In product specifications for polyurethane (PU) casters, rubber casters, and other similar products, you may encounter hardness ratings such as “75±5” or “70±5.” These do not represent two separate hardness values; rather, they indicate a nominal central value accompanied by an allowable tolerance range, which defines the acceptable hardness range for both incoming materials and finished products. Below, we will explain in a more intuitive way what “75±5” signifies and how it relates to the performance of casters in practical applications.

First, what does “75±5” represent?

In the expression of caster hardness:

• “75” typically indicates the target hardness (center value).
• “±5” denotes the permissible tolerance range, meaning that a measured hardness between 70 and 80 is considered acceptable (subject to the hardness-testing method/standard agreed upon by both parties).
• This notation is commonly found in technical agreements, incoming material inspection reports, and factory inspection certificates, where it is used to define acceptable variation.

For example, if a PU castor is labeled as 75±5 and the on-site measured hardness is 78, it is generally still considered within the normal range.

II. What aspects of performance are affected by hardness?

Hardness is neither better the higher it is nor the softer the better. It typically affects the following aspects:

1) Load-bearing and Deformation

• Wheels with higher hardness are less likely to be crushed under heavy loads, resulting in more consistent rolling resistance; wheels with lower hardness, by contrast, deform more easily and may feel “heavier and harder to push” under heavy loads.

2) Shock Absorption and Noise

• Wheels with lower hardness are more elastic, absorb more vibration, and typically produce less noise; wheels with higher hardness offer poorer shock absorption, are more prone to bouncing on hard surfaces, and generate more rolling noise.

3) Wear Resistance and Cut Resistance

• Under conditions involving rough surfaces, abundant debris, or frequent turns, an appropriate hardness and formulation can enhance wear resistance and tear strength. In general, wheels with higher hardness offer better wear resistance; however, this also depends on the material formulation and the wheel core structure.

4) Ground Protection

• For tile, epoxy, and self-leveling floors, wheels with lower hardness or those featuring a “no-mark formula” are less likely to cause scratches or leave marks; when using wheels with higher hardness, greater attention should be paid to floor protection.

3. Temperature can make hardness “appear to change”: lower temperatures make materials harder.

Many elastomeric materials become harder and exhibit reduced resilience at low temperatures; this is a normal physical phenomenon. Consequently, the same castor wheel may register different hardness values when tested in summer versus winter.

Selection recommendation: When used in low-temperature environments such as cold storage facilities or outdoor settings during winter, it is recommended to confirm a “low-temperature-compatible formulation” with the supplier in advance or conduct sample verification.

IV. How to Use the “Hardness Range” for Practical Selection?

You can determine the hardness in three steps: “load—ground—environment.”

• Consider the load: Heavy-duty equipment typically requires wheels with higher hardness and reduced deformation (while also taking into account wheel diameter, wheel width, and hub strength).
• Consider the floor surface: For applications with stringent requirements for floor protection and noise reduction—such as hospitals, shopping malls, and office buildings—prioritize casters with lower hardness or non-marking treads. On rough floors or surfaces with abundant debris, it is advisable to increase caster hardness and wheel diameter accordingly.
• Consider the operating environment: low temperatures can cause the wheels to become brittle; in environments containing oil, cleaning agents, or weak acids and bases, it is also necessary to evaluate the material’s resistance to these media (even if the hardness is appropriate, an incompatible formulation can still shorten service life).

V. Inspection and Communication Recommendations: Clearly Specify the “Measurement Conditions”

• Clarify materials and hardness scales: Different materials typically use different hardness scales (e.g., Shore A), and the agreed-upon standard between the parties shall prevail.
• Note that test conditions—including temperature, placement time, measurement point location, and number of tests—can all affect the readings.
• Monitor whether the bias persists over the long term: if the bias remains consistently too stiff or too soft, it may compromise the feel of operation and the consistency of service life.

Conclusion

“75±5” essentially represents the permissible tolerance range for hardness, which is used to determine whether a product meets quality standards. However, whether the product performs well in practice depends on a combination of operating conditions, including load capacity, floor surface, and ambient temperature. If you provide the supplier with information on the equipment’s total weight, the type of flooring, frequency of use, and ambient temperature, they will typically be able to more quickly recommend a caster solution with the appropriate hardness and formulation.