Calculating a cylinder’s air consumption is often necessary on fast-cycling equipment to ensure enough supply air is available. There are two parts to cylinder air consumption. One is the volume the piston displaces. The other is the unswept volume from end-cover cavities, cylinder ports, connecting tubing, and valves. The unswept portion is likely to be a small percentage of the total and will vary with the installation.
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It’s best to ensure the compressor has sufficient capacity to supply pneumatic equipment under “worst-case” conditions. Otherwise, air starvation at critical times will cause performance to suffer.
Additional considerations
After sizing a cylinder for force and stroke, engineers have a lot of leeway in tweaking a cylinder so it best fits an application. Here are a few considerations.
Port sizes and locations are usually dictated by bore size, but can be adjusted in custom designs.
Envelope dimensions. The National Fluid Power Assn. and International Standards Organization have established standards for many cylinder dimensions, letting engineers interchange cylinders from different manufacturers. Many models also have unique dimensions.
Mounting configuration refers to how a cylinder attaches to the adjacent equipment. The large number of standard mountings — both rigid and articulated — usually ensures a cylinder can execute the specific movements an application requires.
Cylinder materials. The operating environment is the major factor that governs material choice. Pneumatic cylinders are typically made of steel, aluminum, stainless steel, brass, or engineered plastics. Some models combine several materials.
Seal materials. Cylinder manufacturers use a variety of methods to seal the end caps and rod. Designers can specify alternative seal materials for applications that operate in extreme high or low ambient temperatures or are exposed to caustic chemicals.
Position feedback. Magnetic cylinders have a band of magnetic material around the circumference of the piston and a nonmagnetic cylinder barrel. The magnetic field travels with the piston as the rod strokes in and out. Mounting reed switches on the outside of the barrel — one at each end, for example — generates a signal each time the piston completes a stroke. More-advanced versions use LVDT transducers or LRT linear potentiometers to provide continuous indication of piston position.
Cushions. If the piston makes metal-to-metal contact with the end covers, the result is noise and potential mechanical damage. Cushions in cylinders prevent such contact. Adjustable cushions let operators control the rate at which cylinders decelerate at the end of stroke.
Some cylinders have integral fixed cushions. They have a preengineered fixed cushion orifice that restricts exhaust airflow to slow the piston at the end of stroke. The amount of cushioning is repeatable but cannot be altered in the field.
Only use noncushioned cylinders at slow speeds. To operate noncushioned cylinders faster, install external stops with shock absorbers. Position these to prevent contact between the piston and end covers.
Cautionary tips
An important design consideration is to keep cylinder thrust as close as possible to the centerline of the piston rod and free from misalignment or side thrust. Cylinders are normally intended to push and pull without excessive side loads. Off-center loads can substantially reduce the service life of rod bearings and seals.
Off-center and side loading are caused by improper mounting, cylinder deflection under load, machine frame deflection, and rod bending or sagging — as well as by poor machine design. Sometimes adding an optional internal stop tube can reduce a cylinder’s bearing load. A stop tube is a spacer between the piston and rod-end head. It increases the distance between the piston bearing and rod bearing when the rod fully extends. This configuration also helps in long-stroke applications.
If the piston rod sees compressive axial loads, care must be taken to ensure its length, diameter, and load are within safe limits to prevent the exposed rod from buckling.
Most pneumatic cylinders are assembled with a coating of grease on the bore of the barrel and seals for service with nonlubricated air. If the compressed air supply is clean and dry, the grease will give the seals a long life without adding oil through an airline lubricator. However, contaminated air will gradually compromise the original grease lubricant and shorten seal life.
Lubricated air will extend the life of the cylinder, but it will also wash out the original lubrication. So once lubricated air is introduced, it must always be used, and the lubricator should be regularly checked and maintained.
