Cylinder mounting dangers
By Ted Grove
Fluid power cylinder manufacturers generally offer a huge assortment of cylinder mounting styles, which is extremely handy for the machine designer. All too often, however, in their haste to finish th...
Fluid power cylinder manufacturers generally offer a huge assortment of cylinder mounting styles, which is extremely handy for the machine designer. All too often, however, in their haste to finish their projects, the designers fail to consult the application information sections of the cylinder catalogue they are using and end up selecting the most convenient but perhaps the least serviceable mounting for their application. This oversight can result in improper operation of the machine, premature failure of the cylinder or even catastrophic failure of the cylinder or machine, perhaps threatening the personal safety of employees.
Pneumatic cylinders, because of their lower pressures, can generally be used at full pressure, regardless of mounting. Medium and heavy-duty hydraulic cylinders, in some mounting styles, may not be used at full rated pressure.
Cylinder mountings are generally classified into three groups: Group 1 — Straight-line force transfer with fixed mounts, which absorb force on the cylinder centre line; Group 2 — Pivot force transfer which absorbs force on the cylinder centre line but allows swivelling in one plane; and Group 3 — Straight line force transfer with fixed mounts, which do not absorb force on the centre line.
Group 1 includes front and rear flange mount as well as tie rod mount cylinders. Selection of front or rear mounting should be based on whether the cylinder will be used primarily in compression or tension. In compression, the back end of a single-rod cylinder is pressurized. Using a front mount will put undue tension on the tie rods and mounting bolts and generally result in derating the maximum permissible pressure for the cylinder. The same logic can be applied to tension loading with rear-mount cylinders.
Group 2, pivot force cylinders, will normally allow for misalignment in one plane only. The use of spherical bearings in single lug rod and cap clevis mountings will allow for minor misalignment in the second plane. Rear clevis and rear trunion cylinders should only be used in shorter stroke applications. In long-stroke applications, the entire weight of the cylinder will be supported by the rod bearing, causing undue wear and damage, especially when extended. Judicial use of stop tubes in the cylinder to prevent full extension can help a little but the best solution is to use a mid-cylinder trunion positioned to optimally balance the load.
Group 3 cylinders are generally described as side-mounted cylinders. They include side lug, end lug, side tapped, and end angle mountings. All of these mountings produce an off-centre or twisting motion to the cylinder structure, which may cause leaks and premature bearing failure. They are, however, convenient and when needed, the side lug design should be selected if possible. The welded or bolted connection points of end lug and end angle mounts will be severely stressed by the rocking action of the end caps and may fail. End-mounted side connections should be avoided in all but the lightest of applications
The off-centre twisting motion can also create structural problems in the machine. The machine base, to which the cylinder is side-mounted, must be strong enough to withstand the resulting eccentric loading without flexing. Any movement of the machine structure at this point will tend to amplify the cylinder misalignment.
Be careful when selecting cylinder mounts. By being competitive in offering a wide range of cylinder mounts, the cylinder manufacturer may be doing himself and his customer a disservice. More emphasis must be placed on the derating factors or load carrying capacities of various mountings to prevent their misapplication. Having said this, however, does little to solve the problem. The onus, as usual, rests on the shoulders of the user. The use of a little common sense and mechanical intuition, as well as referring to the cylinder manufacturers’ engineering recommendations when designing a machine, can save you a lot of grief later on.
Ted Grove is corporate training manager for Wainbee Limited, Mississauga, Ont., and an widely experienced fluid power trainer.