[Zirok]

hi

As you can see in figure, there is a motion between Part A & B controlled by spring action.

I was curious to know that why such ENGGINERING PRACTICE is followed where a PIN attached in assembly...Actually the pin is undersized by 2 to 3 mm.

( Is it only to guide the spring..{if the spring is long } so as the spring doesnt bends due to extra lenght )



I have not drawn the surrounding structures / elements which guide Part A & B, so please dont assume that A & B are only connected by PIN. Part A & B are different


Zirok

[Geof]

..... ( Is it only to guide the spring..{if the spring is long } so as the spring doesnt bends due to extra lenght )....

Probably just to guide the spring.

When you compress a spring just between two flat surface it will often pop out sideways and the pin inside prevents this. It depends on the length and diameter of the spring and the wire diameter used for the spring. Short, fat springs using thick wire probably don't need a pin to guide them; long skinny springs using thin wire certainly do.

[Zirok]

Thanks,

My spring is shorter in length.

But if I do add a pin inside my spring (which is short) & do this only because to avoid the extra operation of riveting spring to the parts.

Actually the spring is going to be installed in a mechanism which has a long life. This mechanism will operate for 10 to 20 years.

I think that rivets of springs can wear & come out over a period of time.
So, its better to keep the spring loose itself on the pin & avoid riveting.
The spring action will work as the pin inserts to a opening in the counterpart.

Is it a good idea and a acceptable industry standard ??

Zirok

[Boots]

YES !

[Mike Everman]

Very creative, and quite simple and reliable. I'd have to say that 90% of engineers would guess wrong why it is there.

[RICHARD ZASTROW]

Mike, I must be part of the other 10%. LOL

Dick Z

[Mike Everman]

I however, was not. Didn't eat my cheerios or something.

[Zirok]

Thanks all ,

For your wonderful clarification & support.

[wildwestpat]

Hi Zirok

There are several factors to take into consideration. Long life is dependent upon the operating cycles and is normally specified as such not in terms of years. Factors to consider include:-

1. The spring will try and rotate as it is compresed. Look at the design of the valve springs in a car engine. These have ground flat ends to minimise the fretting on the surfaces on which they bear.

2. The spring should be bearing on a hardened surface as well as having ground flat ends. This defect is often seen in mechanisms made of plastic where assembly costs mean that steel washers have been omitted under the spring ends. The springs cutting into the plastic untill failure results. The use of ground flat pring ends is often dispensed with as these springs are much more expensive to make.

3. The spring pressure at the joint surfaces infulences the amount of wear as well as the materials used.

4. There are alternative types of spring other than the compression helical variety. Some times these will offer advatages. The humble washing line clothes peg design is but one every day example where compression vv torsion springs achieve the same end. Think about using a simple loop of spring wire with the ends bent at right angles and inserted into holes in the parts - this can achieve both clamping and separatng forces. The weak point in any spring with bent ends is the bend! The design of springs is a very technical subject particularly if the operating cycles are many and the part is stressed. Look at a stock springs catalogue and try and design arround these - cheaper to buy - more readily obtained both now and as spares!

5. Putting pegs into the ends of the spring is another way of achieving location accuracy but again the ends can wear away the support as the spring winds and unwinds each cycle. An example here is the garden clippers where a coil spring is used to make the clipper self opening.

There is no real substitute for keeping an open mind and observing failed mechanisms as well as working ones.

Hope this helps - Regards Pat

[Zirok]

Hi

If my Pin is of dia 8 mm, than what should be my spring ID...(I mean the clearence between them.


Zirok

[wildwestpat]

You ned to design the spring and pin together as when compressed the spring changes its diameter slightly. The degree of change depends on the material and tempering of the spring as well as the gauge of the wire, number of turns and the load.

Yes you will need to have some clearance between the parts after considering the tolerances to which the parts are manufactured. This is standard design for production proceedure and there is no one size fits all answer.

Regards

Pat

[Geof]

Hi

If my Pin is of dia 8 mm, than what should be my spring ID...(I mean the clearence between them.


Zirok

Don't get too worried about this clearance. The pin is simply there to stop the spring popping out sideways. A clearance of 10% of the ID of the spring is plenty and 30% would not be too much.

