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

Reply to
m
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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.

Reply to
That70sTick

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

-M

Reply to
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

Reply to
Bo

Thanks for sharing, no doubt this will come in handy at some stage in the future.

John Layne

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Reply to
John Layne

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

Reply to
John H

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.

Reply to
R.H. (Rick) Mason

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

Thanks,

-M

Reply to
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

Reply to
Bo

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