Prototype Development - Problems With Commercially Available Air Cylinders.

Patents, Prototypes, Manufacturing, and Marketing New Inventions

None of the commercially available pneumatic cylinders were suitable for our needs, so we developed our own.






Prototype Development - Problems With Commercially Available Air Cylinders

Continued from previous page: 

Developing a Prototype Air Cylinder

Prototype Development - Problems With Commercially Available Air Cylinders

Problems with commercially available pneumatic cylinders

Common and frequent problems we encountered when using commercially available air cylinders included the blade mount coming loose from the piston rod.  We tried every kind of locking nut, locking washer, and chemical thread locking adhesives with no luck. Not even combinations worked.  When we field tested the machine we left a breadcrumb trail of parts behind us. We spent more time repairing cylinders than we did harvesting. Such is the life of an inventor I guess... well, at least this inventor.

We finally solved the problem of the blade mounts falling off by using a 1/4" NPT tapered pipe thread on the piston rod and blade mount.  We had a similar problem with the piston rod and piston. Again, the tapered pipe thread did the trick. In fact, it worked so well that I decided to try and obtain a patent on it.

I was going to try to get a patent on the method of locking threaded parts together using parts with mating tapered threads.  I was all set to file a provisional patent on the method when I discovered a patent through Google Patents that was issued in 1956 that describes using tapered pipe threads to keep the piston rod from coming loose from the piston and the mount on the other end of the piston rod. Darn.  Talk about prior art... jeeze, So much for patenting that idea.

When we used tie rod cylinders the tie rods would stretch and the tie rod nuts would come loose. Even when we used the highest strength Locktite the tie rod nuts would soon fall off.  The same thing would occur with any threaded parts.  The spring cylinder on the rear, the piston rod bushing retainer, and the blade retaining bolts would all vibrate loose and fall off. We did not have a problem with the parts that had tapered pipe threads like the port fittings, which were NPT threads.

All of these problems have been solved by using the tapered threads and with some welding.  We constructed the cylinder by welding the front and rear heads to a heavy wall steel tube and putting 1" pipe threads in the heads for the rear spring cylinder and the piston rod bushing retainer.  It's a lot stronger than most hydraulic cylinders.

 It is simpler, easier to disassemble and assemble, and has fewer parts than other air cylinders.  It's much faster than any other pneumatic cylinders I've ever seen as well, probably in a large part due to the huge 1/2" pipe ports in the cylinder heads. I guarantee you won't find 1/2" ports in a 1" bore cylinder anywhere except at this point, in my garage. 


Life Expectancy

A heavy yielding asparagus field might produce around 75,000 spears per acre per season, with the center 6 cylinders doing most of the cutting.  The machine should be able to do about 50 acres per day.  50 acres times 75,000 spears per acre per season = 3,750,000 spears per season.  Divide that by 6 cylinders and you get about 625,000 spears per cylinder per season.

I need to find out a number of things.  I need the actual time it takes for the cylinder to stroke down, and up, so that I can program the correct information into the computer that sets up the timing for the cutting.

The machine senses the spear about 16 inches ahead of where the spear gets cut.  The time it takes for the cylinder to reach it's fully down position is fixed. Therefore the delay in firing the air cylinder to provide accurate cut timing depends on how fast the machine is moving forward.  I've programmed the circuitry to take the output of a shaft encoder geared to one of the wheels and calculate the proper delay for the cylinders.  To do that, I need to know the down stroke time as accurately as possible.

The circuitry is programmed to prevent the cylinder from firing until it has had time to fully retract. If it fires before it is fully retracted it could travel too far on the down stroke and end up bottoming out.  The timing will be wrong as well, so you won't harvest the spear anyway. I therefore need to know the retract time as accurately as possible.

Other useful information would be what the terminal velocity is, how long it spends at the last inch of stroke as it decelerates, stops, reverses, and accelerates on the retract stroke.  In other words, how long does the blade spend in the soil as the machine moves forward, and how the mass of the blade and blade mount affect the cycle time of the cylinder.


Seal and bushing wear

Obviously I need to find out if the cylinders will survive 650,000 strokes without suffering from metal fatigue somewhere or some similar structurally related problem.

I also need to see how long the seals and bushings will last.  Bimba air cylinder company has on their website information about cylinder life.  The Bimba cylinders should wear the same as mine as far as the seals and rod bushings go.

Here is what Bimba has to say:

"Bimba cylinders have been designed and tested for a rated life of 1,400 miles of travel when properly applied and lubricated per recommendations. Bemba's option E has been designed and tested for a rated life of 2,800 miles of travel when properly applied in an un-lubricated environment."

Option E is for using special permanent grease during assembly.

If my cylinders travel 20 inches down and 20 inches up 650,000 times, they will have traveled 410 miles.  Doing a little more math, if the cylinders will make it to the 1,400 mile mark, they will have survived over 2 million strokes.

The only way to find out is to test, so I shall.

Summing it all up, I need to obtain the following information about the pneumatic cutting cylinders:

  • Stroke extend time
  • Stroke retract time
  • Time blade spends in soil
  • Number of strokes until failure
  • The effects of Lubrication
  • The effects of different blade mount masses


Continued on next page - Setting up testing


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