How Does Spring Coil Count Affect Shock Absorber Performance?

[Mechaman]

Hi All,

I'm trying to design a spring for use in a shock absorber for a college project but my calculations aren't making any sense. I know this is a big ask, but if anyone knows about this stuff I'd appreciate a look.

The maximum load is 16.5kN and when it's sitting it will be 5.5kN The overall height is about 400mm and the rest can be anything needed to get the load value.

It looks like when I lower the number of coils (revolutions) the load needed to deflect the spring increases. This seems wrong, surely more material means more force to deflect?

I have an excel sheet I've been working on with values on there and the relevant pages on coils copied into it (bottom left hand side) if anyone cares to try it?

https://1drv.ms/x/s!ArXz3GkpwfgW02B07v7QxRxfQI0m

 

[jrmichler]

The wire in a coil spring is subject to torsion when the spring is loaded. The spring wire twists under load. More turns makes for longer wire, which twists more under the same load. More twist under the same load means the spring deflects more. More deflection under the same load is a weaker spring.

A good way to verify your calculations is to look up some springs from a manufacturer such as Lee Spring (www.leespring.com) and compare to your calculations. The catalog may not give the number of active turns, but you can calculate that from the solid height and number of end turns.

 

[Mechaman]

Ok thanks I think I understand that part. I was using the formula for finding w but I should be finding strain energy.

If strain energy is measured in n/m how would I say it needs to absorb 16kN at that point? Would the moment of the spring at radius R to the mean of the wire be what takes the point load at the centre of the spring?

 

[jrmichler]

It sounds like you are overthinking this problem. Start with your knowns and constraints: force at displacement, solid height, free length, max or min ID and/or OD, etc. Then use that information to calculate spring rate (kN/m) and free length.

Then, and only then, start designing your spring. Your variables can include wire diameter, number of active turns, and OD. Then check stress at peak load. Iterate as needed.

 

[Mechaman]

I have a static reaction force of the wheel at 5.5kN. The shock load will be around 16.5kN. Even at 5.5kN and 200mm travel I'm getting:

k = 5.5kN / 0.2
k = 2.75kN/m
k = 2750N/m
k = 2750000N/mm

The largest spring I see is rated at 437N/mm here https://www.leespring.com//product_spec.asp?partnum=LHL2000D01&springType=C&subType=

Am I right to be sizing the spring to take the entire shock load? I was under the impression the spring takes the full load then the damper dissipates the energy to slow oscillation?

 

[jrmichler]

It is unclear what problem you are really trying to solve. It's time for a diagram that describes the problem. Then describe in words, not equations, how you came up with your spring forces.

 

[Mechaman]

The problem is a trailer with 2 wheels at the back and one at the front. I used statics to find the reaction force for each axle. The front comes in at 5.5kN. The G force then if hitting a bump I'm setting to 3 gs. 5.5kN * 3 = 16.5kN maximum load at the front axle.

The spring and shock being looked at is <400mm in length and can be any other variable.

So far all attempts at getting a realistic answer have not worked out, seems that the load 16.5kN is way too much and the even 5.5kN seems unrealistic for finding stock springs. Looks like I'm doing something wrong?

Static equation below:

 

[jrmichler]

If you are trying to design a suspension for a vehicle, the better approach is to define a speed and a bump, then design the suspension to absorb that bump with a defined peak displacement or peak acceleration of the vehicle. A very quick Google search using the terms road vehicle acceleration bumps came up with this: http://www.archiwummotoryzacji.pl/images/AM/vol67/vol67-janczur-en-47-60.pdf. When designing a suspension, the peak acceleration of your vehicle is the result of a speed and a bump, not an input.

Your wheel load is roughly comparable to that of an American full size pickup truck. Take a look at some large cars or trucks to get an idea of the sizes of spring used, then measure up a few springs, and calculate their spring constants. That will give you a starting point and a sanity check.