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bottom view what is the name of this nut, I need to find a way to loosen it .

From the top

Side view

Charm_quark
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2 Answers2

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I find using the correct “c” spanner, in fact I would use two together to undo that as it means no hammering to possibly damage bearings etc.

This method of using two nuts is used to set a limit or stop position - if only one nut was used it could easily move. A "nyloc" nut may also move but the two nuts locked together do not move. Some nuts can have a set-screw inserted to prevent movement but that is less secure than the two nuts...

Called a c spanner because it looks like a c but has a tooth at the end to engage in the slot on the nut.

two examples:

Solar Mike
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    Thank you, I have honestly never seen anything like that before, The nut is for a machine from china. So it just baffled me. – Charm_quark Sep 29 '19 at 18:45
  • Also called a hook spanner – Chris H Sep 30 '19 at 09:58
  • @Charm_quark Depending on the kind of machinery, they are (or used to be) somewhat common. Not just in China, so don't be surprised if you see them again. – Mast Sep 30 '19 at 14:13
  • @mast : Is there any advantage over the standard nut? – Charm_quark Oct 02 '19 at 05:37
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    @Charm_quark see edit, but it provides a stop position which cannot move unless left loose. – Solar Mike Oct 02 '19 at 05:42
  • @SolarMike, thank you. I see but can this not be achieved with two normal nuts ? – Charm_quark Oct 02 '19 at 05:47
  • @Charm_quark "normal" nuts have a hexagon shape - these nuts are basically circular, this might be important for the lever or stop that it works against. Have you looked to see what it contacts? – Solar Mike Oct 02 '19 at 05:53
  • @SolarMike; there Green/Blue "plate/brace' on top of the c nut, and hexagonal nut on top of the plate to stop the 'Plat/brace' from moving upwards. – Charm_quark Oct 02 '19 at 06:00
  • @Charm_quark does your picture show the top of the plate? – Solar Mike Oct 02 '19 at 06:01
  • @Charm_quark dies not change how it should be undone, read my answer. – Solar Mike Oct 02 '19 at 06:32
  • @SolarMike you are absolutely right, I can seem to find the tool to undo the nut, and was wondering what would happen in case I damage it by opening. Thank you for you time :) – Charm_quark Oct 02 '19 at 06:44
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It's a spanner nut. There's a special wrench to loosen and tighten it but putting a slotted screwdriver in the groove and tapping it with a hammer usually works. it's a double nut so loosen one first, then the other.

JACK
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    Thank you. , I thought it was a more complicated but .. not sure why I thought that. – Charm_quark Sep 28 '19 at 21:26
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    You can a find spanner wrenches of all sizes in a bicycle shop in the tool area. A pair of vise grips may work also but you may to use something to hold the top one while you loosen the bottom on. – Alaska Man Sep 28 '19 at 21:50
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    That's got to be the US term for it. In the UK all nuts are 'spanner nuts', because a spanner is what we call a wrench ;-) – Tetsujin Sep 29 '19 at 13:25
  • @Tetsujin In the UK, spanners are called spanners, and adjustable ones are called adjustable spanners... Spanner may be also used to refer to someone but that is not polite... Wrench is more American... And given I spent my apprenticeship in the UK “on the tools” I know a spanner when I see one. – Solar Mike Sep 29 '19 at 18:07
  • Is it worth adding that there are two nuts here, which have been tightened against each other, and will be binding to stop motion in either direction on the Acme thread ? – Criggie Sep 30 '19 at 01:09
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    @SolarMike I do know one US supplier that calls a tube spanner a "spanner wrench". It always seems like whoever named it is a bit of a spanner spanner – Chris H Sep 30 '19 at 09:59
  • In the UK, my impression is that a spanner _engages_ with the nut or whatever it's turning (having matching flat surfaces or slots or whatever), while a wrench _grips_ it by pure friction.  But I'm no expert; is there any truth to that? – gidds Oct 01 '19 at 08:24
  • @gidds - A spanner is the British name for what the US calls a wrench. It really is that simple. No need to look any deeper. The lines may have blurred in recent years as the world gets smaller, but that's the original distinction. Like bonnet/boot trunk/hood… tramp/bum/hobo [don't get me started explaining *that* one ;) Brits generally understand both terminologies, being exposed to it a lot on TV/Movies. – Tetsujin Oct 01 '19 at 13:37