Rod eye and clevis mount types: selecting the right end fitting

The rod eye and the clevis mounting comprise the two most commonly used fittings for pivoting ends that connect the rod of a hydraulic cylinder or barrel to a machine's structure. Rod eyes are one-hole fittings that are typically mounted on or welded to the piston rod. On the other hand, the clevis is a two-eared, forked bracket that is mounted to the body of the cylinder or pins that connect to the drive component. Selecting between them and also between their sub-variants (spherical bearing rod eyes, basic rod eye, female clevis, male clevis, and double-clevis) will depend on the amount of misalignment it will tolerate, the side-load conditions, the pin's size and bearing material, as well as whether the mount is required to be able to rotate in a single plane or be able to accommodate multi-axis motion.

Hydraulic cylinders are rarely operating in a straight line. Machine frames stretch and linkages move through arcs, and the mounting points can shift slightly during the load. The fitting at the end is responsible for absorbing this geometry mismatch, and if it's not done correctly, it isn't just a matter of looking sloppy; it creates side-loading onto the rod. This accelerates seal wear and may cause bending of a piston rod that was never intended to handle bends. This article explains the major types of mounts that are available and the place each one should be on your list of materials, as well as the elements that determine the selection.

What's the point of fitting at the end more than the way it appears?

Hydraulic cylinders are constructed to pull and push across its central line. Any force that is applied to the centerline—such as bending or twisting, or an error in alignment—is absorbed somewhere else in the system. If the mounting isn't able to permit angular movement, "somewhere" becomes the rod and the gland bushing as well as the seal on the rod. Over time this manifests as uneven wear on the seal as well as a scratch on the surface of the rod and, eventually, rod bends or even gland failure.

The correct pivoting mount lets the cylinder auto-align as the linkage is moved, ensuring that loads remain from being displaced and securing internal components. This is the reason why the end fitting choice should be given the same consideration as bore size and pressure rating, despite the fact that it is often viewed as a last resort.

Mounts for rod eyes

Simple (Solid) rod eye

A rod eye that is plain is a single ring of steel that is internally threaded in order to correspond with the thread of the rod and an unadorned bore that houses the pin for the clevis. It's the most simple and economical choice and is suited to situations where the misalignment of the rod is not a problem and the linkage operates within a clear, single plane. Think of easy lift arms or tailgate cylinders or fixed-geometry links.

Since it is a bushing, not bearing the plain rod eye has an insufficient tolerance to angular misalignment. Inducing angular motion through a plain eye can accelerate bore wear and can cause tension at the pin.

Eyes on spherical bearing rods

If the linkage geometry is compound motion, that is, moving in more than one plane, or a structure for mounting that isn't rigid enough, a spherical bearing rod eye is the best option. The inner race of the bearing is spherical. It provides a range of angular rotation while still transmitting an axial load in a clean manner, which keeps the rod from being a victim of bending strain even as the mounting points change.

Spherical eyes are more expensive than normal eyes and include an element of wear (the bearing), which eventually requires replacement; however, on agricultural machinery, mobile equipment, as well as any other linkage that has three-dimensional movement, the trade-off is generally worth it. Choose the material of the bearing, typically a steel-on-steel or steel-on-bronze mixture, often using a liner made of PTFE, depending on speed, load, and whether the system is subject to washdown or other contamination that could increase wear on the bearing.

Female, female. Eyes of threaded rods

Eyes on rods also differ based on how they are attached to the rod. They are or they can be threaded (male thread in the rod and female inside the eye) or welded/pinned for instances where a threaded attachment isn't feasible. Threaded rod eyes can be replaced in the field and permit fine-tuning of stroke length through jam nut adjustment, which is helpful to set up linkage and replacement of rod eyes without disassembling the entire rod.

Clevis mounts

Female clevis (Cylinder-end mount)

Female clevis brackets, also known as the clevis mount, are attached to the cap or the head end of the body of the cylinder and are a two-eared yoke into which a mating male pin or lug fits. This is the most common mount for cylindrical units that pivot at both ends, for example, lift cylinders that are mounted on dump bodies, loaders, or booms.

Clevis mounts are usually evaluated by the diameter of the pin and ear width, both of which have to be in line with the expected shock and side load of the device. Pins that are not sized correctly on a clevis mount can be the most frequent cause of failure in mobile devices that are used frequently—the pin is bent or the ears stretch when subjected to repeated shock loads ahead of time that the entire cylinder exhibits signs of wear and tear.

Double clevis/Trunnion-style combinations

Certain designs include a female clevis on the cap end with an eye rod at the rod's end, creating a pin-to-pin pivoting mount—this is the most common configuration for loader arms as well as excavator-style linkages. Some use double clevis and "detachable" clevis brackets that bolt on instead of welding on, which makes it easier to do replacement of cylinders on the job but requires a careful specification of torque on the bolts used to mount them in order to ensure that they are not loosening due to the force of vibration.

Bushing and pin selection

No matter the style of clevis, regardless of clevis style, the pin-and-bushing connection is where wear and tear occurs. Ground pins that are hardened and ground with interchangeable brass or plastic bushings inside the clevis' ears allow their wear areas to be repaired without having to replace the entire bracket—a feature that is worth mentioning in the beginning as welded clevis mounts without bushings are more costly to repair when the ears get worn out.

Comparing different mount types at a glance

Selection checklist

Before you finalize an end fitting, verify:

• An angular range the linkage needs when in use, which includes the worst-case frame flex and wear-induced play
• Side-load and shock-loading exposure applications that are off-highway and mobile generally require hardened pins and bearings instead of plain bushings
• The pin's diameter and the material in relation to the cylinder's push and thrust, not only the diameter of the bore
• Serviceability, i.e. whether bushings, pins, and bearings are able to be replaced without removing the cylinder from the machine
• Exposure to environmental conditions—washdown corrosion-causing atmospheres, or severe contamination—favors sealed spherical bearings or composite bushings that are placed over steel-on-steel joints

Making sure the mount is aligned to the actual motion of the linkage, not only the cylinder's stroke or bore, is what keeps the rod seals, pins, bushings, and seals from being worn evenly rather than concentrating the stress at a single point within the cycle.

What is the difference between rod eyes and mounts for the clevis?

The rod's eye can be described as a single hole fitting that is attached to the piston rod, while a clevis can be described as a two-eared, forked bracket that is typically attached to the cylinder body. The majority of double-pivot cylinders employ one of them, which is joined by pins to the structure of the machine.

When should I utilize a spherical bearing rod instead of a regular rod?

Make use of a spherical bearing rod every time the linkage moves in multiple planes or the structure of the mounting can be slack or shift during load. Plain rod eyes work for single-plane motion only with little disalignment.

What causes clevis pins to wear faster than the entire cylinder?

Clevis pins as well as ear bushings are the sacrificial wear areas inside the mounts, helping to absorb side load and shock. Pins that are too small or without replacement bushings accelerate wear, and so pins usually require maintenance well before the internal seals of the cylinder do.

Do I have the ability to change a clevis mount that is welded to a bolt-on style?

It is contingent on the cylinder's design However, many manufacturers provide bolt-on or detachable brackets for clevises as an alternative to welding mounts. It trades a little bit of rigidity in exchange for a much simpler field replacement.

How can I tell whether my current mount is damaging the rod?

Find wear patterns with irregularities on the rod's surface and scoring that is aligned in the direction of alignment or leakage from the seal of the rod focused on one end of the gland. These are common indicators that the mounting isn't accommodating to the linkage's real motion.