Knob interference

I'm designing a simple knob for a potentiometer with a 1/4 inch round
shaft (no slots or cuts at all). It will be injection molded out of
ABS.

Can anyone suggest how the hole should be sized in order for the knob
to stay on the shaft and not slip with normal use? The hole in the
knob has four small ribs (about 0.025in wide) that are the points of
contact with the shafts outer surface. Due to draft I currently have
about 0.010in of interference about 0.2 in before the shaft bottoms
against the stop. Is that enough?

Thanks,

-Martin

 

I don't have an exact answer. Having "crushable" features like ribs
should buy you a lot of latitude.

Otherwise, "Stay Steel Safe". (or, "you can Always Add plastic").
Start with your features on the smal side and the mold can be cut to
add plastic if you need it.

 

"Stay Steel Safe". (or, "you can Always Add plastic").

Very good point. I'll keep that in mind.

-M

 

By my similar use of such intereferences, I would say that .005" per
rib on that diameter would be more reliably and less likely to want to
shear the ribs.

As a rough gage of interference, A safe bet would start with 4% strain
on a .25" diameter that is .010" on diameter. Given that they are
ribs, they will not put a lot of stress on the hub. With some other
experience, you might go higher interference, but I don't know the
types of loads and potential for creep & loosening over time, etc.


Since the ribs take ALL the strain (practically speaking the hub will
take very little) the ribs are going to go beyond the yield point of
the plastic, so you wind up learning by experience on this part.

Bo

 

That's a sackable offence round here....

 

Get a grip!

But seriously ...

If the potentiometer shaft-end is sharp, it will shear off the grip-ribs
instead of crushing them. Another problem is that if the knob has a
pointer to indicate the setting, there is no correlation between the
shaft (pot.) position & knob. If the pot is frequently adjusted, there
is also a risk that the knob will loosen over time - at which point
someone usually grabs the nearest tube of cyanoacrylate!!

Tradiitonally, a grubscrew or flatted shaft with a 'D'-shaped hole in
the knob (with or without spring-clip) is the more reliable way to go.
Splitting the shaft-boss in the knob & fitting a wire clip is also a tried
& true way of making the knob a reliable push-fit ...

Rick.

 

Tradiitonally
<snip>

I'm having a few variants 3D printed in ABS in order to better
understand how to do this.

Thanks,

-M

 

I have actually just done a few 3D rapid prototypes with external
splines on about a .25" OD, though used in a slightly different way.

What I found out using STL and the 3D Systems Accura grades is that I
had to use the Accura 25 grade which is the most flexible they make to
try the spline use trials.

In the end I found that the small thin splines would easily fracture
and strip, as the particle to particle structure integrity of STL
parts is not anywhere what a molded part has, so I could not judge the
end function very well from the STL parts.

I had to mold actual parts to judge the final size and shape of the
splines for my particular use.

Bo