Schlumpf button fixings

[semach.the.monkey]

Ok, I've had my Schlumpf for about 4 months now, and have lost 4 buttons* so far - which at £6 each, is turning in to quite a high running cost.

The last one I lost was one of the original plastic ones, which I think I may have over-tightened, but that was my last reserve one, so I've had to commute in 1:1 mode this week while I wait for the replacements to be shipped over :-(

Does anyone have any tips on how to attach them securely? The instructions say tighten the screw within the button very firmily (torque 1.1Nm) and then goes on to say Please check that you can't unscrew the buttons by hand. If so, you will loose them soon! Now, I'm not exactly sure what 1.1Nm translates to in terms of brute force, but I'm sure I've done at least that, and they certainly haven't been undoable by hand.

Any suggestions? Oh, and bare in mind that they will need unscrewing if/when I change my cranks over.

Or should I just put a 'please return to...' note on the inside of each one?

STM




* Ok, so one of them I managed to find by retracing my path, so I could reuse that

[harper]

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.

[hell-on-wheel]

Originally Posted by harper

...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.

[Borgschulze]

Originally Posted by hell-on-wheel

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

[joemarshall]

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

[joemarshall]

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

Joe

[semach.the.monkey]

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;



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

[rob.northcott]

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

[joemarshall]

Originally Posted by rob.northcott

, 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

[rob.northcott]

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.

[semach.the.monkey]

Originally Posted by rob.northcott

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

[rob.northcott]

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

Rob

[joemarshall]

Originally Posted by rob.northcott

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

[harper]

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.

[mark williamson]

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?