Molding your own engine mount isolators

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Flow

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Hi folks has anyone done this before?

For 50 to 55 shore durometer elastomer it looks like they need to be molded rather than machined.
Also heat is an issue when that soft. So I am thinking Silicon rather than Urethane?
Can anyone point to a recipe for a silicone based highly damp compound I could use?
Target Vibes are 80-100Hz engine is 100hp and ~70kg

Also should I use a higher durometer for the lower mounts?

The design is essentially rear mounted with 4 two piece doughnuts and through bolts.

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rv7charlie

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You can sometimes freeze soft stuff to machine it, but rolling your own engine mount cushions seems like carrying the homebuilding thing to extremes. Is it a supply chain thing?
edit: Have you looked into 'shackle bushings'? They're basically donut shaped. You might be able to get what you want for that mount design by using 3 bushings at each point (2 large & one small), instead of a pair of stepped bushings.
 

Flow

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Awesome sugestions!

What do you think of using say 55 shore for the bottom mounts and 50 for the top?

Does anyone know if it is possible to source the BTR or BTR II Silicon compound that Lord uses?
 
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Flow

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The next questions are:

How do you tweak the brew to increase the damping coefficient. With polyurethanes I understand you can use ethers or esters but not sure which. I see the Sorbothane compound uses a poly ether mix.
How do I calculate the target viscoelastic properties like Young's modulus, Tan delta etc.
And how do I calculate the target Shore A durometer?

Do I start with expected deformation and through bolt torques, installation weight and elastomer loading and then target the peak Tan delta for the temperature and target frequency?

Does anyone know of any tools to help with this?
 
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Basil

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The next questions are:

How do you tweak the brew to increase the damping coefficient. With polyurethanes I understand you can use ethers or esters but not sure which. I see the Sorbothane compound uses a poly ether mix.
How do I calculate the target viscoelastic properties like Young's modulus, Tan delta etc.
And how do I calculate the target Shore A durometer?

Do I start with expected deformation and through bolt torques, installation weight and elastomer loading and then target the peak Tan delta for the temperature and target frequency?

Does anyone know of any tools to help with this?
Don't know about tweeking the brew but casting the mounts thicker will make them softer and pre-compressing them with the through bolt will make them stiffer. The idea is to get the natural frequency of the engine on its mounts lower than the idle frequency of the running motor.
 

Flow

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Thanks Basil, by Natural Frequency do you mean the resonant frequency of the motor on its mounts?
Any thoughts on how to match peak Tan delta to temperature and target vibes. Are we working in the glass transition or in the viscoelastic region? This DMA engineering is a bit of a learning curve.

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Flow

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I am thinking of going with EMD135 polyether urethane which has better physical properties if slightly off on visco elastic properties when compared to EME165 which is a polyester. See TDS for both attached along with DMA test data for 25-60C up to100Hz

Do we think I could get away with the polyester physical properties for use in an engine mount or better to just stick with the polyether?
 

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Dana

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Do we think I could get away with the polyester physical properties for use in an engine mount or better to just stick with the polyether?
Look at the chemical compatability... I forget which is which, but one is better with fuels and oils. Of course mounts should never see fuel or oil but leaks happen.
 

Flow

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I think they are both better in that department than natural rubber which is in there at the moment.
I am liking the tan delta of the polyester EME165, have to say.
Please someone talk me out of it!
 

Toobuilder

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I machined my own mounts for the Rocket from a billet of polyurethane but found it too stiff. Research indicated that casting my own blend of poly was promising, but would clearly burn a bunch of time to get right. Plenty of info on the web concerning casting polyurethane with varying durometer values.
 

Flow

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Nice! Did you find any DMA data on the Xencast products? Nothing in the TDS. Do you know if they are an ester or ether based product. Lastly how have they held up over the 3 years?

BTW I know the Era Poly EME165 and EMD135 are both 4 part systems where you dial the ratios to achieve the required durometer.
 

trimtab

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I made replacement engine mounts for several Chevys pretty quickly with Smooth On products, cutting the molds out of UMHW on the CNC and using the original metal flanges in the molds.

It was very successful.

I used a simple force gauge and vertical caliper to measure the elastic modulus of some of the astoundingly expensive off the shelf replacements, molded a coupon to verify the smooth on version was going to be really close, assumed the non-newtonian character of the material would not be a significant problem, and went for it.

The flanges were recovered by cutting rubber away with a band saw and then using a torch to finish the job.

To do four isolators for a '56 sedan used up a full day of labor over about 3 evenings. The geometry was far tricker than simple aircraft engine mounts. I still have the molds around somewhere because the person I made them for pointed out they went for $250 a set, but I wasn't interested in going into the vintage car engine mount biz.
 
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