In this section we'll present information that will help in the successful design and implementation of Fluid Power Solutions

ALL ABOUT Actuators – WHITE PAPER #10, Actuators by Thomas W. Kreher, Applied Pneumatic Controls ©

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PNEUMATIC CYLINDERS AS STRAIGHT LINE ACTUATORS

THE PRIME MOVER

Pneumatic Actuators make it happen. They put the motion in motion control. Steven Covey, the author of the best seller, Seven Habits of Highly Successful People, suggested that we begin (life) with the end in mind. We should also begin a successful pneumatic motion control application with the Actuator in mind. The Actuator(s) is the end game and all the preceding plumbing, controls and devices are selected and sized  to achieve the desired results based on the Actuator.

THE FUNDAMENTAL CYLINDER ACTUATOR

The straight line, double acting cylinder is by far the most common actuator. This humble pneumatic cylinder pushes, pulls, clamps and acts to produce and packages many of the items used in our every day life. They press, lift, block and provide simple, quick and effective straight-line motion with adequate force when properly sized and applied.

Making a  fundamental air cylinder does not require rocket science. Many good

pneumatic cylinders are made by firms who are not primarily manufacturers of cylinders. They make their own for a specific need or to fit in an unusual envelope. 

With focused engineering and innovation the humble cylinders offered by commercial manufacturers is a very impressive device.

There are two basic standards for interchangeability of cylinder form, function and fit. The first grew out of dimensions that were standardized for hydraulic cylinders by the Joint Industrial Council (JIC) half a century ago. The JIC specifications established common envelope dimensions and specified materials that would withstand the higher pressure of hydraulic service. The National Fluid Power Association waived the material specifications for pneumatic cylinders to allow materials such as aluminum to replace the steel for lower pressure pneumatic cylinders and birthed the hybrid NFPA Pneumatic Tie Rod Cylinder Standard with JIC dimensions and NFPA revision.

The second standard was not designated by a committee. To give credit where It is due; we hereby declare the DEFACTO STANDARD FOR NON REPAIRABLE CYLINDERS to be the Charles Bimba innovation or BIMBA STANDARD.  Almost all manufacturers of simple non-repairable cylinders conform to Bimba dimensions. The cylinders Mr. Bimba pioneered have become a standard of industry. We would do well to keep that in mind and continually seek better practices.

THE MAIN FACTS

Earth science, basic physics and fudge factors come together when sizing the cylinder actuator(s) required assuming we know what the requirement is. In many applications cylinders are applied in what may be called the “toast” technique, “burn it and scrape it.”

A successful educated guess technique basically requires an actuator that is over sized.

The application with carefully calculated requirements also requires an oversized actuator. An undersized actuator may pose performance problems. Very rarely will the

size of a standard commercial cylinder match the calculated requirements exactly. Selecting the next larger size is the practical answer.

SWAG SIZING AND CONTROL

1. Oversize by 25% minimum.  Force required by 1.25

2. Oversize by 100% minimum for speed. Force required by 2

3. For extreme speed order over sized ports and increase cylinder and plumbing size.

4. Meter out, control the exhaust rate, to control cylinder speed.

5. For the best control, meter out at the valve exhaust ports (Except Poppet Valves).

FORCE CALCULATIONS

1. Area = .7854 x Bore Dia² (Squared)

2. Rod end Area = .7854 x [(Bore Dia²) – (Rod Dia²)]

3. Area (Sq In) x Pressure (Lb/Sq In) = Force (lbs)

Force = Mass x Acceleration

With a 100 (lb) box, a cylinder that produces 100 lbsf will balance the weight. 125 lbsf will raise it at moderate speed. 200 lbsf will move it rapidly. 400 lbsf would launch it. 

To overcome the resistance to slide the 100 lb box across a flat smooth surface with a coeficient of friction of .2 multiply the Cf (friction) x weight x 125% = .2 x 100 x 1.25 = 25 pounds of force to move the box a moderate speed. 

“GET ‘ER DONE”

• To move the box fast but  not  smash it with an initial slam from the cylinder use a smooth start valve to apply the pressure gradually at first and then full force.

• To move the box at top speed but not crush it when it hits the stop use linear decelerators (hydraulic shock absorbers) to decelerate the load.

• The optional CUSHIONS available with many cylinders are intended to protect the cylinder not cushion the over running load.

CAUTIONS AND CONSIDERATIONS

As you know the cylinder moves when the difference in pressure from one side of the piston to the other produces enough force to break inertia and place enough force on the load to cause acceleration. A common misconception that the pressure is zero on one side of the piston and line pressure on the other side almost never happens. The difference in pressure may be only 10 or ?? PSIG when the cylinder is moving the load.

• To apply a cylinder that will jump forward and move the load rapidly try dual pressure.

• When the cylinder does not wait for the exhaust pressure to drop things happen fast.

• The velocity of the load will moderate as the driving force starts to increase the exhausting pressure by pushing faster than the air can escape to exhaust.

CAUTION

This valuable technique also directs our attention to the situation when a cylinder is mechanically blocked the exhaust pressure may drop to zero. The force increases to the maximum on the piston. At break away the driving piston with no opposing pressure and no remaining air volume to control the speed may be violent, destructive and dangerous.

This also may happen with vertical cylinders that drift down over night while idle. In the morning when pressure is first applied there is no resisting pressure in the other side of the cylinder and a violent reaction may occur without a smooth start valve.

DON’Ts

• When you unpack a cylinder and are curious, don’t put pressure on only one port of a cylinder to see how fast it moves. As described in Caution above this could break things.

• Zero side load on the rod and rod gland is just right.

• Dirt and contamination on the rod may wear rod glands rapidly and cause premature failure. Consider a rod wiper or boot in dirty environments.

• A dent in the barrel of a cylinder may cause an internal deformation and interference.

• “No added lubrication required” means the cylinder will last as long as the grease does.

• We believe in lubrication and consider it mandatory in heavy-duty industrial applications. 

We would be pleased to hear about your unusual applications or experience with pneumatic cylinders. Also for suggestions about your requirements or possible assistance contact us through the Fluid Power Journal.