Small thread advice - anti loosening

[YorkshireDave]

Hi

First off I have searched. The searching has made me fully realise (vs only suspecting) that this subject is not simple but that I want to learn, to understand. Thanks.

I have a small circular shaft (8mm dia) that I need to attach to another circular shaft (12mm dia).
Once connected the 8mm dia shaft will very often be rotated back and forth by a handle (max 100mm long) at its end which can be up to 150mm.

Quite obviously, the 'leverage' (apologies but don't know the correct terms) from a handle 100mm long on a shaft 150mm long is much more than on a shaft 30mm long. As the handle is used to open and close something on the end of the 12mm shaft human beings will try to 'make sure' it fully opens and closes. This would put great stress on the join of the two shafts eventually, at least in my mind, causing the joint to loosen or fail.

My current idea is to thread (5mm so it has a shoulder and sits square against the 12mm shaft) the joint and apply a lock nut and then put in a grub screw at 90 degrees to the thread through the 12mm shaft to completely lock the joint so it never works loose.

However, my instinct is that it will eventually fail. Does anyone have any advice please?

[jboggs]

1) Post a diagram for better understanding. For example, you did not say this, but I am assuming the two shafts are being joined end to end and are on the same axis. Not perpendicular to each other.
2) Do the shafts HAVE to be two different diameters? If not, you could have one continuous shaft with no joint.
3) Weld them.

There are other options depending on the details of your arrangement.

[YorkshireDave]

Shafts drwg 1.pdf

Drawing attached. Hopefully it now makes more sense. Apologies for the confusion, thought everyone had crystal balls these days :-)

The 8mm dia shaft is added to the 12mm shaft on a maint visit. As such it cannot be welded or even Loctited due to H&S issues. It really needs to have a robust mechanical solution.

The shafts can be brass (pref) or stainless as they could possibly contact drinkable water. The grub screw shown is nothing more than my original idea. I have already found that if its stainless it cannot be tightened sufficiently ;-(

[JAlberts]

If you are using two connecting shafts, it would seem that their connection point must be inside some type of enclosure and the 8 mm shaft and handle will only be connected periodically "for maintenance purposes"; so, it seems it would be preferable to have some type of simple slip on connection.for the connection to be a simple slip-on type of connection.
Can the 12 mm shaft have some type of larger diameter collar mounted on its connection end? If so the connection could be a slot and tang type of connection (slot in 12 mm shaft end and mating tang on the 8 mm shaft end with a collar extension mounted on the end of the 12 mm shaft to guide the 8 mm shaft.

[Hudson]

Can you machine a 10mm hex on the end of the 12mm shaft and let maintenance use a socket wrench and extension?

[YorkshireDave]

If you are using two connecting shafts, it would seem that their connection point must be inside some type of enclosure and the 8 mm shaft and handle will only be connected periodically "for maintenance purposes"; so, it seems it would be preferable to have some type of simple slip on connection.for the connection to be a simple slip-on type of connection.
Can the 12 mm shaft have some type of larger diameter collar mounted on its connection end? If so the connection could be a slot and tang type of connection (slot in 12 mm shaft end and mating tang on the 8 mm shaft end with a collar extension mounted on the end of the 12 mm shaft to guide the 8 mm shaft.

Thank you for the idea. For what YOU have described, it's a great possibility.

I fear however that I have inadvertently mislead you all so time to be a little clearer.

I am working on a potential product. The joining of these two shafts is the last piece of quite a large jigsaw.

The product will be in use every day with the 'handle' being turned many many times a day by users.

The maint I referred to is where the part containing the 12mm shaft is repaired/replaced/set back to working as new. The shaft with the 'handle' on is then reused after each refurb. A loose 'handle' is not an option from a user perspective so the join must feel, and be, very robust. Cost, as always, plays a part too.

As non-skilled labour may be carrying out the refurbishments, it is important the mechanism used is 'idiot proof'.

We seem to have explored many rabbit holes ourselves. Some are:

• tap out 12mm shaft. Thread 8mm shaft. Turn together tightly and lock with a grub screw (as image above shows)
• punch out hex in 12mm shaft. create matching hex on 8mm shaft and use grub screws to lock together
• drill 12mm shaft. turn down 8mm shaft. use pin through both to lock

Hope that helps people understand