The rule that governs every selection
Isolation efficiency depends on the ratio between the disturbing frequency and the mount's natural frequency. The disturbing frequency for rotating plant is its running speed: a 1450 rpm pump disturbs at roughly 24 Hz, a 960 rpm fan at 16 Hz.
To achieve around 95% isolation you need a frequency ratio of about 4.5, which means static deflection in the region of 25 mm for typical rooftop plant. Neoprene pads deflecting 2–3 mm cannot deliver that on low-speed equipment — a common cause of persistent complaints on UAE towers.
Choosing the isolator
| Isolator | Static deflection | Typical use |
|---|---|---|
| Neoprene pad / waffle | 2–6 mm | Small pumps, high-speed compact fans, transformers |
| Rubber-in-shear mount | 5–10 mm | Small AHUs, condensers, packaged units |
| Open spring mount | 25–50 mm | Rooftop AHUs, chillers, large fans |
| Restrained / housed spring mount | 25–50 mm | Outdoor plant, cooling towers, wind and seismic exposure |
| Spring hanger | 25–50 mm | Suspended fans, in-line pumps, ducts near equipment |
| Concrete or steel inertia base | with springs | Base-mounted pumps, high-vibration and thrust-loaded plant |
Do not forget the connected services
- Rigid pipework and ductwork bypass the isolators completely — fit flexible connectors at every equipment interface
- Isolate the first three pipe hangers downstream of any pump with spring hangers
- Electrical conduit must include a flexible loop at the equipment
- Drain lines and refrigerant runs cross-bridge more often than any other service on site
Inertia bases and thrust
An inertia base adds mass to lower the assembly's centre of gravity, resist starting torque and limit motion. Specify one for base-mounted pumps, for fans with high discharge thrust, and wherever the equipment is light relative to its dynamic forces. Typical base mass is 1.5 to 2 times the equipment weight for pumps.
Seismic and wind restraint in the UAE
Rooftop plant in Dubai and the northern Emirates must be restrained against wind, and many specifications now require seismic restraint to the project's design category. Use restrained spring mounts and cable or strut restraints selected together with the isolators — a restraint that bottoms out under normal operation destroys the isolation it was meant to protect.
Frequently asked questions
What static deflection do I need for a rooftop chiller?
For rooftop plant over occupied space, 25 mm deflection restrained spring mounts are the usual minimum, increasing to 50 mm where the structure is lightweight or long-span. Confirm against the equipment's lowest running speed and the structural natural frequency.
Are neoprene pads ever sufficient?
Yes, for small, high-speed equipment on stiff slabs at grade or where no occupied space is nearby. On flexible floors or for low-speed plant they provide too little deflection to isolate effectively.
Do flexible connectors really matter?
They are essential. A single rigid pipe connection can transmit as much vibration into the structure as the unisolated machine, which is why plant that appears correctly mounted still generates complaints.
Vibration Isolation
Spring, rubber, neoprene, inertia bases, seismic restraints.
View Vibration Isolation