Twelve Tips for Better Cylinder Selection - Valin Corporation

Here's how to design hydraulic cylinders that improve performance, last longer, and cost less.

Hydraulic cylinders harness fluid pressure and flow to generate linear motion and force, and they work well in both industrial machines, like presses and plastic-molding machines, and in mobile equipment, like excavators and mining trucks. And when compared with pneumatic, mechanical, or electric linear-motion systems, hydraulics can be simpler, more durable, and offer significantly greater power density.

Hydraulic cylinders are available in an impressive array of types and sizes to meet a wide range of application needs. Choosing the right cylinder is critical for maximum performance and reliability. Here are 12 practical tips for selecting, sizing, and operating the best one for a job.

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Selection Considerations

1. Choose the right cylinder type. Two basic hydraulic cylinder designs for industrial applications are tie-rod and welded cylinders.

Tie-rod cylinders use high-strength threaded steel tie rods on the outside of the cylinder housing for additional strength and stability. In the U.S., this is the most common cylinder type. They're used on most general industrial applications, such as plastics machinery and machine tools, although they tend to be limited to 3,000 psi maximum operating pressure. The cylinders are built to NFPA standards, which makes their dimensions and pressure ratings interchangeable with any other cylinder built to that standard.

Welded or mill-type cylinders have a heavy-duty housing with a barrel welded or bolted directly to the end caps and require no tie rods. Designed for higher pressures, to 5,000 psi or greater, they are generally preferred in more rugged applications such as presses, steel mills, and offshore settings with harsh environments and wide temperature swings.

Unlike U.S. OEMs, European manufacturers typically use mill-type cylinders in almost all general industrial applications. (They also use tie-rod cylinders, but generally for lower-pressure tasks up to 160 bar (2,350 psi).) However, due to the design, tie-rod cylinders are less expensive than mill-type cylinders - another reason for widespread use in the U.S.

Also, keep in mind that cylinders are often customized. NFPA cylinder standards dictate dimensions, pressure ratings, type of mountings, and so on - they're standard catalog products. However, engineers designing custom machinery often need to deviate from the standards with special mountings, port sizes, or configurations to suit a particular application. About 60% of the cylinders sold in the U.S. are catalog items, while 40% are modified products with unique requirements.

2. Select the proper mountings. Mounting methods also play an important role in cylinder performance. The cylinder mounting method first depends on whether the cylinder body is stationary or pivots.

For stationary cylinders, fixed mounts on the centerline of the cylinder are usually best for straight-line force transfer and minimal wear. Among the different variations, flange mounts are generally preferred. Loads are centered on the cylinder and opposing forces are equally balanced on rectangular or round flanges. They're strong and rigid but have little tolerance for misalignment. Experts recommend cap-end mounts for thrust loads and rod-end mounts for pull loads.

Centerline lug mounts also absorb force on the centerline but require dowel pins to secure the lugs to prevent movement at higher pressures or under shock conditions.
 

Side-mounted or foot-mounted cylinders are relatively easy to install and service, but they generate offset loads. The mounts experience a bending moment as the cylinder applies force to a load, potentially increasing wear and tear. Heavy loading tends to make long-stroke, small-bore cylinders unstable.

Side and foot mounts need to be well aligned and on the same plane, and the load supported and guided. Otherwise, induced side loads due to misalignment lead to cylinder wear and seal leaks. Engineers also must be concerned with shear forces on the bolts. Add a dowel or shear pin and keyway behind the feet to prevent the forces from potentially shearing the mounting bolts. If necessary for extra support, add another set of foot mounts in the cylinder midsection in addition to those on the head and cap ends.

3. Select the right pivot mountings when the cylinder body moves. Pivot mounts absorb force on the cylinder centerline and let a cylinder change alignment in one plane. Common types include clevis, trunnion, and spherical-bearing mounts.

Clevis mounts can be used in any orientation and are generally recommended for short strokes and small to medium-bore cylinders. Cylinder engineers prefer clevis mounts with spherical bearings over those with plain bearings because they allow for a bit more misalignment and are, thus, a bit more forgiving. However, if using a spherical bearing on a rear clevis, they also recommend a rod-end attachment that pivots - such as a spherical rod eye. The combination helps compensate for any side loading or potential misalignment.

