Bonded rubber vibration isolation mount for pump and motor baseframes

01

Natural frequency against running speed: about 80% attenuation from a 10 Hz mount at 1500 rpm, about 85% from 7 Hz at 1200 rpm

Paulstra's catalogue gives the attenuation of a rubber suspension as E% = 100 × (r² − 2)/(r² − 1), where r is the ratio of the exciting frequency to the suspension's natural frequency, and sets an attenuation above 50% as the requirement to aim past. [1] Its chart shows 80% attenuation at 1500 rpm for a natural frequency of 10 Hz. [1] Trelleborg states that its RA mount gives 75–85% isolation at normal speeds of about 1500 rpm and points to its RAEM or M-Series mounts for better isolation. [2]

Paulstra notes that the natural frequency of a rubber mount follows its dynamic stiffness, which can differ from the static stiffness depending on amplitude, frequency and elastomer, so the frequency stated at nominal load is an indication; for another load in the linear part of the curve it scales as f0 × √(nominal load / actual load). [1] Editorial calculation: a mount rated 10 Hz at its nominal load and run at half that load would sit near 10 × √2 ≈ 14 Hz, which at 1500 rpm gives r ≈ 1.77 and, by the same formula, about 53% attenuation, barely above Paulstra's 50% threshold. An underloaded mount isolates less, not more.

02

Load per mount and rubber hardness: 105–1300 kg per RA mount at 40 or 60 IRHD, shocks to 5 g

Trelleborg's RA mount works its rubber in shear and compression for vertical flexibility with horizontal stability, and its maximum load depends on size and on 40° or 60° IRH rubber. [2] Editorial calculation from the table below: the 40° IRH rating is 44–62% of the 60° IRH rating across these sizes.

The RA has an integral fail-safe device with a resilient stop, accepts occasional shock loads to 5 g relative to the supported weight in 60° IRH rubber and withstands shocks up to 2 g without plastic deformation, and has a domed cover against oil contamination. [2] Small cylindrical bobbin mounts sit at the other end of the range: AMC Mecanocaucho's diabolo mounts sold by RS run from the 129001, rated 4 daN in compression at 1.5 mm, to the 60 mm 129009, rated 150 daN in compression at 8 mm and 30 daN in shear at 10 mm. [3]

Editorial calculation: a 1,200 kg pump set on four mounts loads each to about 300 kg if the centre of gravity is central; that exceeds the RA 350's 250 kg in 40° IRH rubber but is within its 450 kg in 60° IRH, and Paulstra warns that when one size of mount is used with different rubber hardnesses there is a high risk of the mounts being interchanged, which may degrade the attenuation. [1]

03

Creep, heat and oil: realign the shafts at least two days after mounting, and treat 70 °C as abnormal

Paulstra estimates rubber creep as static deflection at time t = initial deflection × (1 + Cm × f(t)), with f(t) read from a curve measured on mounts compressed by 10% of their height at 30 °C and Cm a correction for the compound and hardness; it adds that creep also depends on the mount's shape. [1] Where alignment matters, it recommends aligning shafts at least two days after the machine has been mounted, to get past initial creep. [1]

Its standard mounts use natural rubber, and Paulstra lists as abnormal conditions temperatures above 70 °C, prolonged contact with corrosive liquids, acids or alkalis, oils and fuels, and corrosive gases such as ozone and chlorine, which can age the rubber and increase creep; special compounds are offered for these. [1] It also notes that an elastic suspension spreads the static load evenly where a machine on more than three rigid points would be unpredictably loaded, and, in its recommendations for STABIFLEX mounts, that all external connections must be flexible so as not to affect the performance of the mounting system. [1]

Editorial note: on a pump set this means flexible pipe connectors on suction and discharge and slack in cable and conduit runs; a rigid pipe bolted to an isolated baseframe carries the vibration around the mounts.

04

Data for a baseframe isolation mount enquiry

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Machine: [pump, motor, fan or compressor; running speed and any lower speeds passed through]
Mass: [total mass with frame and fluid; centre of gravity; number and position of mounts]
Target: [attenuation or natural frequency wanted]
Mount: [type; rubber compound and hardness; fail-safe stop; bolt size]
Loads: [shock in g; horizontal loads; pipe forces]
Environment: [temperature; oil, ozone or chemical exposure; indoor or outdoor]

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