Installing a pneumatic cylinder correctly is a vital design consideration. Failure to address this properly can result in inaccurate operation, compromised reliability and premature failure.
Depending on the specific application, cylinders such as those from Parker’s P1F series, are either going to be rigidly fixed to the structure of a machine bench, or allowed to swivel to form part of a linkage. The two fixing points will be the cylinder body and its piston rod end. In many applications, the mechanism attached to the piston rod end will be allowed to hinge in one or more planes. In a few applications however, the piston rod end is left free, such as in a simple pushing application.
The mechanics of cylinder installations will vary considerably from one application type to another. A cylinder should be installed so that side loads on the piston rod bearing are reduced to an absolute minimum or eliminated entirely. A side load is a force component acting laterally across the axis of the bearing and can cause premature wear and failure.
Rigid mountings on body cylinders
A cylinder can be rigidly fixed by side mountings, or front or rear flange plates. Alternatively, if the cylinder has a thread on the front or rear end cover, it can be clamped to a structure with a locknut. Special tie rod ISO or CNOMO cylinders can be fitted with tie rod extensions for fixing through a flat plate.
Rigid mounting options for the cylinder are shown in the diagram below using components annotated 1-9. A semi-rigid trunnion option can be achieved using supports (10 or 11), pivots and clevis brackets (3 & 5, 7 & 8 & 4, 5 & 6) allowing the cylinder to rotate by following the piston rod extension or retraction during the work task of the cylinder.
Articulated mountings on cylinder body or piston rod end
If the cylinder is forming part of a linkage, then it must be free to swivel in one or more planes at the mounting point. Different degrees of balance can be achieved for the cylinder and load system by choosing between a rear hinge, front clevis, and central trunnion (using mountings 12, 13 & 14). A front hinge, clevis or universal eye allows swiveling attachments at the end of the piston rod.
There several areas where caution is appropriate when installing a cylinder.
- Avoid attaching an unsupported load to the piston rod; wherever possible support the load on a slide or roller guides.
- The weight of a long out-stroked piston rod alone can produce a high bending moment. It may be possible and prudent to hang the rod end from a roller track or provide other external guidance to minimise bending moments.
- Misalignment of the cylinder and a guided load can easily jam the cylinder completely. The installation of a front fork and slot will eliminate this type of side load on the front-end cap bearing in the cylinder.
- An offset load is a common source cause of a bending moment on the piston rod of the cylinder. Installing external bearings will relieve the side load as well.
- A horizontal mounted rear hinged cylinder will cause the weight of the body cylinder to create a bending moment. This will be helped if a central trunnion is fitted at the point the cylinder balances.
It is difficult to eliminate side loads completely, but by employing good practice and basic mechanical design knowledge, they can be reduced to a minimum helping to increase the lifetime and performance of the cylinder in the application.
Parker’s P1F series pneumatic cylinders are ISO15552-certified and suited to a wide range of industrial applications with bore sizes ranging from 32 to 125mm. The smooth profile version P1F-S cylinder is complemented by the P1F-T tie-rod version, with the P1F-C version for ‘clean’ applications due for release soon. A wide range of cylinder and piston rod mountings and sensors can be supplied for use with the P1F series to suit all customer applications.
Article contributed by Franck Roussillon, Product Manager for Actuators Europe, Parker Hannifin, Pneumatic Division Europe