Mass damper for automotive exhaust- Lead shot??

[galinette]

Hello,

I'm working on a automotive exhaust application where there was a vibration issue. To solve it, a mass damper was added. It is simply welded on the exhaust pipe. The damper is made of a beam welded onto a short cylinder filled with lead shots and closed by 2 end caps.
This was the recommendation from our supplier. When I calculate the mass I can get with a simple bar, I can get the same mass or more.

Details: 0.185Kg tube and 0.650g lead shots => 0.835Kg. With the beam and caps, total weight is about 1Kg.

Tube is 57.2mm diameter, 64mm lenght and 2.11mm thickness.
A full bar with the same dimensions would give me ~1.280Kg vs 0.835Kg today.

I understood from some articles that lead shot is interesting to reduce noise. But my understanding is that it works when you fill the parts that create the noise. In this case, I don't think it is the case (we obsiouly cannot fill the exhaust pipe), unless the mass damper itself would generate noise.

My questions are:
- Is the lead shot really necessary in this case?

Thanks in advance for your help.
Laurent

 

[GregLocock]

Well, why not do the experiment, and get a surprise.

Sand is often used instead of lead shot. Don't fill the container right up.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[galinette]

What kind of surprise would you expect? Noise or no impact?

I will check if I can get the necessary mass with sand.

Thanks,
Laurent

 

[GregLocock]

You could also try steel shot.

The point I was making is that sometimes it is worth reading the literature or doing the experiment (or even asking the person who designed the part).

You think it is a mass damper - it may be. But granular fillings also provide a damping mechanism as the balls or particles rub against each other.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Chimera5]

Similar problem on the same application I have seen steel mass dampers welded directly onto the tubes and more recently smaller dampers incorporated into the hanger bar design.
Cannot think of a logical reason to use lead shot or any shot for that matter. For the shot to have any effect above what a solid mass provides it must be allowed to move in the container, thus providing a noise source. Swapping one problem for another.
Regards
Steve

 

[galinette]

Hello,

I've checked if I can replace lead shot with sand: It is not possible since I cannot obtain the same mass with the available space.

Also I've been reading some more and it might be possible that the lead shot has similar effect to what exist on mass dampers used on building filled with water. Imagine shaking a container half full of water. It might absorb some energy (damping) as Greg said (fritcion, movements inside...). The cylinder is not filled completely. When shaked, it is effectively noisy.
But I'm still not sure it wouldn't work without lead shot. So even if it has an effect, it might be over-engineered.

As Greg said, the easiest way is certainly to try. Bad suprises should be fairly easy to detect. I'll double check with the supplier. But it is not really his interest to remove the lead shots. That's why I wanted a second opinion.

Thanks for the tips Steve on possible new options. I'll keep in mind the damper incorporated into hanger rods.

Laurent