Thread wear and failure 1

[Bobmat]

Hi, i'm really a computer guy but need some advice for a leisure time project, I am making a part for a new kind of adjustable headstock for a bicycle. The handlebars will be able to be turned parallel to the frame making it easier to store and transport with just a few rotations of a nut which slackens the arrangement and allows the rider to rotate the handlebars 90 degrees. The operation requires that a bolt will be screwed handtight in and out of a thread that will be cut internally within a tube/pipe. Over its lifetime it will be screwed in and out 1000's of times (although never actually leaving the thread entirely) I have been told that i could have problems with the amount of usage in relation to the thread. Any thoughts or comments would be much appreciated

 

[CorBlimeyLimey]

It might be easier to use the quick release wheel nuts to remove the front wheel, which would allow the handlebars to be rotated.

 

[electricpete]

Good point! Every bike I've had in the last 20 years had that quick release feature for the front wheel.

Steering, breaks, wheels... all kind of important for controlling the bike, which is kind of important for personal safety. Personally, I would stay away from a home-made modification to these parts of the bike.

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[SomptingGuy]

I can see the issue here. Removing the front wheel generally involves fiddling around with the front brake to provide clearance for the tyre. Then you have a bike that drags its chain on the ground. So you get your hands and floor dirty. Putting the wheel back in is never entirely painless either - a few minutes of jiggling and getting your hands dirty again. I store my bike in my hallway and just live with the handlebars or front wheel being in the way.

I doubt you could build a new kind of headstock arrangement that can be tightened enough by hand ... unless it was positively located (lockable into two or three distinct positions).

I don't see how fiddling with the headstock could compromise the front wheel or brake though??

- Steve

 

[Bobmat]

Hi to you all. CorBlimeyLimey and electricpete - SomptingGuy has, for me, answered the question about quick release front wheel problems. On my idea the nut that tightens the arrangement down is not also the bolt that tightens up the headstock, so i am pretty certain hand tight will do - the headstock is tightened seperately. The only forces the hand tight nut would have to deal with is the initial hand tightening, which locks the arrangement together followed by the upward pull and the downward push of the rider as the bike is rode. I have been told that constant doing/undoing of the nut, and the push/pull of the rider is detrimental to a thread. I wish i could explain better but if this works i am going to look at getting it patented. Are there different types of thread, if so which type is the most durable?

 

[SomptingGuy]

Does it need to be a thread/nut that's used? Why not emulate the quick release mechanism that wheels use? They are intended for repeated tightening/loosening.

- Steve

 

[Bobmat]

I'm pretty certain that it would have to be a flat-topped nut, whatever is used it would have to be on the top of the headstock and i don't think that anything protruding from the headstock (other than something already flat, like a headstock) would pass safety standards. My idea would mean that cyclists would be able to undo a nut and turn the handlebars 90 degrees, use folding pedals and turn them up. The bike would then go from over 2ft wide to just 6 inches wide - by the sounds of it you might be my first customer if it takes off

 

[vims]

I would not worry that much about doing / undoing the nut. I would be concerned about the pull and push from the rider though. With only a hand tightened nut you run a big risk of it undoing itself. Why not look at the previosly discussed quick release feature and copy that?

 

[Bobmat]

vims - The quick release system would be a lot better if i could incorporate it but, as discussed previously, i don't think i can. The push/pull effect would always be encountered when the thread and nut were under tension from the hand-tightening. Would the fact that they would be under tension make the problem better or worse?

 

[Bobmat]

Sorry vim, the nut undoing problem has been dealt with. The push/pull effect and the damage to the thread is what i am refering to.

 

[vims]

Ok, so the undoing is no issue. Under tension is better, but when you only tighten it by hand you dont get that much preload, so I dont know if it makes any difference.
I think what i would do is to calculate what kind of load your first thread will see. In a nut it is said that the first thread takes about 1/3 of the total force. What kind of force would a biker be able to create on the handle bar, 2000N in push?

 

[vims]

Thinking about it you could look at a linear actuator.

http://en.wikipedia.org/wiki/Linear_actuator

They have to withstand alot of abuse and many cycles in their lifetime. I´m sure the threads in a an actuator gets worn, but it is after alot of cycles.

 

[Bobmat]

vims - a linear actuator is something i have never seen before, therefore never considered. Having seen how the theory works i will think about whether or not it is more applicable than the system i have for what i am doing, thanks a lot! In the meantime i would still like to find out more about potential wear and failure in my current system, just in case i decide to keep it. People have spoken to me referring to things such as acme threads, helicoils, wallowing, V threads and the like. I'm not an engineer so i'm still in the dark about wear, failure, best solution, strongest threads etc.

 

[vims]

What I ment was for you to check what kind of thread an actuator uses. The same type of thread should work for you. I checked around a bit and most actuators seem to use acme threads. Still you have to get some idea of what kind of forces your thread will have to withstand. Have you thought about useing some kind of bushing instead of makeing the threads internally in a tube?

 

[Bobmat]

Aah i see. I am going to see about having a propotype made, probably next week and i want to specify as much detail as i can beforehand. If linear actuators get lots of use and abuse, and use an acme thread then i will have the prototype made with this thread type. With regard to the forces involved - I don't know, I presume these will be ascertained when the prototype is made and tested. Sorry i'm not an engineer, what do you mean by 'bushing'? thanks

 

[vims]

Sorry, I think bushing is the wrong word in english. Adapter could be more correct I think? My thought was that if you are concerned abot wear it could be good to have the thread in an adapter, that could be easier replaced. If you have the treads in a tube that is a part of the whole construction it will be more expensive to replace. Just a thought...

 

[Bobmat]

Sorry - busy day yesreday. I see your point, and its a good one. When the prototype is made i will talk to the makers about having the thread in a sleeve/adaptor and definitely suggest an acme thread. You've been very helpful, thanks. I will post back after the prototype has been made.