That depends on a number of factors such as:
Load – what is the weight or mass as determined by pounds or kilograms?
Friction – how much resistance? Smooth surface or rough?
Speed – how many inches per second will the load be traveling?
In general, the air cylinder bore should generate at least 25% more force than the load you plan to overcome. For higher speed applications, over-sizing the bore by more than 50% greater than the total needed to overcome the load is required.
Example: Your load needs to travel 12” in one second. The load is on a platform that you plan to lift 12 inches. The total weight of the platform and the load is 65 pounds. The platform is well guided with little resistance or drag
To overpower the load by 25% as a minimum, you will need a cylinder capable of providing approximately 82 pounds of output force. A 1-1/8” bore cylinder operating at 82 PSI, will produce approximately 82 pounds of output force. If it requires high-speed travel (such as 6 or more inches per second) it is advisable to over size the load by a minimum of 50% and or even double. Over-powering by 50% would require the same 1-1/8” bore cylinder to be operated by 98 PSI air pressure or higher. Since most plant compressed air systems tend to supply less than 90 PSI, then the wiser alternative would be to move up in bore size. A 1-1/4” bore cylinder operating at 80 PSI will deliver 98 pounds of output force.
NOTE: This is the force generated on the “blind” end or NON-rod side of the piston. The output force on the ROD end side of the piston is reduced due to the loss of the annular area of the piston rod, itself.
You can short-cut the sizing process by using a “Force Factor” matrix offered by many of the cylinder manufacturers. For a quick reference, click here, to see the Clippard Force Factor Chart found on pages 3 through 7 of their Complete Products Catalog. This easy to use chart helps identify the force generated on the “extend” side of the piston versus the retract side.
You’ll find other useful information in these pages such as the maximum load recommended to avoid “buckling” of a cylinder rod based on its diameter, information on “cushioning” your cylinder load, and various spring recommendations for single acting air cylinders.
And as always, please don’t hesitate to contact Isaacs Fluid Power Equipment Company to assist you in your application or selection process, and of course, to place your order.
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If you’ve taken the right steps to adequately size your cylinder and the associated controls, you may find instances where you did the job too well. Lo and behold, the cylinder and load is actually going faster than you want it to. So, what is the best way to control the speed of an air cylinder?
First, let’s look at your air pressure, and for two reasons. Reason one; to whatever degree you can reduce the air pressure required to operate any device, you are ultimately conserving the energy required to compress your air. Reducing the supply of air pressure to your air cylinder, your air tool, or your air motor saves compressed air and saves you money. Reason two; when you reduce the pressure feeding your output device, you are also “weakening” its power, which in turn, can also cause it to reduce in speed.
However, it’s not the most precise means of controlling speed. In general, the best means of governing the speed of a double acting air cylinder is to “meter” or throttle out the exhausting pressure from the air cylinder. There are a few different devices that can serve this purpose, but in general, using a true “FLOW” control threaded directly into the port of the air cylinder is the best alternative. Speed control can also be accomplished with speed control type exhaust mufflers threaded into the exhaust port of the directional control valve that’s controlling the air cylinder. But depending on the length and I.D. the hose or tubing running from the valve to the cylinder, the exhausting air from the cylinder may not make it to the restricting type muffler soon enough to have an impact on the cylinder’s speed. For short run distances and longer stroke cylinders, however, the speed control type exhaust muffler can indeed have an impact. And you’ve accomplished noise reduction at the same time.
But for most industrial applications, the meter-out style flow control is the superior choice. Please feel free to contact Isaacs Fluid Power Equipment Company if you have any questions or would like us to assist you with the selection of the best speed control products for your next application, and of course, we’re always ready to accept your orders.
