Schlumpf button fixings

Let's play fast and loose with units so it's easier rather than
distinguishing mass and force. I'm guessing you have no torque wrench.
I don't know what units you prefer but let's say that pounds, kilograms
and Newtons "weigh" something for simplicity. A Newton, which is a
force unit, is equivalent to about a quarter pound and about a tenth of
a kilogram. Let's use 11cm instead of 1.1m so the distance is
manageable and is about 4.5 inches or so. Picture putting a wrench on
this screw with a handle that is 11cm or 4.5 inches long. Put something
that is 2.2 pounds or a kilogram on the end of the handle. That's
applying 1.1-N-m of torque. Not a lot.

I don't have a Schlumph so I don't know what this screw looks like.
From the required torque I would guess in is an Allen head cap screw or
button head screw that is of order M2. A small screw, perhaps easily
stripped. Is there a reason that you cannot use Loctite or the British
equivalent if it is different? That's a way to keep the screw from
coming out, maybe even when you're bopping it with your heel. E-mail
Florian and ask him if it would be OK. You can also nick (score) the
last 2 threads toward the head so they bind when tightened but I would
tend to avoid that. Well, I would do it but I would recommend that you
avoid doing it because you have to answer to Herr Schlumph.

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harper said:

...A Newton, which is a force unit, is equivalent to about a quarter
pound...

My Physics 101 professor once suggested we go to McDonald's and order a
"Newton Burger".

That's my 2 cents.

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hell-on-wheel said:

My Physics 101 professor once suggested we go to McDonald's and order a
"Newton Burger".

That's my 2 cents.

That's a very bad joke, but I get a pretty good laugh out of things
like that 😄

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For those that don't know, the Schlumpf button fixing is an either M2 or
M3 grub screw. It screws into the middle of the button, and pushes
against the shifting rod. This pushes the button outwards, so that the
button is stuck onto the threads holding the button on and won't
unscrew. It comes with a special tool (or a pair of pliers does the
job) that lets you hold the button still whilst you do up the
grubscrew.

I'm not sure if loctite would help, as at the point the button loosens
off, it's not that the screw has come loose, it's that the button has
managed to come unscrewed slightly.

It is possible to overtighten it, as mine came like that, and it wasn't
possible to undo without drilling it out.

I tighten my buttons by using allen keys on both at the same time, so
I'm tightening them against each other. I find this better than using
the special tool to tighten them up, or using pliers to hold the button
still.

Joe

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This might be obvious, but just to check, you are holding the button
still when tightening the grub screw aren't you?

Joe

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well, harnessing all the raw power of Microsoft Paint, coupled with my
technical drawing abilities, I've come up with the following diagram
for those that don't know how the buttons are fixed;

22979

The last button I lost was secure for quite a while, but it was the
older plastic type, and it had been sitting in a warm room for quite a
while. Then, on Monday night I took it out on a bitterly cold ride, so
I guess that probably had something to do with it.

The buttons are held still while I tighten the grub screw, but I like
the idea of tightening it against the grub screw on the other side. I
think I'll treat myself to a torque wrench this weekend and give that a
go.

STM

+-------------------------------------------------------------------+
|Filename: Schlumpf button.JPG |
|Download: http://www.unicyclist.com/attachment/22979 |
+-------------------------------------------------------------------+

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Seems too obvious so I've probably completely missed the point, but
couldn't you loctite the button to the shaft it screws onto, rather
than the grub screw? Is there some reason why that would be bad, such
as the shaft being free-turning so you'd never be able to unscrew the
button if it were loctited on?

Rob

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rob.northcott said:

, such as the shaft being free-turning so you'd never be able to unscrew
the button if it were loctited on?

Yes it is free turning. If it gets stuck on because of the grub screw,
you can drill it out, but if the button itself is stuck on, you'd have
to unbuild the wheel and send the hub to Switzerland.

Joe

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I knew there'd be a catch. Still, I would think you'd get away with a
bit of low-strength threadlock, just to stop it falling off so quickly
if the grub screw came loose. Couldn't you hold the shaft from the
other side to unscrew the button?

On second thoughts, perhaps it's too far recessed to be able to hold it
from the other side.

Rob

EDIT: It would be better if there was a slot in each end of the shaft
so you could hold it still with a screwdriver from the other side, then
you could use threadlock on the buttons without so much worry of
getting them stuck. You could get the first button off by holding the
other button still with pliers or the special tool, then hold the shaft
still with a screwdriver to get the second button off.

