Some equipment that sees little movement in daily use may exhibit noticeable periodic jolts when pushed again after prolonged idling: as the wheels rotate to a certain position, it feels as though they’re stepping over a small bump. If the floor is found to be level and no foreign objects are caught in the wheel tread, you should pay attention to a commonly overlooked phenomenon—permanent deformation caused by sustained compression of the wheel surface. This condition resembles the so‑called “flat spot,” but its underlying causes involve a combination of factors, including the elastic recovery of the wheel material, long‑term loading, temperature, and parking practices.

1. What is compression set?

When subjected to a load, an elastic wheel tread undergoes a certain degree of deformation, which is essential for the material to dampen vibrations and protect the underlying surface. However, upon unloading, ideally the tread should fully recover its original shape. If, after prolonged compression, the material fails to regain its full elasticity, persistent deformation may remain at the loaded area. When the equipment moves again, each rotation of the wheel passes over this spot, resulting in a regular, rhythmic undulation.

II. Why are vehicles more likely to reveal problems when parked for extended periods?

When casters are in motion, the point of load application continuously shifts as the wheels rotate; by contrast, during prolonged periods of static rest, the weight remains concentrated on the same area of the wheel tread. The longer the equipment remains stationary and the greater the actual load borne by each wheel, the more pronounced the challenge to the material’s ability to recover its original shape. If the equipment itself is heavy and left unmoved for extended periods, the localized compressive stress on the wheel tread becomes even more critical than it is for standard rolling carts.

III. A material’s softness does not necessarily mean it is more prone to problems.

The hardness of a wheel’s tread affects the extent of deformation, but it cannot be simply assumed that “soft wheels will always flatten, while hard wheels will never do so.” Resilience, heat resistance, and long-term compression performance vary across different material systems, and even the same material can differ depending on its formulation, structure, and manufacturing process. When selecting a wheel, if one relies solely on hardness values while disregarding whether the equipment will remain stationary for extended periods, one may overlook the truly critical operating conditions.

IV. Ambient temperature can also affect material performance.

The mechanical properties of wheel‑tread materials are influenced by ambient temperature. If equipment is stored for extended periods in high‑temperature areas, near heat sources, or subjected to significant thermal cycling, the wheel tread’s elastic recovery may differ from that observed under normal‑temperature conditions. Therefore, when using elastomeric wheels in drying zones, around hot‑working processes, or in high‑temperature storage environments during summer, it is essential to clearly specify the actual operating temperature conditions during procurement discussions.

5. If you encounter periodic vibrations, don’t rush to replace the bearings just yet.

After the equipment is restarted, if regular vibrations occur, first clean the wheel surfaces and safely jack up the unit. Then, slowly rotate each caster one by one to check for localized depressions, deformations, or out-of-roundness on the wheel faces. Next, inspect the bearings, axle, and mounting brackets to ensure smooth operation. If the wheels turn freely when suspended but exhibit a consistent rhythmic wobble with each revolution once loaded, the deformation caused by prolonged pressure on the wheel surface should be given particular attention.

VI. When selecting wheels for equipment that will be parked long-term, a different approach is needed.

For equipment that “spends most of its time stationary and moves only occasionally”—such as tool cabinets, spare‑equipment racks, mold carts, and heavy‑duty test stands—selection should not be based solely on the load‑carrying requirements during movement. In addition to the rated load capacity, it is essential to consider the equipment’s long‑term idle conditions, the recovery capability of the wheel tread material, floor‑protection requirements, and how frequently the equipment is moved. In some applications, it may even be more appropriate to use support feet, adjustable feet, or other structural solutions to distribute the caster load during extended periods of inactivity.

VII. Proper usage and management can also reduce the risk of long-term stress.

If the equipment permits, periodically reposition it to vary the load distribution on the wheel tread. For heavy‑duty equipment that remains idle for extended periods, additional supports should be provided in accordance with the equipment’s design and safety requirements. It is important to emphasize that you should not arbitrarily heat, hammer, or grind the wheel surface in an attempt to “restore” its condition; such practices can compromise the material and structural integrity, potentially creating new safety hazards.

Written at the end

Whether a caster is truly effective depends not only on how easily it rolls but also on its ability to maintain stable rolling performance after prolonged periods of inactivity. Permanent deformation of the wheel tread serves as a reminder: when selecting casters, both how the equipment “stops” and how it “moves” are equally important. Providing suppliers with advance information on parking duration, load conditions, and ambient temperature often proves more valuable than simply aiming for higher load‑carrying ratings.