Which Loads Does Each Hydraulic Cylinder Mounting Style Put on the Rod End?

What load should a rod end see?

Ideally, only a straight push or pull along the centre line of the rod. Side force (a load pushing sideways), bending and twisting are the unwanted ones.

The governing principle is that a hydraulic cylinder is an actuating part, not a structural part, so no side loads should act on the piston rod, either during assembly or in operation. The rest of this article looks at how each mounting style moves a cylinder closer to, or further from, that ideal.

How do fixed and pivoting mounts differ?

A fixed mount holds the cylinder in place and expects a straight, guided load. A pivoting mount lets the cylinder swing, so it suits a load that moves in an arc.

Mounts that take the force along the cylinder's centre line, such as extended tie rods, flanges and centre lugs, give straight-line force transfer. Pivot mounts should be used when the load moves through an arc. For the fixed centre-line types, the usual advice is a cap mount for thrust loads (push), and a head mount where the main load pulls the rod. The cap is the closed back end of the cylinder. The head is the end the rod comes out of. Confirm the direction with the cylinder maker's own guidance.

The table sorts the common styles by what reaches the rod end.

Where does side load come from?

From geometry that is different from what the drawing assumed. The usual sources are: fixed brackets on a linkage that moves in an arc, fork ears that are not parallel, a load applied off-centre, a flexible flange plate, trunnion blocks that are not in line, bent structure, worn pins, and a base that shifts under load.

Two rules prevent most of these. Pivot axes should be parallel, never crossed. And a pivoting mount should use the same type of pivot at the cylinder body and at the rod end. A cylinder with a fixed clevis at the cap and a spherical rod eye still has one end that holds it to a single plane.

How large can the side load be?

It depends on what restrains the cylinder, so there is no general figure. A simple calculation gives a feel for the size.

Take a force F along the rod, and a small angle θ between the force and the cylinder axis. The sideways part of the force is F × sin θ. For F = 100 kN:

For a 50.3 kN cylinder at 2° the sideways part is 1.75 kN. This is a simple illustration. It is not the real side load on the gland, and it is not an allowable misalignment. Real forces depend on lever arms, bracket stiffness, pin play and friction. What the table shows is that a small angle on a large force creates a real sideways load.

Which rod end suits which mounting?

The rod end follows from the mounting.

• Pivoting mount, motion in one plane: a plain eye or clevis can work, if both pin axes are parallel and rigid (Q1-03).
• Pivoting mount, motion that leaves the plane: a spherical rod end at the rod, ideally with a spherical joint at the cap too (Q1-03, Q1-09).
• Trunnion mount (two pins on the sides of the cylinder body): the rod end sees a straight load, but the supports must sit close to the trunnion shoulders. Power & Motion limits front-trunnion cylinders to a stroke of no more than five times the bore, to keep the trunnion bearing loads within limits.
• Flange or foot mount: the rod end does not protect the cylinder. External guidance does. A spherical rod end cannot make up for a missing guide (Q1-10).

On a long stroke in push, the rod's column strength limits a pivoting cylinder. That is covered in Q1-28.

A table like this is a selection guide, not a dimension standard. Two suppliers' "clevis mounts" can have different pin diameters, fork gaps and closed lengths. If parts must swap directly, use a dimension standard such as ISO 6020-2:2015, which defines mounting dimensions for one metric compact cylinder family. It does not make every metric cylinder interchangeable.

What changes on an off-road or steering cylinder?

Off-road mounting differs in one respect: the ends of the cylinder sit on parts that move relative to each other in more than one plane. A steering cylinder is commonly mounted on the axle, so it moves with the axle and sees mainly the arc of the steering arm; some are fitted between the chassis and the axle. Levelling and hydraulic suspension cylinders usually run between the chassis and the axle, and there the pivots leave the swing plane as the axle rolls and pitches. That is the "movement that leaves the plane" case in the table above. It argues for the same type of pivot at both ends, usually spherical, with the pin axes parallel at rest. A frame that flexes in cornering or on uneven ground adds side force as described earlier.

Dirt is the other difference. A rod exposed to mud, sand and water gets scored, and scoring then damages the seal, so a protective boot over the rod is a common accessory.

A feel for the numbers (illustrative only): a steering-assist cylinder pushing 15 kN with its line of force 3° off the cylinder axis puts about 15 × sin 3° ≈ 0.79 kN sideways on the rod. That is not a limit or a measured load. Steering and suspension cylinders are safety parts, so ratings and mounting rules come from the vehicle and cylinder makers. For rod ends in this kind of service, see off-road rod ends. Suspension arms and bushings are covered on syzmachine.com.

FAQ