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More often than not, a designer will find out what port size is on the cylinder or air motor they plan to operate, and purchase an air valve with the same port size. As with most things in life, you may get lucky a few times using this approach and the valve will work just fine. But the way to truly select a valve suitable for an application requires that you first determine how much flow the output device will require at a given pressure. Once that’s accomplished, you can then focus on choosing a valve that will match the flow requirements. However, most valve manufacturers will not provide an actual flow rating. Instead, you will see a “Cv” value displayed. So what is a “Cv” value?. It is the Coefficient of Velocity value. It is a value that allows you, the designer, to estimate how much flow a valve will pass at varying pressures. But how do you translate the Cv value to an estimate of flow? The easiest way is to use a Cv conversion table that lists the various correction factors based on the pressure you plan to operate at. The table will show something like this:
Cv to SCFM Conversion Factors
If you select a valve with a Cv rating of 1.4, and plan to operate the valve at a pressure of 70 PSI, then your estimated flow capacity of the valve can be calculated as:
1.4 Cv / 0.0238 factor = 58.824 CFM
Most 1/4" and 3/8” NPT ported directional control valves will provide a Cv of at least 1.4 and some as high as 2.2 Cv. But, verifying the Cv before you buy is simply good, sound practice. It avoids the purchase of a control valve that’s too weak to do the job, but may also help you avoid purchasing a valve that has FAR more flow capacity than needed for your project.
Let’s look at a real example by choosing a valve to operate a 1.5 HP air motor. The motor manufacturer uses a 1/4” NPT port in the air motor. However, the motor actually requires upwards of 60 CFM at 70 PSI to provide the output power you require. Choosing a valve simply because it has a 1/4" NPT port could be a big mistake since the one in our example above is too small for the job. For the air motor in our example you would want a valve with a minimum of 1.5 Cv and preferably with a flow rating of at least 20% higher (1.8 Cv) so there’s a margin of performance safety. There are indeed valves out there that will provide a 1.8 Cv in a 1/4” NPT size, but it’s not common, so you need to do your homework to confirm the Cv value of the valve you’re getting. Otherwise, the motor will not provide the output power you expected and require.
The same exercise would be true for estimating the flow required to operate air cylinders, and we would be happy to assist you anytime you have such an application. Please feel free to make use of our experience.
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Fortunately, (if you’re one of those annoying optimists) yes, there are dozens of ways. If you want to be a much more normal realist, then the “yes” answer means you’ve probably been wasting a LOT of dollars. Here’s why and how.
1.) Leaks. Various energy department agencies and trade associations agree that as much as 30% of the compressed air you’ve paid for gets passively leaked to atmosphere. Think about that for a minute (if you really want to poke your blood pressure.) Imagine taking all of the electric utility used in your plant and simply turning on a motor with a horsepower rating equal to 30% of your total utility bill and just letting it run continuously doing absolutely nothing all day, every day.
Replace old valves and cylinders, or at least, replace the soft seals and packings that have worn and are now leaking your compressed air to atmosphere.
Every connection (fitting, quick coupler, hose connection) is a potential leak point.
Often, simply tightening the fittings will work. You may need to throw out old, leaking air hoses, and you may need to get rid of the cheap, leaky, quick connect couplings that may have made their way into your plant via one of the website flea markets.
Install an OSHA recognized lock-out/tag-out isolation valve in the air line leading to unused, or idle equipment. Whether in use or sitting idle, that equipment will leak air, but NOT if the air supply feeding it has been disconnected.
Fully close the manual drains on your compressed air line filters and check auto-drains to make sure they’re not just partially sealing off after expelling condensate.
2.) Dual pressure circuits. Often the force required to accomplish a specific task requires the full pressure available. BUT, it often requires only minimal pressure to retract or reset the piece of equipment. Modifying the control circuit so that high pressure is used for the “action” motion and low pressure is used for the re-set or retract motion will reduce your total air consumption.
3.) Use compressed air for high pressure but use blowers for low pressure. It is far more efficient to use a dedicated blower sized for the application when you simply need less than a few pounds of pressure at relatively high volume. Using air that’s been compressed to 120 plus PSI, then regulating it down to perhaps 2 or 3 PSI for applications such as cooling or material transfer is a MAJOR waste of energy and utility.
4.) Use a vacuum pump rather than a compressed air vacuum venturi device. You may find that a ¼ or ½ HP vacuum pump will be FAR more efficient than taking 60 – 80 PSI compressed air from your plant system to create venturi vacuum, especially if you have multiple such devices in use.
There are numerous other ways to detect leaks, reduce consumption, conserve air, and improve the efficiency of your plant compressed air system. Please allow the experienced sales representatives of Isaacs Fluid Power to assist by contacting us.
