PRODUCT DATA Mechanical Filter for Accelerometers UA-0559 and Mechanical Filter Set UA-0553

Mechanical Filter UA-0559 is supplied in packs of five as: Mechanical Filter Set UA-0553.

Uses • Measurement of general vibrations, particularly where instrumentation is not equipped with a ranged of low-pass filters • Measurement of low-frequency, low-level vibration that is masked by high-frequency, high-acceleration levels often found in hand- held power tools • Protection of accelerometer and preamplifier from high-level shocks • Reduction of sensitivity to transverse vibrations

Features • Prevents the accelerometer from detecting high-frequency vibration • Prevents the excitation of accelerometer resonance • Works with all types of piezoelectric accelerometers • Selects cut-off frequency when load mass is adjusted • Breaks ground-loop paths

Benefits of the Mechanical Filter

Mechanical Filter UA-0559 is interposed between an accelerometer and the mounting point on the test object in order to prevent the accelerometer from detecting high-frequency vibration. There are several instances where this may be desirable: • Where measurements of low-frequency vibration, possibly at low levels, and high-level, high- frequency acceleration masks the low-frequency components because of preamplifier overload, distortion, lack of electronic filters, etc. • Where the accelerometer’s resonance frequency is being excited by high-frequency vibration of a possibly insignificant level, causing wideband measurement errors and overloads. Using the mechanical filter would then increase the useful dynamic range of the measuring instrument by more than 20 dB • Where the accelerometer could be subjected to transient shock levels beyond its maximum capability – the mechanical filter could prevent possible damage • Where the measuring instrument has a high-frequency response in excess of that required and is not equipped with suitable low-pass filters. Vibration measurements with a wide-band sound level meter is a notable example of this case • Where a specific cut-off frequency is required without the use of specially built electronic filters or elaborate tunable filters. For example, when measuring vibration as applied to the human hand, arm or body, the measuring range is normally limited to below 1000 Hz Description

The filter consists of upper and lower parts of stainless steel bonded together and electrically separated by a specially formulated butyl rubber core. The two halves are equipped with standard accelerometer threads (10–32 UNF), male and female respectively. A tommy bar can be inserted transversely through the two halves to protect the rubber core from excessive torque when the accelerometer is tightened down. Where the butyl rubber core can be contaminated with hydrocarbon substances such as oil or fuel, a resistant silicone rubber sealer should be used (for example, Dow Corning® Type 733 RTV).

The effect of the mechanical filter on the frequency response of an accelerometer is shown in Fig. 1. Transverse and main axis resonances, which are typically 30 dB in amplitude, are substituted by a highly damped resonance response of only 3 to 4 dB amplitude. The accelerometer’s own axial resonance is suppressed by 25 to 30 dB and shifted up in frequency due to decoupling of the mass above the filter.

Fig. 1 30 Main axis response Typical main axis and of accelerometer 20 transverse axis Main axis response of accelerometer mounted frequency response 10 on Mechanical Filter of Piezoelectric 0 Charge Accelerometer Transverse axis response of accelerometer Type 4370, –10 demonstrating the –20 effect of Mechanical Response (dB) Filter UA-0559 –30

–40 Transverse axis response of accelerometer mounted –50 on Mechanical Filter 50 100 200 500 1k 2k 5k 10k 20k 50k Frequency (Hz) 770805/1

If a particular accelerometer, when combined with the mechanical filter, does not cover a high enough frequency, an accelerometer with a lower mass should be used, see Fig. 2. Conversely, to reduce the cut-off frequency at the high-frequency end, additional mass loads can be added. These can be added in the form of suitable washers or nuts. The curve in Fig. 3 shows the cut-off frequency (–3 dB) as a function of the load mass on the filter.

Fig. 2 5 Typical frequency response of various Brüel & Kjær 0 437 438 0 general-purpose 2, 4383 4371, 4384 4378, 4379 accelerometers when 4381 –5 8318 mounted on UA-0559

–10 Response (dB)

–15

–20 200 Hz 300 Hz 500 Hz 1 kHz 2 3 5 10 kHz 20 30 50 Frequency 770807/2

2 Fig. 3 1000 Typical cut-off 700 frequency (–3 dB) 500 plotted as a function 300 of load mass on filter 200

100 70 50 30 Load Mass (grams) 20

10 0.71 1.52 3.45 7 10 Cut-off Frequency (kHz) 770806

The mechanical filter can be used to isolate the accelerometer from transverse vibrations. This is particularly useful when measuring complex vibration environments.

Although it is possible to use the mechanical filter with accelerometers that have a larger base radius than itself, care must be taken with the choice of mounting location in order to avoid undue bending moments. Often it is unnecessary to use the mechanical filter with Accelerometer Type 8318 due to its built-in low-pass filter.

The stiffness and damping effect of the filter medium, butyl rubber, is dependent on ambient temperatures: optimum damping is obtained between 20 and 50°C. At higher temperatures, the stiffness and damping are reduced resulting in a reduction in resonant frequency and an increase in the amplification at resonance. At lower temperatures, the resonant frequency is increased while the amplification at resonance is also increased. These characteristics are shown in Fig. 4.

Fig. 4 Temperature °C Typical temperature 100 response of UA-0559 Resonance frequency and 80 cut-off frequency as a 60 function of ambient temperature Resonanceamplitude as 40 a function of ambient temperature 20

0

–20 0.3 0.4 0.5 0.6 0.8 1 1.25 1.5 2 2.5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Frequency Factor Relative to 20°C Resonance Amplitude (dB) 770808

3 Specifications – Mechanical Filter UA-0559

Response at Resonance: <4 dB (+ 60%) for temperatures +20°C to Max. Continuous Dynamic Load: 30 NRMS axial (corresponding to 2 + 50°C. See Fig. 4 550 m/s (55 g) with 54 g accelerometer (Type 4370), 15 NmRMS Cut-off Frequency (–3 dB): At 20°C, approx. 35% of accelerometer’s transverse normal mounted resonance frequency. It is a function of load mass and Max. Accelerometer Mounting Torque: 1 Nm. When using a tommy temperature, see Fig. 3 and Fig. 4 bar: 7 Nm Filter Slope: Approx. 12 dB/octave Weight: 16 g Temperature Range: –50°C to 100°C Materials: Stainless steel AlSl 303, Butyl rubber Transmission Linearity: Deviation <2% up to 10 NRMS dynamic load Electrical Resistance: >100 M top to base Dimensions:

10 − 32 UNF-2A 3.1 10 4

10 − 32 UNF

NV 18 In mm 100004 (all dimensions in mm)

Ordering Information

UA-0553 Mechanical Filter Set Includes the following: • UA-0559: 5  Mechanical Filters • DA-1084: 1  Locking Pin (Tommy Bar ) BP 0297 – 15 2012-06

TRADEMARK Dow Corning is a registered trademark of Dow Corning Corporation

Brüel & Kjær reserves the right to change specifications and accessories without notice. © Brüel & Kjær. All rights reserved. ËBP-0297---ZÎ

HEADQUARTERS: Brüel & Kjær Sound & Vibration Measurement A/S · DK-2850 Nærum · Denmark Telephone: +45 7741 2000 · Fax: +45 4580 1405 · www.bksv.com · [email protected] Local representatives and service organisations worldwide