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rob.northcott said:

I knew there'd be a catch. Still, I would think you'd get away with a
bit of low-strength threadlock, just to stop it falling off so quickly
if the grub screw came loose. Couldn't you hold the shaft from the
other side to unscrew the button?

On second thoughts, perhaps it's too far recessed to be able to hold it
from the other side.

Rob

The only way you would be able to hold it from the other side is if the
button and grub screw on the other side were tighter than the
loctitened button on the other side. But if I could get the buttons
and grub screws tighter than loctite, then I wouldn't have a problem
:-)

STM

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Spencer - my edit was too slow... what about if there was a slot in the
ends of the rod?

Rob

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rob.northcott said:

Spencer - my edit was too slow... what about if there was a slot in the
ends of the rod?

Rob

Is a teeny weeny rod, 2 or 3 mm diameter, I'd think it might be hard
not to strip the slot on something so small?

It's very very hard to hold the rod from the other side, I spent some
time trying to do so when I had a stuck on button.

Joe

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Grub screw is an interesting term. I haven't heard that UK version
before. In the US we call them set screws. From the excellent diagram
it appears that the function of the grub screw is to bind the inner
threads of the button against the outer threads of the shaft. Since the
grub screw has no head, the allen wrench required to screw it in is
very small so there will be a limit to the torque you can safely apply.
Putting Loctite in that mechanism looks risky now that I see how it
works. It would have to be applied to the shaft, not the grub screw. If
any got on the grub screw (I'm going to keep writing that...I like that
name) it would be difficult to remove because of the limited torque you
can apply with the tiny allen wrench.

In order to keep grub screws from working loose they are often stacked
if there is enough thread room. You put the first grub screw in to
apply a force to the shaft and bind the button threads onto the shaft
threads. Then insert the second grub screw to bind against the first to
keep it from working loose. In the diagram there doesn't appear to be
enough room for two grub screws. Also, the danger in this is forgetting
that there is one grub screw buried beneath the top grub screw and
doing something stupid when you try to take it apart. Believe me, I
know this from experience.

Another approach would be to use teflon pipe thread tape (a very, very
small amount) on the grub screw threads. This makes it act like a nylon
tipped screw so it is tight going in and tight coming out. It reduces
the possibility of the grub screw working free from vibration. It also
takes more torque to screw in and unscrew so that may be a problem.

Thanks for letting me write grub screw grub screw grub screw grub screw
grub screw over and over again. I'll be using that a lot from now on.

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Thought about getting a screw that's thread compatible but with a proper
big head on it that you can apply lots of torque to?

Alternatively, have you thought about putting tape over the top of the
hole so the grub screw can't fall out so easily?

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harper said:

Grub screw is an interesting term. I haven't heard that UK version
before. In the US we call them set screws.

Set screw also works in the UK, but c'mon, it looks like a little grub
doesn't it?

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kington99 said:

Set screw also works in the UK, but c'mon, it looks like a little grub
doesn't it?

I thought a set screw was what most people (including me) would call a
bolt - i.e. a thing with a head and a shaft, but where the thread goes
all the way up to the head. Apparently if the thread stops before the
head, so there's a smooth bit of shaft, it's a bolt - or something like
that. My engineer wife is always correcting me when I call things
bolts :o

A grub screw is probably a subset of set screws by that definition, I
suppose.

Rob

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STM

On the button you found, was the grub screw still in it?

It seems, from your drawing, that the pressure put on the button in
order to shift, counteracts the small amount of jam force offered by
the grub screw against the shaft.

My guess is that you are losing the right-side buttons, because the
force on the button "stops" it from moving forward, and thus unscrews
from the shaft. The left side upon pushing the button while moving
forward would actually tighten the force betwen the grub screw and
shaft.

Maybe the right side needs to be left-handed threads? Just my .02

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On the button I found, the grub screw was still in. I don't think that
losing the grub screw is the problem, as it only has to undo a fraction
of a turn before the pressure is taken off. Once this happens, the
grub screw and button have no forces acting on them to separate,
however, the button will then be free to unscrew from the shaft.

As for cutting a slot in the end of the shaft, well, as Joe pointed
out, it is very very small, and would be virtually impossible.

I have taken some photos of the various bits, so I'll upload them later
for everyone to look at. In the mean time, I'm going to try some PTFE
tape on the shaft so see if that works ok.

STM

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semach.the.monkey said:

In the mean time, I'm going to try some PTFE tape on the shaft so see if
that works ok.

Try to get the tape on the grub screw as well. Anything to help bind
these threads so they don't work loose.

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