Hardness testing of rubber-coated wheels is a critical quality control experiment that determines whether the elastomer tread meets specified Shore durometer or IRHD values, directly influencing load capacity, rolling resistance, floor protection, and service life. This technical article explains the test principles, applicable international standards, sample preparation, measurement procedures, and result evaluation for polyurethane and rubber bonded wheels used in industrial casters, conveyor rollers, and AGV drive wheels. By following standardized methods such as ISO 7619-1, ISO 48, ASTM D2240, and GB/T 531.1, manufacturers and accredited third-party laboratories can obtain reproducible hardness data for design verification and batch acceptance.
Technical Overview
A rubber-coated wheel (also called a bonded wheel or polyurethane-coated wheel) consists of an elastomeric tread layer, such as natural rubber, SBR, NBR, thermoplastic polyurethane (TPU), or cast polyurethane, chemically or mechanically bonded to a rigid core made of steel, aluminum, cast iron, or engineering plastics like nylon. The hardness of this elastomer layer is one of the most fundamental physical parameters of the finished wheel, because it governs the relationship between load-bearing capacity, deformation under load, rolling resistance, noise damping, and the degree of floor protection the wheel provides.
Hardness determination for coated wheels is typically performed using indentation-type instruments. For soft to medium elastomers, the Shore A scale is the industry standard; for very hard thermoplastics and hard polyurethane treads, the Shore D scale applies; and for precision rubber testing, the International Rubber Hardness Degrees (IRHD) method is used. The experiment must account for several variables that influence readings, including specimen thickness, temperature and humidity conditioning, reading time after pressure application, curved tread surfaces, and the proximity of the measurement point to the wheel core.
- Shore A hardness: applicable range approximately 20 to 90, covering most rubber and polyurethane treads
- Shore D hardness: applicable range approximately 30 to 90, used for hard thermoplastic treads
- Micro-IRHD: used when the tread thickness is insufficient for standard Shore testing
- Key governing standards: ISO 7619-1, ISO 868, ASTM D2240, ISO 48, GB/T 531.1, and GB/T 6031
Testing Samples
Polyurethane-Coated Wheels
Cast polyurethane and TPU coated wheels with tread hardness typically between 80 and 95 Shore A are the most common samples. These wheels are widely used in AGV drive wheels, heavy-duty casters, and conveyor systems. Because polyurethane treads are often relatively thin, the tester must verify that the total elastomer thickness meets the minimum requirement of the chosen standard, or measure hardness on a cured plaque of the same compound when the tread is too thin.
Natural Rubber and SBR Coated Wheels
Soft rubber treads in the range of 60 to 80 Shore A are common on wheels designed for floor protection and shock absorption, such as those used on hospital equipment, trolleys, and light industrial carts. These materials exhibit higher elasticity and stress relaxation, so the dwell time before taking a reading must be strictly controlled to ensure comparability between measurements.
Silicone and Special Elastomer Wheels
Low-hardness silicone or foam-elastomer treads, typically below 40 Shore A, are used in precision transport rollers for glass, electronics, and printing machinery. For these soft materials, the Asker C scale or IRHD micro method may be more appropriate than Shore A, and sample geometry must be evaluated before testing.
Specimen Preparation and Conditioning
- Finished wheels should be conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 16 hours before testing, unless otherwise specified
- The tread surface must be clean, dry, and free of release agents, mold lubricants, talc, or oxide layers
- The minimum elastomer thickness for a valid Shore A test is generally 6 mm; thinner layers require micro-hardness methods or testing on equivalent cured plaques
- Test points must be located at least 12 mm from the wheel edge, at least 6 mm from the wheel core, and at least 15 mm apart from previous indentations
Testing Items
The hardness determination experiment for coated wheels typically includes the following measurable items, which together characterize both the material and the consistency of the bonding and molding process:
- Shore A hardness of the tread (median value of at least five readings)
- Shore D hardness for hard polyurethane or thermoplastic treads
- IRHD value for precision rubber treads tested to ISO 48 or GB/T 6031
- Hardness uniformity measured at multiple positions around the tread circumference and across the tread width
- Hardness change after thermal aging, evaluated by comparing values before and after oven exposure
- Hardness deviation from the nominal specification, commonly required to fall within plus or minus 5 Shore A units
Testing Methods
Shore Durometer Method (ISO 7619-1 / ASTM D2240 / GB/T 531.1)
The durometer presses a calibrated truncated-cone indenter into the tread surface under a defined spring force, and hardness is read from the penetration depth on a scale of 0 to 100. The instrument is applied perpendicular to the tread surface; on finished wheels, the curvature of the tread is acceptable provided the anvil contacts the surface squarely and the test point is far enough from the core. A reading is normally taken at a standardized time, commonly 3 seconds for the specified instant reading or 15 seconds where delayed reading is agreed, and the median of at least five readings distributed around the wheel is reported along with the scale, temperature, and dwell time.
