The Quarterly Engineering Problem – An Actual Case

//The Quarterly Engineering Problem – An Actual Case

Many large hydro-generators have baffle plates on the rotor spider.  The baffle plates induce and control airflow in the unit to cool the rotor rim, pole windings and primarily the stator.  EME recently investigated a unit with a rotor of approximately 300 inches in diameter, which had 8 spider arms, and a span between arms of approximately 80 inches.   Throughout the unit’s history, the operators have discovered cracked bolts or cracked baffle plates or both.

Why were the baffle bolts failing? What design change should be recommended?

A. Baffle plate vibration (such as a natural frequency coinciding with vortex shedding or other flow induced problems.)  Change baffle plate design.

B. Lack of clearance leads to high forces on bolts – increase clearance hole diameter.

C. Spider arms bend due to operating torque, causing baffles to slide under bolt heads, leading to fatigue cracking. Modify fastening to reduce or eliminate friction.

D. Bolts aren’t tight enough, and movement causes cracking – use higher grade bolt or larger diameter bolt and more torque.

E. Bolts are rusting – switch to stainless steel.

Some of the presented options are nearly correct; D, for example.  Choice A was also investigated, however it was determined not to be the source of the failures.  The condition in choice B has also been observed may have occurred at some locations, however EME determined the primary cause to be C.  On this unit, lightweight plates were rigidly bolted between the spider arms.  Under load, torque bends each arm although the amount of deflection will vary along the arms length, being greatest at the Hub end.  This differential movement could not be accommodated, causing the baffles to slide relative to the bolt head and spider arm.  Both static and sliding friction forces impose a side load on the bolt which changes directions with each start-up and shut-down leading to a fatigue crack.  Using stronger bolts and/or increasing bolt torque (as in D) simply causes the baffles to crack – the bending deflection must be accommodated.

The design must be modified so that the relative movement of the spider arms does not transmit loading to the baffles via the fasteners.

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By | 2017-08-29T19:15:32+00:00 June 2nd, 2017|2Q2017|
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