Electric motor / gear reducer mounting plate thickness

[IanFletcher]

Is there any rule of thumb or guidelines for determining the mounting or base plate thickness for electric motors and gear reducers?

We design the supports for large induced draft cooling tower fans (12-40' in diameter). The fans are horizontal and driven by right angle gear reducers which are in turn driven by composite drive shafts and electric motors. We work with equipment in the 50-250HP range. The entire assembly is typically mounted on top of a fiberglass or wood structure.

 

[MotorVib]

I would not feel all that good with mount 50 to 250 hp motors to fiberglass or wood structure. But as for the mounting plate I would just run some hand cal or FEA. For MG set the mounting base was always an over kill.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[electricpete]

I don't know much about it and certainly don't know any thrumbrules. One comment - I imagine the gearbox output is low speed, high torque. The machine torque should not create excessive deflection which would distrub alignment at bearings or between machines.

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[Tmoose]

If the driveshaft is long enough the motor can live in the more pleasant environment outside the shroud, like this.

http://www.truwater.com.my/images/project/p4_3.jpg

I've worked on some towers that had nagging vibration problems at rotor speed, and some where the problems were at motor/drivshaft speed. Aside from the genuine rotor balance and damaged driveshaft cases, the cause ( in my mind) was weak or even worse, resonant mountings.

Mason has their vintage HVAC isolation discussion on line here -

http://www.mason-industries.com/masonind/downloads/spec/complete/ashlec.pdf

Their suggestion of beam depth = length /10 is a good starting point if you have room. To provide torsional stiffness the frame needs close shapes (round/square/rectangular tubing) or X crossmemebers. Buy and refer to Lincoln Electric's "Procedure handbook for Arc welding".

Then, to your question of base plates (to which the motor and gearbox mount) afixed to the frame, providing stiffness with pure thickness is not very efficient. I think its better (necessary !!)to have stiffeners >>>>CLOSE<<< to the mounting points under a more modest base plate.

We have a 1200 rpm fan whose bearings are mounted on pedestals with 25 mm ( ~ 1 inch thick plate - enough to evoke muttered well meaning but sometimes unfounded comments of "those aren't going anywhere") thick base plates. The anchor bolts are at least 6 inches away from the vertical plates that form the pedestal. We had 8 MILs 1X axial bearing vibration, and 3 MILS horizontal, and portions of the base plate (away from the anchors) have over 4 MILS of vibration(!). Stiffeners were welded in place yesterday to finally provide a decent load path to the anchor bolts. I'm, like, all nervous waiting to hear what the new vibration levels are after the fan is restarted this weekend.