IRHD Dead-Load Method (ISO 48 / GB/T 6031)
The IRHD method applies a spherical indenter under a fixed dead load with a small preliminary force, then converts the difference in penetration depth to hardness degrees through a standard table or formula. Because the applied force is independent of penetration, IRHD results are more reproducible than durometer readings and are preferred for specification disputes and interlaboratory comparisons. The micro-IRHD variant uses smaller indenters and lower forces, making it suitable for thin tread sections that cannot be measured with a standard durometer.
Test Conditions and Procedure Summary
- Calibrate or verify the durometer against reference blocks immediately before testing
- Condition the wheel at 23 ± 2 °C and 50 ± 5 % relative humidity; avoid testing within 30 minutes after the sample is touched by hand
- Select at least five evenly spaced points around the tread circumference, avoiding joints, gate marks, bubbles, and visible defects
- Press the instrument smoothly without shock, hold the specified dwell time, and record each reading to the nearest scale unit
- Report the median value, the hardness scale, the temperature, the dwell time, and any deviations in specimen thickness
Result Evaluation
The measured median hardness is compared with the drawing specification or the customer-defined nominal range. A typical acceptance criterion for industrial coated wheels is the nominal value plus or minus 5 Shore A units. If hardness varies by more than the agreed tolerance between positions on the same wheel, the mixing, curing, or molding process should be investigated, since non-uniform hardness usually indicates incomplete cure, inconsistent compound dispersion, or contamination at the bond interface.
Applications
Hardness determination experiments on coated wheels support product development, incoming material control, and batch release across many industries:
- Industrial casters and heavy-duty wheels, where tread hardness directly correlates with load rating and floor protection
- AGV and AMR drive wheels, where a controlled hardness balances traction, wear life, and low rolling resistance on warehouse floors
- Conveyor rollers and guide rollers in logistics, food processing, and packaging lines
- Precision transport rollers for glass, steel coil, electronics, and printing machinery requiring tight hardness uniformity
- Medical and cleanroom equipment wheels, where non-marking soft treads must be verified for consistent compliance
- Research and development of new polyurethane or rubber compounds, where hardness is tracked against aging, compression set, and tensile data
Manufacturers frequently submit prototype and production wheels to accredited third-party laboratories for independent verification against ISO, ASTM, or GB standards, which strengthens supplier declarations and supports customer qualification in export markets.
FAQ
What is the typical hardness range for rubber-coated wheels?
Most industrial coated wheels fall between 70 and 95 Shore A. Softer treads around 60 to 75 Shore A are chosen for floor protection and quiet operation, while harder treads above 90 Shore A are selected for heavy loads and minimal rolling resistance.
Why do Shore A and Shore D results differ for the same wheel?
The two scales use different indenter geometries and spring forces. Shore A is optimized for flexible elastomers, whereas Shore D suits rigid thermoplastics. Values from different scales are not linearly interchangeable, so the scale must always be stated with the result.
Can hardness be measured directly on a thin tread?
If the elastomer layer is thinner than approximately 6 mm, a standard Shore A reading is influenced by the rigid core and will be biased high. In such cases the micro-IRHD method or testing on a cured plaque of the same compound is the correct approach.
What causes hardness variation around the circumference of one wheel?
Common causes include non-uniform cure temperature, compound variation, contamination at the bond line, entrapped air, and differences in mold pressure. Systematic mapping of hardness at fixed positions is an effective way to diagnose these process issues.
How does temperature affect the measured hardness?
Elastomer hardness decreases as temperature rises. All standardized methods therefore require testing at 23 ± 2 °C after adequate conditioning, and results obtained at other temperatures must be reported with the actual test temperature.