Servo Arm Movement

[DBRV10]

Having just received my first SV42 I have checked the attachment dic / arm to see if there is movement.

There is, so reading the instructions it suggests the shear pin screw is loose and how to reinstall it properly with loctite 271.

I believe the screw is firmly in place, but the movement I can feel is actually the difference in diameter from the shearpin screw and the hole in the arm.

There is way too much movement there in my opinion, but not sure how serious that really is.

I am tempted to apply a small amount to the hole in the arm just to lock it up firm, without getting any on the faces and therefore making the shearing function inoperative.

What do you think Dynon?

Cheers
David  

 

[dynonsupport]

The replacement and re-threadlocking instructions are actually designed to remove exactly the play that you are describing, and the only approved procedure is the the full one.

 

[DBRV10]

So maybe as a dumb Mechanical engineer........I missed something, but how on earth does the thread locking on a perfectly locked screw have any effect on the tolerances of the hole/screw head?

Unless it pulls the screw head over a fraction and that tightens it up due to eccentricity.

Cheers

DB

 

[DBRV10]

Any thoughts?

 

[dynonsupport]

So the key assumption that isn't correct there is that the shear screw is "perfectly locked" without the threadlocker. It in fact isn't, by design.

In contrast to a conventional screw, our shear screw / hole is designed so that the actual screwing action of the screw imparts no loading on the screw's threads or head. If the screw's threads and head are used to cinch the screw, you end up with a very unrepeatable and variable breakaway force, yielding a screw that isn't a very good calibrated shearing device. So, the threadlocker essentially glues and immobilizes the screw into the hole (and acts as a bit of a gapfiller at the same time). Note that this is why we're very precise about the torque that should be used on the screw when setting it. If it's over-tightened, you basically pre-load the screw, yielding a lower shear limit. Note that any mistake you make here will yield a weaker screw that shears before you want it to. This, in the grand scheme of things, is more desirable than having one that is too strong (which can be dangerous).

 

[dynonsupport]

If you're talking about the play between the head of the screw and the hole in the arm - there should be just a touch there, but it should be very extremely minimal. Certainly less than the gear play.

 

[DBRV10]

Goodmorning DS

Yes I would agree entirely with the thread locking and not over torqueing the screw. That is all fine.

Your second post is starting to address what I have observed. There may be an insignificant amount of movement however you Service bulletin states.....
"If there is any movement at all between these two
parts, your shear screw is loose and should be repaired per the following instructions.
However, slight movement of the ENTIRE arm/capstan/disc/shaft relative to the
internal gearing is expected."

And there is movement around the head of the screw, albeit not as much as the gearing backlash but it all adds up to toatl system backlash and I wonder if on some peoples installations if that combined with unknown amounts of slop in their linkages could be the cause of some folks oscillating!

Its more that it all adds up, and maybe its worth looking at also.

Cheers

DB

 

[dynonsupport]

We may need to revise the wording there. Its really the ability of the shear screw to move in its threaded hole that we want to eliminate. This effect is orders of magnitude worse than the normal manufacturing tolerance between the shear screw head and the hole bored in the arm to capture it.

 

[DBRV10]

Gotcha!

I did not think you would get that much movement in the screw!

Mind you it all adds up!

Cheers!