Repairable Air Cylinders
Repairable air cylinders are appropriately named simply due to the fact that they can be repaired and or rebuilt when, not if, a piston fractures, a rod breaks, and rod-thread strips, or any other air cylinder components fail.
The most common type of repairable air cylinder are known as tie rod type air cylinders. They have two removable end caps which are affixed to the cylinder barrel by a minimum of four tie rods.
The tie rods are installed parallel to the outside of the cylinder barrel and either penetrate the cylinder end cap at both ends, or are inserted into the end caps at both ends. Tie rod nuts or bolts are then screwed onto or into threads in the tie rods, fixing the end caps to the barrel, and creating a seal between the end caps and the barrel of the air cylinder, so compressed air cannot escape.
Sealing End Caps
Preventing compressed air from escaping from around the end of the cylinder barrel to end cap junction is accomplished in two ways.
In some styles the end caps butt the end of the cylinder barrel. There is a gasket installed between the end of the barrel and the end caps, When the end caps nuts are tightened, this gasket is compressed, to seal in compressed air.
In other styles of tie rod cylinders, part of the end cap slides inside the barrel when the cylinder is being assembled. An O-ring type seal is fixed between the end cap and the barrel to seal compressed air from escaping once the end caps are bolted on securely.
Molded Tie Rods
More modern style tie rod type cylinders have end caps which are installed by self-tapping screws inserted through holes in the end cap and into mating, non-threaded or threaded holes in the cylinder barrel.
These cylinder barrels have tie-rods that are cast or extruded right into the outside of the cylinder barrel. Removing the self-tapping screws from the end caps frees the caps and the barrel for disassembly of the air cylinder.
While there are some tie-rod type cylinders that are completely custom in design and footprint, the majority are built to NFPA or ISO-6431standards.
Not Necessarily Drop In
This means that any manufacturers NFPA cylinder should be a drop-in replacement for any other manufacturer's NFPA type cylinder in terms of rod thread, mounting footprint, port size and other specs.
However, some NFPA cylinders of the same stroke have different overall lengths, due to differences in end cap lengths. So, though the cylinder meets NFPA specifications in terms of critical dimensions, another maker's cylinder may not just drop-in for a different manufacturers cylinder in the same application, even if it is the same stroke length.
ISO-6431 type air cylinders have consistent critical dimensions, and are much more likely to drop-in as a replacement for another manufacturers ISO-6431 cylinder.
NFPA Is Inch Sizes
The NFPA standard is North American in origin, and available in inch sizes (bore and stroke) with imperial threads and fitting ports. NFPA cylinder bore sizes start at 1 1/2" and can exceed 24" in diameter.
ISO Is Metric Sizing
The ISO-6431 tie rod type cylinders are metric with metric bores and strokes, metric threads and metric port sizes. The smallest ISO-6431 bore size is 32 mm and the largest common size is 100 mm.
Correctly Sizing An Air Cylinder
Here are the things you will want to know to correctly size your tie rod type air cylinder:
- What is the bore size (How much force is needed including allowing a 25% extra force allowance for overcoming friction inside the cylinder, and a safety margin of force for the application)
- What is the stroke required (What is the distance you want to move the object / tooling? Note that for longest cylinder life, the cylinder piston should never "bottom" at the rod or back end cap inside the air cylinder. Rather, the piston should be stopped by having the rod travel stopped externally)
- Is cushioning required for
- Does the application require position sensing
- What style of cylinder mounting, and what are the mounting dimensions
- How will you connect the cylinder rod to the tooling
- Ensure that you have the correct fittings for the cylinder ports.
- Do you need to control the cylinder speed? If so, what type of Flow Controls will you need
- Will the application require a lubricator, or will the factory lubrication suffice? (Check with the cylinder vendor if yours is a high speed application.)
Cylinders are mechanical devices, and over time they will fail. Here are some things to check for when the cylinder rod stops when it is not supposed to:
- Is there an obstruction preventing the rod and the rod attachment from traveling? First, turn off the air. Then remove (carefully) the air lines from both sides of the cylinder to ensure that there's no air trapped that could cause the cylinder rod to "jump" when the obstruction is cleared. Carefully clear the obstruction. Reconnect lines and resume work. Bleed air into the circuit slowly with a Soft Start type valve
- Is compressed air getting to the cylinder? Have you air at the valve? Is there air in the line going to the cylinder? Check to see if any of the lines to the cylinder have been jammed or crimped. Manually over-ride the valve to see if the cylinder will cycle then. Always ensure that doing so doesn't create a dangerous situation
- If the cylinder rod is in the extended position examine it to see if it's been bent due to sideloading. This would prevent the rod from retracting even under pressure
- Is compressed air exiting the exhaust port(s) of the valve even though the cylinder isn't moving? If so, this suggests that the piston seal(s) may have failed and sufficient pressure can't build up inside the cylinder to move the piston.