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Accommodating Cylinder Motion

Nov. 11, 2015
When addressing the motion elements of a cylinder, are there special movements, sensing, or side loads being applied that will require special modifications? If so, you'll need to accommodate them.
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Pneumatic technology holds many advantages as a practical form of power transmission. Pneumatics is often the lowest-cost solution for machine automation, and it can’t be beat for quick, repetitive motions back and forth. This is why pneumatics is usually associated with pick-and-place motion installation.

But pneumatics can easily stop at one, two, or more intermediate positions without having to use any sensors, switches, or sophisticated electronics. They are also available in different designs to serve a wide variety of applications. Here is a guide to which cylinders should be used for different situations:

Must the load stop at any intermediate position?

Three-position cylinders. You can get three or more rod positions from a single cylinder. Many cylinder styles are offered with three position options.

The short-stroke tie-rod cylinder (Fig. 1) is essentially two cylinder bodies combined in a single package. You can specify the same or different stroke lengths to set your work positions as required (Fig. 2).

Four-position cylinders. You can also get numerous cylinder styles in back-to-back configurations that enable positioning at up to four endpoints (Fig. 3). As the name implies, two single-rod cylinders are assembled with their back endcaps attached. By anchoring one rod end and allowing the cylinder body to “float,” four distinct endpoints can be obtained (Fig. 4).

Can the load be allowed to rotate slightly?

Non-rotating options. For applications in which anti-rotation and registration are critical, there are solutions. Maintaining the load’s fixed orientation can be accomplished in several ways.

Figure 5 shows one method used on tie-rod cylinders. Two guide pins incorporated inside the cylinder pass through the piston head. These guide pins prevent rotation of the rod with a tolerance of ±1 deg. A rubber disk is included at the end of each guide pin to take up end play and firmly seat the pins in the precision guide-pin holes.

Because the guide pins are inside the cylinder, they are protected from the environment, physical damage, and are lubricated by the system lubrication. They require no additional space, leaving the rod-end area free for attachments and tooling as required by your application.

External non-rotating options. Another solution uses an external guide block securely attached to the piston rod (Fig. 6). A steel guide shaft, attached to the guide block, assures anti-rotation of less than 0.8 deg.

Twin-rod, non-rotating options. In Fig. 7, twin piston rods are incorporated into the cylinder head to provide anti-rotation. The rods are securely fastened to the piston and tied together externally by a rod-end tool bar. The tool bar ensures that the rods move in tandem and provides an ideal mounting surface for attachments required by your application. The tool bar is furnished with threaded mounting holes or optional counter-bored mounting holes.

Stroke adjustment. Stroke-adjust styles may also be needed when the stroke can change either on the extension or the retraction of the unit.

Adjustable retract stroke. An adjusting screw with a thread-sealing locknut mounted in the rear endcap (Fig. 8) provides a simple, yet rugged adjustment of the cylinder stroke in the retract direction. A fine thread on the adjusting screw will provide precision adjustment. Adjustable retract strokes are offered as optional features for many cylinder styles.

Adjustable extend stroke. It is possible to use the back end of a double-rod cylinder to adjust the extend stroke. A stop collar, bumper, and some kind of impact plate could do the trick. However, if taking this approach, use caution and consider a safety cover to avoid leaving the pinch point exposed (Fig. 9).

Position sensing. Sensing can often change the cylinder based on the type of sensing needed. Standard electronic switching will require magnets to be added to the piston. Proximity switching may require internal or external changes to the cylinder so that the sensing probes will have targets which they can read.  Transducers may also require a variety of internal or external changes to a unit.

Side loads. Side loads often suggest a need for items such as stop tubes or heavier bushings because of the wear produced when the cylinder is in motion.

Will the cylinder have strong side loading or heavy overhung loading?

Cylinder piston rods are supported by a bearing in the front head of the cylinder and the piston itself running inside the cylinder walls (Fig. 10). As the rod nears full extension, the distance (“d”) between support surfaces becomes shorter. The piston rod assembly tends to cock, causing uneven wear on the bearing surfaces and shortening seal life.

Stop tubes. One solution to the problem is to install an internal stop tube (Fig. 11). The stop tube blocks the piston from reaching the front head, thereby increasing the minimum distance between support points. Component wear is reduced and cylinder operating life is extended.

However, in order to maintain the same work stroke, the length of the cylinder body must be increased by the length of the stop tube. Dealing with the increased package size may present issues.

Double-rod cylinders. If you have room available, a double-rod cylinder (Fig. 12) gives you the best piston-rod assembly support.

You’ll have rod bearings in both end caps reducing the load on the piston. And you’ll have maximum distance between support points.

Jerry Walling is Marketing Director at Fabco-Air Inc., Gainesville, Fla. For more information, call (352) 373-3578 or visit www.fabco-air.com.

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