Hardness Testing of Vibration-Damping Mounting Pads (Machine Leveling Wedges)

Hardness testing of vibration-damping mounting pads is a critical quality control process that verifies both the metallic wedge body and the rubber damping element of machine leveling mounts meet specified material requirements. By applying recognized methods such as Brinell, Rockwell, Vickers, and Shore A durometer testing in accordance with ISO, GB, and ASTM standards, accredited third-party laboratories can confirm load-bearing capacity, vibration isolation performance, and long-term service reliability of these components.

Technical Overview

Vibration-damping mounting pads—also known as shock-absorbing leveling wedges, anti-vibration mounts, or machine leveling pads—are mechanical installation components widely used to support and level machine tools, CNC equipment, compressors, pumps, and other industrial machinery. A typical mount consists of a metallic wedge or stepped body manufactured from grey cast iron or spheroidal graphite (ductile) iron, combined with a rubber elastomer damping layer bonded to or inserted beneath the metal base.

Hardness is one of the most important quality indicators for both constituents of the assembly. For the cast iron body, hardness reflects the effectiveness of heat treatment, graphite morphology, and matrix microstructure, which directly determine compressive strength, wear resistance, and resistance to deformation under static and dynamic machine loads. For the rubber damping element, Shore A hardness governs the natural frequency, static deflection, and vibration isolation efficiency of the mount.

Hardness evaluation is typically performed against material and product standards such as GB/T 1348 and ISO 1083 for spheroidal graphite cast iron, GB/T 9439 and ISO 185 for grey cast iron, and JB/T 6607 for machine-tool anti-vibration mounting pads. Rubber elements are assessed using GB/T 531.1, ISO 7619-1, or ASTM D2240 Shore hardness methods. Testing is commonly carried out by accredited third-party laboratories to support factory acceptance, incoming inspection, and product certification.

Testing Samples

Metallic Wedge Bodies and Base Plates

Test pieces are taken from the upper wedge block, lower wedge block, and base plate of the mount. These components are usually cast from HT200/HT250 grey iron or QT450-10/QT500-7 ductile iron. Samples may be tested directly on machined flat surfaces or on separately cast test coupons (keel blocks or attached prongs) representative of the production lot.

Rubber Damping Elements

The elastomeric pads, normally made of NBR, NR, or composite vulcanized rubber compounds, are tested as flat tab specimens cut from the pad or on the pad itself when its surface and thickness satisfy the minimum requirements of the applicable durometer method.

Finished Mount Assemblies

Complete commercial mounts may be submitted for verification testing, allowing the laboratory to assess hardness consistency across different casting positions, batch uniformity, and conformance of the bonded rubber layer as delivered to the end user.

Testing Items

Hardness testing programs for vibration-damping mounting pads generally include the following items:

• Brinell hardness (HBW) of the cast iron body—typically 150–230 HBW for grey iron grades and 140–210 HBW for common ductile iron grades
• Rockwell hardness (HRB or HRC) for thinner sections where Brinell indentation is impractical
• Vickers hardness (HV) for microstructure verification, thin cross-sections, or welded/repaired zones
• Shore A hardness of the rubber damping element—commonly specified between 55 and 70 Shore A for machine mounts
• Hardness uniformity testing across multiple positions on the casting to detect chill, segregation, or incomplete annealing
• Hardness-to-strength correlation verification in accordance with GB/T 1172 or ISO 18265 conversion tables
• Re-test and statistical evaluation for lot acceptance when individual readings fall outside the specified tolerance band

Testing Methods

Brinell Hardness Testing

Brinell testing is the preferred method for cast iron because the large indentation averages out the influence of graphite flakes and nodules. Testing is performed per GB/T 231.1 or ISO 6506-1 using a 10 mm carbide ball with a test force of 29.42 kN (3000 kgf), corresponding to a force-to-ball-diameter-squared ratio of 0.102·F/D² = 30. For thinner wedge sections, a 5 mm or 2.5 mm ball with proportionally scaled force may be applied. The test surface must be ground free of scale, sand, and decarburized material, and supported so that no movement occurs during loading.

