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Need help spec'ing a pneumatic cylinder

summersab

Plastic
Joined
Aug 7, 2015
I'm the owner and founder of a company called Say It With Beef that makes floral arrangements ("BROquets") our of beef jerky. Because my business is a bit . . . unique, I've had to build a lot of custom equipment to help speed up production.

One thing I just finished a few weeks ago is my new jerky press which we use to stamp out the needed shapes of jerky. I built what is basically a Welles juice press using 304 stainless with some "jerky specific" additions. The thing works beautifully except for one thing: the bottle jack is SLOOOOOW. I've got a 4 ton Strongway bottle jack (one of the few brands that uses non-lead paint), and it's plenty strong to cut the jerky, but it takes forever to pump it up to get work done.

That got me thinking about pneumatics. They're fast, pretty quiet, and would likely generate enough force to cut through the jerky. To get a rough guess of how much force I would need, I took some of my jerky, put it in the cutter, and figured out how many slices I could cut using my body weight. Doing the math, I would need about 650 lbs to cut through five pieces of jerky at a time (what my press is designed to cut).

However, I know pretty much nothing about pneumatics other than that there are single and double acting cylinders and the force available is derived from the bore size, rod diameter, and available air pressure. I know nothing about reliable brands, what factor I should use to over-design and have enough available force, etc. Any suggestions? I've been looking at 3" bore cylinders by American Cylinder, but I'd like some feedback from someone who knows what they're talking about before I go spending the money.

Thanks for your help!
 
Think of the units... psi...pounds per square inch which is a unit of force

the equation is force = pressure * area of the cross section of the cylinder.

So whatever your supply pressure time the area of the cylinder. If you wanted to get really fancy subtract the rod diameter if it applies depending on the cylinder type and which way its flowing.

Single acting has a spring to return the piston. Double acting tend to be the most common and easiest to work with the valves and not have a valve port free flowing air. Shortest require stroke will limit the air consumption

I like the ones from automation direct, never had a issue with them.
 
Let me phrase my question a LITTLE differently. We all know that a car's rated HP is at a certain RPM with specific operating conditions. Similarly, I'm sure the ratings on pneumatic cylinders are . . . "ideal." I can run the math on them all day and understand the equations, but I've never worked with them in real life, so I don't know what to expect. On one hand, I don't want to under-design - that would certainly suck. On the other hand, I don't want to over-design, either - why spend more money than I need? Does that make sense?
 
air is dangerously fast if its not automated watch some youtube videos of air cylinders stroking. a liquid stroke would be way preferable you could also use house hold water pressure with the right sizing of cylinder then dump to waste.
 
Flow controls will solve the speed problem.A single acting cyl would probably work fine and would simplify the controls.Use a quick exhaust valve in the pressure side to have a quick return.I have used small supply side lines to control speed(instead of flow controls) which makes a tamper proof simple system.They also make tamper proof,locking flow controls.

Also since you are in the food processing business they make non lube air cyl.'s for that.
remember that all cylinders eventually leak so an air cyl would be my choice.A contaminated product is big time problems.

The disadvantage of air compared to hydraulics is the spring effect,which may not be a problem in your application.
 
I'm going to go against the grain a bit on this one: cylinders, especially larger bore cylinders like you're looking at, aren't all that fast - especially if you're running it off a compressor that's anything other than massive. If you need higher forces, you'll be looking at a 5 or 6 inch plus bore cylinders, which consume a boatload of air, and pretty much crawl.

If you really need the force to match a bottle jack, you should look for an air over oil unit. Those use a small cylinder to 'rapid' to the press distance, then 'switch' to a hydraulic system automatically that can generate tons (literal tons) of force using only standard air pressure.

I'm pretty sure Bimba still makes them new, and they're a good industrial brand and reasonable prices. Alternatively, check out Tox stuff on eBay and the typical industrial second hand stores.
 
Not fast? Huge compressor? Did you run the numbers? A typical small industrial compressor can provide around 20 cfm @ 90-100 psi, and that 3" cylinder with a 6" stroke requires about 42.5 cubic inches of air. That means you can get over 40 full-stroke actuations (one way, spring return) for each cubic foot of output, or 800+ strokes per minute using the full capacity of a 7.5hp compressor. Plus, the action will be very fast if sourced with a 3/8" or larger line because the full stroke isn't generally under full load. A restricted orifice, or 'snubber' would be required in most cases to dampen the movement. Air would be my choice, and the aforementioned Bimba cylinders are reasonable priced and disposable. Their rated life is measured in miles of travel of the piston and in most cases it is thousands of miles. No rebuilds, little downtime. Just swap it for a new one as the need arises.

