Prevent buckling in cylinders with these sizing tips

In our previous Practical Automation column (MRO, Nov. 2000, p. 58), we looked at some of the oversights and problems involved with selecting cylinder mounting styles. This issue focuses on cylinder loading and its effect on bearing surfaces and cylinder rod sizing.

Without a doubt, the most popular style of cylinder in use today is the single-rod, double-acting cylinder driven either hydraulically or pneumatically. This device is designed to provide a compressive or tensile force on an axial plane concentric with the body of the cylinder. Its ability to withstand forces on any other plane is extremely limited.

The two bearing surfaces in a single rod cylinder are the rod bearing, located in the front cap of the cylinder, and the piston itself that slides against the cylinder wall as it extends and retracts. Forces applied to the cylinder rod in a radial or eccentric fashion will tend to overload these bearing surfaces. The strength of the rod bearing depends, to a large extent, on the diameter of the rod (the larger the piston rod, the more bearing surface to carry the radial load).

The piston, however, is constantly changing its position in relation to the front rod bearing and thus the moment arm or leverage between the two bearings is affected. The closer the two bearings get to each other, the larger the radial force supported by each bearing surface becomes and, as a result, the potential for accelerated wear increases.

Radial or eccentric loads can be caused by:

– Misalignment of the cylinder and load

– Improper mounting of the cylinder

– An unsupported load cycling in a horizontal plane

– The weight of a long-stroke, oversize piston rod as it fully extends horizontally

– An off-centre load as the cylinder extends in a vertical plane.

One of the “band-aids” used to reduce the severity of radial loading is the stop tube. As the name implies, a stop tube is mounted internally in the rod end of the cylinder to prevent the piston bearing surface from extending too close to the rod bearing. A Rod Diameter and Stop Tube chart, in conjunction with a Stroke Factor Table, are used to select the proper stop tube configuration for an application. Typical charts can be found in the Parker Hannifin catalogue and most other cylinder manufacturers have similar information available.

To use the chart, you must first arrive at a “stroke factor” from the table. Then multiply the actual stroke of the cylinder by the factor to arrive at the “basic length” to be used in the vertical axis of the Rod Diameter and Stop Tube chart. The load in pounds is plotted on the horizontal axis to arrive at the recommended rod size and also the stop tube size (indicated on the right side of the chart).

The stop tube length and rod size numbers, when calculated in this fashion, are designed primarily to ensure the cylinder’s ability to support thrust loads without resulting in the collapse or buckling of the rod. Radial or eccentric loading, if severe, may require a longer stop tube to reduce the bearing load. Consult your cylinder manufacturer if in doubt.

The cylinder rod diameter must be sized to support the total axial load without buckling. Buckling of a cylinder rod is most likely to happen near full extension of the cylinder where the catalystic effect of radial loads and unsupported rod weight is most severe. It can also happen to an undersized piston rod while still inside the cylinder when the piston first starts to extend under high loads.

Proper sizing of piston rods and prudent selection of stop tubes, especially in long-stroke cylinders, can save a lot of downtime and expense. Be sure you fully understand the nature and magnitude of your intended load before you attempt to select a cylinder to do the job.

Ted Grove is corporate training manager for Wainbee Limited, Mississauga, Ont., and a widely experienced fluid power trainer.