How Do I Add Mass to a High Pressure Die Cast Aluminium Bracket?

In order to better evaluate the problem, will it be a one of a kind device, or the mounts are gonna be produced in large scale?

This would be a production type solution with four different brackets and four different added masses.

Presumably the walls of the bracket are flexing? (as it's fixed at base and point)

Adding mass is an awful idea* and 4 different weights is 4 times as bad. Solve the problem not the symptoms.
Put a ridge up each flat side of the bracket, or change the shape to stop whatever is resonating. Have you looked at it under strobe lighting to see what the actual problem is?
Solving problems is easy, the hard bit is identifying the problem.

Maybe a picture or sketch would help, but I'm having trouble picturing a pyramid which resonates, as it's inherently a pretty rigid structure.

You say 'attached' but don't say how....more info would be good

*It's adding cost, complexity, mass and creating more stuff to go wrong...did Bill Gates give you this idea?

If you must fill it with something how about epoxy or rubber? Or make it thicker or from a different material in the first place.

Del (apologies if I've somehow got the wrong end of the stick)

Sorry if I have confused you, but the problem is that the brackets are stiffer than target (1000+ Hz Vs target of 600 Hz).

If more material is removed to get the brackets to "flex" more, the stress levels exceed target and different material; eg nodular iron; does little if anything to lower the resonant frequency.

Adding mass as far as possible from the block appears to be the only way to reduce the freqency. Because of the different shapes/lengths of the brackets means different masses are required to tune to 600 Hz

I still don't actually understand the problem.

Why is 1khz a problem and 600hz isn't? (both are in the audio band)
What is the driving force which is going to cause this resonance?
If it's an engine mount isn't one end rubber mounted? Or is that what a 'hydro mount' is?

Surely if you go for stiffer you could push the resonance up out of the audio band or remove it all together?
Why is 600 hz your 'target' ?

I'd still suggest filling with epoxy or rubber to damp the resonance as a cheaper, simpler.

I am still a confused Cat

Del

You'll have to take my word for it, 600 Hz is the target for reasons to complicated to explain here.

Driving force is engine firing and the hydro mount is the "rubber" mount between engine and body. Epoxy is not dense enough.

Can't the die be changed to cast 'em thicker?

I still think the solution is to change the part, not to add material after it's been cast, but you'll have to take my word for it, for reasons to complicated to explain here.

Del (I'm out)

Run a couple self tapping screws in from the sides then fill the cavity with molten lead.

Why not make the mounts in the shape of an inverted "L"? The wall is the same thinkness, therefore the same stiffness, and the added mass at the top will lower the resonant frequency.

Add Dense Concrete- into the Pyramid shape.Of course your die cast AL should have small anchor/s for holding the Concrete. Cheapest.

While casting-use Super-Plastificator 0.6% and 0.3 water/Cement ratio.

mm

Oh!
I think I was grasping the wrong end of the stick.

It's the resonant frequency of the entire assembly not the bracket?

Del

I resolved a similar problem by mixing lead shot with catalyzed epoxy and mixing everything at once. The material was then added to the body of the part which had been prepared to obtain a good mechanical bond. I purposefully added more mass than we needed.

After being tested for the new natural frequency, we were able to fixture the part in a milling machine and remove material by making uniform cuts into the cast dampening material.

We tested and milled until we got to a natural frequency and harmonics that were well beyond any vibrations likely when operated.

Wasn't at all Hi Tech but it worked!

L. J.

Thanks for the comments.

L.J., I also considered your proposal but could not use lead due to OH&S considerations. Then looked at steel shot used for shot blasting (very cheap) but the density drops to about 3 as shot which meant the volume to achieve the required mass becomes to much.

Bronze filled epoxy is commercially available.

Devcon Corp offers it.

A urethane might offer some damping, also.

Then you should consider Bismuth if lead is not allowed. or Tungsten.