# Spring tension calculations

1. Jun 17, 2014

### Palmbay

I am building a shock damper for various weights which will be added at Mass D below. What I need to calculate is by how much would I need to turn my adjusters thus changing the angle of the spring, which in this case is 14.9 Kg rated, to allow for different weights to remain at a level attitude, and the operating arms to remain horizontal. The attachment should explain better perhaps! This actually allows a 2.5Kg Mass to remain level with a 14.9 Kg rated spring. If I add weight to mass D, then the upper and lower arms take on a parallelogram shape and the mass lowers. I then need to screw my adjusters up or down to then bring the new mass to a level point where the parallelogram is once again a rectangle.

Does anyone know of any calculations that might be of assistance here please?

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2. Jun 17, 2014

### Simon Bridge

When you turn the adjusters so that the spring is more horizontal, you are increasing the tension in the spring - that is why the spring is more horizontal.

I think you need to work a free-body diagram for the masses.

3. Jun 18, 2014

### Palmbay

Hi Simon,
When the spring is more towards the horizontal, and both adjusters are slackened right off, it's spring length is shorter than when both adjusters are at a maximum, when the spring is at say 22 deg to the horizontal. This puts the spring under more tension as it is longer surely? This is the part I don't understand.
Not sure how to draw a free-body diagram for this as it just looks like a box with an arrow up for normal and another down for gravity, I can't see how this helps as this is a compound lever problem, please explain. Thank you for your reply.

Last edited: Jun 18, 2014
4. Jun 18, 2014

### Simon Bridge

The way you've drawn it, the spring force does not act upwards.
You are using adjustment screws to change the way the spring force acts - your fbd should reflect that.

5. Jun 19, 2014

### Palmbay

Thanks Simon. At the point of the mass, the resultant spring force is indeed acting directly upward. The parallelogram is of a fixed length of arm. The left had side is tethered, however the right hand side can only move in a vertical direction upwards or downwards, ok, it will move slightly inwards, I agree, as the angle increases, but I don't want that as I want to maintain stability.

In essence, there must be a calculation that will determine that for each mm of movement of one, or the other adjuster, for a given spring strength, the mass would need to be increased, or decreased by X amount to maintain the rectangular shape of the arrangement and for the mass to remain static. Did you have a look at my calculation? Perhaps you can see the flaw? I have looked at wishbone car suspension systems which have a similar setup to work out my calculation.

6. Jun 19, 2014

### Simon Bridge

The masses are constrained to move vertically? Say - in a frictionless shaft?
The top and lower arms, therefore, change in length with angle?

7. Jun 19, 2014

### Palmbay

Yes, you are right, in respect to the horizontal, of course they shorten, if the mass is increased without any spring adjustment. But what I need to do is maintain the mass at its respective attitude. Therefore the arms remain static, the spring is then adjusted to compensate for the increased mass which then stays at its position. The prototype does actually do this, but we need to increase the adjustment to cope with a larger mass, or change the spring rating. Which I really don't want to do. Re-machining costs a fortune, so without a definitive calculation, it is impossible to state that a certain rated spring will cope with a mass range from x to z with appropriate adjustment. believe me, I have spent many a sleepless night over this one, and I don't wish to burden anyone else from scratch, it is a call for help, from anyone that might have experience in this area, that just might know of a calculation which might fit the bill already invented.

8. Jun 21, 2014

### Simon Bridge

So the masses are not constrained to move vertically?
Please be clear on this point. You have drawn the setup in two positions, in one of them the arms are longer overall than in the other one.
I also do not understand where the spring is attached.
You will probably find that a more careful diagram will help.

I understand that the screws are adjusted to keep the arms horizontal - but, without adjusting the screws, what happens when there is more mass?

Using only the top diagram and statics, the spring supplies a torque which counters the torque due to the mass.
Changing the screws will change the spring's length? I think details of the spring assembly will be helpful here - is this basically a rubber-band that you wind around a shaft or is it a metal spring attached to the screws by a length of flexable wire or string?