During routine inspections of casters, wear, cracks, and chipping are relatively easy to spot. However, there is another type of abnormality that tends to be more subtle: A localized bulge suddenly appears on the wheel surface, giving the impression of a “bump.”

At first, the irregularity may be only slight, and the wheel can still turn; as use continues, the bulge may become increasingly pronounced, potentially accompanied by cracks or localized damage.

This type of issue warrants particular attention, as it may not be ordinary surface wear but rather related to the internal material condition of the wheel tread.

When a caster develops localized bulging, rather than continuing to wonder “can it still turn?”, it’s best to first determine: What exactly happened inside the wheel face?

1. What is the difference between a bulge on the wheel tread and normal wear?

Normal wear typically occurs in the contact area between the wheel tread and the ground, manifesting as gradual material loss and a slow change in the wheel’s profile.

A bulge, by contrast, is a localized outward protrusion.

When the wheel is turned by hand, you can observe this position periodically passing over the ground. If the protrusion is pronounced, the device may even produce a regular undulating sensation during operation.

Therefore, once localized “height increase” on the wheel tread is detected, it should no longer be treated as ordinary wear.

It is more like an abnormal signal that requires further investigation.

II. Where do the bubbles and voids inside the material originate?

Take partially rubber‑coated industrial casters as an example: during manufacturing, the wheel tread undergoes compounding, molding, and subsequent processing steps.

If the relevant process controls are suboptimal, the material may contain internal porosity, voids, or locally non‑uniform microstructures.

When these issues are concealed within the wheel surface, they may not be directly visible on the product’s exterior.

After casters are put into service, their wheel surfaces continuously undergo loading and rolling, while the internal weak zones are subjected to repeated cyclic stresses, allowing defects to gradually manifest on the surface.

This is also why some casters appear perfectly functional when first installed, only to develop localized bulging after a period of use.

Third, “bulging” may also be related to the region of attachment.

For wheels composed of different materials, it is also necessary to determine where the anomaly occurs.

If the bulge is located near the interface between the wheel tread and the wheel hub, the bonding condition between the two materials should be further inspected.

Because once the local contact condition becomes abnormal, the wheel tread may undergo irregular deformation during loading and rolling.

Therefore, when inspecting a bulge, you cannot rely solely on the highest point.

It’s best to inspect the entire area around the bulge:

Are there any fine cracks?

Are there any abnormal gaps along the rim of the wheel face?

Are there any noticeable differences in the state when pressing at different locations?

These phenomena all help to further narrow down the scope of the problem.

IV. Why can’t a bulge simply be “shaved off and the tire reused”?

On-site maintenance sometimes follows a particular line of reasoning:

Since it’s only slightly protruding, why not just grind down the raised area to restore the wheel to its original round shape?

This approach is not recommended as a standard practice.

Because the bulge is merely an external manifestation.

If the issue stems from internal voids, localized structural anomalies, or variations in the bonding zone, removing surface protrusions will not address the underlying internal causes and may even further compromise the original wheel‑surface structure.

Therefore, when a noticeable bulge is observed, the more appropriate course of action is to assess the overall condition of the casters, replace them if necessary, and disassemble and inspect any abnormal wheels for analysis.

5. If only one caster in the same batch is bulging, how should this be diagnosed?

If all the other casters on the same piece of equipment are functioning properly and only one shows a noticeable bulge, first record its installation position and the nature of the damage.

Subsequently, verify whether any special operating conditions exist at that location.

If there is no obvious difference in the operating environment or load conditions, it is worth further investigating the cause by examining the caster’s material and manufacturing quality.

If similar phenomena repeatedly occur in comparable locations on the same batch of products, it is even more important to retain samples and compare the bulging locations, internal conditions, and the usage conditions prior to the onset of the anomaly.

The most dreaded outcome of failure analysis is when all that remains is the single phrase “it’s broken,” with no trace of on-site information.

6. Can internal issues be detected in advance during acceptance?

Relying solely on the naked eye cannot detect all internal defects.

However, in bulk procurement or when deploying critical equipment, risks can still be mitigated through methods such as visual conformity inspections, sample assembly trials, and operational monitoring under real-world conditions.

In particular, when conducting sample testing, one should not focus solely on whether the product “slides smoothly.”

After operating for a period of time, re‑inspect the wheel tread for any abnormal protrusions, cracks, localized deformations, or other changes.

The true value of a trial lies in observing what happens to the product under real-world operating conditions.

7. After discovering a bulge, it’s best to provide three key pieces of information.

First, take photos of the bulge and the entire wheel.

Second, record the location where the casters are installed and how the equipment is used.

Third, if replacement is necessary, try to keep the defective wheel and do not discard it immediately.

Because many internal issues can only reveal the true areas of abnormality after disassembly, sectioning, or further examination.

With complete on-site information, subsequent assessments will be more well-founded.

Written at the end

When a bulge appears on the wheel tread of a caster, the key difference from ordinary wear is:

The problem you see may be on the surface, but the real root lies within the material itself.

Therefore, when a localized protrusion suddenly appears on the wheel surface, don’t simply try to grind it down, nor should you continue to ignore it just because “it can still turn.”

Only by first recording the observations, then conducting a thorough inspection, and identifying the root cause can one determine whether this is an isolated occurrence or a material- or process-related issue that warrants further attention.

When assessing the quality of casters, it’s not enough to simply judge how attractive they look right out of the factory; what really matters is whether their structure remains stable after enduring real-world loads and prolonged operation.