Preferable method to set in wear rings 1

[vesselguy]

Hello,
Wear rings at shaft and impeller can be set in place by either welding, by axial set screw or radial set screw. Which methods are most preferable and why? the inquiring mind wants to know. thanks.

 

[Artisi]

No prefered method as it depends on design of the components you are trying to lock, although welding seems like a bit of over-kill.

 

[vesselguy]

I expect the "welding" is just small fillet of sort. Is that correct?

Would radial screw be acceptable? Wear surface will wear down and any exposed radial screw head would score the shaft.

 

[Artisi]

It would help if we knew what you were actually talking about, can you attach photo or sketch.

 

[bingopin]

While I have used all of the mentioned methods, radial setscrews are my preference. A small weld bead at three places around the face of the ring is fast and easy but many times the ring metallurgy and ring material hardening can lead to poor quality or cracked welds. I've also seen rings plug welded radially but again you have to deal with the metallurgical issues. Axial setscrews are seen many times from the manufacturer but are my least favorite.
Of course all of the methods are compromised if you start with a poor fit of the ring to the impeller.

 

[vesselguy]

Bingopin,

Thank you for your response. I thought radial screwed wear ring would not be the best choice due to concern over local scoring when the screw head are exposed when the wear surface wears down. Can you elaborate on the difference of both radial and axial screwed type? I'm new on this. Look forward to reading your response.

 

[JJPellin]

Our standards call for welding of any rings that are considered weldable. We do not weld brass or bronze or cast iron. If welding is not used, then axial set-screws are preferred. Many of our pumps use radial set-screw as original design from the manufacturer. I would not be concerned about exposed screw heads contacting the mating parts. A wear ring or bushing should never wear that much unless the pump completely wrecks. And if the surface is "worn" down by corrosion or erosion, the head of the set-screw should not contact as long as the bearings are still working. Any set screw needs to be less prone to corrosion attack than the ring. So, beware of carbon steel set-screws used in 400 series stainless steel rings. The set-screw corrodes away and the ring comes loose. This is the main reason that we use three small tack welds on the axial face. Any set-screw creates a discontinuity on the surface that can cause accelerated corrosion-erosion. A properly done tack weld in a small chamfer creates less of a discontinuity. In the end, any of these methods will work. As noted above, proper fit of the ring to the impeller is critical in all cases.

Johnny Pellin

 

[vesselguy]

Thanks mr. Pellin for your reply with good explanation. Based on what you've said, why is it that I've seen project standards in which radial set screws are not permitted for attaching wear rings? I curious as to the base reason for why not.

 

[JJPellin]

Rotating Machinery Engineers are a strange lot. If they were ever once burned by a radial set-screw coming loose, rubbing, corroding away or as an initiation site for a crack, erosion or corrosion, then they would tend to forbid them in the specifications from then on. I have never had a bad experience with a radial set-screw and so they are still allowed in my specifications. But, I have seen set-screws fail from corrosion, so I still have a preference for tack welds. Another engineer may have had a bad experience with a tack weld cracking and thus prefers (or demands) set-screws.

Johnny Pellin