Hydraulic cylinders are available in a variety of styles and can be mounted in numerous ways. While they come in countless variations, they are usually built to certain standards. National Fluid Power Association (NFPA) and International Standards Organization (ISO) standards for hydraulic tie-rod cylinders are among the most common.
The advantage of standard tie-rod cylinders is their modular nature. Parts such as the head (rod-end block), cap (end opposite the head), barrel, tie rods and cylinder rod can be quickly assembled from off-the-shelf parts.
NFPA and metric ISO standards that apply to mounting types and dimensions are quite similar—they even use the same three-character code to designate mounting options.
A basic cylinder has no mounts, so users typically specify the necessary mounting type and hardware when purchasing a cylinder. Three general styles include cylinders that have:
• Fixed mounts that absorb force along the centerline of a cylinder.
• Fixed mounts that do not absorb force along the centerline.
• Pivot mounts that drive a load in a curved path.
Here’s a look at some of the most common variants.
Flange mounts: Front flange mount (designated MF1), front flange extra mount (MF5), and front head flange mount (ME5) are all methods of mounting the cylinder off of, or part of, the head itself. MF1 has a rectangular flange attached to the head, which extends from the sides of the cylinder, and the MF5 has a larger square flange extending in all directions, which provides extra strength. The ME5, instead of being mounted to the head, uses an extra thick and wide head, which itself attaches directly to the machine. These options require the cylinder to be stationary and this fixed-centerline mounting provides them with high column strength.
Rear flange mounts (MF2, MF6 and ME6) are similar to the front flange family, except their respective locations are off the cap rather than the head. MF2 has a rectangular flange attached to the head, but extends only on the sides, the MF6 uses the same larger flange as the MF5, and the ME6 has a beefy cap containing mounting holes for direct attachment to a machine. Being fixed-centerline-type mounts, they offer the same strength advantage of the front flange versions.
Tie-rod mounts: Tie-rod mounts use extended tie rods to attach the cylinder to a machine. They include rods extended at both ends (MX1), cap end (MX2) and head end (MX3). While they provide centerline support, they’re less rigid than flange mountings and often require additional support for long-stroke applications.
Lug mounts: Lug mounting options use rectangular tabs machined from (or welded to) the same block of steel as the head and cap. MS3 center lugs are positioned mid-way up the head and cap and support loads along the centerline of the cylinder. Foot-mounted versions include four MS2 side lugs on the bottoms of the head and cap, and MS7 end lugs mounted to front bottom of the head and back bottom of the cap. Fixed, lug style mounts can be quite rigid, but those off the cylinder centerline can cause misalignment concerns between the front and rear lugs, especially related to bending or torquing of the mounting surface.
Clevis mounts: MP1 (fixed clevis) and MP2 (detachable clevis) are common mounting options, especially in applications where the cylinder must pivot through an arc as it extends and retracts, such as with an excavator boom. The fixed clevis is a part of the cap itself. A detachable clevis is bolted to the cap. Both allow engagement to a clevis mounting bracket with a steel pin held in place with cotter pins or snap rings. Clevis mounts have the advantage of centerline mounting, but they are highly prone to side load and column strength issues.
Front, rear, and intermediate trunnion mounts (MT1, MT2 and MT4, respectively) are similar to clevis style pivots. They let the rod move through an arc as it extends and retracts, although they have a slight advantage in column strength and precision of movement over a clevis mount, especially the MT1 (front trunnion) and MT4 (intermediate trunnion). The MT2 (rear trunnion) mount is slightly weaker, especially if the rod isn’t rigidly guided. Trunnion cylinders experience smoother movement because the trunnion on either side of the cylinder is fixed to the machine with special mounting brackets and bushings, and has less play in the joint compared to a clevis.
Regardless of how well a hydraulic cylinder is designed and manufactured, it can fail if not mounted correctly. Proper mounting avoids problems like side loads that cause excessive seal and bearing wear, or even bend the rod or bind the load.