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To varying degrees, our atmosphere is saturated with water. We see evidence of it when it rains or on a foggy morning, or in the Great Smokey Mountains. But even when we can’t see it, molecules of gaseous water vapor are entrained in every single cubic foot of air in this atmosphere. Without it, plant and animal life (including us,) would either have to adapt or our forms would simply desiccate. The air or other gas that you are evacuating from your tank also includes molecules of moisture be it water or various other chemical compounds present in your process. In order to create vacuum, you are changing the atmospheric pressure inside the tank which results in drawing cubic feet of air (or air/gas mixture) through your vacuum pump. If the pump is comparatively cool when you open up the vacuum stream from your processing point, the moisture vapor being transported will quickly cool inside the “cold” pump which causes the vapor to condense and form liquid inside the pump. As your pump heats up (as it will under vacuum,) then the tiny grains of moisture will remain in a heated, vapor state, and for the most part, pass right on through your pump and exhaust to atmosphere. But once you shut down your vacuum pump, and as it cools, the air or gas inside the pump also cools. The cooling process creates condensation and this condensation of vapor is the basis for your liquid inside your pump.
I can pretty well guess what your next question is: Okay, then how do I get rid of it, or better yet, how do I prevent it?
Depending on the volume of moisture or how saturated your air or air/gas mixture is, you may be able to do a few things operationally to alleviate the matter. Starting you vacuum pump for 5 or 10 minutes under a partial vacuum load before you open the valve to your processing point will allow the pump to warm-up. This reduces the likelihood of vapors condensing inside a cool pump at start-up. Similarly, before you shut your process down, close off the valve to the processing point, and allow the pump to run for 5 to 10 minutes while partially open to atmosphere. This allows some purge air to flush the more harmful or damaging vapors through the pump and out to atmosphere.
Along with a high quality, high flow vacuum inlet filter, the use of a basic liquid separator trap installed just ahead of the pump inlet is also recommended. This will aid in collecting the condensate that is similarly forming inside the piping system and inside your receiver since both areas are also subject to the principals of vapor condensation. It also protects the pump from a “slug” of moisture suddenly being drawn into the pump and creating catastrophic damage.
For higher flow, gas and chemical processing systems, a pre-condensing process system is often designed and specified to be installed ahead of your chemical duty vacuum pump. The pre-condensing unit pre-cools the vacuum stream ahead of the vacuum pump which forces the condensation of the vapors before they ever reach the inlet of the pump.
As always, our experienced Sales Associates are available to assist you with your vacuum applications and are able to offer knowledgeable recommendations for a good, optimally performing vacuum pump installation.
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Really? You’re asking someone who sells those things whether or not you should buy one?
Well, actually, the answer is no; you may not really need one. And in some cases, you may actually be creating a few more problems than you’re solving if you do use one.
Unless you specify differently, just about every manufacturer of cylinders or valves will pre-lube their product during the manufacturing process. For one reason, it aids in the assembly and helps prevent scoring or scuffing of soft seals or where metal-to-metal contact may occur. In applications where the cycle rate may be fewer than 1 or 2 times per minute, it is entirely likely that you can dispense with an in-line compressed air lubricator. Even more so, if you have clean, dry compressed air supplying your equipment. The wetter, dirtier the compressed air supply is, the more likely that the pre-lube in the components will get “washed” out. Clean, dry air helps preserve the lube.
Plus, keep in mind, the 10 weight oil that you fill your lubricator with doesn’t just go into the valve or the cylinder and stay there. At some point, that oil condenses and will ultimately “leak” or “spray” it’s way out of the exhaust of your control valve, or via leaks in your fittings or elsewhere. As the oil mixes with debris or dirt in your piping and transfers into your cylinders or coats the spools of your valves, then you can easily wind up with valves that “stick” or cylinders that have excess friction rather than the smooth motion you typically prefer.
If your system is high speed, high cycle, then yes, specify a lubricator but be stingy about how much oil you dispense into the system. It doesn’t take but a drop or two per minute to keep an air cylinder and valve operating smoothly, and just as true for an air tool or air motor. And if you have any questions or would like to place an order, please feel free to call us, or for additional information, click here.