Engineering custom actuators
by: Cliff Martin
Confused? I know I was when I first started working with pneumatics. After a while I found that it really isn't very hard, its just there seems to be a few specfic parts for any air application you can imagine. Air cylinders, solenoids and their associated parts come in ALL shapes and sizes to accomodate thousands of applications in manufacturing, construction, military, and transportation as well as dozens of other industries. This stuff is used everywhere, from tiny air switches in medical equipment to air brakes in big trucks.
All the strange names and terms around pneumatics have evolved through about 100 years of their use in manufacturing. Double acting, four way, quick connect are all terms that were invented to describe (as best as could be) the difference between the parts. Don't let the names discourage you. They're just names. I've used quite colorful terms myself when working with pneumatics, most of which i won't use here...
The first thing to remember is: Pneumatics are easy... really! Its all the different names and parts that seem to be overwhelming. But enough about that... on to the fun stuff.
Notes:
There's only two main kinds of air cylinders: Double acting, and single acting. They come in all variations, shapes and sizes. Both kinds are useful for haunt work. Double acting cylinders are useful when you need to push in both directions, and single acting cylinders are useful when only a push in one direction is needed. And, sometimes 'in a pinch', you can adapt a double to act as a single, and a single to act as a double.
Air cylinders are measured by three main values: "pressure rating", the "bore", and "stroke"
Pressure This is the maximum pressure the air cylinder can safely handle (usually measured in pounds per square inch - psi). More than likely you'll never use it close to the max pressure - and you shouldn't! Bore The interior diameter of the cylinder (usually measured in either inches or centimeters). The bore is what gives the cylinder its power. Stroke The range of movement of the air cylinder's rod (usually measured in either inches or centimeters).
There are lots of calculations to accurately figure the power of a cylinder, but most haunt pop-up applications can be handled by air cylinders in the range of 3/4" to 1-1/2" bore, and 3" to 8" stroke.
Power measurements primarily take into account the air pressure (the higher the pressure, the more power); and the bore (the larger the bore - the more power). The power ratings are usually only quoted at maximum pressure. So if a cylinder produces180 pounds of 'push', it will only deliver that at the maximum pressure (usually 250 psi for commerical cylinders).
Haunters should work their props at much, much lower pressures than the max pressure. A good goal is not to exceed 60-70psi for working props. Going much higher causes more stress on the prop and all parts in the air system, and makes your compressor run more often. Even at lower pressures, air cylinders can still move very fast and deliver quite a lot push, so always be very careful around pneumatics!
Double Acting means the air pushes the rod out, and pushes the rod in. Without air pushing one way or the other, nothing happens!
Every double acting air cylinder has these basic parts. A metal cylinder to hold everything together, a 'plunger' that the air pushes against, two connections to get the air in and out, and a rod connected to the plunger that goes in and out. That's it. Here's a simple animation to illustrate the motion...
As air is sent into the left connection (pressure is shown in yellow), it pushes against the plunger and the rod goes out. At the same time, air is released out of the right connection. To reverse the motion, air is sent into the right connection, pushing against the plunger on the other side and the rod is forced back in.
The trick to the double acting cylinder is that you have to let air OUT of the other side! This is an important feature of the double acting cylinder, and an advantage that gives you great control over the motion of the rod (but, more on that later!).
TIP: the most useful double acting cylinder I've found is one with 1/4" connections, 6"-8" stroke, 1" bore, and end clevis mounts. But that's just me - of course, most any cylinder can be adapted for haunt use!
Single Acting means the air cylinder rod is ONLY pushed in a single direction, either out or in. There is only one connection for air, and a little hole in the other end to let air in and out. A spring is used to push the rod in the opposite direction after air pressure is removed.
Single Acting Air Cylinder,
Single Acting Air Cylinder,
As air is pushed into the connection, the plunger begins to move and compress the spring. Exhaust air exits out the exhaust hole on the other end. When air is released, it exits out the connection, and air is sucked into the exhaust hole as the spring pushes the plunger back to its resting position. Basically, the spring is the 'push' needed to return the plunger and rod back to their starting position.
