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Equipment Isolation

February 5, 2009

Isolating Structures from Vibration and Sound

An elevator can be a very noisy thing, especially if little regard is given to properly isolating things that vibrate or bang.  There are a number of potential noise makers in every type of elevator system.  All hydraulic elevators have a hydraulic pump unit with a motor that might range from 10 hp to 75 hp, which rotates at a speed from 1200 to 3600 rpm.  All electric elevators have a drive machine, either geared or gearless, which propel the hoist ropes (cables).  And all elevators of every type have controllers, some of which have rather large moving contactors that bang or drives that vibrate.  Older elevator systems included hoistway switches and door operation devices that make contact as the elevator travels, causing discomforting sounds.  High speed elevators can make an unpleasant sound, a great whooshing as the car passes pushing a large volume of air.

The major hotel chains have learned long ago never to design a hotel with a sleeping room adjacent to an elevator hoistway.  They also try to design so that the machine rooms are as distant as possible from the guests.  Architects know not to put the CEOs office adjacent to the hoistways nor near the machine rooms.  Hydraulic pump units anchored to a concrete slab over earth will likely be less problematic than locating the pump unit on an upper floor.  New overhead traction drive machines are now commonly isolated whereas basement traction drive machines are not.  The concrete floor slab or anchorage block directly over earth are often effective in deadening vibration.  So, where equipment is located can make a big difference in the transmission of vibration and the severity of the noise problem.

A good first start in reducing objectionable noise and vibration is where the manufacturer provides properly designed and installed internal isolation within the greater assembly.  In this way, the elevator installer can simply anchor the equipment directly to the building structure, often without additional isolation.  Good examples of this is where hydraulic pump assemblies contain their rotating units on an internal carriage or cradle, which is supported from the outer assembly via approved isolators.  There have been a few geared traction drive machines that contained internal isolation, but these were the exception.  Some controller manufacturers mount the larger relay starters and drives on an isolated subplate - some do not.  

Where a piece of equipment is a source of vibration, movement or action which will likely cause unacceptable noise, effectively isolating the unit from the structure is necessary.  A common practice in attempting to accomplish this is to install rubber or neoprene strips under the unit's base or feet.  Often the unit is then bolted or anchored to the structure where the fasteners, typically the bolt, nut or washer, are in direct contact with the metal of the unit's base or feet.  In this type of installation, the isolating material is in compression in part by the mass of the unit but also by the tension of the fasteners or anchors. 

I’ve been involved with a few projects where elevator noise was deemed excessive by the building owners and occupants.  In a couple of cases I've worked with Charles Salter & Associates, Sound Engineers.  Anthony P. Nash, P.E., Vice President, a senior project manager with Salter instructed me that the type of machine isolation I’ve described above provides negligible effective isolation between the machine and the structure.  He advised me that sound (noise) is delivered in two ways:  ambient and structural.  Ambient is sound waves travelling directly from the source to the receiver, such as from my vocal cords to your ear drums.  A stereo speaker is another example.  Structural sound (noise) is where the source of the sound is transmitted to a structure which amplifies the sound.  Typically, the source is something that vibrates (like a motor) or bangs (like a starter relay) and that vibration is directly transmitted to the greater structure via its mountings.  Mr. Nash taught me that concrete floors and walls are very efficient at transmitting and even amplifying the original source vibration.  His analogy was the concrete floor acts like the skin of a drum.  The entire slab will vibrate and amplify the sound.  Just as hitting a drum near the rim makes a different sound than hitting it in the middle, where the offending machine is mounted on the slab relative to adjoining structural walls will affect the type and quantity of sound as well. 

The way to reduce the structural transmission of sound is to properly isolate the source of the vibration from the structure.  Mr. Nash suggested that vibration is like electricity.  Where you have a direct connection between the metal frame or enclosure of the vibrating unit and the building structure, the vibration will transmit to the structure.  In the type of faulty isolation described above, the vibration is transmitted directly through the fasteners or wedge anchors to the structure due to the contact between the unit's feet or base and the anchor, washer & nut – despite the rubber/neoprene below the foot base.  Enlarging the foot base holes and inserting a neoprene or rubber bushing and adding an isolating pad on the top of the base and below the fastener/anchor's washer would potentially work.  However, making up these components and installing them properly more often proves to be difficult and the results are often faulty.  Moreover, for isolation to be effective, the isolating material must be soft enough to absorb the vibrations but hard enough not to be over compressed and transfer the vibration.  This is referred to and measured as the durometer loading.  For this design to work properly, one would have to know the load bearing and isolating value of the material based on the dimensional criteria.  This begs for engineering often not provided, especially not in the field.