Trunnion mounts come in head, mid, and rear-mount versions. The mid-trunnion design is likely the most common, as it offers designers a bit more flexibility. They can be specified exactly in the cylinder mid-section or most anywhere toward the front or rear as the application demands. Once specified, however, the mount is not adjustable.
 

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Sizing Considerations

For all types of cylinders, important parameters include stroke, bore diameter, rod diameter, and pressure rating.

4. Piston-rod diameter is critical. Perhaps the most common error in hydraulic design is underspecifying the piston rod, making a cylinder more prone to stress, wear, and failure. Piston-rod diameters can range from 0.5 to more than 20 in., but they must be sized for the available loads. In a push application, it is extremely important to size the rod diameter properly, based on Euler calculations, to avoid rod buckling or bending.

When designing a cylinder to generate a required force, sizing the rod is always the first consideration. From there, work backward and determine the bore size for the available pressure, and so on.

5. Prevent rod bending. In cylinders with long strokes, a fully extended rod can bend under its own weight. Excessive bending leads to wear and damage to seals and bearings. It could even cock the piston inside the bore, which can score and damage the inner surface of the cylinder. Rod deflection should never exceed 1 to 2 mm.

Cylinder rods that are at risk for bending or misalignment require additional support. Depending on the stroke length, a stop tube - which increases the bearing area of the cylinder - may be required to prevent excessive wear and jack-knifing. Engineers might also consider a larger diameter rod, which increases strength. But that also increases weight and may be self-defeating, so do the math carefully. In extreme cases, users may also need to add external mechanical support for the rod, such as a saddle-type bearing.

6. Watch out for impact loads. Stroke length, the distance needed to push or pull a load, can vary from less than an inch to several feet or more. But when the cylinder extends or retracts, ensure that the piston doesn't bottom out and generate impact loads at the end of the stroke. Engineers have several options: Add internal cushions to decelerate the load near the end of stroke; add an external mechanical stop that prevents the cylinder from bottoming out; or use proportional-valve technology to precisely meter flow and safely decelerate the load.

7. Weigh bore diameter versus operating pressure. To produce a given amount of force, engineers can specify large-bore cylinders that operate at low pressures, or vice versa. Generally, systems that operate at higher pressures but with smaller cylinders are more cost-effective. Also, the benefits cascade. Smaller cylinders require less flow and, in turn, smaller pumps, lines, valves and so on. Many installations see an overall cost reduction by moving to higher pressures.

That said, cylinders are rated for both nominal (standard) pressure and test pressure to account for variations. Systems should never exceed the nominal rated design pressure of a cylinder.

8. Add a factor of safety. While design calculations are essential, real-world operations differ from theoretical results. Always assume peak loads will require additional force. The rule of thumb is to choose a cylinder with a tonnage rating of 20% more than required for the load. That compensates for losses like friction from the load, efficiency losses in the hydraulics, actual pressure below the rated system pressure, slip-stick on cylinder seals and bearings, and so on.
 

Operating Considerations

Cylinder parameters like stroke and force must match machine requirements, but that is only half the challenge. Environmental and operating demands also play a major part in determining a cylinder's ultimate success.
 

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Has anyone here done some experimenting with different types of plastics/nylons/teflons in making their own seals for their air rifles?
My lathe is bored since finishing some small cannon projects and my lovely wife wants me back out in the shop and NOT underfoot.
Any and all experiences with what has and has not worked will be MUCH appreciated.
Thanks,
steve

Decades ago when I started cutting my own piston seals for my .177 R9 (LOL, before I had internet access) I would buy 90 durometer urethane rod and cut my seals using hss tools honed razor sharp. The first thing I did was to machine a "mandrel" to hold the urethane seal blank for machining. The mandrel was shaped like the R9 piston seal retainer, however the "flange" was machined a few thou oversize so the recess machined in the urethane blank wouldn't spin when machining the face. Not a good explanation so here are a couple pics of the "mandrel" (LOL, found it in an ols parts drawer)...........


To make a seal I heated the head of a 3/4" hex bolt and used it to melt the surface pf a 3/4" thick x 1 1/8" diameter urethane "puck". The melted urethane formed a "glue" so that when the bolt cooled the urethane puck was solidly attached for machining.

