage the extension of the 2” cylinder mid-extension, you must limit the system pressure to 54.1 psi.

WHY 6?

The pressure on the minor end becomes your limiting factor – you need to keep it at or below 60 psi. Using the 2” cylinder At 60 psi, the minor end of the cylinder can produce 170 pounds of force at a 60 psi working pressure (2.83 square inches x 60 psi). To be able to hold the cylinder mid-extension, you need to limit the force of the major end to 170 pounds – remember that the FORCES must equal, not the pressures. So, to limit to 170 pounds with a 3.14 square inch area, the pressure must be limited to 54.1 psi (170 divided by 3.14).

For the other two cylinders in the kit:

0.75” Cylinder with 0.250” dia rod – 26.5 pounds Major, 23.5 pounds Minor.
The working area of the minor (rod) end is 0.392 square inches x 60 psi = 23.5 pounds.
The working area of the major end is 0.441 square inches.
23.5 pounds divided by 0.441 square inches = 53.3 psi.
53.3 psi is the maximum allowed system pressure to allow you to hold the cylinder mid-extension and keep below the 60 psi maximum allowable in the closed end of the pneumatics.

1.5” Cylinder with 0.437” dia rod – 106 pounds Major, 97 pounds Minor.
The working area of the minor (rod) end is 1.61 square inches x 60 psi = 97 pounds.
The working area of the major end is 1.76 square inches.
97 pounds divided by 1.76 square inches = 55 psi.
55 psi is the maximum allowed system pressure to allow you to hold the cylider mid-extension and keep below the 60 psi maximum allowable in the closed end of the pneumatics.

It has been awhile since I sat in a fluids class so if I made a wrong statement here please advise and I will correct it.