Does this make anyone else uncomfortable? Motor mount question

[rapandi]

14 minutes ago, jimk said:

This mount is a U channel in section.  No, the load is not torsional, it is a bending moment and a plate added across the back adds greatly to the bending strength of the channel.  The load is cantilevered off the bottom of the mount and it springs ever so much back and forth.  These mounts undergo fatigue cracking from being loaded and unloaded as the engine rocks and vibrates.  A plate buried in the channel does little to add strength and does not take the stress off the channel flanges.

Talk to a structural engineer or pull out you statics of materials book.

I am a structural engineer myself and you are right that welding a plate to the web of the channel is not as effective. It is a bending moment. Which is basically a turning effect the force has on the mount. The engine rocks that mount back and forth causing it to crack. 

 

Having a plate welded to the flange is a better idea because by doing so you change the geometry of the mounting tab from being a C section to a box section. The box will have more rigidity to bending. Which is why I did mine as shown above. 

 

Another reason for doing the box section is because it allows you to place the nut on the rubber mount such that you will get at least 2 threads come sticking out of the nut. By welding a plate to the web your nut will only partially sit on the threads as seen in nicks picture above. Here in Aus in structural situations the rule is to have three threads at least come past the nut.

 

Having said all the above, those who have welded a plate to the web using the prefabbed IE plate will not have any issues. As long as the weld bead is partially connecting the flange section. He he he .... all good. My opinion. 

[M3M3]

brought mine all the way down and welded at bottom too.

[abrokenframe]

Well looks like I'm reinforcing my engine mount! Funny thing is that totally overshadows my original question (I'll just use the mount I have). That's why I love forums... ask a question and end up down a whole different road. I did consider beefing up the engine mount prior to powder coating, but just kind of let it go not realizing how often they actually crack. Too bad I'll have to mess up the finish... oh well.

 

This should be a fun weekend though since I'll try to fabricate one myself. The IE one looks nice, but after all of your input I think I'll weld to the flange and box it.

 

Thanks for the help everyone!

 

-Ryan

[rapandi]

When I made my weld bracket I started with the idea of replicating the IE plate. However I eventually changed to the box type bracket instead. Nevertheless I did take pics of the IE replicating plate process. It is the same if you wish to make a wider plate to weld to the flanges. Just wanted to share cause may help you.

 

 

Me trying to bend this plate to fit the curves

 

 

This is where I realised that the plate is going hinder my nut from being fully wound on the threaded stud. At this point I changed to the box design.

 

The box design below

 

[jimk]

Sometimes a fault is caused from the system resonant frequency falling within the operating range, which may be the case here.  The mount strength based on known operating loads may be sufficient, but when passing through the resonant frequency a small load can cause a lot of motion (same as a tire out of balance bouncing at certain speeds an not at other speeds).  I doubt that BMW did such a complicated and computer intensive analysis in the 60s.

 

If the resonant frequency is moved out of the operating range, the problem goes away.  Adding the IE design plate or the boxing plate stiffens the mount and will raise the resonant frequency.  And so the mount may not exhibit cracking with either method.

 

I was on a project with a large industrial fan mounted on the 4th level of a power house structure.  When on high speed, he structure would tremble.   I had the structural engineer conduct an analysis and he was concerned the whole thing would fall down.  We solved the problem in the same manner by stiffening the structure in certain places to change the resonant frequency out of the fan speed range.

[NYNick]

Makes me feel better. Thanks Jim.

[abrokenframe]

Had a chance to make this today. 1/8" thick steel. Think this will get the job done?

 

-Ryan

 

Edited March 5, 2017 by abrokenframe

[jgerock]

Would you feel really uncomfortable if your subframe was like this one?

[M3M3]

1 hour ago, abrokenframe said:

Had a chance to make this today. 1/8" thick steel. Think this will get the job done?

 

-Ryan

 

perfect.

[abrokenframe]

Ok here's the finished product if anyone is interested 

 

and one of the engine mounted just for fun ?

[xferboy]

7 hours ago, abrokenframe said:

and one of the engine mounted just for fun ?

 

 

This falls in the shiny/pretty category!