An air cylinder automatically 'rapids' to press position, the rate is selectable by adjusting the flow rate.
 
A few more details (I'm going to talk about Bimba, because they're the ones I use. I am not otherwise connected with them in any way.) The cylinders are available in several configurations, including double shaft (one out each end). They may be had with stainless rods, and the body is always stainless in the disposable model and they never need lubrication. I've had occasion to call Bimba and was readily transferred to an engineer who answered my questions on the spot. Can't beat that kind of service.

Oh, as to the OP's remark about 'real world' performance, for all practical purposes the math works out every time. You don't have to worry about the 'chinese horsepower' type issues. The modern cylinders are very low friction and end-point losses are negligible.
 
I like the comments Gordon has posted.

Use a larger diameter cylinder at lower pressure rather than smaller diameter cylinder at higher pressure. I hope the rationale is obvious.

Adjustable flow valves will control the operating speed.

Adjustable pressure regulators will control the total force.

Single acting cylinder return springs are not very hefty so, if you're lifting much product/tooling on the return stroke, either design to use gravity in your favor or use double acting cylinder(s).

Don't use an air cylinder to guide any motion as that will certainly lead to side loads and premature seal failure. In other words, the required motion of your device must be fully constrained without the cylinder(s) installed.
 
The best way of controlling the stroke speed is with a flow valve on the exhaust side of the cylinder, not the inlet. Do that and it will not be "terrifyingly fast" (lol)

I would choose a larger cylinder than ideal, and control the force with a pressure regulator, and the speed with a flow valve.

You could also an use air over oil setup which are more controllable, but also messier.
 
I'm liking what I see with the Bimba Flat-1 series. Those would integrate very well into my design. I'm curious about the air over oil units, but that does sound messy, and I wouldn't mind getting away from oil since food is involved (however, it's not a FDA problem if the oil units are BELOW the food, which they are - it just sounds like more of a pain).
 
If your process involves fingers in harms way, by all means consider a liquid drive. If not, you really can't beat the speed and efficiency of air. You can cycle as fast as your 'feeder' can feed.
 
Good points. I can always add in safety features mitigate finger-removal issues. I guess my other concerns are power and cost. Can I get 1/2 ton of (real-world) force out of an air cylinder with a decent shop compressor? It looks like the answer to that is "yes." With liquid/hydraulic, I could absolutely get enough power, but unless I'm just using a bottle jack like I am now (slow), the costs are pretty high. Maybe I'm not searching for the right type of hydraulic units. If I'm trying to keep the cost under $150 ($200 max because this is still all a big prototype), is there a liquid/hydraulic unit you would recommend?
 
oooOOOOHHHHhhhh - Vaccon is a subsidiary of Bimba. I didn't know that. I have one of those Vaccon Venturi vacuum pumps that I use to seal oxygen absorbers I haven't used. It's a very nice little device. That raises my confidence and interest in Bimba considerably.
 
. . .I'm trying to keep the cost under $150 ($200 max). . . is there a liquid/hydraulic unit you would recommend?

Your hand-operated bottle jack is the only thing you'll find for that money. Hydraulics get expensive very quickly, and high-speed systems start out in the high-dollar zone. Not just the power unit but the controls as well. As for prototyping, you can get great deals on air cylinders on eBay, etc., although the larger 3" and up sizes are not as common.
 
Not fast? Huge compressor? Did you run the numbers? A typical small industrial compressor can provide around 20 cfm @ 90-100 psi, and that 3" cylinder with a 6" stroke requires about 42.5 cubic inches of air. That means you can get over 40 full-stroke actuations (one way, spring return) for each cubic foot of output, or 800+ strokes per minute using the full capacity of a 7.5hp compressor. Plus, the action will be very fast if sourced with a 3/8" or larger line because the full stroke isn't generally under full load. A restricted orifice, or 'snubber' would be required in most cases to dampen the movement. Air would be my choice, and the aforementioned Bimba cylinders are reasonable priced and disposable. Their rated life is measured in miles of travel of the piston and in most cases it is thousands of miles. No rebuilds, little downtime. Just swap it for a new one as the need arises.