[wildwestpat]

You have not indicated the length of the spring and the force it is required to restore. These are the factors that determine the ridgidity of the spring under compression. If the spring is long and the restoring force low and the total movement large - then the spring will try and move side ways and will rub on any central support. This will cause wear on the parts. If the parts you are designing have the requirements of long movement weak compression force which will give a very thin wire gauge - then your coil spring will try and slip sideways. You can accept this and the implied wear or use some other form of spring. I have given you pointers to other types of return spring and the types of mechanism in which they are frequently found. To sum up if the internal support is there to stop the spring snaking sideways and you are looking to achieve millions of operations then redesign the spring - use a different type - use a thicker gauge of wire and reduce the movement by use of a lever - use a tension spring or springs applied elsewhere in the mechanism.

The pleasure of design is there are no set answers and a wealth of alternative ways of achieving the same ends. A good engineering design is both fit for purpose as well as being economical to make and possibly repair.

Good luck with the design and I hope it works out well. Regards Pat

[Zirok]

Thanks for the clearence clarification.


I have some design issues. Need some assistance on the following factors & need your valuable suggestion.

My Project -

I have a very simple application which uses a compression spring. There is a slider attached to one end which pushes (by hand) the spring by 10 mm. After release the spring should throw back a weight of 2 kg(which is the slider weight).

Spring Dimensions -
OD - 14
ID - 9

What should be my -

Closed Length
Free length
Material .......(Which material should I use. Suggest something which is non corrosive)
Hardness........(should i use a hardened spring. My application is simple but the life is 20-30 years)
Pin…I will be using a dowel pin of 8 mm..( Should I use a hardened Dowel Pin ).
Coiling direction......(What should be the coiling direction..Left or right...& what effect it makes on the operation)
Type of end....(There are many types such as plain, square, ground end. Which one is preferable for my application)

Need Help.

[wildwestpat]

The simple answer is to consult the catalogue of the spring maker. If you want to calculate the the spring then you should consult the 'Machinery's Handbook'a published by Industrial Press Inc. - NY. This book is invaluable source of theoretical and practical guidance on mechanical design.

For example a 5/8 spring made from music wire of 0.041" dia has a deflection of 4.63 pounds for a load deflection of 0.2332" per turn. i.e at least 3 active turns would be required to give 10mm deflection. From this you can work out the order of magnitude of your spring BUT there are other factors that come into play such as the force needed to fully compress the spring and of course the material from which it is made and for this reason suggest you need to consult a commercial suppliers catalogue.

However I suspect that your user may have ideas on the maximum force they are prepared to apply to compress your spring and this might mean a much softer spring!

Regards Pat

[Zirok]

Hi

Actually i have designed some spring with some guidance but not sure whether the derived values are within the allowed range.

U seem to me having some designing knowledge about spring.

My Derived Values are -

Spring Constant (K) - 6.045
Shear Stress - 505.367 Mpa
Spring Index (D/d) - 9

Are they OKAY ?

ZIROK

[wildwestpat]

The simple answer is that the maximum stress is related to the material used and the number of flexures you are designing for. This is why I keep on about using stock springs where the material - heat treatment - stress have been fixed by the maker. There is a lot more to making a good spring than just taking a length of music wire and winding it round a rod. If you are going to make your own I strongly suggest you consult an appropriate text book such as the one I mentioned and flog through the calculations.

For a simple helical compression spring the force that has to be exerted to compress the spring obviously increases as the compresed length is reduced. The wire gauge will determine how far the spring compresses as the movement progresses. Whilst your values could be OK there is a lot more to making a good spring than just winding the chosen wire round a rod. This is why I keep suggesting commercial springs where the physical constants have been sorted for you. The difficult bits are finishing the spring so that it has a nice flat end and not a spike that will fret into the other parts of your mechanism.

Regards - Pat

[wildwestpat]

Here take a look! Hope this helps contain costs as well!


http://hardwareproducts.thomasnet.co...ngs?&forward=1

Regards Pat

[Zirok]

hMM....

I will go through some book.

[rowbare]

Zirok,

If you are want to experiment with making some springs, here is a good how to article: http://madmodder.net/index.php?topic=2734.0

bob