Should You Replace Only the Spherical Bearing or the Whole Rod Eye on a Hydraulic Cylinder?

What are the two options?

You can take out the worn bearing and fit a new one in the same eye. Or you can replace the whole eye, bearing included.

Which is possible depends first on how the bearing is held, and second on the state of the eye it sits in.

How is the bearing held in the eye?

In one of two ways: permanently, or with retaining rings.

Hänchen lists the mounting of the bearing for its spherical rod eyes by cylinder series. In some series the bearing is "permanently installed". In others it is "with retaining ring". A bearing that is staked, meaning the metal of the eye is bent over it, is meant to stay where it is. One held by retaining rings can be taken out (K02-Q1-10).

Heavy press fits can also make removal impractical. With a heavy interference fit, the bearing usually cannot be pushed out without galling the bore.

So the first step is to find out how your bearing is held. Check the drawing, or look at the edge of the eye for a groove and ring, or for staking marks.

When can you replace only the bearing?

When the eye itself is still good. Check these:

• The bearing is held by retaining rings. Then it can be removed without harming the eye.
• The bore is round and in specification. Measure the bore and compare it with the maker's specification. In one documented case, a pin looked fine, but the bore was out of round beyond the limit, and that was the source of the knock.
• The bearing has not spun in its seat. A bushing that has turned in its housing shows the press fit has been lost.
• The eye is not cracked, deformed or damaged at the thread.

If all of these hold, a new bearing of the same type restores the joint.

When should you replace the whole rod eye?

When the eye cannot hold a new bearing properly.

• The bearing is permanently staked and cannot be taken out without damaging the eye.
• The bore is out of round, oversize or damaged.
• The old bearing has been turning in the bore.
• The eye is cracked or deformed, or its thread is damaged.

Replacing the eye also gives you a new bore, a new seat and a new bearing in one piece. It is the more reliable choice when the housing is in doubt.

How do you decide, step by step?

Three questions, in this order. The first two are about the eye. The third is about why the bearing failed.

Can the bearing come out without harming the eye? A bearing held by retaining rings can. A staked bearing is held by the eye's own metal, bent over its edge, and removing it destroys that lip. Re-staking a used eye does not restore the original retention, so a staked eye is replaced as a whole.

Can the bore still hold a new bearing? A press-fitted ring stays in place by friction. The holding force is roughly F = μ × p × π × D × L, where p is the contact pressure the interference creates. For an illustrative 47 mm ring, 20 mm wide, μ = 0.15 and p = 30 MPa, the holding force is about 13.3 kN. If wear or ovality cuts the interference so that p falls to a third, 10 MPa, the force falls to about 4.4 kN, and the ring can creep or turn under side load. This is why the bore must be measured (two readings at right angles, compared with the specification) and why fretting marks or a spun ring are decisive. A bore that has lost its interference cannot be rescued by fitting a new bearing.

Has the cause been removed? A bearing that wore out from misalignment, dirt or a dry joint will wear out again if nothing changes. Check the geometry (Q1-09) and the lubrication (Q1-22) before fitting anything new. If the eye is cracked at the hole edge, the neck or the thread, or the hole edge is deformed by a hard or loose pin, replace the eye, because a crack does not stop at the bearing.

What else should you replace with it?

Replace related parts as a set when wear is far along: a new pin in a worn bushing will fail just like a new bushing on a worn pin. In most cases, replacing the pin, bushing, seals and retaining hardware together is the most reliable repair. Do not fit an undersized pin to make up for a worn bore, because that creates point loading and faster wear.

What should the new part match?

• Type. Do not swap a spherical bearing for a straight bushing just because it is cheaper and fits the bore. A joint that must follow an angle out of the pin plane needs the spherical bearing; otherwise the rod can be scored as well.
• Bearing clearance. Cylinder makers build eyes with normal (N) or reduced (C2) bearing clearance depending on the series. Replace like for like and follow the bearing maker's advice on fits (Q1-22).
• Load rating, material and grease path. The pin and bushing must handle the cylinder's push and pull, not just the weight, and the grease fitting must line up with the grease grooves of the new part.
• Hardness and material of the pin. Do not choose by dimensions alone (Q1-14).

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