Re: trantorque gt for rsg motor attachment and maybe electrodes?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FIFTYGUY@xxxxxxx 

In a message dated 4/4/06 1:16:28 AM Eastern Daylight Time, Dr. Cox writes:

>This sounds like unsafe engineering practice.
    I would argue this. Let's do some actual engineering before 
condemning anything:

    Refer to 
<http://fennerdrives.com/keyless_bushings/ttqtech.pdf>http://fennerdrives.com/keyless_bushings/ttqtech.pdf

    Let's say our RSG uses a NEMA 56 framed motor. Shaft diameter is 
5/8". Let's say we're using a 1 HP motor, and use that figure for the 
caluclations, although that's a little on the large side for the 
average RSG and we know the HP drops dramatically with the 
salient-pole modification. We'll also say typical 1800 RPM motor.

    Running torque = 1 * 5750/1800 = 3.2 ft-lbs or 38 in-lbs. Let's 
worst-case the starting torque for our synch motor at an extremely 
conservative four times that, at 153 in-lbs. Let's get ultra-paranoid 
and call it a "heavy shock load" (as the manufacturer would call 
"punch presses", "hammer mills", "crushers", "pulverizers", or 
"piston compressors" driven by a "multi-cylinder engine"). The 
service factor then doubles the torque requirements, in our case to 
306 inch-lbs.

    Now looking at the specs for the Trantorque "Mini" series, the 
smallest construction they offer, the 5/8" shaft size is rated for 
450 in-lbs, well over our extremely worst case. The equivalent 
"standard" bushing is rated for 1750 in-lbs. Even the "Narrow" 
series, for thin rotors (3/8" in "Mini" and 1/2" in "Standard") are 
still within the above requirements.
    They also make "Non-Transversing" models, which prevent axial 
movement while being tightened. This would be great for maintaining 
electrode spacing when removing/installing the rotor. And even these 
are within our requirements.

    Minimum hub diameter for above applications (to resist rotor 
breakage from the Trantorque expansion on installation) is also well 
within the bounds of good taste for any reasonable RSG rotor - 
nobody's running a 3" diameter rotor off a 1 HP motor are they?

    In your favor, however, the application data for the Trantorque says:

    "Synthetic Mounted Components

    Trantorque GT units are not recommended for use with any 
component completely constructed of a synthetic material. Most of 
these types of materials have a certain amount of creep under load 
which will cause loosening over time. A Trantorque GT unit can be 
used if the bore of the synthetic component incorporates a 
reinforcing metal sleeve."

    G-10, "red fiberglass", etc. probably have very low creepage. 
After all, they're widely used in critical structural applications 
for commercial electrical equipment (and switchgear). Pressing in a 
metal bushing would be an easy way to comply with the manufacturer's 
recommendations. Not to mention, if creepage is an issue, the 
"throughbolts and hub" method would suffer as well. So would the 
typical "radial setscrew" method for the flying electrodes.
    And my original caveat still stands - lock everything down with 
retaining compound.

>It's probably best to use a 3 inch dia. alum. hub with 5-6 equally
>spaced fasteners that extend all the way thru the G-10 and then
>thread into an aluminum "backblock".

    One could argue it would be safer engineering practice to 
accomplish the same task with fewer parts, thus reducing the 
possibilities of failure. In this case the multiple fastening bolts 
wouldn't necessarily be redundant, as the loosening or failure of 
only one would cause catastrophic failure of the entire system 
(off-balance rotor would wobble and crash electrodes).

-Phil LaBudde