Rockwell Hardness Testing

Rockwell testing per GB/T 230.1, ISO 6508-1, or ASTM E18 is applied where section thickness is insufficient for Brinell indentation. The HRB scale (1.588 mm ball, 100 kgf major load) is suitable for annealed or normalized iron, while the HRC scale (diamond cone, 150 kgf) is reserved for quenched-and-tempered or surface-hardened components. The minimum thickness rule requires the specimen to be at least ten times the permanent indentation depth.

Vickers Hardness Testing

Vickers testing per GB/T 4340.1 or ISO 6507-1, typically at HV10 or HV30, is used for laboratory verification, matrix micro-hardness measurement, and evaluation of heat-affected or repaired areas. It provides a precise indentation suitable for metallographic cross-sections cut from failed or disputed mounts.

Shore A Durometer Testing of Rubber Elements

The rubber damping layer is tested with a Shore A durometer in accordance with GB/T 531.1, ISO 7619-1, or ASTM D2240. The specimen must have a flat test area of sufficient size, a thickness of at least 6 mm (stacked plies are not permitted), and readings are taken at least 12 mm from the edge. The median of at least five measurements taken at different points is reported, with the measurement time (typically 3 seconds) recorded.

Portable and On-Site Hardness Testing

For installed mounts or large production batches, portable Leeb impact testers (ISO 16859, ASTM A956) or UCI instruments (ASTM A1038) may be used, provided results are validated against stationary bench testers and converted using recognized conversion standards rather than used directly for acceptance.

Applications

Hardness testing of vibration-damping mounting pads is essential wherever machinery is installed on such mounts, since incorrect hardness can lead to wedge collapse, accelerated wear, loosening of leveling adjustment, or degraded vibration isolation. Typical application sectors include:

• Machine tools: lathes, milling machines, grinding machines, and machining centers
• Plastics and rubber machinery: injection molding machines and extruders
• Compressors, blowers, fans, and industrial pumps
• Printing, packaging, and textile machinery
• Generators, machine shop equipment, and metal forming presses
• Quality control by manufacturers, incoming inspection by equipment builders, and failure analysis by end users

FAQ

What hardness value should a ductile iron mount body have?

For the most common ductile iron grades used in mounting pads, the body hardness is typically 140–210 HBW for QT450-10 and approximately 170–230 HBW for QT500-7. The exact requirement must be taken from the drawing, the purchase specification, or the relevant grade tables in GB/T 1348 or ISO 1083.

Why is Brinell testing preferred over Rockwell for cast iron pads?

The large Brinell ball indentation spans many graphite particles and measures the average response of the matrix, giving a stable, reproducible value. Rockwell or Vickers indentations are small and can be disproportionately affected by individual graphite flakes or nodules, increasing data scatter.

What Shore A hardness is typical for the rubber damping layer?

Most machine mounts use rubber in the range of 55–70 Shore A. Softer compounds give better low-frequency isolation but greater static deflection, while harder compounds provide higher load capacity. The specified value is chosen to match the machine weight and the disturbance frequency of each application.

How should samples be prepared before testing?

The test surface should be ground flat and free of scale, paint, sand inclusions, and decarburized or chilled layers. For Brinell testing a surface roughness of Ra 1.6 µm or better is recommended, with progressively finer finishes required for Rockwell and Vickers testing. The opposite face must be clean and supported firmly to prevent any movement or elastic spring-back during load application.

How many hardness readings are required for lot acceptance?

Standard practice is to take at least three readings per specimen, spaced at least three indentation diameters apart, and to sample multiple positions or components per production lot according to the sampling plan agreed in the inspection specification. If any single result falls outside the specified range, additional testing and a metallographic check for chill, carbides, or matrix anomalies are normally required before making an acceptance decision.