The hex bolt was then chucked in the lathe jaws and the "recess" of the seal base was begun using a custom ground HSS bit sized to cut the piston seal retaining groove. Here is a pic of the tools used............


When properly sized the home cut urethane seals worked as well as the aftermarket seals I bought at that time however I found that, like the urethane aftermarket seals, the urethane didn't hold up well to heat at the transfer port. They only lasted about 6 months of a 10,000 shot per year season before the face of the seal was cratered to the point that velocity was affected. Actually, this was also an issue with HW factory piston seals where the face was eroded at the transfer port..........


Since the "flat faced urethane seals" I cut performed better than the thin edged HW piston seals of that time I simply cut a new urethane seal whenever needed till a certain early season field target match in Virginia where the sight in was done during snow flurries and the temp rose to the upper 50s during the match. Well, it seemed that the combination of "tar on the spring", molly paste on the seal and the 7/16" thick seal sides caused poi shifts due to lube viscosity and urethane durometer changes due to the 25 degree temp shift during the match.

It then seemed that if there wasn't 7/16" of lubricated rubber sliding against the receiver ID under pressure there also wouldn't be as much "seal material" affecting the sliding with atmospheric temperature changes. It was then that I went to an ACE home center, bought a few 1" dia x 1/16" "Plain Jane" orings, cut an oring sealed piston cap to replace the "rubber" seal, then tested and found that indeed temperature related poi shifting was greatly reduced (even the number of shots required to stabilize the poi) so I've been using oring sealed piston caps ever since. I've refined the oring seal fit (amount of squish), the material (75 or 80 durometer military spec Viton), and even the lube used. Gone are the dieseling prone "dinosaur oil based lubes" replaced with non-dieseling "space station lubes" called Dupont Krytox. Here are a few early piston cap pics and a couple recent versions..........
The very first cap I tested for a few thousand shots when I was using molly bearing lubes next to a new old design HW95 piston seal..........


Later piston caps...........


Recent piston caps...........


I have found that the current HW95 factory piston seals function almost as well as a properly fitted piston cap to the point that if you get a "good one" it really isn't worth the hassles of fitting an oring sealed cap, however the issue with current HW piston seals is getting one that "fits properly" (some are undersized) and a seal that doesn't crack on the face (LOL, aluminum oring sealed piston caps don't crack or erode). The first "new style" HW95 piston cap I tested worked out OK and here is a pic..........


Since I did that test I've received some HW factory seals in guns previously tuned by others and from "online pics" that were degraded like this............

Ed, Great work.  I like your hybrid metal oring piston seal.  Is the plastic to reduce weight ?  Seems like better for attachment too.

The newer piston seals still have " T6 cores", the Delrin sleeve is simply there for a "slippery surface". Here are a few pics of the "bearing sleeve" being pressed onto the core for machining..........


I learned in the beginning that the "seal retaining button" on HW pistons aren't always concentric with the piston shell which created "alignment issues" with the rather close "cap to receiver clearance" I use for my size 020 orings (.005ish piston cap to receiver ID). Also, like HW piston seals & pistons, HW95 receivers aren't especially precise at times creating "oring cap setup issues". Because of this I fit an oring sealed piston cap blank to the piston with a light press fit, mount the piston in my "4 independent jaw lathe chuck" and machine the cap to fit and cut the oring groove to suit the receiver. This was the piston cap and piston shell are "aligned" even if the factory piston has a seal retaining flange off center. HW internal component variations have little affect on a factory parachute seal but it can affect the function of an oring seal with less than recommended compression that I use. Here are some oring fitting info. Instead of "15% minimum compression" I only use about 10% compression for my guns...........
 

As a side note. years ago I bough two Chinese B3s at a Cummins Truckload Sale for $19.95 each, cobbled together one B3 from the best parts of the two, then machined an oring sealed piston cap to replace the leather seal. The result was very good as these 25 yard targets show, one with factory irons and one with a 6x Burris scope after stripping off the factory sights.........


Factory piston was "cleaned up" on the lathe & the leather seal was replaced with and oring sealed piston cap. The crooked spring guide was replaced with a home turned Delrin guide fitted to the factory spring. The mods increased the factory 550fps velocity to 700fps with the original spring......