ISO 5348: Mechanical vibration and shock – Mechanical mounting of accelerometers

Summary

ISO 5348 is a fundamental and highly practical standard for any vibration analyst. It addresses a critical factor that directly impacts data quality: how the accelerometer is physically attached to the machine. The standard specifies various mounting methods and describes how each method affects the frequency response of the measurement. Following the guidance in ISO 5348 is essential for obtaining accurate and repeatable vibration data, especially when measuring high-frequency vibrations.

Table of Contents (Conceptual Structure)

The standard is structured to provide clear, practical advice on mounting techniques:

1. 1. Scope and Mounting Methods:

   This initial section establishes the standard’s purpose: to provide clear, technical guidance on the methods of attaching accelerometers to a vibrating surface to ensure accurate data. The central thesis of the standard is introduced here: the mounting method is a critical part of the measurement system and directly determines the highest frequency at which reliable data can be collected. A poor mounting technique will act as a mechanical filter, attenuating or damping high-frequency vibrations before they can be measured. The section then introduces the primary mounting methods that will be evaluated in detail: stud mounting, adhesive mounting, and magnetic mounting, establishing the framework for the rest of the document.

2. 2. Stud Mounting:

   This method is presented as the optimal, reference-grade technique for accelerometer attachment. It involves drilling a hole into the machine structure, tapping it with a thread, and then screwing the accelerometer’s mounting stud directly into the hole. The standard specifies that the mounting surface must be clean, flat, and smooth, with a spot face machined if necessary to achieve this. A thin layer of silicone grease or a similar coupling fluid should be applied to the base of the sensor to fill any microscopic voids, maximizing the surface contact area and improving the transmission of high-frequency energy. This method provides the highest possible mounting stiffness, which in turn results in the highest mounted resonant frequency. This ensures that the sensor can accurately measure the widest possible range of frequencies without its measurement being corrupted by the resonance of the mounting itself. It is considered the benchmark for all other methods and is essential for permanent monitoring installations, high-frequency diagnostic tests (like for bearings and gears), and for sensor calibration.

3. 3. Adhesive Mounting:

   This section details the use of adhesives as a semi-permanent mounting solution, often used when drilling into the machine is not practical or permitted. The standard differentiates between different types of adhesives. For the best results, a hard, rigid adhesive such as a cyanoacrylate (“super glue”) or a two-part epoxy is recommended. The key principle is to use a minimal amount of adhesive to create a very thin, rigid bond line between the sensor’s base and the machine surface. A thick or soft adhesive (like a silicone rubber) will act as a damper, severely limiting the high-frequency response. When performed correctly on a properly prepared surface, a rigid adhesive mount can achieve a usable frequency range that is nearly as high as a stud mount, making it a viable alternative for many diagnostic applications. The standard also covers the use of adhesive-mounted bases, which are small metal pads glued to the machine to provide a repeatable location for attaching a stud-mount sensor.

4. 4. Magnetic Mounting:

   This chapter discusses the use of magnetic bases, which are extremely common for portable, route-based data collection due to their convenience. However, the standard emphasizes that this convenience comes at a significant cost to data quality. A magnetic mount is inherently less stiff than a stud or adhesive mount. Furthermore, the magnet adds significant mass to the accelerometer. This combination of lower stiffness and higher mass dramatically lowers the mounted resonant frequency of the sensor system, which severely limits the usable upper frequency range of the measurement. The standard makes it clear that high-frequency data (typically above 2,000 Hz) collected with a magnet is often unreliable. It provides practical guidance for maximizing the quality of a magnetic mount: use a strong, “two-pole” magnet, ensure the contact surfaces are perfectly clean and flat, and apply firm pressure when attaching the magnet to the machine.

5. 5. Other Methods (Probes):

   This section addresses the use of hand-held probes, often called “stingers,” which are sometimes used for quick checks or in hard-to-reach areas. The standard strongly discourages this practice for any serious diagnostic work. The human body is a very effective low-pass filter and damper, and it is impossible to hold a probe with consistent pressure or at a perfectly perpendicular angle. As a result, this method is shown to be highly unrepeatable and its frequency response is severely limited, often to less than 1,000 Hz. While a probe might be able to confirm the presence of a very large, low-frequency vibration (like a severe unbalance), it is completely unsuitable for reliable trend analysis or for the detection of high-frequency faults like bearing and gear defects.

6. 6. Surface Preparation and Cabling:

   This final section provides critical, practical advice for ensuring data quality, regardless of the mounting method used. It emphasizes that the mounting surface must be properly prepared. This includes ensuring the surface is as flat and smooth as possible, and that any paint, rust, or dirt is removed to ensure direct metal-to-metal contact (or metal-to-adhesive-to-metal). For stud mounting, it specifies the need for a spot face to be machined if the surface is not perfectly flat. The standard also provides important guidance on sensor cabling. It recommends that the cable be firmly tied down to the structure a short distance from the sensor. This provides strain relief for the connector and, more importantly, prevents cable motion. If a cable is allowed to whip around during measurement, it can generate a low-frequency electrical signal due to the triboelectric effect, which can contaminate the true vibration signal and lead to erroneous data.

Key Concepts

• Frequency Response is Key: The central theme of the standard is that the mounting method acts as a mechanical filter. A poor mount (like a magnet) adds mass and reduces stiffness, creating a low-pass filter that cuts off the high-frequency vibration before it can even reach the sensor.
• Stiffness is Paramount: To accurately transmit high-frequency vibration, the connection between the sensor and the machine must be as stiff and lightweight as possible. This is why a direct stud mount is superior to all other methods.
• Trade-off Between Convenience and Accuracy: The standard makes it clear there is a direct trade-off. Magnetic mounts are convenient for route-based data collection, but the analyst must accept that the usable frequency range is compromised. For high-frequency bearing or gear analysis, a stud or adhesive mount is strongly preferred.
• Repeatability: Following the standard’s guidance, such as using mounting pads for repeatable sensor placement, is crucial for good trend analysis, as it ensures that changes in the data are due to the machine’s condition, not variations in the measurement technique.

← Back to Main Index