Tilting Pad Bearings - Split line at angle in running position

[tlwrigh1]

I recently came across a tilt pad bearing on a GE A10 steam turbine where the split line of the bearing is rotated approx 30° with respect to the horizontal joint of the fit that the bearing ring slides into in the front standard.

I talked with someone else that told me a lot of Alstom turbines have bearings that have their split lines rotated with respect to the horizontal joint.

I was wanting to know why the bearings were designed to be oriented in such a manor.

Thanks

 

[electricpete]

I don't know the reason. However, if you wanted to change load-on-pad to load-between pad or vice versa for a 6-pad bearing, you would rotate the bearing by approximately 30 degrees.

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(2B)+(2B)' ?

 

[stgrme]

I don't know for sure, but I suspect that GE would like the bearing reaction to fall "on-pad". If the turbine is a partial-arc admission type, keeping the reaction "on-pad" becomes more difficult since the angle of the bearing reaction will change depending on how many control (governing) valves are open.

Best of luck!

 

[JJPellin]

I have seen this many times for radial and thrust bearings. I believe it is just an assembly issue. The bearing has to have a key or pin to act as anti-rotation. That pin or key would be placed top-dead-center and fit into a recess in the bearing cover. Or it could be placed to one side and fit into a recess milled into the split line.

The bearing cover has to be positioned so that the bore is a true circle. This is usually accomplished with two or more tapered dowel pins. If the bearing split line is aligned with the housing split line, then those dowel pins are the only thing to position the upper housing to the lower housing. However, by rotating the split line of the bearing away from the split line of the housing, the bearing cage can also serve to help align the upper housing to the lower.

This arrangement also allows the anti-rotation pin or key to be placed into the bearing cage further away from the spit line of the bearing. One section of the bearing (without the pin or key) is placed into the lower housing. The other section of the bearing (with the pin or key), is placed on top of the other section. Then, both sections are rotated together to place the pin or key down into the milled slot at the split line. In this position, the two halves of the bearing are locked into position and are quite stable.

When the cover is brought down onto the bearing, it will tend to position itself to the bearing. There would still be dowel pins, but this arrangement is more certain to result in a perfectly circular bore for the bearing to ride within.

Actually, designs with the housing and bearing split line in the same line are problematic. We have had problems with dowel pins or dowel pin holes that did not correctly position the cover. Since the cover is going down blind, once it is down, you cannot confirm that it resulted in a circular bore for the bearing.

If they just wanted to go from a load on pad to a load between pads, they would most likely redesign the cage so that its mounting was unchanged. I would not expect an OEM to just rotate the cage 30 degrees to accomplish this change. Plus, it is more likely a five pad bearing. I don't recall ever seeing a six pad bearing in a large steam turbine.


Johnny Pellin