A better solution is to use pre-manufactured, ready to use isolators that are designed for the type of loading (compression, tension or shear) and are sized by a range of loading.  Such isolators are also specifically engineered for the type of vibration, typically measured in hertz (cycles per second).  Some machines vibrate at a very low speed, such as a device that bangs or pounds.  Other machines vibrate at a high speed, such as a high speed motor or transformer.  Some isolators include natural rubber, which can deteriorate when exposed to oils or petroleum products.  Neoprene isolators are available for such installations.  Some isolators are pre-certified for use in seismic conditions, especially for OSHPD and DSA projects.

There are a number of manufacturers of pre-engineered isolators.  These include Mason Industries, RPM Mechanical, Inc., Tech Products Corp., Barry Controls, and Lord Corp.  Distributers include Western Rubber & Supply, Inc. and ERA Industrial Sales Inc.  I’ve specified Mason Industries #HMIB-1/2 Neoprene Isolators for foot mounted controllers and transformers.  These allow the wedge anchor to directly penetrate the isolator (see drawing below).  Note that these and many other isolators are only to be used in compression and tension – not in shear.  Loading the bushing part of the isolator provides too little isolation.  An isolator that works well in support of heavier components, such as hydraulic pump units, is the Lord Machinery Mount #J-2919-1.  Another very good isolator for moderately heavy items such as motor-generators is the Mason Industries model #BR, which works in compression, tension or shear and is OSHPD approved.  To isolate a heavy motor starter unit or motor drive within a controller enclosure, look at the Lord, Flex-Bolt Sandwich Mount isolators.  Depending on the weight of the item and mounting orientation, models #J-11729-125 or #J-4624-10 might work. 

In summary, there are very good off-the-shelf products available to effectively isolate equipment and minimize the transmission of offensive vibration and noise.  The time to install isolators is during the initial elevator equipment installation, not later in response to an unhappy client.  Dealing with revising electrical connections (which must be flexible for isolation), anchorages and fit issues by adding isolators after the initial equipment installation will likely be a much more difficult task.  Proper planning and design can prevent noise and vibration from becoming an issue. 

Experience has shown, once an issue has become a problem it can be hard to put the issue to rest, even if the noise and sound problems have been effectively mitigated.  It's much better to spend the few dollars up front to add effective isolation to assure satisfied customers than to spend considerably more sums later to correct a problem.

Designing elevator equipment installation which includes isolation is a service provided by RCB Elevator Consulting, LLC.  If you have a project in need of design engineering, which requires or would benefit from the addition of proper isolation, feel free to call.

Richard C. Blaska
Principal
RCB Elevator Consulting, LLC

Isolation Examples


This is a very good example of ineffective equipment isolation.  Note the equipment base rests upon an isolating pad, which might provide good isolation, assuming the durometer and loading calc out properly.  The problem with this isolation is the concrete wedge anchor and its washer & nut are in direct contact with the inner web of the hydraulic pump unit base channel.  Any vibration the unit emits will be transmitted to the slab of the structure through the wedge anchor. 

This ineffectual method of isolation may not be a problem if the concrete slab is directly over the earth, as the combined masses may effectively deaden any transmitted vibration.  However, if this method were used on an upper floor of a building, the transmission of vibration would very likely be a problem.


The detail above is a better solution for isolating the same piece of equipment, the hydraulic pump unit from the picture at the top.  It uses a total of four Lord Shock Mount #J-2919-1 isolators.  Care must be taken to determine the total gross weight of the unit, the distribution of the weight over the unit's supports and to follow the manufacturer's loading guidelines to choose the correct isolator.


This detail shows the use of a Mason Industries #HMB-1/2 neoprene isolator rated at 100 lbs.  These are used in sufficient quantities and properly located where the weight is applied to support elevator controllers, transformers, line filters and similar components.  A great advantage of these isolators is the anchor or bolt passes through the isolator and the unit base foot.  Typically the OEM holes in the base feet must be enlarged to the proper size to accommodate the isolator internal bushing.  These particular isolators can be used in compression or tension but not in shear.


Comments:  All comments and suggestions are welcome.  Feel free to write, call or email the author.


Disclaimer:  Any use of information contained herein is solely the responsibility of the user.  RCB Elevator Consulting, LLC and its associates accepts no liability for any information contained herein.

 

 

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