When selecting a cylinder for an application, remember that a double acting cylinder pushes in both directions, while a single acting cylinder only pushes in one direction.
There are about as many ways to mount an air cylinder as there are different types of air cylinders. Again, this is because of all the uses. My personal favorite is the clevis mount. (see photo below) Clevis mounts give the greatest amount of movement, flexibility, and ease of mounting over other mounts. You can adapt one type of mount to another by rigging up an adapter. You'll need to evaluate how to build or buy an adapter - every situation is different. The key thing to remember is that "There is a way!".
Buying on eBay
is a great way to save money when purchasing air cylinders. Often you
can find air cylinders that include all the fittings and mounting hardware.
This can easily save you $20 in extra parts. I'd recommend not spending
more than $10-$15 for an air cylinder without fittings and mounting hardware,
and certainly no more than $20-$25 for an air cylinder with all the hardware.
(this includes shipping/handling!). There's just too many air cylinders
being sold on the surplus market (eBay sellers) to pay anywhere near retail
for an air cylinder. Read the descriptions carefully, and pay special
attention to the shipping details. Often shipping will be equal to or
more than the purchase price! If the description doesn't match or doesn't
have enough information, DONT buy it! There will be more for sale
tomorrow. The secret to successful purchases is to take your time. Never
buy something because you need it immediately - especially from eBay.
There are folks who have paid more than retail for used equipment because
they got caught up in the 'auction frenzy'.
eBay update - Things have changed since I wrote the above about purchasing
on eBay. A lot more folks are bidding, and are bidding prices up beyond
what this cheap haunter is willing to pay. So I'll add another note of
caution to using eBay - don't pay too much! It's really worthwhile to
learn the retail price on a particular cylinder before bidding. There's
also several dealers on the web that offer the same products at good prices
- www.evilusions.com
is one example.
If you must have a particular air cylinder to fit a need, McMaster Carr, and Grainger are good local places to purchase a variety of air cylinders. Check the phone book, there is probably a local supplier of hydraulic and air equipment in your area. Building a long-term relationship with a local supplier is invaluable when you need advice or help with a sticky problem.
Here's the most confusing part of dealing with pneumatics - solenoids (they're also called 'valves', 'air valves', air solenoids', etc.). Just like air cylinders, they come in all sizes, styles, shapes, sizes, and combinations. There's literally something for everyone when it comes to solenoids.
TIP: Don't worry about what the solenoid looks like, just what it can do.
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The whole 'four port', 'five port', 'two way', 'three way', naming comes from the action of the air as it moves through the solenoid. Again, the names aren't as important as what it does. The best combination of flexibility and use for double acting cylinders is what's called a "five port, four way" solenoid.
A five port solenoid has just that - five connections called ports. Usually, they are labeled A, B, E1, E2, and In. There are (of course) variations of this too. In most cases, any pair of ports that have a label that has an 'A & B', '1 & 2', 'A1 & A2' - are the ports that connect to the air cylinder. Exhaust ports almost always have an 'E' in the name. There is almost always a single 'In'. Notice the phrase 'almost always' - that's because there are cases where solenoids have several sets of in's and out's to fill a particular application. Ok, that's the 'five port' part...
The 'four way' term describes the paths that air can take when the solenoid is in operation. Use the drawing to follow this desription. When a four way solenoid is 'off' or 'deenergized', air will flow through from the In to the A port (that's one way), and also let out of port B through port E1 (two ways). So air goes in through the In port, and out the A port to push the cylinder, and it lets air out of port B through E1.
And when the solenoid is 'on', (energized), air pressure from the In port flows to the B port (the third way), and exhaust air is let out of port A through port E2 (the fourth way).
