Because it has only one bearing supporting the rod, and everything off-axis is carried by it and by the piston.

A standard cylinder's rod passes through a bearing and a seal in the front cover. That bearing is designed to guide the rod axially, not to resist a load pushing it sideways or a moment trying to tilt it.

Apply side load and the rod levers against that bearing, which wears it and the rod seal. The piston is forced against the tube wall, scoring the bore and wearing the piston seal. The rod may bend on longer strokes. The symptoms are leakage past the rod seal, sticking, and eventually a cylinder that no longer holds position or pressure.

Manufacturers publish permissible side load figures, and they are small - and they reduce as stroke increases, because the leverage grows.

Where a load must be carried rather than simply pushed, the correct answer is either a guided cylinder or an external guide arrangement, not a larger standard cylinder.

Side load, bending moments and rotation - leaving the piston rod to provide axial force only.

The two guide rods are rigidly fixed to the tool plate at the front and run in bearings in the cylinder body. Because they are spaced apart, any moment applied to the tool plate becomes a push-pull couple on the two rods, which their bearings resist easily.

The same geometry prevents rotation. A single round rod can twist; two parallel rods cannot, so the tool plate maintains its angular orientation throughout the stroke.

That matters enormously for anything carrying a gripper, a tool or a sensor, where orientation must be repeatable.

The piston rod then does one job - generating axial thrust - and is not loaded off-axis at all, so its seal and bearing last as designed. Manufacturers publish permissible load and moment figures for the guided assembly, and they are far higher than for the bare cylinder.

By load, speed, accuracy and duty cycle, and the two have quite different characteristics.

Plain bearing guides use a bushing - typically bronze or a polymer - running on the guide rod. They tolerate shock and contamination well, are quieter, cost less, and are forgiving of poor alignment. Friction is higher, so they suit slower movement and shorter duty cycles, and they wear gradually, opening up clearance and reducing accuracy over time.

Ball bearing guides use recirculating ball bushings. Friction is much lower, so they suit high speed, high cycle rates and continuous duty, and they hold accuracy better through their life. They are less tolerant of shock loading and of contamination, and they cost more.

The practical rule: high cycle rate, high precision or high speed points to ball bearings; heavy shock, dirty environments, occasional movement and cost sensitivity point to plain bearings.

Check the manufacturer's load ratings for each - they differ substantially for the same body size.

From the load, its distance from the cylinder axis, and the stroke position - not from weight alone.

A guided cylinder's rating is expressed as permissible load together with permissible moments about the axes. A modest load mounted well out from the axis produces a large moment, and it is usually the moment rather than the weight that governs.

Stroke position matters too. With the cylinder fully extended, the guide bearings are closest together relative to the applied load, so the reaction forces on them are highest. Manufacturers therefore publish capacity as a function of extension, and the figure at full stroke is the one to design against.

Dynamic effects add to it: accelerating and decelerating a load creates forces beyond its static weight, and on a fast pick-and-place those can dominate.

Provide the load, its offset in each direction, the stroke and the cycle time when specifying, and let the supplier check against their data - it is a calculation that is easy to get optimistically wrong by hand.

Good repeatability at the stroke ends, with the guides determining how well position holds under load.

A pneumatic cylinder driven between its end stops repeats those end positions well, because the stops are mechanical. Repeatability figures quoted for guided cylinders generally refer to that - returning to the same end position cycle after cycle.

What the guides add is that the tool plate arrives in the same orientation and lateral position as well, not just at the same extension, and that it holds that position when the load is applied. A bare cylinder can reach the same extension while the tool has deflected sideways under load.

Mid-stroke positioning is a different matter: pneumatics are poor at stopping accurately part way, because air is compressible. Where mid-stroke positions are needed, external stops, a locking device or an electric actuator are the appropriate answers.

For two-position work, which is most of it, a guided cylinder is both accurate and repeatable.

Yes - bolt-on guide units exist - but an integrated guided cylinder is usually better and often cheaper overall.

Add-on guide packages clamp to a standard cylinder and provide guide rods and a tool plate. They are useful for retrofitting an existing installation where a side-load problem has emerged, and they avoid replacing the cylinder.

The difficulty is alignment. The guide rods and the cylinder axis must be parallel, and any error puts the two systems in conflict - the guides try to hold one line and the cylinder another, which binds and wears both. Achieving that alignment on an assembly of separate components is harder than it looks.

An integrated guided cylinder has the guide bearings machined into the same body as the cylinder, so alignment is inherent. It is also more compact and generally has higher published load capacity.

For a new design, specify the integrated product. Keep bolt-on guides for fixing something already built.

Bellows, scrapers, corrosion-resistant materials and sealed bearings, depending on what the guide rods are exposed to.

The guide rods are precision-ground surfaces running in bearings, and they are exposed along the stroke. Abrasive dust, weld spatter, coolant and swarf all damage them, and once a rod is scored the bearing wears rapidly and accuracy is lost.

Bellows over the exposed rod length are the usual protection in dirty environments, and rod scrapers deal with lighter contamination.

Stainless rods, corrosion-resistant bodies and appropriate seal materials are specified for washdown, food and chemical environments, where the cylinder is cleaned regularly and the water is sometimes hot and chemically aggressive.

High temperature and cleanroom variants exist with appropriate seals and low-particulate bearings.

Specify the environment as carefully as the load. A guided cylinder that would last for years in a clean assembly cell can be destroyed in months on an unprotected welding or grinding application.