Torsion Spring

A small, one turn torsion spring in a door latch/lock I have, has broken. The lock supplier can't provide a replacement spring [but will sell me a replacement latch for £100!]. I cant find a matching, off-the-shelf spring anywhere

I thought therefore, I would make one but have a few questions. The wire diameter is 0.95mm. The spring has one 7mm [int dim] turn with 20mm arms. I assume I need spring steel but have been offered piano wire. Are they the same? I assume it will be better to heat and form rather than cold form? Will I need to temper/harden the spring before use and if so how? Stainless steel may better as the environment the lock is in is damp.

Thankyou

Ian R

I have boxes of compression springs from "Lewis Springs"

One is marked music wire the other stainless steel wire.

Music wire is known as piano wire Ian and has better strength properties than the stainless wire but in your case I would plump for the stainless.

Len. P.

Hi Ian,

In damp locations go with stainless for sure.

You might find the link below to Lee Springs UK helpful.

**LINK**

JD

I suppose what is worrying me is whether 1mm stainless will have a sustained springiness? The door/lock is used fairly heavily.

Piano wire is pretty good at rust resisting, most good quality modern stuff is coated too which helps. Locks are semi-protected so a quick squirt of paint and wipe over with a super sticky grease (I use red rubber grease 'cos I have a large tin and use very little for its proper purpose) should be sufficient protection. As for durability just make one or two more for spares. Bound to have to buy enough material for a couple or three and its well known that having one on the shelf ready go is an excellent specific against Gremlins. I guess its no fun for them if you can just fixit right now! Just remember to lubricate the screws and replace any with damaged heads so you can get in next time.

Clive

If you use 302 or 316 spring temper stainless wire (or get a spring from Lee) it is very unlikely you will have trouble with spring life /retained temper.

My advice is to form it cold if you make it yourself. Most small springs made at springmakers' factories are bent cold in springmaking machines from pre-tempered wire. You will destroy the factory temper if you heat it at all. Wire is hard to heat treat yourself, as the thin section loses heat rapidly so temp control is very difficult.

How long did the original spring last? How did it fail? In the coil or at the ends? Are the ends bent?

Photos of failed spring and mechanism it operates would help.

JD

Edited By Jeff Dayman on 15/04/2013 13:21:20

Herewith a photo of the lock. I have added in red [on the opposite side] how I think the spring fits. I am not entirely clear how the latch actually works. You would think that the 2 no. helical springs would suffice but with them in position and working, the latch itself just flops about. You can see the rough broken end of the spring in the photo. It lasted perhaps 2 - 3 years.

Thanks for the replies to date which as always are very helpful.

Edited By Ian Robinson 4 on 15/04/2013 16:30:35

Hi Ian,

The pix were a great help to understand the spring failure. I think this lock has a major design problem. It looks like they need the torsion springs to increase the latching force, because they didn't leave enough room for adequately sized extension springs. The torsion springs would be under extreme stress to work with large tension as shown. A replacement will likely also break after a few years.

More importantly though, I don't see a cam or space piece between the latch bar and something solid to prevent the latch bar from being pushed back in, say with a credit card, to jimmy the lock open. It looks like the lock depends only on the torsion and extension springs to counteract pushback forces, rather than a cam, spacer or block placed behind the latch on locking.

I would invest in a simple swing type deadbolt (no springs, just a big cam inside) if you are protecting something valuable with this lock. Good deadbolts are simple and can not be pushed back. No springs either, and usually cheaper than a complicated lock like the torsion spring one.

Just my $0.02

JD

Ian,

You might find this page useful regarding spring design.

So far as I am aware, springs are usually wound cold.

MichaelG.

.

P.S.   Just had a thought ... Could you sleeve the broken end with a length of good steel tube?    There seems to be plenty of room in the lock, and 638 Loctite, or similar, should hold spring and tube together nicely.

Edited By Michael Gilligan on 15/04/2013 17:38:39

Ian, which part 'flops about' when you dont have the 3rd spring in place. From the picture it looks like the two springs should pull the plate (with the 'bolt' attached to it) pretty firmly in the down direction.

I would not describe the thing as a lock, its more just a latch that draws back when you turn the handle.

There are online sellers of replacement springs for achitectural and door funiture who will certainly have a generic spring if not the exact one assuming the hardware has a makers name on it.

At least you have got something you can dismantle and repair, more and more these days the products are deliberately made to be non serviceable (as a by product of the quest for cheaper maunfacture) usually defined by the phrase 'continuous product improvement'

Ian

Your right , it is a mortice latch - but with a snib lock, operated by turning one or other of the square spindles on either side of the handle spindle. The latch is part of a plate below the top plate - you can just see it in the photograph but the two plates are independant. It is the latch that 'flops about'.

I thought it would be easy to find a generic spring but so far all enquiries have drawn a blank. A supplier has offered to make some but at a min charge of £60 for 10. I have found something similar but with 6 turns rather than 1.

MG's suggestion is a very good idea - though I think I will try and cold form a few so I have some spares.

Reeves sells 20SWG stainless steel spring wire which hopefully will be suitable?

You may find what you want Here

Edited By Stick on 15/04/2013 19:11:17

Thanks Stick. I tried the Spring Man [and also Johnstones Tools which Springman links to] without succes.

Edited By Ian Robinson 4 on 15/04/2013 19:10:59

Hi Ian R4, I don't want to spoil your fun, but , if we had some dimensions, I may have a suitable one, from an inkjet printer or video recorder. Regards, Bear..

I don't think you should think this is special or difficult because it is a commercial device. This is the sort of thing aeromodellers do every weekend ('cos they crashed last week's plane)
. Get ordinary 'piano wire' from a model shop and bend it cold. Never heat piano wire as it is impossible to reharden. To cut it don't try to cut it with any kind of pliers or cutters - it just leaves a dent in their edge. Use a Junior Hacksaw, not a normal hacksaw as the JH has a a harder, finer tooth. For very thin wire you have to nick it with a triangular file and bend., Don't use the cutters just becaue it is thin.

The failure probably occured because it was nicked and fatigued, not just because it was stressed so be carefull not to damage it.

I have used a pair of similar torsion springs on my flash steamer with no problems they are made from piano wire bought from a model shop.

The only heat treatment was to stress relieve them after bending them on a mandril.

Paul

I have added copy of the dimensioned drawing of the spring I sent to various suppliers to the album. It has one turn only.

I dont think it is difficult but I do think the latch is poorly designed - particulary the way the spring [which seems undersized for what it has to do] acts on the latch. I just want to replace it with something that isn't going to fail in 5 min.

Edited By Ian Robinson 4 on 16/04/2013 07:31:57

Hi Ian R4, Thank you for the dimensions, very precise. The best I can do, is a five coil spring. The five coils measure 10mm in width. Regards, Bear..

Hi Ian,

The spring out of a large clothes peg may do the job if you bend the ends straight and open it up a little.

Neil

Hi Bear - I thought that this might work but having just measured the inside dimension of the latch case it is 8.86mm so a 10mm wide spring + the end loop of the helical spring is going to be too wide. Thanks anyway - very much appreciated.

I'll go to a shop that sells clothes pegs and see if they can be adapted as Neil suggests. If not, I'll have a shot at making some.

Edited By Ian Robinson 4 on 17/04/2013 08:48:04