Help ID compounds on Strain Gauge in hub motor? - photo

Thanks for reading. Any suggestions are appreciated. Sorry the text is a little hard to read.



I'm replacing the strain gauge on my 8 year old rear hub bionx motor and can don't know what two of the compounds are. The strain gauge is attached to the axle beneath all of these glues / compounds.

Mystery 1. the bright white putty-like material shown covering the strain gauge and beneath the clear silicone

this material does not dry like a glue would, it is still tacky after 8 years

it doesn't flow or feel slippery like a grease either, it is a little sticky like a putty

if I touch it, it will retain my fingerprint on it's surface

Mystery 2. the black foam-like material

it has the feeling of foam rubber, compressible with medium force from a finger

it needs to seal a gap which would otherwise allow water into the motor housing - should expand slightly or least not shrink when it dries

I tried Permatex "Black Rubber Sealant" but it shrinks alot and becomes hard when it dries

black shoe goo also shrinks and feels more like a hard silicone when dry

"Liquid Electrical Tape" also becomes stiffer and is not compressible when dry

Any ideas out there?

silentflight said:

Mystery 1. the bright white putty-like material shown covering the strain gauge and beneath the clear silicone

this material does not dry like a glue would, it is still tacky after 8 years

it doesn't flow or feel slippery like a grease either, it is a little sticky like a putty

if I touch it, it will retain my fingerprint on it's surface

Click to expand...

Possibly Siloxane? (like silly putty) Sorry I dont' recall the exact name, but there's another thread talking about stuff like that right now.

Thanks amberwolf, I'll look for it.

neutral cure silicone RTV is often used for covering strain gauges but your stuff sounds like something a little different, like AW says.

Thanks, Punx0r. It still has me baffled, the clear outer layer is silicone RTV (at least I am fairly certain it is) but the underlying tacky putty is new to me. I'm considering using a dielectric grease in it's place.

That would probably work. The normal reason for gooping RTV onto gauges is to provide strain relief for the lead wires and seal the gauge from moisture. The Bionix system seems to be doing that with the epoxy and the clear silicone, so what function the white stuff is fulfilling I'm not sure. Someone clearly put some thought into it though.

I've seen the tacky white mystery compound in blood pressure transducers. I think it is there to prevent forces from being transmitted to the strain gauge from the overlying glue. Something like silly putty would probably work like that.

fechter said:

I've seen the tacky white mystery compound in blood pressure transducers. I think it is there to prevent forces from being transmitted to the strain gauge from the overlying glue. Something like silly putty would probably work like that.

Click to expand...

Thank you, fechter.

I also believe the purpose of the white compound is to provide freedom for the strain gauge to lengthen with the flexing of the axle without the overlying glue trying to prevent it from expanding. Over time, the forces from the glue might contribute to failure of the glue holding the strain gauge to the steel axle.

I must be over-thinking this at this point, I worry about whether the compound might carry micro-currents and affect the fidelity of the strain gauge whose resistance only varies a tiny amount when it is functioning normally.

I am tempted to use Silly Putty instead of dielectric grease.

Both silly putty and dielectric grease are non conductive. And the strain gauge is a fairly low resistance resistor, so it would take something really conductive to mess it up. The most important thing is to get a good bond between the strain gauge and the metal. What goes over it shouldn't effect it that much as long as it's elastic. I'd think you could just use silicone glue and call it a day.

Here is a place that sells that kind of stuff.

https://www.omega.co.uk/pptst/STRAIN_GAGE_ADHESIVES.html