Cylinder Mounting & Column Loading
“I need 100,000 pounds of force on a 120” stroke with a 2” diameter piston rod and clevis mount.” A click and then there is no one on the other end of the phone.
These words will cause any cylinder manufacturer to cringe, run away, and then talk about a crazy customer requirement at the water cooler as if it was the latest escapade from their continued saga of made up fishing adventures. But why is this such a scary scenario? Other than the fact that a 2” diameter rod is not a standard size in any cylinder that will produce 100,000 pounds at standard 3000 psi hydraulic pressure, the 2” diameter rod will not handle the column loading.
Imagine for a moment an uncooked spaghetti noddle. If you stand it up lengthwise on the counter and then push down on it, it bends and eventually snaps. This is the same problem the cylinder requirement mentioned above will have if it reaches that force at the full 120” of stroke. This issue is compounded by the clevis mount.
There are six basic mounting classes that affect column strength. The class is determined by both the cylinder mounting and the rod end mounting. For instance a fixed cylinder mount, such as a front head flange or a side lug mount, pushing on a load that is both fixed to the rod end, and guided through its range of motion would be considered a class 1 mount. A cylinder with a clevis mount and a rod end with a pinned connection to its load would be considered a class 5 mount. This is the difference between holding both ends of the spaghetti noddle and pushing, verses just pushing on both ends. Enough force will still cause it to buckle, but if you hold both ends rigid, it will take more force before buckling occurs.
Image credit: Getideaka
Where have you been all my life?
While this is nice to understand conceptually there is a considerable amount of calculation that goes into knowing just when a column will break under load. Thankfully, our engineers have spent the time calculating this out and have put it into a chart form in our free download: 12 Fluid Power Engineering Charts and Guides for Everyday Use. Page EN-9 shows all six classes of mounting, and pages EN-11 and EN-12 show a loading chart for each standard rod size, class, and strokes up to 120”. It shows that our 2” diameter rod in question above at 120” of stroke on a clevis mount cylinder (class 5) will only handle 1,340 pounds of force before it has gone beyond its safe operating capacity. If the 120” of stroke is the critical requirement, either a class 1 mount with a 3” diameter rod would do the trick (109,000 pounds), or a class 5 mount with a 7” diameter rod (201,950 pounds). If a requirement does not show up on the chart, Sheffer engineers are happy to help assess the feasibility of your needs.
The long and short of it
Long cylinder strokes can add another layer of design consideration to any project, but with the right tools and understanding these potential problems can be easily mitigated and a good solid design can easily be obtained.
Now it’s time for lunch, we're thinking Italian!