An important characteristic of the 5 port, 4way solenoid is called "orifice" size. This is the size of the internal air holes - the internal air paths through the solenoid. Its usually quoted as diameter. An orifice of at least 1/8" is recommended, with a size of 1/4" preferred. The orifice size directly affect the solenoid air flow. The more air it can move, the faster an air cylinder can move.
REMEMBER:
In a 5 port, 4 way solenoid, full pressure air is always being
sent out of one of the two air cylinder ports. There isn't a position
where the air cylinder does not have air going to it. So be sure everything's
connected before applying air pressure.
TIP: The most flexible solenoid for double acting cylinders is a 5 port, 4 way solenoid with a 1/4" orifice.
Solenoids come with different voltage ratings also. The most popular voltage is 24 volt DC because that's what most manufacturing plants use, so you'll see lots of them on the surplus market. A 24 volt plug in power supply will easily drive several solenoids. You can find a good selection of wall mount power supplies from Allied Electronics.
A variation of the five port, four way solenoid combines the two exhaust ports into one single exhaust. So its called a four port. Its only difference is in the number of exhaust ports. It's used the same as the five port version.
The most popular solenoid used with single acting cylinders is the three way solenoid. Remember to make a single acting solenoid work, you only need to push from one side. The three way solenoid does just that. When the solenoid is on (activated), the E (exhaust port) is shut off. Air enters the In port, and goes out the A port to the cylinder. And, when off (deactivated), the In port is turned off, and air from the A port is sent to the E (exhaust) port. So in a three way, air only goes in three directions.
Then there's the 2 Way solenoid. It's like a light switch for air. Its either on (sending air to the 'out'), or off (no air out). This valve is really useful in air blasters and air cannons.
The 'plumbing' is fairly straightforward. The air compressor supplies the air pressure. The solenoid directs which side of the air cylinder gets the air pressure. The air cylinder moves the rod. Here's a couple of examples:
There are two very useful fittings: Push-in (or Push-on), and quick-connect. The quick-connect are the standard fittings seen at gas stations. They are extremely useful to 'quickly' connect and disconnect air to props. The Push-in connectors are designed to let you simply "push in" the air hose into the fitting. It has little clips that catch the hose and seal it. Push in fittings are very useful to connect solenoids to air cylinders. Both of these connector types are highly recommended. (this is where a local supplier comes in handy to show you "what's what" in fittings)
Most air systems include air regulators, particle filters, water filters, and manual valves to deliver 'good' air to the solenoids and air cylinders.
The most important of these is the air regulator. This unit sets the overall pressure for your air system. A good starting pressure to run a few popups is 60 psi or less. ("psi" is pounds per square inch) If you're supporting a large air system with lots of popups and long air lines, 70-80 psi is not excessive. However, pressures beyond 80 psi will begin to 'stress' the entire system, and show itself in small leaks around fittings, wear and tear of popup mechanisms, and long running times for the compressor. Pressures over 80 psi can be extremely dangerous if something breaks suddenly. I'd highly recommend using the lowest overall pressure you can - and not go over 80 psi.
You may also consider having several regulators in your air system. This gives you the option of optimizing just the right amount of air pressure to each prop. For example, a jumper popup may only need 30 psi to work. Although the prop's movement would be very exciting to see, running it on your 60 psi system will eventually wear it out and cause a failure. It's always a balancing act to pick the right air pressure to make a prop move - but not too fast that it tears itself apart. Placing a regulator just before the jumper's solenoid lets you reduce air pressure and give the jumper just the right pressure it needs to run.
Particle and water filters are useful items to use to keep your air lines free from debris and moisture. Debris will clog air lines and solenoids and cause erratic or intermittent operation. Moisture in the air line will cause rust to form.
There are also lubricators that add a small amount of oil to the air to keep the mechanics working smoothly.
If you decide on any of these filters, please take some time to read up on them, because just like all the other air parts, there are hundreds of different kinds!
Use your judgement here, and always lean towards safety!
Oh, did I mention that - ScaryGuys has a DVD on Pneumatics?
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