An air cylinder automatically 'rapids' to press position, the rate is selectable by adjusting the flow rate.

I guess it all depends on what someone's definitely of 'fast' is. I'm a professional automation engineer - cylinders are the slowest devices we use. Typical rule of thumb for calculating cycle time on a station is 12 inches per second for a pneumatic device, and that often proves to be optimistic. On a 3-4"+ bore cylinder running on plant air we would typically use 6-9" per second when it comes to calculating cycle times. The shorter the stroke, the less this matters, obviously. So if you can keep your press stroke short, you'll be a lot better off for a bunch of different reasons.

Of course, this is all in an actual industrial automation setting, which has to be controlled and safe. Although I'm not convinced dropping an airline on the floor directly from the compressor will actually make things much faster.

For the sake of comparison, a typical servo motion will be 2-3 times the speed of pneumatics (as a rule of thumb) and I'm currently working on a linear motor system that's completing a ~48" motion in about 0.8s (the carriage assembly weighs about 500lbs as well, so it's a real eye-opener the first time you watch it take off.)

If you want to go waaaay too deep down the rabbit hole of pneumatic system design, SMC has two different free software programs that are very useful. If you Google 'SMC Model Selection Software' it will bring you right to the page that can calculate speeds, outputs, sizing information, and about 100 other things you never knew you wanted to know based on every conceivable piece of your pneumatic system. (Out of curiosity, I checked your application and it spits out a 1.03s extend time to achieve 90% of desired force with a 2 inch stroke. Figure the same time on the return stroke for a ~2s cycle time per press cycle.)

In the future, if you run into a situation where you want a second opinion on something, are looking to increase efficiency, or to automate more fully, feel free to give me a shout. You've got this one in hand, but I still do consulting and design work on the side for folks.
 
It appears your speed calc's are starting and ending with a full load which is not typical for a 'load and press' operation of the type described by the OP. The speed factor diminishes rapidly (no pun intended) when cost is considered in the realm of a small 'mom and pop' organization. Not just the power and control, but general installation as well. Running a hundred feet of PEX air line is a fraction of the cost of rigid hydraulic tubing, and then there is the noise factor to consider as well.

I am NOT an engineer of any kind, though engineering smallish tools and equipment is a large part of my job so I defer to you on all points. My comments come from direct personal experience of that 'small operation' variety and I believe they apply to the situation described by the OP.
 
It appears your speed calc's are starting and ending with a full load which is not typical for a 'load and press' operation of the type described by the OP. The speed factor diminishes rapidly (no pun intended) when cost is considered in the realm of a small 'mom and pop' organization. Not just the power and control, but general installation as well. Running a hundred feet of PEX air line is a fraction of the cost of rigid hydraulic tubing, and then there is the noise factor to consider as well.

I am NOT an engineer of any kind, though engineering smallish tools and equipment is a large part of my job so I defer to you on all points. My comments come from direct personal experience of that 'small operation' variety and I believe they apply to the situation described by the OP.

I'd have to go back and double check, but the results seemed about right to me, so I wasn't looking all that hard.

I definitely agree - pneumatics is going to be cheaper than hydraulic across the board, and much less of a headache as well. But the 'air over oil' cylinders (sometimes called 'booster' cylinders or 'hybrids') have an entirely internal hydraulic system - you just have to run air to them, same as a normal cylinder. I agree that an all hydraulic setup would not be the right solution here.

The TOX website has a very cool graphic demonstrating how they work if you're not familiar, or haven't seen an air-hydraulic cylinder in a while: Air Hydraulic Cylinders - Air/Oil Cylinders | TOX(R) PRESSOTECHNIK

But nothing TOX is cheap. At all. There are some cheapo knockoffs available via eBay, but I wouldn't recommend them, just on principle.

And I agree that a purely pneumatic solution may do everything the OP wants and needs - I just thought I'd suggest another option that would have the added benefit of reduced cycle time and reduced air consumption. I'm still not clear on what his required press forces are though - if he can figure that out and a reasonably sized cylinder will generate the force he wants, it'll probably be a lot cheaper and easier than an air-hydraulic cylinder.
 
Thanks for the link. I haven't looked at those systems for years. As I recall, the OP's force requirement was near, but under, 700lbs. The 100psi figure is just a round number, all the mentioned cylinders are rated well above that.
 








 
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