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Mechanical forces or holding down a generator
2

Mechanical forces or holding down a generator

Mechanical forces or holding down a generator

(OP)
A client of mine is installing an old 1966 2MVA 11kV 50HZ horizontally mounted synch generator. He wants to know what the forces are on the the holding down bolts for civil pad design. Does anyone have any typical information for a generator about this size. Unit is AEI manufacture, almost nothing else known
 

RE: Mechanical forces or holding down a generator

I would use power divided by speed to get shaft torque. For a synchronous generator, speed is almost always either 1500 or 3000 RPM. That is roughly 150 or 300 rad/s. So, torque is around 2000000/150 or 2000000/300 = 13 kNm or 6 kNm.

That torque acts on the feet. If feet are at distance x m, then force will be 13/x kNm or 6/x kNm. There are usually four feet, so divide with two (assuming torque load being taken up equally by the two pairs of feet) to get around 6/x or 3/x kN.

If x = 1 m, then forces will be 6 or 3 kN, or 600 respectively 300 kgf.

That is steady state force. Transient forces, when synchronising and if things like shorts happen on the line, will bring that load up around ten times. So, each bolt shold be able to hold down 6 or 3 metric tonnes of force.

Then, a good safety factor should also be used. A positive thing is that the weight of the generator helps keeping it in place. Still, I would contact a local consultant to get a second opinion.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Mechanical forces or holding down a generator

I agree with Skoggs. Ten times steady state torque plus a generous safety factor. My experience is with smaller machines, 300 kW to 600 kW. I have seen a number of sheared couplings mostly due to out of sync closing. The worst was a 400 HP motor when the starter arced over phase to phase. The motor became an induction generator and slowed down faster than the fan that it was driving. The result was a bent fan shaft, a bent motor shaft and parts of an exploded coupling in all parts of the fan room. It could have been worse if the mounting bolts on either the motor or fan had failed.
As a reality check, what is the largest bolt that will easily pass through the holes in the mounting feet? Beware of overtorquing the hold down bolts and subtract the preload from the ultimate strength of the bolts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Mechanical forces or holding down a generator

The preload does not subtract from the strength of bolts. When the applied load exceeds the preload then the clamping force simply drops to zero.

RE: Mechanical forces or holding down a generator

On the other side of the machine, the generator tries to lift and the load adds to the preload until something fails.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Mechanical forces or holding down a generator

Certainly ultimate tensile strength Su and preload all need to be considered in developing a bolting strategy, but there is no logic to subtract preload directly from Su unless you are imagining that total load = preload + external load, which would completely be incorrect.

A change in external load causes an incremental change in bolt load which is given by:
deltaFbolt = deltaFexternal * Kbolt/(Kbolt+Kjoint)
where Kjoint is stiffness of the clamped component.

This relationship applies until joint separation occurs at which time bolt will see 100% of the external load...of course at the bolt load is the external load, still no need to subtract preload.  

http://www.boltscience.com/pages/decomp1.htm
 

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RE: Mechanical forces or holding down a generator

(OP)
Thank you, that has been most helpful

RE: Mechanical forces or holding down a generator

Hi Rodmcm

After you have worked out the bolt loads you need to ensure your preload exceeds all the external loads and the method of how you preload your bolts is important, here is a link that gives accuracies of presetting bolt loads as well as theory on bolted joints.

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

desertfox

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